Adding upstream version 1.16.10.upstream/1.16.10upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

362 files changed, 77464 insertions, 0 deletions

diff --git a/src/go/ast/ast.go b/src/go/ast/ast.go
new file mode 100644
index 0000000..300966a
--- /dev/null
+++ b/src/go/ast/ast.go
@@ -0,0 +1,1063 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package ast declares the types used to represent syntax trees for Go
+// packages.
+//
+package ast
+
+import (
+	"go/token"
+	"strings"
+)
+
+// ----------------------------------------------------------------------------
+// Interfaces
+//
+// There are 3 main classes of nodes: Expressions and type nodes,
+// statement nodes, and declaration nodes. The node names usually
+// match the corresponding Go spec production names to which they
+// correspond. The node fields correspond to the individual parts
+// of the respective productions.
+//
+// All nodes contain position information marking the beginning of
+// the corresponding source text segment; it is accessible via the
+// Pos accessor method. Nodes may contain additional position info
+// for language constructs where comments may be found between parts
+// of the construct (typically any larger, parenthesized subpart).
+// That position information is needed to properly position comments
+// when printing the construct.
+
+// All node types implement the Node interface.
+type Node interface {
+	Pos() token.Pos // position of first character belonging to the node
+	End() token.Pos // position of first character immediately after the node
+}
+
+// All expression nodes implement the Expr interface.
+type Expr interface {
+	Node
+	exprNode()
+}
+
+// All statement nodes implement the Stmt interface.
+type Stmt interface {
+	Node
+	stmtNode()
+}
+
+// All declaration nodes implement the Decl interface.
+type Decl interface {
+	Node
+	declNode()
+}
+
+// ----------------------------------------------------------------------------
+// Comments
+
+// A Comment node represents a single //-style or /*-style comment.
+//
+// The Text field contains the comment text without carriage returns (\r) that
+// may have been present in the source. Because a comment's end position is
+// computed using len(Text), the position reported by End() does not match the
+// true source end position for comments containing carriage returns.
+type Comment struct {
+	Slash token.Pos // position of "/" starting the comment
+	Text  string    // comment text (excluding '\n' for //-style comments)
+}
+
+func (c *Comment) Pos() token.Pos { return c.Slash }
+func (c *Comment) End() token.Pos { return token.Pos(int(c.Slash) + len(c.Text)) }
+
+// A CommentGroup represents a sequence of comments
+// with no other tokens and no empty lines between.
+//
+type CommentGroup struct {
+	List []*Comment // len(List) > 0
+}
+
+func (g *CommentGroup) Pos() token.Pos { return g.List[0].Pos() }
+func (g *CommentGroup) End() token.Pos { return g.List[len(g.List)-1].End() }
+
+func isWhitespace(ch byte) bool { return ch == ' ' || ch == '\t' || ch == '\n' || ch == '\r' }
+
+func stripTrailingWhitespace(s string) string {
+	i := len(s)
+	for i > 0 && isWhitespace(s[i-1]) {
+		i--
+	}
+	return s[0:i]
+}
+
+// Text returns the text of the comment.
+// Comment markers (//, /*, and */), the first space of a line comment, and
+// leading and trailing empty lines are removed.
+// Comment directives like "//line" and "//go:noinline" are also removed.
+// Multiple empty lines are reduced to one, and trailing space on lines is trimmed.
+// Unless the result is empty, it is newline-terminated.
+func (g *CommentGroup) Text() string {
+	if g == nil {
+		return ""
+	}
+	comments := make([]string, len(g.List))
+	for i, c := range g.List {
+		comments[i] = c.Text
+	}
+
+	lines := make([]string, 0, 10) // most comments are less than 10 lines
+	for _, c := range comments {
+		// Remove comment markers.
+		// The parser has given us exactly the comment text.
+		switch c[1] {
+		case '/':
+			//-style comment (no newline at the end)
+			c = c[2:]
+			if len(c) == 0 {
+				// empty line
+				break
+			}
+			if c[0] == ' ' {
+				// strip first space - required for Example tests
+				c = c[1:]
+				break
+			}
+			if isDirective(c) {
+				// Ignore //go:noinline, //line, and so on.
+				continue
+			}
+		case '*':
+			/*-style comment */
+			c = c[2 : len(c)-2]
+		}
+
+		// Split on newlines.
+		cl := strings.Split(c, "\n")
+
+		// Walk lines, stripping trailing white space and adding to list.
+		for _, l := range cl {
+			lines = append(lines, stripTrailingWhitespace(l))
+		}
+	}
+
+	// Remove leading blank lines; convert runs of
+	// interior blank lines to a single blank line.
+	n := 0
+	for _, line := range lines {
+		if line != "" || n > 0 && lines[n-1] != "" {
+			lines[n] = line
+			n++
+		}
+	}
+	lines = lines[0:n]
+
+	// Add final "" entry to get trailing newline from Join.
+	if n > 0 && lines[n-1] != "" {
+		lines = append(lines, "")
+	}
+
+	return strings.Join(lines, "\n")
+}
+
+// isDirective reports whether c is a comment directive.
+func isDirective(c string) bool {
+	// "//line " is a line directive.
+	// (The // has been removed.)
+	if strings.HasPrefix(c, "line ") {
+		return true
+	}
+
+	// "//[a-z0-9]+:[a-z0-9]"
+	// (The // has been removed.)
+	colon := strings.Index(c, ":")
+	if colon <= 0 || colon+1 >= len(c) {
+		return false
+	}
+	for i := 0; i <= colon+1; i++ {
+		if i == colon {
+			continue
+		}
+		b := c[i]
+		if !('a' <= b && b <= 'z' || '0' <= b && b <= '9') {
+			return false
+		}
+	}
+	return true
+}
+
+// ----------------------------------------------------------------------------
+// Expressions and types
+
+// A Field represents a Field declaration list in a struct type,
+// a method list in an interface type, or a parameter/result declaration
+// in a signature.
+// Field.Names is nil for unnamed parameters (parameter lists which only contain types)
+// and embedded struct fields. In the latter case, the field name is the type name.
+//
+type Field struct {
+	Doc     *CommentGroup // associated documentation; or nil
+	Names   []*Ident      // field/method/parameter names; or nil
+	Type    Expr          // field/method/parameter type
+	Tag     *BasicLit     // field tag; or nil
+	Comment *CommentGroup // line comments; or nil
+}
+
+func (f *Field) Pos() token.Pos {
+	if len(f.Names) > 0 {
+		return f.Names[0].Pos()
+	}
+	return f.Type.Pos()
+}
+
+func (f *Field) End() token.Pos {
+	if f.Tag != nil {
+		return f.Tag.End()
+	}
+	return f.Type.End()
+}
+
+// A FieldList represents a list of Fields, enclosed by parentheses or braces.
+type FieldList struct {
+	Opening token.Pos // position of opening parenthesis/brace, if any
+	List    []*Field  // field list; or nil
+	Closing token.Pos // position of closing parenthesis/brace, if any
+}
+
+func (f *FieldList) Pos() token.Pos {
+	if f.Opening.IsValid() {
+		return f.Opening
+	}
+	// the list should not be empty in this case;
+	// be conservative and guard against bad ASTs
+	if len(f.List) > 0 {
+		return f.List[0].Pos()
+	}
+	return token.NoPos
+}
+
+func (f *FieldList) End() token.Pos {
+	if f.Closing.IsValid() {
+		return f.Closing + 1
+	}
+	// the list should not be empty in this case;
+	// be conservative and guard against bad ASTs
+	if n := len(f.List); n > 0 {
+		return f.List[n-1].End()
+	}
+	return token.NoPos
+}
+
+// NumFields returns the number of parameters or struct fields represented by a FieldList.
+func (f *FieldList) NumFields() int {
+	n := 0
+	if f != nil {
+		for _, g := range f.List {
+			m := len(g.Names)
+			if m == 0 {
+				m = 1
+			}
+			n += m
+		}
+	}
+	return n
+}
+
+// An expression is represented by a tree consisting of one
+// or more of the following concrete expression nodes.
+//
+type (
+	// A BadExpr node is a placeholder for an expression containing
+	// syntax errors for which a correct expression node cannot be
+	// created.
+	//
+	BadExpr struct {
+		From, To token.Pos // position range of bad expression
+	}
+
+	// An Ident node represents an identifier.
+	Ident struct {
+		NamePos token.Pos // identifier position
+		Name    string    // identifier name
+		Obj     *Object   // denoted object; or nil
+	}
+
+	// An Ellipsis node stands for the "..." type in a
+	// parameter list or the "..." length in an array type.
+	//
+	Ellipsis struct {
+		Ellipsis token.Pos // position of "..."
+		Elt      Expr      // ellipsis element type (parameter lists only); or nil
+	}
+
+	// A BasicLit node represents a literal of basic type.
+	//
+	// Note that for the CHAR and STRING kinds, the literal is stored
+	// with its quotes. For example, for a double-quoted STRING, the
+	// first and the last rune in the Value field will be ". The
+	// Unquote and UnquoteChar functions in the strconv package can be
+	// used to unquote STRING and CHAR values, respectively.
+	BasicLit struct {
+		ValuePos token.Pos   // literal position
+		Kind     token.Token // token.INT, token.FLOAT, token.IMAG, token.CHAR, or token.STRING
+		Value    string      // literal string; e.g. 42, 0x7f, 3.14, 1e-9, 2.4i, 'a', '\x7f', "foo" or `\m\n\o`
+	}
+
+	// A FuncLit node represents a function literal.
+	FuncLit struct {
+		Type *FuncType  // function type
+		Body *BlockStmt // function body
+	}
+
+	// A CompositeLit node represents a composite literal.
+	CompositeLit struct {
+		Type       Expr      // literal type; or nil
+		Lbrace     token.Pos // position of "{"
+		Elts       []Expr    // list of composite elements; or nil
+		Rbrace     token.Pos // position of "}"
+		Incomplete bool      // true if (source) expressions are missing in the Elts list
+	}
+
+	// A ParenExpr node represents a parenthesized expression.
+	ParenExpr struct {
+		Lparen token.Pos // position of "("
+		X      Expr      // parenthesized expression
+		Rparen token.Pos // position of ")"
+	}
+
+	// A SelectorExpr node represents an expression followed by a selector.
+	SelectorExpr struct {
+		X   Expr   // expression
+		Sel *Ident // field selector
+	}
+
+	// An IndexExpr node represents an expression followed by an index.
+	IndexExpr struct {
+		X      Expr      // expression
+		Lbrack token.Pos // position of "["
+		Index  Expr      // index expression
+		Rbrack token.Pos // position of "]"
+	}
+
+	// A SliceExpr node represents an expression followed by slice indices.
+	SliceExpr struct {
+		X      Expr      // expression
+		Lbrack token.Pos // position of "["
+		Low    Expr      // begin of slice range; or nil
+		High   Expr      // end of slice range; or nil
+		Max    Expr      // maximum capacity of slice; or nil
+		Slice3 bool      // true if 3-index slice (2 colons present)
+		Rbrack token.Pos // position of "]"
+	}
+
+	// A TypeAssertExpr node represents an expression followed by a
+	// type assertion.
+	//
+	TypeAssertExpr struct {
+		X      Expr      // expression
+		Lparen token.Pos // position of "("
+		Type   Expr      // asserted type; nil means type switch X.(type)
+		Rparen token.Pos // position of ")"
+	}
+
+	// A CallExpr node represents an expression followed by an argument list.
+	CallExpr struct {
+		Fun      Expr      // function expression
+		Lparen   token.Pos // position of "("
+		Args     []Expr    // function arguments; or nil
+		Ellipsis token.Pos // position of "..." (token.NoPos if there is no "...")
+		Rparen   token.Pos // position of ")"
+	}
+
+	// A StarExpr node represents an expression of the form "*" Expression.
+	// Semantically it could be a unary "*" expression, or a pointer type.
+	//
+	StarExpr struct {
+		Star token.Pos // position of "*"
+		X    Expr      // operand
+	}
+
+	// A UnaryExpr node represents a unary expression.
+	// Unary "*" expressions are represented via StarExpr nodes.
+	//
+	UnaryExpr struct {
+		OpPos token.Pos   // position of Op
+		Op    token.Token // operator
+		X     Expr        // operand
+	}
+
+	// A BinaryExpr node represents a binary expression.
+	BinaryExpr struct {
+		X     Expr        // left operand
+		OpPos token.Pos   // position of Op
+		Op    token.Token // operator
+		Y     Expr        // right operand
+	}
+
+	// A KeyValueExpr node represents (key : value) pairs
+	// in composite literals.
+	//
+	KeyValueExpr struct {
+		Key   Expr
+		Colon token.Pos // position of ":"
+		Value Expr
+	}
+)
+
+// The direction of a channel type is indicated by a bit
+// mask including one or both of the following constants.
+//
+type ChanDir int
+
+const (
+	SEND ChanDir = 1 << iota
+	RECV
+)
+
+// A type is represented by a tree consisting of one
+// or more of the following type-specific expression
+// nodes.
+//
+type (
+	// An ArrayType node represents an array or slice type.
+	ArrayType struct {
+		Lbrack token.Pos // position of "["
+		Len    Expr      // Ellipsis node for [...]T array types, nil for slice types
+		Elt    Expr      // element type
+	}
+
+	// A StructType node represents a struct type.
+	StructType struct {
+		Struct     token.Pos  // position of "struct" keyword
+		Fields     *FieldList // list of field declarations
+		Incomplete bool       // true if (source) fields are missing in the Fields list
+	}
+
+	// Pointer types are represented via StarExpr nodes.
+
+	// A FuncType node represents a function type.
+	FuncType struct {
+		Func    token.Pos  // position of "func" keyword (token.NoPos if there is no "func")
+		Params  *FieldList // (incoming) parameters; non-nil
+		Results *FieldList // (outgoing) results; or nil
+	}
+
+	// An InterfaceType node represents an interface type.
+	InterfaceType struct {
+		Interface  token.Pos  // position of "interface" keyword
+		Methods    *FieldList // list of methods
+		Incomplete bool       // true if (source) methods are missing in the Methods list
+	}
+
+	// A MapType node represents a map type.
+	MapType struct {
+		Map   token.Pos // position of "map" keyword
+		Key   Expr
+		Value Expr
+	}
+
+	// A ChanType node represents a channel type.
+	ChanType struct {
+		Begin token.Pos // position of "chan" keyword or "<-" (whichever comes first)
+		Arrow token.Pos // position of "<-" (token.NoPos if there is no "<-")
+		Dir   ChanDir   // channel direction
+		Value Expr      // value type
+	}
+)
+
+// Pos and End implementations for expression/type nodes.
+
+func (x *BadExpr) Pos() token.Pos  { return x.From }
+func (x *Ident) Pos() token.Pos    { return x.NamePos }
+func (x *Ellipsis) Pos() token.Pos { return x.Ellipsis }
+func (x *BasicLit) Pos() token.Pos { return x.ValuePos }
+func (x *FuncLit) Pos() token.Pos  { return x.Type.Pos() }
+func (x *CompositeLit) Pos() token.Pos {
+	if x.Type != nil {
+		return x.Type.Pos()
+	}
+	return x.Lbrace
+}
+func (x *ParenExpr) Pos() token.Pos      { return x.Lparen }
+func (x *SelectorExpr) Pos() token.Pos   { return x.X.Pos() }
+func (x *IndexExpr) Pos() token.Pos      { return x.X.Pos() }
+func (x *SliceExpr) Pos() token.Pos      { return x.X.Pos() }
+func (x *TypeAssertExpr) Pos() token.Pos { return x.X.Pos() }
+func (x *CallExpr) Pos() token.Pos       { return x.Fun.Pos() }
+func (x *StarExpr) Pos() token.Pos       { return x.Star }
+func (x *UnaryExpr) Pos() token.Pos      { return x.OpPos }
+func (x *BinaryExpr) Pos() token.Pos     { return x.X.Pos() }
+func (x *KeyValueExpr) Pos() token.Pos   { return x.Key.Pos() }
+func (x *ArrayType) Pos() token.Pos      { return x.Lbrack }
+func (x *StructType) Pos() token.Pos     { return x.Struct }
+func (x *FuncType) Pos() token.Pos {
+	if x.Func.IsValid() || x.Params == nil { // see issue 3870
+		return x.Func
+	}
+	return x.Params.Pos() // interface method declarations have no "func" keyword
+}
+func (x *InterfaceType) Pos() token.Pos { return x.Interface }
+func (x *MapType) Pos() token.Pos       { return x.Map }
+func (x *ChanType) Pos() token.Pos      { return x.Begin }
+
+func (x *BadExpr) End() token.Pos { return x.To }
+func (x *Ident) End() token.Pos   { return token.Pos(int(x.NamePos) + len(x.Name)) }
+func (x *Ellipsis) End() token.Pos {
+	if x.Elt != nil {
+		return x.Elt.End()
+	}
+	return x.Ellipsis + 3 // len("...")
+}
+func (x *BasicLit) End() token.Pos       { return token.Pos(int(x.ValuePos) + len(x.Value)) }
+func (x *FuncLit) End() token.Pos        { return x.Body.End() }
+func (x *CompositeLit) End() token.Pos   { return x.Rbrace + 1 }
+func (x *ParenExpr) End() token.Pos      { return x.Rparen + 1 }
+func (x *SelectorExpr) End() token.Pos   { return x.Sel.End() }
+func (x *IndexExpr) End() token.Pos      { return x.Rbrack + 1 }
+func (x *SliceExpr) End() token.Pos      { return x.Rbrack + 1 }
+func (x *TypeAssertExpr) End() token.Pos { return x.Rparen + 1 }
+func (x *CallExpr) End() token.Pos       { return x.Rparen + 1 }
+func (x *StarExpr) End() token.Pos       { return x.X.End() }
+func (x *UnaryExpr) End() token.Pos      { return x.X.End() }
+func (x *BinaryExpr) End() token.Pos     { return x.Y.End() }
+func (x *KeyValueExpr) End() token.Pos   { return x.Value.End() }
+func (x *ArrayType) End() token.Pos      { return x.Elt.End() }
+func (x *StructType) End() token.Pos     { return x.Fields.End() }
+func (x *FuncType) End() token.Pos {
+	if x.Results != nil {
+		return x.Results.End()
+	}
+	return x.Params.End()
+}
+func (x *InterfaceType) End() token.Pos { return x.Methods.End() }
+func (x *MapType) End() token.Pos       { return x.Value.End() }
+func (x *ChanType) End() token.Pos      { return x.Value.End() }
+
+// exprNode() ensures that only expression/type nodes can be
+// assigned to an Expr.
+//
+func (*BadExpr) exprNode()        {}
+func (*Ident) exprNode()          {}
+func (*Ellipsis) exprNode()       {}
+func (*BasicLit) exprNode()       {}
+func (*FuncLit) exprNode()        {}
+func (*CompositeLit) exprNode()   {}
+func (*ParenExpr) exprNode()      {}
+func (*SelectorExpr) exprNode()   {}
+func (*IndexExpr) exprNode()      {}
+func (*SliceExpr) exprNode()      {}
+func (*TypeAssertExpr) exprNode() {}
+func (*CallExpr) exprNode()       {}
+func (*StarExpr) exprNode()       {}
+func (*UnaryExpr) exprNode()      {}
+func (*BinaryExpr) exprNode()     {}
+func (*KeyValueExpr) exprNode()   {}
+
+func (*ArrayType) exprNode()     {}
+func (*StructType) exprNode()    {}
+func (*FuncType) exprNode()      {}
+func (*InterfaceType) exprNode() {}
+func (*MapType) exprNode()       {}
+func (*ChanType) exprNode()      {}
+
+// ----------------------------------------------------------------------------
+// Convenience functions for Idents
+
+// NewIdent creates a new Ident without position.
+// Useful for ASTs generated by code other than the Go parser.
+//
+func NewIdent(name string) *Ident { return &Ident{token.NoPos, name, nil} }
+
+// IsExported reports whether name starts with an upper-case letter.
+//
+func IsExported(name string) bool { return token.IsExported(name) }
+
+// IsExported reports whether id starts with an upper-case letter.
+//
+func (id *Ident) IsExported() bool { return token.IsExported(id.Name) }
+
+func (id *Ident) String() string {
+	if id != nil {
+		return id.Name
+	}
+	return "<nil>"
+}
+
+// ----------------------------------------------------------------------------
+// Statements
+
+// A statement is represented by a tree consisting of one
+// or more of the following concrete statement nodes.
+//
+type (
+	// A BadStmt node is a placeholder for statements containing
+	// syntax errors for which no correct statement nodes can be
+	// created.
+	//
+	BadStmt struct {
+		From, To token.Pos // position range of bad statement
+	}
+
+	// A DeclStmt node represents a declaration in a statement list.
+	DeclStmt struct {
+		Decl Decl // *GenDecl with CONST, TYPE, or VAR token
+	}
+
+	// An EmptyStmt node represents an empty statement.
+	// The "position" of the empty statement is the position
+	// of the immediately following (explicit or implicit) semicolon.
+	//
+	EmptyStmt struct {
+		Semicolon token.Pos // position of following ";"
+		Implicit  bool      // if set, ";" was omitted in the source
+	}
+
+	// A LabeledStmt node represents a labeled statement.
+	LabeledStmt struct {
+		Label *Ident
+		Colon token.Pos // position of ":"
+		Stmt  Stmt
+	}
+
+	// An ExprStmt node represents a (stand-alone) expression
+	// in a statement list.
+	//
+	ExprStmt struct {
+		X Expr // expression
+	}
+
+	// A SendStmt node represents a send statement.
+	SendStmt struct {
+		Chan  Expr
+		Arrow token.Pos // position of "<-"
+		Value Expr
+	}
+
+	// An IncDecStmt node represents an increment or decrement statement.
+	IncDecStmt struct {
+		X      Expr
+		TokPos token.Pos   // position of Tok
+		Tok    token.Token // INC or DEC
+	}
+
+	// An AssignStmt node represents an assignment or
+	// a short variable declaration.
+	//
+	AssignStmt struct {
+		Lhs    []Expr
+		TokPos token.Pos   // position of Tok
+		Tok    token.Token // assignment token, DEFINE
+		Rhs    []Expr
+	}
+
+	// A GoStmt node represents a go statement.
+	GoStmt struct {
+		Go   token.Pos // position of "go" keyword
+		Call *CallExpr
+	}
+
+	// A DeferStmt node represents a defer statement.
+	DeferStmt struct {
+		Defer token.Pos // position of "defer" keyword
+		Call  *CallExpr
+	}
+
+	// A ReturnStmt node represents a return statement.
+	ReturnStmt struct {
+		Return  token.Pos // position of "return" keyword
+		Results []Expr    // result expressions; or nil
+	}
+
+	// A BranchStmt node represents a break, continue, goto,
+	// or fallthrough statement.
+	//
+	BranchStmt struct {
+		TokPos token.Pos   // position of Tok
+		Tok    token.Token // keyword token (BREAK, CONTINUE, GOTO, FALLTHROUGH)
+		Label  *Ident      // label name; or nil
+	}
+
+	// A BlockStmt node represents a braced statement list.
+	BlockStmt struct {
+		Lbrace token.Pos // position of "{"
+		List   []Stmt
+		Rbrace token.Pos // position of "}", if any (may be absent due to syntax error)
+	}
+
+	// An IfStmt node represents an if statement.
+	IfStmt struct {
+		If   token.Pos // position of "if" keyword
+		Init Stmt      // initialization statement; or nil
+		Cond Expr      // condition
+		Body *BlockStmt
+		Else Stmt // else branch; or nil
+	}
+
+	// A CaseClause represents a case of an expression or type switch statement.
+	CaseClause struct {
+		Case  token.Pos // position of "case" or "default" keyword
+		List  []Expr    // list of expressions or types; nil means default case
+		Colon token.Pos // position of ":"
+		Body  []Stmt    // statement list; or nil
+	}
+
+	// A SwitchStmt node represents an expression switch statement.
+	SwitchStmt struct {
+		Switch token.Pos  // position of "switch" keyword
+		Init   Stmt       // initialization statement; or nil
+		Tag    Expr       // tag expression; or nil
+		Body   *BlockStmt // CaseClauses only
+	}
+
+	// A TypeSwitchStmt node represents a type switch statement.
+	TypeSwitchStmt struct {
+		Switch token.Pos  // position of "switch" keyword
+		Init   Stmt       // initialization statement; or nil
+		Assign Stmt       // x := y.(type) or y.(type)
+		Body   *BlockStmt // CaseClauses only
+	}
+
+	// A CommClause node represents a case of a select statement.
+	CommClause struct {
+		Case  token.Pos // position of "case" or "default" keyword
+		Comm  Stmt      // send or receive statement; nil means default case
+		Colon token.Pos // position of ":"
+		Body  []Stmt    // statement list; or nil
+	}
+
+	// A SelectStmt node represents a select statement.
+	SelectStmt struct {
+		Select token.Pos  // position of "select" keyword
+		Body   *BlockStmt // CommClauses only
+	}
+
+	// A ForStmt represents a for statement.
+	ForStmt struct {
+		For  token.Pos // position of "for" keyword
+		Init Stmt      // initialization statement; or nil
+		Cond Expr      // condition; or nil
+		Post Stmt      // post iteration statement; or nil
+		Body *BlockStmt
+	}
+
+	// A RangeStmt represents a for statement with a range clause.
+	RangeStmt struct {
+		For        token.Pos   // position of "for" keyword
+		Key, Value Expr        // Key, Value may be nil
+		TokPos     token.Pos   // position of Tok; invalid if Key == nil
+		Tok        token.Token // ILLEGAL if Key == nil, ASSIGN, DEFINE
+		X          Expr        // value to range over
+		Body       *BlockStmt
+	}
+)
+
+// Pos and End implementations for statement nodes.
+
+func (s *BadStmt) Pos() token.Pos        { return s.From }
+func (s *DeclStmt) Pos() token.Pos       { return s.Decl.Pos() }
+func (s *EmptyStmt) Pos() token.Pos      { return s.Semicolon }
+func (s *LabeledStmt) Pos() token.Pos    { return s.Label.Pos() }
+func (s *ExprStmt) Pos() token.Pos       { return s.X.Pos() }
+func (s *SendStmt) Pos() token.Pos       { return s.Chan.Pos() }
+func (s *IncDecStmt) Pos() token.Pos     { return s.X.Pos() }
+func (s *AssignStmt) Pos() token.Pos     { return s.Lhs[0].Pos() }
+func (s *GoStmt) Pos() token.Pos         { return s.Go }
+func (s *DeferStmt) Pos() token.Pos      { return s.Defer }
+func (s *ReturnStmt) Pos() token.Pos     { return s.Return }
+func (s *BranchStmt) Pos() token.Pos     { return s.TokPos }
+func (s *BlockStmt) Pos() token.Pos      { return s.Lbrace }
+func (s *IfStmt) Pos() token.Pos         { return s.If }
+func (s *CaseClause) Pos() token.Pos     { return s.Case }
+func (s *SwitchStmt) Pos() token.Pos     { return s.Switch }
+func (s *TypeSwitchStmt) Pos() token.Pos { return s.Switch }
+func (s *CommClause) Pos() token.Pos     { return s.Case }
+func (s *SelectStmt) Pos() token.Pos     { return s.Select }
+func (s *ForStmt) Pos() token.Pos        { return s.For }
+func (s *RangeStmt) Pos() token.Pos      { return s.For }
+
+func (s *BadStmt) End() token.Pos  { return s.To }
+func (s *DeclStmt) End() token.Pos { return s.Decl.End() }
+func (s *EmptyStmt) End() token.Pos {
+	if s.Implicit {
+		return s.Semicolon
+	}
+	return s.Semicolon + 1 /* len(";") */
+}
+func (s *LabeledStmt) End() token.Pos { return s.Stmt.End() }
+func (s *ExprStmt) End() token.Pos    { return s.X.End() }
+func (s *SendStmt) End() token.Pos    { return s.Value.End() }
+func (s *IncDecStmt) End() token.Pos {
+	return s.TokPos + 2 /* len("++") */
+}
+func (s *AssignStmt) End() token.Pos { return s.Rhs[len(s.Rhs)-1].End() }
+func (s *GoStmt) End() token.Pos     { return s.Call.End() }
+func (s *DeferStmt) End() token.Pos  { return s.Call.End() }
+func (s *ReturnStmt) End() token.Pos {
+	if n := len(s.Results); n > 0 {
+		return s.Results[n-1].End()
+	}
+	return s.Return + 6 // len("return")
+}
+func (s *BranchStmt) End() token.Pos {
+	if s.Label != nil {
+		return s.Label.End()
+	}
+	return token.Pos(int(s.TokPos) + len(s.Tok.String()))
+}
+func (s *BlockStmt) End() token.Pos {
+	if s.Rbrace.IsValid() {
+		return s.Rbrace + 1
+	}
+	if n := len(s.List); n > 0 {
+		return s.List[n-1].End()
+	}
+	return s.Lbrace + 1
+}
+func (s *IfStmt) End() token.Pos {
+	if s.Else != nil {
+		return s.Else.End()
+	}
+	return s.Body.End()
+}
+func (s *CaseClause) End() token.Pos {
+	if n := len(s.Body); n > 0 {
+		return s.Body[n-1].End()
+	}
+	return s.Colon + 1
+}
+func (s *SwitchStmt) End() token.Pos     { return s.Body.End() }
+func (s *TypeSwitchStmt) End() token.Pos { return s.Body.End() }
+func (s *CommClause) End() token.Pos {
+	if n := len(s.Body); n > 0 {
+		return s.Body[n-1].End()
+	}
+	return s.Colon + 1
+}
+func (s *SelectStmt) End() token.Pos { return s.Body.End() }
+func (s *ForStmt) End() token.Pos    { return s.Body.End() }
+func (s *RangeStmt) End() token.Pos  { return s.Body.End() }
+
+// stmtNode() ensures that only statement nodes can be
+// assigned to a Stmt.
+//
+func (*BadStmt) stmtNode()        {}
+func (*DeclStmt) stmtNode()       {}
+func (*EmptyStmt) stmtNode()      {}
+func (*LabeledStmt) stmtNode()    {}
+func (*ExprStmt) stmtNode()       {}
+func (*SendStmt) stmtNode()       {}
+func (*IncDecStmt) stmtNode()     {}
+func (*AssignStmt) stmtNode()     {}
+func (*GoStmt) stmtNode()         {}
+func (*DeferStmt) stmtNode()      {}
+func (*ReturnStmt) stmtNode()     {}
+func (*BranchStmt) stmtNode()     {}
+func (*BlockStmt) stmtNode()      {}
+func (*IfStmt) stmtNode()         {}
+func (*CaseClause) stmtNode()     {}
+func (*SwitchStmt) stmtNode()     {}
+func (*TypeSwitchStmt) stmtNode() {}
+func (*CommClause) stmtNode()     {}
+func (*SelectStmt) stmtNode()     {}
+func (*ForStmt) stmtNode()        {}
+func (*RangeStmt) stmtNode()      {}
+
+// ----------------------------------------------------------------------------
+// Declarations
+
+// A Spec node represents a single (non-parenthesized) import,
+// constant, type, or variable declaration.
+//
+type (
+	// The Spec type stands for any of *ImportSpec, *ValueSpec, and *TypeSpec.
+	Spec interface {
+		Node
+		specNode()
+	}
+
+	// An ImportSpec node represents a single package import.
+	ImportSpec struct {
+		Doc     *CommentGroup // associated documentation; or nil
+		Name    *Ident        // local package name (including "."); or nil
+		Path    *BasicLit     // import path
+		Comment *CommentGroup // line comments; or nil
+		EndPos  token.Pos     // end of spec (overrides Path.Pos if nonzero)
+	}
+
+	// A ValueSpec node represents a constant or variable declaration
+	// (ConstSpec or VarSpec production).
+	//
+	ValueSpec struct {
+		Doc     *CommentGroup // associated documentation; or nil
+		Names   []*Ident      // value names (len(Names) > 0)
+		Type    Expr          // value type; or nil
+		Values  []Expr        // initial values; or nil
+		Comment *CommentGroup // line comments; or nil
+	}
+
+	// A TypeSpec node represents a type declaration (TypeSpec production).
+	TypeSpec struct {
+		Doc     *CommentGroup // associated documentation; or nil
+		Name    *Ident        // type name
+		Assign  token.Pos     // position of '=', if any
+		Type    Expr          // *Ident, *ParenExpr, *SelectorExpr, *StarExpr, or any of the *XxxTypes
+		Comment *CommentGroup // line comments; or nil
+	}
+)
+
+// Pos and End implementations for spec nodes.
+
+func (s *ImportSpec) Pos() token.Pos {
+	if s.Name != nil {
+		return s.Name.Pos()
+	}
+	return s.Path.Pos()
+}
+func (s *ValueSpec) Pos() token.Pos { return s.Names[0].Pos() }
+func (s *TypeSpec) Pos() token.Pos  { return s.Name.Pos() }
+
+func (s *ImportSpec) End() token.Pos {
+	if s.EndPos != 0 {
+		return s.EndPos
+	}
+	return s.Path.End()
+}
+
+func (s *ValueSpec) End() token.Pos {
+	if n := len(s.Values); n > 0 {
+		return s.Values[n-1].End()
+	}
+	if s.Type != nil {
+		return s.Type.End()
+	}
+	return s.Names[len(s.Names)-1].End()
+}
+func (s *TypeSpec) End() token.Pos { return s.Type.End() }
+
+// specNode() ensures that only spec nodes can be
+// assigned to a Spec.
+//
+func (*ImportSpec) specNode() {}
+func (*ValueSpec) specNode()  {}
+func (*TypeSpec) specNode()   {}
+
+// A declaration is represented by one of the following declaration nodes.
+//
+type (
+	// A BadDecl node is a placeholder for a declaration containing
+	// syntax errors for which a correct declaration node cannot be
+	// created.
+	//
+	BadDecl struct {
+		From, To token.Pos // position range of bad declaration
+	}
+
+	// A GenDecl node (generic declaration node) represents an import,
+	// constant, type or variable declaration. A valid Lparen position
+	// (Lparen.IsValid()) indicates a parenthesized declaration.
+	//
+	// Relationship between Tok value and Specs element type:
+	//
+	//	token.IMPORT  *ImportSpec
+	//	token.CONST   *ValueSpec
+	//	token.TYPE    *TypeSpec
+	//	token.VAR     *ValueSpec
+	//
+	GenDecl struct {
+		Doc    *CommentGroup // associated documentation; or nil
+		TokPos token.Pos     // position of Tok
+		Tok    token.Token   // IMPORT, CONST, TYPE, VAR
+		Lparen token.Pos     // position of '(', if any
+		Specs  []Spec
+		Rparen token.Pos // position of ')', if any
+	}
+
+	// A FuncDecl node represents a function declaration.
+	FuncDecl struct {
+		Doc  *CommentGroup // associated documentation; or nil
+		Recv *FieldList    // receiver (methods); or nil (functions)
+		Name *Ident        // function/method name
+		Type *FuncType     // function signature: parameters, results, and position of "func" keyword
+		Body *BlockStmt    // function body; or nil for external (non-Go) function
+	}
+)
+
+// Pos and End implementations for declaration nodes.
+
+func (d *BadDecl) Pos() token.Pos  { return d.From }
+func (d *GenDecl) Pos() token.Pos  { return d.TokPos }
+func (d *FuncDecl) Pos() token.Pos { return d.Type.Pos() }
+
+func (d *BadDecl) End() token.Pos { return d.To }
+func (d *GenDecl) End() token.Pos {
+	if d.Rparen.IsValid() {
+		return d.Rparen + 1
+	}
+	return d.Specs[0].End()
+}
+func (d *FuncDecl) End() token.Pos {
+	if d.Body != nil {
+		return d.Body.End()
+	}
+	return d.Type.End()
+}
+
+// declNode() ensures that only declaration nodes can be
+// assigned to a Decl.
+//
+func (*BadDecl) declNode()  {}
+func (*GenDecl) declNode()  {}
+func (*FuncDecl) declNode() {}
+
+// ----------------------------------------------------------------------------
+// Files and packages
+
+// A File node represents a Go source file.
+//
+// The Comments list contains all comments in the source file in order of
+// appearance, including the comments that are pointed to from other nodes
+// via Doc and Comment fields.
+//
+// For correct printing of source code containing comments (using packages
+// go/format and go/printer), special care must be taken to update comments
+// when a File's syntax tree is modified: For printing, comments are interspersed
+// between tokens based on their position. If syntax tree nodes are
+// removed or moved, relevant comments in their vicinity must also be removed
+// (from the File.Comments list) or moved accordingly (by updating their
+// positions). A CommentMap may be used to facilitate some of these operations.
+//
+// Whether and how a comment is associated with a node depends on the
+// interpretation of the syntax tree by the manipulating program: Except for Doc
+// and Comment comments directly associated with nodes, the remaining comments
+// are "free-floating" (see also issues #18593, #20744).
+//
+type File struct {
+	Doc        *CommentGroup   // associated documentation; or nil
+	Package    token.Pos       // position of "package" keyword
+	Name       *Ident          // package name
+	Decls      []Decl          // top-level declarations; or nil
+	Scope      *Scope          // package scope (this file only)
+	Imports    []*ImportSpec   // imports in this file
+	Unresolved []*Ident        // unresolved identifiers in this file
+	Comments   []*CommentGroup // list of all comments in the source file
+}
+
+func (f *File) Pos() token.Pos { return f.Package }
+func (f *File) End() token.Pos {
+	if n := len(f.Decls); n > 0 {
+		return f.Decls[n-1].End()
+	}
+	return f.Name.End()
+}
+
+// A Package node represents a set of source files
+// collectively building a Go package.
+//
+type Package struct {
+	Name    string             // package name
+	Scope   *Scope             // package scope across all files
+	Imports map[string]*Object // map of package id -> package object
+	Files   map[string]*File   // Go source files by filename
+}
+
+func (p *Package) Pos() token.Pos { return token.NoPos }
+func (p *Package) End() token.Pos { return token.NoPos }
diff --git a/src/go/ast/ast_test.go b/src/go/ast/ast_test.go
new file mode 100644
index 0000000..71b2d6c
--- /dev/null
+++ b/src/go/ast/ast_test.go
@@ -0,0 +1,79 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import (
+	"testing"
+)
+
+var comments = []struct {
+	list []string
+	text string
+}{
+	{[]string{"//"}, ""},
+	{[]string{"//   "}, ""},
+	{[]string{"//", "//", "//   "}, ""},
+	{[]string{"// foo   "}, "foo\n"},
+	{[]string{"//", "//", "// foo"}, "foo\n"},
+	{[]string{"// foo  bar  "}, "foo  bar\n"},
+	{[]string{"// foo", "// bar"}, "foo\nbar\n"},
+	{[]string{"// foo", "//", "//", "//", "// bar"}, "foo\n\nbar\n"},
+	{[]string{"// foo", "/* bar */"}, "foo\n bar\n"},
+	{[]string{"//", "//", "//", "// foo", "//", "//", "//"}, "foo\n"},
+
+	{[]string{"/**/"}, ""},
+	{[]string{"/*   */"}, ""},
+	{[]string{"/**/", "/**/", "/*   */"}, ""},
+	{[]string{"/* Foo   */"}, " Foo\n"},
+	{[]string{"/* Foo  Bar  */"}, " Foo  Bar\n"},
+	{[]string{"/* Foo*/", "/* Bar*/"}, " Foo\n Bar\n"},
+	{[]string{"/* Foo*/", "/**/", "/**/", "/**/", "// Bar"}, " Foo\n\nBar\n"},
+	{[]string{"/* Foo*/", "/*\n*/", "//", "/*\n*/", "// Bar"}, " Foo\n\nBar\n"},
+	{[]string{"/* Foo*/", "// Bar"}, " Foo\nBar\n"},
+	{[]string{"/* Foo\n Bar*/"}, " Foo\n Bar\n"},
+
+	{[]string{"// foo", "//go:noinline", "// bar", "//:baz"}, "foo\nbar\n:baz\n"},
+	{[]string{"// foo", "//lint123:ignore", "// bar"}, "foo\nbar\n"},
+}
+
+func TestCommentText(t *testing.T) {
+	for i, c := range comments {
+		list := make([]*Comment, len(c.list))
+		for i, s := range c.list {
+			list[i] = &Comment{Text: s}
+		}
+
+		text := (&CommentGroup{list}).Text()
+		if text != c.text {
+			t.Errorf("case %d: got %q; expected %q", i, text, c.text)
+		}
+	}
+}
+
+var isDirectiveTests = []struct {
+	in string
+	ok bool
+}{
+	{"abc", false},
+	{"go:inline", true},
+	{"Go:inline", false},
+	{"go:Inline", false},
+	{":inline", false},
+	{"lint:ignore", true},
+	{"lint:1234", true},
+	{"1234:lint", true},
+	{"go: inline", false},
+	{"go:", false},
+	{"go:*", false},
+	{"go:x*", true},
+}
+
+func TestIsDirective(t *testing.T) {
+	for _, tt := range isDirectiveTests {
+		if ok := isDirective(tt.in); ok != tt.ok {
+			t.Errorf("isDirective(%q) = %v, want %v", tt.in, ok, tt.ok)
+		}
+	}
+}
diff --git a/src/go/ast/commentmap.go b/src/go/ast/commentmap.go
new file mode 100644
index 0000000..2a653a6
--- /dev/null
+++ b/src/go/ast/commentmap.go
@@ -0,0 +1,332 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import (
+	"bytes"
+	"fmt"
+	"go/token"
+	"sort"
+)
+
+type byPos []*CommentGroup
+
+func (a byPos) Len() int           { return len(a) }
+func (a byPos) Less(i, j int) bool { return a[i].Pos() < a[j].Pos() }
+func (a byPos) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+
+// sortComments sorts the list of comment groups in source order.
+//
+func sortComments(list []*CommentGroup) {
+	// TODO(gri): Does it make sense to check for sorted-ness
+	//            first (because we know that sorted-ness is
+	//            very likely)?
+	if orderedList := byPos(list); !sort.IsSorted(orderedList) {
+		sort.Sort(orderedList)
+	}
+}
+
+// A CommentMap maps an AST node to a list of comment groups
+// associated with it. See NewCommentMap for a description of
+// the association.
+//
+type CommentMap map[Node][]*CommentGroup
+
+func (cmap CommentMap) addComment(n Node, c *CommentGroup) {
+	list := cmap[n]
+	if len(list) == 0 {
+		list = []*CommentGroup{c}
+	} else {
+		list = append(list, c)
+	}
+	cmap[n] = list
+}
+
+type byInterval []Node
+
+func (a byInterval) Len() int { return len(a) }
+func (a byInterval) Less(i, j int) bool {
+	pi, pj := a[i].Pos(), a[j].Pos()
+	return pi < pj || pi == pj && a[i].End() > a[j].End()
+}
+func (a byInterval) Swap(i, j int) { a[i], a[j] = a[j], a[i] }
+
+// nodeList returns the list of nodes of the AST n in source order.
+//
+func nodeList(n Node) []Node {
+	var list []Node
+	Inspect(n, func(n Node) bool {
+		// don't collect comments
+		switch n.(type) {
+		case nil, *CommentGroup, *Comment:
+			return false
+		}
+		list = append(list, n)
+		return true
+	})
+	// Note: The current implementation assumes that Inspect traverses the
+	//       AST in depth-first and thus _source_ order. If AST traversal
+	//       does not follow source order, the sorting call below will be
+	//       required.
+	// sort.Sort(byInterval(list))
+	return list
+}
+
+// A commentListReader helps iterating through a list of comment groups.
+//
+type commentListReader struct {
+	fset     *token.FileSet
+	list     []*CommentGroup
+	index    int
+	comment  *CommentGroup  // comment group at current index
+	pos, end token.Position // source interval of comment group at current index
+}
+
+func (r *commentListReader) eol() bool {
+	return r.index >= len(r.list)
+}
+
+func (r *commentListReader) next() {
+	if !r.eol() {
+		r.comment = r.list[r.index]
+		r.pos = r.fset.Position(r.comment.Pos())
+		r.end = r.fset.Position(r.comment.End())
+		r.index++
+	}
+}
+
+// A nodeStack keeps track of nested nodes.
+// A node lower on the stack lexically contains the nodes higher on the stack.
+//
+type nodeStack []Node
+
+// push pops all nodes that appear lexically before n
+// and then pushes n on the stack.
+//
+func (s *nodeStack) push(n Node) {
+	s.pop(n.Pos())
+	*s = append((*s), n)
+}
+
+// pop pops all nodes that appear lexically before pos
+// (i.e., whose lexical extent has ended before or at pos).
+// It returns the last node popped.
+//
+func (s *nodeStack) pop(pos token.Pos) (top Node) {
+	i := len(*s)
+	for i > 0 && (*s)[i-1].End() <= pos {
+		top = (*s)[i-1]
+		i--
+	}
+	*s = (*s)[0:i]
+	return top
+}
+
+// NewCommentMap creates a new comment map by associating comment groups
+// of the comments list with the nodes of the AST specified by node.
+//
+// A comment group g is associated with a node n if:
+//
+//   - g starts on the same line as n ends
+//   - g starts on the line immediately following n, and there is
+//     at least one empty line after g and before the next node
+//   - g starts before n and is not associated to the node before n
+//     via the previous rules
+//
+// NewCommentMap tries to associate a comment group to the "largest"
+// node possible: For instance, if the comment is a line comment
+// trailing an assignment, the comment is associated with the entire
+// assignment rather than just the last operand in the assignment.
+//
+func NewCommentMap(fset *token.FileSet, node Node, comments []*CommentGroup) CommentMap {
+	if len(comments) == 0 {
+		return nil // no comments to map
+	}
+
+	cmap := make(CommentMap)
+
+	// set up comment reader r
+	tmp := make([]*CommentGroup, len(comments))
+	copy(tmp, comments) // don't change incoming comments
+	sortComments(tmp)
+	r := commentListReader{fset: fset, list: tmp} // !r.eol() because len(comments) > 0
+	r.next()
+
+	// create node list in lexical order
+	nodes := nodeList(node)
+	nodes = append(nodes, nil) // append sentinel
+
+	// set up iteration variables
+	var (
+		p     Node           // previous node
+		pend  token.Position // end of p
+		pg    Node           // previous node group (enclosing nodes of "importance")
+		pgend token.Position // end of pg
+		stack nodeStack      // stack of node groups
+	)
+
+	for _, q := range nodes {
+		var qpos token.Position
+		if q != nil {
+			qpos = fset.Position(q.Pos()) // current node position
+		} else {
+			// set fake sentinel position to infinity so that
+			// all comments get processed before the sentinel
+			const infinity = 1 << 30
+			qpos.Offset = infinity
+			qpos.Line = infinity
+		}
+
+		// process comments before current node
+		for r.end.Offset <= qpos.Offset {
+			// determine recent node group
+			if top := stack.pop(r.comment.Pos()); top != nil {
+				pg = top
+				pgend = fset.Position(pg.End())
+			}
+			// Try to associate a comment first with a node group
+			// (i.e., a node of "importance" such as a declaration);
+			// if that fails, try to associate it with the most recent
+			// node.
+			// TODO(gri) try to simplify the logic below
+			var assoc Node
+			switch {
+			case pg != nil &&
+				(pgend.Line == r.pos.Line ||
+					pgend.Line+1 == r.pos.Line && r.end.Line+1 < qpos.Line):
+				// 1) comment starts on same line as previous node group ends, or
+				// 2) comment starts on the line immediately after the
+				//    previous node group and there is an empty line before
+				//    the current node
+				// => associate comment with previous node group
+				assoc = pg
+			case p != nil &&
+				(pend.Line == r.pos.Line ||
+					pend.Line+1 == r.pos.Line && r.end.Line+1 < qpos.Line ||
+					q == nil):
+				// same rules apply as above for p rather than pg,
+				// but also associate with p if we are at the end (q == nil)
+				assoc = p
+			default:
+				// otherwise, associate comment with current node
+				if q == nil {
+					// we can only reach here if there was no p
+					// which would imply that there were no nodes
+					panic("internal error: no comments should be associated with sentinel")
+				}
+				assoc = q
+			}
+			cmap.addComment(assoc, r.comment)
+			if r.eol() {
+				return cmap
+			}
+			r.next()
+		}
+
+		// update previous node
+		p = q
+		pend = fset.Position(p.End())
+
+		// update previous node group if we see an "important" node
+		switch q.(type) {
+		case *File, *Field, Decl, Spec, Stmt:
+			stack.push(q)
+		}
+	}
+
+	return cmap
+}
+
+// Update replaces an old node in the comment map with the new node
+// and returns the new node. Comments that were associated with the
+// old node are associated with the new node.
+//
+func (cmap CommentMap) Update(old, new Node) Node {
+	if list := cmap[old]; len(list) > 0 {
+		delete(cmap, old)
+		cmap[new] = append(cmap[new], list...)
+	}
+	return new
+}
+
+// Filter returns a new comment map consisting of only those
+// entries of cmap for which a corresponding node exists in
+// the AST specified by node.
+//
+func (cmap CommentMap) Filter(node Node) CommentMap {
+	umap := make(CommentMap)
+	Inspect(node, func(n Node) bool {
+		if g := cmap[n]; len(g) > 0 {
+			umap[n] = g
+		}
+		return true
+	})
+	return umap
+}
+
+// Comments returns the list of comment groups in the comment map.
+// The result is sorted in source order.
+//
+func (cmap CommentMap) Comments() []*CommentGroup {
+	list := make([]*CommentGroup, 0, len(cmap))
+	for _, e := range cmap {
+		list = append(list, e...)
+	}
+	sortComments(list)
+	return list
+}
+
+func summary(list []*CommentGroup) string {
+	const maxLen = 40
+	var buf bytes.Buffer
+
+	// collect comments text
+loop:
+	for _, group := range list {
+		// Note: CommentGroup.Text() does too much work for what we
+		//       need and would only replace this innermost loop.
+		//       Just do it explicitly.
+		for _, comment := range group.List {
+			if buf.Len() >= maxLen {
+				break loop
+			}
+			buf.WriteString(comment.Text)
+		}
+	}
+
+	// truncate if too long
+	if buf.Len() > maxLen {
+		buf.Truncate(maxLen - 3)
+		buf.WriteString("...")
+	}
+
+	// replace any invisibles with blanks
+	bytes := buf.Bytes()
+	for i, b := range bytes {
+		switch b {
+		case '\t', '\n', '\r':
+			bytes[i] = ' '
+		}
+	}
+
+	return string(bytes)
+}
+
+func (cmap CommentMap) String() string {
+	var buf bytes.Buffer
+	fmt.Fprintln(&buf, "CommentMap {")
+	for node, comment := range cmap {
+		// print name of identifiers; print node type for other nodes
+		var s string
+		if ident, ok := node.(*Ident); ok {
+			s = ident.Name
+		} else {
+			s = fmt.Sprintf("%T", node)
+		}
+		fmt.Fprintf(&buf, "\t%p  %20s:  %s\n", node, s, summary(comment))
+	}
+	fmt.Fprintln(&buf, "}")
+	return buf.String()
+}
diff --git a/src/go/ast/commentmap_test.go b/src/go/ast/commentmap_test.go
new file mode 100644
index 0000000..38c62b0
--- /dev/null
+++ b/src/go/ast/commentmap_test.go
@@ -0,0 +1,170 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// To avoid a cyclic dependency with go/parser, this file is in a separate package.
+
+package ast_test
+
+import (
+	"bytes"
+	"fmt"
+	. "go/ast"
+	"go/parser"
+	"go/token"
+	"sort"
+	"testing"
+)
+
+const src = `
+// the very first comment
+
+// package p
+package p /* the name is p */
+
+// imports
+import (
+	"bytes"     // bytes
+	"fmt"       // fmt
+	"go/ast"
+	"go/parser"
+)
+
+// T
+type T struct {
+	a, b, c int // associated with a, b, c
+	// associated with x, y
+	x, y float64    // float values
+	z    complex128 // complex value
+}
+// also associated with T
+
+// x
+var x = 0 // x = 0
+// also associated with x
+
+// f1
+func f1() {
+	/* associated with s1 */
+	s1()
+	// also associated with s1
+	
+	// associated with s2
+	
+	// also associated with s2
+	s2() // line comment for s2
+}
+// associated with f1
+// also associated with f1
+
+// associated with f2
+
+// f2
+func f2() {
+}
+
+func f3() {
+	i := 1 /* 1 */ + 2 // addition
+	_ = i
+}
+
+// the very last comment
+`
+
+// res maps a key of the form "line number: node type"
+// to the associated comments' text.
+//
+var res = map[string]string{
+	" 5: *ast.File":       "the very first comment\npackage p\n",
+	" 5: *ast.Ident":      " the name is p\n",
+	" 8: *ast.GenDecl":    "imports\n",
+	" 9: *ast.ImportSpec": "bytes\n",
+	"10: *ast.ImportSpec": "fmt\n",
+	"16: *ast.GenDecl":    "T\nalso associated with T\n",
+	"17: *ast.Field":      "associated with a, b, c\n",
+	"19: *ast.Field":      "associated with x, y\nfloat values\n",
+	"20: *ast.Field":      "complex value\n",
+	"25: *ast.GenDecl":    "x\nx = 0\nalso associated with x\n",
+	"29: *ast.FuncDecl":   "f1\nassociated with f1\nalso associated with f1\n",
+	"31: *ast.ExprStmt":   " associated with s1\nalso associated with s1\n",
+	"37: *ast.ExprStmt":   "associated with s2\nalso associated with s2\nline comment for s2\n",
+	"45: *ast.FuncDecl":   "associated with f2\nf2\n",
+	"49: *ast.AssignStmt": "addition\n",
+	"49: *ast.BasicLit":   " 1\n",
+	"50: *ast.Ident":      "the very last comment\n",
+}
+
+func ctext(list []*CommentGroup) string {
+	var buf bytes.Buffer
+	for _, g := range list {
+		buf.WriteString(g.Text())
+	}
+	return buf.String()
+}
+
+func TestCommentMap(t *testing.T) {
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	cmap := NewCommentMap(fset, f, f.Comments)
+
+	// very correct association of comments
+	for n, list := range cmap {
+		key := fmt.Sprintf("%2d: %T", fset.Position(n.Pos()).Line, n)
+		got := ctext(list)
+		want := res[key]
+		if got != want {
+			t.Errorf("%s: got %q; want %q", key, got, want)
+		}
+	}
+
+	// verify that no comments got lost
+	if n := len(cmap.Comments()); n != len(f.Comments) {
+		t.Errorf("got %d comment groups in map; want %d", n, len(f.Comments))
+	}
+
+	// support code to update test:
+	// set genMap to true to generate res map
+	const genMap = false
+	if genMap {
+		out := make([]string, 0, len(cmap))
+		for n, list := range cmap {
+			out = append(out, fmt.Sprintf("\t\"%2d: %T\":\t%q,", fset.Position(n.Pos()).Line, n, ctext(list)))
+		}
+		sort.Strings(out)
+		for _, s := range out {
+			fmt.Println(s)
+		}
+	}
+}
+
+func TestFilter(t *testing.T) {
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	cmap := NewCommentMap(fset, f, f.Comments)
+
+	// delete variable declaration
+	for i, decl := range f.Decls {
+		if gen, ok := decl.(*GenDecl); ok && gen.Tok == token.VAR {
+			copy(f.Decls[i:], f.Decls[i+1:])
+			f.Decls = f.Decls[:len(f.Decls)-1]
+			break
+		}
+	}
+
+	// check if comments are filtered correctly
+	cc := cmap.Filter(f)
+	for n, list := range cc {
+		key := fmt.Sprintf("%2d: %T", fset.Position(n.Pos()).Line, n)
+		got := ctext(list)
+		want := res[key]
+		if key == "25: *ast.GenDecl" || got != want {
+			t.Errorf("%s: got %q; want %q", key, got, want)
+		}
+	}
+}
diff --git a/src/go/ast/example_test.go b/src/go/ast/example_test.go
new file mode 100644
index 0000000..e3013f6
--- /dev/null
+++ b/src/go/ast/example_test.go
@@ -0,0 +1,206 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast_test
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/format"
+	"go/parser"
+	"go/token"
+)
+
+// This example demonstrates how to inspect the AST of a Go program.
+func ExampleInspect() {
+	// src is the input for which we want to inspect the AST.
+	src := `
+package p
+const c = 1.0
+var X = f(3.14)*2 + c
+`
+
+	// Create the AST by parsing src.
+	fset := token.NewFileSet() // positions are relative to fset
+	f, err := parser.ParseFile(fset, "src.go", src, 0)
+	if err != nil {
+		panic(err)
+	}
+
+	// Inspect the AST and print all identifiers and literals.
+	ast.Inspect(f, func(n ast.Node) bool {
+		var s string
+		switch x := n.(type) {
+		case *ast.BasicLit:
+			s = x.Value
+		case *ast.Ident:
+			s = x.Name
+		}
+		if s != "" {
+			fmt.Printf("%s:\t%s\n", fset.Position(n.Pos()), s)
+		}
+		return true
+	})
+
+	// Output:
+	// src.go:2:9:	p
+	// src.go:3:7:	c
+	// src.go:3:11:	1.0
+	// src.go:4:5:	X
+	// src.go:4:9:	f
+	// src.go:4:11:	3.14
+	// src.go:4:17:	2
+	// src.go:4:21:	c
+}
+
+// This example shows what an AST looks like when printed for debugging.
+func ExamplePrint() {
+	// src is the input for which we want to print the AST.
+	src := `
+package main
+func main() {
+	println("Hello, World!")
+}
+`
+
+	// Create the AST by parsing src.
+	fset := token.NewFileSet() // positions are relative to fset
+	f, err := parser.ParseFile(fset, "", src, 0)
+	if err != nil {
+		panic(err)
+	}
+
+	// Print the AST.
+	ast.Print(fset, f)
+
+	// Output:
+	//      0  *ast.File {
+	//      1  .  Package: 2:1
+	//      2  .  Name: *ast.Ident {
+	//      3  .  .  NamePos: 2:9
+	//      4  .  .  Name: "main"
+	//      5  .  }
+	//      6  .  Decls: []ast.Decl (len = 1) {
+	//      7  .  .  0: *ast.FuncDecl {
+	//      8  .  .  .  Name: *ast.Ident {
+	//      9  .  .  .  .  NamePos: 3:6
+	//     10  .  .  .  .  Name: "main"
+	//     11  .  .  .  .  Obj: *ast.Object {
+	//     12  .  .  .  .  .  Kind: func
+	//     13  .  .  .  .  .  Name: "main"
+	//     14  .  .  .  .  .  Decl: *(obj @ 7)
+	//     15  .  .  .  .  }
+	//     16  .  .  .  }
+	//     17  .  .  .  Type: *ast.FuncType {
+	//     18  .  .  .  .  Func: 3:1
+	//     19  .  .  .  .  Params: *ast.FieldList {
+	//     20  .  .  .  .  .  Opening: 3:10
+	//     21  .  .  .  .  .  Closing: 3:11
+	//     22  .  .  .  .  }
+	//     23  .  .  .  }
+	//     24  .  .  .  Body: *ast.BlockStmt {
+	//     25  .  .  .  .  Lbrace: 3:13
+	//     26  .  .  .  .  List: []ast.Stmt (len = 1) {
+	//     27  .  .  .  .  .  0: *ast.ExprStmt {
+	//     28  .  .  .  .  .  .  X: *ast.CallExpr {
+	//     29  .  .  .  .  .  .  .  Fun: *ast.Ident {
+	//     30  .  .  .  .  .  .  .  .  NamePos: 4:2
+	//     31  .  .  .  .  .  .  .  .  Name: "println"
+	//     32  .  .  .  .  .  .  .  }
+	//     33  .  .  .  .  .  .  .  Lparen: 4:9
+	//     34  .  .  .  .  .  .  .  Args: []ast.Expr (len = 1) {
+	//     35  .  .  .  .  .  .  .  .  0: *ast.BasicLit {
+	//     36  .  .  .  .  .  .  .  .  .  ValuePos: 4:10
+	//     37  .  .  .  .  .  .  .  .  .  Kind: STRING
+	//     38  .  .  .  .  .  .  .  .  .  Value: "\"Hello, World!\""
+	//     39  .  .  .  .  .  .  .  .  }
+	//     40  .  .  .  .  .  .  .  }
+	//     41  .  .  .  .  .  .  .  Ellipsis: -
+	//     42  .  .  .  .  .  .  .  Rparen: 4:25
+	//     43  .  .  .  .  .  .  }
+	//     44  .  .  .  .  .  }
+	//     45  .  .  .  .  }
+	//     46  .  .  .  .  Rbrace: 5:1
+	//     47  .  .  .  }
+	//     48  .  .  }
+	//     49  .  }
+	//     50  .  Scope: *ast.Scope {
+	//     51  .  .  Objects: map[string]*ast.Object (len = 1) {
+	//     52  .  .  .  "main": *(obj @ 11)
+	//     53  .  .  }
+	//     54  .  }
+	//     55  .  Unresolved: []*ast.Ident (len = 1) {
+	//     56  .  .  0: *(obj @ 29)
+	//     57  .  }
+	//     58  }
+}
+
+// This example illustrates how to remove a variable declaration
+// in a Go program while maintaining correct comment association
+// using an ast.CommentMap.
+func ExampleCommentMap() {
+	// src is the input for which we create the AST that we
+	// are going to manipulate.
+	src := `
+// This is the package comment.
+package main
+
+// This comment is associated with the hello constant.
+const hello = "Hello, World!" // line comment 1
+
+// This comment is associated with the foo variable.
+var foo = hello // line comment 2
+
+// This comment is associated with the main function.
+func main() {
+	fmt.Println(hello) // line comment 3
+}
+`
+
+	// Create the AST by parsing src.
+	fset := token.NewFileSet() // positions are relative to fset
+	f, err := parser.ParseFile(fset, "src.go", src, parser.ParseComments)
+	if err != nil {
+		panic(err)
+	}
+
+	// Create an ast.CommentMap from the ast.File's comments.
+	// This helps keeping the association between comments
+	// and AST nodes.
+	cmap := ast.NewCommentMap(fset, f, f.Comments)
+
+	// Remove the first variable declaration from the list of declarations.
+	for i, decl := range f.Decls {
+		if gen, ok := decl.(*ast.GenDecl); ok && gen.Tok == token.VAR {
+			copy(f.Decls[i:], f.Decls[i+1:])
+			f.Decls = f.Decls[:len(f.Decls)-1]
+			break
+		}
+	}
+
+	// Use the comment map to filter comments that don't belong anymore
+	// (the comments associated with the variable declaration), and create
+	// the new comments list.
+	f.Comments = cmap.Filter(f).Comments()
+
+	// Print the modified AST.
+	var buf bytes.Buffer
+	if err := format.Node(&buf, fset, f); err != nil {
+		panic(err)
+	}
+	fmt.Printf("%s", buf.Bytes())
+
+	// Output:
+	// // This is the package comment.
+	// package main
+	//
+	// // This comment is associated with the hello constant.
+	// const hello = "Hello, World!" // line comment 1
+	//
+	// // This comment is associated with the main function.
+	// func main() {
+	// 	fmt.Println(hello) // line comment 3
+	// }
+}
diff --git a/src/go/ast/filter.go b/src/go/ast/filter.go
new file mode 100644
index 0000000..c398e6e
--- /dev/null
+++ b/src/go/ast/filter.go
@@ -0,0 +1,497 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import (
+	"go/token"
+	"sort"
+)
+
+// ----------------------------------------------------------------------------
+// Export filtering
+
+// exportFilter is a special filter function to extract exported nodes.
+func exportFilter(name string) bool {
+	return IsExported(name)
+}
+
+// FileExports trims the AST for a Go source file in place such that
+// only exported nodes remain: all top-level identifiers which are not exported
+// and their associated information (such as type, initial value, or function
+// body) are removed. Non-exported fields and methods of exported types are
+// stripped. The File.Comments list is not changed.
+//
+// FileExports reports whether there are exported declarations.
+//
+func FileExports(src *File) bool {
+	return filterFile(src, exportFilter, true)
+}
+
+// PackageExports trims the AST for a Go package in place such that
+// only exported nodes remain. The pkg.Files list is not changed, so that
+// file names and top-level package comments don't get lost.
+//
+// PackageExports reports whether there are exported declarations;
+// it returns false otherwise.
+//
+func PackageExports(pkg *Package) bool {
+	return filterPackage(pkg, exportFilter, true)
+}
+
+// ----------------------------------------------------------------------------
+// General filtering
+
+type Filter func(string) bool
+
+func filterIdentList(list []*Ident, f Filter) []*Ident {
+	j := 0
+	for _, x := range list {
+		if f(x.Name) {
+			list[j] = x
+			j++
+		}
+	}
+	return list[0:j]
+}
+
+// fieldName assumes that x is the type of an anonymous field and
+// returns the corresponding field name. If x is not an acceptable
+// anonymous field, the result is nil.
+//
+func fieldName(x Expr) *Ident {
+	switch t := x.(type) {
+	case *Ident:
+		return t
+	case *SelectorExpr:
+		if _, ok := t.X.(*Ident); ok {
+			return t.Sel
+		}
+	case *StarExpr:
+		return fieldName(t.X)
+	}
+	return nil
+}
+
+func filterFieldList(fields *FieldList, filter Filter, export bool) (removedFields bool) {
+	if fields == nil {
+		return false
+	}
+	list := fields.List
+	j := 0
+	for _, f := range list {
+		keepField := false
+		if len(f.Names) == 0 {
+			// anonymous field
+			name := fieldName(f.Type)
+			keepField = name != nil && filter(name.Name)
+		} else {
+			n := len(f.Names)
+			f.Names = filterIdentList(f.Names, filter)
+			if len(f.Names) < n {
+				removedFields = true
+			}
+			keepField = len(f.Names) > 0
+		}
+		if keepField {
+			if export {
+				filterType(f.Type, filter, export)
+			}
+			list[j] = f
+			j++
+		}
+	}
+	if j < len(list) {
+		removedFields = true
+	}
+	fields.List = list[0:j]
+	return
+}
+
+func filterCompositeLit(lit *CompositeLit, filter Filter, export bool) {
+	n := len(lit.Elts)
+	lit.Elts = filterExprList(lit.Elts, filter, export)
+	if len(lit.Elts) < n {
+		lit.Incomplete = true
+	}
+}
+
+func filterExprList(list []Expr, filter Filter, export bool) []Expr {
+	j := 0
+	for _, exp := range list {
+		switch x := exp.(type) {
+		case *CompositeLit:
+			filterCompositeLit(x, filter, export)
+		case *KeyValueExpr:
+			if x, ok := x.Key.(*Ident); ok && !filter(x.Name) {
+				continue
+			}
+			if x, ok := x.Value.(*CompositeLit); ok {
+				filterCompositeLit(x, filter, export)
+			}
+		}
+		list[j] = exp
+		j++
+	}
+	return list[0:j]
+}
+
+func filterParamList(fields *FieldList, filter Filter, export bool) bool {
+	if fields == nil {
+		return false
+	}
+	var b bool
+	for _, f := range fields.List {
+		if filterType(f.Type, filter, export) {
+			b = true
+		}
+	}
+	return b
+}
+
+func filterType(typ Expr, f Filter, export bool) bool {
+	switch t := typ.(type) {
+	case *Ident:
+		return f(t.Name)
+	case *ParenExpr:
+		return filterType(t.X, f, export)
+	case *ArrayType:
+		return filterType(t.Elt, f, export)
+	case *StructType:
+		if filterFieldList(t.Fields, f, export) {
+			t.Incomplete = true
+		}
+		return len(t.Fields.List) > 0
+	case *FuncType:
+		b1 := filterParamList(t.Params, f, export)
+		b2 := filterParamList(t.Results, f, export)
+		return b1 || b2
+	case *InterfaceType:
+		if filterFieldList(t.Methods, f, export) {
+			t.Incomplete = true
+		}
+		return len(t.Methods.List) > 0
+	case *MapType:
+		b1 := filterType(t.Key, f, export)
+		b2 := filterType(t.Value, f, export)
+		return b1 || b2
+	case *ChanType:
+		return filterType(t.Value, f, export)
+	}
+	return false
+}
+
+func filterSpec(spec Spec, f Filter, export bool) bool {
+	switch s := spec.(type) {
+	case *ValueSpec:
+		s.Names = filterIdentList(s.Names, f)
+		s.Values = filterExprList(s.Values, f, export)
+		if len(s.Names) > 0 {
+			if export {
+				filterType(s.Type, f, export)
+			}
+			return true
+		}
+	case *TypeSpec:
+		if f(s.Name.Name) {
+			if export {
+				filterType(s.Type, f, export)
+			}
+			return true
+		}
+		if !export {
+			// For general filtering (not just exports),
+			// filter type even if name is not filtered
+			// out.
+			// If the type contains filtered elements,
+			// keep the declaration.
+			return filterType(s.Type, f, export)
+		}
+	}
+	return false
+}
+
+func filterSpecList(list []Spec, f Filter, export bool) []Spec {
+	j := 0
+	for _, s := range list {
+		if filterSpec(s, f, export) {
+			list[j] = s
+			j++
+		}
+	}
+	return list[0:j]
+}
+
+// FilterDecl trims the AST for a Go declaration in place by removing
+// all names (including struct field and interface method names, but
+// not from parameter lists) that don't pass through the filter f.
+//
+// FilterDecl reports whether there are any declared names left after
+// filtering.
+//
+func FilterDecl(decl Decl, f Filter) bool {
+	return filterDecl(decl, f, false)
+}
+
+func filterDecl(decl Decl, f Filter, export bool) bool {
+	switch d := decl.(type) {
+	case *GenDecl:
+		d.Specs = filterSpecList(d.Specs, f, export)
+		return len(d.Specs) > 0
+	case *FuncDecl:
+		return f(d.Name.Name)
+	}
+	return false
+}
+
+// FilterFile trims the AST for a Go file in place by removing all
+// names from top-level declarations (including struct field and
+// interface method names, but not from parameter lists) that don't
+// pass through the filter f. If the declaration is empty afterwards,
+// the declaration is removed from the AST. Import declarations are
+// always removed. The File.Comments list is not changed.
+//
+// FilterFile reports whether there are any top-level declarations
+// left after filtering.
+//
+func FilterFile(src *File, f Filter) bool {
+	return filterFile(src, f, false)
+}
+
+func filterFile(src *File, f Filter, export bool) bool {
+	j := 0
+	for _, d := range src.Decls {
+		if filterDecl(d, f, export) {
+			src.Decls[j] = d
+			j++
+		}
+	}
+	src.Decls = src.Decls[0:j]
+	return j > 0
+}
+
+// FilterPackage trims the AST for a Go package in place by removing
+// all names from top-level declarations (including struct field and
+// interface method names, but not from parameter lists) that don't
+// pass through the filter f. If the declaration is empty afterwards,
+// the declaration is removed from the AST. The pkg.Files list is not
+// changed, so that file names and top-level package comments don't get
+// lost.
+//
+// FilterPackage reports whether there are any top-level declarations
+// left after filtering.
+//
+func FilterPackage(pkg *Package, f Filter) bool {
+	return filterPackage(pkg, f, false)
+}
+
+func filterPackage(pkg *Package, f Filter, export bool) bool {
+	hasDecls := false
+	for _, src := range pkg.Files {
+		if filterFile(src, f, export) {
+			hasDecls = true
+		}
+	}
+	return hasDecls
+}
+
+// ----------------------------------------------------------------------------
+// Merging of package files
+
+// The MergeMode flags control the behavior of MergePackageFiles.
+type MergeMode uint
+
+const (
+	// If set, duplicate function declarations are excluded.
+	FilterFuncDuplicates MergeMode = 1 << iota
+	// If set, comments that are not associated with a specific
+	// AST node (as Doc or Comment) are excluded.
+	FilterUnassociatedComments
+	// If set, duplicate import declarations are excluded.
+	FilterImportDuplicates
+)
+
+// nameOf returns the function (foo) or method name (foo.bar) for
+// the given function declaration. If the AST is incorrect for the
+// receiver, it assumes a function instead.
+//
+func nameOf(f *FuncDecl) string {
+	if r := f.Recv; r != nil && len(r.List) == 1 {
+		// looks like a correct receiver declaration
+		t := r.List[0].Type
+		// dereference pointer receiver types
+		if p, _ := t.(*StarExpr); p != nil {
+			t = p.X
+		}
+		// the receiver type must be a type name
+		if p, _ := t.(*Ident); p != nil {
+			return p.Name + "." + f.Name.Name
+		}
+		// otherwise assume a function instead
+	}
+	return f.Name.Name
+}
+
+// separator is an empty //-style comment that is interspersed between
+// different comment groups when they are concatenated into a single group
+//
+var separator = &Comment{token.NoPos, "//"}
+
+// MergePackageFiles creates a file AST by merging the ASTs of the
+// files belonging to a package. The mode flags control merging behavior.
+//
+func MergePackageFiles(pkg *Package, mode MergeMode) *File {
+	// Count the number of package docs, comments and declarations across
+	// all package files. Also, compute sorted list of filenames, so that
+	// subsequent iterations can always iterate in the same order.
+	ndocs := 0
+	ncomments := 0
+	ndecls := 0
+	filenames := make([]string, len(pkg.Files))
+	i := 0
+	for filename, f := range pkg.Files {
+		filenames[i] = filename
+		i++
+		if f.Doc != nil {
+			ndocs += len(f.Doc.List) + 1 // +1 for separator
+		}
+		ncomments += len(f.Comments)
+		ndecls += len(f.Decls)
+	}
+	sort.Strings(filenames)
+
+	// Collect package comments from all package files into a single
+	// CommentGroup - the collected package documentation. In general
+	// there should be only one file with a package comment; but it's
+	// better to collect extra comments than drop them on the floor.
+	var doc *CommentGroup
+	var pos token.Pos
+	if ndocs > 0 {
+		list := make([]*Comment, ndocs-1) // -1: no separator before first group
+		i := 0
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
+			if f.Doc != nil {
+				if i > 0 {
+					// not the first group - add separator
+					list[i] = separator
+					i++
+				}
+				for _, c := range f.Doc.List {
+					list[i] = c
+					i++
+				}
+				if f.Package > pos {
+					// Keep the maximum package clause position as
+					// position for the package clause of the merged
+					// files.
+					pos = f.Package
+				}
+			}
+		}
+		doc = &CommentGroup{list}
+	}
+
+	// Collect declarations from all package files.
+	var decls []Decl
+	if ndecls > 0 {
+		decls = make([]Decl, ndecls)
+		funcs := make(map[string]int) // map of func name -> decls index
+		i := 0                        // current index
+		n := 0                        // number of filtered entries
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
+			for _, d := range f.Decls {
+				if mode&FilterFuncDuplicates != 0 {
+					// A language entity may be declared multiple
+					// times in different package files; only at
+					// build time declarations must be unique.
+					// For now, exclude multiple declarations of
+					// functions - keep the one with documentation.
+					//
+					// TODO(gri): Expand this filtering to other
+					//            entities (const, type, vars) if
+					//            multiple declarations are common.
+					if f, isFun := d.(*FuncDecl); isFun {
+						name := nameOf(f)
+						if j, exists := funcs[name]; exists {
+							// function declared already
+							if decls[j] != nil && decls[j].(*FuncDecl).Doc == nil {
+								// existing declaration has no documentation;
+								// ignore the existing declaration
+								decls[j] = nil
+							} else {
+								// ignore the new declaration
+								d = nil
+							}
+							n++ // filtered an entry
+						} else {
+							funcs[name] = i
+						}
+					}
+				}
+				decls[i] = d
+				i++
+			}
+		}
+
+		// Eliminate nil entries from the decls list if entries were
+		// filtered. We do this using a 2nd pass in order to not disturb
+		// the original declaration order in the source (otherwise, this
+		// would also invalidate the monotonically increasing position
+		// info within a single file).
+		if n > 0 {
+			i = 0
+			for _, d := range decls {
+				if d != nil {
+					decls[i] = d
+					i++
+				}
+			}
+			decls = decls[0:i]
+		}
+	}
+
+	// Collect import specs from all package files.
+	var imports []*ImportSpec
+	if mode&FilterImportDuplicates != 0 {
+		seen := make(map[string]bool)
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
+			for _, imp := range f.Imports {
+				if path := imp.Path.Value; !seen[path] {
+					// TODO: consider handling cases where:
+					// - 2 imports exist with the same import path but
+					//   have different local names (one should probably
+					//   keep both of them)
+					// - 2 imports exist but only one has a comment
+					// - 2 imports exist and they both have (possibly
+					//   different) comments
+					imports = append(imports, imp)
+					seen[path] = true
+				}
+			}
+		}
+	} else {
+		// Iterate over filenames for deterministic order.
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
+			imports = append(imports, f.Imports...)
+		}
+	}
+
+	// Collect comments from all package files.
+	var comments []*CommentGroup
+	if mode&FilterUnassociatedComments == 0 {
+		comments = make([]*CommentGroup, ncomments)
+		i := 0
+		for _, filename := range filenames {
+			f := pkg.Files[filename]
+			i += copy(comments[i:], f.Comments)
+		}
+	}
+
+	// TODO(gri) need to compute unresolved identifiers!
+	return &File{doc, pos, NewIdent(pkg.Name), decls, pkg.Scope, imports, nil, comments}
+}
diff --git a/src/go/ast/filter_test.go b/src/go/ast/filter_test.go
new file mode 100644
index 0000000..9fd86cb
--- /dev/null
+++ b/src/go/ast/filter_test.go
@@ -0,0 +1,86 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// To avoid a cyclic dependency with go/parser, this file is in a separate package.
+
+package ast_test
+
+import (
+	"bytes"
+	"go/ast"
+	"go/format"
+	"go/parser"
+	"go/token"
+	"testing"
+)
+
+const input = `package p
+
+type t1 struct{}
+type t2 struct{}
+
+func f1() {}
+func f1() {}
+func f2() {}
+
+func (*t1) f1() {}
+func (t1) f1() {}
+func (t1) f2() {}
+
+func (t2) f1() {}
+func (t2) f2() {}
+func (x *t2) f2() {}
+`
+
+// Calling ast.MergePackageFiles with ast.FilterFuncDuplicates
+// keeps a duplicate entry with attached documentation in favor
+// of one without, and it favors duplicate entries appearing
+// later in the source over ones appearing earlier. This is why
+// (*t2).f2 is kept and t2.f2 is eliminated in this test case.
+//
+const golden = `package p
+
+type t1 struct{}
+type t2 struct{}
+
+func f1() {}
+func f2() {}
+
+func (t1) f1() {}
+func (t1) f2() {}
+
+func (t2) f1() {}
+
+func (x *t2) f2() {}
+`
+
+func TestFilterDuplicates(t *testing.T) {
+	// parse input
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "", input, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// create package
+	files := map[string]*ast.File{"": file}
+	pkg, err := ast.NewPackage(fset, files, nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// filter
+	merged := ast.MergePackageFiles(pkg, ast.FilterFuncDuplicates)
+
+	// pretty-print
+	var buf bytes.Buffer
+	if err := format.Node(&buf, fset, merged); err != nil {
+		t.Fatal(err)
+	}
+	output := buf.String()
+
+	if output != golden {
+		t.Errorf("incorrect output:\n%s", output)
+	}
+}
diff --git a/src/go/ast/import.go b/src/go/ast/import.go
new file mode 100644
index 0000000..7fdf137
--- /dev/null
+++ b/src/go/ast/import.go
@@ -0,0 +1,230 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import (
+	"go/token"
+	"sort"
+	"strconv"
+)
+
+// SortImports sorts runs of consecutive import lines in import blocks in f.
+// It also removes duplicate imports when it is possible to do so without data loss.
+func SortImports(fset *token.FileSet, f *File) {
+	for _, d := range f.Decls {
+		d, ok := d.(*GenDecl)
+		if !ok || d.Tok != token.IMPORT {
+			// Not an import declaration, so we're done.
+			// Imports are always first.
+			break
+		}
+
+		if !d.Lparen.IsValid() {
+			// Not a block: sorted by default.
+			continue
+		}
+
+		// Identify and sort runs of specs on successive lines.
+		i := 0
+		specs := d.Specs[:0]
+		for j, s := range d.Specs {
+			if j > i && lineAt(fset, s.Pos()) > 1+lineAt(fset, d.Specs[j-1].End()) {
+				// j begins a new run. End this one.
+				specs = append(specs, sortSpecs(fset, f, d.Specs[i:j])...)
+				i = j
+			}
+		}
+		specs = append(specs, sortSpecs(fset, f, d.Specs[i:])...)
+		d.Specs = specs
+
+		// Deduping can leave a blank line before the rparen; clean that up.
+		if len(d.Specs) > 0 {
+			lastSpec := d.Specs[len(d.Specs)-1]
+			lastLine := lineAt(fset, lastSpec.Pos())
+			rParenLine := lineAt(fset, d.Rparen)
+			for rParenLine > lastLine+1 {
+				rParenLine--
+				fset.File(d.Rparen).MergeLine(rParenLine)
+			}
+		}
+	}
+}
+
+func lineAt(fset *token.FileSet, pos token.Pos) int {
+	return fset.PositionFor(pos, false).Line
+}
+
+func importPath(s Spec) string {
+	t, err := strconv.Unquote(s.(*ImportSpec).Path.Value)
+	if err == nil {
+		return t
+	}
+	return ""
+}
+
+func importName(s Spec) string {
+	n := s.(*ImportSpec).Name
+	if n == nil {
+		return ""
+	}
+	return n.Name
+}
+
+func importComment(s Spec) string {
+	c := s.(*ImportSpec).Comment
+	if c == nil {
+		return ""
+	}
+	return c.Text()
+}
+
+// collapse indicates whether prev may be removed, leaving only next.
+func collapse(prev, next Spec) bool {
+	if importPath(next) != importPath(prev) || importName(next) != importName(prev) {
+		return false
+	}
+	return prev.(*ImportSpec).Comment == nil
+}
+
+type posSpan struct {
+	Start token.Pos
+	End   token.Pos
+}
+
+type cgPos struct {
+	left bool // true if comment is to the left of the spec, false otherwise.
+	cg   *CommentGroup
+}
+
+func sortSpecs(fset *token.FileSet, f *File, specs []Spec) []Spec {
+	// Can't short-circuit here even if specs are already sorted,
+	// since they might yet need deduplication.
+	// A lone import, however, may be safely ignored.
+	if len(specs) <= 1 {
+		return specs
+	}
+
+	// Record positions for specs.
+	pos := make([]posSpan, len(specs))
+	for i, s := range specs {
+		pos[i] = posSpan{s.Pos(), s.End()}
+	}
+
+	// Identify comments in this range.
+	begSpecs := pos[0].Start
+	endSpecs := pos[len(pos)-1].End
+	beg := fset.File(begSpecs).LineStart(lineAt(fset, begSpecs))
+	endLine := lineAt(fset, endSpecs)
+	endFile := fset.File(endSpecs)
+	var end token.Pos
+	if endLine == endFile.LineCount() {
+		end = endSpecs
+	} else {
+		end = endFile.LineStart(endLine + 1) // beginning of next line
+	}
+	first := len(f.Comments)
+	last := -1
+	for i, g := range f.Comments {
+		if g.End() >= end {
+			break
+		}
+		// g.End() < end
+		if beg <= g.Pos() {
+			// comment is within the range [beg, end[ of import declarations
+			if i < first {
+				first = i
+			}
+			if i > last {
+				last = i
+			}
+		}
+	}
+
+	var comments []*CommentGroup
+	if last >= 0 {
+		comments = f.Comments[first : last+1]
+	}
+
+	// Assign each comment to the import spec on the same line.
+	importComments := map[*ImportSpec][]cgPos{}
+	specIndex := 0
+	for _, g := range comments {
+		for specIndex+1 < len(specs) && pos[specIndex+1].Start <= g.Pos() {
+			specIndex++
+		}
+		var left bool
+		// A block comment can appear before the first import spec.
+		if specIndex == 0 && pos[specIndex].Start > g.Pos() {
+			left = true
+		} else if specIndex+1 < len(specs) && // Or it can appear on the left of an import spec.
+			lineAt(fset, pos[specIndex].Start)+1 == lineAt(fset, g.Pos()) {
+			specIndex++
+			left = true
+		}
+		s := specs[specIndex].(*ImportSpec)
+		importComments[s] = append(importComments[s], cgPos{left: left, cg: g})
+	}
+
+	// Sort the import specs by import path.
+	// Remove duplicates, when possible without data loss.
+	// Reassign the import paths to have the same position sequence.
+	// Reassign each comment to the spec on the same line.
+	// Sort the comments by new position.
+	sort.Slice(specs, func(i, j int) bool {
+		ipath := importPath(specs[i])
+		jpath := importPath(specs[j])
+		if ipath != jpath {
+			return ipath < jpath
+		}
+		iname := importName(specs[i])
+		jname := importName(specs[j])
+		if iname != jname {
+			return iname < jname
+		}
+		return importComment(specs[i]) < importComment(specs[j])
+	})
+
+	// Dedup. Thanks to our sorting, we can just consider
+	// adjacent pairs of imports.
+	deduped := specs[:0]
+	for i, s := range specs {
+		if i == len(specs)-1 || !collapse(s, specs[i+1]) {
+			deduped = append(deduped, s)
+		} else {
+			p := s.Pos()
+			fset.File(p).MergeLine(lineAt(fset, p))
+		}
+	}
+	specs = deduped
+
+	// Fix up comment positions
+	for i, s := range specs {
+		s := s.(*ImportSpec)
+		if s.Name != nil {
+			s.Name.NamePos = pos[i].Start
+		}
+		s.Path.ValuePos = pos[i].Start
+		s.EndPos = pos[i].End
+		for _, g := range importComments[s] {
+			for _, c := range g.cg.List {
+				if g.left {
+					c.Slash = pos[i].Start - 1
+				} else {
+					// An import spec can have both block comment and a line comment
+					// to its right. In that case, both of them will have the same pos.
+					// But while formatting the AST, the line comment gets moved to
+					// after the block comment.
+					c.Slash = pos[i].End
+				}
+			}
+		}
+	}
+
+	sort.Slice(comments, func(i, j int) bool {
+		return comments[i].Pos() < comments[j].Pos()
+	})
+
+	return specs
+}
diff --git a/src/go/ast/issues_test.go b/src/go/ast/issues_test.go
new file mode 100644
index 0000000..788c557
--- /dev/null
+++ b/src/go/ast/issues_test.go
@@ -0,0 +1,42 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast_test
+
+import (
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"testing"
+)
+
+func TestIssue33649(t *testing.T) {
+	for _, src := range []string{
+		`package p; func _()`,
+		`package p; func _() {`,
+		`package p; func _() { _ = 0`,
+		`package p; func _() { _ = 0 }`,
+	} {
+		fset := token.NewFileSet()
+		f, _ := parser.ParseFile(fset, "", src, parser.AllErrors)
+		if f == nil {
+			panic("invalid test setup: parser didn't return an AST")
+		}
+
+		// find corresponding token.File
+		var tf *token.File
+		fset.Iterate(func(f *token.File) bool {
+			tf = f
+			return true
+		})
+		tfEnd := tf.Base() + tf.Size()
+
+		fd := f.Decls[len(f.Decls)-1].(*ast.FuncDecl)
+		fdEnd := int(fd.End())
+
+		if fdEnd != tfEnd {
+			t.Errorf("%q: got fdEnd = %d; want %d (base = %d, size = %d)", src, fdEnd, tfEnd, tf.Base(), tf.Size())
+		}
+	}
+}
diff --git a/src/go/ast/print.go b/src/go/ast/print.go
new file mode 100644
index 0000000..d86d9ba
--- /dev/null
+++ b/src/go/ast/print.go
@@ -0,0 +1,254 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains printing support for ASTs.
+
+package ast
+
+import (
+	"fmt"
+	"go/token"
+	"io"
+	"os"
+	"reflect"
+)
+
+// A FieldFilter may be provided to Fprint to control the output.
+type FieldFilter func(name string, value reflect.Value) bool
+
+// NotNilFilter returns true for field values that are not nil;
+// it returns false otherwise.
+func NotNilFilter(_ string, v reflect.Value) bool {
+	switch v.Kind() {
+	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice:
+		return !v.IsNil()
+	}
+	return true
+}
+
+// Fprint prints the (sub-)tree starting at AST node x to w.
+// If fset != nil, position information is interpreted relative
+// to that file set. Otherwise positions are printed as integer
+// values (file set specific offsets).
+//
+// A non-nil FieldFilter f may be provided to control the output:
+// struct fields for which f(fieldname, fieldvalue) is true are
+// printed; all others are filtered from the output. Unexported
+// struct fields are never printed.
+func Fprint(w io.Writer, fset *token.FileSet, x interface{}, f FieldFilter) error {
+	return fprint(w, fset, x, f)
+}
+
+func fprint(w io.Writer, fset *token.FileSet, x interface{}, f FieldFilter) (err error) {
+	// setup printer
+	p := printer{
+		output: w,
+		fset:   fset,
+		filter: f,
+		ptrmap: make(map[interface{}]int),
+		last:   '\n', // force printing of line number on first line
+	}
+
+	// install error handler
+	defer func() {
+		if e := recover(); e != nil {
+			err = e.(localError).err // re-panics if it's not a localError
+		}
+	}()
+
+	// print x
+	if x == nil {
+		p.printf("nil\n")
+		return
+	}
+	p.print(reflect.ValueOf(x))
+	p.printf("\n")
+
+	return
+}
+
+// Print prints x to standard output, skipping nil fields.
+// Print(fset, x) is the same as Fprint(os.Stdout, fset, x, NotNilFilter).
+func Print(fset *token.FileSet, x interface{}) error {
+	return Fprint(os.Stdout, fset, x, NotNilFilter)
+}
+
+type printer struct {
+	output io.Writer
+	fset   *token.FileSet
+	filter FieldFilter
+	ptrmap map[interface{}]int // *T -> line number
+	indent int                 // current indentation level
+	last   byte                // the last byte processed by Write
+	line   int                 // current line number
+}
+
+var indent = []byte(".  ")
+
+func (p *printer) Write(data []byte) (n int, err error) {
+	var m int
+	for i, b := range data {
+		// invariant: data[0:n] has been written
+		if b == '\n' {
+			m, err = p.output.Write(data[n : i+1])
+			n += m
+			if err != nil {
+				return
+			}
+			p.line++
+		} else if p.last == '\n' {
+			_, err = fmt.Fprintf(p.output, "%6d  ", p.line)
+			if err != nil {
+				return
+			}
+			for j := p.indent; j > 0; j-- {
+				_, err = p.output.Write(indent)
+				if err != nil {
+					return
+				}
+			}
+		}
+		p.last = b
+	}
+	if len(data) > n {
+		m, err = p.output.Write(data[n:])
+		n += m
+	}
+	return
+}
+
+// localError wraps locally caught errors so we can distinguish
+// them from genuine panics which we don't want to return as errors.
+type localError struct {
+	err error
+}
+
+// printf is a convenience wrapper that takes care of print errors.
+func (p *printer) printf(format string, args ...interface{}) {
+	if _, err := fmt.Fprintf(p, format, args...); err != nil {
+		panic(localError{err})
+	}
+}
+
+// Implementation note: Print is written for AST nodes but could be
+// used to print arbitrary data structures; such a version should
+// probably be in a different package.
+//
+// Note: This code detects (some) cycles created via pointers but
+// not cycles that are created via slices or maps containing the
+// same slice or map. Code for general data structures probably
+// should catch those as well.
+
+func (p *printer) print(x reflect.Value) {
+	if !NotNilFilter("", x) {
+		p.printf("nil")
+		return
+	}
+
+	switch x.Kind() {
+	case reflect.Interface:
+		p.print(x.Elem())
+
+	case reflect.Map:
+		p.printf("%s (len = %d) {", x.Type(), x.Len())
+		if x.Len() > 0 {
+			p.indent++
+			p.printf("\n")
+			for _, key := range x.MapKeys() {
+				p.print(key)
+				p.printf(": ")
+				p.print(x.MapIndex(key))
+				p.printf("\n")
+			}
+			p.indent--
+		}
+		p.printf("}")
+
+	case reflect.Ptr:
+		p.printf("*")
+		// type-checked ASTs may contain cycles - use ptrmap
+		// to keep track of objects that have been printed
+		// already and print the respective line number instead
+		ptr := x.Interface()
+		if line, exists := p.ptrmap[ptr]; exists {
+			p.printf("(obj @ %d)", line)
+		} else {
+			p.ptrmap[ptr] = p.line
+			p.print(x.Elem())
+		}
+
+	case reflect.Array:
+		p.printf("%s {", x.Type())
+		if x.Len() > 0 {
+			p.indent++
+			p.printf("\n")
+			for i, n := 0, x.Len(); i < n; i++ {
+				p.printf("%d: ", i)
+				p.print(x.Index(i))
+				p.printf("\n")
+			}
+			p.indent--
+		}
+		p.printf("}")
+
+	case reflect.Slice:
+		if s, ok := x.Interface().([]byte); ok {
+			p.printf("%#q", s)
+			return
+		}
+		p.printf("%s (len = %d) {", x.Type(), x.Len())
+		if x.Len() > 0 {
+			p.indent++
+			p.printf("\n")
+			for i, n := 0, x.Len(); i < n; i++ {
+				p.printf("%d: ", i)
+				p.print(x.Index(i))
+				p.printf("\n")
+			}
+			p.indent--
+		}
+		p.printf("}")
+
+	case reflect.Struct:
+		t := x.Type()
+		p.printf("%s {", t)
+		p.indent++
+		first := true
+		for i, n := 0, t.NumField(); i < n; i++ {
+			// exclude non-exported fields because their
+			// values cannot be accessed via reflection
+			if name := t.Field(i).Name; IsExported(name) {
+				value := x.Field(i)
+				if p.filter == nil || p.filter(name, value) {
+					if first {
+						p.printf("\n")
+						first = false
+					}
+					p.printf("%s: ", name)
+					p.print(value)
+					p.printf("\n")
+				}
+			}
+		}
+		p.indent--
+		p.printf("}")
+
+	default:
+		v := x.Interface()
+		switch v := v.(type) {
+		case string:
+			// print strings in quotes
+			p.printf("%q", v)
+			return
+		case token.Pos:
+			// position values can be printed nicely if we have a file set
+			if p.fset != nil {
+				p.printf("%s", p.fset.Position(v))
+				return
+			}
+		}
+		// default
+		p.printf("%v", v)
+	}
+}
diff --git a/src/go/ast/print_test.go b/src/go/ast/print_test.go
new file mode 100644
index 0000000..210f164
--- /dev/null
+++ b/src/go/ast/print_test.go
@@ -0,0 +1,97 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+var tests = []struct {
+	x interface{} // x is printed as s
+	s string
+}{
+	// basic types
+	{nil, "0  nil"},
+	{true, "0  true"},
+	{42, "0  42"},
+	{3.14, "0  3.14"},
+	{1 + 2.718i, "0  (1+2.718i)"},
+	{"foobar", "0  \"foobar\""},
+
+	// maps
+	{map[Expr]string{}, `0  map[ast.Expr]string (len = 0) {}`},
+	{map[string]int{"a": 1},
+		`0  map[string]int (len = 1) {
+		1  .  "a": 1
+		2  }`},
+
+	// pointers
+	{new(int), "0  *0"},
+
+	// arrays
+	{[0]int{}, `0  [0]int {}`},
+	{[3]int{1, 2, 3},
+		`0  [3]int {
+		1  .  0: 1
+		2  .  1: 2
+		3  .  2: 3
+		4  }`},
+	{[...]int{42},
+		`0  [1]int {
+		1  .  0: 42
+		2  }`},
+
+	// slices
+	{[]int{}, `0  []int (len = 0) {}`},
+	{[]int{1, 2, 3},
+		`0  []int (len = 3) {
+		1  .  0: 1
+		2  .  1: 2
+		3  .  2: 3
+		4  }`},
+
+	// structs
+	{struct{}{}, `0  struct {} {}`},
+	{struct{ x int }{007}, `0  struct { x int } {}`},
+	{struct{ X, y int }{42, 991},
+		`0  struct { X int; y int } {
+		1  .  X: 42
+		2  }`},
+	{struct{ X, Y int }{42, 991},
+		`0  struct { X int; Y int } {
+		1  .  X: 42
+		2  .  Y: 991
+		3  }`},
+}
+
+// Split s into lines, trim whitespace from all lines, and return
+// the concatenated non-empty lines.
+func trim(s string) string {
+	lines := strings.Split(s, "\n")
+	i := 0
+	for _, line := range lines {
+		line = strings.TrimSpace(line)
+		if line != "" {
+			lines[i] = line
+			i++
+		}
+	}
+	return strings.Join(lines[0:i], "\n")
+}
+
+func TestPrint(t *testing.T) {
+	var buf bytes.Buffer
+	for _, test := range tests {
+		buf.Reset()
+		if err := Fprint(&buf, nil, test.x, nil); err != nil {
+			t.Errorf("Fprint failed: %s", err)
+		}
+		if s, ts := trim(buf.String()), trim(test.s); s != ts {
+			t.Errorf("got:\n%s\nexpected:\n%s\n", s, ts)
+		}
+	}
+}
diff --git a/src/go/ast/resolve.go b/src/go/ast/resolve.go
new file mode 100644
index 0000000..c1830b5
--- /dev/null
+++ b/src/go/ast/resolve.go
@@ -0,0 +1,174 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements NewPackage.
+
+package ast
+
+import (
+	"fmt"
+	"go/scanner"
+	"go/token"
+	"strconv"
+)
+
+type pkgBuilder struct {
+	fset   *token.FileSet
+	errors scanner.ErrorList
+}
+
+func (p *pkgBuilder) error(pos token.Pos, msg string) {
+	p.errors.Add(p.fset.Position(pos), msg)
+}
+
+func (p *pkgBuilder) errorf(pos token.Pos, format string, args ...interface{}) {
+	p.error(pos, fmt.Sprintf(format, args...))
+}
+
+func (p *pkgBuilder) declare(scope, altScope *Scope, obj *Object) {
+	alt := scope.Insert(obj)
+	if alt == nil && altScope != nil {
+		// see if there is a conflicting declaration in altScope
+		alt = altScope.Lookup(obj.Name)
+	}
+	if alt != nil {
+		prevDecl := ""
+		if pos := alt.Pos(); pos.IsValid() {
+			prevDecl = fmt.Sprintf("\n\tprevious declaration at %s", p.fset.Position(pos))
+		}
+		p.error(obj.Pos(), fmt.Sprintf("%s redeclared in this block%s", obj.Name, prevDecl))
+	}
+}
+
+func resolve(scope *Scope, ident *Ident) bool {
+	for ; scope != nil; scope = scope.Outer {
+		if obj := scope.Lookup(ident.Name); obj != nil {
+			ident.Obj = obj
+			return true
+		}
+	}
+	return false
+}
+
+// An Importer resolves import paths to package Objects.
+// The imports map records the packages already imported,
+// indexed by package id (canonical import path).
+// An Importer must determine the canonical import path and
+// check the map to see if it is already present in the imports map.
+// If so, the Importer can return the map entry. Otherwise, the
+// Importer should load the package data for the given path into
+// a new *Object (pkg), record pkg in the imports map, and then
+// return pkg.
+type Importer func(imports map[string]*Object, path string) (pkg *Object, err error)
+
+// NewPackage creates a new Package node from a set of File nodes. It resolves
+// unresolved identifiers across files and updates each file's Unresolved list
+// accordingly. If a non-nil importer and universe scope are provided, they are
+// used to resolve identifiers not declared in any of the package files. Any
+// remaining unresolved identifiers are reported as undeclared. If the files
+// belong to different packages, one package name is selected and files with
+// different package names are reported and then ignored.
+// The result is a package node and a scanner.ErrorList if there were errors.
+//
+func NewPackage(fset *token.FileSet, files map[string]*File, importer Importer, universe *Scope) (*Package, error) {
+	var p pkgBuilder
+	p.fset = fset
+
+	// complete package scope
+	pkgName := ""
+	pkgScope := NewScope(universe)
+	for _, file := range files {
+		// package names must match
+		switch name := file.Name.Name; {
+		case pkgName == "":
+			pkgName = name
+		case name != pkgName:
+			p.errorf(file.Package, "package %s; expected %s", name, pkgName)
+			continue // ignore this file
+		}
+
+		// collect top-level file objects in package scope
+		for _, obj := range file.Scope.Objects {
+			p.declare(pkgScope, nil, obj)
+		}
+	}
+
+	// package global mapping of imported package ids to package objects
+	imports := make(map[string]*Object)
+
+	// complete file scopes with imports and resolve identifiers
+	for _, file := range files {
+		// ignore file if it belongs to a different package
+		// (error has already been reported)
+		if file.Name.Name != pkgName {
+			continue
+		}
+
+		// build file scope by processing all imports
+		importErrors := false
+		fileScope := NewScope(pkgScope)
+		for _, spec := range file.Imports {
+			if importer == nil {
+				importErrors = true
+				continue
+			}
+			path, _ := strconv.Unquote(spec.Path.Value)
+			pkg, err := importer(imports, path)
+			if err != nil {
+				p.errorf(spec.Path.Pos(), "could not import %s (%s)", path, err)
+				importErrors = true
+				continue
+			}
+			// TODO(gri) If a local package name != "." is provided,
+			// global identifier resolution could proceed even if the
+			// import failed. Consider adjusting the logic here a bit.
+
+			// local name overrides imported package name
+			name := pkg.Name
+			if spec.Name != nil {
+				name = spec.Name.Name
+			}
+
+			// add import to file scope
+			if name == "." {
+				// merge imported scope with file scope
+				for _, obj := range pkg.Data.(*Scope).Objects {
+					p.declare(fileScope, pkgScope, obj)
+				}
+			} else if name != "_" {
+				// declare imported package object in file scope
+				// (do not re-use pkg in the file scope but create
+				// a new object instead; the Decl field is different
+				// for different files)
+				obj := NewObj(Pkg, name)
+				obj.Decl = spec
+				obj.Data = pkg.Data
+				p.declare(fileScope, pkgScope, obj)
+			}
+		}
+
+		// resolve identifiers
+		if importErrors {
+			// don't use the universe scope without correct imports
+			// (objects in the universe may be shadowed by imports;
+			// with missing imports, identifiers might get resolved
+			// incorrectly to universe objects)
+			pkgScope.Outer = nil
+		}
+		i := 0
+		for _, ident := range file.Unresolved {
+			if !resolve(fileScope, ident) {
+				p.errorf(ident.Pos(), "undeclared name: %s", ident.Name)
+				file.Unresolved[i] = ident
+				i++
+			}
+
+		}
+		file.Unresolved = file.Unresolved[0:i]
+		pkgScope.Outer = universe // reset universe scope
+	}
+
+	p.errors.Sort()
+	return &Package{pkgName, pkgScope, imports, files}, p.errors.Err()
+}
diff --git a/src/go/ast/scope.go b/src/go/ast/scope.go
new file mode 100644
index 0000000..a400c71
--- /dev/null
+++ b/src/go/ast/scope.go
@@ -0,0 +1,160 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements scopes and the objects they contain.
+
+package ast
+
+import (
+	"bytes"
+	"fmt"
+	"go/token"
+)
+
+// A Scope maintains the set of named language entities declared
+// in the scope and a link to the immediately surrounding (outer)
+// scope.
+//
+type Scope struct {
+	Outer   *Scope
+	Objects map[string]*Object
+}
+
+// NewScope creates a new scope nested in the outer scope.
+func NewScope(outer *Scope) *Scope {
+	const n = 4 // initial scope capacity
+	return &Scope{outer, make(map[string]*Object, n)}
+}
+
+// Lookup returns the object with the given name if it is
+// found in scope s, otherwise it returns nil. Outer scopes
+// are ignored.
+//
+func (s *Scope) Lookup(name string) *Object {
+	return s.Objects[name]
+}
+
+// Insert attempts to insert a named object obj into the scope s.
+// If the scope already contains an object alt with the same name,
+// Insert leaves the scope unchanged and returns alt. Otherwise
+// it inserts obj and returns nil.
+//
+func (s *Scope) Insert(obj *Object) (alt *Object) {
+	if alt = s.Objects[obj.Name]; alt == nil {
+		s.Objects[obj.Name] = obj
+	}
+	return
+}
+
+// Debugging support
+func (s *Scope) String() string {
+	var buf bytes.Buffer
+	fmt.Fprintf(&buf, "scope %p {", s)
+	if s != nil && len(s.Objects) > 0 {
+		fmt.Fprintln(&buf)
+		for _, obj := range s.Objects {
+			fmt.Fprintf(&buf, "\t%s %s\n", obj.Kind, obj.Name)
+		}
+	}
+	fmt.Fprintf(&buf, "}\n")
+	return buf.String()
+}
+
+// ----------------------------------------------------------------------------
+// Objects
+
+// An Object describes a named language entity such as a package,
+// constant, type, variable, function (incl. methods), or label.
+//
+// The Data fields contains object-specific data:
+//
+//	Kind    Data type         Data value
+//	Pkg     *Scope            package scope
+//	Con     int               iota for the respective declaration
+//
+type Object struct {
+	Kind ObjKind
+	Name string      // declared name
+	Decl interface{} // corresponding Field, XxxSpec, FuncDecl, LabeledStmt, AssignStmt, Scope; or nil
+	Data interface{} // object-specific data; or nil
+	Type interface{} // placeholder for type information; may be nil
+}
+
+// NewObj creates a new object of a given kind and name.
+func NewObj(kind ObjKind, name string) *Object {
+	return &Object{Kind: kind, Name: name}
+}
+
+// Pos computes the source position of the declaration of an object name.
+// The result may be an invalid position if it cannot be computed
+// (obj.Decl may be nil or not correct).
+func (obj *Object) Pos() token.Pos {
+	name := obj.Name
+	switch d := obj.Decl.(type) {
+	case *Field:
+		for _, n := range d.Names {
+			if n.Name == name {
+				return n.Pos()
+			}
+		}
+	case *ImportSpec:
+		if d.Name != nil && d.Name.Name == name {
+			return d.Name.Pos()
+		}
+		return d.Path.Pos()
+	case *ValueSpec:
+		for _, n := range d.Names {
+			if n.Name == name {
+				return n.Pos()
+			}
+		}
+	case *TypeSpec:
+		if d.Name.Name == name {
+			return d.Name.Pos()
+		}
+	case *FuncDecl:
+		if d.Name.Name == name {
+			return d.Name.Pos()
+		}
+	case *LabeledStmt:
+		if d.Label.Name == name {
+			return d.Label.Pos()
+		}
+	case *AssignStmt:
+		for _, x := range d.Lhs {
+			if ident, isIdent := x.(*Ident); isIdent && ident.Name == name {
+				return ident.Pos()
+			}
+		}
+	case *Scope:
+		// predeclared object - nothing to do for now
+	}
+	return token.NoPos
+}
+
+// ObjKind describes what an object represents.
+type ObjKind int
+
+// The list of possible Object kinds.
+const (
+	Bad ObjKind = iota // for error handling
+	Pkg                // package
+	Con                // constant
+	Typ                // type
+	Var                // variable
+	Fun                // function or method
+	Lbl                // label
+)
+
+var objKindStrings = [...]string{
+	Bad: "bad",
+	Pkg: "package",
+	Con: "const",
+	Typ: "type",
+	Var: "var",
+	Fun: "func",
+	Lbl: "label",
+}
+
+func (kind ObjKind) String() string { return objKindStrings[kind] }
diff --git a/src/go/ast/walk.go b/src/go/ast/walk.go
new file mode 100644
index 0000000..8ca2195
--- /dev/null
+++ b/src/go/ast/walk.go
@@ -0,0 +1,386 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ast
+
+import "fmt"
+
+// A Visitor's Visit method is invoked for each node encountered by Walk.
+// If the result visitor w is not nil, Walk visits each of the children
+// of node with the visitor w, followed by a call of w.Visit(nil).
+type Visitor interface {
+	Visit(node Node) (w Visitor)
+}
+
+// Helper functions for common node lists. They may be empty.
+
+func walkIdentList(v Visitor, list []*Ident) {
+	for _, x := range list {
+		Walk(v, x)
+	}
+}
+
+func walkExprList(v Visitor, list []Expr) {
+	for _, x := range list {
+		Walk(v, x)
+	}
+}
+
+func walkStmtList(v Visitor, list []Stmt) {
+	for _, x := range list {
+		Walk(v, x)
+	}
+}
+
+func walkDeclList(v Visitor, list []Decl) {
+	for _, x := range list {
+		Walk(v, x)
+	}
+}
+
+// TODO(gri): Investigate if providing a closure to Walk leads to
+//            simpler use (and may help eliminate Inspect in turn).
+
+// Walk traverses an AST in depth-first order: It starts by calling
+// v.Visit(node); node must not be nil. If the visitor w returned by
+// v.Visit(node) is not nil, Walk is invoked recursively with visitor
+// w for each of the non-nil children of node, followed by a call of
+// w.Visit(nil).
+//
+func Walk(v Visitor, node Node) {
+	if v = v.Visit(node); v == nil {
+		return
+	}
+
+	// walk children
+	// (the order of the cases matches the order
+	// of the corresponding node types in ast.go)
+	switch n := node.(type) {
+	// Comments and fields
+	case *Comment:
+		// nothing to do
+
+	case *CommentGroup:
+		for _, c := range n.List {
+			Walk(v, c)
+		}
+
+	case *Field:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		walkIdentList(v, n.Names)
+		Walk(v, n.Type)
+		if n.Tag != nil {
+			Walk(v, n.Tag)
+		}
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
+
+	case *FieldList:
+		for _, f := range n.List {
+			Walk(v, f)
+		}
+
+	// Expressions
+	case *BadExpr, *Ident, *BasicLit:
+		// nothing to do
+
+	case *Ellipsis:
+		if n.Elt != nil {
+			Walk(v, n.Elt)
+		}
+
+	case *FuncLit:
+		Walk(v, n.Type)
+		Walk(v, n.Body)
+
+	case *CompositeLit:
+		if n.Type != nil {
+			Walk(v, n.Type)
+		}
+		walkExprList(v, n.Elts)
+
+	case *ParenExpr:
+		Walk(v, n.X)
+
+	case *SelectorExpr:
+		Walk(v, n.X)
+		Walk(v, n.Sel)
+
+	case *IndexExpr:
+		Walk(v, n.X)
+		Walk(v, n.Index)
+
+	case *SliceExpr:
+		Walk(v, n.X)
+		if n.Low != nil {
+			Walk(v, n.Low)
+		}
+		if n.High != nil {
+			Walk(v, n.High)
+		}
+		if n.Max != nil {
+			Walk(v, n.Max)
+		}
+
+	case *TypeAssertExpr:
+		Walk(v, n.X)
+		if n.Type != nil {
+			Walk(v, n.Type)
+		}
+
+	case *CallExpr:
+		Walk(v, n.Fun)
+		walkExprList(v, n.Args)
+
+	case *StarExpr:
+		Walk(v, n.X)
+
+	case *UnaryExpr:
+		Walk(v, n.X)
+
+	case *BinaryExpr:
+		Walk(v, n.X)
+		Walk(v, n.Y)
+
+	case *KeyValueExpr:
+		Walk(v, n.Key)
+		Walk(v, n.Value)
+
+	// Types
+	case *ArrayType:
+		if n.Len != nil {
+			Walk(v, n.Len)
+		}
+		Walk(v, n.Elt)
+
+	case *StructType:
+		Walk(v, n.Fields)
+
+	case *FuncType:
+		if n.Params != nil {
+			Walk(v, n.Params)
+		}
+		if n.Results != nil {
+			Walk(v, n.Results)
+		}
+
+	case *InterfaceType:
+		Walk(v, n.Methods)
+
+	case *MapType:
+		Walk(v, n.Key)
+		Walk(v, n.Value)
+
+	case *ChanType:
+		Walk(v, n.Value)
+
+	// Statements
+	case *BadStmt:
+		// nothing to do
+
+	case *DeclStmt:
+		Walk(v, n.Decl)
+
+	case *EmptyStmt:
+		// nothing to do
+
+	case *LabeledStmt:
+		Walk(v, n.Label)
+		Walk(v, n.Stmt)
+
+	case *ExprStmt:
+		Walk(v, n.X)
+
+	case *SendStmt:
+		Walk(v, n.Chan)
+		Walk(v, n.Value)
+
+	case *IncDecStmt:
+		Walk(v, n.X)
+
+	case *AssignStmt:
+		walkExprList(v, n.Lhs)
+		walkExprList(v, n.Rhs)
+
+	case *GoStmt:
+		Walk(v, n.Call)
+
+	case *DeferStmt:
+		Walk(v, n.Call)
+
+	case *ReturnStmt:
+		walkExprList(v, n.Results)
+
+	case *BranchStmt:
+		if n.Label != nil {
+			Walk(v, n.Label)
+		}
+
+	case *BlockStmt:
+		walkStmtList(v, n.List)
+
+	case *IfStmt:
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
+		Walk(v, n.Cond)
+		Walk(v, n.Body)
+		if n.Else != nil {
+			Walk(v, n.Else)
+		}
+
+	case *CaseClause:
+		walkExprList(v, n.List)
+		walkStmtList(v, n.Body)
+
+	case *SwitchStmt:
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
+		if n.Tag != nil {
+			Walk(v, n.Tag)
+		}
+		Walk(v, n.Body)
+
+	case *TypeSwitchStmt:
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
+		Walk(v, n.Assign)
+		Walk(v, n.Body)
+
+	case *CommClause:
+		if n.Comm != nil {
+			Walk(v, n.Comm)
+		}
+		walkStmtList(v, n.Body)
+
+	case *SelectStmt:
+		Walk(v, n.Body)
+
+	case *ForStmt:
+		if n.Init != nil {
+			Walk(v, n.Init)
+		}
+		if n.Cond != nil {
+			Walk(v, n.Cond)
+		}
+		if n.Post != nil {
+			Walk(v, n.Post)
+		}
+		Walk(v, n.Body)
+
+	case *RangeStmt:
+		if n.Key != nil {
+			Walk(v, n.Key)
+		}
+		if n.Value != nil {
+			Walk(v, n.Value)
+		}
+		Walk(v, n.X)
+		Walk(v, n.Body)
+
+	// Declarations
+	case *ImportSpec:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		if n.Name != nil {
+			Walk(v, n.Name)
+		}
+		Walk(v, n.Path)
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
+
+	case *ValueSpec:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		walkIdentList(v, n.Names)
+		if n.Type != nil {
+			Walk(v, n.Type)
+		}
+		walkExprList(v, n.Values)
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
+
+	case *TypeSpec:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		Walk(v, n.Name)
+		Walk(v, n.Type)
+		if n.Comment != nil {
+			Walk(v, n.Comment)
+		}
+
+	case *BadDecl:
+		// nothing to do
+
+	case *GenDecl:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		for _, s := range n.Specs {
+			Walk(v, s)
+		}
+
+	case *FuncDecl:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		if n.Recv != nil {
+			Walk(v, n.Recv)
+		}
+		Walk(v, n.Name)
+		Walk(v, n.Type)
+		if n.Body != nil {
+			Walk(v, n.Body)
+		}
+
+	// Files and packages
+	case *File:
+		if n.Doc != nil {
+			Walk(v, n.Doc)
+		}
+		Walk(v, n.Name)
+		walkDeclList(v, n.Decls)
+		// don't walk n.Comments - they have been
+		// visited already through the individual
+		// nodes
+
+	case *Package:
+		for _, f := range n.Files {
+			Walk(v, f)
+		}
+
+	default:
+		panic(fmt.Sprintf("ast.Walk: unexpected node type %T", n))
+	}
+
+	v.Visit(nil)
+}
+
+type inspector func(Node) bool
+
+func (f inspector) Visit(node Node) Visitor {
+	if f(node) {
+		return f
+	}
+	return nil
+}
+
+// Inspect traverses an AST in depth-first order: It starts by calling
+// f(node); node must not be nil. If f returns true, Inspect invokes f
+// recursively for each of the non-nil children of node, followed by a
+// call of f(nil).
+//
+func Inspect(node Node, f func(Node) bool) {
+	Walk(inspector(f), node)
+}
diff --git a/src/go/build/build.go b/src/go/build/build.go
new file mode 100644
index 0000000..217fadf
--- /dev/null
+++ b/src/go/build/build.go
@@ -0,0 +1,1986 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"go/ast"
+	"go/doc"
+	"go/token"
+	exec "internal/execabs"
+	"internal/goroot"
+	"internal/goversion"
+	"io"
+	"io/fs"
+	"io/ioutil"
+	"os"
+	pathpkg "path"
+	"path/filepath"
+	"runtime"
+	"sort"
+	"strconv"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// A Context specifies the supporting context for a build.
+type Context struct {
+	GOARCH string // target architecture
+	GOOS   string // target operating system
+	GOROOT string // Go root
+	GOPATH string // Go path
+
+	// Dir is the caller's working directory, or the empty string to use
+	// the current directory of the running process. In module mode, this is used
+	// to locate the main module.
+	//
+	// If Dir is non-empty, directories passed to Import and ImportDir must
+	// be absolute.
+	Dir string
+
+	CgoEnabled  bool   // whether cgo files are included
+	UseAllFiles bool   // use files regardless of +build lines, file names
+	Compiler    string // compiler to assume when computing target paths
+
+	// The build and release tags specify build constraints
+	// that should be considered satisfied when processing +build lines.
+	// Clients creating a new context may customize BuildTags, which
+	// defaults to empty, but it is usually an error to customize ReleaseTags,
+	// which defaults to the list of Go releases the current release is compatible with.
+	// BuildTags is not set for the Default build Context.
+	// In addition to the BuildTags and ReleaseTags, build constraints
+	// consider the values of GOARCH and GOOS as satisfied tags.
+	// The last element in ReleaseTags is assumed to be the current release.
+	BuildTags   []string
+	ReleaseTags []string
+
+	// The install suffix specifies a suffix to use in the name of the installation
+	// directory. By default it is empty, but custom builds that need to keep
+	// their outputs separate can set InstallSuffix to do so. For example, when
+	// using the race detector, the go command uses InstallSuffix = "race", so
+	// that on a Linux/386 system, packages are written to a directory named
+	// "linux_386_race" instead of the usual "linux_386".
+	InstallSuffix string
+
+	// By default, Import uses the operating system's file system calls
+	// to read directories and files. To read from other sources,
+	// callers can set the following functions. They all have default
+	// behaviors that use the local file system, so clients need only set
+	// the functions whose behaviors they wish to change.
+
+	// JoinPath joins the sequence of path fragments into a single path.
+	// If JoinPath is nil, Import uses filepath.Join.
+	JoinPath func(elem ...string) string
+
+	// SplitPathList splits the path list into a slice of individual paths.
+	// If SplitPathList is nil, Import uses filepath.SplitList.
+	SplitPathList func(list string) []string
+
+	// IsAbsPath reports whether path is an absolute path.
+	// If IsAbsPath is nil, Import uses filepath.IsAbs.
+	IsAbsPath func(path string) bool
+
+	// IsDir reports whether the path names a directory.
+	// If IsDir is nil, Import calls os.Stat and uses the result's IsDir method.
+	IsDir func(path string) bool
+
+	// HasSubdir reports whether dir is lexically a subdirectory of
+	// root, perhaps multiple levels below. It does not try to check
+	// whether dir exists.
+	// If so, HasSubdir sets rel to a slash-separated path that
+	// can be joined to root to produce a path equivalent to dir.
+	// If HasSubdir is nil, Import uses an implementation built on
+	// filepath.EvalSymlinks.
+	HasSubdir func(root, dir string) (rel string, ok bool)
+
+	// ReadDir returns a slice of fs.FileInfo, sorted by Name,
+	// describing the content of the named directory.
+	// If ReadDir is nil, Import uses ioutil.ReadDir.
+	ReadDir func(dir string) ([]fs.FileInfo, error)
+
+	// OpenFile opens a file (not a directory) for reading.
+	// If OpenFile is nil, Import uses os.Open.
+	OpenFile func(path string) (io.ReadCloser, error)
+}
+
+// joinPath calls ctxt.JoinPath (if not nil) or else filepath.Join.
+func (ctxt *Context) joinPath(elem ...string) string {
+	if f := ctxt.JoinPath; f != nil {
+		return f(elem...)
+	}
+	return filepath.Join(elem...)
+}
+
+// splitPathList calls ctxt.SplitPathList (if not nil) or else filepath.SplitList.
+func (ctxt *Context) splitPathList(s string) []string {
+	if f := ctxt.SplitPathList; f != nil {
+		return f(s)
+	}
+	return filepath.SplitList(s)
+}
+
+// isAbsPath calls ctxt.IsAbsPath (if not nil) or else filepath.IsAbs.
+func (ctxt *Context) isAbsPath(path string) bool {
+	if f := ctxt.IsAbsPath; f != nil {
+		return f(path)
+	}
+	return filepath.IsAbs(path)
+}
+
+// isDir calls ctxt.IsDir (if not nil) or else uses os.Stat.
+func (ctxt *Context) isDir(path string) bool {
+	if f := ctxt.IsDir; f != nil {
+		return f(path)
+	}
+	fi, err := os.Stat(path)
+	return err == nil && fi.IsDir()
+}
+
+// hasSubdir calls ctxt.HasSubdir (if not nil) or else uses
+// the local file system to answer the question.
+func (ctxt *Context) hasSubdir(root, dir string) (rel string, ok bool) {
+	if f := ctxt.HasSubdir; f != nil {
+		return f(root, dir)
+	}
+
+	// Try using paths we received.
+	if rel, ok = hasSubdir(root, dir); ok {
+		return
+	}
+
+	// Try expanding symlinks and comparing
+	// expanded against unexpanded and
+	// expanded against expanded.
+	rootSym, _ := filepath.EvalSymlinks(root)
+	dirSym, _ := filepath.EvalSymlinks(dir)
+
+	if rel, ok = hasSubdir(rootSym, dir); ok {
+		return
+	}
+	if rel, ok = hasSubdir(root, dirSym); ok {
+		return
+	}
+	return hasSubdir(rootSym, dirSym)
+}
+
+// hasSubdir reports if dir is within root by performing lexical analysis only.
+func hasSubdir(root, dir string) (rel string, ok bool) {
+	const sep = string(filepath.Separator)
+	root = filepath.Clean(root)
+	if !strings.HasSuffix(root, sep) {
+		root += sep
+	}
+	dir = filepath.Clean(dir)
+	if !strings.HasPrefix(dir, root) {
+		return "", false
+	}
+	return filepath.ToSlash(dir[len(root):]), true
+}
+
+// readDir calls ctxt.ReadDir (if not nil) or else ioutil.ReadDir.
+func (ctxt *Context) readDir(path string) ([]fs.FileInfo, error) {
+	if f := ctxt.ReadDir; f != nil {
+		return f(path)
+	}
+	return ioutil.ReadDir(path)
+}
+
+// openFile calls ctxt.OpenFile (if not nil) or else os.Open.
+func (ctxt *Context) openFile(path string) (io.ReadCloser, error) {
+	if fn := ctxt.OpenFile; fn != nil {
+		return fn(path)
+	}
+
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err // nil interface
+	}
+	return f, nil
+}
+
+// isFile determines whether path is a file by trying to open it.
+// It reuses openFile instead of adding another function to the
+// list in Context.
+func (ctxt *Context) isFile(path string) bool {
+	f, err := ctxt.openFile(path)
+	if err != nil {
+		return false
+	}
+	f.Close()
+	return true
+}
+
+// gopath returns the list of Go path directories.
+func (ctxt *Context) gopath() []string {
+	var all []string
+	for _, p := range ctxt.splitPathList(ctxt.GOPATH) {
+		if p == "" || p == ctxt.GOROOT {
+			// Empty paths are uninteresting.
+			// If the path is the GOROOT, ignore it.
+			// People sometimes set GOPATH=$GOROOT.
+			// Do not get confused by this common mistake.
+			continue
+		}
+		if strings.HasPrefix(p, "~") {
+			// Path segments starting with ~ on Unix are almost always
+			// users who have incorrectly quoted ~ while setting GOPATH,
+			// preventing it from expanding to $HOME.
+			// The situation is made more confusing by the fact that
+			// bash allows quoted ~ in $PATH (most shells do not).
+			// Do not get confused by this, and do not try to use the path.
+			// It does not exist, and printing errors about it confuses
+			// those users even more, because they think "sure ~ exists!".
+			// The go command diagnoses this situation and prints a
+			// useful error.
+			// On Windows, ~ is used in short names, such as c:\progra~1
+			// for c:\program files.
+			continue
+		}
+		all = append(all, p)
+	}
+	return all
+}
+
+// SrcDirs returns a list of package source root directories.
+// It draws from the current Go root and Go path but omits directories
+// that do not exist.
+func (ctxt *Context) SrcDirs() []string {
+	var all []string
+	if ctxt.GOROOT != "" && ctxt.Compiler != "gccgo" {
+		dir := ctxt.joinPath(ctxt.GOROOT, "src")
+		if ctxt.isDir(dir) {
+			all = append(all, dir)
+		}
+	}
+	for _, p := range ctxt.gopath() {
+		dir := ctxt.joinPath(p, "src")
+		if ctxt.isDir(dir) {
+			all = append(all, dir)
+		}
+	}
+	return all
+}
+
+// Default is the default Context for builds.
+// It uses the GOARCH, GOOS, GOROOT, and GOPATH environment variables
+// if set, or else the compiled code's GOARCH, GOOS, and GOROOT.
+var Default Context = defaultContext()
+
+func defaultGOPATH() string {
+	env := "HOME"
+	if runtime.GOOS == "windows" {
+		env = "USERPROFILE"
+	} else if runtime.GOOS == "plan9" {
+		env = "home"
+	}
+	if home := os.Getenv(env); home != "" {
+		def := filepath.Join(home, "go")
+		if filepath.Clean(def) == filepath.Clean(runtime.GOROOT()) {
+			// Don't set the default GOPATH to GOROOT,
+			// as that will trigger warnings from the go tool.
+			return ""
+		}
+		return def
+	}
+	return ""
+}
+
+var defaultReleaseTags []string
+
+func defaultContext() Context {
+	var c Context
+
+	c.GOARCH = envOr("GOARCH", runtime.GOARCH)
+	c.GOOS = envOr("GOOS", runtime.GOOS)
+	c.GOROOT = pathpkg.Clean(runtime.GOROOT())
+	c.GOPATH = envOr("GOPATH", defaultGOPATH())
+	c.Compiler = runtime.Compiler
+
+	// Each major Go release in the Go 1.x series adds a new
+	// "go1.x" release tag. That is, the go1.x tag is present in
+	// all releases >= Go 1.x. Code that requires Go 1.x or later
+	// should say "+build go1.x", and code that should only be
+	// built before Go 1.x (perhaps it is the stub to use in that
+	// case) should say "+build !go1.x".
+	// The last element in ReleaseTags is the current release.
+	for i := 1; i <= goversion.Version; i++ {
+		c.ReleaseTags = append(c.ReleaseTags, "go1."+strconv.Itoa(i))
+	}
+
+	defaultReleaseTags = append([]string{}, c.ReleaseTags...) // our own private copy
+
+	env := os.Getenv("CGO_ENABLED")
+	if env == "" {
+		env = defaultCGO_ENABLED
+	}
+	switch env {
+	case "1":
+		c.CgoEnabled = true
+	case "0":
+		c.CgoEnabled = false
+	default:
+		// cgo must be explicitly enabled for cross compilation builds
+		if runtime.GOARCH == c.GOARCH && runtime.GOOS == c.GOOS {
+			c.CgoEnabled = cgoEnabled[c.GOOS+"/"+c.GOARCH]
+			break
+		}
+		c.CgoEnabled = false
+	}
+
+	return c
+}
+
+func envOr(name, def string) string {
+	s := os.Getenv(name)
+	if s == "" {
+		return def
+	}
+	return s
+}
+
+// An ImportMode controls the behavior of the Import method.
+type ImportMode uint
+
+const (
+	// If FindOnly is set, Import stops after locating the directory
+	// that should contain the sources for a package. It does not
+	// read any files in the directory.
+	FindOnly ImportMode = 1 << iota
+
+	// If AllowBinary is set, Import can be satisfied by a compiled
+	// package object without corresponding sources.
+	//
+	// Deprecated:
+	// The supported way to create a compiled-only package is to
+	// write source code containing a //go:binary-only-package comment at
+	// the top of the file. Such a package will be recognized
+	// regardless of this flag setting (because it has source code)
+	// and will have BinaryOnly set to true in the returned Package.
+	AllowBinary
+
+	// If ImportComment is set, parse import comments on package statements.
+	// Import returns an error if it finds a comment it cannot understand
+	// or finds conflicting comments in multiple source files.
+	// See golang.org/s/go14customimport for more information.
+	ImportComment
+
+	// By default, Import searches vendor directories
+	// that apply in the given source directory before searching
+	// the GOROOT and GOPATH roots.
+	// If an Import finds and returns a package using a vendor
+	// directory, the resulting ImportPath is the complete path
+	// to the package, including the path elements leading up
+	// to and including "vendor".
+	// For example, if Import("y", "x/subdir", 0) finds
+	// "x/vendor/y", the returned package's ImportPath is "x/vendor/y",
+	// not plain "y".
+	// See golang.org/s/go15vendor for more information.
+	//
+	// Setting IgnoreVendor ignores vendor directories.
+	//
+	// In contrast to the package's ImportPath,
+	// the returned package's Imports, TestImports, and XTestImports
+	// are always the exact import paths from the source files:
+	// Import makes no attempt to resolve or check those paths.
+	IgnoreVendor
+)
+
+// A Package describes the Go package found in a directory.
+type Package struct {
+	Dir           string   // directory containing package sources
+	Name          string   // package name
+	ImportComment string   // path in import comment on package statement
+	Doc           string   // documentation synopsis
+	ImportPath    string   // import path of package ("" if unknown)
+	Root          string   // root of Go tree where this package lives
+	SrcRoot       string   // package source root directory ("" if unknown)
+	PkgRoot       string   // package install root directory ("" if unknown)
+	PkgTargetRoot string   // architecture dependent install root directory ("" if unknown)
+	BinDir        string   // command install directory ("" if unknown)
+	Goroot        bool     // package found in Go root
+	PkgObj        string   // installed .a file
+	AllTags       []string // tags that can influence file selection in this directory
+	ConflictDir   string   // this directory shadows Dir in $GOPATH
+	BinaryOnly    bool     // cannot be rebuilt from source (has //go:binary-only-package comment)
+
+	// Source files
+	GoFiles           []string // .go source files (excluding CgoFiles, TestGoFiles, XTestGoFiles)
+	CgoFiles          []string // .go source files that import "C"
+	IgnoredGoFiles    []string // .go source files ignored for this build (including ignored _test.go files)
+	InvalidGoFiles    []string // .go source files with detected problems (parse error, wrong package name, and so on)
+	IgnoredOtherFiles []string // non-.go source files ignored for this build
+	CFiles            []string // .c source files
+	CXXFiles          []string // .cc, .cpp and .cxx source files
+	MFiles            []string // .m (Objective-C) source files
+	HFiles            []string // .h, .hh, .hpp and .hxx source files
+	FFiles            []string // .f, .F, .for and .f90 Fortran source files
+	SFiles            []string // .s source files
+	SwigFiles         []string // .swig files
+	SwigCXXFiles      []string // .swigcxx files
+	SysoFiles         []string // .syso system object files to add to archive
+
+	// Cgo directives
+	CgoCFLAGS    []string // Cgo CFLAGS directives
+	CgoCPPFLAGS  []string // Cgo CPPFLAGS directives
+	CgoCXXFLAGS  []string // Cgo CXXFLAGS directives
+	CgoFFLAGS    []string // Cgo FFLAGS directives
+	CgoLDFLAGS   []string // Cgo LDFLAGS directives
+	CgoPkgConfig []string // Cgo pkg-config directives
+
+	// Test information
+	TestGoFiles  []string // _test.go files in package
+	XTestGoFiles []string // _test.go files outside package
+
+	// Dependency information
+	Imports        []string                    // import paths from GoFiles, CgoFiles
+	ImportPos      map[string][]token.Position // line information for Imports
+	TestImports    []string                    // import paths from TestGoFiles
+	TestImportPos  map[string][]token.Position // line information for TestImports
+	XTestImports   []string                    // import paths from XTestGoFiles
+	XTestImportPos map[string][]token.Position // line information for XTestImports
+
+	// //go:embed patterns found in Go source files
+	// For example, if a source file says
+	//	//go:embed a* b.c
+	// then the list will contain those two strings as separate entries.
+	// (See package embed for more details about //go:embed.)
+	EmbedPatterns        []string                    // patterns from GoFiles, CgoFiles
+	EmbedPatternPos      map[string][]token.Position // line information for EmbedPatterns
+	TestEmbedPatterns    []string                    // patterns from TestGoFiles
+	TestEmbedPatternPos  map[string][]token.Position // line information for TestEmbedPatterns
+	XTestEmbedPatterns   []string                    // patterns from XTestGoFiles
+	XTestEmbedPatternPos map[string][]token.Position // line information for XTestEmbedPatternPos
+}
+
+// IsCommand reports whether the package is considered a
+// command to be installed (not just a library).
+// Packages named "main" are treated as commands.
+func (p *Package) IsCommand() bool {
+	return p.Name == "main"
+}
+
+// ImportDir is like Import but processes the Go package found in
+// the named directory.
+func (ctxt *Context) ImportDir(dir string, mode ImportMode) (*Package, error) {
+	return ctxt.Import(".", dir, mode)
+}
+
+// NoGoError is the error used by Import to describe a directory
+// containing no buildable Go source files. (It may still contain
+// test files, files hidden by build tags, and so on.)
+type NoGoError struct {
+	Dir string
+}
+
+func (e *NoGoError) Error() string {
+	return "no buildable Go source files in " + e.Dir
+}
+
+// MultiplePackageError describes a directory containing
+// multiple buildable Go source files for multiple packages.
+type MultiplePackageError struct {
+	Dir      string   // directory containing files
+	Packages []string // package names found
+	Files    []string // corresponding files: Files[i] declares package Packages[i]
+}
+
+func (e *MultiplePackageError) Error() string {
+	// Error string limited to two entries for compatibility.
+	return fmt.Sprintf("found packages %s (%s) and %s (%s) in %s", e.Packages[0], e.Files[0], e.Packages[1], e.Files[1], e.Dir)
+}
+
+func nameExt(name string) string {
+	i := strings.LastIndex(name, ".")
+	if i < 0 {
+		return ""
+	}
+	return name[i:]
+}
+
+// Import returns details about the Go package named by the import path,
+// interpreting local import paths relative to the srcDir directory.
+// If the path is a local import path naming a package that can be imported
+// using a standard import path, the returned package will set p.ImportPath
+// to that path.
+//
+// In the directory containing the package, .go, .c, .h, and .s files are
+// considered part of the package except for:
+//
+//	- .go files in package documentation
+//	- files starting with _ or . (likely editor temporary files)
+//	- files with build constraints not satisfied by the context
+//
+// If an error occurs, Import returns a non-nil error and a non-nil
+// *Package containing partial information.
+//
+func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Package, error) {
+	p := &Package{
+		ImportPath: path,
+	}
+	if path == "" {
+		return p, fmt.Errorf("import %q: invalid import path", path)
+	}
+
+	var pkgtargetroot string
+	var pkga string
+	var pkgerr error
+	suffix := ""
+	if ctxt.InstallSuffix != "" {
+		suffix = "_" + ctxt.InstallSuffix
+	}
+	switch ctxt.Compiler {
+	case "gccgo":
+		pkgtargetroot = "pkg/gccgo_" + ctxt.GOOS + "_" + ctxt.GOARCH + suffix
+	case "gc":
+		pkgtargetroot = "pkg/" + ctxt.GOOS + "_" + ctxt.GOARCH + suffix
+	default:
+		// Save error for end of function.
+		pkgerr = fmt.Errorf("import %q: unknown compiler %q", path, ctxt.Compiler)
+	}
+	setPkga := func() {
+		switch ctxt.Compiler {
+		case "gccgo":
+			dir, elem := pathpkg.Split(p.ImportPath)
+			pkga = pkgtargetroot + "/" + dir + "lib" + elem + ".a"
+		case "gc":
+			pkga = pkgtargetroot + "/" + p.ImportPath + ".a"
+		}
+	}
+	setPkga()
+
+	binaryOnly := false
+	if IsLocalImport(path) {
+		pkga = "" // local imports have no installed path
+		if srcDir == "" {
+			return p, fmt.Errorf("import %q: import relative to unknown directory", path)
+		}
+		if !ctxt.isAbsPath(path) {
+			p.Dir = ctxt.joinPath(srcDir, path)
+		}
+		// p.Dir directory may or may not exist. Gather partial information first, check if it exists later.
+		// Determine canonical import path, if any.
+		// Exclude results where the import path would include /testdata/.
+		inTestdata := func(sub string) bool {
+			return strings.Contains(sub, "/testdata/") || strings.HasSuffix(sub, "/testdata") || strings.HasPrefix(sub, "testdata/") || sub == "testdata"
+		}
+		if ctxt.GOROOT != "" {
+			root := ctxt.joinPath(ctxt.GOROOT, "src")
+			if sub, ok := ctxt.hasSubdir(root, p.Dir); ok && !inTestdata(sub) {
+				p.Goroot = true
+				p.ImportPath = sub
+				p.Root = ctxt.GOROOT
+				setPkga() // p.ImportPath changed
+				goto Found
+			}
+		}
+		all := ctxt.gopath()
+		for i, root := range all {
+			rootsrc := ctxt.joinPath(root, "src")
+			if sub, ok := ctxt.hasSubdir(rootsrc, p.Dir); ok && !inTestdata(sub) {
+				// We found a potential import path for dir,
+				// but check that using it wouldn't find something
+				// else first.
+				if ctxt.GOROOT != "" && ctxt.Compiler != "gccgo" {
+					if dir := ctxt.joinPath(ctxt.GOROOT, "src", sub); ctxt.isDir(dir) {
+						p.ConflictDir = dir
+						goto Found
+					}
+				}
+				for _, earlyRoot := range all[:i] {
+					if dir := ctxt.joinPath(earlyRoot, "src", sub); ctxt.isDir(dir) {
+						p.ConflictDir = dir
+						goto Found
+					}
+				}
+
+				// sub would not name some other directory instead of this one.
+				// Record it.
+				p.ImportPath = sub
+				p.Root = root
+				setPkga() // p.ImportPath changed
+				goto Found
+			}
+		}
+		// It's okay that we didn't find a root containing dir.
+		// Keep going with the information we have.
+	} else {
+		if strings.HasPrefix(path, "/") {
+			return p, fmt.Errorf("import %q: cannot import absolute path", path)
+		}
+
+		if err := ctxt.importGo(p, path, srcDir, mode); err == nil {
+			goto Found
+		} else if err != errNoModules {
+			return p, err
+		}
+
+		gopath := ctxt.gopath() // needed twice below; avoid computing many times
+
+		// tried records the location of unsuccessful package lookups
+		var tried struct {
+			vendor []string
+			goroot string
+			gopath []string
+		}
+
+		// Vendor directories get first chance to satisfy import.
+		if mode&IgnoreVendor == 0 && srcDir != "" {
+			searchVendor := func(root string, isGoroot bool) bool {
+				sub, ok := ctxt.hasSubdir(root, srcDir)
+				if !ok || !strings.HasPrefix(sub, "src/") || strings.Contains(sub, "/testdata/") {
+					return false
+				}
+				for {
+					vendor := ctxt.joinPath(root, sub, "vendor")
+					if ctxt.isDir(vendor) {
+						dir := ctxt.joinPath(vendor, path)
+						if ctxt.isDir(dir) && hasGoFiles(ctxt, dir) {
+							p.Dir = dir
+							p.ImportPath = strings.TrimPrefix(pathpkg.Join(sub, "vendor", path), "src/")
+							p.Goroot = isGoroot
+							p.Root = root
+							setPkga() // p.ImportPath changed
+							return true
+						}
+						tried.vendor = append(tried.vendor, dir)
+					}
+					i := strings.LastIndex(sub, "/")
+					if i < 0 {
+						break
+					}
+					sub = sub[:i]
+				}
+				return false
+			}
+			if ctxt.Compiler != "gccgo" && searchVendor(ctxt.GOROOT, true) {
+				goto Found
+			}
+			for _, root := range gopath {
+				if searchVendor(root, false) {
+					goto Found
+				}
+			}
+		}
+
+		// Determine directory from import path.
+		if ctxt.GOROOT != "" {
+			// If the package path starts with "vendor/", only search GOROOT before
+			// GOPATH if the importer is also within GOROOT. That way, if the user has
+			// vendored in a package that is subsequently included in the standard
+			// distribution, they'll continue to pick up their own vendored copy.
+			gorootFirst := srcDir == "" || !strings.HasPrefix(path, "vendor/")
+			if !gorootFirst {
+				_, gorootFirst = ctxt.hasSubdir(ctxt.GOROOT, srcDir)
+			}
+			if gorootFirst {
+				dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
+				if ctxt.Compiler != "gccgo" {
+					isDir := ctxt.isDir(dir)
+					binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(ctxt.GOROOT, pkga))
+					if isDir || binaryOnly {
+						p.Dir = dir
+						p.Goroot = true
+						p.Root = ctxt.GOROOT
+						goto Found
+					}
+				}
+				tried.goroot = dir
+			}
+		}
+		if ctxt.Compiler == "gccgo" && goroot.IsStandardPackage(ctxt.GOROOT, ctxt.Compiler, path) {
+			p.Dir = ctxt.joinPath(ctxt.GOROOT, "src", path)
+			p.Goroot = true
+			p.Root = ctxt.GOROOT
+			goto Found
+		}
+		for _, root := range gopath {
+			dir := ctxt.joinPath(root, "src", path)
+			isDir := ctxt.isDir(dir)
+			binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(root, pkga))
+			if isDir || binaryOnly {
+				p.Dir = dir
+				p.Root = root
+				goto Found
+			}
+			tried.gopath = append(tried.gopath, dir)
+		}
+
+		// If we tried GOPATH first due to a "vendor/" prefix, fall back to GOPATH.
+		// That way, the user can still get useful results from 'go list' for
+		// standard-vendored paths passed on the command line.
+		if ctxt.GOROOT != "" && tried.goroot == "" {
+			dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
+			if ctxt.Compiler != "gccgo" {
+				isDir := ctxt.isDir(dir)
+				binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(ctxt.GOROOT, pkga))
+				if isDir || binaryOnly {
+					p.Dir = dir
+					p.Goroot = true
+					p.Root = ctxt.GOROOT
+					goto Found
+				}
+			}
+			tried.goroot = dir
+		}
+
+		// package was not found
+		var paths []string
+		format := "\t%s (vendor tree)"
+		for _, dir := range tried.vendor {
+			paths = append(paths, fmt.Sprintf(format, dir))
+			format = "\t%s"
+		}
+		if tried.goroot != "" {
+			paths = append(paths, fmt.Sprintf("\t%s (from $GOROOT)", tried.goroot))
+		} else {
+			paths = append(paths, "\t($GOROOT not set)")
+		}
+		format = "\t%s (from $GOPATH)"
+		for _, dir := range tried.gopath {
+			paths = append(paths, fmt.Sprintf(format, dir))
+			format = "\t%s"
+		}
+		if len(tried.gopath) == 0 {
+			paths = append(paths, "\t($GOPATH not set. For more details see: 'go help gopath')")
+		}
+		return p, fmt.Errorf("cannot find package %q in any of:\n%s", path, strings.Join(paths, "\n"))
+	}
+
+Found:
+	if p.Root != "" {
+		p.SrcRoot = ctxt.joinPath(p.Root, "src")
+		p.PkgRoot = ctxt.joinPath(p.Root, "pkg")
+		p.BinDir = ctxt.joinPath(p.Root, "bin")
+		if pkga != "" {
+			p.PkgTargetRoot = ctxt.joinPath(p.Root, pkgtargetroot)
+			p.PkgObj = ctxt.joinPath(p.Root, pkga)
+		}
+	}
+
+	// If it's a local import path, by the time we get here, we still haven't checked
+	// that p.Dir directory exists. This is the right time to do that check.
+	// We can't do it earlier, because we want to gather partial information for the
+	// non-nil *Package returned when an error occurs.
+	// We need to do this before we return early on FindOnly flag.
+	if IsLocalImport(path) && !ctxt.isDir(p.Dir) {
+		if ctxt.Compiler == "gccgo" && p.Goroot {
+			// gccgo has no sources for GOROOT packages.
+			return p, nil
+		}
+
+		// package was not found
+		return p, fmt.Errorf("cannot find package %q in:\n\t%s", path, p.Dir)
+	}
+
+	if mode&FindOnly != 0 {
+		return p, pkgerr
+	}
+	if binaryOnly && (mode&AllowBinary) != 0 {
+		return p, pkgerr
+	}
+
+	if ctxt.Compiler == "gccgo" && p.Goroot {
+		// gccgo has no sources for GOROOT packages.
+		return p, nil
+	}
+
+	dirs, err := ctxt.readDir(p.Dir)
+	if err != nil {
+		return p, err
+	}
+
+	var badGoError error
+	var Sfiles []string // files with ".S"(capital S)/.sx(capital s equivalent for case insensitive filesystems)
+	var firstFile, firstCommentFile string
+	embedPos := make(map[string][]token.Position)
+	testEmbedPos := make(map[string][]token.Position)
+	xTestEmbedPos := make(map[string][]token.Position)
+	importPos := make(map[string][]token.Position)
+	testImportPos := make(map[string][]token.Position)
+	xTestImportPos := make(map[string][]token.Position)
+	allTags := make(map[string]bool)
+	fset := token.NewFileSet()
+	for _, d := range dirs {
+		if d.IsDir() {
+			continue
+		}
+		if d.Mode()&fs.ModeSymlink != 0 {
+			if ctxt.isDir(ctxt.joinPath(p.Dir, d.Name())) {
+				// Symlinks to directories are not source files.
+				continue
+			}
+		}
+
+		name := d.Name()
+		ext := nameExt(name)
+
+		badFile := func(err error) {
+			if badGoError == nil {
+				badGoError = err
+			}
+			p.InvalidGoFiles = append(p.InvalidGoFiles, name)
+		}
+
+		info, err := ctxt.matchFile(p.Dir, name, allTags, &p.BinaryOnly, fset)
+		if err != nil {
+			badFile(err)
+			continue
+		}
+		if info == nil {
+			if strings.HasPrefix(name, "_") || strings.HasPrefix(name, ".") {
+				// not due to build constraints - don't report
+			} else if ext == ".go" {
+				p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
+			} else if fileListForExt(p, ext) != nil {
+				p.IgnoredOtherFiles = append(p.IgnoredOtherFiles, name)
+			}
+			continue
+		}
+		data, filename := info.header, info.name
+
+		// Going to save the file. For non-Go files, can stop here.
+		switch ext {
+		case ".go":
+			// keep going
+		case ".S", ".sx":
+			// special case for cgo, handled at end
+			Sfiles = append(Sfiles, name)
+			continue
+		default:
+			if list := fileListForExt(p, ext); list != nil {
+				*list = append(*list, name)
+			}
+			continue
+		}
+
+		if info.parseErr != nil {
+			badFile(info.parseErr)
+			continue
+		}
+		pf := info.parsed
+
+		pkg := pf.Name.Name
+		if pkg == "documentation" {
+			p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
+			continue
+		}
+
+		isTest := strings.HasSuffix(name, "_test.go")
+		isXTest := false
+		if isTest && strings.HasSuffix(pkg, "_test") {
+			isXTest = true
+			pkg = pkg[:len(pkg)-len("_test")]
+		}
+
+		if p.Name == "" {
+			p.Name = pkg
+			firstFile = name
+		} else if pkg != p.Name {
+			badFile(&MultiplePackageError{
+				Dir:      p.Dir,
+				Packages: []string{p.Name, pkg},
+				Files:    []string{firstFile, name},
+			})
+			p.InvalidGoFiles = append(p.InvalidGoFiles, name)
+		}
+		// Grab the first package comment as docs, provided it is not from a test file.
+		if pf.Doc != nil && p.Doc == "" && !isTest && !isXTest {
+			p.Doc = doc.Synopsis(pf.Doc.Text())
+		}
+
+		if mode&ImportComment != 0 {
+			qcom, line := findImportComment(data)
+			if line != 0 {
+				com, err := strconv.Unquote(qcom)
+				if err != nil {
+					badFile(fmt.Errorf("%s:%d: cannot parse import comment", filename, line))
+				} else if p.ImportComment == "" {
+					p.ImportComment = com
+					firstCommentFile = name
+				} else if p.ImportComment != com {
+					badFile(fmt.Errorf("found import comments %q (%s) and %q (%s) in %s", p.ImportComment, firstCommentFile, com, name, p.Dir))
+				}
+			}
+		}
+
+		// Record imports and information about cgo.
+		isCgo := false
+		for _, imp := range info.imports {
+			if imp.path == "C" {
+				if isTest {
+					badFile(fmt.Errorf("use of cgo in test %s not supported", filename))
+					continue
+				}
+				isCgo = true
+				if imp.doc != nil {
+					if err := ctxt.saveCgo(filename, p, imp.doc); err != nil {
+						badFile(err)
+					}
+				}
+			}
+		}
+
+		var fileList *[]string
+		var importMap, embedMap map[string][]token.Position
+		switch {
+		case isCgo:
+			allTags["cgo"] = true
+			if ctxt.CgoEnabled {
+				fileList = &p.CgoFiles
+				importMap = importPos
+				embedMap = embedPos
+			} else {
+				// Ignore imports and embeds from cgo files if cgo is disabled.
+				fileList = &p.IgnoredGoFiles
+			}
+		case isXTest:
+			fileList = &p.XTestGoFiles
+			importMap = xTestImportPos
+			embedMap = xTestEmbedPos
+		case isTest:
+			fileList = &p.TestGoFiles
+			importMap = testImportPos
+			embedMap = testEmbedPos
+		default:
+			fileList = &p.GoFiles
+			importMap = importPos
+			embedMap = embedPos
+		}
+		*fileList = append(*fileList, name)
+		if importMap != nil {
+			for _, imp := range info.imports {
+				importMap[imp.path] = append(importMap[imp.path], fset.Position(imp.pos))
+			}
+		}
+		if embedMap != nil {
+			for _, emb := range info.embeds {
+				embedMap[emb.pattern] = append(embedMap[emb.pattern], emb.pos)
+			}
+		}
+	}
+
+	for tag := range allTags {
+		p.AllTags = append(p.AllTags, tag)
+	}
+	sort.Strings(p.AllTags)
+
+	p.EmbedPatterns, p.EmbedPatternPos = cleanDecls(embedPos)
+	p.TestEmbedPatterns, p.TestEmbedPatternPos = cleanDecls(testEmbedPos)
+	p.XTestEmbedPatterns, p.XTestEmbedPatternPos = cleanDecls(xTestEmbedPos)
+
+	p.Imports, p.ImportPos = cleanDecls(importPos)
+	p.TestImports, p.TestImportPos = cleanDecls(testImportPos)
+	p.XTestImports, p.XTestImportPos = cleanDecls(xTestImportPos)
+
+	// add the .S/.sx files only if we are using cgo
+	// (which means gcc will compile them).
+	// The standard assemblers expect .s files.
+	if len(p.CgoFiles) > 0 {
+		p.SFiles = append(p.SFiles, Sfiles...)
+		sort.Strings(p.SFiles)
+	} else {
+		p.IgnoredOtherFiles = append(p.IgnoredOtherFiles, Sfiles...)
+		sort.Strings(p.IgnoredOtherFiles)
+	}
+
+	if badGoError != nil {
+		return p, badGoError
+	}
+	if len(p.GoFiles)+len(p.CgoFiles)+len(p.TestGoFiles)+len(p.XTestGoFiles) == 0 {
+		return p, &NoGoError{p.Dir}
+	}
+	return p, pkgerr
+}
+
+func fileListForExt(p *Package, ext string) *[]string {
+	switch ext {
+	case ".c":
+		return &p.CFiles
+	case ".cc", ".cpp", ".cxx":
+		return &p.CXXFiles
+	case ".m":
+		return &p.MFiles
+	case ".h", ".hh", ".hpp", ".hxx":
+		return &p.HFiles
+	case ".f", ".F", ".for", ".f90":
+		return &p.FFiles
+	case ".s", ".S", ".sx":
+		return &p.SFiles
+	case ".swig":
+		return &p.SwigFiles
+	case ".swigcxx":
+		return &p.SwigCXXFiles
+	case ".syso":
+		return &p.SysoFiles
+	}
+	return nil
+}
+
+func uniq(list []string) []string {
+	if list == nil {
+		return nil
+	}
+	out := make([]string, len(list))
+	copy(out, list)
+	sort.Strings(out)
+	uniq := out[:0]
+	for _, x := range out {
+		if len(uniq) == 0 || uniq[len(uniq)-1] != x {
+			uniq = append(uniq, x)
+		}
+	}
+	return uniq
+}
+
+var errNoModules = errors.New("not using modules")
+
+// importGo checks whether it can use the go command to find the directory for path.
+// If using the go command is not appropriate, importGo returns errNoModules.
+// Otherwise, importGo tries using the go command and reports whether that succeeded.
+// Using the go command lets build.Import and build.Context.Import find code
+// in Go modules. In the long term we want tools to use go/packages (currently golang.org/x/tools/go/packages),
+// which will also use the go command.
+// Invoking the go command here is not very efficient in that it computes information
+// about the requested package and all dependencies and then only reports about the requested package.
+// Then we reinvoke it for every dependency. But this is still better than not working at all.
+// See golang.org/issue/26504.
+func (ctxt *Context) importGo(p *Package, path, srcDir string, mode ImportMode) error {
+	// To invoke the go command,
+	// we must not being doing special things like AllowBinary or IgnoreVendor,
+	// and all the file system callbacks must be nil (we're meant to use the local file system).
+	if mode&AllowBinary != 0 || mode&IgnoreVendor != 0 ||
+		ctxt.JoinPath != nil || ctxt.SplitPathList != nil || ctxt.IsAbsPath != nil || ctxt.IsDir != nil || ctxt.HasSubdir != nil || ctxt.ReadDir != nil || ctxt.OpenFile != nil || !equal(ctxt.ReleaseTags, defaultReleaseTags) {
+		return errNoModules
+	}
+
+	// Predict whether module aware mode is enabled by checking the value of
+	// GO111MODULE and looking for a go.mod file in the source directory or
+	// one of its parents. Running 'go env GOMOD' in the source directory would
+	// give a canonical answer, but we'd prefer not to execute another command.
+	go111Module := os.Getenv("GO111MODULE")
+	switch go111Module {
+	case "off":
+		return errNoModules
+	default: // "", "on", "auto", anything else
+		// Maybe use modules.
+	}
+
+	if srcDir != "" {
+		var absSrcDir string
+		if filepath.IsAbs(srcDir) {
+			absSrcDir = srcDir
+		} else if ctxt.Dir != "" {
+			return fmt.Errorf("go/build: Dir is non-empty, so relative srcDir is not allowed: %v", srcDir)
+		} else {
+			// Find the absolute source directory. hasSubdir does not handle
+			// relative paths (and can't because the callbacks don't support this).
+			var err error
+			absSrcDir, err = filepath.Abs(srcDir)
+			if err != nil {
+				return errNoModules
+			}
+		}
+
+		// If the source directory is in GOROOT, then the in-process code works fine
+		// and we should keep using it. Moreover, the 'go list' approach below doesn't
+		// take standard-library vendoring into account and will fail.
+		if _, ok := ctxt.hasSubdir(filepath.Join(ctxt.GOROOT, "src"), absSrcDir); ok {
+			return errNoModules
+		}
+	}
+
+	// For efficiency, if path is a standard library package, let the usual lookup code handle it.
+	if ctxt.GOROOT != "" {
+		dir := ctxt.joinPath(ctxt.GOROOT, "src", path)
+		if ctxt.isDir(dir) {
+			return errNoModules
+		}
+	}
+
+	// If GO111MODULE=auto, look to see if there is a go.mod.
+	// Since go1.13, it doesn't matter if we're inside GOPATH.
+	if go111Module == "auto" {
+		var (
+			parent string
+			err    error
+		)
+		if ctxt.Dir == "" {
+			parent, err = os.Getwd()
+			if err != nil {
+				// A nonexistent working directory can't be in a module.
+				return errNoModules
+			}
+		} else {
+			parent, err = filepath.Abs(ctxt.Dir)
+			if err != nil {
+				// If the caller passed a bogus Dir explicitly, that's materially
+				// different from not having modules enabled.
+				return err
+			}
+		}
+		for {
+			if f, err := ctxt.openFile(ctxt.joinPath(parent, "go.mod")); err == nil {
+				buf := make([]byte, 100)
+				_, err := f.Read(buf)
+				f.Close()
+				if err == nil || err == io.EOF {
+					// go.mod exists and is readable (is a file, not a directory).
+					break
+				}
+			}
+			d := filepath.Dir(parent)
+			if len(d) >= len(parent) {
+				return errNoModules // reached top of file system, no go.mod
+			}
+			parent = d
+		}
+	}
+
+	cmd := exec.Command("go", "list", "-e", "-compiler="+ctxt.Compiler, "-tags="+strings.Join(ctxt.BuildTags, ","), "-installsuffix="+ctxt.InstallSuffix, "-f={{.Dir}}\n{{.ImportPath}}\n{{.Root}}\n{{.Goroot}}\n{{if .Error}}{{.Error}}{{end}}\n", "--", path)
+
+	if ctxt.Dir != "" {
+		cmd.Dir = ctxt.Dir
+	}
+
+	var stdout, stderr strings.Builder
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+
+	cgo := "0"
+	if ctxt.CgoEnabled {
+		cgo = "1"
+	}
+	cmd.Env = append(os.Environ(),
+		"GOOS="+ctxt.GOOS,
+		"GOARCH="+ctxt.GOARCH,
+		"GOROOT="+ctxt.GOROOT,
+		"GOPATH="+ctxt.GOPATH,
+		"CGO_ENABLED="+cgo,
+	)
+
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("go/build: go list %s: %v\n%s\n", path, err, stderr.String())
+	}
+
+	f := strings.SplitN(stdout.String(), "\n", 5)
+	if len(f) != 5 {
+		return fmt.Errorf("go/build: importGo %s: unexpected output:\n%s\n", path, stdout.String())
+	}
+	dir := f[0]
+	errStr := strings.TrimSpace(f[4])
+	if errStr != "" && dir == "" {
+		// If 'go list' could not locate the package (dir is empty),
+		// return the same error that 'go list' reported.
+		return errors.New(errStr)
+	}
+
+	// If 'go list' did locate the package, ignore the error.
+	// It was probably related to loading source files, and we'll
+	// encounter it ourselves shortly if the FindOnly flag isn't set.
+	p.Dir = dir
+	p.ImportPath = f[1]
+	p.Root = f[2]
+	p.Goroot = f[3] == "true"
+	return nil
+}
+
+func equal(x, y []string) bool {
+	if len(x) != len(y) {
+		return false
+	}
+	for i, xi := range x {
+		if xi != y[i] {
+			return false
+		}
+	}
+	return true
+}
+
+// hasGoFiles reports whether dir contains any files with names ending in .go.
+// For a vendor check we must exclude directories that contain no .go files.
+// Otherwise it is not possible to vendor just a/b/c and still import the
+// non-vendored a/b. See golang.org/issue/13832.
+func hasGoFiles(ctxt *Context, dir string) bool {
+	ents, _ := ctxt.readDir(dir)
+	for _, ent := range ents {
+		if !ent.IsDir() && strings.HasSuffix(ent.Name(), ".go") {
+			return true
+		}
+	}
+	return false
+}
+
+func findImportComment(data []byte) (s string, line int) {
+	// expect keyword package
+	word, data := parseWord(data)
+	if string(word) != "package" {
+		return "", 0
+	}
+
+	// expect package name
+	_, data = parseWord(data)
+
+	// now ready for import comment, a // or /* */ comment
+	// beginning and ending on the current line.
+	for len(data) > 0 && (data[0] == ' ' || data[0] == '\t' || data[0] == '\r') {
+		data = data[1:]
+	}
+
+	var comment []byte
+	switch {
+	case bytes.HasPrefix(data, slashSlash):
+		i := bytes.Index(data, newline)
+		if i < 0 {
+			i = len(data)
+		}
+		comment = data[2:i]
+	case bytes.HasPrefix(data, slashStar):
+		data = data[2:]
+		i := bytes.Index(data, starSlash)
+		if i < 0 {
+			// malformed comment
+			return "", 0
+		}
+		comment = data[:i]
+		if bytes.Contains(comment, newline) {
+			return "", 0
+		}
+	}
+	comment = bytes.TrimSpace(comment)
+
+	// split comment into `import`, `"pkg"`
+	word, arg := parseWord(comment)
+	if string(word) != "import" {
+		return "", 0
+	}
+
+	line = 1 + bytes.Count(data[:cap(data)-cap(arg)], newline)
+	return strings.TrimSpace(string(arg)), line
+}
+
+var (
+	slashSlash = []byte("//")
+	slashStar  = []byte("/*")
+	starSlash  = []byte("*/")
+	newline    = []byte("\n")
+)
+
+// skipSpaceOrComment returns data with any leading spaces or comments removed.
+func skipSpaceOrComment(data []byte) []byte {
+	for len(data) > 0 {
+		switch data[0] {
+		case ' ', '\t', '\r', '\n':
+			data = data[1:]
+			continue
+		case '/':
+			if bytes.HasPrefix(data, slashSlash) {
+				i := bytes.Index(data, newline)
+				if i < 0 {
+					return nil
+				}
+				data = data[i+1:]
+				continue
+			}
+			if bytes.HasPrefix(data, slashStar) {
+				data = data[2:]
+				i := bytes.Index(data, starSlash)
+				if i < 0 {
+					return nil
+				}
+				data = data[i+2:]
+				continue
+			}
+		}
+		break
+	}
+	return data
+}
+
+// parseWord skips any leading spaces or comments in data
+// and then parses the beginning of data as an identifier or keyword,
+// returning that word and what remains after the word.
+func parseWord(data []byte) (word, rest []byte) {
+	data = skipSpaceOrComment(data)
+
+	// Parse past leading word characters.
+	rest = data
+	for {
+		r, size := utf8.DecodeRune(rest)
+		if unicode.IsLetter(r) || '0' <= r && r <= '9' || r == '_' {
+			rest = rest[size:]
+			continue
+		}
+		break
+	}
+
+	word = data[:len(data)-len(rest)]
+	if len(word) == 0 {
+		return nil, nil
+	}
+
+	return word, rest
+}
+
+// MatchFile reports whether the file with the given name in the given directory
+// matches the context and would be included in a Package created by ImportDir
+// of that directory.
+//
+// MatchFile considers the name of the file and may use ctxt.OpenFile to
+// read some or all of the file's content.
+func (ctxt *Context) MatchFile(dir, name string) (match bool, err error) {
+	info, err := ctxt.matchFile(dir, name, nil, nil, nil)
+	return info != nil, err
+}
+
+var dummyPkg Package
+
+// fileInfo records information learned about a file included in a build.
+type fileInfo struct {
+	name     string // full name including dir
+	header   []byte
+	fset     *token.FileSet
+	parsed   *ast.File
+	parseErr error
+	imports  []fileImport
+	embeds   []fileEmbed
+	embedErr error
+}
+
+type fileImport struct {
+	path string
+	pos  token.Pos
+	doc  *ast.CommentGroup
+}
+
+type fileEmbed struct {
+	pattern string
+	pos     token.Position
+}
+
+// matchFile determines whether the file with the given name in the given directory
+// should be included in the package being constructed.
+// If the file should be included, matchFile returns a non-nil *fileInfo (and a nil error).
+// Non-nil errors are reserved for unexpected problems.
+//
+// If name denotes a Go program, matchFile reads until the end of the
+// imports and returns that section of the file in the fileInfo's header field,
+// even though it only considers text until the first non-comment
+// for +build lines.
+//
+// If allTags is non-nil, matchFile records any encountered build tag
+// by setting allTags[tag] = true.
+func (ctxt *Context) matchFile(dir, name string, allTags map[string]bool, binaryOnly *bool, fset *token.FileSet) (*fileInfo, error) {
+	if strings.HasPrefix(name, "_") ||
+		strings.HasPrefix(name, ".") {
+		return nil, nil
+	}
+
+	i := strings.LastIndex(name, ".")
+	if i < 0 {
+		i = len(name)
+	}
+	ext := name[i:]
+
+	if !ctxt.goodOSArchFile(name, allTags) && !ctxt.UseAllFiles {
+		return nil, nil
+	}
+
+	if ext != ".go" && fileListForExt(&dummyPkg, ext) == nil {
+		// skip
+		return nil, nil
+	}
+
+	info := &fileInfo{name: ctxt.joinPath(dir, name), fset: fset}
+	if ext == ".syso" {
+		// binary, no reading
+		return info, nil
+	}
+
+	f, err := ctxt.openFile(info.name)
+	if err != nil {
+		return nil, err
+	}
+
+	if strings.HasSuffix(name, ".go") {
+		err = readGoInfo(f, info)
+		if strings.HasSuffix(name, "_test.go") {
+			binaryOnly = nil // ignore //go:binary-only-package comments in _test.go files
+		}
+	} else {
+		binaryOnly = nil // ignore //go:binary-only-package comments in non-Go sources
+		info.header, err = readComments(f)
+	}
+	f.Close()
+	if err != nil {
+		return nil, fmt.Errorf("read %s: %v", info.name, err)
+	}
+
+	// Look for +build comments to accept or reject the file.
+	ok, sawBinaryOnly, err := ctxt.shouldBuild(info.header, allTags)
+	if err != nil {
+		return nil, err
+	}
+	if !ok && !ctxt.UseAllFiles {
+		return nil, nil
+	}
+
+	if binaryOnly != nil && sawBinaryOnly {
+		*binaryOnly = true
+	}
+
+	return info, nil
+}
+
+func cleanDecls(m map[string][]token.Position) ([]string, map[string][]token.Position) {
+	all := make([]string, 0, len(m))
+	for path := range m {
+		all = append(all, path)
+	}
+	sort.Strings(all)
+	return all, m
+}
+
+// Import is shorthand for Default.Import.
+func Import(path, srcDir string, mode ImportMode) (*Package, error) {
+	return Default.Import(path, srcDir, mode)
+}
+
+// ImportDir is shorthand for Default.ImportDir.
+func ImportDir(dir string, mode ImportMode) (*Package, error) {
+	return Default.ImportDir(dir, mode)
+}
+
+var (
+	bSlashSlash = []byte(slashSlash)
+	bStarSlash  = []byte(starSlash)
+	bSlashStar  = []byte(slashStar)
+
+	goBuildComment = []byte("//go:build")
+
+	errGoBuildWithoutBuild = errors.New("//go:build comment without // +build comment")
+	errMultipleGoBuild     = errors.New("multiple //go:build comments") // unused in Go 1.(N-1)
+)
+
+func isGoBuildComment(line []byte) bool {
+	if !bytes.HasPrefix(line, goBuildComment) {
+		return false
+	}
+	line = bytes.TrimSpace(line)
+	rest := line[len(goBuildComment):]
+	return len(rest) == 0 || len(bytes.TrimSpace(rest)) < len(rest)
+}
+
+// Special comment denoting a binary-only package.
+// See https://golang.org/design/2775-binary-only-packages
+// for more about the design of binary-only packages.
+var binaryOnlyComment = []byte("//go:binary-only-package")
+
+// shouldBuild reports whether it is okay to use this file,
+// The rule is that in the file's leading run of // comments
+// and blank lines, which must be followed by a blank line
+// (to avoid including a Go package clause doc comment),
+// lines beginning with '// +build' are taken as build directives.
+//
+// The file is accepted only if each such line lists something
+// matching the file. For example:
+//
+//	// +build windows linux
+//
+// marks the file as applicable only on Windows and Linux.
+//
+// For each build tag it consults, shouldBuild sets allTags[tag] = true.
+//
+// shouldBuild reports whether the file should be built
+// and whether a //go:binary-only-package comment was found.
+func (ctxt *Context) shouldBuild(content []byte, allTags map[string]bool) (shouldBuild, binaryOnly bool, err error) {
+
+	// Pass 1. Identify leading run of // comments and blank lines,
+	// which must be followed by a blank line.
+	// Also identify any //go:build comments.
+	content, goBuild, sawBinaryOnly, err := parseFileHeader(content)
+	if err != nil {
+		return false, false, err
+	}
+
+	// Pass 2.  Process each +build line in the run.
+	p := content
+	shouldBuild = true
+	sawBuild := false
+	for len(p) > 0 {
+		line := p
+		if i := bytes.IndexByte(line, '\n'); i >= 0 {
+			line, p = line[:i], p[i+1:]
+		} else {
+			p = p[len(p):]
+		}
+		line = bytes.TrimSpace(line)
+		if !bytes.HasPrefix(line, bSlashSlash) {
+			continue
+		}
+		line = bytes.TrimSpace(line[len(bSlashSlash):])
+		if len(line) > 0 && line[0] == '+' {
+			// Looks like a comment +line.
+			f := strings.Fields(string(line))
+			if f[0] == "+build" {
+				sawBuild = true
+				ok := false
+				for _, tok := range f[1:] {
+					if ctxt.match(tok, allTags) {
+						ok = true
+					}
+				}
+				if !ok {
+					shouldBuild = false
+				}
+			}
+		}
+	}
+
+	if goBuild != nil && !sawBuild {
+		return false, false, errGoBuildWithoutBuild
+	}
+
+	return shouldBuild, sawBinaryOnly, nil
+}
+
+func parseFileHeader(content []byte) (trimmed, goBuild []byte, sawBinaryOnly bool, err error) {
+	end := 0
+	p := content
+	ended := false       // found non-blank, non-// line, so stopped accepting // +build lines
+	inSlashStar := false // in /* */ comment
+
+Lines:
+	for len(p) > 0 {
+		line := p
+		if i := bytes.IndexByte(line, '\n'); i >= 0 {
+			line, p = line[:i], p[i+1:]
+		} else {
+			p = p[len(p):]
+		}
+		line = bytes.TrimSpace(line)
+		if len(line) == 0 && !ended { // Blank line
+			// Remember position of most recent blank line.
+			// When we find the first non-blank, non-// line,
+			// this "end" position marks the latest file position
+			// where a // +build line can appear.
+			// (It must appear _before_ a blank line before the non-blank, non-// line.
+			// Yes, that's confusing, which is part of why we moved to //go:build lines.)
+			// Note that ended==false here means that inSlashStar==false,
+			// since seeing a /* would have set ended==true.
+			end = len(content) - len(p)
+			continue Lines
+		}
+		if !bytes.HasPrefix(line, slashSlash) { // Not comment line
+			ended = true
+		}
+
+		if !inSlashStar && isGoBuildComment(line) {
+			if false && goBuild != nil { // enabled in Go 1.N
+				return nil, nil, false, errMultipleGoBuild
+			}
+			goBuild = line
+		}
+		if !inSlashStar && bytes.Equal(line, binaryOnlyComment) {
+			sawBinaryOnly = true
+		}
+
+	Comments:
+		for len(line) > 0 {
+			if inSlashStar {
+				if i := bytes.Index(line, starSlash); i >= 0 {
+					inSlashStar = false
+					line = bytes.TrimSpace(line[i+len(starSlash):])
+					continue Comments
+				}
+				continue Lines
+			}
+			if bytes.HasPrefix(line, bSlashSlash) {
+				continue Lines
+			}
+			if bytes.HasPrefix(line, bSlashStar) {
+				inSlashStar = true
+				line = bytes.TrimSpace(line[len(bSlashStar):])
+				continue Comments
+			}
+			// Found non-comment text.
+			break Lines
+		}
+	}
+
+	return content[:end], goBuild, sawBinaryOnly, nil
+}
+
+// saveCgo saves the information from the #cgo lines in the import "C" comment.
+// These lines set CFLAGS, CPPFLAGS, CXXFLAGS and LDFLAGS and pkg-config directives
+// that affect the way cgo's C code is built.
+func (ctxt *Context) saveCgo(filename string, di *Package, cg *ast.CommentGroup) error {
+	text := cg.Text()
+	for _, line := range strings.Split(text, "\n") {
+		orig := line
+
+		// Line is
+		//	#cgo [GOOS/GOARCH...] LDFLAGS: stuff
+		//
+		line = strings.TrimSpace(line)
+		if len(line) < 5 || line[:4] != "#cgo" || (line[4] != ' ' && line[4] != '\t') {
+			continue
+		}
+
+		// Split at colon.
+		line = strings.TrimSpace(line[4:])
+		i := strings.Index(line, ":")
+		if i < 0 {
+			return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
+		}
+		line, argstr := line[:i], line[i+1:]
+
+		// Parse GOOS/GOARCH stuff.
+		f := strings.Fields(line)
+		if len(f) < 1 {
+			return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
+		}
+
+		cond, verb := f[:len(f)-1], f[len(f)-1]
+		if len(cond) > 0 {
+			ok := false
+			for _, c := range cond {
+				if ctxt.match(c, nil) {
+					ok = true
+					break
+				}
+			}
+			if !ok {
+				continue
+			}
+		}
+
+		args, err := splitQuoted(argstr)
+		if err != nil {
+			return fmt.Errorf("%s: invalid #cgo line: %s", filename, orig)
+		}
+		var ok bool
+		for i, arg := range args {
+			if arg, ok = expandSrcDir(arg, di.Dir); !ok {
+				return fmt.Errorf("%s: malformed #cgo argument: %s", filename, arg)
+			}
+			args[i] = arg
+		}
+
+		switch verb {
+		case "CFLAGS", "CPPFLAGS", "CXXFLAGS", "FFLAGS", "LDFLAGS":
+			// Change relative paths to absolute.
+			ctxt.makePathsAbsolute(args, di.Dir)
+		}
+
+		switch verb {
+		case "CFLAGS":
+			di.CgoCFLAGS = append(di.CgoCFLAGS, args...)
+		case "CPPFLAGS":
+			di.CgoCPPFLAGS = append(di.CgoCPPFLAGS, args...)
+		case "CXXFLAGS":
+			di.CgoCXXFLAGS = append(di.CgoCXXFLAGS, args...)
+		case "FFLAGS":
+			di.CgoFFLAGS = append(di.CgoFFLAGS, args...)
+		case "LDFLAGS":
+			di.CgoLDFLAGS = append(di.CgoLDFLAGS, args...)
+		case "pkg-config":
+			di.CgoPkgConfig = append(di.CgoPkgConfig, args...)
+		default:
+			return fmt.Errorf("%s: invalid #cgo verb: %s", filename, orig)
+		}
+	}
+	return nil
+}
+
+// expandSrcDir expands any occurrence of ${SRCDIR}, making sure
+// the result is safe for the shell.
+func expandSrcDir(str string, srcdir string) (string, bool) {
+	// "\" delimited paths cause safeCgoName to fail
+	// so convert native paths with a different delimiter
+	// to "/" before starting (eg: on windows).
+	srcdir = filepath.ToSlash(srcdir)
+
+	chunks := strings.Split(str, "${SRCDIR}")
+	if len(chunks) < 2 {
+		return str, safeCgoName(str)
+	}
+	ok := true
+	for _, chunk := range chunks {
+		ok = ok && (chunk == "" || safeCgoName(chunk))
+	}
+	ok = ok && (srcdir == "" || safeCgoName(srcdir))
+	res := strings.Join(chunks, srcdir)
+	return res, ok && res != ""
+}
+
+// makePathsAbsolute looks for compiler options that take paths and
+// makes them absolute. We do this because through the 1.8 release we
+// ran the compiler in the package directory, so any relative -I or -L
+// options would be relative to that directory. In 1.9 we changed to
+// running the compiler in the build directory, to get consistent
+// build results (issue #19964). To keep builds working, we change any
+// relative -I or -L options to be absolute.
+//
+// Using filepath.IsAbs and filepath.Join here means the results will be
+// different on different systems, but that's OK: -I and -L options are
+// inherently system-dependent.
+func (ctxt *Context) makePathsAbsolute(args []string, srcDir string) {
+	nextPath := false
+	for i, arg := range args {
+		if nextPath {
+			if !filepath.IsAbs(arg) {
+				args[i] = filepath.Join(srcDir, arg)
+			}
+			nextPath = false
+		} else if strings.HasPrefix(arg, "-I") || strings.HasPrefix(arg, "-L") {
+			if len(arg) == 2 {
+				nextPath = true
+			} else {
+				if !filepath.IsAbs(arg[2:]) {
+					args[i] = arg[:2] + filepath.Join(srcDir, arg[2:])
+				}
+			}
+		}
+	}
+}
+
+// NOTE: $ is not safe for the shell, but it is allowed here because of linker options like -Wl,$ORIGIN.
+// We never pass these arguments to a shell (just to programs we construct argv for), so this should be okay.
+// See golang.org/issue/6038.
+// The @ is for OS X. See golang.org/issue/13720.
+// The % is for Jenkins. See golang.org/issue/16959.
+// The ! is because module paths may use them. See golang.org/issue/26716.
+// The ~ and ^ are for sr.ht. See golang.org/issue/32260.
+const safeString = "+-.,/0123456789=ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz:$@%! ~^"
+
+func safeCgoName(s string) bool {
+	if s == "" {
+		return false
+	}
+	for i := 0; i < len(s); i++ {
+		if c := s[i]; c < utf8.RuneSelf && strings.IndexByte(safeString, c) < 0 {
+			return false
+		}
+	}
+	return true
+}
+
+// splitQuoted splits the string s around each instance of one or more consecutive
+// white space characters while taking into account quotes and escaping, and
+// returns an array of substrings of s or an empty list if s contains only white space.
+// Single quotes and double quotes are recognized to prevent splitting within the
+// quoted region, and are removed from the resulting substrings. If a quote in s
+// isn't closed err will be set and r will have the unclosed argument as the
+// last element. The backslash is used for escaping.
+//
+// For example, the following string:
+//
+//     a b:"c d" 'e''f'  "g\""
+//
+// Would be parsed as:
+//
+//     []string{"a", "b:c d", "ef", `g"`}
+//
+func splitQuoted(s string) (r []string, err error) {
+	var args []string
+	arg := make([]rune, len(s))
+	escaped := false
+	quoted := false
+	quote := '\x00'
+	i := 0
+	for _, rune := range s {
+		switch {
+		case escaped:
+			escaped = false
+		case rune == '\\':
+			escaped = true
+			continue
+		case quote != '\x00':
+			if rune == quote {
+				quote = '\x00'
+				continue
+			}
+		case rune == '"' || rune == '\'':
+			quoted = true
+			quote = rune
+			continue
+		case unicode.IsSpace(rune):
+			if quoted || i > 0 {
+				quoted = false
+				args = append(args, string(arg[:i]))
+				i = 0
+			}
+			continue
+		}
+		arg[i] = rune
+		i++
+	}
+	if quoted || i > 0 {
+		args = append(args, string(arg[:i]))
+	}
+	if quote != 0 {
+		err = errors.New("unclosed quote")
+	} else if escaped {
+		err = errors.New("unfinished escaping")
+	}
+	return args, err
+}
+
+// match reports whether the name is one of:
+//
+//	$GOOS
+//	$GOARCH
+//	cgo (if cgo is enabled)
+//	!cgo (if cgo is disabled)
+//	ctxt.Compiler
+//	!ctxt.Compiler
+//	tag (if tag is listed in ctxt.BuildTags or ctxt.ReleaseTags)
+//	!tag (if tag is not listed in ctxt.BuildTags or ctxt.ReleaseTags)
+//	a comma-separated list of any of these
+//
+func (ctxt *Context) match(name string, allTags map[string]bool) bool {
+	if name == "" {
+		if allTags != nil {
+			allTags[name] = true
+		}
+		return false
+	}
+	if i := strings.Index(name, ","); i >= 0 {
+		// comma-separated list
+		ok1 := ctxt.match(name[:i], allTags)
+		ok2 := ctxt.match(name[i+1:], allTags)
+		return ok1 && ok2
+	}
+	if strings.HasPrefix(name, "!!") { // bad syntax, reject always
+		return false
+	}
+	if strings.HasPrefix(name, "!") { // negation
+		return len(name) > 1 && !ctxt.match(name[1:], allTags)
+	}
+
+	if allTags != nil {
+		allTags[name] = true
+	}
+
+	// Tags must be letters, digits, underscores or dots.
+	// Unlike in Go identifiers, all digits are fine (e.g., "386").
+	for _, c := range name {
+		if !unicode.IsLetter(c) && !unicode.IsDigit(c) && c != '_' && c != '.' {
+			return false
+		}
+	}
+
+	// special tags
+	if ctxt.CgoEnabled && name == "cgo" {
+		return true
+	}
+	if name == ctxt.GOOS || name == ctxt.GOARCH || name == ctxt.Compiler {
+		return true
+	}
+	if ctxt.GOOS == "android" && name == "linux" {
+		return true
+	}
+	if ctxt.GOOS == "illumos" && name == "solaris" {
+		return true
+	}
+	if ctxt.GOOS == "ios" && name == "darwin" {
+		return true
+	}
+
+	// other tags
+	for _, tag := range ctxt.BuildTags {
+		if tag == name {
+			return true
+		}
+	}
+	for _, tag := range ctxt.ReleaseTags {
+		if tag == name {
+			return true
+		}
+	}
+
+	return false
+}
+
+// goodOSArchFile returns false if the name contains a $GOOS or $GOARCH
+// suffix which does not match the current system.
+// The recognized name formats are:
+//
+//     name_$(GOOS).*
+//     name_$(GOARCH).*
+//     name_$(GOOS)_$(GOARCH).*
+//     name_$(GOOS)_test.*
+//     name_$(GOARCH)_test.*
+//     name_$(GOOS)_$(GOARCH)_test.*
+//
+// Exceptions:
+// if GOOS=android, then files with GOOS=linux are also matched.
+// if GOOS=illumos, then files with GOOS=solaris are also matched.
+// if GOOS=ios, then files with GOOS=darwin are also matched.
+func (ctxt *Context) goodOSArchFile(name string, allTags map[string]bool) bool {
+	if dot := strings.Index(name, "."); dot != -1 {
+		name = name[:dot]
+	}
+
+	// Before Go 1.4, a file called "linux.go" would be equivalent to having a
+	// build tag "linux" in that file. For Go 1.4 and beyond, we require this
+	// auto-tagging to apply only to files with a non-empty prefix, so
+	// "foo_linux.go" is tagged but "linux.go" is not. This allows new operating
+	// systems, such as android, to arrive without breaking existing code with
+	// innocuous source code in "android.go". The easiest fix: cut everything
+	// in the name before the initial _.
+	i := strings.Index(name, "_")
+	if i < 0 {
+		return true
+	}
+	name = name[i:] // ignore everything before first _
+
+	l := strings.Split(name, "_")
+	if n := len(l); n > 0 && l[n-1] == "test" {
+		l = l[:n-1]
+	}
+	n := len(l)
+	if n >= 2 && knownOS[l[n-2]] && knownArch[l[n-1]] {
+		return ctxt.match(l[n-1], allTags) && ctxt.match(l[n-2], allTags)
+	}
+	if n >= 1 && (knownOS[l[n-1]] || knownArch[l[n-1]]) {
+		return ctxt.match(l[n-1], allTags)
+	}
+	return true
+}
+
+var knownOS = make(map[string]bool)
+var knownArch = make(map[string]bool)
+
+func init() {
+	for _, v := range strings.Fields(goosList) {
+		knownOS[v] = true
+	}
+	for _, v := range strings.Fields(goarchList) {
+		knownArch[v] = true
+	}
+}
+
+// ToolDir is the directory containing build tools.
+var ToolDir = getToolDir()
+
+// IsLocalImport reports whether the import path is
+// a local import path, like ".", "..", "./foo", or "../foo".
+func IsLocalImport(path string) bool {
+	return path == "." || path == ".." ||
+		strings.HasPrefix(path, "./") || strings.HasPrefix(path, "../")
+}
+
+// ArchChar returns "?" and an error.
+// In earlier versions of Go, the returned string was used to derive
+// the compiler and linker tool names, the default object file suffix,
+// and the default linker output name. As of Go 1.5, those strings
+// no longer vary by architecture; they are compile, link, .o, and a.out, respectively.
+func ArchChar(goarch string) (string, error) {
+	return "?", errors.New("architecture letter no longer used")
+}
diff --git a/src/go/build/build_test.go b/src/go/build/build_test.go
new file mode 100644
index 0000000..d13ea81
--- /dev/null
+++ b/src/go/build/build_test.go
@@ -0,0 +1,712 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+import (
+	"flag"
+	"internal/testenv"
+	"io"
+	"os"
+	"path/filepath"
+	"reflect"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	flag.Parse()
+	if goTool, err := testenv.GoTool(); err == nil {
+		os.Setenv("PATH", filepath.Dir(goTool)+string(os.PathListSeparator)+os.Getenv("PATH"))
+	}
+	os.Exit(m.Run())
+}
+
+func TestMatch(t *testing.T) {
+	ctxt := Default
+	what := "default"
+	match := func(tag string, want map[string]bool) {
+		t.Helper()
+		m := make(map[string]bool)
+		if !ctxt.match(tag, m) {
+			t.Errorf("%s context should match %s, does not", what, tag)
+		}
+		if !reflect.DeepEqual(m, want) {
+			t.Errorf("%s tags = %v, want %v", tag, m, want)
+		}
+	}
+	nomatch := func(tag string, want map[string]bool) {
+		t.Helper()
+		m := make(map[string]bool)
+		if ctxt.match(tag, m) {
+			t.Errorf("%s context should NOT match %s, does", what, tag)
+		}
+		if !reflect.DeepEqual(m, want) {
+			t.Errorf("%s tags = %v, want %v", tag, m, want)
+		}
+	}
+
+	match(runtime.GOOS+","+runtime.GOARCH, map[string]bool{runtime.GOOS: true, runtime.GOARCH: true})
+	match(runtime.GOOS+","+runtime.GOARCH+",!foo", map[string]bool{runtime.GOOS: true, runtime.GOARCH: true, "foo": true})
+	nomatch(runtime.GOOS+","+runtime.GOARCH+",foo", map[string]bool{runtime.GOOS: true, runtime.GOARCH: true, "foo": true})
+
+	what = "modified"
+	ctxt.BuildTags = []string{"foo"}
+	match(runtime.GOOS+","+runtime.GOARCH, map[string]bool{runtime.GOOS: true, runtime.GOARCH: true})
+	match(runtime.GOOS+","+runtime.GOARCH+",foo", map[string]bool{runtime.GOOS: true, runtime.GOARCH: true, "foo": true})
+	nomatch(runtime.GOOS+","+runtime.GOARCH+",!foo", map[string]bool{runtime.GOOS: true, runtime.GOARCH: true, "foo": true})
+	match(runtime.GOOS+","+runtime.GOARCH+",!bar", map[string]bool{runtime.GOOS: true, runtime.GOARCH: true, "bar": true})
+	nomatch(runtime.GOOS+","+runtime.GOARCH+",bar", map[string]bool{runtime.GOOS: true, runtime.GOARCH: true, "bar": true})
+}
+
+func TestDotSlashImport(t *testing.T) {
+	p, err := ImportDir("testdata/other", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(p.Imports) != 1 || p.Imports[0] != "./file" {
+		t.Fatalf("testdata/other: Imports=%v, want [./file]", p.Imports)
+	}
+
+	p1, err := Import("./file", "testdata/other", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if p1.Name != "file" {
+		t.Fatalf("./file: Name=%q, want %q", p1.Name, "file")
+	}
+	dir := filepath.Clean("testdata/other/file") // Clean to use \ on Windows
+	if p1.Dir != dir {
+		t.Fatalf("./file: Dir=%q, want %q", p1.Name, dir)
+	}
+}
+
+func TestEmptyImport(t *testing.T) {
+	p, err := Import("", Default.GOROOT, FindOnly)
+	if err == nil {
+		t.Fatal(`Import("") returned nil error.`)
+	}
+	if p == nil {
+		t.Fatal(`Import("") returned nil package.`)
+	}
+	if p.ImportPath != "" {
+		t.Fatalf("ImportPath=%q, want %q.", p.ImportPath, "")
+	}
+}
+
+func TestEmptyFolderImport(t *testing.T) {
+	_, err := Import(".", "testdata/empty", 0)
+	if _, ok := err.(*NoGoError); !ok {
+		t.Fatal(`Import("testdata/empty") did not return NoGoError.`)
+	}
+}
+
+func TestMultiplePackageImport(t *testing.T) {
+	_, err := Import(".", "testdata/multi", 0)
+	mpe, ok := err.(*MultiplePackageError)
+	if !ok {
+		t.Fatal(`Import("testdata/multi") did not return MultiplePackageError.`)
+	}
+	want := &MultiplePackageError{
+		Dir:      filepath.FromSlash("testdata/multi"),
+		Packages: []string{"main", "test_package"},
+		Files:    []string{"file.go", "file_appengine.go"},
+	}
+	if !reflect.DeepEqual(mpe, want) {
+		t.Errorf("got %#v; want %#v", mpe, want)
+	}
+}
+
+func TestLocalDirectory(t *testing.T) {
+	if runtime.GOOS == "ios" {
+		t.Skipf("skipping on %s/%s, no valid GOROOT", runtime.GOOS, runtime.GOARCH)
+	}
+
+	cwd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	p, err := ImportDir(cwd, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if p.ImportPath != "go/build" {
+		t.Fatalf("ImportPath=%q, want %q", p.ImportPath, "go/build")
+	}
+}
+
+var shouldBuildTests = []struct {
+	name        string
+	content     string
+	tags        map[string]bool
+	binaryOnly  bool
+	shouldBuild bool
+	err         error
+}{
+	{
+		name: "Yes",
+		content: "// +build yes\n\n" +
+			"package main\n",
+		tags:        map[string]bool{"yes": true},
+		shouldBuild: true,
+	},
+	{
+		name: "Or",
+		content: "// +build no yes\n\n" +
+			"package main\n",
+		tags:        map[string]bool{"yes": true, "no": true},
+		shouldBuild: true,
+	},
+	{
+		name: "And",
+		content: "// +build no,yes\n\n" +
+			"package main\n",
+		tags:        map[string]bool{"yes": true, "no": true},
+		shouldBuild: false,
+	},
+	{
+		name: "Cgo",
+		content: "// +build cgo\n\n" +
+			"// Copyright The Go Authors.\n\n" +
+			"// This package implements parsing of tags like\n" +
+			"// +build tag1\n" +
+			"package build",
+		tags:        map[string]bool{"cgo": true},
+		shouldBuild: false,
+	},
+	{
+		name: "AfterPackage",
+		content: "// Copyright The Go Authors.\n\n" +
+			"package build\n\n" +
+			"// shouldBuild checks tags given by lines of the form\n" +
+			"// +build tag\n" +
+			"func shouldBuild(content []byte)\n",
+		tags:        map[string]bool{},
+		shouldBuild: true,
+	},
+	{
+		name: "TooClose",
+		content: "// +build yes\n" +
+			"package main\n",
+		tags:        map[string]bool{},
+		shouldBuild: true,
+	},
+	{
+		name: "TooCloseNo",
+		content: "// +build no\n" +
+			"package main\n",
+		tags:        map[string]bool{},
+		shouldBuild: true,
+	},
+	{
+		name: "BinaryOnly",
+		content: "//go:binary-only-package\n" +
+			"// +build yes\n" +
+			"package main\n",
+		tags:        map[string]bool{},
+		binaryOnly:  true,
+		shouldBuild: true,
+	},
+	{
+		name: "ValidGoBuild",
+		content: "// +build yes\n\n" +
+			"//go:build no\n" +
+			"package main\n",
+		tags:        map[string]bool{"yes": true},
+		shouldBuild: true,
+	},
+	{
+		name: "MissingBuild",
+		content: "//go:build no\n" +
+			"package main\n",
+		tags:        map[string]bool{},
+		shouldBuild: false,
+		err:         errGoBuildWithoutBuild,
+	},
+	{
+		name: "MissingBuild2",
+		content: "/* */\n" +
+			"// +build yes\n\n" +
+			"//go:build no\n" +
+			"package main\n",
+		tags:        map[string]bool{},
+		shouldBuild: false,
+		err:         errGoBuildWithoutBuild,
+	},
+	{
+		name: "MissingBuild2",
+		content: "/*\n" +
+			"// +build yes\n\n" +
+			"*/\n" +
+			"//go:build no\n" +
+			"package main\n",
+		tags:        map[string]bool{},
+		shouldBuild: false,
+		err:         errGoBuildWithoutBuild,
+	},
+	{
+		name: "Comment1",
+		content: "/*\n" +
+			"//go:build no\n" +
+			"*/\n\n" +
+			"package main\n",
+		tags:        map[string]bool{},
+		shouldBuild: true,
+	},
+	{
+		name: "Comment2",
+		content: "/*\n" +
+			"text\n" +
+			"*/\n\n" +
+			"//go:build no\n" +
+			"package main\n",
+		tags:        map[string]bool{},
+		shouldBuild: false,
+		err:         errGoBuildWithoutBuild,
+	},
+	{
+		name: "Comment3",
+		content: "/*/*/ /* hi *//* \n" +
+			"text\n" +
+			"*/\n\n" +
+			"//go:build no\n" +
+			"package main\n",
+		tags:        map[string]bool{},
+		shouldBuild: false,
+		err:         errGoBuildWithoutBuild,
+	},
+	{
+		name: "Comment4",
+		content: "/**///go:build no\n" +
+			"package main\n",
+		tags:        map[string]bool{},
+		shouldBuild: true,
+	},
+	{
+		name: "Comment5",
+		content: "/**/\n" +
+			"//go:build no\n" +
+			"package main\n",
+		tags:        map[string]bool{},
+		shouldBuild: false,
+		err:         errGoBuildWithoutBuild,
+	},
+}
+
+func TestShouldBuild(t *testing.T) {
+	for _, tt := range shouldBuildTests {
+		t.Run(tt.name, func(t *testing.T) {
+			ctx := &Context{BuildTags: []string{"yes"}}
+			tags := map[string]bool{}
+			shouldBuild, binaryOnly, err := ctx.shouldBuild([]byte(tt.content), tags)
+			if shouldBuild != tt.shouldBuild || binaryOnly != tt.binaryOnly || !reflect.DeepEqual(tags, tt.tags) || err != tt.err {
+				t.Errorf("mismatch:\n"+
+					"have shouldBuild=%v, binaryOnly=%v, tags=%v, err=%v\n"+
+					"want shouldBuild=%v, binaryOnly=%v, tags=%v, err=%v",
+					shouldBuild, binaryOnly, tags, err,
+					tt.shouldBuild, tt.binaryOnly, tt.tags, tt.err)
+			}
+		})
+	}
+}
+
+func TestGoodOSArchFile(t *testing.T) {
+	ctx := &Context{BuildTags: []string{"linux"}, GOOS: "darwin"}
+	m := map[string]bool{}
+	want := map[string]bool{"linux": true}
+	if !ctx.goodOSArchFile("hello_linux.go", m) {
+		t.Errorf("goodOSArchFile(hello_linux.go) = false, want true")
+	}
+	if !reflect.DeepEqual(m, want) {
+		t.Errorf("goodOSArchFile(hello_linux.go) tags = %v, want %v", m, want)
+	}
+}
+
+type readNopCloser struct {
+	io.Reader
+}
+
+func (r readNopCloser) Close() error {
+	return nil
+}
+
+var (
+	ctxtP9      = Context{GOARCH: "arm", GOOS: "plan9"}
+	ctxtAndroid = Context{GOARCH: "arm", GOOS: "android"}
+)
+
+var matchFileTests = []struct {
+	ctxt  Context
+	name  string
+	data  string
+	match bool
+}{
+	{ctxtP9, "foo_arm.go", "", true},
+	{ctxtP9, "foo1_arm.go", "// +build linux\n\npackage main\n", false},
+	{ctxtP9, "foo_darwin.go", "", false},
+	{ctxtP9, "foo.go", "", true},
+	{ctxtP9, "foo1.go", "// +build linux\n\npackage main\n", false},
+	{ctxtP9, "foo.badsuffix", "", false},
+	{ctxtAndroid, "foo_linux.go", "", true},
+	{ctxtAndroid, "foo_android.go", "", true},
+	{ctxtAndroid, "foo_plan9.go", "", false},
+	{ctxtAndroid, "android.go", "", true},
+	{ctxtAndroid, "plan9.go", "", true},
+	{ctxtAndroid, "plan9_test.go", "", true},
+	{ctxtAndroid, "arm.s", "", true},
+	{ctxtAndroid, "amd64.s", "", true},
+}
+
+func TestMatchFile(t *testing.T) {
+	for _, tt := range matchFileTests {
+		ctxt := tt.ctxt
+		ctxt.OpenFile = func(path string) (r io.ReadCloser, err error) {
+			if path != "x+"+tt.name {
+				t.Fatalf("OpenFile asked for %q, expected %q", path, "x+"+tt.name)
+			}
+			return &readNopCloser{strings.NewReader(tt.data)}, nil
+		}
+		ctxt.JoinPath = func(elem ...string) string {
+			return strings.Join(elem, "+")
+		}
+		match, err := ctxt.MatchFile("x", tt.name)
+		if match != tt.match || err != nil {
+			t.Fatalf("MatchFile(%q) = %v, %v, want %v, nil", tt.name, match, err, tt.match)
+		}
+	}
+}
+
+func TestImportCmd(t *testing.T) {
+	if runtime.GOOS == "ios" {
+		t.Skipf("skipping on %s/%s, no valid GOROOT", runtime.GOOS, runtime.GOARCH)
+	}
+
+	p, err := Import("cmd/internal/objfile", "", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.HasSuffix(filepath.ToSlash(p.Dir), "src/cmd/internal/objfile") {
+		t.Fatalf("Import cmd/internal/objfile returned Dir=%q, want %q", filepath.ToSlash(p.Dir), ".../src/cmd/internal/objfile")
+	}
+}
+
+var (
+	expandSrcDirPath = filepath.Join(string(filepath.Separator)+"projects", "src", "add")
+)
+
+var expandSrcDirTests = []struct {
+	input, expected string
+}{
+	{"-L ${SRCDIR}/libs -ladd", "-L /projects/src/add/libs -ladd"},
+	{"${SRCDIR}/add_linux_386.a -pthread -lstdc++", "/projects/src/add/add_linux_386.a -pthread -lstdc++"},
+	{"Nothing to expand here!", "Nothing to expand here!"},
+	{"$", "$"},
+	{"$$", "$$"},
+	{"${", "${"},
+	{"$}", "$}"},
+	{"$FOO ${BAR}", "$FOO ${BAR}"},
+	{"Find me the $SRCDIRECTORY.", "Find me the $SRCDIRECTORY."},
+	{"$SRCDIR is missing braces", "$SRCDIR is missing braces"},
+}
+
+func TestExpandSrcDir(t *testing.T) {
+	for _, test := range expandSrcDirTests {
+		output, _ := expandSrcDir(test.input, expandSrcDirPath)
+		if output != test.expected {
+			t.Errorf("%q expands to %q with SRCDIR=%q when %q is expected", test.input, output, expandSrcDirPath, test.expected)
+		} else {
+			t.Logf("%q expands to %q with SRCDIR=%q", test.input, output, expandSrcDirPath)
+		}
+	}
+}
+
+func TestShellSafety(t *testing.T) {
+	tests := []struct {
+		input, srcdir, expected string
+		result                  bool
+	}{
+		{"-I${SRCDIR}/../include", "/projects/src/issue 11868", "-I/projects/src/issue 11868/../include", true},
+		{"-I${SRCDIR}", "~wtf$@%^", "-I~wtf$@%^", true},
+		{"-X${SRCDIR}/1,${SRCDIR}/2", "/projects/src/issue 11868", "-X/projects/src/issue 11868/1,/projects/src/issue 11868/2", true},
+		{"-I/tmp -I/tmp", "/tmp2", "-I/tmp -I/tmp", true},
+		{"-I/tmp", "/tmp/[0]", "-I/tmp", true},
+		{"-I${SRCDIR}/dir", "/tmp/[0]", "-I/tmp/[0]/dir", false},
+		{"-I${SRCDIR}/dir", "/tmp/go go", "-I/tmp/go go/dir", true},
+		{"-I${SRCDIR}/dir dir", "/tmp/go", "-I/tmp/go/dir dir", true},
+	}
+	for _, test := range tests {
+		output, ok := expandSrcDir(test.input, test.srcdir)
+		if ok != test.result {
+			t.Errorf("Expected %t while %q expands to %q with SRCDIR=%q; got %t", test.result, test.input, output, test.srcdir, ok)
+		}
+		if output != test.expected {
+			t.Errorf("Expected %q while %q expands with SRCDIR=%q; got %q", test.expected, test.input, test.srcdir, output)
+		}
+	}
+}
+
+// Want to get a "cannot find package" error when directory for package does not exist.
+// There should be valid partial information in the returned non-nil *Package.
+func TestImportDirNotExist(t *testing.T) {
+	testenv.MustHaveGoBuild(t) // really must just have source
+	ctxt := Default
+
+	emptyDir, err := os.MkdirTemp("", t.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(emptyDir)
+
+	ctxt.GOPATH = emptyDir
+	ctxt.Dir = emptyDir
+
+	tests := []struct {
+		label        string
+		path, srcDir string
+		mode         ImportMode
+	}{
+		{"Import(full, 0)", "go/build/doesnotexist", "", 0},
+		{"Import(local, 0)", "./doesnotexist", filepath.Join(ctxt.GOROOT, "src/go/build"), 0},
+		{"Import(full, FindOnly)", "go/build/doesnotexist", "", FindOnly},
+		{"Import(local, FindOnly)", "./doesnotexist", filepath.Join(ctxt.GOROOT, "src/go/build"), FindOnly},
+	}
+
+	defer os.Setenv("GO111MODULE", os.Getenv("GO111MODULE"))
+
+	for _, GO111MODULE := range []string{"off", "on"} {
+		t.Run("GO111MODULE="+GO111MODULE, func(t *testing.T) {
+			os.Setenv("GO111MODULE", GO111MODULE)
+
+			for _, test := range tests {
+				p, err := ctxt.Import(test.path, test.srcDir, test.mode)
+
+				errOk := (err != nil && strings.HasPrefix(err.Error(), "cannot find package"))
+				wantErr := `"cannot find package" error`
+				if test.srcDir == "" {
+					if err != nil && strings.Contains(err.Error(), "is not in GOROOT") {
+						errOk = true
+					}
+					wantErr = `"cannot find package" or "is not in GOROOT" error`
+				}
+				if !errOk {
+					t.Errorf("%s got error: %q, want %s", test.label, err, wantErr)
+				}
+				// If an error occurs, build.Import is documented to return
+				// a non-nil *Package containing partial information.
+				if p == nil {
+					t.Fatalf(`%s got nil p, want non-nil *Package`, test.label)
+				}
+				// Verify partial information in p.
+				if p.ImportPath != "go/build/doesnotexist" {
+					t.Errorf(`%s got p.ImportPath: %q, want "go/build/doesnotexist"`, test.label, p.ImportPath)
+				}
+			}
+		})
+	}
+}
+
+func TestImportVendor(t *testing.T) {
+	testenv.MustHaveGoBuild(t) // really must just have source
+
+	defer os.Setenv("GO111MODULE", os.Getenv("GO111MODULE"))
+	os.Setenv("GO111MODULE", "off")
+
+	ctxt := Default
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	ctxt.GOPATH = filepath.Join(wd, "testdata/withvendor")
+	p, err := ctxt.Import("c/d", filepath.Join(ctxt.GOPATH, "src/a/b"), 0)
+	if err != nil {
+		t.Fatalf("cannot find vendored c/d from testdata src/a/b directory: %v", err)
+	}
+	want := "a/vendor/c/d"
+	if p.ImportPath != want {
+		t.Fatalf("Import succeeded but found %q, want %q", p.ImportPath, want)
+	}
+}
+
+func TestImportVendorFailure(t *testing.T) {
+	testenv.MustHaveGoBuild(t) // really must just have source
+
+	defer os.Setenv("GO111MODULE", os.Getenv("GO111MODULE"))
+	os.Setenv("GO111MODULE", "off")
+
+	ctxt := Default
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	ctxt.GOPATH = filepath.Join(wd, "testdata/withvendor")
+	p, err := ctxt.Import("x.com/y/z", filepath.Join(ctxt.GOPATH, "src/a/b"), 0)
+	if err == nil {
+		t.Fatalf("found made-up package x.com/y/z in %s", p.Dir)
+	}
+
+	e := err.Error()
+	if !strings.Contains(e, " (vendor tree)") {
+		t.Fatalf("error on failed import does not mention GOROOT/src/vendor directory:\n%s", e)
+	}
+}
+
+func TestImportVendorParentFailure(t *testing.T) {
+	testenv.MustHaveGoBuild(t) // really must just have source
+
+	defer os.Setenv("GO111MODULE", os.Getenv("GO111MODULE"))
+	os.Setenv("GO111MODULE", "off")
+
+	ctxt := Default
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	ctxt.GOPATH = filepath.Join(wd, "testdata/withvendor")
+	// This import should fail because the vendor/c directory has no source code.
+	p, err := ctxt.Import("c", filepath.Join(ctxt.GOPATH, "src/a/b"), 0)
+	if err == nil {
+		t.Fatalf("found empty parent in %s", p.Dir)
+	}
+	if p != nil && p.Dir != "" {
+		t.Fatalf("decided to use %s", p.Dir)
+	}
+	e := err.Error()
+	if !strings.Contains(e, " (vendor tree)") {
+		t.Fatalf("error on failed import does not mention GOROOT/src/vendor directory:\n%s", e)
+	}
+}
+
+// Check that a package is loaded in module mode if GO111MODULE=on, even when
+// no go.mod file is present. It should fail to resolve packages outside std.
+// Verifies golang.org/issue/34669.
+func TestImportPackageOutsideModule(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	// Disable module fetching for this test so that 'go list' fails quickly
+	// without trying to find the latest version of a module.
+	defer os.Setenv("GOPROXY", os.Getenv("GOPROXY"))
+	os.Setenv("GOPROXY", "off")
+
+	// Create a GOPATH in a temporary directory. We don't use testdata
+	// because it's in GOROOT, which interferes with the module heuristic.
+	gopath, err := os.MkdirTemp("", "gobuild-notmodule")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(gopath)
+	if err := os.MkdirAll(filepath.Join(gopath, "src/example.com/p"), 0777); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(gopath, "src/example.com/p/p.go"), []byte("package p"), 0666); err != nil {
+		t.Fatal(err)
+	}
+
+	defer os.Setenv("GO111MODULE", os.Getenv("GO111MODULE"))
+	os.Setenv("GO111MODULE", "on")
+	defer os.Setenv("GOPATH", os.Getenv("GOPATH"))
+	os.Setenv("GOPATH", gopath)
+	ctxt := Default
+	ctxt.GOPATH = gopath
+	ctxt.Dir = filepath.Join(gopath, "src/example.com/p")
+
+	want := "go.mod file not found in current directory or any parent directory"
+	if _, err := ctxt.Import("example.com/p", gopath, FindOnly); err == nil {
+		t.Fatal("importing package when no go.mod is present succeeded unexpectedly")
+	} else if errStr := err.Error(); !strings.Contains(errStr, want) {
+		t.Fatalf("error when importing package when no go.mod is present: got %q; want %q", errStr, want)
+	} else {
+		t.Logf(`ctxt.Import("example.com/p", _, FindOnly): %v`, err)
+	}
+}
+
+func TestImportDirTarget(t *testing.T) {
+	testenv.MustHaveGoBuild(t) // really must just have source
+	ctxt := Default
+	ctxt.GOPATH = ""
+	p, err := ctxt.ImportDir(filepath.Join(ctxt.GOROOT, "src/path"), 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if p.PkgTargetRoot == "" || p.PkgObj == "" {
+		t.Errorf("p.PkgTargetRoot == %q, p.PkgObj == %q, want non-empty", p.PkgTargetRoot, p.PkgObj)
+	}
+}
+
+// TestIssue23594 prevents go/build from regressing and populating Package.Doc
+// from comments in test files.
+func TestIssue23594(t *testing.T) {
+	// Package testdata/doc contains regular and external test files
+	// with comments attached to their package declarations. The names of the files
+	// ensure that we see the comments from the test files first.
+	p, err := ImportDir("testdata/doc", 0)
+	if err != nil {
+		t.Fatalf("could not import testdata: %v", err)
+	}
+
+	if p.Doc != "Correct" {
+		t.Fatalf("incorrectly set .Doc to %q", p.Doc)
+	}
+}
+
+// TestMissingImportErrorRepetition checks that when an unknown package is
+// imported, the package path is only shown once in the error.
+// Verifies golang.org/issue/34752.
+func TestMissingImportErrorRepetition(t *testing.T) {
+	testenv.MustHaveGoBuild(t) // need 'go list' internally
+	tmp, err := os.MkdirTemp("", "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmp)
+	if err := os.WriteFile(filepath.Join(tmp, "go.mod"), []byte("module m"), 0666); err != nil {
+		t.Fatal(err)
+	}
+	defer os.Setenv("GO111MODULE", os.Getenv("GO111MODULE"))
+	os.Setenv("GO111MODULE", "on")
+	defer os.Setenv("GOPROXY", os.Getenv("GOPROXY"))
+	os.Setenv("GOPROXY", "off")
+	defer os.Setenv("GONOPROXY", os.Getenv("GONOPROXY"))
+	os.Setenv("GONOPROXY", "none")
+
+	ctxt := Default
+	ctxt.Dir = tmp
+
+	pkgPath := "example.com/hello"
+	_, err = ctxt.Import(pkgPath, tmp, FindOnly)
+	if err == nil {
+		t.Fatal("unexpected success")
+	}
+
+	// Don't count the package path with a URL like https://...?go-get=1.
+	// See golang.org/issue/35986.
+	errStr := strings.ReplaceAll(err.Error(), "://"+pkgPath+"?go-get=1", "://...?go-get=1")
+
+	// Also don't count instances in suggested "go get" or similar commands
+	// (see https://golang.org/issue/41576). The suggested command typically
+	// follows a semicolon.
+	errStr = strings.SplitN(errStr, ";", 2)[0]
+
+	if n := strings.Count(errStr, pkgPath); n != 1 {
+		t.Fatalf("package path %q appears in error %d times; should appear once\nerror: %v", pkgPath, n, err)
+	}
+}
+
+// TestCgoImportsIgnored checks that imports in cgo files are not included
+// in the imports list when cgo is disabled.
+// Verifies golang.org/issue/35946.
+func TestCgoImportsIgnored(t *testing.T) {
+	ctxt := Default
+	ctxt.CgoEnabled = false
+	p, err := ctxt.ImportDir("testdata/cgo_disabled", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, path := range p.Imports {
+		if path == "should/be/ignored" {
+			t.Errorf("found import %q in ignored cgo file", path)
+		}
+	}
+}
diff --git a/src/go/build/constraint/expr.go b/src/go/build/constraint/expr.go
new file mode 100644
index 0000000..3b27870
--- /dev/null
+++ b/src/go/build/constraint/expr.go
@@ -0,0 +1,574 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package constraint implements parsing and evaluation of build constraint lines.
+// See https://golang.org/cmd/go/#hdr-Build_constraints for documentation about build constraints themselves.
+//
+// This package parses both the original “// +build” syntax and the “//go:build” syntax that will be added in Go 1.17.
+// The parser is being included in Go 1.16 to allow tools that need to process Go 1.17 source code
+// to still be built against the Go 1.16 release.
+// See https://golang.org/design/draft-gobuild for details about the “//go:build” syntax.
+package constraint
+
+import (
+	"errors"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// An Expr is a build tag constraint expression.
+// The underlying concrete type is *AndExpr, *OrExpr, *NotExpr, or *TagExpr.
+type Expr interface {
+	// String returns the string form of the expression,
+	// using the boolean syntax used in //go:build lines.
+	String() string
+
+	// Eval reports whether the expression evaluates to true.
+	// It calls ok(tag) as needed to find out whether a given build tag
+	// is satisfied by the current build configuration.
+	Eval(ok func(tag string) bool) bool
+
+	// The presence of an isExpr method explicitly marks the type as an Expr.
+	// Only implementations in this package should be used as Exprs.
+	isExpr()
+}
+
+// A TagExpr is an Expr for the single tag Tag.
+type TagExpr struct {
+	Tag string // for example, “linux” or “cgo”
+}
+
+func (x *TagExpr) isExpr() {}
+
+func (x *TagExpr) Eval(ok func(tag string) bool) bool {
+	return ok(x.Tag)
+}
+
+func (x *TagExpr) String() string {
+	return x.Tag
+}
+
+func tag(tag string) Expr { return &TagExpr{tag} }
+
+// A NotExpr represents the expression !X (the negation of X).
+type NotExpr struct {
+	X Expr
+}
+
+func (x *NotExpr) isExpr() {}
+
+func (x *NotExpr) Eval(ok func(tag string) bool) bool {
+	return !x.X.Eval(ok)
+}
+
+func (x *NotExpr) String() string {
+	s := x.X.String()
+	switch x.X.(type) {
+	case *AndExpr, *OrExpr:
+		s = "(" + s + ")"
+	}
+	return "!" + s
+}
+
+func not(x Expr) Expr { return &NotExpr{x} }
+
+// An AndExpr represents the expression X && Y.
+type AndExpr struct {
+	X, Y Expr
+}
+
+func (x *AndExpr) isExpr() {}
+
+func (x *AndExpr) Eval(ok func(tag string) bool) bool {
+	// Note: Eval both, to make sure ok func observes all tags.
+	xok := x.X.Eval(ok)
+	yok := x.Y.Eval(ok)
+	return xok && yok
+}
+
+func (x *AndExpr) String() string {
+	return andArg(x.X) + " && " + andArg(x.Y)
+}
+
+func andArg(x Expr) string {
+	s := x.String()
+	if _, ok := x.(*OrExpr); ok {
+		s = "(" + s + ")"
+	}
+	return s
+}
+
+func and(x, y Expr) Expr {
+	return &AndExpr{x, y}
+}
+
+// An OrExpr represents the expression X || Y.
+type OrExpr struct {
+	X, Y Expr
+}
+
+func (x *OrExpr) isExpr() {}
+
+func (x *OrExpr) Eval(ok func(tag string) bool) bool {
+	// Note: Eval both, to make sure ok func observes all tags.
+	xok := x.X.Eval(ok)
+	yok := x.Y.Eval(ok)
+	return xok || yok
+}
+
+func (x *OrExpr) String() string {
+	return orArg(x.X) + " || " + orArg(x.Y)
+}
+
+func orArg(x Expr) string {
+	s := x.String()
+	if _, ok := x.(*AndExpr); ok {
+		s = "(" + s + ")"
+	}
+	return s
+}
+
+func or(x, y Expr) Expr {
+	return &OrExpr{x, y}
+}
+
+// A SyntaxError reports a syntax error in a parsed build expression.
+type SyntaxError struct {
+	Offset int    // byte offset in input where error was detected
+	Err    string // description of error
+}
+
+func (e *SyntaxError) Error() string {
+	return e.Err
+}
+
+var errNotConstraint = errors.New("not a build constraint")
+
+// Parse parses a single build constraint line of the form “//go:build ...” or “// +build ...”
+// and returns the corresponding boolean expression.
+func Parse(line string) (Expr, error) {
+	if text, ok := splitGoBuild(line); ok {
+		return parseExpr(text)
+	}
+	if text, ok := splitPlusBuild(line); ok {
+		return parsePlusBuildExpr(text), nil
+	}
+	return nil, errNotConstraint
+}
+
+// IsGoBuild reports whether the line of text is a “//go:build” constraint.
+// It only checks the prefix of the text, not that the expression itself parses.
+func IsGoBuild(line string) bool {
+	_, ok := splitGoBuild(line)
+	return ok
+}
+
+// splitGoBuild splits apart the leading //go:build prefix in line from the build expression itself.
+// It returns "", false if the input is not a //go:build line or if the input contains multiple lines.
+func splitGoBuild(line string) (expr string, ok bool) {
+	// A single trailing newline is OK; otherwise multiple lines are not.
+	if len(line) > 0 && line[len(line)-1] == '\n' {
+		line = line[:len(line)-1]
+	}
+	if strings.Contains(line, "\n") {
+		return "", false
+	}
+
+	if !strings.HasPrefix(line, "//go:build") {
+		return "", false
+	}
+
+	line = strings.TrimSpace(line)
+	line = line[len("//go:build"):]
+
+	// If strings.TrimSpace finds more to trim after removing the //go:build prefix,
+	// it means that the prefix was followed by a space, making this a //go:build line
+	// (as opposed to a //go:buildsomethingelse line).
+	// If line is empty, we had "//go:build" by itself, which also counts.
+	trim := strings.TrimSpace(line)
+	if len(line) == len(trim) && line != "" {
+		return "", false
+	}
+
+	return trim, true
+}
+
+// An exprParser holds state for parsing a build expression.
+type exprParser struct {
+	s string // input string
+	i int    // next read location in s
+
+	tok   string // last token read
+	isTag bool
+	pos   int // position (start) of last token
+}
+
+// parseExpr parses a boolean build tag expression.
+func parseExpr(text string) (x Expr, err error) {
+	defer func() {
+		if e := recover(); e != nil {
+			if e, ok := e.(*SyntaxError); ok {
+				err = e
+				return
+			}
+			panic(e) // unreachable unless parser has a bug
+		}
+	}()
+
+	p := &exprParser{s: text}
+	x = p.or()
+	if p.tok != "" {
+		panic(&SyntaxError{Offset: p.pos, Err: "unexpected token " + p.tok})
+	}
+	return x, nil
+}
+
+// or parses a sequence of || expressions.
+// On entry, the next input token has not yet been lexed.
+// On exit, the next input token has been lexed and is in p.tok.
+func (p *exprParser) or() Expr {
+	x := p.and()
+	for p.tok == "||" {
+		x = or(x, p.and())
+	}
+	return x
+}
+
+// and parses a sequence of && expressions.
+// On entry, the next input token has not yet been lexed.
+// On exit, the next input token has been lexed and is in p.tok.
+func (p *exprParser) and() Expr {
+	x := p.not()
+	for p.tok == "&&" {
+		x = and(x, p.not())
+	}
+	return x
+}
+
+// not parses a ! expression.
+// On entry, the next input token has not yet been lexed.
+// On exit, the next input token has been lexed and is in p.tok.
+func (p *exprParser) not() Expr {
+	p.lex()
+	if p.tok == "!" {
+		p.lex()
+		if p.tok == "!" {
+			panic(&SyntaxError{Offset: p.pos, Err: "double negation not allowed"})
+		}
+		return not(p.atom())
+	}
+	return p.atom()
+}
+
+// atom parses a tag or a parenthesized expression.
+// On entry, the next input token HAS been lexed.
+// On exit, the next input token has been lexed and is in p.tok.
+func (p *exprParser) atom() Expr {
+	// first token already in p.tok
+	if p.tok == "(" {
+		pos := p.pos
+		defer func() {
+			if e := recover(); e != nil {
+				if e, ok := e.(*SyntaxError); ok && e.Err == "unexpected end of expression" {
+					e.Err = "missing close paren"
+				}
+				panic(e)
+			}
+		}()
+		x := p.or()
+		if p.tok != ")" {
+			panic(&SyntaxError{Offset: pos, Err: "missing close paren"})
+		}
+		p.lex()
+		return x
+	}
+
+	if !p.isTag {
+		if p.tok == "" {
+			panic(&SyntaxError{Offset: p.pos, Err: "unexpected end of expression"})
+		}
+		panic(&SyntaxError{Offset: p.pos, Err: "unexpected token " + p.tok})
+	}
+	tok := p.tok
+	p.lex()
+	return tag(tok)
+}
+
+// lex finds and consumes the next token in the input stream.
+// On return, p.tok is set to the token text,
+// p.isTag reports whether the token was a tag,
+// and p.pos records the byte offset of the start of the token in the input stream.
+// If lex reaches the end of the input, p.tok is set to the empty string.
+// For any other syntax error, lex panics with a SyntaxError.
+func (p *exprParser) lex() {
+	p.isTag = false
+	for p.i < len(p.s) && (p.s[p.i] == ' ' || p.s[p.i] == '\t') {
+		p.i++
+	}
+	if p.i >= len(p.s) {
+		p.tok = ""
+		p.pos = p.i
+		return
+	}
+	switch p.s[p.i] {
+	case '(', ')', '!':
+		p.pos = p.i
+		p.i++
+		p.tok = p.s[p.pos:p.i]
+		return
+
+	case '&', '|':
+		if p.i+1 >= len(p.s) || p.s[p.i+1] != p.s[p.i] {
+			panic(&SyntaxError{Offset: p.i, Err: "invalid syntax at " + string(rune(p.s[p.i]))})
+		}
+		p.pos = p.i
+		p.i += 2
+		p.tok = p.s[p.pos:p.i]
+		return
+	}
+
+	tag := p.s[p.i:]
+	for i, c := range tag {
+		if !unicode.IsLetter(c) && !unicode.IsDigit(c) && c != '_' && c != '.' {
+			tag = tag[:i]
+			break
+		}
+	}
+	if tag == "" {
+		c, _ := utf8.DecodeRuneInString(p.s[p.i:])
+		panic(&SyntaxError{Offset: p.i, Err: "invalid syntax at " + string(c)})
+	}
+
+	p.pos = p.i
+	p.i += len(tag)
+	p.tok = p.s[p.pos:p.i]
+	p.isTag = true
+	return
+}
+
+// IsPlusBuild reports whether the line of text is a “// +build” constraint.
+// It only checks the prefix of the text, not that the expression itself parses.
+func IsPlusBuild(line string) bool {
+	_, ok := splitPlusBuild(line)
+	return ok
+}
+
+// splitGoBuild splits apart the leading //go:build prefix in line from the build expression itself.
+// It returns "", false if the input is not a //go:build line or if the input contains multiple lines.
+func splitPlusBuild(line string) (expr string, ok bool) {
+	// A single trailing newline is OK; otherwise multiple lines are not.
+	if len(line) > 0 && line[len(line)-1] == '\n' {
+		line = line[:len(line)-1]
+	}
+	if strings.Contains(line, "\n") {
+		return "", false
+	}
+
+	if !strings.HasPrefix(line, "//") {
+		return "", false
+	}
+	line = line[len("//"):]
+	// Note the space is optional; "//+build" is recognized too.
+	line = strings.TrimSpace(line)
+
+	if !strings.HasPrefix(line, "+build") {
+		return "", false
+	}
+	line = line[len("+build"):]
+
+	// If strings.TrimSpace finds more to trim after removing the +build prefix,
+	// it means that the prefix was followed by a space, making this a +build line
+	// (as opposed to a +buildsomethingelse line).
+	// If line is empty, we had "// +build" by itself, which also counts.
+	trim := strings.TrimSpace(line)
+	if len(line) == len(trim) && line != "" {
+		return "", false
+	}
+
+	return trim, true
+}
+
+// parsePlusBuildExpr parses a legacy build tag expression (as used with “// +build”).
+func parsePlusBuildExpr(text string) Expr {
+	var x Expr
+	for _, clause := range strings.Fields(text) {
+		var y Expr
+		for _, lit := range strings.Split(clause, ",") {
+			var z Expr
+			var neg bool
+			if strings.HasPrefix(lit, "!!") || lit == "!" {
+				z = tag("ignore")
+			} else {
+				if strings.HasPrefix(lit, "!") {
+					neg = true
+					lit = lit[len("!"):]
+				}
+				if isValidTag(lit) {
+					z = tag(lit)
+				} else {
+					z = tag("ignore")
+				}
+				if neg {
+					z = not(z)
+				}
+			}
+			if y == nil {
+				y = z
+			} else {
+				y = and(y, z)
+			}
+		}
+		if x == nil {
+			x = y
+		} else {
+			x = or(x, y)
+		}
+	}
+	return x
+}
+
+// isValidTag reports whether the word is a valid build tag.
+// Tags must be letters, digits, underscores or dots.
+// Unlike in Go identifiers, all digits are fine (e.g., "386").
+func isValidTag(word string) bool {
+	if word == "" {
+		return false
+	}
+	for _, c := range word {
+		if !unicode.IsLetter(c) && !unicode.IsDigit(c) && c != '_' && c != '.' {
+			return false
+		}
+	}
+	return true
+}
+
+var errComplex = errors.New("expression too complex for // +build lines")
+
+// PlusBuildLines returns a sequence of “// +build” lines that evaluate to the build expression x.
+// If the expression is too complex to convert directly to “// +build” lines, PlusBuildLines returns an error.
+func PlusBuildLines(x Expr) ([]string, error) {
+	// Push all NOTs to the expression leaves, so that //go:build !(x && y) can be treated as !x || !y.
+	// This rewrite is both efficient and commonly needed, so it's worth doing.
+	// Essentially all other possible rewrites are too expensive and too rarely needed.
+	x = pushNot(x, false)
+
+	// Split into AND of ORs of ANDs of literals (tag or NOT tag).
+	var split [][][]Expr
+	for _, or := range appendSplitAnd(nil, x) {
+		var ands [][]Expr
+		for _, and := range appendSplitOr(nil, or) {
+			var lits []Expr
+			for _, lit := range appendSplitAnd(nil, and) {
+				switch lit.(type) {
+				case *TagExpr, *NotExpr:
+					lits = append(lits, lit)
+				default:
+					return nil, errComplex
+				}
+			}
+			ands = append(ands, lits)
+		}
+		split = append(split, ands)
+	}
+
+	// If all the ORs have length 1 (no actual OR'ing going on),
+	// push the top-level ANDs to the bottom level, so that we get
+	// one // +build line instead of many.
+	maxOr := 0
+	for _, or := range split {
+		if maxOr < len(or) {
+			maxOr = len(or)
+		}
+	}
+	if maxOr == 1 {
+		var lits []Expr
+		for _, or := range split {
+			lits = append(lits, or[0]...)
+		}
+		split = [][][]Expr{{lits}}
+	}
+
+	// Prepare the +build lines.
+	var lines []string
+	for _, or := range split {
+		line := "// +build"
+		for _, and := range or {
+			clause := ""
+			for i, lit := range and {
+				if i > 0 {
+					clause += ","
+				}
+				clause += lit.String()
+			}
+			line += " " + clause
+		}
+		lines = append(lines, line)
+	}
+
+	return lines, nil
+}
+
+// pushNot applies DeMorgan's law to push negations down the expression,
+// so that only tags are negated in the result.
+// (It applies the rewrites !(X && Y) => (!X || !Y) and !(X || Y) => (!X && !Y).)
+func pushNot(x Expr, not bool) Expr {
+	switch x := x.(type) {
+	default:
+		// unreachable
+		return x
+	case *NotExpr:
+		if _, ok := x.X.(*TagExpr); ok && !not {
+			return x
+		}
+		return pushNot(x.X, !not)
+	case *TagExpr:
+		if not {
+			return &NotExpr{X: x}
+		}
+		return x
+	case *AndExpr:
+		x1 := pushNot(x.X, not)
+		y1 := pushNot(x.Y, not)
+		if not {
+			return or(x1, y1)
+		}
+		if x1 == x.X && y1 == x.Y {
+			return x
+		}
+		return and(x1, y1)
+	case *OrExpr:
+		x1 := pushNot(x.X, not)
+		y1 := pushNot(x.Y, not)
+		if not {
+			return and(x1, y1)
+		}
+		if x1 == x.X && y1 == x.Y {
+			return x
+		}
+		return or(x1, y1)
+	}
+}
+
+// appendSplitAnd appends x to list while splitting apart any top-level && expressions.
+// For example, appendSplitAnd({W}, X && Y && Z) = {W, X, Y, Z}.
+func appendSplitAnd(list []Expr, x Expr) []Expr {
+	if x, ok := x.(*AndExpr); ok {
+		list = appendSplitAnd(list, x.X)
+		list = appendSplitAnd(list, x.Y)
+		return list
+	}
+	return append(list, x)
+}
+
+// appendSplitOr appends x to list while splitting apart any top-level || expressions.
+// For example, appendSplitOr({W}, X || Y || Z) = {W, X, Y, Z}.
+func appendSplitOr(list []Expr, x Expr) []Expr {
+	if x, ok := x.(*OrExpr); ok {
+		list = appendSplitOr(list, x.X)
+		list = appendSplitOr(list, x.Y)
+		return list
+	}
+	return append(list, x)
+}
diff --git a/src/go/build/constraint/expr_test.go b/src/go/build/constraint/expr_test.go
new file mode 100644
index 0000000..4979f8b
--- /dev/null
+++ b/src/go/build/constraint/expr_test.go
@@ -0,0 +1,317 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package constraint
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+var exprStringTests = []struct {
+	x   Expr
+	out string
+}{
+	{
+		x:   tag("abc"),
+		out: "abc",
+	},
+	{
+		x:   not(tag("abc")),
+		out: "!abc",
+	},
+	{
+		x:   not(and(tag("abc"), tag("def"))),
+		out: "!(abc && def)",
+	},
+	{
+		x:   and(tag("abc"), or(tag("def"), tag("ghi"))),
+		out: "abc && (def || ghi)",
+	},
+	{
+		x:   or(and(tag("abc"), tag("def")), tag("ghi")),
+		out: "(abc && def) || ghi",
+	},
+}
+
+func TestExprString(t *testing.T) {
+	for i, tt := range exprStringTests {
+		t.Run(fmt.Sprint(i), func(t *testing.T) {
+			s := tt.x.String()
+			if s != tt.out {
+				t.Errorf("String() mismatch:\nhave %s\nwant %s", s, tt.out)
+			}
+		})
+	}
+}
+
+var lexTests = []struct {
+	in  string
+	out string
+}{
+	{"", ""},
+	{"x", "x"},
+	{"x.y", "x.y"},
+	{"x_y", "x_y"},
+	{"αx", "αx"},
+	{"αx²", "αx err: invalid syntax at ²"},
+	{"go1.2", "go1.2"},
+	{"x y", "x y"},
+	{"x!y", "x ! y"},
+	{"&&||!()xy yx ", "&& || ! ( ) xy yx"},
+	{"x~", "x err: invalid syntax at ~"},
+	{"x ~", "x err: invalid syntax at ~"},
+	{"x &", "x err: invalid syntax at &"},
+	{"x &y", "x err: invalid syntax at &"},
+}
+
+func TestLex(t *testing.T) {
+	for i, tt := range lexTests {
+		t.Run(fmt.Sprint(i), func(t *testing.T) {
+			p := &exprParser{s: tt.in}
+			out := ""
+			for {
+				tok, err := lexHelp(p)
+				if tok == "" && err == nil {
+					break
+				}
+				if out != "" {
+					out += " "
+				}
+				if err != nil {
+					out += "err: " + err.Error()
+					break
+				}
+				out += tok
+			}
+			if out != tt.out {
+				t.Errorf("lex(%q):\nhave %s\nwant %s", tt.in, out, tt.out)
+			}
+		})
+	}
+}
+
+func lexHelp(p *exprParser) (tok string, err error) {
+	defer func() {
+		if e := recover(); e != nil {
+			if e, ok := e.(*SyntaxError); ok {
+				err = e
+				return
+			}
+			panic(e)
+		}
+	}()
+
+	p.lex()
+	return p.tok, nil
+}
+
+var parseExprTests = []struct {
+	in string
+	x  Expr
+}{
+	{"x", tag("x")},
+	{"x&&y", and(tag("x"), tag("y"))},
+	{"x||y", or(tag("x"), tag("y"))},
+	{"(x)", tag("x")},
+	{"x||y&&z", or(tag("x"), and(tag("y"), tag("z")))},
+	{"x&&y||z", or(and(tag("x"), tag("y")), tag("z"))},
+	{"x&&(y||z)", and(tag("x"), or(tag("y"), tag("z")))},
+	{"(x||y)&&z", and(or(tag("x"), tag("y")), tag("z"))},
+	{"!(x&&y)", not(and(tag("x"), tag("y")))},
+}
+
+func TestParseExpr(t *testing.T) {
+	for i, tt := range parseExprTests {
+		t.Run(fmt.Sprint(i), func(t *testing.T) {
+			x, err := parseExpr(tt.in)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if x.String() != tt.x.String() {
+				t.Errorf("parseExpr(%q):\nhave %s\nwant %s", tt.in, x, tt.x)
+			}
+		})
+	}
+}
+
+var parseExprErrorTests = []struct {
+	in  string
+	err error
+}{
+	{"x && ", &SyntaxError{Offset: 5, Err: "unexpected end of expression"}},
+	{"x && (", &SyntaxError{Offset: 6, Err: "missing close paren"}},
+	{"x && ||", &SyntaxError{Offset: 5, Err: "unexpected token ||"}},
+	{"x && !", &SyntaxError{Offset: 6, Err: "unexpected end of expression"}},
+	{"x && !!", &SyntaxError{Offset: 6, Err: "double negation not allowed"}},
+	{"x !", &SyntaxError{Offset: 2, Err: "unexpected token !"}},
+	{"x && (y", &SyntaxError{Offset: 5, Err: "missing close paren"}},
+}
+
+func TestParseError(t *testing.T) {
+	for i, tt := range parseExprErrorTests {
+		t.Run(fmt.Sprint(i), func(t *testing.T) {
+			x, err := parseExpr(tt.in)
+			if err == nil {
+				t.Fatalf("parseExpr(%q) = %v, want error", tt.in, x)
+			}
+			if !reflect.DeepEqual(err, tt.err) {
+				t.Fatalf("parseExpr(%q): wrong error:\nhave %#v\nwant %#v", tt.in, err, tt.err)
+			}
+		})
+	}
+}
+
+var exprEvalTests = []struct {
+	in   string
+	ok   bool
+	tags string
+}{
+	{"x", false, "x"},
+	{"x && y", false, "x y"},
+	{"x || y", false, "x y"},
+	{"!x && yes", true, "x yes"},
+	{"yes || y", true, "y yes"},
+}
+
+func TestExprEval(t *testing.T) {
+	for i, tt := range exprEvalTests {
+		t.Run(fmt.Sprint(i), func(t *testing.T) {
+			x, err := parseExpr(tt.in)
+			if err != nil {
+				t.Fatal(err)
+			}
+			tags := make(map[string]bool)
+			wantTags := make(map[string]bool)
+			for _, tag := range strings.Fields(tt.tags) {
+				wantTags[tag] = true
+			}
+			hasTag := func(tag string) bool {
+				tags[tag] = true
+				return tag == "yes"
+			}
+			ok := x.Eval(hasTag)
+			if ok != tt.ok || !reflect.DeepEqual(tags, wantTags) {
+				t.Errorf("Eval(%#q):\nhave ok=%v, tags=%v\nwant ok=%v, tags=%v",
+					tt.in, ok, tags, tt.ok, wantTags)
+			}
+		})
+	}
+}
+
+var parsePlusBuildExprTests = []struct {
+	in string
+	x  Expr
+}{
+	{"x", tag("x")},
+	{"x,y", and(tag("x"), tag("y"))},
+	{"x y", or(tag("x"), tag("y"))},
+	{"x y,z", or(tag("x"), and(tag("y"), tag("z")))},
+	{"x,y z", or(and(tag("x"), tag("y")), tag("z"))},
+	{"x,!y !z", or(and(tag("x"), not(tag("y"))), not(tag("z")))},
+	{"!! x", or(tag("ignore"), tag("x"))},
+	{"!!x", tag("ignore")},
+	{"!x", not(tag("x"))},
+	{"!", tag("ignore")},
+}
+
+func TestParsePlusBuildExpr(t *testing.T) {
+	for i, tt := range parsePlusBuildExprTests {
+		t.Run(fmt.Sprint(i), func(t *testing.T) {
+			x := parsePlusBuildExpr(tt.in)
+			if x.String() != tt.x.String() {
+				t.Errorf("parsePlusBuildExpr(%q):\nhave %v\nwant %v", tt.in, x, tt.x)
+			}
+		})
+	}
+}
+
+var constraintTests = []struct {
+	in  string
+	x   Expr
+	err error
+}{
+	{"//+build x y", or(tag("x"), tag("y")), nil},
+	{"// +build x y \n", or(tag("x"), tag("y")), nil},
+	{"// +build x y \n ", nil, errNotConstraint},
+	{"// +build x y \nmore", nil, errNotConstraint},
+	{" //+build x y", nil, errNotConstraint},
+
+	{"//go:build x && y", and(tag("x"), tag("y")), nil},
+	{"//go:build x && y\n", and(tag("x"), tag("y")), nil},
+	{"//go:build x && y\n ", nil, errNotConstraint},
+	{"//go:build x && y\nmore", nil, errNotConstraint},
+	{" //go:build x && y", nil, errNotConstraint},
+}
+
+func TestParse(t *testing.T) {
+	for i, tt := range constraintTests {
+		t.Run(fmt.Sprint(i), func(t *testing.T) {
+			x, err := Parse(tt.in)
+			if err != nil {
+				if tt.err == nil {
+					t.Errorf("Constraint(%q): unexpected error: %v", tt.in, err)
+				} else if tt.err != err {
+					t.Errorf("Constraint(%q): error %v, want %v", tt.in, err, tt.err)
+				}
+				return
+			}
+			if tt.err != nil {
+				t.Errorf("Constraint(%q) = %v, want error %v", tt.in, x, tt.err)
+				return
+			}
+			if x.String() != tt.x.String() {
+				t.Errorf("Constraint(%q):\nhave %v\nwant %v", tt.in, x, tt.x)
+			}
+		})
+	}
+}
+
+var plusBuildLinesTests = []struct {
+	in  string
+	out []string
+	err error
+}{
+	{"x", []string{"x"}, nil},
+	{"x && !y", []string{"x,!y"}, nil},
+	{"x || y", []string{"x y"}, nil},
+	{"x && (y || z)", []string{"x", "y z"}, nil},
+	{"!(x && y)", []string{"!x !y"}, nil},
+	{"x || (y && z)", []string{"x y,z"}, nil},
+	{"w && (x || (y && z))", []string{"w", "x y,z"}, nil},
+	{"v || (w && (x || (y && z)))", nil, errComplex},
+}
+
+func TestPlusBuildLines(t *testing.T) {
+	for i, tt := range plusBuildLinesTests {
+		t.Run(fmt.Sprint(i), func(t *testing.T) {
+			x, err := parseExpr(tt.in)
+			if err != nil {
+				t.Fatal(err)
+			}
+			lines, err := PlusBuildLines(x)
+			if err != nil {
+				if tt.err == nil {
+					t.Errorf("PlusBuildLines(%q): unexpected error: %v", tt.in, err)
+				} else if tt.err != err {
+					t.Errorf("PlusBuildLines(%q): error %v, want %v", tt.in, err, tt.err)
+				}
+				return
+			}
+			if tt.err != nil {
+				t.Errorf("PlusBuildLines(%q) = %v, want error %v", tt.in, lines, tt.err)
+				return
+			}
+			var want []string
+			for _, line := range tt.out {
+				want = append(want, "// +build "+line)
+			}
+			if !reflect.DeepEqual(lines, want) {
+				t.Errorf("PlusBuildLines(%q):\nhave %q\nwant %q", tt.in, lines, want)
+			}
+		})
+	}
+}
diff --git a/src/go/build/deps_test.go b/src/go/build/deps_test.go
new file mode 100644
index 0000000..c97c668
--- /dev/null
+++ b/src/go/build/deps_test.go
@@ -0,0 +1,877 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file exercises the import parser but also checks that
+// some low-level packages do not have new dependencies added.
+
+package build
+
+import (
+	"bytes"
+	"fmt"
+	"go/token"
+	"internal/testenv"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"runtime"
+	"sort"
+	"strings"
+	"testing"
+)
+
+// depsRules defines the expected dependencies between packages in
+// the Go source tree. It is a statement of policy.
+//
+// DO NOT CHANGE THIS DATA TO FIX BUILDS.
+// Existing packages should not have their constraints relaxed
+// without prior discussion.
+// Negative assertions should almost never be removed.
+//
+// The general syntax of a rule is:
+//
+//		a, b < c, d;
+//
+// which means c and d come after a and b in the partial order
+// (that is, c and d can import a and b),
+// but doesn't provide a relative order between a vs b or c vs d.
+//
+// The rules can chain together, as in:
+//
+//		e < f, g < h;
+//
+// which is equivalent to
+//
+//		e < f, g;
+//		f, g < h;
+//
+// Except for the special bottom element "NONE", each name
+// must appear exactly once on the right-hand side of a rule.
+// That rule serves as the definition of the allowed dependencies
+// for that name. The definition must appear before any uses
+// of the name on the left-hand side of a rule. (That is, the
+// rules themselves must be ordered according to the partial
+// order, for easier reading by people.)
+//
+// Negative assertions double-check the partial order:
+//
+//		i !< j
+//
+// means that it must NOT be the case that i < j.
+// Negative assertions may appear anywhere in the rules,
+// even before i and j have been defined.
+//
+// Comments begin with #.
+//
+// All-caps names are pseudo-names for specific points
+// in the dependency lattice.
+//
+var depsRules = `
+	# No dependencies allowed for any of these packages.
+	NONE
+	< container/list, container/ring,
+	  internal/cfg, internal/cpu,
+	  internal/goversion, internal/nettrace,
+	  unicode/utf8, unicode/utf16, unicode,
+	  unsafe;
+
+	# RUNTIME is the core runtime group of packages, all of them very light-weight.
+	internal/cpu, unsafe
+	< internal/bytealg
+	< internal/unsafeheader
+	< runtime/internal/sys
+	< runtime/internal/atomic
+	< runtime/internal/math
+	< runtime
+	< sync/atomic
+	< internal/race
+	< sync
+	< internal/reflectlite
+	< errors
+	< internal/oserror, math/bits
+	< RUNTIME;
+
+	RUNTIME
+	< sort
+	< container/heap;
+
+	RUNTIME
+	< io;
+
+	syscall !< io;
+	reflect !< sort;
+
+	RUNTIME, unicode/utf8
+	< path;
+
+	unicode !< path;
+
+	# SYSCALL is RUNTIME plus the packages necessary for basic system calls.
+	RUNTIME, unicode/utf8, unicode/utf16
+	< internal/syscall/windows/sysdll, syscall/js
+	< syscall
+	< internal/syscall/unix, internal/syscall/windows, internal/syscall/windows/registry
+	< internal/syscall/execenv
+	< SYSCALL;
+
+	# TIME is SYSCALL plus the core packages about time, including context.
+	SYSCALL
+	< time/tzdata
+	< time
+	< context
+	< TIME;
+
+	TIME, io, path, sort
+	< io/fs;
+
+	# MATH is RUNTIME plus the basic math packages.
+	RUNTIME
+	< math
+	< MATH;
+
+	unicode !< math;
+
+	MATH
+	< math/cmplx;
+
+	MATH
+	< math/rand;
+
+	MATH
+	< runtime/metrics;
+
+	MATH, unicode/utf8
+	< strconv;
+
+	unicode !< strconv;
+
+	# STR is basic string and buffer manipulation.
+	RUNTIME, io, unicode/utf8, unicode/utf16, unicode
+	< bytes, strings
+	< bufio;
+
+	bufio, path, strconv
+	< STR;
+
+	# OS is basic OS access, including helpers (path/filepath, os/exec, etc).
+	# OS includes string routines, but those must be layered above package os.
+	# OS does not include reflection.
+	io/fs
+	< internal/testlog
+	< internal/poll
+	< os
+	< os/signal;
+
+	io/fs
+	< embed;
+
+	unicode, fmt !< os, os/signal;
+
+	os/signal, STR
+	< path/filepath
+	< io/ioutil, os/exec;
+
+	io/ioutil, os/exec, os/signal
+	< OS;
+
+	reflect !< OS;
+
+	OS
+	< golang.org/x/sys/cpu;
+
+	# FMT is OS (which includes string routines) plus reflect and fmt.
+	# It does not include package log, which should be avoided in core packages.
+	strconv, unicode
+	< reflect;
+
+	os, reflect
+	< internal/fmtsort
+	< fmt;
+
+	OS, fmt
+	< FMT;
+
+	log !< FMT;
+
+	OS, FMT
+	< internal/execabs;
+
+	OS, internal/execabs
+	< internal/goroot;
+
+	# Misc packages needing only FMT.
+	FMT
+	< flag,
+	  html,
+	  mime/quotedprintable,
+	  net/internal/socktest,
+	  net/url,
+	  runtime/debug,
+	  runtime/trace,
+	  text/scanner,
+	  text/tabwriter;
+
+	# encodings
+	# core ones do not use fmt.
+	io, strconv
+	< encoding;
+
+	encoding, reflect
+	< encoding/binary
+	< encoding/base32, encoding/base64;
+
+	fmt !< encoding/base32, encoding/base64;
+
+	FMT, encoding/base32, encoding/base64
+	< encoding/ascii85, encoding/csv, encoding/gob, encoding/hex,
+	  encoding/json, encoding/pem, encoding/xml, mime;
+
+	# hashes
+	io
+	< hash
+	< hash/adler32, hash/crc32, hash/crc64, hash/fnv, hash/maphash;
+
+	# math/big
+	FMT, encoding/binary, math/rand
+	< math/big;
+
+	# compression
+	FMT, encoding/binary, hash/adler32, hash/crc32
+	< compress/bzip2, compress/flate, compress/lzw
+	< archive/zip, compress/gzip, compress/zlib;
+
+	# templates
+	FMT
+	< text/template/parse;
+
+	net/url, text/template/parse
+	< text/template
+	< internal/lazytemplate;
+
+	encoding/json, html, text/template
+	< html/template;
+
+	# regexp
+	FMT
+	< regexp/syntax
+	< regexp
+	< internal/lazyregexp;
+
+	# suffix array
+	encoding/binary, regexp
+	< index/suffixarray;
+
+	# executable parsing
+	FMT, encoding/binary, compress/zlib
+	< debug/dwarf
+	< debug/elf, debug/gosym, debug/macho, debug/pe, debug/plan9obj, internal/xcoff
+	< DEBUG;
+
+	# go parser and friends.
+	FMT
+	< go/token
+	< go/scanner
+	< go/ast
+	< go/parser;
+
+	go/parser, text/tabwriter
+	< go/printer
+	< go/format;
+
+	go/parser, internal/lazyregexp, text/template
+	< go/doc;
+
+	math/big, go/token
+	< go/constant;
+
+	container/heap, go/constant, go/parser
+	< go/types;
+
+	FMT
+	< go/build/constraint;
+
+	go/doc, go/parser, internal/goroot, internal/goversion
+	< go/build;
+
+	DEBUG, go/build, go/types, text/scanner
+	< go/internal/gcimporter, go/internal/gccgoimporter, go/internal/srcimporter
+	< go/importer;
+
+	# databases
+	FMT
+	< database/sql/internal
+	< database/sql/driver
+	< database/sql;
+
+	# images
+	FMT, compress/lzw, compress/zlib
+	< image/color
+	< image, image/color/palette
+	< image/internal/imageutil
+	< image/draw
+	< image/gif, image/jpeg, image/png;
+
+	# cgo, delayed as long as possible.
+	# If you add a dependency on CGO, you must add the package
+	# to cgoPackages in cmd/dist/test.go as well.
+	RUNTIME
+	< C
+	< runtime/cgo
+	< CGO
+	< runtime/race, runtime/msan;
+
+	# Bulk of the standard library must not use cgo.
+	# The prohibition stops at net and os/user.
+	C !< fmt, go/types, CRYPTO-MATH;
+
+	CGO, OS
+	< plugin;
+
+	CGO, FMT
+	< os/user
+	< archive/tar;
+
+	sync
+	< internal/singleflight;
+
+	os
+	< golang.org/x/net/dns/dnsmessage,
+	  golang.org/x/net/lif,
+	  golang.org/x/net/route;
+
+	# net is unavoidable when doing any networking,
+	# so large dependencies must be kept out.
+	# This is a long-looking list but most of these
+	# are small with few dependencies.
+	CGO,
+	golang.org/x/net/dns/dnsmessage,
+	golang.org/x/net/lif,
+	golang.org/x/net/route,
+	internal/nettrace,
+	internal/poll,
+	internal/singleflight,
+	internal/race,
+	os
+	< net;
+
+	fmt, unicode !< net;
+	math/rand !< net; # net uses runtime instead
+
+	# NET is net plus net-helper packages.
+	FMT, net
+	< net/textproto;
+
+	mime, net/textproto, net/url
+	< NET;
+
+	# logging - most packages should not import; http and up is allowed
+	FMT
+	< log;
+
+	log !< crypto/tls, database/sql, go/importer, testing;
+
+	FMT, log, net
+	< log/syslog;
+
+	NET, log
+	< net/mail;
+
+	# CRYPTO is core crypto algorithms - no cgo, fmt, net.
+	# Unfortunately, stuck with reflect via encoding/binary.
+	encoding/binary, golang.org/x/sys/cpu, hash
+	< crypto
+	< crypto/subtle
+	< crypto/internal/subtle
+	< crypto/cipher
+	< crypto/aes, crypto/des, crypto/hmac, crypto/md5, crypto/rc4,
+	  crypto/sha1, crypto/sha256, crypto/sha512
+	< CRYPTO;
+
+	CGO, fmt, net !< CRYPTO;
+
+	# CRYPTO-MATH is core bignum-based crypto - no cgo, net; fmt now ok.
+	CRYPTO, FMT, math/big
+	< crypto/rand
+	< crypto/internal/randutil
+	< crypto/ed25519/internal/edwards25519
+	< crypto/ed25519
+	< encoding/asn1
+	< golang.org/x/crypto/cryptobyte/asn1
+	< golang.org/x/crypto/cryptobyte
+	< golang.org/x/crypto/curve25519
+	< crypto/dsa, crypto/elliptic, crypto/rsa
+	< crypto/ecdsa
+	< CRYPTO-MATH;
+
+	CGO, net !< CRYPTO-MATH;
+
+	# TLS, Prince of Dependencies.
+	CRYPTO-MATH, NET, container/list, encoding/hex, encoding/pem
+	< golang.org/x/crypto/internal/subtle
+	< golang.org/x/crypto/chacha20
+	< golang.org/x/crypto/poly1305
+	< golang.org/x/crypto/chacha20poly1305
+	< golang.org/x/crypto/hkdf
+	< crypto/x509/internal/macos
+	< crypto/x509/pkix
+	< crypto/x509
+	< crypto/tls;
+
+	# crypto-aware packages
+
+	NET, crypto/rand, mime/quotedprintable
+	< mime/multipart;
+
+	crypto/tls
+	< net/smtp;
+
+	# HTTP, King of Dependencies.
+
+	FMT
+	< golang.org/x/net/http2/hpack, net/http/internal;
+
+	FMT, NET, container/list, encoding/binary, log
+	< golang.org/x/text/transform
+	< golang.org/x/text/unicode/norm
+	< golang.org/x/text/unicode/bidi
+	< golang.org/x/text/secure/bidirule
+	< golang.org/x/net/idna
+	< golang.org/x/net/http/httpguts, golang.org/x/net/http/httpproxy;
+
+	NET, crypto/tls
+	< net/http/httptrace;
+
+	compress/gzip,
+	golang.org/x/net/http/httpguts,
+	golang.org/x/net/http/httpproxy,
+	golang.org/x/net/http2/hpack,
+	net/http/internal,
+	net/http/httptrace,
+	mime/multipart,
+	log
+	< net/http;
+
+	# HTTP-aware packages
+
+	encoding/json, net/http
+	< expvar;
+
+	net/http
+	< net/http/cookiejar, net/http/httputil;
+
+	net/http, flag
+	< net/http/httptest;
+
+	net/http, regexp
+	< net/http/cgi
+	< net/http/fcgi;
+
+	# Profiling
+	FMT, compress/gzip, encoding/binary, text/tabwriter
+	< runtime/pprof;
+
+	OS, compress/gzip, regexp
+	< internal/profile;
+
+	html, internal/profile, net/http, runtime/pprof, runtime/trace
+	< net/http/pprof;
+
+	# RPC
+	encoding/gob, encoding/json, go/token, html/template, net/http
+	< net/rpc
+	< net/rpc/jsonrpc;
+
+	# System Information
+	internal/cpu, sync
+	< internal/sysinfo;
+
+	# Test-only
+	log
+	< testing/iotest
+	< testing/fstest;
+
+	FMT, flag, math/rand
+	< testing/quick;
+
+	FMT, flag, runtime/debug, runtime/trace, internal/sysinfo
+	< testing;
+
+	internal/testlog, runtime/pprof, regexp
+	< testing/internal/testdeps;
+
+	OS, flag, testing, internal/cfg
+	< internal/testenv;
+
+	OS, encoding/base64
+	< internal/obscuretestdata;
+
+	CGO, OS, fmt
+	< os/signal/internal/pty;
+
+	NET, testing, math/rand
+	< golang.org/x/net/nettest;
+
+	FMT, container/heap, math/rand
+	< internal/trace;
+`
+
+// listStdPkgs returns the same list of packages as "go list std".
+func listStdPkgs(goroot string) ([]string, error) {
+	// Based on cmd/go's matchPackages function.
+	var pkgs []string
+
+	src := filepath.Join(goroot, "src") + string(filepath.Separator)
+	walkFn := func(path string, d fs.DirEntry, err error) error {
+		if err != nil || !d.IsDir() || path == src {
+			return nil
+		}
+
+		base := filepath.Base(path)
+		if strings.HasPrefix(base, ".") || strings.HasPrefix(base, "_") || base == "testdata" {
+			return filepath.SkipDir
+		}
+
+		name := filepath.ToSlash(path[len(src):])
+		if name == "builtin" || name == "cmd" {
+			return filepath.SkipDir
+		}
+
+		pkgs = append(pkgs, strings.TrimPrefix(name, "vendor/"))
+		return nil
+	}
+	if err := filepath.WalkDir(src, walkFn); err != nil {
+		return nil, err
+	}
+	return pkgs, nil
+}
+
+func TestDependencies(t *testing.T) {
+	if !testenv.HasSrc() {
+		// Tests run in a limited file system and we do not
+		// provide access to every source file.
+		t.Skipf("skipping on %s/%s, missing full GOROOT", runtime.GOOS, runtime.GOARCH)
+	}
+
+	ctxt := Default
+	all, err := listStdPkgs(ctxt.GOROOT)
+	if err != nil {
+		t.Fatal(err)
+	}
+	sort.Strings(all)
+
+	sawImport := map[string]map[string]bool{} // from package => to package => true
+	policy := depsPolicy(t)
+
+	for _, pkg := range all {
+		imports, err := findImports(pkg)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+		if sawImport[pkg] == nil {
+			sawImport[pkg] = map[string]bool{}
+		}
+		ok := policy[pkg]
+		var bad []string
+		for _, imp := range imports {
+			sawImport[pkg][imp] = true
+			if !ok[imp] {
+				bad = append(bad, imp)
+			}
+		}
+		if bad != nil {
+			t.Errorf("unexpected dependency: %s imports %v", pkg, bad)
+		}
+	}
+
+	// depPath returns the path between the given from and to packages.
+	// It returns the empty string if there's no dependency path.
+	var depPath func(string, string) string
+	depPath = func(from, to string) string {
+		if sawImport[from][to] {
+			return from + " => " + to
+		}
+		for pkg := range sawImport[from] {
+			if p := depPath(pkg, to); p != "" {
+				return from + " => " + p
+			}
+		}
+		return ""
+	}
+}
+
+var buildIgnore = []byte("\n// +build ignore")
+
+func findImports(pkg string) ([]string, error) {
+	vpkg := pkg
+	if strings.HasPrefix(pkg, "golang.org") {
+		vpkg = "vendor/" + pkg
+	}
+	dir := filepath.Join(Default.GOROOT, "src", vpkg)
+	files, err := os.ReadDir(dir)
+	if err != nil {
+		return nil, err
+	}
+	var imports []string
+	var haveImport = map[string]bool{}
+	fset := token.NewFileSet()
+	for _, file := range files {
+		name := file.Name()
+		if name == "slice_go14.go" || name == "slice_go18.go" {
+			// These files are for compiler bootstrap with older versions of Go and not built in the standard build.
+			continue
+		}
+		if !strings.HasSuffix(name, ".go") || strings.HasSuffix(name, "_test.go") {
+			continue
+		}
+		info := fileInfo{
+			name: filepath.Join(dir, name),
+			fset: fset,
+		}
+		f, err := os.Open(info.name)
+		if err != nil {
+			return nil, err
+		}
+		err = readGoInfo(f, &info)
+		f.Close()
+		if err != nil {
+			return nil, fmt.Errorf("reading %v: %v", name, err)
+		}
+		if bytes.Contains(info.header, buildIgnore) {
+			continue
+		}
+		for _, imp := range info.imports {
+			path := imp.path
+			if !haveImport[path] {
+				haveImport[path] = true
+				imports = append(imports, path)
+			}
+		}
+	}
+	sort.Strings(imports)
+	return imports, nil
+}
+
+// depsPolicy returns a map m such that m[p][d] == true when p can import d.
+func depsPolicy(t *testing.T) map[string]map[string]bool {
+	allowed := map[string]map[string]bool{"NONE": {}}
+	disallowed := [][2][]string{}
+
+	parseDepsRules(t, func(deps []string, op string, users []string) {
+		if op == "!<" {
+			disallowed = append(disallowed, [2][]string{deps, users})
+			return
+		}
+		for _, u := range users {
+			if allowed[u] != nil {
+				t.Errorf("multiple deps lists for %s", u)
+			}
+			allowed[u] = make(map[string]bool)
+			for _, d := range deps {
+				if allowed[d] == nil {
+					t.Errorf("use of %s before its deps list", d)
+				}
+				allowed[u][d] = true
+			}
+		}
+	})
+
+	// Check for missing deps info.
+	for _, deps := range allowed {
+		for d := range deps {
+			if allowed[d] == nil {
+				t.Errorf("missing deps list for %s", d)
+			}
+		}
+	}
+
+	// Complete transitive allowed deps.
+	for k := range allowed {
+		for i := range allowed {
+			for j := range allowed {
+				if i != k && k != j && allowed[i][k] && allowed[k][j] {
+					if i == j {
+						// Can only happen along with a "use of X before deps" error above,
+						// but this error is more specific - it makes clear that reordering the
+						// rules will not be enough to fix the problem.
+						t.Errorf("deps policy cycle: %s < %s < %s", j, k, i)
+					}
+					allowed[i][j] = true
+				}
+			}
+		}
+	}
+
+	// Check negative assertions against completed allowed deps.
+	for _, bad := range disallowed {
+		deps, users := bad[0], bad[1]
+		for _, d := range deps {
+			for _, u := range users {
+				if allowed[u][d] {
+					t.Errorf("deps policy incorrect: assertion failed: %s !< %s", d, u)
+				}
+			}
+		}
+	}
+
+	if t.Failed() {
+		t.FailNow()
+	}
+
+	return allowed
+}
+
+// parseDepsRules parses depsRules, calling save(deps, op, users)
+// for each deps < users or deps !< users rule
+// (op is "<" or "!<").
+func parseDepsRules(t *testing.T, save func(deps []string, op string, users []string)) {
+	p := &depsParser{t: t, lineno: 1, text: depsRules}
+
+	var prev []string
+	var op string
+	for {
+		list, tok := p.nextList()
+		if tok == "" {
+			if prev == nil {
+				break
+			}
+			p.syntaxError("unexpected EOF")
+		}
+		if prev != nil {
+			save(prev, op, list)
+		}
+		prev = list
+		if tok == ";" {
+			prev = nil
+			op = ""
+			continue
+		}
+		if tok != "<" && tok != "!<" {
+			p.syntaxError("missing <")
+		}
+		op = tok
+	}
+}
+
+// A depsParser parses the depsRules syntax described above.
+type depsParser struct {
+	t        *testing.T
+	lineno   int
+	lastWord string
+	text     string
+}
+
+// syntaxError reports a parsing error.
+func (p *depsParser) syntaxError(msg string) {
+	p.t.Fatalf("deps:%d: syntax error: %s near %s", p.lineno, msg, p.lastWord)
+}
+
+// nextList parses and returns a comma-separated list of names.
+func (p *depsParser) nextList() (list []string, token string) {
+	for {
+		tok := p.nextToken()
+		switch tok {
+		case "":
+			if len(list) == 0 {
+				return nil, ""
+			}
+			fallthrough
+		case ",", "<", "!<", ";":
+			p.syntaxError("bad list syntax")
+		}
+		list = append(list, tok)
+
+		tok = p.nextToken()
+		if tok != "," {
+			return list, tok
+		}
+	}
+}
+
+// nextToken returns the next token in the deps rules,
+// one of ";" "," "<" "!<" or a name.
+func (p *depsParser) nextToken() string {
+	for {
+		if p.text == "" {
+			return ""
+		}
+		switch p.text[0] {
+		case ';', ',', '<':
+			t := p.text[:1]
+			p.text = p.text[1:]
+			return t
+
+		case '!':
+			if len(p.text) < 2 || p.text[1] != '<' {
+				p.syntaxError("unexpected token !")
+			}
+			p.text = p.text[2:]
+			return "!<"
+
+		case '#':
+			i := strings.Index(p.text, "\n")
+			if i < 0 {
+				i = len(p.text)
+			}
+			p.text = p.text[i:]
+			continue
+
+		case '\n':
+			p.lineno++
+			fallthrough
+		case ' ', '\t':
+			p.text = p.text[1:]
+			continue
+
+		default:
+			i := strings.IndexAny(p.text, "!;,<#\n \t")
+			if i < 0 {
+				i = len(p.text)
+			}
+			t := p.text[:i]
+			p.text = p.text[i:]
+			p.lastWord = t
+			return t
+		}
+	}
+}
+
+// TestStdlibLowercase tests that all standard library package names are
+// lowercase. See Issue 40065.
+func TestStdlibLowercase(t *testing.T) {
+	if !testenv.HasSrc() {
+		t.Skipf("skipping on %s/%s, missing full GOROOT", runtime.GOOS, runtime.GOARCH)
+	}
+
+	ctxt := Default
+	all, err := listStdPkgs(ctxt.GOROOT)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, pkgname := range all {
+		if strings.ToLower(pkgname) != pkgname {
+			t.Errorf("package %q should not use upper-case path", pkgname)
+		}
+	}
+}
+
+// TestFindImports tests that findImports works.  See #43249.
+func TestFindImports(t *testing.T) {
+	imports, err := findImports("go/build")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("go/build imports %q", imports)
+	want := []string{"bytes", "os", "path/filepath", "strings"}
+wantLoop:
+	for _, w := range want {
+		for _, imp := range imports {
+			if imp == w {
+				continue wantLoop
+			}
+		}
+		t.Errorf("expected to find %q in import list", w)
+	}
+}
diff --git a/src/go/build/doc.go b/src/go/build/doc.go
new file mode 100644
index 0000000..2c6f0a8
--- /dev/null
+++ b/src/go/build/doc.go
@@ -0,0 +1,98 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package build gathers information about Go packages.
+//
+// Go Path
+//
+// The Go path is a list of directory trees containing Go source code.
+// It is consulted to resolve imports that cannot be found in the standard
+// Go tree. The default path is the value of the GOPATH environment
+// variable, interpreted as a path list appropriate to the operating system
+// (on Unix, the variable is a colon-separated string;
+// on Windows, a semicolon-separated string;
+// on Plan 9, a list).
+//
+// Each directory listed in the Go path must have a prescribed structure:
+//
+// The src/ directory holds source code. The path below 'src' determines
+// the import path or executable name.
+//
+// The pkg/ directory holds installed package objects.
+// As in the Go tree, each target operating system and
+// architecture pair has its own subdirectory of pkg
+// (pkg/GOOS_GOARCH).
+//
+// If DIR is a directory listed in the Go path, a package with
+// source in DIR/src/foo/bar can be imported as "foo/bar" and
+// has its compiled form installed to "DIR/pkg/GOOS_GOARCH/foo/bar.a"
+// (or, for gccgo, "DIR/pkg/gccgo/foo/libbar.a").
+//
+// The bin/ directory holds compiled commands.
+// Each command is named for its source directory, but only
+// using the final element, not the entire path. That is, the
+// command with source in DIR/src/foo/quux is installed into
+// DIR/bin/quux, not DIR/bin/foo/quux. The foo/ is stripped
+// so that you can add DIR/bin to your PATH to get at the
+// installed commands.
+//
+// Here's an example directory layout:
+//
+//	GOPATH=/home/user/gocode
+//
+//	/home/user/gocode/
+//	    src/
+//	        foo/
+//	            bar/               (go code in package bar)
+//	                x.go
+//	            quux/              (go code in package main)
+//	                y.go
+//	    bin/
+//	        quux                   (installed command)
+//	    pkg/
+//	        linux_amd64/
+//	            foo/
+//	                bar.a          (installed package object)
+//
+// Build Constraints
+//
+// A build constraint, also known as a build tag, is a line comment that begins
+//
+// 	// +build
+//
+// that lists the conditions under which a file should be included in the
+// package. Build constraints may also be part of a file's name
+// (for example, source_windows.go will only be included if the target
+// operating system is windows).
+//
+// See 'go help buildconstraint'
+// (https://golang.org/cmd/go/#hdr-Build_constraints) for details.
+//
+// Binary-Only Packages
+//
+// In Go 1.12 and earlier, it was possible to distribute packages in binary
+// form without including the source code used for compiling the package.
+// The package was distributed with a source file not excluded by build
+// constraints and containing a "//go:binary-only-package" comment. Like a
+// build constraint, this comment appeared at the top of a file, preceded
+// only by blank lines and other line comments and with a blank line
+// following the comment, to separate it from the package documentation.
+// Unlike build constraints, this comment is only recognized in non-test
+// Go source files.
+//
+// The minimal source code for a binary-only package was therefore:
+//
+//	//go:binary-only-package
+//
+//	package mypkg
+//
+// The source code could include additional Go code. That code was never
+// compiled but would be processed by tools like godoc and might be useful
+// as end-user documentation.
+//
+// "go build" and other commands no longer support binary-only-packages.
+// Import and ImportDir will still set the BinaryOnly flag in packages
+// containing these comments for use in tools and error messages.
+//
+package build
diff --git a/src/go/build/gc.go b/src/go/build/gc.go
new file mode 100644
index 0000000..3025cd5
--- /dev/null
+++ b/src/go/build/gc.go
@@ -0,0 +1,17 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build gc
+
+package build
+
+import (
+	"path/filepath"
+	"runtime"
+)
+
+// getToolDir returns the default value of ToolDir.
+func getToolDir() string {
+	return filepath.Join(runtime.GOROOT(), "pkg/tool/"+runtime.GOOS+"_"+runtime.GOARCH)
+}
diff --git a/src/go/build/gccgo.go b/src/go/build/gccgo.go
new file mode 100644
index 0000000..c6aac9a
--- /dev/null
+++ b/src/go/build/gccgo.go
@@ -0,0 +1,14 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build gccgo
+
+package build
+
+import "runtime"
+
+// getToolDir returns the default value of ToolDir.
+func getToolDir() string {
+	return envOr("GCCGOTOOLDIR", runtime.GCCGOTOOLDIR)
+}
diff --git a/src/go/build/read.go b/src/go/build/read.go
new file mode 100644
index 0000000..aa7c6ee
--- /dev/null
+++ b/src/go/build/read.go
@@ -0,0 +1,546 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"io"
+	"strconv"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+type importReader struct {
+	b    *bufio.Reader
+	buf  []byte
+	peek byte
+	err  error
+	eof  bool
+	nerr int
+	pos  token.Position
+}
+
+func newImportReader(name string, r io.Reader) *importReader {
+	return &importReader{
+		b: bufio.NewReader(r),
+		pos: token.Position{
+			Filename: name,
+			Line:     1,
+			Column:   1,
+		},
+	}
+}
+
+func isIdent(c byte) bool {
+	return 'A' <= c && c <= 'Z' || 'a' <= c && c <= 'z' || '0' <= c && c <= '9' || c == '_' || c >= utf8.RuneSelf
+}
+
+var (
+	errSyntax = errors.New("syntax error")
+	errNUL    = errors.New("unexpected NUL in input")
+)
+
+// syntaxError records a syntax error, but only if an I/O error has not already been recorded.
+func (r *importReader) syntaxError() {
+	if r.err == nil {
+		r.err = errSyntax
+	}
+}
+
+// readByte reads the next byte from the input, saves it in buf, and returns it.
+// If an error occurs, readByte records the error in r.err and returns 0.
+func (r *importReader) readByte() byte {
+	c, err := r.b.ReadByte()
+	if err == nil {
+		r.buf = append(r.buf, c)
+		if c == 0 {
+			err = errNUL
+		}
+	}
+	if err != nil {
+		if err == io.EOF {
+			r.eof = true
+		} else if r.err == nil {
+			r.err = err
+		}
+		c = 0
+	}
+	return c
+}
+
+// readByteNoBuf is like readByte but doesn't buffer the byte.
+// It exhausts r.buf before reading from r.b.
+func (r *importReader) readByteNoBuf() byte {
+	var c byte
+	var err error
+	if len(r.buf) > 0 {
+		c = r.buf[0]
+		r.buf = r.buf[1:]
+	} else {
+		c, err = r.b.ReadByte()
+		if err == nil && c == 0 {
+			err = errNUL
+		}
+	}
+
+	if err != nil {
+		if err == io.EOF {
+			r.eof = true
+		} else if r.err == nil {
+			r.err = err
+		}
+		return 0
+	}
+	r.pos.Offset++
+	if c == '\n' {
+		r.pos.Line++
+		r.pos.Column = 1
+	} else {
+		r.pos.Column++
+	}
+	return c
+}
+
+// peekByte returns the next byte from the input reader but does not advance beyond it.
+// If skipSpace is set, peekByte skips leading spaces and comments.
+func (r *importReader) peekByte(skipSpace bool) byte {
+	if r.err != nil {
+		if r.nerr++; r.nerr > 10000 {
+			panic("go/build: import reader looping")
+		}
+		return 0
+	}
+
+	// Use r.peek as first input byte.
+	// Don't just return r.peek here: it might have been left by peekByte(false)
+	// and this might be peekByte(true).
+	c := r.peek
+	if c == 0 {
+		c = r.readByte()
+	}
+	for r.err == nil && !r.eof {
+		if skipSpace {
+			// For the purposes of this reader, semicolons are never necessary to
+			// understand the input and are treated as spaces.
+			switch c {
+			case ' ', '\f', '\t', '\r', '\n', ';':
+				c = r.readByte()
+				continue
+
+			case '/':
+				c = r.readByte()
+				if c == '/' {
+					for c != '\n' && r.err == nil && !r.eof {
+						c = r.readByte()
+					}
+				} else if c == '*' {
+					var c1 byte
+					for (c != '*' || c1 != '/') && r.err == nil {
+						if r.eof {
+							r.syntaxError()
+						}
+						c, c1 = c1, r.readByte()
+					}
+				} else {
+					r.syntaxError()
+				}
+				c = r.readByte()
+				continue
+			}
+		}
+		break
+	}
+	r.peek = c
+	return r.peek
+}
+
+// nextByte is like peekByte but advances beyond the returned byte.
+func (r *importReader) nextByte(skipSpace bool) byte {
+	c := r.peekByte(skipSpace)
+	r.peek = 0
+	return c
+}
+
+var goEmbed = []byte("go:embed")
+
+// findEmbed advances the input reader to the next //go:embed comment.
+// It reports whether it found a comment.
+// (Otherwise it found an error or EOF.)
+func (r *importReader) findEmbed(first bool) bool {
+	// The import block scan stopped after a non-space character,
+	// so the reader is not at the start of a line on the first call.
+	// After that, each //go:embed extraction leaves the reader
+	// at the end of a line.
+	startLine := !first
+	var c byte
+	for r.err == nil && !r.eof {
+		c = r.readByteNoBuf()
+	Reswitch:
+		switch c {
+		default:
+			startLine = false
+
+		case '\n':
+			startLine = true
+
+		case ' ', '\t':
+			// leave startLine alone
+
+		case '"':
+			startLine = false
+			for r.err == nil {
+				if r.eof {
+					r.syntaxError()
+				}
+				c = r.readByteNoBuf()
+				if c == '\\' {
+					r.readByteNoBuf()
+					if r.err != nil {
+						r.syntaxError()
+						return false
+					}
+					continue
+				}
+				if c == '"' {
+					c = r.readByteNoBuf()
+					goto Reswitch
+				}
+			}
+			goto Reswitch
+
+		case '`':
+			startLine = false
+			for r.err == nil {
+				if r.eof {
+					r.syntaxError()
+				}
+				c = r.readByteNoBuf()
+				if c == '`' {
+					c = r.readByteNoBuf()
+					goto Reswitch
+				}
+			}
+
+		case '/':
+			c = r.readByteNoBuf()
+			switch c {
+			default:
+				startLine = false
+				goto Reswitch
+
+			case '*':
+				var c1 byte
+				for (c != '*' || c1 != '/') && r.err == nil {
+					if r.eof {
+						r.syntaxError()
+					}
+					c, c1 = c1, r.readByteNoBuf()
+				}
+				startLine = false
+
+			case '/':
+				if startLine {
+					// Try to read this as a //go:embed comment.
+					for i := range goEmbed {
+						c = r.readByteNoBuf()
+						if c != goEmbed[i] {
+							goto SkipSlashSlash
+						}
+					}
+					c = r.readByteNoBuf()
+					if c == ' ' || c == '\t' {
+						// Found one!
+						return true
+					}
+				}
+			SkipSlashSlash:
+				for c != '\n' && r.err == nil && !r.eof {
+					c = r.readByteNoBuf()
+				}
+				startLine = true
+			}
+		}
+	}
+	return false
+}
+
+// readKeyword reads the given keyword from the input.
+// If the keyword is not present, readKeyword records a syntax error.
+func (r *importReader) readKeyword(kw string) {
+	r.peekByte(true)
+	for i := 0; i < len(kw); i++ {
+		if r.nextByte(false) != kw[i] {
+			r.syntaxError()
+			return
+		}
+	}
+	if isIdent(r.peekByte(false)) {
+		r.syntaxError()
+	}
+}
+
+// readIdent reads an identifier from the input.
+// If an identifier is not present, readIdent records a syntax error.
+func (r *importReader) readIdent() {
+	c := r.peekByte(true)
+	if !isIdent(c) {
+		r.syntaxError()
+		return
+	}
+	for isIdent(r.peekByte(false)) {
+		r.peek = 0
+	}
+}
+
+// readString reads a quoted string literal from the input.
+// If an identifier is not present, readString records a syntax error.
+func (r *importReader) readString() {
+	switch r.nextByte(true) {
+	case '`':
+		for r.err == nil {
+			if r.nextByte(false) == '`' {
+				break
+			}
+			if r.eof {
+				r.syntaxError()
+			}
+		}
+	case '"':
+		for r.err == nil {
+			c := r.nextByte(false)
+			if c == '"' {
+				break
+			}
+			if r.eof || c == '\n' {
+				r.syntaxError()
+			}
+			if c == '\\' {
+				r.nextByte(false)
+			}
+		}
+	default:
+		r.syntaxError()
+	}
+}
+
+// readImport reads an import clause - optional identifier followed by quoted string -
+// from the input.
+func (r *importReader) readImport() {
+	c := r.peekByte(true)
+	if c == '.' {
+		r.peek = 0
+	} else if isIdent(c) {
+		r.readIdent()
+	}
+	r.readString()
+}
+
+// readComments is like io.ReadAll, except that it only reads the leading
+// block of comments in the file.
+func readComments(f io.Reader) ([]byte, error) {
+	r := newImportReader("", f)
+	r.peekByte(true)
+	if r.err == nil && !r.eof {
+		// Didn't reach EOF, so must have found a non-space byte. Remove it.
+		r.buf = r.buf[:len(r.buf)-1]
+	}
+	return r.buf, r.err
+}
+
+// readGoInfo expects a Go file as input and reads the file up to and including the import section.
+// It records what it learned in *info.
+// If info.fset is non-nil, readGoInfo parses the file and sets info.parsed, info.parseErr,
+// info.imports, info.embeds, and info.embedErr.
+//
+// It only returns an error if there are problems reading the file,
+// not for syntax errors in the file itself.
+func readGoInfo(f io.Reader, info *fileInfo) error {
+	r := newImportReader(info.name, f)
+
+	r.readKeyword("package")
+	r.readIdent()
+	for r.peekByte(true) == 'i' {
+		r.readKeyword("import")
+		if r.peekByte(true) == '(' {
+			r.nextByte(false)
+			for r.peekByte(true) != ')' && r.err == nil {
+				r.readImport()
+			}
+			r.nextByte(false)
+		} else {
+			r.readImport()
+		}
+	}
+
+	info.header = r.buf
+
+	// If we stopped successfully before EOF, we read a byte that told us we were done.
+	// Return all but that last byte, which would cause a syntax error if we let it through.
+	if r.err == nil && !r.eof {
+		info.header = r.buf[:len(r.buf)-1]
+	}
+
+	// If we stopped for a syntax error, consume the whole file so that
+	// we are sure we don't change the errors that go/parser returns.
+	if r.err == errSyntax {
+		r.err = nil
+		for r.err == nil && !r.eof {
+			r.readByte()
+		}
+		info.header = r.buf
+	}
+	if r.err != nil {
+		return r.err
+	}
+
+	if info.fset == nil {
+		return nil
+	}
+
+	// Parse file header & record imports.
+	info.parsed, info.parseErr = parser.ParseFile(info.fset, info.name, info.header, parser.ImportsOnly|parser.ParseComments)
+	if info.parseErr != nil {
+		return nil
+	}
+
+	hasEmbed := false
+	for _, decl := range info.parsed.Decls {
+		d, ok := decl.(*ast.GenDecl)
+		if !ok {
+			continue
+		}
+		for _, dspec := range d.Specs {
+			spec, ok := dspec.(*ast.ImportSpec)
+			if !ok {
+				continue
+			}
+			quoted := spec.Path.Value
+			path, err := strconv.Unquote(quoted)
+			if err != nil {
+				return fmt.Errorf("parser returned invalid quoted string: <%s>", quoted)
+			}
+			if path == "embed" {
+				hasEmbed = true
+			}
+
+			doc := spec.Doc
+			if doc == nil && len(d.Specs) == 1 {
+				doc = d.Doc
+			}
+			info.imports = append(info.imports, fileImport{path, spec.Pos(), doc})
+		}
+	}
+
+	// If the file imports "embed",
+	// we have to look for //go:embed comments
+	// in the remainder of the file.
+	// The compiler will enforce the mapping of comments to
+	// declared variables. We just need to know the patterns.
+	// If there were //go:embed comments earlier in the file
+	// (near the package statement or imports), the compiler
+	// will reject them. They can be (and have already been) ignored.
+	if hasEmbed {
+		var line []byte
+		for first := true; r.findEmbed(first); first = false {
+			line = line[:0]
+			pos := r.pos
+			for {
+				c := r.readByteNoBuf()
+				if c == '\n' || r.err != nil || r.eof {
+					break
+				}
+				line = append(line, c)
+			}
+			// Add args if line is well-formed.
+			// Ignore badly-formed lines - the compiler will report them when it finds them,
+			// and we can pretend they are not there to help go list succeed with what it knows.
+			embs, err := parseGoEmbed(string(line), pos)
+			if err == nil {
+				info.embeds = append(info.embeds, embs...)
+			}
+		}
+	}
+
+	return nil
+}
+
+// parseGoEmbed parses the text following "//go:embed" to extract the glob patterns.
+// It accepts unquoted space-separated patterns as well as double-quoted and back-quoted Go strings.
+// This is based on a similar function in cmd/compile/internal/gc/noder.go;
+// this version calculates position information as well.
+func parseGoEmbed(args string, pos token.Position) ([]fileEmbed, error) {
+	trimBytes := func(n int) {
+		pos.Offset += n
+		pos.Column += utf8.RuneCountInString(args[:n])
+		args = args[n:]
+	}
+	trimSpace := func() {
+		trim := strings.TrimLeftFunc(args, unicode.IsSpace)
+		trimBytes(len(args) - len(trim))
+	}
+
+	var list []fileEmbed
+	for trimSpace(); args != ""; trimSpace() {
+		var path string
+		pathPos := pos
+	Switch:
+		switch args[0] {
+		default:
+			i := len(args)
+			for j, c := range args {
+				if unicode.IsSpace(c) {
+					i = j
+					break
+				}
+			}
+			path = args[:i]
+			trimBytes(i)
+
+		case '`':
+			i := strings.Index(args[1:], "`")
+			if i < 0 {
+				return nil, fmt.Errorf("invalid quoted string in //go:embed: %s", args)
+			}
+			path = args[1 : 1+i]
+			trimBytes(1 + i + 1)
+
+		case '"':
+			i := 1
+			for ; i < len(args); i++ {
+				if args[i] == '\\' {
+					i++
+					continue
+				}
+				if args[i] == '"' {
+					q, err := strconv.Unquote(args[:i+1])
+					if err != nil {
+						return nil, fmt.Errorf("invalid quoted string in //go:embed: %s", args[:i+1])
+					}
+					path = q
+					trimBytes(i + 1)
+					break Switch
+				}
+			}
+			if i >= len(args) {
+				return nil, fmt.Errorf("invalid quoted string in //go:embed: %s", args)
+			}
+		}
+
+		if args != "" {
+			r, _ := utf8.DecodeRuneInString(args)
+			if !unicode.IsSpace(r) {
+				return nil, fmt.Errorf("invalid quoted string in //go:embed: %s", args)
+			}
+		}
+		list = append(list, fileEmbed{path, pathPos})
+	}
+	return list, nil
+}
diff --git a/src/go/build/read_test.go b/src/go/build/read_test.go
new file mode 100644
index 0000000..32e6bae
--- /dev/null
+++ b/src/go/build/read_test.go
@@ -0,0 +1,321 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+import (
+	"fmt"
+	"go/token"
+	"io"
+	"strings"
+	"testing"
+)
+
+const quote = "`"
+
+type readTest struct {
+	// Test input contains ℙ where readGoInfo should stop.
+	in  string
+	err string
+}
+
+var readGoInfoTests = []readTest{
+	{
+		`package p`,
+		"",
+	},
+	{
+		`package p; import "x"`,
+		"",
+	},
+	{
+		`package p; import . "x"`,
+		"",
+	},
+	{
+		`package p; import "x";ℙvar x = 1`,
+		"",
+	},
+	{
+		`package p
+
+		// comment
+
+		import "x"
+		import _ "x"
+		import a "x"
+
+		/* comment */
+
+		import (
+			"x" /* comment */
+			_ "x"
+			a "x" // comment
+			` + quote + `x` + quote + `
+			_ /*comment*/ ` + quote + `x` + quote + `
+			a ` + quote + `x` + quote + `
+		)
+		import (
+		)
+		import ()
+		import()import()import()
+		import();import();import()
+
+		ℙvar x = 1
+		`,
+		"",
+	},
+}
+
+var readCommentsTests = []readTest{
+	{
+		`ℙpackage p`,
+		"",
+	},
+	{
+		`ℙpackage p; import "x"`,
+		"",
+	},
+	{
+		`ℙpackage p; import . "x"`,
+		"",
+	},
+	{
+		`// foo
+
+		/* bar */
+
+		/* quux */ // baz
+
+		/*/ zot */
+
+		// asdf
+		ℙHello, world`,
+		"",
+	},
+}
+
+func testRead(t *testing.T, tests []readTest, read func(io.Reader) ([]byte, error)) {
+	for i, tt := range tests {
+		var in, testOut string
+		j := strings.Index(tt.in, "ℙ")
+		if j < 0 {
+			in = tt.in
+			testOut = tt.in
+		} else {
+			in = tt.in[:j] + tt.in[j+len("ℙ"):]
+			testOut = tt.in[:j]
+		}
+		r := strings.NewReader(in)
+		buf, err := read(r)
+		if err != nil {
+			if tt.err == "" {
+				t.Errorf("#%d: err=%q, expected success (%q)", i, err, string(buf))
+			} else if !strings.Contains(err.Error(), tt.err) {
+				t.Errorf("#%d: err=%q, expected %q", i, err, tt.err)
+			}
+			continue
+		}
+		if tt.err != "" {
+			t.Errorf("#%d: success, expected %q", i, tt.err)
+			continue
+		}
+
+		out := string(buf)
+		if out != testOut {
+			t.Errorf("#%d: wrong output:\nhave %q\nwant %q\n", i, out, testOut)
+		}
+	}
+}
+
+func TestReadGoInfo(t *testing.T) {
+	testRead(t, readGoInfoTests, func(r io.Reader) ([]byte, error) {
+		var info fileInfo
+		err := readGoInfo(r, &info)
+		return info.header, err
+	})
+}
+
+func TestReadComments(t *testing.T) {
+	testRead(t, readCommentsTests, readComments)
+}
+
+var readFailuresTests = []readTest{
+	{
+		`package`,
+		"syntax error",
+	},
+	{
+		"package p\n\x00\nimport `math`\n",
+		"unexpected NUL in input",
+	},
+	{
+		`package p; import`,
+		"syntax error",
+	},
+	{
+		`package p; import "`,
+		"syntax error",
+	},
+	{
+		"package p; import ` \n\n",
+		"syntax error",
+	},
+	{
+		`package p; import "x`,
+		"syntax error",
+	},
+	{
+		`package p; import _`,
+		"syntax error",
+	},
+	{
+		`package p; import _ "`,
+		"syntax error",
+	},
+	{
+		`package p; import _ "x`,
+		"syntax error",
+	},
+	{
+		`package p; import .`,
+		"syntax error",
+	},
+	{
+		`package p; import . "`,
+		"syntax error",
+	},
+	{
+		`package p; import . "x`,
+		"syntax error",
+	},
+	{
+		`package p; import (`,
+		"syntax error",
+	},
+	{
+		`package p; import ("`,
+		"syntax error",
+	},
+	{
+		`package p; import ("x`,
+		"syntax error",
+	},
+	{
+		`package p; import ("x"`,
+		"syntax error",
+	},
+}
+
+func TestReadFailuresIgnored(t *testing.T) {
+	// Syntax errors should not be reported (false arg to readImports).
+	// Instead, entire file should be the output and no error.
+	// Convert tests not to return syntax errors.
+	tests := make([]readTest, len(readFailuresTests))
+	copy(tests, readFailuresTests)
+	for i := range tests {
+		tt := &tests[i]
+		if !strings.Contains(tt.err, "NUL") {
+			tt.err = ""
+		}
+	}
+	testRead(t, tests, func(r io.Reader) ([]byte, error) {
+		var info fileInfo
+		err := readGoInfo(r, &info)
+		return info.header, err
+	})
+}
+
+var readEmbedTests = []struct {
+	in, out string
+}{
+	{
+		"package p\n",
+		"",
+	},
+	{
+		"package p\nimport \"embed\"\nvar i int\n//go:embed x y z\nvar files embed.FS",
+		`test:4:12:x
+		 test:4:14:y
+		 test:4:16:z`,
+	},
+	{
+		"package p\nimport \"embed\"\nvar i int\n//go:embed x \"\\x79\" `z`\nvar files embed.FS",
+		`test:4:12:x
+		 test:4:14:y
+		 test:4:21:z`,
+	},
+	{
+		"package p\nimport \"embed\"\nvar i int\n//go:embed x y\n//go:embed z\nvar files embed.FS",
+		`test:4:12:x
+		 test:4:14:y
+		 test:5:12:z`,
+	},
+	{
+		"package p\nimport \"embed\"\nvar i int\n\t //go:embed x y\n\t //go:embed z\n\t var files embed.FS",
+		`test:4:14:x
+		 test:4:16:y
+		 test:5:14:z`,
+	},
+	{
+		"package p\nimport \"embed\"\n//go:embed x y z\nvar files embed.FS",
+		`test:3:12:x
+		 test:3:14:y
+		 test:3:16:z`,
+	},
+	{
+		"package p\nimport \"embed\"\nvar s = \"/*\"\n//go:embed x\nvar files embed.FS",
+		`test:4:12:x`,
+	},
+	{
+		`package p
+		 import "embed"
+		 var s = "\"\\\\"
+		 //go:embed x
+		 var files embed.FS`,
+		`test:4:15:x`,
+	},
+	{
+		"package p\nimport \"embed\"\nvar s = `/*`\n//go:embed x\nvar files embed.FS",
+		`test:4:12:x`,
+	},
+	{
+		"package p\nimport \"embed\"\nvar s = z/ *y\n//go:embed pointer\nvar pointer embed.FS",
+		"test:4:12:pointer",
+	},
+	{
+		"package p\n//go:embed x y z\n", // no import, no scan
+		"",
+	},
+	{
+		"package p\n//go:embed x y z\nvar files embed.FS", // no import, no scan
+		"",
+	},
+}
+
+func TestReadEmbed(t *testing.T) {
+	fset := token.NewFileSet()
+	for i, tt := range readEmbedTests {
+		info := fileInfo{
+			name: "test",
+			fset: fset,
+		}
+		err := readGoInfo(strings.NewReader(tt.in), &info)
+		if err != nil {
+			t.Errorf("#%d: %v", i, err)
+			continue
+		}
+		b := &strings.Builder{}
+		sep := ""
+		for _, emb := range info.embeds {
+			fmt.Fprintf(b, "%s%v:%s", sep, emb.pos, emb.pattern)
+			sep = "\n"
+		}
+		got := b.String()
+		want := strings.Join(strings.Fields(tt.out), "\n")
+		if got != want {
+			t.Errorf("#%d: embeds:\n%s\nwant:\n%s", i, got, want)
+		}
+	}
+}
diff --git a/src/go/build/syslist.go b/src/go/build/syslist.go
new file mode 100644
index 0000000..1275f7c
--- /dev/null
+++ b/src/go/build/syslist.go
@@ -0,0 +1,11 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+// List of past, present, and future known GOOS and GOARCH values.
+// Do not remove from this list, as these are used for go/build filename matching.
+
+const goosList = "aix android darwin dragonfly freebsd hurd illumos ios js linux nacl netbsd openbsd plan9 solaris windows zos "
+const goarchList = "386 amd64 amd64p32 arm armbe arm64 arm64be ppc64 ppc64le mips mipsle mips64 mips64le mips64p32 mips64p32le ppc riscv riscv64 s390 s390x sparc sparc64 wasm "
diff --git a/src/go/build/syslist_test.go b/src/go/build/syslist_test.go
new file mode 100644
index 0000000..2b7b4c7
--- /dev/null
+++ b/src/go/build/syslist_test.go
@@ -0,0 +1,62 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package build
+
+import (
+	"runtime"
+	"testing"
+)
+
+var (
+	thisOS    = runtime.GOOS
+	thisArch  = runtime.GOARCH
+	otherOS   = anotherOS()
+	otherArch = anotherArch()
+)
+
+func anotherOS() string {
+	if thisOS != "darwin" && thisOS != "ios" {
+		return "darwin"
+	}
+	return "linux"
+}
+
+func anotherArch() string {
+	if thisArch != "amd64" {
+		return "amd64"
+	}
+	return "386"
+}
+
+type GoodFileTest struct {
+	name   string
+	result bool
+}
+
+var tests = []GoodFileTest{
+	{"file.go", true},
+	{"file.c", true},
+	{"file_foo.go", true},
+	{"file_" + thisArch + ".go", true},
+	{"file_" + otherArch + ".go", false},
+	{"file_" + thisOS + ".go", true},
+	{"file_" + otherOS + ".go", false},
+	{"file_" + thisOS + "_" + thisArch + ".go", true},
+	{"file_" + otherOS + "_" + thisArch + ".go", false},
+	{"file_" + thisOS + "_" + otherArch + ".go", false},
+	{"file_" + otherOS + "_" + otherArch + ".go", false},
+	{"file_foo_" + thisArch + ".go", true},
+	{"file_foo_" + otherArch + ".go", false},
+	{"file_" + thisOS + ".c", true},
+	{"file_" + otherOS + ".c", false},
+}
+
+func TestGoodOSArch(t *testing.T) {
+	for _, test := range tests {
+		if Default.goodOSArchFile(test.name, make(map[string]bool)) != test.result {
+			t.Fatalf("goodOSArchFile(%q) != %v", test.name, test.result)
+		}
+	}
+}
diff --git a/src/go/build/testdata/cgo_disabled/cgo_disabled.go b/src/go/build/testdata/cgo_disabled/cgo_disabled.go
new file mode 100644
index 0000000..d1edb99
--- /dev/null
+++ b/src/go/build/testdata/cgo_disabled/cgo_disabled.go
@@ -0,0 +1,5 @@
+package cgo_disabled
+
+import "C"
+
+import _ "should/be/ignored"
diff --git a/src/go/build/testdata/cgo_disabled/empty.go b/src/go/build/testdata/cgo_disabled/empty.go
new file mode 100644
index 0000000..63afe42
--- /dev/null
+++ b/src/go/build/testdata/cgo_disabled/empty.go
@@ -0,0 +1 @@
+package cgo_disabled
diff --git a/src/go/build/testdata/doc/a_test.go b/src/go/build/testdata/doc/a_test.go
new file mode 100644
index 0000000..1c07b56
--- /dev/null
+++ b/src/go/build/testdata/doc/a_test.go
@@ -0,0 +1,2 @@
+// Doc from xtests
+package doc_test
diff --git a/src/go/build/testdata/doc/b_test.go b/src/go/build/testdata/doc/b_test.go
new file mode 100644
index 0000000..0cf1605
--- /dev/null
+++ b/src/go/build/testdata/doc/b_test.go
@@ -0,0 +1 @@
+package doc_test
diff --git a/src/go/build/testdata/doc/c_test.go b/src/go/build/testdata/doc/c_test.go
new file mode 100644
index 0000000..1025707
--- /dev/null
+++ b/src/go/build/testdata/doc/c_test.go
@@ -0,0 +1 @@
+package doc
diff --git a/src/go/build/testdata/doc/d_test.go b/src/go/build/testdata/doc/d_test.go
new file mode 100644
index 0000000..ec19564
--- /dev/null
+++ b/src/go/build/testdata/doc/d_test.go
@@ -0,0 +1,2 @@
+// Doc from regular tests.
+package doc
diff --git a/src/go/build/testdata/doc/e.go b/src/go/build/testdata/doc/e.go
new file mode 100644
index 0000000..1025707
--- /dev/null
+++ b/src/go/build/testdata/doc/e.go
@@ -0,0 +1 @@
+package doc
diff --git a/src/go/build/testdata/doc/f.go b/src/go/build/testdata/doc/f.go
new file mode 100644
index 0000000..ab1d0bc
--- /dev/null
+++ b/src/go/build/testdata/doc/f.go
@@ -0,0 +1,2 @@
+// Correct
+package doc
diff --git a/src/go/build/testdata/empty/dummy b/src/go/build/testdata/empty/dummy
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/src/go/build/testdata/empty/dummy
diff --git a/src/go/build/testdata/multi/file.go b/src/go/build/testdata/multi/file.go
new file mode 100644
index 0000000..ee946eb
--- /dev/null
+++ b/src/go/build/testdata/multi/file.go
@@ -0,0 +1,5 @@
+// Test data - not compiled.
+
+package main
+
+func main() {}
diff --git a/src/go/build/testdata/multi/file_appengine.go b/src/go/build/testdata/multi/file_appengine.go
new file mode 100644
index 0000000..4ea31e7
--- /dev/null
+++ b/src/go/build/testdata/multi/file_appengine.go
@@ -0,0 +1,5 @@
+// Test data - not compiled.
+
+package test_package
+
+func init() {}
diff --git a/src/go/build/testdata/other/file/file.go b/src/go/build/testdata/other/file/file.go
new file mode 100644
index 0000000..bbfd3e9
--- /dev/null
+++ b/src/go/build/testdata/other/file/file.go
@@ -0,0 +1,5 @@
+// Test data - not compiled.
+
+package file
+
+func F() {}
diff --git a/src/go/build/testdata/other/main.go b/src/go/build/testdata/other/main.go
new file mode 100644
index 0000000..e090435
--- /dev/null
+++ b/src/go/build/testdata/other/main.go
@@ -0,0 +1,11 @@
+// Test data - not compiled.
+
+package main
+
+import (
+	"./file"
+)
+
+func main() {
+	file.F()
+}
diff --git a/src/go/build/testdata/withvendor/src/a/b/b.go b/src/go/build/testdata/withvendor/src/a/b/b.go
new file mode 100644
index 0000000..4405d54
--- /dev/null
+++ b/src/go/build/testdata/withvendor/src/a/b/b.go
@@ -0,0 +1,3 @@
+package b
+
+import _ "c/d"
diff --git a/src/go/build/testdata/withvendor/src/a/vendor/c/d/d.go b/src/go/build/testdata/withvendor/src/a/vendor/c/d/d.go
new file mode 100644
index 0000000..142fb42
--- /dev/null
+++ b/src/go/build/testdata/withvendor/src/a/vendor/c/d/d.go
@@ -0,0 +1 @@
+package d
diff --git a/src/go/constant/example_test.go b/src/go/constant/example_test.go
new file mode 100644
index 0000000..6443ee6
--- /dev/null
+++ b/src/go/constant/example_test.go
@@ -0,0 +1,180 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package constant_test
+
+import (
+	"fmt"
+	"go/constant"
+	"go/token"
+	"math"
+	"sort"
+)
+
+func Example_complexNumbers() {
+	// Create the complex number 2.3 + 5i.
+	ar := constant.MakeFloat64(2.3)
+	ai := constant.MakeImag(constant.MakeInt64(5))
+	a := constant.BinaryOp(ar, token.ADD, ai)
+
+	// Compute (2.3 + 5i) * 11.
+	b := constant.MakeUint64(11)
+	c := constant.BinaryOp(a, token.MUL, b)
+
+	// Convert c into a complex128.
+	Ar, exact := constant.Float64Val(constant.Real(c))
+	if !exact {
+		fmt.Printf("Could not represent real part %s exactly as float64\n", constant.Real(c))
+	}
+	Ai, exact := constant.Float64Val(constant.Imag(c))
+	if !exact {
+		fmt.Printf("Could not represent imaginary part %s as exactly as float64\n", constant.Imag(c))
+	}
+	C := complex(Ar, Ai)
+
+	fmt.Println("literal", 25.3+55i)
+	fmt.Println("go/constant", c)
+	fmt.Println("complex128", C)
+
+	// Output:
+	//
+	// Could not represent real part 25.3 exactly as float64
+	// literal (25.3+55i)
+	// go/constant (25.3 + 55i)
+	// complex128 (25.299999999999997+55i)
+}
+
+func ExampleBinaryOp() {
+	// 11 / 0.5
+	a := constant.MakeUint64(11)
+	b := constant.MakeFloat64(0.5)
+	c := constant.BinaryOp(a, token.QUO, b)
+	fmt.Println(c)
+
+	// Output: 22
+}
+
+func ExampleUnaryOp() {
+	vs := []constant.Value{
+		constant.MakeBool(true),
+		constant.MakeFloat64(2.7),
+		constant.MakeUint64(42),
+	}
+
+	for i, v := range vs {
+		switch v.Kind() {
+		case constant.Bool:
+			vs[i] = constant.UnaryOp(token.NOT, v, 0)
+
+		case constant.Float:
+			vs[i] = constant.UnaryOp(token.SUB, v, 0)
+
+		case constant.Int:
+			// Use 16-bit precision.
+			// This would be equivalent to ^uint16(v).
+			vs[i] = constant.UnaryOp(token.XOR, v, 16)
+		}
+	}
+
+	for _, v := range vs {
+		fmt.Println(v)
+	}
+
+	// Output:
+	//
+	// false
+	// -2.7
+	// 65493
+}
+
+func ExampleCompare() {
+	vs := []constant.Value{
+		constant.MakeString("Z"),
+		constant.MakeString("bacon"),
+		constant.MakeString("go"),
+		constant.MakeString("Frame"),
+		constant.MakeString("defer"),
+		constant.MakeFromLiteral(`"a"`, token.STRING, 0),
+	}
+
+	sort.Slice(vs, func(i, j int) bool {
+		// Equivalent to vs[i] <= vs[j].
+		return constant.Compare(vs[i], token.LEQ, vs[j])
+	})
+
+	for _, v := range vs {
+		fmt.Println(constant.StringVal(v))
+	}
+
+	// Output:
+	//
+	// Frame
+	// Z
+	// a
+	// bacon
+	// defer
+	// go
+}
+
+func ExampleSign() {
+	zero := constant.MakeInt64(0)
+	one := constant.MakeInt64(1)
+	negOne := constant.MakeInt64(-1)
+
+	mkComplex := func(a, b constant.Value) constant.Value {
+		b = constant.MakeImag(b)
+		return constant.BinaryOp(a, token.ADD, b)
+	}
+
+	vs := []constant.Value{
+		negOne,
+		mkComplex(zero, negOne),
+		mkComplex(one, negOne),
+		mkComplex(negOne, one),
+		mkComplex(negOne, negOne),
+		zero,
+		mkComplex(zero, zero),
+		one,
+		mkComplex(zero, one),
+		mkComplex(one, one),
+	}
+
+	for _, v := range vs {
+		fmt.Printf("% d %s\n", constant.Sign(v), v)
+	}
+
+	// Output:
+	//
+	// -1 -1
+	// -1 (0 + -1i)
+	// -1 (1 + -1i)
+	// -1 (-1 + 1i)
+	// -1 (-1 + -1i)
+	//  0 0
+	//  0 (0 + 0i)
+	//  1 1
+	//  1 (0 + 1i)
+	//  1 (1 + 1i)
+}
+
+func ExampleVal() {
+	maxint := constant.MakeInt64(math.MaxInt64)
+	fmt.Printf("%v\n", constant.Val(maxint))
+
+	e := constant.MakeFloat64(math.E)
+	fmt.Printf("%v\n", constant.Val(e))
+
+	b := constant.MakeBool(true)
+	fmt.Printf("%v\n", constant.Val(b))
+
+	b = constant.Make(false)
+	fmt.Printf("%v\n", constant.Val(b))
+
+	// Output:
+	//
+	// 9223372036854775807
+	// 6121026514868073/2251799813685248
+	// true
+	// false
+}
diff --git a/src/go/constant/value.go b/src/go/constant/value.go
new file mode 100644
index 0000000..4641442
--- /dev/null
+++ b/src/go/constant/value.go
@@ -0,0 +1,1391 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package constant implements Values representing untyped
+// Go constants and their corresponding operations.
+//
+// A special Unknown value may be used when a value
+// is unknown due to an error. Operations on unknown
+// values produce unknown values unless specified
+// otherwise.
+//
+package constant
+
+import (
+	"fmt"
+	"go/token"
+	"math"
+	"math/big"
+	"strconv"
+	"strings"
+	"sync"
+	"unicode/utf8"
+)
+
+// Kind specifies the kind of value represented by a Value.
+type Kind int
+
+const (
+	// unknown values
+	Unknown Kind = iota
+
+	// non-numeric values
+	Bool
+	String
+
+	// numeric values
+	Int
+	Float
+	Complex
+)
+
+// A Value represents the value of a Go constant.
+type Value interface {
+	// Kind returns the value kind.
+	Kind() Kind
+
+	// String returns a short, quoted (human-readable) form of the value.
+	// For numeric values, the result may be an approximation;
+	// for String values the result may be a shortened string.
+	// Use ExactString for a string representing a value exactly.
+	String() string
+
+	// ExactString returns an exact, quoted (human-readable) form of the value.
+	// If the Value is of Kind String, use StringVal to obtain the unquoted string.
+	ExactString() string
+
+	// Prevent external implementations.
+	implementsValue()
+}
+
+// ----------------------------------------------------------------------------
+// Implementations
+
+// Maximum supported mantissa precision.
+// The spec requires at least 256 bits; typical implementations use 512 bits.
+const prec = 512
+
+// TODO(gri) Consider storing "error" information in an unknownVal so clients
+//           can provide better error messages. For instance, if a number is
+//           too large (incl. infinity), that could be recorded in unknownVal.
+//           See also #20583 and #42695 for use cases.
+
+type (
+	unknownVal struct{}
+	boolVal    bool
+	stringVal  struct {
+		// Lazy value: either a string (l,r==nil) or an addition (l,r!=nil).
+		mu   sync.Mutex
+		s    string
+		l, r *stringVal
+	}
+	int64Val   int64                    // Int values representable as an int64
+	intVal     struct{ val *big.Int }   // Int values not representable as an int64
+	ratVal     struct{ val *big.Rat }   // Float values representable as a fraction
+	floatVal   struct{ val *big.Float } // Float values not representable as a fraction
+	complexVal struct{ re, im Value }
+)
+
+func (unknownVal) Kind() Kind { return Unknown }
+func (boolVal) Kind() Kind    { return Bool }
+func (*stringVal) Kind() Kind { return String }
+func (int64Val) Kind() Kind   { return Int }
+func (intVal) Kind() Kind     { return Int }
+func (ratVal) Kind() Kind     { return Float }
+func (floatVal) Kind() Kind   { return Float }
+func (complexVal) Kind() Kind { return Complex }
+
+func (unknownVal) String() string { return "unknown" }
+func (x boolVal) String() string  { return strconv.FormatBool(bool(x)) }
+
+// String returns a possibly shortened quoted form of the String value.
+func (x *stringVal) String() string {
+	const maxLen = 72 // a reasonable length
+	s := strconv.Quote(x.string())
+	if utf8.RuneCountInString(s) > maxLen {
+		// The string without the enclosing quotes is greater than maxLen-2 runes
+		// long. Remove the last 3 runes (including the closing '"') by keeping
+		// only the first maxLen-3 runes; then add "...".
+		i := 0
+		for n := 0; n < maxLen-3; n++ {
+			_, size := utf8.DecodeRuneInString(s[i:])
+			i += size
+		}
+		s = s[:i] + "..."
+	}
+	return s
+}
+
+// string constructs and returns the actual string literal value.
+// If x represents an addition, then it rewrites x to be a single
+// string, to speed future calls. This lazy construction avoids
+// building different string values for all subpieces of a large
+// concatenation. See golang.org/issue/23348.
+func (x *stringVal) string() string {
+	x.mu.Lock()
+	if x.l != nil {
+		x.s = strings.Join(reverse(x.appendReverse(nil)), "")
+		x.l = nil
+		x.r = nil
+	}
+	s := x.s
+	x.mu.Unlock()
+
+	return s
+}
+
+// reverse reverses x in place and returns it.
+func reverse(x []string) []string {
+	n := len(x)
+	for i := 0; i+i < n; i++ {
+		x[i], x[n-1-i] = x[n-1-i], x[i]
+	}
+	return x
+}
+
+// appendReverse appends to list all of x's subpieces, but in reverse,
+// and returns the result. Appending the reversal allows processing
+// the right side in a recursive call and the left side in a loop.
+// Because a chain like a + b + c + d + e is actually represented
+// as ((((a + b) + c) + d) + e), the left-side loop avoids deep recursion.
+// x must be locked.
+func (x *stringVal) appendReverse(list []string) []string {
+	y := x
+	for y.r != nil {
+		y.r.mu.Lock()
+		list = y.r.appendReverse(list)
+		y.r.mu.Unlock()
+
+		l := y.l
+		if y != x {
+			y.mu.Unlock()
+		}
+		l.mu.Lock()
+		y = l
+	}
+	s := y.s
+	if y != x {
+		y.mu.Unlock()
+	}
+	return append(list, s)
+}
+
+func (x int64Val) String() string { return strconv.FormatInt(int64(x), 10) }
+func (x intVal) String() string   { return x.val.String() }
+func (x ratVal) String() string   { return rtof(x).String() }
+
+// String returns a decimal approximation of the Float value.
+func (x floatVal) String() string {
+	f := x.val
+
+	// Don't try to convert infinities (will not terminate).
+	if f.IsInf() {
+		return f.String()
+	}
+
+	// Use exact fmt formatting if in float64 range (common case):
+	// proceed if f doesn't underflow to 0 or overflow to inf.
+	if x, _ := f.Float64(); f.Sign() == 0 == (x == 0) && !math.IsInf(x, 0) {
+		return fmt.Sprintf("%.6g", x)
+	}
+
+	// Out of float64 range. Do approximate manual to decimal
+	// conversion to avoid precise but possibly slow Float
+	// formatting.
+	// f = mant * 2**exp
+	var mant big.Float
+	exp := f.MantExp(&mant) // 0.5 <= |mant| < 1.0
+
+	// approximate float64 mantissa m and decimal exponent d
+	// f ~ m * 10**d
+	m, _ := mant.Float64()                     // 0.5 <= |m| < 1.0
+	d := float64(exp) * (math.Ln2 / math.Ln10) // log_10(2)
+
+	// adjust m for truncated (integer) decimal exponent e
+	e := int64(d)
+	m *= math.Pow(10, d-float64(e))
+
+	// ensure 1 <= |m| < 10
+	switch am := math.Abs(m); {
+	case am < 1-0.5e-6:
+		// The %.6g format below rounds m to 5 digits after the
+		// decimal point. Make sure that m*10 < 10 even after
+		// rounding up: m*10 + 0.5e-5 < 10 => m < 1 - 0.5e6.
+		m *= 10
+		e--
+	case am >= 10:
+		m /= 10
+		e++
+	}
+
+	return fmt.Sprintf("%.6ge%+d", m, e)
+}
+
+func (x complexVal) String() string { return fmt.Sprintf("(%s + %si)", x.re, x.im) }
+
+func (x unknownVal) ExactString() string { return x.String() }
+func (x boolVal) ExactString() string    { return x.String() }
+func (x *stringVal) ExactString() string { return strconv.Quote(x.string()) }
+func (x int64Val) ExactString() string   { return x.String() }
+func (x intVal) ExactString() string     { return x.String() }
+
+func (x ratVal) ExactString() string {
+	r := x.val
+	if r.IsInt() {
+		return r.Num().String()
+	}
+	return r.String()
+}
+
+func (x floatVal) ExactString() string { return x.val.Text('p', 0) }
+
+func (x complexVal) ExactString() string {
+	return fmt.Sprintf("(%s + %si)", x.re.ExactString(), x.im.ExactString())
+}
+
+func (unknownVal) implementsValue() {}
+func (boolVal) implementsValue()    {}
+func (*stringVal) implementsValue() {}
+func (int64Val) implementsValue()   {}
+func (ratVal) implementsValue()     {}
+func (intVal) implementsValue()     {}
+func (floatVal) implementsValue()   {}
+func (complexVal) implementsValue() {}
+
+func newInt() *big.Int     { return new(big.Int) }
+func newRat() *big.Rat     { return new(big.Rat) }
+func newFloat() *big.Float { return new(big.Float).SetPrec(prec) }
+
+func i64toi(x int64Val) intVal   { return intVal{newInt().SetInt64(int64(x))} }
+func i64tor(x int64Val) ratVal   { return ratVal{newRat().SetInt64(int64(x))} }
+func i64tof(x int64Val) floatVal { return floatVal{newFloat().SetInt64(int64(x))} }
+func itor(x intVal) ratVal       { return ratVal{newRat().SetInt(x.val)} }
+func itof(x intVal) floatVal     { return floatVal{newFloat().SetInt(x.val)} }
+
+func rtof(x ratVal) floatVal {
+	a := newFloat().SetInt(x.val.Num())
+	b := newFloat().SetInt(x.val.Denom())
+	return floatVal{a.Quo(a, b)}
+}
+
+func vtoc(x Value) complexVal { return complexVal{x, int64Val(0)} }
+
+func makeInt(x *big.Int) Value {
+	if x.IsInt64() {
+		return int64Val(x.Int64())
+	}
+	return intVal{x}
+}
+
+// Permit fractions with component sizes up to maxExp
+// before switching to using floating-point numbers.
+const maxExp = 4 << 10
+
+func makeRat(x *big.Rat) Value {
+	a := x.Num()
+	b := x.Denom()
+	if a.BitLen() < maxExp && b.BitLen() < maxExp {
+		// ok to remain fraction
+		return ratVal{x}
+	}
+	// components too large => switch to float
+	fa := newFloat().SetInt(a)
+	fb := newFloat().SetInt(b)
+	return floatVal{fa.Quo(fa, fb)}
+}
+
+var floatVal0 = floatVal{newFloat()}
+
+func makeFloat(x *big.Float) Value {
+	// convert -0
+	if x.Sign() == 0 {
+		return floatVal0
+	}
+	if x.IsInf() {
+		return unknownVal{}
+	}
+	return floatVal{x}
+}
+
+func makeComplex(re, im Value) Value {
+	if re.Kind() == Unknown || im.Kind() == Unknown {
+		return unknownVal{}
+	}
+	return complexVal{re, im}
+}
+
+func makeFloatFromLiteral(lit string) Value {
+	if f, ok := newFloat().SetString(lit); ok {
+		if smallRat(f) {
+			// ok to use rationals
+			if f.Sign() == 0 {
+				// Issue 20228: If the float underflowed to zero, parse just "0".
+				// Otherwise, lit might contain a value with a large negative exponent,
+				// such as -6e-1886451601. As a float, that will underflow to 0,
+				// but it'll take forever to parse as a Rat.
+				lit = "0"
+			}
+			if r, ok := newRat().SetString(lit); ok {
+				return ratVal{r}
+			}
+		}
+		// otherwise use floats
+		return makeFloat(f)
+	}
+	return nil
+}
+
+// smallRat reports whether x would lead to "reasonably"-sized fraction
+// if converted to a *big.Rat.
+func smallRat(x *big.Float) bool {
+	if !x.IsInf() {
+		e := x.MantExp(nil)
+		return -maxExp < e && e < maxExp
+	}
+	return false
+}
+
+// ----------------------------------------------------------------------------
+// Factories
+
+// MakeUnknown returns the Unknown value.
+func MakeUnknown() Value { return unknownVal{} }
+
+// MakeBool returns the Bool value for b.
+func MakeBool(b bool) Value { return boolVal(b) }
+
+// MakeString returns the String value for s.
+func MakeString(s string) Value { return &stringVal{s: s} }
+
+// MakeInt64 returns the Int value for x.
+func MakeInt64(x int64) Value { return int64Val(x) }
+
+// MakeUint64 returns the Int value for x.
+func MakeUint64(x uint64) Value {
+	if x < 1<<63 {
+		return int64Val(int64(x))
+	}
+	return intVal{newInt().SetUint64(x)}
+}
+
+// MakeFloat64 returns the Float value for x.
+// If x is -0.0, the result is 0.0.
+// If x is not finite, the result is an Unknown.
+func MakeFloat64(x float64) Value {
+	if math.IsInf(x, 0) || math.IsNaN(x) {
+		return unknownVal{}
+	}
+	return ratVal{newRat().SetFloat64(x + 0)} // convert -0 to 0
+}
+
+// MakeFromLiteral returns the corresponding integer, floating-point,
+// imaginary, character, or string value for a Go literal string. The
+// tok value must be one of token.INT, token.FLOAT, token.IMAG,
+// token.CHAR, or token.STRING. The final argument must be zero.
+// If the literal string syntax is invalid, the result is an Unknown.
+func MakeFromLiteral(lit string, tok token.Token, zero uint) Value {
+	if zero != 0 {
+		panic("MakeFromLiteral called with non-zero last argument")
+	}
+
+	switch tok {
+	case token.INT:
+		if x, err := strconv.ParseInt(lit, 0, 64); err == nil {
+			return int64Val(x)
+		}
+		if x, ok := newInt().SetString(lit, 0); ok {
+			return intVal{x}
+		}
+
+	case token.FLOAT:
+		if x := makeFloatFromLiteral(lit); x != nil {
+			return x
+		}
+
+	case token.IMAG:
+		if n := len(lit); n > 0 && lit[n-1] == 'i' {
+			if im := makeFloatFromLiteral(lit[:n-1]); im != nil {
+				return makeComplex(int64Val(0), im)
+			}
+		}
+
+	case token.CHAR:
+		if n := len(lit); n >= 2 {
+			if code, _, _, err := strconv.UnquoteChar(lit[1:n-1], '\''); err == nil {
+				return MakeInt64(int64(code))
+			}
+		}
+
+	case token.STRING:
+		if s, err := strconv.Unquote(lit); err == nil {
+			return MakeString(s)
+		}
+
+	default:
+		panic(fmt.Sprintf("%v is not a valid token", tok))
+	}
+
+	return unknownVal{}
+}
+
+// ----------------------------------------------------------------------------
+// Accessors
+//
+// For unknown arguments the result is the zero value for the respective
+// accessor type, except for Sign, where the result is 1.
+
+// BoolVal returns the Go boolean value of x, which must be a Bool or an Unknown.
+// If x is Unknown, the result is false.
+func BoolVal(x Value) bool {
+	switch x := x.(type) {
+	case boolVal:
+		return bool(x)
+	case unknownVal:
+		return false
+	default:
+		panic(fmt.Sprintf("%v not a Bool", x))
+	}
+}
+
+// StringVal returns the Go string value of x, which must be a String or an Unknown.
+// If x is Unknown, the result is "".
+func StringVal(x Value) string {
+	switch x := x.(type) {
+	case *stringVal:
+		return x.string()
+	case unknownVal:
+		return ""
+	default:
+		panic(fmt.Sprintf("%v not a String", x))
+	}
+}
+
+// Int64Val returns the Go int64 value of x and whether the result is exact;
+// x must be an Int or an Unknown. If the result is not exact, its value is undefined.
+// If x is Unknown, the result is (0, false).
+func Int64Val(x Value) (int64, bool) {
+	switch x := x.(type) {
+	case int64Val:
+		return int64(x), true
+	case intVal:
+		return x.val.Int64(), false // not an int64Val and thus not exact
+	case unknownVal:
+		return 0, false
+	default:
+		panic(fmt.Sprintf("%v not an Int", x))
+	}
+}
+
+// Uint64Val returns the Go uint64 value of x and whether the result is exact;
+// x must be an Int or an Unknown. If the result is not exact, its value is undefined.
+// If x is Unknown, the result is (0, false).
+func Uint64Val(x Value) (uint64, bool) {
+	switch x := x.(type) {
+	case int64Val:
+		return uint64(x), x >= 0
+	case intVal:
+		return x.val.Uint64(), x.val.IsUint64()
+	case unknownVal:
+		return 0, false
+	default:
+		panic(fmt.Sprintf("%v not an Int", x))
+	}
+}
+
+// Float32Val is like Float64Val but for float32 instead of float64.
+func Float32Val(x Value) (float32, bool) {
+	switch x := x.(type) {
+	case int64Val:
+		f := float32(x)
+		return f, int64Val(f) == x
+	case intVal:
+		f, acc := newFloat().SetInt(x.val).Float32()
+		return f, acc == big.Exact
+	case ratVal:
+		return x.val.Float32()
+	case floatVal:
+		f, acc := x.val.Float32()
+		return f, acc == big.Exact
+	case unknownVal:
+		return 0, false
+	default:
+		panic(fmt.Sprintf("%v not a Float", x))
+	}
+}
+
+// Float64Val returns the nearest Go float64 value of x and whether the result is exact;
+// x must be numeric or an Unknown, but not Complex. For values too small (too close to 0)
+// to represent as float64, Float64Val silently underflows to 0. The result sign always
+// matches the sign of x, even for 0.
+// If x is Unknown, the result is (0, false).
+func Float64Val(x Value) (float64, bool) {
+	switch x := x.(type) {
+	case int64Val:
+		f := float64(int64(x))
+		return f, int64Val(f) == x
+	case intVal:
+		f, acc := newFloat().SetInt(x.val).Float64()
+		return f, acc == big.Exact
+	case ratVal:
+		return x.val.Float64()
+	case floatVal:
+		f, acc := x.val.Float64()
+		return f, acc == big.Exact
+	case unknownVal:
+		return 0, false
+	default:
+		panic(fmt.Sprintf("%v not a Float", x))
+	}
+}
+
+// Val returns the underlying value for a given constant. Since it returns an
+// interface, it is up to the caller to type assert the result to the expected
+// type. The possible dynamic return types are:
+//
+//    x Kind             type of result
+//    -----------------------------------------
+//    Bool               bool
+//    String             string
+//    Int                int64 or *big.Int
+//    Float              *big.Float or *big.Rat
+//    everything else    nil
+//
+func Val(x Value) interface{} {
+	switch x := x.(type) {
+	case boolVal:
+		return bool(x)
+	case *stringVal:
+		return x.string()
+	case int64Val:
+		return int64(x)
+	case intVal:
+		return x.val
+	case ratVal:
+		return x.val
+	case floatVal:
+		return x.val
+	default:
+		return nil
+	}
+}
+
+// Make returns the Value for x.
+//
+//    type of x        result Kind
+//    ----------------------------
+//    bool             Bool
+//    string           String
+//    int64            Int
+//    *big.Int         Int
+//    *big.Float       Float
+//    *big.Rat         Float
+//    anything else    Unknown
+//
+func Make(x interface{}) Value {
+	switch x := x.(type) {
+	case bool:
+		return boolVal(x)
+	case string:
+		return &stringVal{s: x}
+	case int64:
+		return int64Val(x)
+	case *big.Int:
+		return makeInt(x)
+	case *big.Rat:
+		return makeRat(x)
+	case *big.Float:
+		return makeFloat(x)
+	default:
+		return unknownVal{}
+	}
+}
+
+// BitLen returns the number of bits required to represent
+// the absolute value x in binary representation; x must be an Int or an Unknown.
+// If x is Unknown, the result is 0.
+func BitLen(x Value) int {
+	switch x := x.(type) {
+	case int64Val:
+		return i64toi(x).val.BitLen()
+	case intVal:
+		return x.val.BitLen()
+	case unknownVal:
+		return 0
+	default:
+		panic(fmt.Sprintf("%v not an Int", x))
+	}
+}
+
+// Sign returns -1, 0, or 1 depending on whether x < 0, x == 0, or x > 0;
+// x must be numeric or Unknown. For complex values x, the sign is 0 if x == 0,
+// otherwise it is != 0. If x is Unknown, the result is 1.
+func Sign(x Value) int {
+	switch x := x.(type) {
+	case int64Val:
+		switch {
+		case x < 0:
+			return -1
+		case x > 0:
+			return 1
+		}
+		return 0
+	case intVal:
+		return x.val.Sign()
+	case ratVal:
+		return x.val.Sign()
+	case floatVal:
+		return x.val.Sign()
+	case complexVal:
+		return Sign(x.re) | Sign(x.im)
+	case unknownVal:
+		return 1 // avoid spurious division by zero errors
+	default:
+		panic(fmt.Sprintf("%v not numeric", x))
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Support for assembling/disassembling numeric values
+
+const (
+	// Compute the size of a Word in bytes.
+	_m       = ^big.Word(0)
+	_log     = _m>>8&1 + _m>>16&1 + _m>>32&1
+	wordSize = 1 << _log
+)
+
+// Bytes returns the bytes for the absolute value of x in little-
+// endian binary representation; x must be an Int.
+func Bytes(x Value) []byte {
+	var t intVal
+	switch x := x.(type) {
+	case int64Val:
+		t = i64toi(x)
+	case intVal:
+		t = x
+	default:
+		panic(fmt.Sprintf("%v not an Int", x))
+	}
+
+	words := t.val.Bits()
+	bytes := make([]byte, len(words)*wordSize)
+
+	i := 0
+	for _, w := range words {
+		for j := 0; j < wordSize; j++ {
+			bytes[i] = byte(w)
+			w >>= 8
+			i++
+		}
+	}
+	// remove leading 0's
+	for i > 0 && bytes[i-1] == 0 {
+		i--
+	}
+
+	return bytes[:i]
+}
+
+// MakeFromBytes returns the Int value given the bytes of its little-endian
+// binary representation. An empty byte slice argument represents 0.
+func MakeFromBytes(bytes []byte) Value {
+	words := make([]big.Word, (len(bytes)+(wordSize-1))/wordSize)
+
+	i := 0
+	var w big.Word
+	var s uint
+	for _, b := range bytes {
+		w |= big.Word(b) << s
+		if s += 8; s == wordSize*8 {
+			words[i] = w
+			i++
+			w = 0
+			s = 0
+		}
+	}
+	// store last word
+	if i < len(words) {
+		words[i] = w
+		i++
+	}
+	// remove leading 0's
+	for i > 0 && words[i-1] == 0 {
+		i--
+	}
+
+	return makeInt(newInt().SetBits(words[:i]))
+}
+
+// Num returns the numerator of x; x must be Int, Float, or Unknown.
+// If x is Unknown, or if it is too large or small to represent as a
+// fraction, the result is Unknown. Otherwise the result is an Int
+// with the same sign as x.
+func Num(x Value) Value {
+	switch x := x.(type) {
+	case int64Val, intVal:
+		return x
+	case ratVal:
+		return makeInt(x.val.Num())
+	case floatVal:
+		if smallRat(x.val) {
+			r, _ := x.val.Rat(nil)
+			return makeInt(r.Num())
+		}
+	case unknownVal:
+		break
+	default:
+		panic(fmt.Sprintf("%v not Int or Float", x))
+	}
+	return unknownVal{}
+}
+
+// Denom returns the denominator of x; x must be Int, Float, or Unknown.
+// If x is Unknown, or if it is too large or small to represent as a
+// fraction, the result is Unknown. Otherwise the result is an Int >= 1.
+func Denom(x Value) Value {
+	switch x := x.(type) {
+	case int64Val, intVal:
+		return int64Val(1)
+	case ratVal:
+		return makeInt(x.val.Denom())
+	case floatVal:
+		if smallRat(x.val) {
+			r, _ := x.val.Rat(nil)
+			return makeInt(r.Denom())
+		}
+	case unknownVal:
+		break
+	default:
+		panic(fmt.Sprintf("%v not Int or Float", x))
+	}
+	return unknownVal{}
+}
+
+// MakeImag returns the Complex value x*i;
+// x must be Int, Float, or Unknown.
+// If x is Unknown, the result is Unknown.
+func MakeImag(x Value) Value {
+	switch x.(type) {
+	case unknownVal:
+		return x
+	case int64Val, intVal, ratVal, floatVal:
+		return makeComplex(int64Val(0), x)
+	default:
+		panic(fmt.Sprintf("%v not Int or Float", x))
+	}
+}
+
+// Real returns the real part of x, which must be a numeric or unknown value.
+// If x is Unknown, the result is Unknown.
+func Real(x Value) Value {
+	switch x := x.(type) {
+	case unknownVal, int64Val, intVal, ratVal, floatVal:
+		return x
+	case complexVal:
+		return x.re
+	default:
+		panic(fmt.Sprintf("%v not numeric", x))
+	}
+}
+
+// Imag returns the imaginary part of x, which must be a numeric or unknown value.
+// If x is Unknown, the result is Unknown.
+func Imag(x Value) Value {
+	switch x := x.(type) {
+	case unknownVal:
+		return x
+	case int64Val, intVal, ratVal, floatVal:
+		return int64Val(0)
+	case complexVal:
+		return x.im
+	default:
+		panic(fmt.Sprintf("%v not numeric", x))
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Numeric conversions
+
+// ToInt converts x to an Int value if x is representable as an Int.
+// Otherwise it returns an Unknown.
+func ToInt(x Value) Value {
+	switch x := x.(type) {
+	case int64Val, intVal:
+		return x
+
+	case ratVal:
+		if x.val.IsInt() {
+			return makeInt(x.val.Num())
+		}
+
+	case floatVal:
+		// avoid creation of huge integers
+		// (Existing tests require permitting exponents of at least 1024;
+		// allow any value that would also be permissible as a fraction.)
+		if smallRat(x.val) {
+			i := newInt()
+			if _, acc := x.val.Int(i); acc == big.Exact {
+				return makeInt(i)
+			}
+
+			// If we can get an integer by rounding up or down,
+			// assume x is not an integer because of rounding
+			// errors in prior computations.
+
+			const delta = 4 // a small number of bits > 0
+			var t big.Float
+			t.SetPrec(prec - delta)
+
+			// try rounding down a little
+			t.SetMode(big.ToZero)
+			t.Set(x.val)
+			if _, acc := t.Int(i); acc == big.Exact {
+				return makeInt(i)
+			}
+
+			// try rounding up a little
+			t.SetMode(big.AwayFromZero)
+			t.Set(x.val)
+			if _, acc := t.Int(i); acc == big.Exact {
+				return makeInt(i)
+			}
+		}
+
+	case complexVal:
+		if re := ToFloat(x); re.Kind() == Float {
+			return ToInt(re)
+		}
+	}
+
+	return unknownVal{}
+}
+
+// ToFloat converts x to a Float value if x is representable as a Float.
+// Otherwise it returns an Unknown.
+func ToFloat(x Value) Value {
+	switch x := x.(type) {
+	case int64Val:
+		return i64tof(x)
+	case intVal:
+		return itof(x)
+	case ratVal, floatVal:
+		return x
+	case complexVal:
+		if im := ToInt(x.im); im.Kind() == Int && Sign(im) == 0 {
+			// imaginary component is 0
+			return ToFloat(x.re)
+		}
+	}
+	return unknownVal{}
+}
+
+// ToComplex converts x to a Complex value if x is representable as a Complex.
+// Otherwise it returns an Unknown.
+func ToComplex(x Value) Value {
+	switch x := x.(type) {
+	case int64Val:
+		return vtoc(i64tof(x))
+	case intVal:
+		return vtoc(itof(x))
+	case ratVal:
+		return vtoc(x)
+	case floatVal:
+		return vtoc(x)
+	case complexVal:
+		return x
+	}
+	return unknownVal{}
+}
+
+// ----------------------------------------------------------------------------
+// Operations
+
+// is32bit reports whether x can be represented using 32 bits.
+func is32bit(x int64) bool {
+	const s = 32
+	return -1<<(s-1) <= x && x <= 1<<(s-1)-1
+}
+
+// is63bit reports whether x can be represented using 63 bits.
+func is63bit(x int64) bool {
+	const s = 63
+	return -1<<(s-1) <= x && x <= 1<<(s-1)-1
+}
+
+// UnaryOp returns the result of the unary expression op y.
+// The operation must be defined for the operand.
+// If prec > 0 it specifies the ^ (xor) result size in bits.
+// If y is Unknown, the result is Unknown.
+//
+func UnaryOp(op token.Token, y Value, prec uint) Value {
+	switch op {
+	case token.ADD:
+		switch y.(type) {
+		case unknownVal, int64Val, intVal, ratVal, floatVal, complexVal:
+			return y
+		}
+
+	case token.SUB:
+		switch y := y.(type) {
+		case unknownVal:
+			return y
+		case int64Val:
+			if z := -y; z != y {
+				return z // no overflow
+			}
+			return makeInt(newInt().Neg(big.NewInt(int64(y))))
+		case intVal:
+			return makeInt(newInt().Neg(y.val))
+		case ratVal:
+			return makeRat(newRat().Neg(y.val))
+		case floatVal:
+			return makeFloat(newFloat().Neg(y.val))
+		case complexVal:
+			re := UnaryOp(token.SUB, y.re, 0)
+			im := UnaryOp(token.SUB, y.im, 0)
+			return makeComplex(re, im)
+		}
+
+	case token.XOR:
+		z := newInt()
+		switch y := y.(type) {
+		case unknownVal:
+			return y
+		case int64Val:
+			z.Not(big.NewInt(int64(y)))
+		case intVal:
+			z.Not(y.val)
+		default:
+			goto Error
+		}
+		// For unsigned types, the result will be negative and
+		// thus "too large": We must limit the result precision
+		// to the type's precision.
+		if prec > 0 {
+			z.AndNot(z, newInt().Lsh(big.NewInt(-1), prec)) // z &^= (-1)<<prec
+		}
+		return makeInt(z)
+
+	case token.NOT:
+		switch y := y.(type) {
+		case unknownVal:
+			return y
+		case boolVal:
+			return !y
+		}
+	}
+
+Error:
+	panic(fmt.Sprintf("invalid unary operation %s%v", op, y))
+}
+
+func ord(x Value) int {
+	switch x.(type) {
+	default:
+		// force invalid value into "x position" in match
+		// (don't panic here so that callers can provide a better error message)
+		return -1
+	case unknownVal:
+		return 0
+	case boolVal, *stringVal:
+		return 1
+	case int64Val:
+		return 2
+	case intVal:
+		return 3
+	case ratVal:
+		return 4
+	case floatVal:
+		return 5
+	case complexVal:
+		return 6
+	}
+}
+
+// match returns the matching representation (same type) with the
+// smallest complexity for two values x and y. If one of them is
+// numeric, both of them must be numeric. If one of them is Unknown
+// or invalid (say, nil) both results are that value.
+//
+func match(x, y Value) (_, _ Value) {
+	if ord(x) > ord(y) {
+		y, x = match(y, x)
+		return x, y
+	}
+	// ord(x) <= ord(y)
+
+	switch x := x.(type) {
+	case boolVal, *stringVal, complexVal:
+		return x, y
+
+	case int64Val:
+		switch y := y.(type) {
+		case int64Val:
+			return x, y
+		case intVal:
+			return i64toi(x), y
+		case ratVal:
+			return i64tor(x), y
+		case floatVal:
+			return i64tof(x), y
+		case complexVal:
+			return vtoc(x), y
+		}
+
+	case intVal:
+		switch y := y.(type) {
+		case intVal:
+			return x, y
+		case ratVal:
+			return itor(x), y
+		case floatVal:
+			return itof(x), y
+		case complexVal:
+			return vtoc(x), y
+		}
+
+	case ratVal:
+		switch y := y.(type) {
+		case ratVal:
+			return x, y
+		case floatVal:
+			return rtof(x), y
+		case complexVal:
+			return vtoc(x), y
+		}
+
+	case floatVal:
+		switch y := y.(type) {
+		case floatVal:
+			return x, y
+		case complexVal:
+			return vtoc(x), y
+		}
+	}
+
+	// force unknown and invalid values into "x position" in callers of match
+	// (don't panic here so that callers can provide a better error message)
+	return x, x
+}
+
+// BinaryOp returns the result of the binary expression x op y.
+// The operation must be defined for the operands. If one of the
+// operands is Unknown, the result is Unknown.
+// BinaryOp doesn't handle comparisons or shifts; use Compare
+// or Shift instead.
+//
+// To force integer division of Int operands, use op == token.QUO_ASSIGN
+// instead of token.QUO; the result is guaranteed to be Int in this case.
+// Division by zero leads to a run-time panic.
+//
+func BinaryOp(x_ Value, op token.Token, y_ Value) Value {
+	x, y := match(x_, y_)
+
+	switch x := x.(type) {
+	case unknownVal:
+		return x
+
+	case boolVal:
+		y := y.(boolVal)
+		switch op {
+		case token.LAND:
+			return x && y
+		case token.LOR:
+			return x || y
+		}
+
+	case int64Val:
+		a := int64(x)
+		b := int64(y.(int64Val))
+		var c int64
+		switch op {
+		case token.ADD:
+			if !is63bit(a) || !is63bit(b) {
+				return makeInt(newInt().Add(big.NewInt(a), big.NewInt(b)))
+			}
+			c = a + b
+		case token.SUB:
+			if !is63bit(a) || !is63bit(b) {
+				return makeInt(newInt().Sub(big.NewInt(a), big.NewInt(b)))
+			}
+			c = a - b
+		case token.MUL:
+			if !is32bit(a) || !is32bit(b) {
+				return makeInt(newInt().Mul(big.NewInt(a), big.NewInt(b)))
+			}
+			c = a * b
+		case token.QUO:
+			return makeRat(big.NewRat(a, b))
+		case token.QUO_ASSIGN: // force integer division
+			c = a / b
+		case token.REM:
+			c = a % b
+		case token.AND:
+			c = a & b
+		case token.OR:
+			c = a | b
+		case token.XOR:
+			c = a ^ b
+		case token.AND_NOT:
+			c = a &^ b
+		default:
+			goto Error
+		}
+		return int64Val(c)
+
+	case intVal:
+		a := x.val
+		b := y.(intVal).val
+		c := newInt()
+		switch op {
+		case token.ADD:
+			c.Add(a, b)
+		case token.SUB:
+			c.Sub(a, b)
+		case token.MUL:
+			c.Mul(a, b)
+		case token.QUO:
+			return makeRat(newRat().SetFrac(a, b))
+		case token.QUO_ASSIGN: // force integer division
+			c.Quo(a, b)
+		case token.REM:
+			c.Rem(a, b)
+		case token.AND:
+			c.And(a, b)
+		case token.OR:
+			c.Or(a, b)
+		case token.XOR:
+			c.Xor(a, b)
+		case token.AND_NOT:
+			c.AndNot(a, b)
+		default:
+			goto Error
+		}
+		return makeInt(c)
+
+	case ratVal:
+		a := x.val
+		b := y.(ratVal).val
+		c := newRat()
+		switch op {
+		case token.ADD:
+			c.Add(a, b)
+		case token.SUB:
+			c.Sub(a, b)
+		case token.MUL:
+			c.Mul(a, b)
+		case token.QUO:
+			c.Quo(a, b)
+		default:
+			goto Error
+		}
+		return makeRat(c)
+
+	case floatVal:
+		a := x.val
+		b := y.(floatVal).val
+		c := newFloat()
+		switch op {
+		case token.ADD:
+			c.Add(a, b)
+		case token.SUB:
+			c.Sub(a, b)
+		case token.MUL:
+			c.Mul(a, b)
+		case token.QUO:
+			c.Quo(a, b)
+		default:
+			goto Error
+		}
+		return makeFloat(c)
+
+	case complexVal:
+		y := y.(complexVal)
+		a, b := x.re, x.im
+		c, d := y.re, y.im
+		var re, im Value
+		switch op {
+		case token.ADD:
+			// (a+c) + i(b+d)
+			re = add(a, c)
+			im = add(b, d)
+		case token.SUB:
+			// (a-c) + i(b-d)
+			re = sub(a, c)
+			im = sub(b, d)
+		case token.MUL:
+			// (ac-bd) + i(bc+ad)
+			ac := mul(a, c)
+			bd := mul(b, d)
+			bc := mul(b, c)
+			ad := mul(a, d)
+			re = sub(ac, bd)
+			im = add(bc, ad)
+		case token.QUO:
+			// (ac+bd)/s + i(bc-ad)/s, with s = cc + dd
+			ac := mul(a, c)
+			bd := mul(b, d)
+			bc := mul(b, c)
+			ad := mul(a, d)
+			cc := mul(c, c)
+			dd := mul(d, d)
+			s := add(cc, dd)
+			re = add(ac, bd)
+			re = quo(re, s)
+			im = sub(bc, ad)
+			im = quo(im, s)
+		default:
+			goto Error
+		}
+		return makeComplex(re, im)
+
+	case *stringVal:
+		if op == token.ADD {
+			return &stringVal{l: x, r: y.(*stringVal)}
+		}
+	}
+
+Error:
+	panic(fmt.Sprintf("invalid binary operation %v %s %v", x_, op, y_))
+}
+
+func add(x, y Value) Value { return BinaryOp(x, token.ADD, y) }
+func sub(x, y Value) Value { return BinaryOp(x, token.SUB, y) }
+func mul(x, y Value) Value { return BinaryOp(x, token.MUL, y) }
+func quo(x, y Value) Value { return BinaryOp(x, token.QUO, y) }
+
+// Shift returns the result of the shift expression x op s
+// with op == token.SHL or token.SHR (<< or >>). x must be
+// an Int or an Unknown. If x is Unknown, the result is x.
+//
+func Shift(x Value, op token.Token, s uint) Value {
+	switch x := x.(type) {
+	case unknownVal:
+		return x
+
+	case int64Val:
+		if s == 0 {
+			return x
+		}
+		switch op {
+		case token.SHL:
+			z := i64toi(x).val
+			return makeInt(z.Lsh(z, s))
+		case token.SHR:
+			return x >> s
+		}
+
+	case intVal:
+		if s == 0 {
+			return x
+		}
+		z := newInt()
+		switch op {
+		case token.SHL:
+			return makeInt(z.Lsh(x.val, s))
+		case token.SHR:
+			return makeInt(z.Rsh(x.val, s))
+		}
+	}
+
+	panic(fmt.Sprintf("invalid shift %v %s %d", x, op, s))
+}
+
+func cmpZero(x int, op token.Token) bool {
+	switch op {
+	case token.EQL:
+		return x == 0
+	case token.NEQ:
+		return x != 0
+	case token.LSS:
+		return x < 0
+	case token.LEQ:
+		return x <= 0
+	case token.GTR:
+		return x > 0
+	case token.GEQ:
+		return x >= 0
+	}
+	panic(fmt.Sprintf("invalid comparison %v %s 0", x, op))
+}
+
+// Compare returns the result of the comparison x op y.
+// The comparison must be defined for the operands.
+// If one of the operands is Unknown, the result is
+// false.
+//
+func Compare(x_ Value, op token.Token, y_ Value) bool {
+	x, y := match(x_, y_)
+
+	switch x := x.(type) {
+	case unknownVal:
+		return false
+
+	case boolVal:
+		y := y.(boolVal)
+		switch op {
+		case token.EQL:
+			return x == y
+		case token.NEQ:
+			return x != y
+		}
+
+	case int64Val:
+		y := y.(int64Val)
+		switch op {
+		case token.EQL:
+			return x == y
+		case token.NEQ:
+			return x != y
+		case token.LSS:
+			return x < y
+		case token.LEQ:
+			return x <= y
+		case token.GTR:
+			return x > y
+		case token.GEQ:
+			return x >= y
+		}
+
+	case intVal:
+		return cmpZero(x.val.Cmp(y.(intVal).val), op)
+
+	case ratVal:
+		return cmpZero(x.val.Cmp(y.(ratVal).val), op)
+
+	case floatVal:
+		return cmpZero(x.val.Cmp(y.(floatVal).val), op)
+
+	case complexVal:
+		y := y.(complexVal)
+		re := Compare(x.re, token.EQL, y.re)
+		im := Compare(x.im, token.EQL, y.im)
+		switch op {
+		case token.EQL:
+			return re && im
+		case token.NEQ:
+			return !re || !im
+		}
+
+	case *stringVal:
+		xs := x.string()
+		ys := y.(*stringVal).string()
+		switch op {
+		case token.EQL:
+			return xs == ys
+		case token.NEQ:
+			return xs != ys
+		case token.LSS:
+			return xs < ys
+		case token.LEQ:
+			return xs <= ys
+		case token.GTR:
+			return xs > ys
+		case token.GEQ:
+			return xs >= ys
+		}
+	}
+
+	panic(fmt.Sprintf("invalid comparison %v %s %v", x_, op, y_))
+}
diff --git a/src/go/constant/value_test.go b/src/go/constant/value_test.go
new file mode 100644
index 0000000..2866774
--- /dev/null
+++ b/src/go/constant/value_test.go
@@ -0,0 +1,708 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package constant
+
+import (
+	"fmt"
+	"go/token"
+	"math"
+	"math/big"
+	"strings"
+	"testing"
+)
+
+var intTests = []string{
+	// 0-octals
+	`0_123 = 0123`,
+	`0123_456 = 0123456`,
+
+	// decimals
+	`1_234 = 1234`,
+	`1_234_567 = 1234567`,
+
+	// hexadecimals
+	`0X_0 = 0`,
+	`0X_1234 = 0x1234`,
+	`0X_CAFE_f00d = 0xcafef00d`,
+
+	// octals
+	`0o0 = 0`,
+	`0o1234 = 01234`,
+	`0o01234567 = 01234567`,
+
+	`0O0 = 0`,
+	`0O1234 = 01234`,
+	`0O01234567 = 01234567`,
+
+	`0o_0 = 0`,
+	`0o_1234 = 01234`,
+	`0o0123_4567 = 01234567`,
+
+	`0O_0 = 0`,
+	`0O_1234 = 01234`,
+	`0O0123_4567 = 01234567`,
+
+	// binaries
+	`0b0 = 0`,
+	`0b1011 = 0xb`,
+	`0b00101101 = 0x2d`,
+
+	`0B0 = 0`,
+	`0B1011 = 0xb`,
+	`0B00101101 = 0x2d`,
+
+	`0b_0 = 0`,
+	`0b10_11 = 0xb`,
+	`0b_0010_1101 = 0x2d`,
+}
+
+// The RHS operand may be a floating-point quotient n/d of two integer values n and d.
+var floatTests = []string{
+	// decimal floats
+	`1_2_3. = 123.`,
+	`0_123. = 123.`,
+
+	`0_0e0 = 0.`,
+	`1_2_3e0 = 123.`,
+	`0_123e0 = 123.`,
+
+	`0e-0_0 = 0.`,
+	`1_2_3E+0 = 123.`,
+	`0123E1_2_3 = 123e123`,
+
+	`0.e+1 = 0.`,
+	`123.E-1_0 = 123e-10`,
+	`01_23.e123 = 123e123`,
+
+	`.0e-1 = .0`,
+	`.123E+10 = .123e10`,
+	`.0123E123 = .0123e123`,
+
+	`1_2_3.123 = 123.123`,
+	`0123.01_23 = 123.0123`,
+
+	`1e-1000000000 = 0`,
+	`1e+1000000000 = ?`,
+	`6e5518446744 = ?`,
+	`-6e5518446744 = ?`,
+
+	// hexadecimal floats
+	`0x0.p+0 = 0.`,
+	`0Xdeadcafe.p-10 = 0xdeadcafe/1024`,
+	`0x1234.P84 = 0x1234000000000000000000000`,
+
+	`0x.1p-0 = 1/16`,
+	`0X.deadcafep4 = 0xdeadcafe/0x10000000`,
+	`0x.1234P+12 = 0x1234/0x10`,
+
+	`0x0p0 = 0.`,
+	`0Xdeadcafep+1 = 0x1bd5b95fc`,
+	`0x1234P-10 = 0x1234/1024`,
+
+	`0x0.0p0 = 0.`,
+	`0Xdead.cafep+1 = 0x1bd5b95fc/0x10000`,
+	`0x12.34P-10 = 0x1234/0x40000`,
+
+	`0Xdead_cafep+1 = 0xdeadcafep+1`,
+	`0x_1234P-10 = 0x1234p-10`,
+
+	`0X_dead_cafe.p-10 = 0xdeadcafe.p-10`,
+	`0x12_34.P1_2_3 = 0x1234.p123`,
+}
+
+var imagTests = []string{
+	`1_234i = 1234i`,
+	`1_234_567i = 1234567i`,
+
+	`0.i = 0i`,
+	`123.i = 123i`,
+	`0123.i = 123i`,
+
+	`0.e+1i = 0i`,
+	`123.E-1_0i = 123e-10i`,
+	`01_23.e123i = 123e123i`,
+
+	`1e-1000000000i = 0i`,
+	`1e+1000000000i = ?`,
+	`6e5518446744i = ?`,
+	`-6e5518446744i = ?`,
+}
+
+func testNumbers(t *testing.T, kind token.Token, tests []string) {
+	for _, test := range tests {
+		a := strings.Split(test, " = ")
+		if len(a) != 2 {
+			t.Errorf("invalid test case: %s", test)
+			continue
+		}
+
+		x := MakeFromLiteral(a[0], kind, 0)
+		var y Value
+		if a[1] == "?" {
+			y = MakeUnknown()
+		} else {
+			if i := strings.Index(a[1], "/"); i >= 0 && kind == token.FLOAT {
+				n := MakeFromLiteral(a[1][:i], token.INT, 0)
+				d := MakeFromLiteral(a[1][i+1:], token.INT, 0)
+				y = BinaryOp(n, token.QUO, d)
+			} else {
+				y = MakeFromLiteral(a[1], kind, 0)
+			}
+			if y.Kind() == Unknown {
+				panic(fmt.Sprintf("invalid test case: %s %d", test, y.Kind()))
+			}
+		}
+
+		xk := x.Kind()
+		yk := y.Kind()
+		if xk != yk {
+			t.Errorf("%s: got kind %d != %d", test, xk, yk)
+			continue
+		}
+
+		if yk == Unknown {
+			continue
+		}
+
+		if !Compare(x, token.EQL, y) {
+			t.Errorf("%s: %s != %s", test, x, y)
+		}
+	}
+}
+
+// TestNumbers verifies that differently written literals
+// representing the same number do have the same value.
+func TestNumbers(t *testing.T) {
+	testNumbers(t, token.INT, intTests)
+	testNumbers(t, token.FLOAT, floatTests)
+	testNumbers(t, token.IMAG, imagTests)
+}
+
+var opTests = []string{
+	// unary operations
+	`+ 0 = 0`,
+	`+ ? = ?`,
+	`- 1 = -1`,
+	`- ? = ?`,
+	`^ 0 = -1`,
+	`^ ? = ?`,
+
+	`! true = false`,
+	`! false = true`,
+	`! ? = ?`,
+
+	// etc.
+
+	// binary operations
+	`"" + "" = ""`,
+	`"foo" + "" = "foo"`,
+	`"" + "bar" = "bar"`,
+	`"foo" + "bar" = "foobar"`,
+
+	`0 + 0 = 0`,
+	`0 + 0.1 = 0.1`,
+	`0 + 0.1i = 0.1i`,
+	`0.1 + 0.9 = 1`,
+	`1e100 + 1e100 = 2e100`,
+	`? + 0 = ?`,
+	`0 + ? = ?`,
+
+	`0 - 0 = 0`,
+	`0 - 0.1 = -0.1`,
+	`0 - 0.1i = -0.1i`,
+	`1e100 - 1e100 = 0`,
+	`? - 0 = ?`,
+	`0 - ? = ?`,
+
+	`0 * 0 = 0`,
+	`1 * 0.1 = 0.1`,
+	`1 * 0.1i = 0.1i`,
+	`1i * 1i = -1`,
+	`? * 0 = ?`,
+	`0 * ? = ?`,
+	`0 * 1e+1000000000 = ?`,
+
+	`0 / 0 = "division_by_zero"`,
+	`10 / 2 = 5`,
+	`5 / 3 = 5/3`,
+	`5i / 3i = 5/3`,
+	`? / 0 = ?`,
+	`0 / ? = ?`,
+	`0 * 1e+1000000000i = ?`,
+
+	`0 % 0 = "runtime_error:_integer_divide_by_zero"`, // TODO(gri) should be the same as for /
+	`10 % 3 = 1`,
+	`? % 0 = ?`,
+	`0 % ? = ?`,
+
+	`0 & 0 = 0`,
+	`12345 & 0 = 0`,
+	`0xff & 0xf = 0xf`,
+	`? & 0 = ?`,
+	`0 & ? = ?`,
+
+	`0 | 0 = 0`,
+	`12345 | 0 = 12345`,
+	`0xb | 0xa0 = 0xab`,
+	`? | 0 = ?`,
+	`0 | ? = ?`,
+
+	`0 ^ 0 = 0`,
+	`1 ^ -1 = -2`,
+	`? ^ 0 = ?`,
+	`0 ^ ? = ?`,
+
+	`0 &^ 0 = 0`,
+	`0xf &^ 1 = 0xe`,
+	`1 &^ 0xf = 0`,
+	// etc.
+
+	// shifts
+	`0 << 0 = 0`,
+	`1 << 10 = 1024`,
+	`0 >> 0 = 0`,
+	`1024 >> 10 == 1`,
+	`? << 0 == ?`,
+	`? >> 10 == ?`,
+	// etc.
+
+	// comparisons
+	`false == false = true`,
+	`false == true = false`,
+	`true == false = false`,
+	`true == true = true`,
+
+	`false != false = false`,
+	`false != true = true`,
+	`true != false = true`,
+	`true != true = false`,
+
+	`"foo" == "bar" = false`,
+	`"foo" != "bar" = true`,
+	`"foo" < "bar" = false`,
+	`"foo" <= "bar" = false`,
+	`"foo" > "bar" = true`,
+	`"foo" >= "bar" = true`,
+
+	`0 == 0 = true`,
+	`0 != 0 = false`,
+	`0 < 10 = true`,
+	`10 <= 10 = true`,
+	`0 > 10 = false`,
+	`10 >= 10 = true`,
+
+	`1/123456789 == 1/123456789 == true`,
+	`1/123456789 != 1/123456789 == false`,
+	`1/123456789 < 1/123456788 == true`,
+	`1/123456788 <= 1/123456789 == false`,
+	`0.11 > 0.11 = false`,
+	`0.11 >= 0.11 = true`,
+
+	`? == 0 = false`,
+	`? != 0 = false`,
+	`? < 10 = false`,
+	`? <= 10 = false`,
+	`? > 10 = false`,
+	`? >= 10 = false`,
+
+	`0 == ? = false`,
+	`0 != ? = false`,
+	`0 < ? = false`,
+	`10 <= ? = false`,
+	`0 > ? = false`,
+	`10 >= ? = false`,
+
+	// etc.
+}
+
+func TestOps(t *testing.T) {
+	for _, test := range opTests {
+		a := strings.Split(test, " ")
+		i := 0 // operator index
+
+		var x, x0 Value
+		switch len(a) {
+		case 4:
+			// unary operation
+		case 5:
+			// binary operation
+			x, x0 = val(a[0]), val(a[0])
+			i = 1
+		default:
+			t.Errorf("invalid test case: %s", test)
+			continue
+		}
+
+		op, ok := optab[a[i]]
+		if !ok {
+			panic("missing optab entry for " + a[i])
+		}
+
+		y, y0 := val(a[i+1]), val(a[i+1])
+
+		got := doOp(x, op, y)
+		want := val(a[i+3])
+		if !eql(got, want) {
+			t.Errorf("%s: got %s; want %s", test, got, want)
+			continue
+		}
+
+		if x0 != nil && !eql(x, x0) {
+			t.Errorf("%s: x changed to %s", test, x)
+			continue
+		}
+
+		if !eql(y, y0) {
+			t.Errorf("%s: y changed to %s", test, y)
+			continue
+		}
+	}
+}
+
+func eql(x, y Value) bool {
+	_, ux := x.(unknownVal)
+	_, uy := y.(unknownVal)
+	if ux || uy {
+		return ux == uy
+	}
+	return Compare(x, token.EQL, y)
+}
+
+// ----------------------------------------------------------------------------
+// String tests
+
+var xxx = strings.Repeat("x", 68)
+var issue14262 = `"بموجب الشروط التالية نسب المصنف — يجب عليك أن تنسب العمل بالطريقة التي تحددها المؤلف أو المرخص (ولكن ليس بأي حال من الأحوال أن توحي وتقترح بتحول أو استخدامك للعمل).  المشاركة على قدم المساواة — إذا كنت يعدل ، والتغيير ، أو الاستفادة من هذا العمل ، قد ينتج عن توزيع العمل إلا في ظل تشابه او تطابق فى واحد لهذا الترخيص."`
+
+var stringTests = []struct {
+	input, short, exact string
+}{
+	// Unknown
+	{"", "unknown", "unknown"},
+	{"0x", "unknown", "unknown"},
+	{"'", "unknown", "unknown"},
+	{"1f0", "unknown", "unknown"},
+	{"unknown", "unknown", "unknown"},
+
+	// Bool
+	{"true", "true", "true"},
+	{"false", "false", "false"},
+
+	// String
+	{`""`, `""`, `""`},
+	{`"foo"`, `"foo"`, `"foo"`},
+	{`"` + xxx + `xx"`, `"` + xxx + `xx"`, `"` + xxx + `xx"`},
+	{`"` + xxx + `xxx"`, `"` + xxx + `...`, `"` + xxx + `xxx"`},
+	{`"` + xxx + xxx + `xxx"`, `"` + xxx + `...`, `"` + xxx + xxx + `xxx"`},
+	{issue14262, `"بموجب الشروط التالية نسب المصنف — يجب عليك أن تنسب العمل بالطريقة ال...`, issue14262},
+
+	// Int
+	{"0", "0", "0"},
+	{"-1", "-1", "-1"},
+	{"12345", "12345", "12345"},
+	{"-12345678901234567890", "-12345678901234567890", "-12345678901234567890"},
+	{"12345678901234567890", "12345678901234567890", "12345678901234567890"},
+
+	// Float
+	{"0.", "0", "0"},
+	{"-0.0", "0", "0"},
+	{"10.0", "10", "10"},
+	{"2.1", "2.1", "21/10"},
+	{"-2.1", "-2.1", "-21/10"},
+	{"1e9999", "1e+9999", "0x.f8d4a9da224650a8cb2959e10d985ad92adbd44c62917e608b1f24c0e1b76b6f61edffeb15c135a4b601637315f7662f325f82325422b244286a07663c9415d2p+33216"},
+	{"1e-9999", "1e-9999", "0x.83b01ba6d8c0425eec1b21e96f7742d63c2653ed0a024cf8a2f9686df578d7b07d7a83d84df6a2ec70a921d1f6cd5574893a7eda4d28ee719e13a5dce2700759p-33215"},
+	{"2.71828182845904523536028747135266249775724709369995957496696763", "2.71828", "271828182845904523536028747135266249775724709369995957496696763/100000000000000000000000000000000000000000000000000000000000000"},
+	{"0e9999999999", "0", "0"},   // issue #16176
+	{"-6e-1886451601", "0", "0"}, // issue #20228
+
+	// Complex
+	{"0i", "(0 + 0i)", "(0 + 0i)"},
+	{"-0i", "(0 + 0i)", "(0 + 0i)"},
+	{"10i", "(0 + 10i)", "(0 + 10i)"},
+	{"-10i", "(0 + -10i)", "(0 + -10i)"},
+	{"1e9999i", "(0 + 1e+9999i)", "(0 + 0x.f8d4a9da224650a8cb2959e10d985ad92adbd44c62917e608b1f24c0e1b76b6f61edffeb15c135a4b601637315f7662f325f82325422b244286a07663c9415d2p+33216i)"},
+}
+
+func TestString(t *testing.T) {
+	for _, test := range stringTests {
+		x := val(test.input)
+		if got := x.String(); got != test.short {
+			t.Errorf("%s: got %q; want %q as short string", test.input, got, test.short)
+		}
+		if got := x.ExactString(); got != test.exact {
+			t.Errorf("%s: got %q; want %q as exact string", test.input, got, test.exact)
+		}
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Support functions
+
+func val(lit string) Value {
+	if len(lit) == 0 {
+		return MakeUnknown()
+	}
+
+	switch lit {
+	case "?":
+		return MakeUnknown()
+	case "true":
+		return MakeBool(true)
+	case "false":
+		return MakeBool(false)
+	}
+
+	if i := strings.IndexByte(lit, '/'); i >= 0 {
+		// assume fraction
+		a := MakeFromLiteral(lit[:i], token.INT, 0)
+		b := MakeFromLiteral(lit[i+1:], token.INT, 0)
+		return BinaryOp(a, token.QUO, b)
+	}
+
+	tok := token.INT
+	switch first, last := lit[0], lit[len(lit)-1]; {
+	case first == '"' || first == '`':
+		tok = token.STRING
+		lit = strings.ReplaceAll(lit, "_", " ")
+	case first == '\'':
+		tok = token.CHAR
+	case last == 'i':
+		tok = token.IMAG
+	default:
+		if !strings.HasPrefix(lit, "0x") && strings.ContainsAny(lit, "./Ee") {
+			tok = token.FLOAT
+		}
+	}
+
+	return MakeFromLiteral(lit, tok, 0)
+}
+
+var optab = map[string]token.Token{
+	"!": token.NOT,
+
+	"+": token.ADD,
+	"-": token.SUB,
+	"*": token.MUL,
+	"/": token.QUO,
+	"%": token.REM,
+
+	"<<": token.SHL,
+	">>": token.SHR,
+
+	"&":  token.AND,
+	"|":  token.OR,
+	"^":  token.XOR,
+	"&^": token.AND_NOT,
+
+	"==": token.EQL,
+	"!=": token.NEQ,
+	"<":  token.LSS,
+	"<=": token.LEQ,
+	">":  token.GTR,
+	">=": token.GEQ,
+}
+
+func panicHandler(v *Value) {
+	switch p := recover().(type) {
+	case nil:
+		// nothing to do
+	case string:
+		*v = MakeString(p)
+	case error:
+		*v = MakeString(p.Error())
+	default:
+		panic(p)
+	}
+}
+
+func doOp(x Value, op token.Token, y Value) (z Value) {
+	defer panicHandler(&z)
+
+	if x == nil {
+		return UnaryOp(op, y, 0)
+	}
+
+	switch op {
+	case token.EQL, token.NEQ, token.LSS, token.LEQ, token.GTR, token.GEQ:
+		return MakeBool(Compare(x, op, y))
+	case token.SHL, token.SHR:
+		s, _ := Int64Val(y)
+		return Shift(x, op, uint(s))
+	default:
+		return BinaryOp(x, op, y)
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Other tests
+
+var fracTests = []string{
+	"0",
+	"1",
+	"-1",
+	"1.2",
+	"-0.991",
+	"2.718281828",
+	"3.14159265358979323e-10",
+	"1e100",
+	"1e1000",
+}
+
+func TestFractions(t *testing.T) {
+	for _, test := range fracTests {
+		x := val(test)
+		// We don't check the actual numerator and denominator because they
+		// are unlikely to be 100% correct due to floatVal rounding errors.
+		// Instead, we compute the fraction again and compare the rounded
+		// result.
+		q := BinaryOp(Num(x), token.QUO, Denom(x))
+		got := q.String()
+		want := x.String()
+		if got != want {
+			t.Errorf("%s: got quotient %s, want %s", x, got, want)
+		}
+	}
+}
+
+var bytesTests = []string{
+	"0",
+	"1",
+	"123456789",
+	"123456789012345678901234567890123456789012345678901234567890",
+}
+
+func TestBytes(t *testing.T) {
+	for _, test := range bytesTests {
+		x := val(test)
+		bytes := Bytes(x)
+
+		// special case 0
+		if Sign(x) == 0 && len(bytes) != 0 {
+			t.Errorf("%s: got %v; want empty byte slice", test, bytes)
+		}
+
+		if n := len(bytes); n > 0 && bytes[n-1] == 0 {
+			t.Errorf("%s: got %v; want no leading 0 byte", test, bytes)
+		}
+
+		if got := MakeFromBytes(bytes); !eql(got, x) {
+			t.Errorf("%s: got %s; want %s (bytes = %v)", test, got, x, bytes)
+		}
+	}
+}
+
+func TestUnknown(t *testing.T) {
+	u := MakeUnknown()
+	var values = []Value{
+		u,
+		MakeBool(false), // token.ADD ok below, operation is never considered
+		MakeString(""),
+		MakeInt64(1),
+		MakeFromLiteral("''", token.CHAR, 0),
+		MakeFromLiteral("-1234567890123456789012345678901234567890", token.INT, 0),
+		MakeFloat64(1.2),
+		MakeImag(MakeFloat64(1.2)),
+	}
+	for _, val := range values {
+		x, y := val, u
+		for i := range [2]int{} {
+			if i == 1 {
+				x, y = y, x
+			}
+			if got := BinaryOp(x, token.ADD, y); got.Kind() != Unknown {
+				t.Errorf("%s + %s: got %s; want %s", x, y, got, u)
+			}
+			if got := Compare(x, token.EQL, y); got {
+				t.Errorf("%s == %s: got true; want false", x, y)
+			}
+		}
+	}
+}
+
+func TestMakeFloat64(t *testing.T) {
+	var zero float64
+	for _, arg := range []float64{
+		-math.MaxFloat32,
+		-10,
+		-0.5,
+		-zero,
+		zero,
+		1,
+		10,
+		123456789.87654321e-23,
+		1e10,
+		math.MaxFloat64,
+	} {
+		val := MakeFloat64(arg)
+		if val.Kind() != Float {
+			t.Errorf("%v: got kind = %d; want %d", arg, val.Kind(), Float)
+		}
+
+		// -0.0 is mapped to 0.0
+		got, exact := Float64Val(val)
+		if !exact || math.Float64bits(got) != math.Float64bits(arg+0) {
+			t.Errorf("%v: got %v (exact = %v)", arg, got, exact)
+		}
+	}
+
+	// infinity
+	for sign := range []int{-1, 1} {
+		arg := math.Inf(sign)
+		val := MakeFloat64(arg)
+		if val.Kind() != Unknown {
+			t.Errorf("%v: got kind = %d; want %d", arg, val.Kind(), Unknown)
+		}
+	}
+}
+
+type makeTestCase struct {
+	kind      Kind
+	arg, want interface{}
+}
+
+func dup(k Kind, x interface{}) makeTestCase { return makeTestCase{k, x, x} }
+
+func TestMake(t *testing.T) {
+	for _, test := range []makeTestCase{
+		{Bool, false, false},
+		{String, "hello", "hello"},
+
+		{Int, int64(1), int64(1)},
+		{Int, big.NewInt(10), int64(10)},
+		{Int, new(big.Int).Lsh(big.NewInt(1), 62), int64(1 << 62)},
+		dup(Int, new(big.Int).Lsh(big.NewInt(1), 63)),
+
+		{Float, big.NewFloat(0), floatVal0.val},
+		dup(Float, big.NewFloat(2.0)),
+		dup(Float, big.NewRat(1, 3)),
+	} {
+		val := Make(test.arg)
+		got := Val(val)
+		if val.Kind() != test.kind || got != test.want {
+			t.Errorf("got %v (%T, kind = %d); want %v (%T, kind = %d)",
+				got, got, val.Kind(), test.want, test.want, test.kind)
+		}
+	}
+}
+
+func BenchmarkStringAdd(b *testing.B) {
+	for size := 1; size <= 65536; size *= 4 {
+		b.Run(fmt.Sprint(size), func(b *testing.B) {
+			b.ReportAllocs()
+			n := int64(0)
+			for i := 0; i < b.N; i++ {
+				x := MakeString(strings.Repeat("x", 100))
+				y := x
+				for j := 0; j < size-1; j++ {
+					y = BinaryOp(y, token.ADD, x)
+				}
+				n += int64(len(StringVal(y)))
+			}
+			if n != int64(b.N)*int64(size)*100 {
+				b.Fatalf("bad string %d != %d", n, int64(b.N)*int64(size)*100)
+			}
+		})
+	}
+}
diff --git a/src/go/doc/Makefile b/src/go/doc/Makefile
new file mode 100644
index 0000000..ca4948f
--- /dev/null
+++ b/src/go/doc/Makefile
@@ -0,0 +1,7 @@
+# Copyright 2009 The Go Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+# Script to test heading detection heuristic
+headscan: headscan.go
+	go build headscan.go
diff --git a/src/go/doc/comment.go b/src/go/doc/comment.go
new file mode 100644
index 0000000..92131a3
--- /dev/null
+++ b/src/go/doc/comment.go
@@ -0,0 +1,513 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Godoc comment extraction and comment -> HTML formatting.
+
+package doc
+
+import (
+	"bytes"
+	"internal/lazyregexp"
+	"io"
+	"strings"
+	"text/template" // for HTMLEscape
+	"unicode"
+	"unicode/utf8"
+)
+
+const (
+	ldquo = "&ldquo;"
+	rdquo = "&rdquo;"
+	ulquo = "“"
+	urquo = "”"
+)
+
+var (
+	htmlQuoteReplacer    = strings.NewReplacer(ulquo, ldquo, urquo, rdquo)
+	unicodeQuoteReplacer = strings.NewReplacer("``", ulquo, "''", urquo)
+)
+
+// Escape comment text for HTML. If nice is set,
+// also turn `` into &ldquo; and '' into &rdquo;.
+func commentEscape(w io.Writer, text string, nice bool) {
+	if nice {
+		// In the first pass, we convert `` and '' into their unicode equivalents.
+		// This prevents them from being escaped in HTMLEscape.
+		text = convertQuotes(text)
+		var buf bytes.Buffer
+		template.HTMLEscape(&buf, []byte(text))
+		// Now we convert the unicode quotes to their HTML escaped entities to maintain old behavior.
+		// We need to use a temp buffer to read the string back and do the conversion,
+		// otherwise HTMLEscape will escape & to &amp;
+		htmlQuoteReplacer.WriteString(w, buf.String())
+		return
+	}
+	template.HTMLEscape(w, []byte(text))
+}
+
+func convertQuotes(text string) string {
+	return unicodeQuoteReplacer.Replace(text)
+}
+
+const (
+	// Regexp for Go identifiers
+	identRx = `[\pL_][\pL_0-9]*`
+
+	// Regexp for URLs
+	// Match parens, and check later for balance - see #5043, #22285
+	// Match .,:;?! within path, but not at end - see #18139, #16565
+	// This excludes some rare yet valid urls ending in common punctuation
+	// in order to allow sentences ending in URLs.
+
+	// protocol (required) e.g. http
+	protoPart = `(https?|ftp|file|gopher|mailto|nntp)`
+	// host (required) e.g. www.example.com or [::1]:8080
+	hostPart = `([a-zA-Z0-9_@\-.\[\]:]+)`
+	// path+query+fragment (optional) e.g. /path/index.html?q=foo#bar
+	pathPart = `([.,:;?!]*[a-zA-Z0-9$'()*+&#=@~_/\-\[\]%])*`
+
+	urlRx = protoPart + `://` + hostPart + pathPart
+)
+
+var matchRx = lazyregexp.New(`(` + urlRx + `)|(` + identRx + `)`)
+
+var (
+	html_a      = []byte(`<a href="`)
+	html_aq     = []byte(`">`)
+	html_enda   = []byte("</a>")
+	html_i      = []byte("<i>")
+	html_endi   = []byte("</i>")
+	html_p      = []byte("<p>\n")
+	html_endp   = []byte("</p>\n")
+	html_pre    = []byte("<pre>")
+	html_endpre = []byte("</pre>\n")
+	html_h      = []byte(`<h3 id="`)
+	html_hq     = []byte(`">`)
+	html_endh   = []byte("</h3>\n")
+)
+
+// Emphasize and escape a line of text for HTML. URLs are converted into links;
+// if the URL also appears in the words map, the link is taken from the map (if
+// the corresponding map value is the empty string, the URL is not converted
+// into a link). Go identifiers that appear in the words map are italicized; if
+// the corresponding map value is not the empty string, it is considered a URL
+// and the word is converted into a link. If nice is set, the remaining text's
+// appearance is improved where it makes sense (e.g., `` is turned into &ldquo;
+// and '' into &rdquo;).
+func emphasize(w io.Writer, line string, words map[string]string, nice bool) {
+	for {
+		m := matchRx.FindStringSubmatchIndex(line)
+		if m == nil {
+			break
+		}
+		// m >= 6 (two parenthesized sub-regexps in matchRx, 1st one is urlRx)
+
+		// write text before match
+		commentEscape(w, line[0:m[0]], nice)
+
+		// adjust match for URLs
+		match := line[m[0]:m[1]]
+		if strings.Contains(match, "://") {
+			m0, m1 := m[0], m[1]
+			for _, s := range []string{"()", "{}", "[]"} {
+				open, close := s[:1], s[1:] // E.g., "(" and ")"
+				// require opening parentheses before closing parentheses (#22285)
+				if i := strings.Index(match, close); i >= 0 && i < strings.Index(match, open) {
+					m1 = m0 + i
+					match = line[m0:m1]
+				}
+				// require balanced pairs of parentheses (#5043)
+				for i := 0; strings.Count(match, open) != strings.Count(match, close) && i < 10; i++ {
+					m1 = strings.LastIndexAny(line[:m1], s)
+					match = line[m0:m1]
+				}
+			}
+			if m1 != m[1] {
+				// redo matching with shortened line for correct indices
+				m = matchRx.FindStringSubmatchIndex(line[:m[0]+len(match)])
+			}
+		}
+
+		// analyze match
+		url := ""
+		italics := false
+		if words != nil {
+			url, italics = words[match]
+		}
+		if m[2] >= 0 {
+			// match against first parenthesized sub-regexp; must be match against urlRx
+			if !italics {
+				// no alternative URL in words list, use match instead
+				url = match
+			}
+			italics = false // don't italicize URLs
+		}
+
+		// write match
+		if len(url) > 0 {
+			w.Write(html_a)
+			template.HTMLEscape(w, []byte(url))
+			w.Write(html_aq)
+		}
+		if italics {
+			w.Write(html_i)
+		}
+		commentEscape(w, match, nice)
+		if italics {
+			w.Write(html_endi)
+		}
+		if len(url) > 0 {
+			w.Write(html_enda)
+		}
+
+		// advance
+		line = line[m[1]:]
+	}
+	commentEscape(w, line, nice)
+}
+
+func indentLen(s string) int {
+	i := 0
+	for i < len(s) && (s[i] == ' ' || s[i] == '\t') {
+		i++
+	}
+	return i
+}
+
+func isBlank(s string) bool {
+	return len(s) == 0 || (len(s) == 1 && s[0] == '\n')
+}
+
+func commonPrefix(a, b string) string {
+	i := 0
+	for i < len(a) && i < len(b) && a[i] == b[i] {
+		i++
+	}
+	return a[0:i]
+}
+
+func unindent(block []string) {
+	if len(block) == 0 {
+		return
+	}
+
+	// compute maximum common white prefix
+	prefix := block[0][0:indentLen(block[0])]
+	for _, line := range block {
+		if !isBlank(line) {
+			prefix = commonPrefix(prefix, line[0:indentLen(line)])
+		}
+	}
+	n := len(prefix)
+
+	// remove
+	for i, line := range block {
+		if !isBlank(line) {
+			block[i] = line[n:]
+		}
+	}
+}
+
+// heading returns the trimmed line if it passes as a section heading;
+// otherwise it returns the empty string.
+func heading(line string) string {
+	line = strings.TrimSpace(line)
+	if len(line) == 0 {
+		return ""
+	}
+
+	// a heading must start with an uppercase letter
+	r, _ := utf8.DecodeRuneInString(line)
+	if !unicode.IsLetter(r) || !unicode.IsUpper(r) {
+		return ""
+	}
+
+	// it must end in a letter or digit:
+	r, _ = utf8.DecodeLastRuneInString(line)
+	if !unicode.IsLetter(r) && !unicode.IsDigit(r) {
+		return ""
+	}
+
+	// exclude lines with illegal characters. we allow "(),"
+	if strings.ContainsAny(line, ";:!?+*/=[]{}_^°&§~%#@<\">\\") {
+		return ""
+	}
+
+	// allow "'" for possessive "'s" only
+	for b := line; ; {
+		i := strings.IndexRune(b, '\'')
+		if i < 0 {
+			break
+		}
+		if i+1 >= len(b) || b[i+1] != 's' || (i+2 < len(b) && b[i+2] != ' ') {
+			return "" // not followed by "s "
+		}
+		b = b[i+2:]
+	}
+
+	// allow "." when followed by non-space
+	for b := line; ; {
+		i := strings.IndexRune(b, '.')
+		if i < 0 {
+			break
+		}
+		if i+1 >= len(b) || b[i+1] == ' ' {
+			return "" // not followed by non-space
+		}
+		b = b[i+1:]
+	}
+
+	return line
+}
+
+type op int
+
+const (
+	opPara op = iota
+	opHead
+	opPre
+)
+
+type block struct {
+	op    op
+	lines []string
+}
+
+var nonAlphaNumRx = lazyregexp.New(`[^a-zA-Z0-9]`)
+
+func anchorID(line string) string {
+	// Add a "hdr-" prefix to avoid conflicting with IDs used for package symbols.
+	return "hdr-" + nonAlphaNumRx.ReplaceAllString(line, "_")
+}
+
+// ToHTML converts comment text to formatted HTML.
+// The comment was prepared by DocReader,
+// so it is known not to have leading, trailing blank lines
+// nor to have trailing spaces at the end of lines.
+// The comment markers have already been removed.
+//
+// Each span of unindented non-blank lines is converted into
+// a single paragraph. There is one exception to the rule: a span that
+// consists of a single line, is followed by another paragraph span,
+// begins with a capital letter, and contains no punctuation
+// other than parentheses and commas is formatted as a heading.
+//
+// A span of indented lines is converted into a <pre> block,
+// with the common indent prefix removed.
+//
+// URLs in the comment text are converted into links; if the URL also appears
+// in the words map, the link is taken from the map (if the corresponding map
+// value is the empty string, the URL is not converted into a link).
+//
+// A pair of (consecutive) backticks (`) is converted to a unicode left quote (“), and a pair of (consecutive)
+// single quotes (') is converted to a unicode right quote (”).
+//
+// Go identifiers that appear in the words map are italicized; if the corresponding
+// map value is not the empty string, it is considered a URL and the word is converted
+// into a link.
+func ToHTML(w io.Writer, text string, words map[string]string) {
+	for _, b := range blocks(text) {
+		switch b.op {
+		case opPara:
+			w.Write(html_p)
+			for _, line := range b.lines {
+				emphasize(w, line, words, true)
+			}
+			w.Write(html_endp)
+		case opHead:
+			w.Write(html_h)
+			id := ""
+			for _, line := range b.lines {
+				if id == "" {
+					id = anchorID(line)
+					w.Write([]byte(id))
+					w.Write(html_hq)
+				}
+				commentEscape(w, line, true)
+			}
+			if id == "" {
+				w.Write(html_hq)
+			}
+			w.Write(html_endh)
+		case opPre:
+			w.Write(html_pre)
+			for _, line := range b.lines {
+				emphasize(w, line, nil, false)
+			}
+			w.Write(html_endpre)
+		}
+	}
+}
+
+func blocks(text string) []block {
+	var (
+		out  []block
+		para []string
+
+		lastWasBlank   = false
+		lastWasHeading = false
+	)
+
+	close := func() {
+		if para != nil {
+			out = append(out, block{opPara, para})
+			para = nil
+		}
+	}
+
+	lines := strings.SplitAfter(text, "\n")
+	unindent(lines)
+	for i := 0; i < len(lines); {
+		line := lines[i]
+		if isBlank(line) {
+			// close paragraph
+			close()
+			i++
+			lastWasBlank = true
+			continue
+		}
+		if indentLen(line) > 0 {
+			// close paragraph
+			close()
+
+			// count indented or blank lines
+			j := i + 1
+			for j < len(lines) && (isBlank(lines[j]) || indentLen(lines[j]) > 0) {
+				j++
+			}
+			// but not trailing blank lines
+			for j > i && isBlank(lines[j-1]) {
+				j--
+			}
+			pre := lines[i:j]
+			i = j
+
+			unindent(pre)
+
+			// put those lines in a pre block
+			out = append(out, block{opPre, pre})
+			lastWasHeading = false
+			continue
+		}
+
+		if lastWasBlank && !lastWasHeading && i+2 < len(lines) &&
+			isBlank(lines[i+1]) && !isBlank(lines[i+2]) && indentLen(lines[i+2]) == 0 {
+			// current line is non-blank, surrounded by blank lines
+			// and the next non-blank line is not indented: this
+			// might be a heading.
+			if head := heading(line); head != "" {
+				close()
+				out = append(out, block{opHead, []string{head}})
+				i += 2
+				lastWasHeading = true
+				continue
+			}
+		}
+
+		// open paragraph
+		lastWasBlank = false
+		lastWasHeading = false
+		para = append(para, lines[i])
+		i++
+	}
+	close()
+
+	return out
+}
+
+// ToText prepares comment text for presentation in textual output.
+// It wraps paragraphs of text to width or fewer Unicode code points
+// and then prefixes each line with the indent. In preformatted sections
+// (such as program text), it prefixes each non-blank line with preIndent.
+//
+// A pair of (consecutive) backticks (`) is converted to a unicode left quote (“), and a pair of (consecutive)
+// single quotes (') is converted to a unicode right quote (”).
+func ToText(w io.Writer, text string, indent, preIndent string, width int) {
+	l := lineWrapper{
+		out:    w,
+		width:  width,
+		indent: indent,
+	}
+	for _, b := range blocks(text) {
+		switch b.op {
+		case opPara:
+			// l.write will add leading newline if required
+			for _, line := range b.lines {
+				line = convertQuotes(line)
+				l.write(line)
+			}
+			l.flush()
+		case opHead:
+			w.Write(nl)
+			for _, line := range b.lines {
+				line = convertQuotes(line)
+				l.write(line + "\n")
+			}
+			l.flush()
+		case opPre:
+			w.Write(nl)
+			for _, line := range b.lines {
+				if isBlank(line) {
+					w.Write([]byte("\n"))
+				} else {
+					w.Write([]byte(preIndent))
+					w.Write([]byte(line))
+				}
+			}
+		}
+	}
+}
+
+type lineWrapper struct {
+	out       io.Writer
+	printed   bool
+	width     int
+	indent    string
+	n         int
+	pendSpace int
+}
+
+var nl = []byte("\n")
+var space = []byte(" ")
+var prefix = []byte("// ")
+
+func (l *lineWrapper) write(text string) {
+	if l.n == 0 && l.printed {
+		l.out.Write(nl) // blank line before new paragraph
+	}
+	l.printed = true
+
+	needsPrefix := false
+	isComment := strings.HasPrefix(text, "//")
+	for _, f := range strings.Fields(text) {
+		w := utf8.RuneCountInString(f)
+		// wrap if line is too long
+		if l.n > 0 && l.n+l.pendSpace+w > l.width {
+			l.out.Write(nl)
+			l.n = 0
+			l.pendSpace = 0
+			needsPrefix = isComment && !strings.HasPrefix(f, "//")
+		}
+		if l.n == 0 {
+			l.out.Write([]byte(l.indent))
+		}
+		if needsPrefix {
+			l.out.Write(prefix)
+			needsPrefix = false
+		}
+		l.out.Write(space[:l.pendSpace])
+		l.out.Write([]byte(f))
+		l.n += l.pendSpace + w
+		l.pendSpace = 1
+	}
+}
+
+func (l *lineWrapper) flush() {
+	if l.n == 0 {
+		return
+	}
+	l.out.Write(nl)
+	l.pendSpace = 0
+	l.n = 0
+}
diff --git a/src/go/doc/comment_test.go b/src/go/doc/comment_test.go
new file mode 100644
index 0000000..6d1b209
--- /dev/null
+++ b/src/go/doc/comment_test.go
@@ -0,0 +1,247 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import (
+	"bytes"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+var headingTests = []struct {
+	line string
+	ok   bool
+}{
+	{"Section", true},
+	{"A typical usage", true},
+	{"ΔΛΞ is Greek", true},
+	{"Foo 42", true},
+	{"", false},
+	{"section", false},
+	{"A typical usage:", false},
+	{"This code:", false},
+	{"δ is Greek", false},
+	{"Foo §", false},
+	{"Fermat's Last Sentence", true},
+	{"Fermat's", true},
+	{"'sX", false},
+	{"Ted 'Too' Bar", false},
+	{"Use n+m", false},
+	{"Scanning:", false},
+	{"N:M", false},
+}
+
+func TestIsHeading(t *testing.T) {
+	for _, tt := range headingTests {
+		if h := heading(tt.line); (len(h) > 0) != tt.ok {
+			t.Errorf("isHeading(%q) = %v, want %v", tt.line, h, tt.ok)
+		}
+	}
+}
+
+var blocksTests = []struct {
+	in   string
+	out  []block
+	text string
+}{
+	{
+		in: `Para 1.
+Para 1 line 2.
+
+Para 2.
+
+Section
+
+Para 3.
+
+	pre
+	pre1
+
+Para 4.
+
+	pre
+	pre1
+
+	pre2
+
+Para 5.
+
+
+	pre
+
+
+	pre1
+	pre2
+
+Para 6.
+	pre
+	pre2
+`,
+		out: []block{
+			{opPara, []string{"Para 1.\n", "Para 1 line 2.\n"}},
+			{opPara, []string{"Para 2.\n"}},
+			{opHead, []string{"Section"}},
+			{opPara, []string{"Para 3.\n"}},
+			{opPre, []string{"pre\n", "pre1\n"}},
+			{opPara, []string{"Para 4.\n"}},
+			{opPre, []string{"pre\n", "pre1\n", "\n", "pre2\n"}},
+			{opPara, []string{"Para 5.\n"}},
+			{opPre, []string{"pre\n", "\n", "\n", "pre1\n", "pre2\n"}},
+			{opPara, []string{"Para 6.\n"}},
+			{opPre, []string{"pre\n", "pre2\n"}},
+		},
+		text: `.   Para 1. Para 1 line 2.
+
+.   Para 2.
+
+
+.   Section
+
+.   Para 3.
+
+$	pre
+$	pre1
+
+.   Para 4.
+
+$	pre
+$	pre1
+
+$	pre2
+
+.   Para 5.
+
+$	pre
+
+
+$	pre1
+$	pre2
+
+.   Para 6.
+
+$	pre
+$	pre2
+`,
+	},
+	{
+		in: "Para.\n\tshould not be ``escaped''",
+		out: []block{
+			{opPara, []string{"Para.\n"}},
+			{opPre, []string{"should not be ``escaped''"}},
+		},
+		text: ".   Para.\n\n$	should not be ``escaped''",
+	},
+	{
+		in: "// A very long line of 46 char for line wrapping.",
+		out: []block{
+			{opPara, []string{"// A very long line of 46 char for line wrapping."}},
+		},
+		text: `.   // A very long line of 46 char for line
+.   // wrapping.
+`,
+	},
+	{
+		in: `/* A very long line of 46 char for line wrapping.
+A very long line of 46 char for line wrapping. */`,
+		out: []block{
+			{opPara, []string{"/* A very long line of 46 char for line wrapping.\n", "A very long line of 46 char for line wrapping. */"}},
+		},
+		text: `.   /* A very long line of 46 char for line
+.   wrapping. A very long line of 46 char
+.   for line wrapping. */
+`,
+	},
+	{
+		in: `A line of 36 char for line wrapping.
+//Another line starting with //`,
+		out: []block{
+			{opPara, []string{"A line of 36 char for line wrapping.\n",
+				"//Another line starting with //"}},
+		},
+		text: `.   A line of 36 char for line wrapping.
+.   //Another line starting with //
+`,
+	},
+}
+
+func TestBlocks(t *testing.T) {
+	for i, tt := range blocksTests {
+		b := blocks(tt.in)
+		if !reflect.DeepEqual(b, tt.out) {
+			t.Errorf("#%d: mismatch\nhave: %v\nwant: %v", i, b, tt.out)
+		}
+	}
+}
+
+func TestToText(t *testing.T) {
+	var buf bytes.Buffer
+	for i, tt := range blocksTests {
+		ToText(&buf, tt.in, ".   ", "$\t", 40)
+		if have := buf.String(); have != tt.text {
+			t.Errorf("#%d: mismatch\nhave: %s\nwant: %s\nhave vs want:\n%q\n%q", i, have, tt.text, have, tt.text)
+		}
+		buf.Reset()
+	}
+}
+
+var emphasizeTests = []struct {
+	in, out string
+}{
+	{"", ""},
+	{"http://[::1]:8080/foo.txt", `<a href="http://[::1]:8080/foo.txt">http://[::1]:8080/foo.txt</a>`},
+	{"before (https://www.google.com) after", `before (<a href="https://www.google.com">https://www.google.com</a>) after`},
+	{"before https://www.google.com:30/x/y/z:b::c. After", `before <a href="https://www.google.com:30/x/y/z:b::c">https://www.google.com:30/x/y/z:b::c</a>. After`},
+	{"http://www.google.com/path/:;!-/?query=%34b#093124", `<a href="http://www.google.com/path/:;!-/?query=%34b#093124">http://www.google.com/path/:;!-/?query=%34b#093124</a>`},
+	{"http://www.google.com/path/:;!-/?query=%34bar#093124", `<a href="http://www.google.com/path/:;!-/?query=%34bar#093124">http://www.google.com/path/:;!-/?query=%34bar#093124</a>`},
+	{"http://www.google.com/index.html! After", `<a href="http://www.google.com/index.html">http://www.google.com/index.html</a>! After`},
+	{"http://www.google.com/", `<a href="http://www.google.com/">http://www.google.com/</a>`},
+	{"https://www.google.com/", `<a href="https://www.google.com/">https://www.google.com/</a>`},
+	{"http://www.google.com/path.", `<a href="http://www.google.com/path">http://www.google.com/path</a>.`},
+	{"http://en.wikipedia.org/wiki/Camellia_(cipher)", `<a href="http://en.wikipedia.org/wiki/Camellia_(cipher)">http://en.wikipedia.org/wiki/Camellia_(cipher)</a>`},
+	{"(http://www.google.com/)", `(<a href="http://www.google.com/">http://www.google.com/</a>)`},
+	{"http://gmail.com)", `<a href="http://gmail.com">http://gmail.com</a>)`},
+	{"((http://gmail.com))", `((<a href="http://gmail.com">http://gmail.com</a>))`},
+	{"http://gmail.com ((http://gmail.com)) ()", `<a href="http://gmail.com">http://gmail.com</a> ((<a href="http://gmail.com">http://gmail.com</a>)) ()`},
+	{"Foo bar http://example.com/ quux!", `Foo bar <a href="http://example.com/">http://example.com/</a> quux!`},
+	{"Hello http://example.com/%2f/ /world.", `Hello <a href="http://example.com/%2f/">http://example.com/%2f/</a> /world.`},
+	{"Lorem http: ipsum //host/path", "Lorem http: ipsum //host/path"},
+	{"javascript://is/not/linked", "javascript://is/not/linked"},
+	{"http://foo", `<a href="http://foo">http://foo</a>`},
+	{"art by [[https://www.example.com/person/][Person Name]]", `art by [[<a href="https://www.example.com/person/">https://www.example.com/person/</a>][Person Name]]`},
+	{"please visit (http://golang.org/)", `please visit (<a href="http://golang.org/">http://golang.org/</a>)`},
+	{"please visit http://golang.org/hello())", `please visit <a href="http://golang.org/hello()">http://golang.org/hello()</a>)`},
+	{"http://git.qemu.org/?p=qemu.git;a=blob;f=qapi-schema.json;hb=HEAD", `<a href="http://git.qemu.org/?p=qemu.git;a=blob;f=qapi-schema.json;hb=HEAD">http://git.qemu.org/?p=qemu.git;a=blob;f=qapi-schema.json;hb=HEAD</a>`},
+	{"https://foo.bar/bal/x(])", `<a href="https://foo.bar/bal/x(">https://foo.bar/bal/x(</a>])`}, // inner ] causes (]) to be cut off from URL
+	{"foo [ http://bar(])", `foo [ <a href="http://bar(">http://bar(</a>])`},                      // outer [ causes ]) to be cut off from URL
+}
+
+func TestEmphasize(t *testing.T) {
+	for i, tt := range emphasizeTests {
+		var buf bytes.Buffer
+		emphasize(&buf, tt.in, nil, true)
+		out := buf.String()
+		if out != tt.out {
+			t.Errorf("#%d: mismatch\nhave: %v\nwant: %v", i, out, tt.out)
+		}
+	}
+}
+
+func TestCommentEscape(t *testing.T) {
+	commentTests := []struct {
+		in, out string
+	}{
+		{"typically invoked as ``go tool asm'',", "typically invoked as " + ldquo + "go tool asm" + rdquo + ","},
+		{"For more detail, run ``go help test'' and ``go help testflag''", "For more detail, run " + ldquo + "go help test" + rdquo + " and " + ldquo + "go help testflag" + rdquo},
+	}
+	for i, tt := range commentTests {
+		var buf strings.Builder
+		commentEscape(&buf, tt.in, true)
+		out := buf.String()
+		if out != tt.out {
+			t.Errorf("#%d: mismatch\nhave: %q\nwant: %q", i, out, tt.out)
+		}
+	}
+}
diff --git a/src/go/doc/doc.go b/src/go/doc/doc.go
new file mode 100644
index 0000000..79d3899
--- /dev/null
+++ b/src/go/doc/doc.go
@@ -0,0 +1,222 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package doc extracts source code documentation from a Go AST.
+package doc
+
+import (
+	"fmt"
+	"go/ast"
+	"go/token"
+	"strings"
+)
+
+// Package is the documentation for an entire package.
+type Package struct {
+	Doc        string
+	Name       string
+	ImportPath string
+	Imports    []string
+	Filenames  []string
+	Notes      map[string][]*Note
+
+	// Deprecated: For backward compatibility Bugs is still populated,
+	// but all new code should use Notes instead.
+	Bugs []string
+
+	// declarations
+	Consts []*Value
+	Types  []*Type
+	Vars   []*Value
+	Funcs  []*Func
+
+	// Examples is a sorted list of examples associated with
+	// the package. Examples are extracted from _test.go files
+	// provided to NewFromFiles.
+	Examples []*Example
+}
+
+// Value is the documentation for a (possibly grouped) var or const declaration.
+type Value struct {
+	Doc   string
+	Names []string // var or const names in declaration order
+	Decl  *ast.GenDecl
+
+	order int
+}
+
+// Type is the documentation for a type declaration.
+type Type struct {
+	Doc  string
+	Name string
+	Decl *ast.GenDecl
+
+	// associated declarations
+	Consts  []*Value // sorted list of constants of (mostly) this type
+	Vars    []*Value // sorted list of variables of (mostly) this type
+	Funcs   []*Func  // sorted list of functions returning this type
+	Methods []*Func  // sorted list of methods (including embedded ones) of this type
+
+	// Examples is a sorted list of examples associated with
+	// this type. Examples are extracted from _test.go files
+	// provided to NewFromFiles.
+	Examples []*Example
+}
+
+// Func is the documentation for a func declaration.
+type Func struct {
+	Doc  string
+	Name string
+	Decl *ast.FuncDecl
+
+	// methods
+	// (for functions, these fields have the respective zero value)
+	Recv  string // actual   receiver "T" or "*T"
+	Orig  string // original receiver "T" or "*T"
+	Level int    // embedding level; 0 means not embedded
+
+	// Examples is a sorted list of examples associated with this
+	// function or method. Examples are extracted from _test.go files
+	// provided to NewFromFiles.
+	Examples []*Example
+}
+
+// A Note represents a marked comment starting with "MARKER(uid): note body".
+// Any note with a marker of 2 or more upper case [A-Z] letters and a uid of
+// at least one character is recognized. The ":" following the uid is optional.
+// Notes are collected in the Package.Notes map indexed by the notes marker.
+type Note struct {
+	Pos, End token.Pos // position range of the comment containing the marker
+	UID      string    // uid found with the marker
+	Body     string    // note body text
+}
+
+// Mode values control the operation of New and NewFromFiles.
+type Mode int
+
+const (
+	// AllDecls says to extract documentation for all package-level
+	// declarations, not just exported ones.
+	AllDecls Mode = 1 << iota
+
+	// AllMethods says to show all embedded methods, not just the ones of
+	// invisible (unexported) anonymous fields.
+	AllMethods
+
+	// PreserveAST says to leave the AST unmodified. Originally, pieces of
+	// the AST such as function bodies were nil-ed out to save memory in
+	// godoc, but not all programs want that behavior.
+	PreserveAST
+)
+
+// New computes the package documentation for the given package AST.
+// New takes ownership of the AST pkg and may edit or overwrite it.
+// To have the Examples fields populated, use NewFromFiles and include
+// the package's _test.go files.
+//
+func New(pkg *ast.Package, importPath string, mode Mode) *Package {
+	var r reader
+	r.readPackage(pkg, mode)
+	r.computeMethodSets()
+	r.cleanupTypes()
+	return &Package{
+		Doc:        r.doc,
+		Name:       pkg.Name,
+		ImportPath: importPath,
+		Imports:    sortedKeys(r.imports),
+		Filenames:  r.filenames,
+		Notes:      r.notes,
+		Bugs:       noteBodies(r.notes["BUG"]),
+		Consts:     sortedValues(r.values, token.CONST),
+		Types:      sortedTypes(r.types, mode&AllMethods != 0),
+		Vars:       sortedValues(r.values, token.VAR),
+		Funcs:      sortedFuncs(r.funcs, true),
+	}
+}
+
+// NewFromFiles computes documentation for a package.
+//
+// The package is specified by a list of *ast.Files and corresponding
+// file set, which must not be nil.
+// NewFromFiles uses all provided files when computing documentation,
+// so it is the caller's responsibility to provide only the files that
+// match the desired build context. "go/build".Context.MatchFile can
+// be used for determining whether a file matches a build context with
+// the desired GOOS and GOARCH values, and other build constraints.
+// The import path of the package is specified by importPath.
+//
+// Examples found in _test.go files are associated with the corresponding
+// type, function, method, or the package, based on their name.
+// If the example has a suffix in its name, it is set in the
+// Example.Suffix field. Examples with malformed names are skipped.
+//
+// Optionally, a single extra argument of type Mode can be provided to
+// control low-level aspects of the documentation extraction behavior.
+//
+// NewFromFiles takes ownership of the AST files and may edit them,
+// unless the PreserveAST Mode bit is on.
+//
+func NewFromFiles(fset *token.FileSet, files []*ast.File, importPath string, opts ...interface{}) (*Package, error) {
+	// Check for invalid API usage.
+	if fset == nil {
+		panic(fmt.Errorf("doc.NewFromFiles: no token.FileSet provided (fset == nil)"))
+	}
+	var mode Mode
+	switch len(opts) { // There can only be 0 or 1 options, so a simple switch works for now.
+	case 0:
+		// Nothing to do.
+	case 1:
+		m, ok := opts[0].(Mode)
+		if !ok {
+			panic(fmt.Errorf("doc.NewFromFiles: option argument type must be doc.Mode"))
+		}
+		mode = m
+	default:
+		panic(fmt.Errorf("doc.NewFromFiles: there must not be more than 1 option argument"))
+	}
+
+	// Collect .go and _test.go files.
+	var (
+		goFiles     = make(map[string]*ast.File)
+		testGoFiles []*ast.File
+	)
+	for i := range files {
+		f := fset.File(files[i].Pos())
+		if f == nil {
+			return nil, fmt.Errorf("file files[%d] is not found in the provided file set", i)
+		}
+		switch name := f.Name(); {
+		case strings.HasSuffix(name, ".go") && !strings.HasSuffix(name, "_test.go"):
+			goFiles[name] = files[i]
+		case strings.HasSuffix(name, "_test.go"):
+			testGoFiles = append(testGoFiles, files[i])
+		default:
+			return nil, fmt.Errorf("file files[%d] filename %q does not have a .go extension", i, name)
+		}
+	}
+
+	// TODO(dmitshur,gri): A relatively high level call to ast.NewPackage with a simpleImporter
+	// ast.Importer implementation is made below. It might be possible to short-circuit and simplify.
+
+	// Compute package documentation.
+	pkg, _ := ast.NewPackage(fset, goFiles, simpleImporter, nil) // Ignore errors that can happen due to unresolved identifiers.
+	p := New(pkg, importPath, mode)
+	classifyExamples(p, Examples(testGoFiles...))
+	return p, nil
+}
+
+// simpleImporter returns a (dummy) package object named by the last path
+// component of the provided package path (as is the convention for packages).
+// This is sufficient to resolve package identifiers without doing an actual
+// import. It never returns an error.
+func simpleImporter(imports map[string]*ast.Object, path string) (*ast.Object, error) {
+	pkg := imports[path]
+	if pkg == nil {
+		// note that strings.LastIndex returns -1 if there is no "/"
+		pkg = ast.NewObj(ast.Pkg, path[strings.LastIndex(path, "/")+1:])
+		pkg.Data = ast.NewScope(nil) // required by ast.NewPackage for dot-import
+		imports[path] = pkg
+	}
+	return pkg, nil
+}
diff --git a/src/go/doc/doc_test.go b/src/go/doc/doc_test.go
new file mode 100644
index 0000000..cbdca62
--- /dev/null
+++ b/src/go/doc/doc_test.go
@@ -0,0 +1,164 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/printer"
+	"go/token"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+	"text/template"
+)
+
+var update = flag.Bool("update", false, "update golden (.out) files")
+var files = flag.String("files", "", "consider only Go test files matching this regular expression")
+
+const dataDir = "testdata"
+
+var templateTxt = readTemplate("template.txt")
+
+func readTemplate(filename string) *template.Template {
+	t := template.New(filename)
+	t.Funcs(template.FuncMap{
+		"node":     nodeFmt,
+		"synopsis": synopsisFmt,
+		"indent":   indentFmt,
+	})
+	return template.Must(t.ParseFiles(filepath.Join(dataDir, filename)))
+}
+
+func nodeFmt(node interface{}, fset *token.FileSet) string {
+	var buf bytes.Buffer
+	printer.Fprint(&buf, fset, node)
+	return strings.ReplaceAll(strings.TrimSpace(buf.String()), "\n", "\n\t")
+}
+
+func synopsisFmt(s string) string {
+	const n = 64
+	if len(s) > n {
+		// cut off excess text and go back to a word boundary
+		s = s[0:n]
+		if i := strings.LastIndexAny(s, "\t\n "); i >= 0 {
+			s = s[0:i]
+		}
+		s = strings.TrimSpace(s) + " ..."
+	}
+	return "// " + strings.ReplaceAll(s, "\n", " ")
+}
+
+func indentFmt(indent, s string) string {
+	end := ""
+	if strings.HasSuffix(s, "\n") {
+		end = "\n"
+		s = s[:len(s)-1]
+	}
+	return indent + strings.ReplaceAll(s, "\n", "\n"+indent) + end
+}
+
+func isGoFile(fi fs.FileInfo) bool {
+	name := fi.Name()
+	return !fi.IsDir() &&
+		len(name) > 0 && name[0] != '.' && // ignore .files
+		filepath.Ext(name) == ".go"
+}
+
+type bundle struct {
+	*Package
+	FSet *token.FileSet
+}
+
+func test(t *testing.T, mode Mode) {
+	// determine file filter
+	filter := isGoFile
+	if *files != "" {
+		rx, err := regexp.Compile(*files)
+		if err != nil {
+			t.Fatal(err)
+		}
+		filter = func(fi fs.FileInfo) bool {
+			return isGoFile(fi) && rx.MatchString(fi.Name())
+		}
+	}
+
+	// get packages
+	fset := token.NewFileSet()
+	pkgs, err := parser.ParseDir(fset, dataDir, filter, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// test packages
+	for _, pkg := range pkgs {
+		importPath := dataDir + "/" + pkg.Name
+		var files []*ast.File
+		for _, f := range pkg.Files {
+			files = append(files, f)
+		}
+		doc, err := NewFromFiles(fset, files, importPath, mode)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+
+		// golden files always use / in filenames - canonicalize them
+		for i, filename := range doc.Filenames {
+			doc.Filenames[i] = filepath.ToSlash(filename)
+		}
+
+		// print documentation
+		var buf bytes.Buffer
+		if err := templateTxt.Execute(&buf, bundle{doc, fset}); err != nil {
+			t.Error(err)
+			continue
+		}
+		got := buf.Bytes()
+
+		// update golden file if necessary
+		golden := filepath.Join(dataDir, fmt.Sprintf("%s.%d.golden", pkg.Name, mode))
+		if *update {
+			err := os.WriteFile(golden, got, 0644)
+			if err != nil {
+				t.Error(err)
+			}
+			continue
+		}
+
+		// get golden file
+		want, err := os.ReadFile(golden)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+
+		// compare
+		if !bytes.Equal(got, want) {
+			t.Errorf("package %s\n\tgot:\n%s\n\twant:\n%s", pkg.Name, got, want)
+		}
+	}
+}
+
+func Test(t *testing.T) {
+	test(t, 0)
+	test(t, AllDecls)
+	test(t, AllMethods)
+}
+
+func TestAnchorID(t *testing.T) {
+	const in = "Important Things 2 Know & Stuff"
+	const want = "hdr-Important_Things_2_Know___Stuff"
+	got := anchorID(in)
+	if got != want {
+		t.Errorf("anchorID(%q) = %q; want %q", in, got, want)
+	}
+}
diff --git a/src/go/doc/example.go b/src/go/doc/example.go
new file mode 100644
index 0000000..125fd53
--- /dev/null
+++ b/src/go/doc/example.go
@@ -0,0 +1,547 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Extract example functions from file ASTs.
+
+package doc
+
+import (
+	"go/ast"
+	"go/token"
+	"internal/lazyregexp"
+	"path"
+	"sort"
+	"strconv"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// An Example represents an example function found in a test source file.
+type Example struct {
+	Name        string // name of the item being exemplified (including optional suffix)
+	Suffix      string // example suffix, without leading '_' (only populated by NewFromFiles)
+	Doc         string // example function doc string
+	Code        ast.Node
+	Play        *ast.File // a whole program version of the example
+	Comments    []*ast.CommentGroup
+	Output      string // expected output
+	Unordered   bool
+	EmptyOutput bool // expect empty output
+	Order       int  // original source code order
+}
+
+// Examples returns the examples found in testFiles, sorted by Name field.
+// The Order fields record the order in which the examples were encountered.
+// The Suffix field is not populated when Examples is called directly, it is
+// only populated by NewFromFiles for examples it finds in _test.go files.
+//
+// Playable Examples must be in a package whose name ends in "_test".
+// An Example is "playable" (the Play field is non-nil) in either of these
+// circumstances:
+//   - The example function is self-contained: the function references only
+//     identifiers from other packages (or predeclared identifiers, such as
+//     "int") and the test file does not include a dot import.
+//   - The entire test file is the example: the file contains exactly one
+//     example function, zero test or benchmark functions, and at least one
+//     top-level function, type, variable, or constant declaration other
+//     than the example function.
+func Examples(testFiles ...*ast.File) []*Example {
+	var list []*Example
+	for _, file := range testFiles {
+		hasTests := false // file contains tests or benchmarks
+		numDecl := 0      // number of non-import declarations in the file
+		var flist []*Example
+		for _, decl := range file.Decls {
+			if g, ok := decl.(*ast.GenDecl); ok && g.Tok != token.IMPORT {
+				numDecl++
+				continue
+			}
+			f, ok := decl.(*ast.FuncDecl)
+			if !ok || f.Recv != nil {
+				continue
+			}
+			numDecl++
+			name := f.Name.Name
+			if isTest(name, "Test") || isTest(name, "Benchmark") {
+				hasTests = true
+				continue
+			}
+			if !isTest(name, "Example") {
+				continue
+			}
+			if params := f.Type.Params; len(params.List) != 0 {
+				continue // function has params; not a valid example
+			}
+			if f.Body == nil { // ast.File.Body nil dereference (see issue 28044)
+				continue
+			}
+			var doc string
+			if f.Doc != nil {
+				doc = f.Doc.Text()
+			}
+			output, unordered, hasOutput := exampleOutput(f.Body, file.Comments)
+			flist = append(flist, &Example{
+				Name:        name[len("Example"):],
+				Doc:         doc,
+				Code:        f.Body,
+				Play:        playExample(file, f),
+				Comments:    file.Comments,
+				Output:      output,
+				Unordered:   unordered,
+				EmptyOutput: output == "" && hasOutput,
+				Order:       len(flist),
+			})
+		}
+		if !hasTests && numDecl > 1 && len(flist) == 1 {
+			// If this file only has one example function, some
+			// other top-level declarations, and no tests or
+			// benchmarks, use the whole file as the example.
+			flist[0].Code = file
+			flist[0].Play = playExampleFile(file)
+		}
+		list = append(list, flist...)
+	}
+	// sort by name
+	sort.Slice(list, func(i, j int) bool {
+		return list[i].Name < list[j].Name
+	})
+	return list
+}
+
+var outputPrefix = lazyregexp.New(`(?i)^[[:space:]]*(unordered )?output:`)
+
+// Extracts the expected output and whether there was a valid output comment
+func exampleOutput(b *ast.BlockStmt, comments []*ast.CommentGroup) (output string, unordered, ok bool) {
+	if _, last := lastComment(b, comments); last != nil {
+		// test that it begins with the correct prefix
+		text := last.Text()
+		if loc := outputPrefix.FindStringSubmatchIndex(text); loc != nil {
+			if loc[2] != -1 {
+				unordered = true
+			}
+			text = text[loc[1]:]
+			// Strip zero or more spaces followed by \n or a single space.
+			text = strings.TrimLeft(text, " ")
+			if len(text) > 0 && text[0] == '\n' {
+				text = text[1:]
+			}
+			return text, unordered, true
+		}
+	}
+	return "", false, false // no suitable comment found
+}
+
+// isTest tells whether name looks like a test, example, or benchmark.
+// It is a Test (say) if there is a character after Test that is not a
+// lower-case letter. (We don't want Testiness.)
+func isTest(name, prefix string) bool {
+	if !strings.HasPrefix(name, prefix) {
+		return false
+	}
+	if len(name) == len(prefix) { // "Test" is ok
+		return true
+	}
+	rune, _ := utf8.DecodeRuneInString(name[len(prefix):])
+	return !unicode.IsLower(rune)
+}
+
+// playExample synthesizes a new *ast.File based on the provided
+// file with the provided function body as the body of main.
+func playExample(file *ast.File, f *ast.FuncDecl) *ast.File {
+	body := f.Body
+
+	if !strings.HasSuffix(file.Name.Name, "_test") {
+		// We don't support examples that are part of the
+		// greater package (yet).
+		return nil
+	}
+
+	// Collect top-level declarations in the file.
+	topDecls := make(map[*ast.Object]ast.Decl)
+	typMethods := make(map[string][]ast.Decl)
+
+	for _, decl := range file.Decls {
+		switch d := decl.(type) {
+		case *ast.FuncDecl:
+			if d.Recv == nil {
+				topDecls[d.Name.Obj] = d
+			} else {
+				if len(d.Recv.List) == 1 {
+					t := d.Recv.List[0].Type
+					tname, _ := baseTypeName(t)
+					typMethods[tname] = append(typMethods[tname], d)
+				}
+			}
+		case *ast.GenDecl:
+			for _, spec := range d.Specs {
+				switch s := spec.(type) {
+				case *ast.TypeSpec:
+					topDecls[s.Name.Obj] = d
+				case *ast.ValueSpec:
+					for _, name := range s.Names {
+						topDecls[name.Obj] = d
+					}
+				}
+			}
+		}
+	}
+
+	// Find unresolved identifiers and uses of top-level declarations.
+	unresolved := make(map[string]bool)
+	var depDecls []ast.Decl
+	hasDepDecls := make(map[ast.Decl]bool)
+
+	var inspectFunc func(ast.Node) bool
+	inspectFunc = func(n ast.Node) bool {
+		switch e := n.(type) {
+		case *ast.Ident:
+			if e.Obj == nil && e.Name != "_" {
+				unresolved[e.Name] = true
+			} else if d := topDecls[e.Obj]; d != nil {
+				if !hasDepDecls[d] {
+					hasDepDecls[d] = true
+					depDecls = append(depDecls, d)
+				}
+			}
+			return true
+		case *ast.SelectorExpr:
+			// For selector expressions, only inspect the left hand side.
+			// (For an expression like fmt.Println, only add "fmt" to the
+			// set of unresolved names, not "Println".)
+			ast.Inspect(e.X, inspectFunc)
+			return false
+		case *ast.KeyValueExpr:
+			// For key value expressions, only inspect the value
+			// as the key should be resolved by the type of the
+			// composite literal.
+			ast.Inspect(e.Value, inspectFunc)
+			return false
+		}
+		return true
+	}
+	ast.Inspect(body, inspectFunc)
+	for i := 0; i < len(depDecls); i++ {
+		switch d := depDecls[i].(type) {
+		case *ast.FuncDecl:
+			// Inspect types of parameters and results. See #28492.
+			if d.Type.Params != nil {
+				for _, p := range d.Type.Params.List {
+					ast.Inspect(p.Type, inspectFunc)
+				}
+			}
+			if d.Type.Results != nil {
+				for _, r := range d.Type.Results.List {
+					ast.Inspect(r.Type, inspectFunc)
+				}
+			}
+
+			ast.Inspect(d.Body, inspectFunc)
+		case *ast.GenDecl:
+			for _, spec := range d.Specs {
+				switch s := spec.(type) {
+				case *ast.TypeSpec:
+					ast.Inspect(s.Type, inspectFunc)
+
+					depDecls = append(depDecls, typMethods[s.Name.Name]...)
+				case *ast.ValueSpec:
+					if s.Type != nil {
+						ast.Inspect(s.Type, inspectFunc)
+					}
+					for _, val := range s.Values {
+						ast.Inspect(val, inspectFunc)
+					}
+				}
+			}
+		}
+	}
+
+	// Remove predeclared identifiers from unresolved list.
+	for n := range unresolved {
+		if predeclaredTypes[n] || predeclaredConstants[n] || predeclaredFuncs[n] {
+			delete(unresolved, n)
+		}
+	}
+
+	// Use unresolved identifiers to determine the imports used by this
+	// example. The heuristic assumes package names match base import
+	// paths for imports w/o renames (should be good enough most of the time).
+	namedImports := make(map[string]string) // [name]path
+	var blankImports []ast.Spec             // _ imports
+	for _, s := range file.Imports {
+		p, err := strconv.Unquote(s.Path.Value)
+		if err != nil {
+			continue
+		}
+		if p == "syscall/js" {
+			// We don't support examples that import syscall/js,
+			// because the package syscall/js is not available in the playground.
+			return nil
+		}
+		n := path.Base(p)
+		if s.Name != nil {
+			n = s.Name.Name
+			switch n {
+			case "_":
+				blankImports = append(blankImports, s)
+				continue
+			case ".":
+				// We can't resolve dot imports (yet).
+				return nil
+			}
+		}
+		if unresolved[n] {
+			namedImports[n] = p
+			delete(unresolved, n)
+		}
+	}
+
+	// If there are other unresolved identifiers, give up because this
+	// synthesized file is not going to build.
+	if len(unresolved) > 0 {
+		return nil
+	}
+
+	// Include documentation belonging to blank imports.
+	var comments []*ast.CommentGroup
+	for _, s := range blankImports {
+		if c := s.(*ast.ImportSpec).Doc; c != nil {
+			comments = append(comments, c)
+		}
+	}
+
+	// Include comments that are inside the function body.
+	for _, c := range file.Comments {
+		if body.Pos() <= c.Pos() && c.End() <= body.End() {
+			comments = append(comments, c)
+		}
+	}
+
+	// Strip the "Output:" or "Unordered output:" comment and adjust body
+	// end position.
+	body, comments = stripOutputComment(body, comments)
+
+	// Include documentation belonging to dependent declarations.
+	for _, d := range depDecls {
+		switch d := d.(type) {
+		case *ast.GenDecl:
+			if d.Doc != nil {
+				comments = append(comments, d.Doc)
+			}
+		case *ast.FuncDecl:
+			if d.Doc != nil {
+				comments = append(comments, d.Doc)
+			}
+		}
+	}
+
+	// Synthesize import declaration.
+	importDecl := &ast.GenDecl{
+		Tok:    token.IMPORT,
+		Lparen: 1, // Need non-zero Lparen and Rparen so that printer
+		Rparen: 1, // treats this as a factored import.
+	}
+	for n, p := range namedImports {
+		s := &ast.ImportSpec{Path: &ast.BasicLit{Value: strconv.Quote(p)}}
+		if path.Base(p) != n {
+			s.Name = ast.NewIdent(n)
+		}
+		importDecl.Specs = append(importDecl.Specs, s)
+	}
+	importDecl.Specs = append(importDecl.Specs, blankImports...)
+
+	// Synthesize main function.
+	funcDecl := &ast.FuncDecl{
+		Name: ast.NewIdent("main"),
+		Type: f.Type,
+		Body: body,
+	}
+
+	decls := make([]ast.Decl, 0, 2+len(depDecls))
+	decls = append(decls, importDecl)
+	decls = append(decls, depDecls...)
+	decls = append(decls, funcDecl)
+
+	sort.Slice(decls, func(i, j int) bool {
+		return decls[i].Pos() < decls[j].Pos()
+	})
+
+	sort.Slice(comments, func(i, j int) bool {
+		return comments[i].Pos() < comments[j].Pos()
+	})
+
+	// Synthesize file.
+	return &ast.File{
+		Name:     ast.NewIdent("main"),
+		Decls:    decls,
+		Comments: comments,
+	}
+}
+
+// playExampleFile takes a whole file example and synthesizes a new *ast.File
+// such that the example is function main in package main.
+func playExampleFile(file *ast.File) *ast.File {
+	// Strip copyright comment if present.
+	comments := file.Comments
+	if len(comments) > 0 && strings.HasPrefix(comments[0].Text(), "Copyright") {
+		comments = comments[1:]
+	}
+
+	// Copy declaration slice, rewriting the ExampleX function to main.
+	var decls []ast.Decl
+	for _, d := range file.Decls {
+		if f, ok := d.(*ast.FuncDecl); ok && isTest(f.Name.Name, "Example") {
+			// Copy the FuncDecl, as it may be used elsewhere.
+			newF := *f
+			newF.Name = ast.NewIdent("main")
+			newF.Body, comments = stripOutputComment(f.Body, comments)
+			d = &newF
+		}
+		decls = append(decls, d)
+	}
+
+	// Copy the File, as it may be used elsewhere.
+	f := *file
+	f.Name = ast.NewIdent("main")
+	f.Decls = decls
+	f.Comments = comments
+	return &f
+}
+
+// stripOutputComment finds and removes the "Output:" or "Unordered output:"
+// comment from body and comments, and adjusts the body block's end position.
+func stripOutputComment(body *ast.BlockStmt, comments []*ast.CommentGroup) (*ast.BlockStmt, []*ast.CommentGroup) {
+	// Do nothing if there is no "Output:" or "Unordered output:" comment.
+	i, last := lastComment(body, comments)
+	if last == nil || !outputPrefix.MatchString(last.Text()) {
+		return body, comments
+	}
+
+	// Copy body and comments, as the originals may be used elsewhere.
+	newBody := &ast.BlockStmt{
+		Lbrace: body.Lbrace,
+		List:   body.List,
+		Rbrace: last.Pos(),
+	}
+	newComments := make([]*ast.CommentGroup, len(comments)-1)
+	copy(newComments, comments[:i])
+	copy(newComments[i:], comments[i+1:])
+	return newBody, newComments
+}
+
+// lastComment returns the last comment inside the provided block.
+func lastComment(b *ast.BlockStmt, c []*ast.CommentGroup) (i int, last *ast.CommentGroup) {
+	if b == nil {
+		return
+	}
+	pos, end := b.Pos(), b.End()
+	for j, cg := range c {
+		if cg.Pos() < pos {
+			continue
+		}
+		if cg.End() > end {
+			break
+		}
+		i, last = j, cg
+	}
+	return
+}
+
+// classifyExamples classifies examples and assigns them to the Examples field
+// of the relevant Func, Type, or Package that the example is associated with.
+//
+// The classification process is ambiguous in some cases:
+//
+// 	- ExampleFoo_Bar matches a type named Foo_Bar
+// 	  or a method named Foo.Bar.
+// 	- ExampleFoo_bar matches a type named Foo_bar
+// 	  or Foo (with a "bar" suffix).
+//
+// Examples with malformed names are not associated with anything.
+//
+func classifyExamples(p *Package, examples []*Example) {
+	if len(examples) == 0 {
+		return
+	}
+
+	// Mapping of names for funcs, types, and methods to the example listing.
+	ids := make(map[string]*[]*Example)
+	ids[""] = &p.Examples // package-level examples have an empty name
+	for _, f := range p.Funcs {
+		if !token.IsExported(f.Name) {
+			continue
+		}
+		ids[f.Name] = &f.Examples
+	}
+	for _, t := range p.Types {
+		if !token.IsExported(t.Name) {
+			continue
+		}
+		ids[t.Name] = &t.Examples
+		for _, f := range t.Funcs {
+			if !token.IsExported(f.Name) {
+				continue
+			}
+			ids[f.Name] = &f.Examples
+		}
+		for _, m := range t.Methods {
+			if !token.IsExported(m.Name) {
+				continue
+			}
+			ids[strings.TrimPrefix(m.Recv, "*")+"_"+m.Name] = &m.Examples
+		}
+	}
+
+	// Group each example with the associated func, type, or method.
+	for _, ex := range examples {
+		// Consider all possible split points for the suffix
+		// by starting at the end of string (no suffix case),
+		// then trying all positions that contain a '_' character.
+		//
+		// An association is made on the first successful match.
+		// Examples with malformed names that match nothing are skipped.
+		for i := len(ex.Name); i >= 0; i = strings.LastIndexByte(ex.Name[:i], '_') {
+			prefix, suffix, ok := splitExampleName(ex.Name, i)
+			if !ok {
+				continue
+			}
+			exs, ok := ids[prefix]
+			if !ok {
+				continue
+			}
+			ex.Suffix = suffix
+			*exs = append(*exs, ex)
+			break
+		}
+	}
+
+	// Sort list of example according to the user-specified suffix name.
+	for _, exs := range ids {
+		sort.Slice((*exs), func(i, j int) bool {
+			return (*exs)[i].Suffix < (*exs)[j].Suffix
+		})
+	}
+}
+
+// splitExampleName attempts to split example name s at index i,
+// and reports if that produces a valid split. The suffix may be
+// absent. Otherwise, it must start with a lower-case letter and
+// be preceded by '_'.
+//
+// One of i == len(s) or s[i] == '_' must be true.
+func splitExampleName(s string, i int) (prefix, suffix string, ok bool) {
+	if i == len(s) {
+		return s, "", true
+	}
+	if i == len(s)-1 {
+		return "", "", false
+	}
+	prefix, suffix = s[:i], s[i+1:]
+	return prefix, suffix, isExampleSuffix(suffix)
+}
+
+func isExampleSuffix(s string) bool {
+	r, size := utf8.DecodeRuneInString(s)
+	return size > 0 && unicode.IsLower(r)
+}
diff --git a/src/go/doc/example_test.go b/src/go/doc/example_test.go
new file mode 100644
index 0000000..7c96f03
--- /dev/null
+++ b/src/go/doc/example_test.go
@@ -0,0 +1,718 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc_test
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/doc"
+	"go/format"
+	"go/parser"
+	"go/token"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+const exampleTestFile = `
+package foo_test
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"sort"
+	"os/exec"
+)
+
+func ExampleHello() {
+	fmt.Println("Hello, world!")
+	// Output: Hello, world!
+}
+
+func ExampleImport() {
+	out, err := exec.Command("date").Output()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("The date is %s\n", out)
+}
+
+func ExampleKeyValue() {
+	v := struct {
+		a string
+		b int
+	}{
+		a: "A",
+		b: 1,
+	}
+	fmt.Print(v)
+	// Output: a: "A", b: 1
+}
+
+func ExampleKeyValueImport() {
+	f := flag.Flag{
+		Name: "play",
+	}
+	fmt.Print(f)
+	// Output: Name: "play"
+}
+
+var keyValueTopDecl = struct {
+	a string
+	b int
+}{
+	a: "B",
+	b: 2,
+}
+
+func ExampleKeyValueTopDecl() {
+	fmt.Print(keyValueTopDecl)
+	// Output: a: "B", b: 2
+}
+
+// Person represents a person by name and age.
+type Person struct {
+    Name string
+    Age  int
+}
+
+// String returns a string representation of the Person.
+func (p Person) String() string {
+    return fmt.Sprintf("%s: %d", p.Name, p.Age)
+}
+
+// ByAge implements sort.Interface for []Person based on
+// the Age field.
+type ByAge []Person
+
+// Len returns the number of elements in ByAge.
+func (a (ByAge)) Len() int { return len(a) }
+
+// Swap swaps the elements in ByAge.
+func (a ByAge) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+func (a ByAge) Less(i, j int) bool { return a[i].Age < a[j].Age }
+
+// people is the array of Person
+var people = []Person{
+	{"Bob", 31},
+	{"John", 42},
+	{"Michael", 17},
+	{"Jenny", 26},
+}
+
+func ExampleSort() {
+    fmt.Println(people)
+    sort.Sort(ByAge(people))
+    fmt.Println(people)
+    // Output:
+    // [Bob: 31 John: 42 Michael: 17 Jenny: 26]
+    // [Michael: 17 Jenny: 26 Bob: 31 John: 42]
+}
+`
+
+var exampleTestCases = []struct {
+	Name, Play, Output string
+}{
+	{
+		Name:   "Hello",
+		Play:   exampleHelloPlay,
+		Output: "Hello, world!\n",
+	},
+	{
+		Name: "Import",
+		Play: exampleImportPlay,
+	},
+	{
+		Name:   "KeyValue",
+		Play:   exampleKeyValuePlay,
+		Output: "a: \"A\", b: 1\n",
+	},
+	{
+		Name:   "KeyValueImport",
+		Play:   exampleKeyValueImportPlay,
+		Output: "Name: \"play\"\n",
+	},
+	{
+		Name:   "KeyValueTopDecl",
+		Play:   exampleKeyValueTopDeclPlay,
+		Output: "a: \"B\", b: 2\n",
+	},
+	{
+		Name:   "Sort",
+		Play:   exampleSortPlay,
+		Output: "[Bob: 31 John: 42 Michael: 17 Jenny: 26]\n[Michael: 17 Jenny: 26 Bob: 31 John: 42]\n",
+	},
+}
+
+const exampleHelloPlay = `package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	fmt.Println("Hello, world!")
+}
+`
+const exampleImportPlay = `package main
+
+import (
+	"fmt"
+	"log"
+	"os/exec"
+)
+
+func main() {
+	out, err := exec.Command("date").Output()
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("The date is %s\n", out)
+}
+`
+
+const exampleKeyValuePlay = `package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	v := struct {
+		a string
+		b int
+	}{
+		a: "A",
+		b: 1,
+	}
+	fmt.Print(v)
+}
+`
+
+const exampleKeyValueImportPlay = `package main
+
+import (
+	"flag"
+	"fmt"
+)
+
+func main() {
+	f := flag.Flag{
+		Name: "play",
+	}
+	fmt.Print(f)
+}
+`
+
+const exampleKeyValueTopDeclPlay = `package main
+
+import (
+	"fmt"
+)
+
+var keyValueTopDecl = struct {
+	a string
+	b int
+}{
+	a: "B",
+	b: 2,
+}
+
+func main() {
+	fmt.Print(keyValueTopDecl)
+}
+`
+
+const exampleSortPlay = `package main
+
+import (
+	"fmt"
+	"sort"
+)
+
+// Person represents a person by name and age.
+type Person struct {
+	Name string
+	Age  int
+}
+
+// String returns a string representation of the Person.
+func (p Person) String() string {
+	return fmt.Sprintf("%s: %d", p.Name, p.Age)
+}
+
+// ByAge implements sort.Interface for []Person based on
+// the Age field.
+type ByAge []Person
+
+// Len returns the number of elements in ByAge.
+func (a ByAge) Len() int { return len(a) }
+
+// Swap swaps the elements in ByAge.
+func (a ByAge) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+func (a ByAge) Less(i, j int) bool { return a[i].Age < a[j].Age }
+
+// people is the array of Person
+var people = []Person{
+	{"Bob", 31},
+	{"John", 42},
+	{"Michael", 17},
+	{"Jenny", 26},
+}
+
+func main() {
+	fmt.Println(people)
+	sort.Sort(ByAge(people))
+	fmt.Println(people)
+}
+`
+
+func TestExamples(t *testing.T) {
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "test.go", strings.NewReader(exampleTestFile), parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, e := range doc.Examples(file) {
+		c := exampleTestCases[i]
+		if e.Name != c.Name {
+			t.Errorf("got Name == %q, want %q", e.Name, c.Name)
+		}
+		if w := c.Play; w != "" {
+			g := formatFile(t, fset, e.Play)
+			if g != w {
+				t.Errorf("%s: got Play == %q, want %q", c.Name, g, w)
+			}
+		}
+		if g, w := e.Output, c.Output; g != w {
+			t.Errorf("%s: got Output == %q, want %q", c.Name, g, w)
+		}
+	}
+}
+
+const exampleWholeFile = `package foo_test
+
+type X int
+
+func (X) Foo() {
+}
+
+func (X) TestBlah() {
+}
+
+func (X) BenchmarkFoo() {
+}
+
+func Example() {
+	fmt.Println("Hello, world!")
+	// Output: Hello, world!
+}
+`
+
+const exampleWholeFileOutput = `package main
+
+type X int
+
+func (X) Foo() {
+}
+
+func (X) TestBlah() {
+}
+
+func (X) BenchmarkFoo() {
+}
+
+func main() {
+	fmt.Println("Hello, world!")
+}
+`
+
+const exampleWholeFileFunction = `package foo_test
+
+func Foo(x int) {
+}
+
+func Example() {
+	fmt.Println("Hello, world!")
+	// Output: Hello, world!
+}
+`
+
+const exampleWholeFileFunctionOutput = `package main
+
+func Foo(x int) {
+}
+
+func main() {
+	fmt.Println("Hello, world!")
+}
+`
+
+var exampleWholeFileTestCases = []struct {
+	Title, Source, Play, Output string
+}{
+	{
+		"Methods",
+		exampleWholeFile,
+		exampleWholeFileOutput,
+		"Hello, world!\n",
+	},
+	{
+		"Function",
+		exampleWholeFileFunction,
+		exampleWholeFileFunctionOutput,
+		"Hello, world!\n",
+	},
+}
+
+func TestExamplesWholeFile(t *testing.T) {
+	for _, c := range exampleWholeFileTestCases {
+		fset := token.NewFileSet()
+		file, err := parser.ParseFile(fset, "test.go", strings.NewReader(c.Source), parser.ParseComments)
+		if err != nil {
+			t.Fatal(err)
+		}
+		es := doc.Examples(file)
+		if len(es) != 1 {
+			t.Fatalf("%s: wrong number of examples; got %d want 1", c.Title, len(es))
+		}
+		e := es[0]
+		if e.Name != "" {
+			t.Errorf("%s: got Name == %q, want %q", c.Title, e.Name, "")
+		}
+		if g, w := formatFile(t, fset, e.Play), c.Play; g != w {
+			t.Errorf("%s: got Play == %q, want %q", c.Title, g, w)
+		}
+		if g, w := e.Output, c.Output; g != w {
+			t.Errorf("%s: got Output == %q, want %q", c.Title, g, w)
+		}
+	}
+}
+
+const exampleInspectSignature = `package foo_test
+
+import (
+	"bytes"
+	"io"
+)
+
+func getReader() io.Reader { return nil }
+
+func do(b bytes.Reader) {}
+
+func Example() {
+	getReader()
+	do()
+	// Output:
+}
+
+func ExampleIgnored() {
+}
+`
+
+const exampleInspectSignatureOutput = `package main
+
+import (
+	"bytes"
+	"io"
+)
+
+func getReader() io.Reader { return nil }
+
+func do(b bytes.Reader) {}
+
+func main() {
+	getReader()
+	do()
+}
+`
+
+func TestExampleInspectSignature(t *testing.T) {
+	// Verify that "bytes" and "io" are imported. See issue #28492.
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "test.go", strings.NewReader(exampleInspectSignature), parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	es := doc.Examples(file)
+	if len(es) != 2 {
+		t.Fatalf("wrong number of examples; got %d want 2", len(es))
+	}
+	// We are interested in the first example only.
+	e := es[0]
+	if e.Name != "" {
+		t.Errorf("got Name == %q, want %q", e.Name, "")
+	}
+	if g, w := formatFile(t, fset, e.Play), exampleInspectSignatureOutput; g != w {
+		t.Errorf("got Play == %q, want %q", g, w)
+	}
+	if g, w := e.Output, ""; g != w {
+		t.Errorf("got Output == %q, want %q", g, w)
+	}
+}
+
+const exampleEmpty = `
+package p
+func Example() {}
+func Example_a()
+`
+
+const exampleEmptyOutput = `package main
+
+func main() {}
+func main()
+`
+
+func TestExampleEmpty(t *testing.T) {
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "test.go", strings.NewReader(exampleEmpty), parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	es := doc.Examples(file)
+	if len(es) != 1 {
+		t.Fatalf("wrong number of examples; got %d want 1", len(es))
+	}
+	e := es[0]
+	if e.Name != "" {
+		t.Errorf("got Name == %q, want %q", e.Name, "")
+	}
+	if g, w := formatFile(t, fset, e.Play), exampleEmptyOutput; g != w {
+		t.Errorf("got Play == %q, want %q", g, w)
+	}
+	if g, w := e.Output, ""; g != w {
+		t.Errorf("got Output == %q, want %q", g, w)
+	}
+}
+
+func formatFile(t *testing.T, fset *token.FileSet, n *ast.File) string {
+	if n == nil {
+		return "<nil>"
+	}
+	var buf bytes.Buffer
+	if err := format.Node(&buf, fset, n); err != nil {
+		t.Fatal(err)
+	}
+	return buf.String()
+}
+
+// This example illustrates how to use NewFromFiles
+// to compute package documentation with examples.
+func ExampleNewFromFiles() {
+	// src and test are two source files that make up
+	// a package whose documentation will be computed.
+	const src = `
+// This is the package comment.
+package p
+
+import "fmt"
+
+// This comment is associated with the Greet function.
+func Greet(who string) {
+	fmt.Printf("Hello, %s!\n", who)
+}
+`
+	const test = `
+package p_test
+
+// This comment is associated with the ExampleGreet_world example.
+func ExampleGreet_world() {
+	Greet("world")
+}
+`
+
+	// Create the AST by parsing src and test.
+	fset := token.NewFileSet()
+	files := []*ast.File{
+		mustParse(fset, "src.go", src),
+		mustParse(fset, "src_test.go", test),
+	}
+
+	// Compute package documentation with examples.
+	p, err := doc.NewFromFiles(fset, files, "example.com/p")
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Printf("package %s - %s", p.Name, p.Doc)
+	fmt.Printf("func %s - %s", p.Funcs[0].Name, p.Funcs[0].Doc)
+	fmt.Printf(" ⤷ example with suffix %q - %s", p.Funcs[0].Examples[0].Suffix, p.Funcs[0].Examples[0].Doc)
+
+	// Output:
+	// package p - This is the package comment.
+	// func Greet - This comment is associated with the Greet function.
+	//  ⤷ example with suffix "world" - This comment is associated with the ExampleGreet_world example.
+}
+
+func TestClassifyExamples(t *testing.T) {
+	const src = `
+package p
+
+const Const1 = 0
+var   Var1   = 0
+
+type (
+	Type1     int
+	Type1_Foo int
+	Type1_foo int
+	type2     int
+
+	Embed struct { Type1 }
+	Uembed struct { type2 }
+)
+
+func Func1()     {}
+func Func1_Foo() {}
+func Func1_foo() {}
+func func2()     {}
+
+func (Type1) Func1() {}
+func (Type1) Func1_Foo() {}
+func (Type1) Func1_foo() {}
+func (Type1) func2() {}
+
+func (type2) Func1() {}
+
+type (
+	Conflict          int
+	Conflict_Conflict int
+	Conflict_conflict int
+)
+
+func (Conflict) Conflict() {}
+`
+	const test = `
+package p_test
+
+func ExampleConst1() {} // invalid - no support for consts and vars
+func ExampleVar1()   {} // invalid - no support for consts and vars
+
+func Example()               {}
+func Example_()              {} // invalid - suffix must start with a lower-case letter
+func Example_suffix()        {}
+func Example_suffix_xX_X_x() {}
+func Example_世界()           {} // invalid - suffix must start with a lower-case letter
+func Example_123()           {} // invalid - suffix must start with a lower-case letter
+func Example_BadSuffix()     {} // invalid - suffix must start with a lower-case letter
+
+func ExampleType1()               {}
+func ExampleType1_()              {} // invalid - suffix must start with a lower-case letter
+func ExampleType1_suffix()        {}
+func ExampleType1_BadSuffix()     {} // invalid - suffix must start with a lower-case letter
+func ExampleType1_Foo()           {}
+func ExampleType1_Foo_suffix()    {}
+func ExampleType1_Foo_BadSuffix() {} // invalid - suffix must start with a lower-case letter
+func ExampleType1_foo()           {}
+func ExampleType1_foo_suffix()    {}
+func ExampleType1_foo_Suffix()    {} // matches Type1, instead of Type1_foo
+func Exampletype2()               {} // invalid - cannot match unexported
+
+func ExampleFunc1()               {}
+func ExampleFunc1_()              {} // invalid - suffix must start with a lower-case letter
+func ExampleFunc1_suffix()        {}
+func ExampleFunc1_BadSuffix()     {} // invalid - suffix must start with a lower-case letter
+func ExampleFunc1_Foo()           {}
+func ExampleFunc1_Foo_suffix()    {}
+func ExampleFunc1_Foo_BadSuffix() {} // invalid - suffix must start with a lower-case letter
+func ExampleFunc1_foo()           {}
+func ExampleFunc1_foo_suffix()    {}
+func ExampleFunc1_foo_Suffix()    {} // matches Func1, instead of Func1_foo
+func Examplefunc1()               {} // invalid - cannot match unexported
+
+func ExampleType1_Func1()               {}
+func ExampleType1_Func1_()              {} // invalid - suffix must start with a lower-case letter
+func ExampleType1_Func1_suffix()        {}
+func ExampleType1_Func1_BadSuffix()     {} // invalid - suffix must start with a lower-case letter
+func ExampleType1_Func1_Foo()           {}
+func ExampleType1_Func1_Foo_suffix()    {}
+func ExampleType1_Func1_Foo_BadSuffix() {} // invalid - suffix must start with a lower-case letter
+func ExampleType1_Func1_foo()           {}
+func ExampleType1_Func1_foo_suffix()    {}
+func ExampleType1_Func1_foo_Suffix()    {} // matches Type1.Func1, instead of Type1.Func1_foo
+func ExampleType1_func2()               {} // matches Type1, instead of Type1.func2
+
+func ExampleEmbed_Func1()         {} // invalid - no support for forwarded methods from embedding exported type
+func ExampleUembed_Func1()        {} // methods from embedding unexported types are OK
+func ExampleUembed_Func1_suffix() {}
+
+func ExampleConflict_Conflict()        {} // ambiguous with either Conflict or Conflict_Conflict type
+func ExampleConflict_conflict()        {} // ambiguous with either Conflict or Conflict_conflict type
+func ExampleConflict_Conflict_suffix() {} // ambiguous with either Conflict or Conflict_Conflict type
+func ExampleConflict_conflict_suffix() {} // ambiguous with either Conflict or Conflict_conflict type
+`
+
+	// Parse literal source code as a *doc.Package.
+	fset := token.NewFileSet()
+	files := []*ast.File{
+		mustParse(fset, "src.go", src),
+		mustParse(fset, "src_test.go", test),
+	}
+	p, err := doc.NewFromFiles(fset, files, "example.com/p")
+	if err != nil {
+		t.Fatalf("doc.NewFromFiles: %v", err)
+	}
+
+	// Collect the association of examples to top-level identifiers.
+	got := map[string][]string{}
+	got[""] = exampleNames(p.Examples)
+	for _, f := range p.Funcs {
+		got[f.Name] = exampleNames(f.Examples)
+	}
+	for _, t := range p.Types {
+		got[t.Name] = exampleNames(t.Examples)
+		for _, f := range t.Funcs {
+			got[f.Name] = exampleNames(f.Examples)
+		}
+		for _, m := range t.Methods {
+			got[t.Name+"."+m.Name] = exampleNames(m.Examples)
+		}
+	}
+
+	want := map[string][]string{
+		"": {"", "suffix", "suffix_xX_X_x"}, // Package-level examples.
+
+		"Type1":     {"", "foo_Suffix", "func2", "suffix"},
+		"Type1_Foo": {"", "suffix"},
+		"Type1_foo": {"", "suffix"},
+
+		"Func1":     {"", "foo_Suffix", "suffix"},
+		"Func1_Foo": {"", "suffix"},
+		"Func1_foo": {"", "suffix"},
+
+		"Type1.Func1":     {"", "foo_Suffix", "suffix"},
+		"Type1.Func1_Foo": {"", "suffix"},
+		"Type1.Func1_foo": {"", "suffix"},
+
+		"Uembed.Func1": {"", "suffix"},
+
+		// These are implementation dependent due to the ambiguous parsing.
+		"Conflict_Conflict": {"", "suffix"},
+		"Conflict_conflict": {"", "suffix"},
+	}
+
+	for id := range got {
+		if !reflect.DeepEqual(got[id], want[id]) {
+			t.Errorf("classification mismatch for %q:\ngot  %q\nwant %q", id, got[id], want[id])
+		}
+	}
+}
+
+func exampleNames(exs []*doc.Example) (out []string) {
+	for _, ex := range exs {
+		out = append(out, ex.Suffix)
+	}
+	return out
+}
+
+func mustParse(fset *token.FileSet, filename, src string) *ast.File {
+	f, err := parser.ParseFile(fset, filename, src, parser.ParseComments)
+	if err != nil {
+		panic(err)
+	}
+	return f
+}
diff --git a/src/go/doc/exports.go b/src/go/doc/exports.go
new file mode 100644
index 0000000..819c030
--- /dev/null
+++ b/src/go/doc/exports.go
@@ -0,0 +1,309 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements export filtering of an AST.
+
+package doc
+
+import (
+	"go/ast"
+	"go/token"
+)
+
+// filterIdentList removes unexported names from list in place
+// and returns the resulting list.
+//
+func filterIdentList(list []*ast.Ident) []*ast.Ident {
+	j := 0
+	for _, x := range list {
+		if token.IsExported(x.Name) {
+			list[j] = x
+			j++
+		}
+	}
+	return list[0:j]
+}
+
+var underscore = ast.NewIdent("_")
+
+func filterCompositeLit(lit *ast.CompositeLit, filter Filter, export bool) {
+	n := len(lit.Elts)
+	lit.Elts = filterExprList(lit.Elts, filter, export)
+	if len(lit.Elts) < n {
+		lit.Incomplete = true
+	}
+}
+
+func filterExprList(list []ast.Expr, filter Filter, export bool) []ast.Expr {
+	j := 0
+	for _, exp := range list {
+		switch x := exp.(type) {
+		case *ast.CompositeLit:
+			filterCompositeLit(x, filter, export)
+		case *ast.KeyValueExpr:
+			if x, ok := x.Key.(*ast.Ident); ok && !filter(x.Name) {
+				continue
+			}
+			if x, ok := x.Value.(*ast.CompositeLit); ok {
+				filterCompositeLit(x, filter, export)
+			}
+		}
+		list[j] = exp
+		j++
+	}
+	return list[0:j]
+}
+
+// updateIdentList replaces all unexported identifiers with underscore
+// and reports whether at least one exported name exists.
+func updateIdentList(list []*ast.Ident) (hasExported bool) {
+	for i, x := range list {
+		if token.IsExported(x.Name) {
+			hasExported = true
+		} else {
+			list[i] = underscore
+		}
+	}
+	return hasExported
+}
+
+// hasExportedName reports whether list contains any exported names.
+//
+func hasExportedName(list []*ast.Ident) bool {
+	for _, x := range list {
+		if x.IsExported() {
+			return true
+		}
+	}
+	return false
+}
+
+// removeErrorField removes anonymous fields named "error" from an interface.
+// This is called when "error" has been determined to be a local name,
+// not the predeclared type.
+//
+func removeErrorField(ityp *ast.InterfaceType) {
+	list := ityp.Methods.List // we know that ityp.Methods != nil
+	j := 0
+	for _, field := range list {
+		keepField := true
+		if n := len(field.Names); n == 0 {
+			// anonymous field
+			if fname, _ := baseTypeName(field.Type); fname == "error" {
+				keepField = false
+			}
+		}
+		if keepField {
+			list[j] = field
+			j++
+		}
+	}
+	if j < len(list) {
+		ityp.Incomplete = true
+	}
+	ityp.Methods.List = list[0:j]
+}
+
+// filterFieldList removes unexported fields (field names) from the field list
+// in place and reports whether fields were removed. Anonymous fields are
+// recorded with the parent type. filterType is called with the types of
+// all remaining fields.
+//
+func (r *reader) filterFieldList(parent *namedType, fields *ast.FieldList, ityp *ast.InterfaceType) (removedFields bool) {
+	if fields == nil {
+		return
+	}
+	list := fields.List
+	j := 0
+	for _, field := range list {
+		keepField := false
+		if n := len(field.Names); n == 0 {
+			// anonymous field
+			fname := r.recordAnonymousField(parent, field.Type)
+			if token.IsExported(fname) {
+				keepField = true
+			} else if ityp != nil && fname == "error" {
+				// possibly the predeclared error interface; keep
+				// it for now but remember this interface so that
+				// it can be fixed if error is also defined locally
+				keepField = true
+				r.remember(ityp)
+			}
+		} else {
+			field.Names = filterIdentList(field.Names)
+			if len(field.Names) < n {
+				removedFields = true
+			}
+			if len(field.Names) > 0 {
+				keepField = true
+			}
+		}
+		if keepField {
+			r.filterType(nil, field.Type)
+			list[j] = field
+			j++
+		}
+	}
+	if j < len(list) {
+		removedFields = true
+	}
+	fields.List = list[0:j]
+	return
+}
+
+// filterParamList applies filterType to each parameter type in fields.
+//
+func (r *reader) filterParamList(fields *ast.FieldList) {
+	if fields != nil {
+		for _, f := range fields.List {
+			r.filterType(nil, f.Type)
+		}
+	}
+}
+
+// filterType strips any unexported struct fields or method types from typ
+// in place. If fields (or methods) have been removed, the corresponding
+// struct or interface type has the Incomplete field set to true.
+//
+func (r *reader) filterType(parent *namedType, typ ast.Expr) {
+	switch t := typ.(type) {
+	case *ast.Ident:
+		// nothing to do
+	case *ast.ParenExpr:
+		r.filterType(nil, t.X)
+	case *ast.ArrayType:
+		r.filterType(nil, t.Elt)
+	case *ast.StructType:
+		if r.filterFieldList(parent, t.Fields, nil) {
+			t.Incomplete = true
+		}
+	case *ast.FuncType:
+		r.filterParamList(t.Params)
+		r.filterParamList(t.Results)
+	case *ast.InterfaceType:
+		if r.filterFieldList(parent, t.Methods, t) {
+			t.Incomplete = true
+		}
+	case *ast.MapType:
+		r.filterType(nil, t.Key)
+		r.filterType(nil, t.Value)
+	case *ast.ChanType:
+		r.filterType(nil, t.Value)
+	}
+}
+
+func (r *reader) filterSpec(spec ast.Spec) bool {
+	switch s := spec.(type) {
+	case *ast.ImportSpec:
+		// always keep imports so we can collect them
+		return true
+	case *ast.ValueSpec:
+		s.Values = filterExprList(s.Values, token.IsExported, true)
+		if len(s.Values) > 0 || s.Type == nil && len(s.Values) == 0 {
+			// If there are values declared on RHS, just replace the unexported
+			// identifiers on the LHS with underscore, so that it matches
+			// the sequence of expression on the RHS.
+			//
+			// Similarly, if there are no type and values, then this expression
+			// must be following an iota expression, where order matters.
+			if updateIdentList(s.Names) {
+				r.filterType(nil, s.Type)
+				return true
+			}
+		} else {
+			s.Names = filterIdentList(s.Names)
+			if len(s.Names) > 0 {
+				r.filterType(nil, s.Type)
+				return true
+			}
+		}
+	case *ast.TypeSpec:
+		if name := s.Name.Name; token.IsExported(name) {
+			r.filterType(r.lookupType(s.Name.Name), s.Type)
+			return true
+		} else if name == "error" {
+			// special case: remember that error is declared locally
+			r.errorDecl = true
+		}
+	}
+	return false
+}
+
+// copyConstType returns a copy of typ with position pos.
+// typ must be a valid constant type.
+// In practice, only (possibly qualified) identifiers are possible.
+//
+func copyConstType(typ ast.Expr, pos token.Pos) ast.Expr {
+	switch typ := typ.(type) {
+	case *ast.Ident:
+		return &ast.Ident{Name: typ.Name, NamePos: pos}
+	case *ast.SelectorExpr:
+		if id, ok := typ.X.(*ast.Ident); ok {
+			// presumably a qualified identifier
+			return &ast.SelectorExpr{
+				Sel: ast.NewIdent(typ.Sel.Name),
+				X:   &ast.Ident{Name: id.Name, NamePos: pos},
+			}
+		}
+	}
+	return nil // shouldn't happen, but be conservative and don't panic
+}
+
+func (r *reader) filterSpecList(list []ast.Spec, tok token.Token) []ast.Spec {
+	if tok == token.CONST {
+		// Propagate any type information that would get lost otherwise
+		// when unexported constants are filtered.
+		var prevType ast.Expr
+		for _, spec := range list {
+			spec := spec.(*ast.ValueSpec)
+			if spec.Type == nil && len(spec.Values) == 0 && prevType != nil {
+				// provide current spec with an explicit type
+				spec.Type = copyConstType(prevType, spec.Pos())
+			}
+			if hasExportedName(spec.Names) {
+				// exported names are preserved so there's no need to propagate the type
+				prevType = nil
+			} else {
+				prevType = spec.Type
+			}
+		}
+	}
+
+	j := 0
+	for _, s := range list {
+		if r.filterSpec(s) {
+			list[j] = s
+			j++
+		}
+	}
+	return list[0:j]
+}
+
+func (r *reader) filterDecl(decl ast.Decl) bool {
+	switch d := decl.(type) {
+	case *ast.GenDecl:
+		d.Specs = r.filterSpecList(d.Specs, d.Tok)
+		return len(d.Specs) > 0
+	case *ast.FuncDecl:
+		// ok to filter these methods early because any
+		// conflicting method will be filtered here, too -
+		// thus, removing these methods early will not lead
+		// to the false removal of possible conflicts
+		return token.IsExported(d.Name.Name)
+	}
+	return false
+}
+
+// fileExports removes unexported declarations from src in place.
+//
+func (r *reader) fileExports(src *ast.File) {
+	j := 0
+	for _, d := range src.Decls {
+		if r.filterDecl(d) {
+			src.Decls[j] = d
+			j++
+		}
+	}
+	src.Decls = src.Decls[0:j]
+}
diff --git a/src/go/doc/filter.go b/src/go/doc/filter.go
new file mode 100644
index 0000000..a6f243f
--- /dev/null
+++ b/src/go/doc/filter.go
@@ -0,0 +1,105 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import "go/ast"
+
+type Filter func(string) bool
+
+func matchFields(fields *ast.FieldList, f Filter) bool {
+	if fields != nil {
+		for _, field := range fields.List {
+			for _, name := range field.Names {
+				if f(name.Name) {
+					return true
+				}
+			}
+		}
+	}
+	return false
+}
+
+func matchDecl(d *ast.GenDecl, f Filter) bool {
+	for _, d := range d.Specs {
+		switch v := d.(type) {
+		case *ast.ValueSpec:
+			for _, name := range v.Names {
+				if f(name.Name) {
+					return true
+				}
+			}
+		case *ast.TypeSpec:
+			if f(v.Name.Name) {
+				return true
+			}
+			switch t := v.Type.(type) {
+			case *ast.StructType:
+				if matchFields(t.Fields, f) {
+					return true
+				}
+			case *ast.InterfaceType:
+				if matchFields(t.Methods, f) {
+					return true
+				}
+			}
+		}
+	}
+	return false
+}
+
+func filterValues(a []*Value, f Filter) []*Value {
+	w := 0
+	for _, vd := range a {
+		if matchDecl(vd.Decl, f) {
+			a[w] = vd
+			w++
+		}
+	}
+	return a[0:w]
+}
+
+func filterFuncs(a []*Func, f Filter) []*Func {
+	w := 0
+	for _, fd := range a {
+		if f(fd.Name) {
+			a[w] = fd
+			w++
+		}
+	}
+	return a[0:w]
+}
+
+func filterTypes(a []*Type, f Filter) []*Type {
+	w := 0
+	for _, td := range a {
+		n := 0 // number of matches
+		if matchDecl(td.Decl, f) {
+			n = 1
+		} else {
+			// type name doesn't match, but we may have matching consts, vars, factories or methods
+			td.Consts = filterValues(td.Consts, f)
+			td.Vars = filterValues(td.Vars, f)
+			td.Funcs = filterFuncs(td.Funcs, f)
+			td.Methods = filterFuncs(td.Methods, f)
+			n += len(td.Consts) + len(td.Vars) + len(td.Funcs) + len(td.Methods)
+		}
+		if n > 0 {
+			a[w] = td
+			w++
+		}
+	}
+	return a[0:w]
+}
+
+// Filter eliminates documentation for names that don't pass through the filter f.
+// TODO(gri): Recognize "Type.Method" as a name.
+//
+func (p *Package) Filter(f Filter) {
+	p.Consts = filterValues(p.Consts, f)
+	p.Vars = filterValues(p.Vars, f)
+	p.Types = filterTypes(p.Types, f)
+	p.Funcs = filterFuncs(p.Funcs, f)
+	p.Doc = "" // don't show top-level package doc
+}
diff --git a/src/go/doc/headscan.go b/src/go/doc/headscan.go
new file mode 100644
index 0000000..fe26a0e
--- /dev/null
+++ b/src/go/doc/headscan.go
@@ -0,0 +1,115 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build ignore
+
+/*
+	The headscan command extracts comment headings from package files;
+	it is used to detect false positives which may require an adjustment
+	to the comment formatting heuristics in comment.go.
+
+	Usage: headscan [-root root_directory]
+
+	By default, the $GOROOT/src directory is scanned.
+*/
+package main
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"go/doc"
+	"go/parser"
+	"go/token"
+	"internal/lazyregexp"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+)
+
+var (
+	root    = flag.String("root", filepath.Join(runtime.GOROOT(), "src"), "root of filesystem tree to scan")
+	verbose = flag.Bool("v", false, "verbose mode")
+)
+
+// ToHTML in comment.go assigns a (possibly blank) ID to each heading
+var html_h = lazyregexp.New(`<h3 id="[^"]*">`)
+
+const html_endh = "</h3>\n"
+
+func isGoFile(fi fs.FileInfo) bool {
+	return strings.HasSuffix(fi.Name(), ".go") &&
+		!strings.HasSuffix(fi.Name(), "_test.go")
+}
+
+func appendHeadings(list []string, comment string) []string {
+	var buf bytes.Buffer
+	doc.ToHTML(&buf, comment, nil)
+	for s := buf.String(); ; {
+		loc := html_h.FindStringIndex(s)
+		if len(loc) == 0 {
+			break
+		}
+		i := loc[1]
+		j := strings.Index(s, html_endh)
+		if j < 0 {
+			list = append(list, s[i:]) // incorrect HTML
+			break
+		}
+		list = append(list, s[i:j])
+		s = s[j+len(html_endh):]
+	}
+	return list
+}
+
+func main() {
+	flag.Parse()
+	fset := token.NewFileSet()
+	nheadings := 0
+	err := filepath.WalkDir(*root, func(path string, info fs.DirEntry, err error) error {
+		if !info.IsDir() {
+			return nil
+		}
+		pkgs, err := parser.ParseDir(fset, path, isGoFile, parser.ParseComments)
+		if err != nil {
+			if *verbose {
+				fmt.Fprintln(os.Stderr, err)
+			}
+			return nil
+		}
+		for _, pkg := range pkgs {
+			d := doc.New(pkg, path, doc.Mode(0))
+			list := appendHeadings(nil, d.Doc)
+			for _, d := range d.Consts {
+				list = appendHeadings(list, d.Doc)
+			}
+			for _, d := range d.Types {
+				list = appendHeadings(list, d.Doc)
+			}
+			for _, d := range d.Vars {
+				list = appendHeadings(list, d.Doc)
+			}
+			for _, d := range d.Funcs {
+				list = appendHeadings(list, d.Doc)
+			}
+			if len(list) > 0 {
+				// directories may contain multiple packages;
+				// print path and package name
+				fmt.Printf("%s (package %s)\n", path, pkg.Name)
+				for _, h := range list {
+					fmt.Printf("\t%s\n", h)
+				}
+				nheadings += len(list)
+			}
+		}
+		return nil
+	})
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+	fmt.Println(nheadings, "headings found")
+}
diff --git a/src/go/doc/reader.go b/src/go/doc/reader.go
new file mode 100644
index 0000000..c277b35
--- /dev/null
+++ b/src/go/doc/reader.go
@@ -0,0 +1,917 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import (
+	"go/ast"
+	"go/token"
+	"internal/lazyregexp"
+	"sort"
+	"strconv"
+)
+
+// ----------------------------------------------------------------------------
+// function/method sets
+//
+// Internally, we treat functions like methods and collect them in method sets.
+
+// A methodSet describes a set of methods. Entries where Decl == nil are conflict
+// entries (more than one method with the same name at the same embedding level).
+//
+type methodSet map[string]*Func
+
+// recvString returns a string representation of recv of the
+// form "T", "*T", or "BADRECV" (if not a proper receiver type).
+//
+func recvString(recv ast.Expr) string {
+	switch t := recv.(type) {
+	case *ast.Ident:
+		return t.Name
+	case *ast.StarExpr:
+		return "*" + recvString(t.X)
+	}
+	return "BADRECV"
+}
+
+// set creates the corresponding Func for f and adds it to mset.
+// If there are multiple f's with the same name, set keeps the first
+// one with documentation; conflicts are ignored. The boolean
+// specifies whether to leave the AST untouched.
+//
+func (mset methodSet) set(f *ast.FuncDecl, preserveAST bool) {
+	name := f.Name.Name
+	if g := mset[name]; g != nil && g.Doc != "" {
+		// A function with the same name has already been registered;
+		// since it has documentation, assume f is simply another
+		// implementation and ignore it. This does not happen if the
+		// caller is using go/build.ScanDir to determine the list of
+		// files implementing a package.
+		return
+	}
+	// function doesn't exist or has no documentation; use f
+	recv := ""
+	if f.Recv != nil {
+		var typ ast.Expr
+		// be careful in case of incorrect ASTs
+		if list := f.Recv.List; len(list) == 1 {
+			typ = list[0].Type
+		}
+		recv = recvString(typ)
+	}
+	mset[name] = &Func{
+		Doc:  f.Doc.Text(),
+		Name: name,
+		Decl: f,
+		Recv: recv,
+		Orig: recv,
+	}
+	if !preserveAST {
+		f.Doc = nil // doc consumed - remove from AST
+	}
+}
+
+// add adds method m to the method set; m is ignored if the method set
+// already contains a method with the same name at the same or a higher
+// level than m.
+//
+func (mset methodSet) add(m *Func) {
+	old := mset[m.Name]
+	if old == nil || m.Level < old.Level {
+		mset[m.Name] = m
+		return
+	}
+	if m.Level == old.Level {
+		// conflict - mark it using a method with nil Decl
+		mset[m.Name] = &Func{
+			Name:  m.Name,
+			Level: m.Level,
+		}
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Named types
+
+// baseTypeName returns the name of the base type of x (or "")
+// and whether the type is imported or not.
+//
+func baseTypeName(x ast.Expr) (name string, imported bool) {
+	switch t := x.(type) {
+	case *ast.Ident:
+		return t.Name, false
+	case *ast.SelectorExpr:
+		if _, ok := t.X.(*ast.Ident); ok {
+			// only possible for qualified type names;
+			// assume type is imported
+			return t.Sel.Name, true
+		}
+	case *ast.ParenExpr:
+		return baseTypeName(t.X)
+	case *ast.StarExpr:
+		return baseTypeName(t.X)
+	}
+	return
+}
+
+// An embeddedSet describes a set of embedded types.
+type embeddedSet map[*namedType]bool
+
+// A namedType represents a named unqualified (package local, or possibly
+// predeclared) type. The namedType for a type name is always found via
+// reader.lookupType.
+//
+type namedType struct {
+	doc  string       // doc comment for type
+	name string       // type name
+	decl *ast.GenDecl // nil if declaration hasn't been seen yet
+
+	isEmbedded bool        // true if this type is embedded
+	isStruct   bool        // true if this type is a struct
+	embedded   embeddedSet // true if the embedded type is a pointer
+
+	// associated declarations
+	values  []*Value // consts and vars
+	funcs   methodSet
+	methods methodSet
+}
+
+// ----------------------------------------------------------------------------
+// AST reader
+
+// reader accumulates documentation for a single package.
+// It modifies the AST: Comments (declaration documentation)
+// that have been collected by the reader are set to nil
+// in the respective AST nodes so that they are not printed
+// twice (once when printing the documentation and once when
+// printing the corresponding AST node).
+//
+type reader struct {
+	mode Mode
+
+	// package properties
+	doc       string // package documentation, if any
+	filenames []string
+	notes     map[string][]*Note
+
+	// declarations
+	imports   map[string]int
+	hasDotImp bool     // if set, package contains a dot import
+	values    []*Value // consts and vars
+	order     int      // sort order of const and var declarations (when we can't use a name)
+	types     map[string]*namedType
+	funcs     methodSet
+
+	// support for package-local error type declarations
+	errorDecl bool                 // if set, type "error" was declared locally
+	fixlist   []*ast.InterfaceType // list of interfaces containing anonymous field "error"
+}
+
+func (r *reader) isVisible(name string) bool {
+	return r.mode&AllDecls != 0 || token.IsExported(name)
+}
+
+// lookupType returns the base type with the given name.
+// If the base type has not been encountered yet, a new
+// type with the given name but no associated declaration
+// is added to the type map.
+//
+func (r *reader) lookupType(name string) *namedType {
+	if name == "" || name == "_" {
+		return nil // no type docs for anonymous types
+	}
+	if typ, found := r.types[name]; found {
+		return typ
+	}
+	// type not found - add one without declaration
+	typ := &namedType{
+		name:     name,
+		embedded: make(embeddedSet),
+		funcs:    make(methodSet),
+		methods:  make(methodSet),
+	}
+	r.types[name] = typ
+	return typ
+}
+
+// recordAnonymousField registers fieldType as the type of an
+// anonymous field in the parent type. If the field is imported
+// (qualified name) or the parent is nil, the field is ignored.
+// The function returns the field name.
+//
+func (r *reader) recordAnonymousField(parent *namedType, fieldType ast.Expr) (fname string) {
+	fname, imp := baseTypeName(fieldType)
+	if parent == nil || imp {
+		return
+	}
+	if ftype := r.lookupType(fname); ftype != nil {
+		ftype.isEmbedded = true
+		_, ptr := fieldType.(*ast.StarExpr)
+		parent.embedded[ftype] = ptr
+	}
+	return
+}
+
+func (r *reader) readDoc(comment *ast.CommentGroup) {
+	// By convention there should be only one package comment
+	// but collect all of them if there are more than one.
+	text := comment.Text()
+	if r.doc == "" {
+		r.doc = text
+		return
+	}
+	r.doc += "\n" + text
+}
+
+func (r *reader) remember(typ *ast.InterfaceType) {
+	r.fixlist = append(r.fixlist, typ)
+}
+
+func specNames(specs []ast.Spec) []string {
+	names := make([]string, 0, len(specs)) // reasonable estimate
+	for _, s := range specs {
+		// s guaranteed to be an *ast.ValueSpec by readValue
+		for _, ident := range s.(*ast.ValueSpec).Names {
+			names = append(names, ident.Name)
+		}
+	}
+	return names
+}
+
+// readValue processes a const or var declaration.
+//
+func (r *reader) readValue(decl *ast.GenDecl) {
+	// determine if decl should be associated with a type
+	// Heuristic: For each typed entry, determine the type name, if any.
+	//            If there is exactly one type name that is sufficiently
+	//            frequent, associate the decl with the respective type.
+	domName := ""
+	domFreq := 0
+	prev := ""
+	n := 0
+	for _, spec := range decl.Specs {
+		s, ok := spec.(*ast.ValueSpec)
+		if !ok {
+			continue // should not happen, but be conservative
+		}
+		name := ""
+		switch {
+		case s.Type != nil:
+			// a type is present; determine its name
+			if n, imp := baseTypeName(s.Type); !imp {
+				name = n
+			}
+		case decl.Tok == token.CONST && len(s.Values) == 0:
+			// no type or value is present but we have a constant declaration;
+			// use the previous type name (possibly the empty string)
+			name = prev
+		}
+		if name != "" {
+			// entry has a named type
+			if domName != "" && domName != name {
+				// more than one type name - do not associate
+				// with any type
+				domName = ""
+				break
+			}
+			domName = name
+			domFreq++
+		}
+		prev = name
+		n++
+	}
+
+	// nothing to do w/o a legal declaration
+	if n == 0 {
+		return
+	}
+
+	// determine values list with which to associate the Value for this decl
+	values := &r.values
+	const threshold = 0.75
+	if domName != "" && r.isVisible(domName) && domFreq >= int(float64(len(decl.Specs))*threshold) {
+		// typed entries are sufficiently frequent
+		if typ := r.lookupType(domName); typ != nil {
+			values = &typ.values // associate with that type
+		}
+	}
+
+	*values = append(*values, &Value{
+		Doc:   decl.Doc.Text(),
+		Names: specNames(decl.Specs),
+		Decl:  decl,
+		order: r.order,
+	})
+	if r.mode&PreserveAST == 0 {
+		decl.Doc = nil // doc consumed - remove from AST
+	}
+	// Note: It's important that the order used here is global because the cleanupTypes
+	// methods may move values associated with types back into the global list. If the
+	// order is list-specific, sorting is not deterministic because the same order value
+	// may appear multiple times (was bug, found when fixing #16153).
+	r.order++
+}
+
+// fields returns a struct's fields or an interface's methods.
+//
+func fields(typ ast.Expr) (list []*ast.Field, isStruct bool) {
+	var fields *ast.FieldList
+	switch t := typ.(type) {
+	case *ast.StructType:
+		fields = t.Fields
+		isStruct = true
+	case *ast.InterfaceType:
+		fields = t.Methods
+	}
+	if fields != nil {
+		list = fields.List
+	}
+	return
+}
+
+// readType processes a type declaration.
+//
+func (r *reader) readType(decl *ast.GenDecl, spec *ast.TypeSpec) {
+	typ := r.lookupType(spec.Name.Name)
+	if typ == nil {
+		return // no name or blank name - ignore the type
+	}
+
+	// A type should be added at most once, so typ.decl
+	// should be nil - if it is not, simply overwrite it.
+	typ.decl = decl
+
+	// compute documentation
+	doc := spec.Doc
+	if doc == nil {
+		// no doc associated with the spec, use the declaration doc, if any
+		doc = decl.Doc
+	}
+	if r.mode&PreserveAST == 0 {
+		spec.Doc = nil // doc consumed - remove from AST
+		decl.Doc = nil // doc consumed - remove from AST
+	}
+	typ.doc = doc.Text()
+
+	// record anonymous fields (they may contribute methods)
+	// (some fields may have been recorded already when filtering
+	// exports, but that's ok)
+	var list []*ast.Field
+	list, typ.isStruct = fields(spec.Type)
+	for _, field := range list {
+		if len(field.Names) == 0 {
+			r.recordAnonymousField(typ, field.Type)
+		}
+	}
+}
+
+// isPredeclared reports whether n denotes a predeclared type.
+//
+func (r *reader) isPredeclared(n string) bool {
+	return predeclaredTypes[n] && r.types[n] == nil
+}
+
+// readFunc processes a func or method declaration.
+//
+func (r *reader) readFunc(fun *ast.FuncDecl) {
+	// strip function body if requested.
+	if r.mode&PreserveAST == 0 {
+		fun.Body = nil
+	}
+
+	// associate methods with the receiver type, if any
+	if fun.Recv != nil {
+		// method
+		if len(fun.Recv.List) == 0 {
+			// should not happen (incorrect AST); (See issue 17788)
+			// don't show this method
+			return
+		}
+		recvTypeName, imp := baseTypeName(fun.Recv.List[0].Type)
+		if imp {
+			// should not happen (incorrect AST);
+			// don't show this method
+			return
+		}
+		if typ := r.lookupType(recvTypeName); typ != nil {
+			typ.methods.set(fun, r.mode&PreserveAST != 0)
+		}
+		// otherwise ignore the method
+		// TODO(gri): There may be exported methods of non-exported types
+		// that can be called because of exported values (consts, vars, or
+		// function results) of that type. Could determine if that is the
+		// case and then show those methods in an appropriate section.
+		return
+	}
+
+	// Associate factory functions with the first visible result type, as long as
+	// others are predeclared types.
+	if fun.Type.Results.NumFields() >= 1 {
+		var typ *namedType // type to associate the function with
+		numResultTypes := 0
+		for _, res := range fun.Type.Results.List {
+			factoryType := res.Type
+			if t, ok := factoryType.(*ast.ArrayType); ok {
+				// We consider functions that return slices or arrays of type
+				// T (or pointers to T) as factory functions of T.
+				factoryType = t.Elt
+			}
+			if n, imp := baseTypeName(factoryType); !imp && r.isVisible(n) && !r.isPredeclared(n) {
+				if t := r.lookupType(n); t != nil {
+					typ = t
+					numResultTypes++
+					if numResultTypes > 1 {
+						break
+					}
+				}
+			}
+		}
+		// If there is exactly one result type,
+		// associate the function with that type.
+		if numResultTypes == 1 {
+			typ.funcs.set(fun, r.mode&PreserveAST != 0)
+			return
+		}
+	}
+
+	// just an ordinary function
+	r.funcs.set(fun, r.mode&PreserveAST != 0)
+}
+
+var (
+	noteMarker    = `([A-Z][A-Z]+)\(([^)]+)\):?`                // MARKER(uid), MARKER at least 2 chars, uid at least 1 char
+	noteMarkerRx  = lazyregexp.New(`^[ \t]*` + noteMarker)      // MARKER(uid) at text start
+	noteCommentRx = lazyregexp.New(`^/[/*][ \t]*` + noteMarker) // MARKER(uid) at comment start
+)
+
+// readNote collects a single note from a sequence of comments.
+//
+func (r *reader) readNote(list []*ast.Comment) {
+	text := (&ast.CommentGroup{List: list}).Text()
+	if m := noteMarkerRx.FindStringSubmatchIndex(text); m != nil {
+		// The note body starts after the marker.
+		// We remove any formatting so that we don't
+		// get spurious line breaks/indentation when
+		// showing the TODO body.
+		body := clean(text[m[1]:], keepNL)
+		if body != "" {
+			marker := text[m[2]:m[3]]
+			r.notes[marker] = append(r.notes[marker], &Note{
+				Pos:  list[0].Pos(),
+				End:  list[len(list)-1].End(),
+				UID:  text[m[4]:m[5]],
+				Body: body,
+			})
+		}
+	}
+}
+
+// readNotes extracts notes from comments.
+// A note must start at the beginning of a comment with "MARKER(uid):"
+// and is followed by the note body (e.g., "// BUG(gri): fix this").
+// The note ends at the end of the comment group or at the start of
+// another note in the same comment group, whichever comes first.
+//
+func (r *reader) readNotes(comments []*ast.CommentGroup) {
+	for _, group := range comments {
+		i := -1 // comment index of most recent note start, valid if >= 0
+		list := group.List
+		for j, c := range list {
+			if noteCommentRx.MatchString(c.Text) {
+				if i >= 0 {
+					r.readNote(list[i:j])
+				}
+				i = j
+			}
+		}
+		if i >= 0 {
+			r.readNote(list[i:])
+		}
+	}
+}
+
+// readFile adds the AST for a source file to the reader.
+//
+func (r *reader) readFile(src *ast.File) {
+	// add package documentation
+	if src.Doc != nil {
+		r.readDoc(src.Doc)
+		if r.mode&PreserveAST == 0 {
+			src.Doc = nil // doc consumed - remove from AST
+		}
+	}
+
+	// add all declarations but for functions which are processed in a separate pass
+	for _, decl := range src.Decls {
+		switch d := decl.(type) {
+		case *ast.GenDecl:
+			switch d.Tok {
+			case token.IMPORT:
+				// imports are handled individually
+				for _, spec := range d.Specs {
+					if s, ok := spec.(*ast.ImportSpec); ok {
+						if import_, err := strconv.Unquote(s.Path.Value); err == nil {
+							r.imports[import_] = 1
+							if s.Name != nil && s.Name.Name == "." {
+								r.hasDotImp = true
+							}
+						}
+					}
+				}
+			case token.CONST, token.VAR:
+				// constants and variables are always handled as a group
+				r.readValue(d)
+			case token.TYPE:
+				// types are handled individually
+				if len(d.Specs) == 1 && !d.Lparen.IsValid() {
+					// common case: single declaration w/o parentheses
+					// (if a single declaration is parenthesized,
+					// create a new fake declaration below, so that
+					// go/doc type declarations always appear w/o
+					// parentheses)
+					if s, ok := d.Specs[0].(*ast.TypeSpec); ok {
+						r.readType(d, s)
+					}
+					break
+				}
+				for _, spec := range d.Specs {
+					if s, ok := spec.(*ast.TypeSpec); ok {
+						// use an individual (possibly fake) declaration
+						// for each type; this also ensures that each type
+						// gets to (re-)use the declaration documentation
+						// if there's none associated with the spec itself
+						fake := &ast.GenDecl{
+							Doc: d.Doc,
+							// don't use the existing TokPos because it
+							// will lead to the wrong selection range for
+							// the fake declaration if there are more
+							// than one type in the group (this affects
+							// src/cmd/godoc/godoc.go's posLink_urlFunc)
+							TokPos: s.Pos(),
+							Tok:    token.TYPE,
+							Specs:  []ast.Spec{s},
+						}
+						r.readType(fake, s)
+					}
+				}
+			}
+		}
+	}
+
+	// collect MARKER(...): annotations
+	r.readNotes(src.Comments)
+	if r.mode&PreserveAST == 0 {
+		src.Comments = nil // consumed unassociated comments - remove from AST
+	}
+}
+
+func (r *reader) readPackage(pkg *ast.Package, mode Mode) {
+	// initialize reader
+	r.filenames = make([]string, len(pkg.Files))
+	r.imports = make(map[string]int)
+	r.mode = mode
+	r.types = make(map[string]*namedType)
+	r.funcs = make(methodSet)
+	r.notes = make(map[string][]*Note)
+
+	// sort package files before reading them so that the
+	// result does not depend on map iteration order
+	i := 0
+	for filename := range pkg.Files {
+		r.filenames[i] = filename
+		i++
+	}
+	sort.Strings(r.filenames)
+
+	// process files in sorted order
+	for _, filename := range r.filenames {
+		f := pkg.Files[filename]
+		if mode&AllDecls == 0 {
+			r.fileExports(f)
+		}
+		r.readFile(f)
+	}
+
+	// process functions now that we have better type information
+	for _, f := range pkg.Files {
+		for _, decl := range f.Decls {
+			if d, ok := decl.(*ast.FuncDecl); ok {
+				r.readFunc(d)
+			}
+		}
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Types
+
+func customizeRecv(f *Func, recvTypeName string, embeddedIsPtr bool, level int) *Func {
+	if f == nil || f.Decl == nil || f.Decl.Recv == nil || len(f.Decl.Recv.List) != 1 {
+		return f // shouldn't happen, but be safe
+	}
+
+	// copy existing receiver field and set new type
+	newField := *f.Decl.Recv.List[0]
+	origPos := newField.Type.Pos()
+	_, origRecvIsPtr := newField.Type.(*ast.StarExpr)
+	newIdent := &ast.Ident{NamePos: origPos, Name: recvTypeName}
+	var typ ast.Expr = newIdent
+	if !embeddedIsPtr && origRecvIsPtr {
+		newIdent.NamePos++ // '*' is one character
+		typ = &ast.StarExpr{Star: origPos, X: newIdent}
+	}
+	newField.Type = typ
+
+	// copy existing receiver field list and set new receiver field
+	newFieldList := *f.Decl.Recv
+	newFieldList.List = []*ast.Field{&newField}
+
+	// copy existing function declaration and set new receiver field list
+	newFuncDecl := *f.Decl
+	newFuncDecl.Recv = &newFieldList
+
+	// copy existing function documentation and set new declaration
+	newF := *f
+	newF.Decl = &newFuncDecl
+	newF.Recv = recvString(typ)
+	// the Orig field never changes
+	newF.Level = level
+
+	return &newF
+}
+
+// collectEmbeddedMethods collects the embedded methods of typ in mset.
+//
+func (r *reader) collectEmbeddedMethods(mset methodSet, typ *namedType, recvTypeName string, embeddedIsPtr bool, level int, visited embeddedSet) {
+	visited[typ] = true
+	for embedded, isPtr := range typ.embedded {
+		// Once an embedded type is embedded as a pointer type
+		// all embedded types in those types are treated like
+		// pointer types for the purpose of the receiver type
+		// computation; i.e., embeddedIsPtr is sticky for this
+		// embedding hierarchy.
+		thisEmbeddedIsPtr := embeddedIsPtr || isPtr
+		for _, m := range embedded.methods {
+			// only top-level methods are embedded
+			if m.Level == 0 {
+				mset.add(customizeRecv(m, recvTypeName, thisEmbeddedIsPtr, level))
+			}
+		}
+		if !visited[embedded] {
+			r.collectEmbeddedMethods(mset, embedded, recvTypeName, thisEmbeddedIsPtr, level+1, visited)
+		}
+	}
+	delete(visited, typ)
+}
+
+// computeMethodSets determines the actual method sets for each type encountered.
+//
+func (r *reader) computeMethodSets() {
+	for _, t := range r.types {
+		// collect embedded methods for t
+		if t.isStruct {
+			// struct
+			r.collectEmbeddedMethods(t.methods, t, t.name, false, 1, make(embeddedSet))
+		} else {
+			// interface
+			// TODO(gri) fix this
+		}
+	}
+
+	// if error was declared locally, don't treat it as exported field anymore
+	if r.errorDecl {
+		for _, ityp := range r.fixlist {
+			removeErrorField(ityp)
+		}
+	}
+}
+
+// cleanupTypes removes the association of functions and methods with
+// types that have no declaration. Instead, these functions and methods
+// are shown at the package level. It also removes types with missing
+// declarations or which are not visible.
+//
+func (r *reader) cleanupTypes() {
+	for _, t := range r.types {
+		visible := r.isVisible(t.name)
+		predeclared := predeclaredTypes[t.name]
+
+		if t.decl == nil && (predeclared || visible && (t.isEmbedded || r.hasDotImp)) {
+			// t.name is a predeclared type (and was not redeclared in this package),
+			// or it was embedded somewhere but its declaration is missing (because
+			// the AST is incomplete), or we have a dot-import (and all bets are off):
+			// move any associated values, funcs, and methods back to the top-level so
+			// that they are not lost.
+			// 1) move values
+			r.values = append(r.values, t.values...)
+			// 2) move factory functions
+			for name, f := range t.funcs {
+				// in a correct AST, package-level function names
+				// are all different - no need to check for conflicts
+				r.funcs[name] = f
+			}
+			// 3) move methods
+			if !predeclared {
+				for name, m := range t.methods {
+					// don't overwrite functions with the same name - drop them
+					if _, found := r.funcs[name]; !found {
+						r.funcs[name] = m
+					}
+				}
+			}
+		}
+		// remove types w/o declaration or which are not visible
+		if t.decl == nil || !visible {
+			delete(r.types, t.name)
+		}
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Sorting
+
+type data struct {
+	n    int
+	swap func(i, j int)
+	less func(i, j int) bool
+}
+
+func (d *data) Len() int           { return d.n }
+func (d *data) Swap(i, j int)      { d.swap(i, j) }
+func (d *data) Less(i, j int) bool { return d.less(i, j) }
+
+// sortBy is a helper function for sorting
+func sortBy(less func(i, j int) bool, swap func(i, j int), n int) {
+	sort.Sort(&data{n, swap, less})
+}
+
+func sortedKeys(m map[string]int) []string {
+	list := make([]string, len(m))
+	i := 0
+	for key := range m {
+		list[i] = key
+		i++
+	}
+	sort.Strings(list)
+	return list
+}
+
+// sortingName returns the name to use when sorting d into place.
+//
+func sortingName(d *ast.GenDecl) string {
+	if len(d.Specs) == 1 {
+		if s, ok := d.Specs[0].(*ast.ValueSpec); ok {
+			return s.Names[0].Name
+		}
+	}
+	return ""
+}
+
+func sortedValues(m []*Value, tok token.Token) []*Value {
+	list := make([]*Value, len(m)) // big enough in any case
+	i := 0
+	for _, val := range m {
+		if val.Decl.Tok == tok {
+			list[i] = val
+			i++
+		}
+	}
+	list = list[0:i]
+
+	sortBy(
+		func(i, j int) bool {
+			if ni, nj := sortingName(list[i].Decl), sortingName(list[j].Decl); ni != nj {
+				return ni < nj
+			}
+			return list[i].order < list[j].order
+		},
+		func(i, j int) { list[i], list[j] = list[j], list[i] },
+		len(list),
+	)
+
+	return list
+}
+
+func sortedTypes(m map[string]*namedType, allMethods bool) []*Type {
+	list := make([]*Type, len(m))
+	i := 0
+	for _, t := range m {
+		list[i] = &Type{
+			Doc:     t.doc,
+			Name:    t.name,
+			Decl:    t.decl,
+			Consts:  sortedValues(t.values, token.CONST),
+			Vars:    sortedValues(t.values, token.VAR),
+			Funcs:   sortedFuncs(t.funcs, true),
+			Methods: sortedFuncs(t.methods, allMethods),
+		}
+		i++
+	}
+
+	sortBy(
+		func(i, j int) bool { return list[i].Name < list[j].Name },
+		func(i, j int) { list[i], list[j] = list[j], list[i] },
+		len(list),
+	)
+
+	return list
+}
+
+func removeStar(s string) string {
+	if len(s) > 0 && s[0] == '*' {
+		return s[1:]
+	}
+	return s
+}
+
+func sortedFuncs(m methodSet, allMethods bool) []*Func {
+	list := make([]*Func, len(m))
+	i := 0
+	for _, m := range m {
+		// determine which methods to include
+		switch {
+		case m.Decl == nil:
+			// exclude conflict entry
+		case allMethods, m.Level == 0, !token.IsExported(removeStar(m.Orig)):
+			// forced inclusion, method not embedded, or method
+			// embedded but original receiver type not exported
+			list[i] = m
+			i++
+		}
+	}
+	list = list[0:i]
+	sortBy(
+		func(i, j int) bool { return list[i].Name < list[j].Name },
+		func(i, j int) { list[i], list[j] = list[j], list[i] },
+		len(list),
+	)
+	return list
+}
+
+// noteBodies returns a list of note body strings given a list of notes.
+// This is only used to populate the deprecated Package.Bugs field.
+//
+func noteBodies(notes []*Note) []string {
+	var list []string
+	for _, n := range notes {
+		list = append(list, n.Body)
+	}
+	return list
+}
+
+// ----------------------------------------------------------------------------
+// Predeclared identifiers
+
+// IsPredeclared reports whether s is a predeclared identifier.
+func IsPredeclared(s string) bool {
+	return predeclaredTypes[s] || predeclaredFuncs[s] || predeclaredConstants[s]
+}
+
+var predeclaredTypes = map[string]bool{
+	"bool":       true,
+	"byte":       true,
+	"complex64":  true,
+	"complex128": true,
+	"error":      true,
+	"float32":    true,
+	"float64":    true,
+	"int":        true,
+	"int8":       true,
+	"int16":      true,
+	"int32":      true,
+	"int64":      true,
+	"rune":       true,
+	"string":     true,
+	"uint":       true,
+	"uint8":      true,
+	"uint16":     true,
+	"uint32":     true,
+	"uint64":     true,
+	"uintptr":    true,
+}
+
+var predeclaredFuncs = map[string]bool{
+	"append":  true,
+	"cap":     true,
+	"close":   true,
+	"complex": true,
+	"copy":    true,
+	"delete":  true,
+	"imag":    true,
+	"len":     true,
+	"make":    true,
+	"new":     true,
+	"panic":   true,
+	"print":   true,
+	"println": true,
+	"real":    true,
+	"recover": true,
+}
+
+var predeclaredConstants = map[string]bool{
+	"false": true,
+	"iota":  true,
+	"nil":   true,
+	"true":  true,
+}
diff --git a/src/go/doc/synopsis.go b/src/go/doc/synopsis.go
new file mode 100644
index 0000000..3fa1616
--- /dev/null
+++ b/src/go/doc/synopsis.go
@@ -0,0 +1,83 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import (
+	"strings"
+	"unicode"
+)
+
+// firstSentenceLen returns the length of the first sentence in s.
+// The sentence ends after the first period followed by space and
+// not preceded by exactly one uppercase letter.
+//
+func firstSentenceLen(s string) int {
+	var ppp, pp, p rune
+	for i, q := range s {
+		if q == '\n' || q == '\r' || q == '\t' {
+			q = ' '
+		}
+		if q == ' ' && p == '.' && (!unicode.IsUpper(pp) || unicode.IsUpper(ppp)) {
+			return i
+		}
+		if p == '。' || p == '．' {
+			return i
+		}
+		ppp, pp, p = pp, p, q
+	}
+	return len(s)
+}
+
+const (
+	keepNL = 1 << iota
+)
+
+// clean replaces each sequence of space, \n, \r, or \t characters
+// with a single space and removes any trailing and leading spaces.
+// If the keepNL flag is set, newline characters are passed through
+// instead of being change to spaces.
+func clean(s string, flags int) string {
+	var b []byte
+	p := byte(' ')
+	for i := 0; i < len(s); i++ {
+		q := s[i]
+		if (flags&keepNL) == 0 && q == '\n' || q == '\r' || q == '\t' {
+			q = ' '
+		}
+		if q != ' ' || p != ' ' {
+			b = append(b, q)
+			p = q
+		}
+	}
+	// remove trailing blank, if any
+	if n := len(b); n > 0 && p == ' ' {
+		b = b[0 : n-1]
+	}
+	return string(b)
+}
+
+// Synopsis returns a cleaned version of the first sentence in s.
+// That sentence ends after the first period followed by space and
+// not preceded by exactly one uppercase letter. The result string
+// has no \n, \r, or \t characters and uses only single spaces between
+// words. If s starts with any of the IllegalPrefixes, the result
+// is the empty string.
+//
+func Synopsis(s string) string {
+	s = clean(s[0:firstSentenceLen(s)], 0)
+	for _, prefix := range IllegalPrefixes {
+		if strings.HasPrefix(strings.ToLower(s), prefix) {
+			return ""
+		}
+	}
+	s = convertQuotes(s)
+	return s
+}
+
+var IllegalPrefixes = []string{
+	"copyright",
+	"all rights",
+	"author",
+}
diff --git a/src/go/doc/synopsis_test.go b/src/go/doc/synopsis_test.go
new file mode 100644
index 0000000..3f443dc
--- /dev/null
+++ b/src/go/doc/synopsis_test.go
@@ -0,0 +1,52 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import "testing"
+
+var tests = []struct {
+	txt string
+	fsl int
+	syn string
+}{
+	{"", 0, ""},
+	{"foo", 3, "foo"},
+	{"foo.", 4, "foo."},
+	{"foo.bar", 7, "foo.bar"},
+	{"  foo.  ", 6, "foo."},
+	{"  foo\t  bar.\n", 12, "foo bar."},
+	{"  foo\t  bar.\n", 12, "foo bar."},
+	{"a  b\n\nc\r\rd\t\t", 12, "a b c d"},
+	{"a  b\n\nc\r\rd\t\t  . BLA", 15, "a b c d ."},
+	{"Package poems by T.S.Eliot. To rhyme...", 27, "Package poems by T.S.Eliot."},
+	{"Package poems by T. S. Eliot. To rhyme...", 29, "Package poems by T. S. Eliot."},
+	{"foo implements the foo ABI. The foo ABI is...", 27, "foo implements the foo ABI."},
+	{"Package\nfoo. ..", 12, "Package foo."},
+	{"P . Q.", 3, "P ."},
+	{"P. Q.   ", 8, "P. Q."},
+	{"Package Καλημέρα κόσμε.", 36, "Package Καλημέρα κόσμε."},
+	{"Package こんにちは 世界\n", 31, "Package こんにちは 世界"},
+	{"Package こんにちは。世界", 26, "Package こんにちは。"},
+	{"Package 안녕．世界", 17, "Package 안녕．"},
+	{"Package foo does bar.", 21, "Package foo does bar."},
+	{"Copyright 2012 Google, Inc. Package foo does bar.", 27, ""},
+	{"All Rights reserved. Package foo does bar.", 20, ""},
+	{"All rights reserved. Package foo does bar.", 20, ""},
+	{"Authors: foo@bar.com. Package foo does bar.", 21, ""},
+	{"typically invoked as ``go tool asm'',", 37, "typically invoked as " + ulquo + "go tool asm" + urquo + ","},
+}
+
+func TestSynopsis(t *testing.T) {
+	for _, e := range tests {
+		fsl := firstSentenceLen(e.txt)
+		if fsl != e.fsl {
+			t.Errorf("got fsl = %d; want %d for %q\n", fsl, e.fsl, e.txt)
+		}
+		syn := Synopsis(e.txt)
+		if syn != e.syn {
+			t.Errorf("got syn = %q; want %q for %q\n", syn, e.syn, e.txt)
+		}
+	}
+}
diff --git a/src/go/doc/testdata/a.0.golden b/src/go/doc/testdata/a.0.golden
new file mode 100644
index 0000000..7e680b8
--- /dev/null
+++ b/src/go/doc/testdata/a.0.golden
@@ -0,0 +1,52 @@
+// comment 0  comment 1 
+PACKAGE a
+
+IMPORTPATH
+	testdata/a
+
+FILENAMES
+	testdata/a0.go
+	testdata/a1.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	bug0
+
+	bug1
+
+
+BUGS
+BUG(uid)	bug0
+
+BUG(uid)	bug1
+
+
+NOTES
+NOTE(uid)	
+
+NOTE(foo)	1 of 4 - this is the first line of note 1
+	- note 1 continues on this 2nd line
+	- note 1 continues on this 3rd line
+
+NOTE(foo)	2 of 4
+
+NOTE(bar)	3 of 4
+
+NOTE(bar)	4 of 4
+	- this is the last line of note 4
+
+NOTE(bam)	This note which contains a (parenthesized) subphrase
+	 must appear in its entirety.
+
+NOTE(xxx)	The ':' after the marker and uid is optional.
+
+
+SECBUGS
+SECBUG(uid)	sec hole 0
+	need to fix asap
+
+
+TODOS
+TODO(uid)	todo0
+
+TODO(uid)	todo1
+
diff --git a/src/go/doc/testdata/a.1.golden b/src/go/doc/testdata/a.1.golden
new file mode 100644
index 0000000..7e680b8
--- /dev/null
+++ b/src/go/doc/testdata/a.1.golden
@@ -0,0 +1,52 @@
+// comment 0  comment 1 
+PACKAGE a
+
+IMPORTPATH
+	testdata/a
+
+FILENAMES
+	testdata/a0.go
+	testdata/a1.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	bug0
+
+	bug1
+
+
+BUGS
+BUG(uid)	bug0
+
+BUG(uid)	bug1
+
+
+NOTES
+NOTE(uid)	
+
+NOTE(foo)	1 of 4 - this is the first line of note 1
+	- note 1 continues on this 2nd line
+	- note 1 continues on this 3rd line
+
+NOTE(foo)	2 of 4
+
+NOTE(bar)	3 of 4
+
+NOTE(bar)	4 of 4
+	- this is the last line of note 4
+
+NOTE(bam)	This note which contains a (parenthesized) subphrase
+	 must appear in its entirety.
+
+NOTE(xxx)	The ':' after the marker and uid is optional.
+
+
+SECBUGS
+SECBUG(uid)	sec hole 0
+	need to fix asap
+
+
+TODOS
+TODO(uid)	todo0
+
+TODO(uid)	todo1
+
diff --git a/src/go/doc/testdata/a.2.golden b/src/go/doc/testdata/a.2.golden
new file mode 100644
index 0000000..7e680b8
--- /dev/null
+++ b/src/go/doc/testdata/a.2.golden
@@ -0,0 +1,52 @@
+// comment 0  comment 1 
+PACKAGE a
+
+IMPORTPATH
+	testdata/a
+
+FILENAMES
+	testdata/a0.go
+	testdata/a1.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	bug0
+
+	bug1
+
+
+BUGS
+BUG(uid)	bug0
+
+BUG(uid)	bug1
+
+
+NOTES
+NOTE(uid)	
+
+NOTE(foo)	1 of 4 - this is the first line of note 1
+	- note 1 continues on this 2nd line
+	- note 1 continues on this 3rd line
+
+NOTE(foo)	2 of 4
+
+NOTE(bar)	3 of 4
+
+NOTE(bar)	4 of 4
+	- this is the last line of note 4
+
+NOTE(bam)	This note which contains a (parenthesized) subphrase
+	 must appear in its entirety.
+
+NOTE(xxx)	The ':' after the marker and uid is optional.
+
+
+SECBUGS
+SECBUG(uid)	sec hole 0
+	need to fix asap
+
+
+TODOS
+TODO(uid)	todo0
+
+TODO(uid)	todo1
+
diff --git a/src/go/doc/testdata/a0.go b/src/go/doc/testdata/a0.go
new file mode 100644
index 0000000..2420c8a
--- /dev/null
+++ b/src/go/doc/testdata/a0.go
@@ -0,0 +1,40 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// comment 0
+package a
+
+//BUG(uid): bug0
+
+//TODO(uid): todo0
+
+// A note with some spaces after it, should be ignored (watch out for
+// emacs modes that remove trailing whitespace).
+//NOTE(uid):
+
+// SECBUG(uid): sec hole 0
+// need to fix asap
+
+// Multiple notes may be in the same comment group and should be
+// recognized individually. Notes may start in the middle of a
+// comment group as long as they start at the beginning of an
+// individual comment.
+//
+// NOTE(foo): 1 of 4 - this is the first line of note 1
+// - note 1 continues on this 2nd line
+// - note 1 continues on this 3rd line
+// NOTE(foo): 2 of 4
+// NOTE(bar): 3 of 4
+/* NOTE(bar): 4 of 4 */
+// - this is the last line of note 4
+//
+//
+
+// NOTE(bam): This note which contains a (parenthesized) subphrase
+//            must appear in its entirety.
+
+// NOTE(xxx) The ':' after the marker and uid is optional.
+
+// NOTE(): NO uid - should not show up.
+// NOTE()  NO uid - should not show up.
diff --git a/src/go/doc/testdata/a1.go b/src/go/doc/testdata/a1.go
new file mode 100644
index 0000000..9fad1e0
--- /dev/null
+++ b/src/go/doc/testdata/a1.go
@@ -0,0 +1,12 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// comment 1
+package a
+
+//BUG(uid): bug1
+
+//TODO(uid): todo1
+
+//TODO(): ignored
diff --git a/src/go/doc/testdata/b.0.golden b/src/go/doc/testdata/b.0.golden
new file mode 100644
index 0000000..9d93392
--- /dev/null
+++ b/src/go/doc/testdata/b.0.golden
@@ -0,0 +1,71 @@
+// 
+PACKAGE b
+
+IMPORTPATH
+	testdata/b
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/b.go
+
+CONSTANTS
+	// 
+	const (
+		C1	notExported	= iota
+		C2
+	
+		C4
+		C5
+	)
+
+	// 
+	const C notExported = 0
+
+	// 
+	const Pi = 3.14	// Pi
+
+
+VARIABLES
+	// 
+	var (
+		U1, U2, U4, U5	notExported
+	
+		U7	notExported	= 7
+	)
+
+	// 
+	var MaxInt int	// MaxInt
+
+	// 
+	var V notExported
+
+	// 
+	var V1, V2, V4, V5 notExported
+
+
+FUNCTIONS
+	// 
+	func F(x int) int
+
+	// 
+	func F1() notExported
+
+	// Always under the package functions list. 
+	func NotAFactory() int
+
+	// Associated with uint type if AllDecls is set. 
+	func UintFactory() uint
+
+
+TYPES
+	// 
+	type T struct{}	// T
+
+	// 
+	var V T	// v
+
+	// 
+	func (x *T) M()
+
diff --git a/src/go/doc/testdata/b.1.golden b/src/go/doc/testdata/b.1.golden
new file mode 100644
index 0000000..66c47b5
--- /dev/null
+++ b/src/go/doc/testdata/b.1.golden
@@ -0,0 +1,83 @@
+// 
+PACKAGE b
+
+IMPORTPATH
+	testdata/b
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/b.go
+
+CONSTANTS
+	// 
+	const Pi = 3.14	// Pi
+
+
+VARIABLES
+	// 
+	var MaxInt int	// MaxInt
+
+
+FUNCTIONS
+	// 
+	func F(x int) int
+
+	// Always under the package functions list. 
+	func NotAFactory() int
+
+
+TYPES
+	// 
+	type T struct{}	// T
+
+	// 
+	var V T	// v
+
+	// 
+	func (x *T) M()
+
+	// 
+	type notExported int
+
+	// 
+	const (
+		C1	notExported	= iota
+		C2
+		c3
+		C4
+		C5
+	)
+
+	// 
+	const C notExported = 0
+
+	// 
+	var (
+		U1, U2, u3, U4, U5	notExported
+		u6			notExported
+		U7			notExported	= 7
+	)
+
+	// 
+	var V notExported
+
+	// 
+	var V1, V2, v3, V4, V5 notExported
+
+	// 
+	func F1() notExported
+
+	// 
+	func f2() notExported
+
+	// Should only appear if AllDecls is set. 
+	type uint struct{}	// overrides a predeclared type uint
+
+	// Associated with uint type if AllDecls is set. 
+	func UintFactory() uint
+
+	// Associated with uint type if AllDecls is set. 
+	func uintFactory() uint
+
diff --git a/src/go/doc/testdata/b.2.golden b/src/go/doc/testdata/b.2.golden
new file mode 100644
index 0000000..9d93392
--- /dev/null
+++ b/src/go/doc/testdata/b.2.golden
@@ -0,0 +1,71 @@
+// 
+PACKAGE b
+
+IMPORTPATH
+	testdata/b
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/b.go
+
+CONSTANTS
+	// 
+	const (
+		C1	notExported	= iota
+		C2
+	
+		C4
+		C5
+	)
+
+	// 
+	const C notExported = 0
+
+	// 
+	const Pi = 3.14	// Pi
+
+
+VARIABLES
+	// 
+	var (
+		U1, U2, U4, U5	notExported
+	
+		U7	notExported	= 7
+	)
+
+	// 
+	var MaxInt int	// MaxInt
+
+	// 
+	var V notExported
+
+	// 
+	var V1, V2, V4, V5 notExported
+
+
+FUNCTIONS
+	// 
+	func F(x int) int
+
+	// 
+	func F1() notExported
+
+	// Always under the package functions list. 
+	func NotAFactory() int
+
+	// Associated with uint type if AllDecls is set. 
+	func UintFactory() uint
+
+
+TYPES
+	// 
+	type T struct{}	// T
+
+	// 
+	var V T	// v
+
+	// 
+	func (x *T) M()
+
diff --git a/src/go/doc/testdata/b.go b/src/go/doc/testdata/b.go
new file mode 100644
index 0000000..e50663b
--- /dev/null
+++ b/src/go/doc/testdata/b.go
@@ -0,0 +1,58 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package b
+
+import "a"
+
+// ----------------------------------------------------------------------------
+// Basic declarations
+
+const Pi = 3.14   // Pi
+var MaxInt int    // MaxInt
+type T struct{}   // T
+var V T           // v
+func F(x int) int {} // F
+func (x *T) M()   {} // M
+
+// Corner cases: association with (presumed) predeclared types
+
+// Always under the package functions list.
+func NotAFactory() int {}
+
+// Associated with uint type if AllDecls is set.
+func UintFactory() uint {}
+
+// Associated with uint type if AllDecls is set.
+func uintFactory() uint {}
+
+// Should only appear if AllDecls is set.
+type uint struct{} // overrides a predeclared type uint
+
+// ----------------------------------------------------------------------------
+// Exported declarations associated with non-exported types must always be shown.
+
+type notExported int
+
+const C notExported = 0
+
+const (
+	C1 notExported = iota
+	C2
+	c3
+	C4
+	C5
+)
+
+var V notExported
+var V1, V2, v3, V4, V5 notExported
+
+var (
+	U1, U2, u3, U4, U5 notExported
+	u6                 notExported
+	U7                 notExported = 7
+)
+
+func F1() notExported {}
+func f2() notExported {}
diff --git a/src/go/doc/testdata/benchmark.go b/src/go/doc/testdata/benchmark.go
new file mode 100644
index 0000000..1d581f0
--- /dev/null
+++ b/src/go/doc/testdata/benchmark.go
@@ -0,0 +1,293 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"runtime"
+	"time"
+)
+
+var matchBenchmarks = flag.String("test.bench", "", "regular expression to select benchmarks to run")
+var benchTime = flag.Duration("test.benchtime", 1*time.Second, "approximate run time for each benchmark")
+
+// An internal type but exported because it is cross-package; part of the implementation
+// of go test.
+type InternalBenchmark struct {
+	Name string
+	F    func(b *B)
+}
+
+// B is a type passed to Benchmark functions to manage benchmark
+// timing and to specify the number of iterations to run.
+type B struct {
+	common
+	N         int
+	benchmark InternalBenchmark
+	bytes     int64
+	timerOn   bool
+	result    BenchmarkResult
+}
+
+// StartTimer starts timing a test. This function is called automatically
+// before a benchmark starts, but it can also used to resume timing after
+// a call to StopTimer.
+func (b *B) StartTimer() {
+	if !b.timerOn {
+		b.start = time.Now()
+		b.timerOn = true
+	}
+}
+
+// StopTimer stops timing a test. This can be used to pause the timer
+// while performing complex initialization that you don't
+// want to measure.
+func (b *B) StopTimer() {
+	if b.timerOn {
+		b.duration += time.Now().Sub(b.start)
+		b.timerOn = false
+	}
+}
+
+// ResetTimer sets the elapsed benchmark time to zero.
+// It does not affect whether the timer is running.
+func (b *B) ResetTimer() {
+	if b.timerOn {
+		b.start = time.Now()
+	}
+	b.duration = 0
+}
+
+// SetBytes records the number of bytes processed in a single operation.
+// If this is called, the benchmark will report ns/op and MB/s.
+func (b *B) SetBytes(n int64) { b.bytes = n }
+
+func (b *B) nsPerOp() int64 {
+	if b.N <= 0 {
+		return 0
+	}
+	return b.duration.Nanoseconds() / int64(b.N)
+}
+
+// runN runs a single benchmark for the specified number of iterations.
+func (b *B) runN(n int) {
+	// Try to get a comparable environment for each run
+	// by clearing garbage from previous runs.
+	runtime.GC()
+	b.N = n
+	b.ResetTimer()
+	b.StartTimer()
+	b.benchmark.F(b)
+	b.StopTimer()
+}
+
+func min(x, y int) int {
+	if x > y {
+		return y
+	}
+	return x
+}
+
+func max(x, y int) int {
+	if x < y {
+		return y
+	}
+	return x
+}
+
+// roundDown10 rounds a number down to the nearest power of 10.
+func roundDown10(n int) int {
+	var tens = 0
+	// tens = floor(log_10(n))
+	for n > 10 {
+		n = n / 10
+		tens++
+	}
+	// result = 10^tens
+	result := 1
+	for i := 0; i < tens; i++ {
+		result *= 10
+	}
+	return result
+}
+
+// roundUp rounds x up to a number of the form [1eX, 2eX, 5eX].
+func roundUp(n int) int {
+	base := roundDown10(n)
+	if n < (2 * base) {
+		return 2 * base
+	}
+	if n < (5 * base) {
+		return 5 * base
+	}
+	return 10 * base
+}
+
+// run times the benchmark function in a separate goroutine.
+func (b *B) run() BenchmarkResult {
+	go b.launch()
+	<-b.signal
+	return b.result
+}
+
+// launch launches the benchmark function. It gradually increases the number
+// of benchmark iterations until the benchmark runs for a second in order
+// to get a reasonable measurement. It prints timing information in this form
+//		testing.BenchmarkHello	100000		19 ns/op
+// launch is run by the fun function as a separate goroutine.
+func (b *B) launch() {
+	// Run the benchmark for a single iteration in case it's expensive.
+	n := 1
+
+	// Signal that we're done whether we return normally
+	// or by FailNow's runtime.Goexit.
+	defer func() {
+		b.signal <- b
+	}()
+
+	b.runN(n)
+	// Run the benchmark for at least the specified amount of time.
+	d := *benchTime
+	for !b.failed && b.duration < d && n < 1e9 {
+		last := n
+		// Predict iterations/sec.
+		if b.nsPerOp() == 0 {
+			n = 1e9
+		} else {
+			n = int(d.Nanoseconds() / b.nsPerOp())
+		}
+		// Run more iterations than we think we'll need for a second (1.5x).
+		// Don't grow too fast in case we had timing errors previously.
+		// Be sure to run at least one more than last time.
+		n = max(min(n+n/2, 100*last), last+1)
+		// Round up to something easy to read.
+		n = roundUp(n)
+		b.runN(n)
+	}
+	b.result = BenchmarkResult{b.N, b.duration, b.bytes}
+}
+
+// The results of a benchmark run.
+type BenchmarkResult struct {
+	N     int           // The number of iterations.
+	T     time.Duration // The total time taken.
+	Bytes int64         // Bytes processed in one iteration.
+}
+
+func (r BenchmarkResult) NsPerOp() int64 {
+	if r.N <= 0 {
+		return 0
+	}
+	return r.T.Nanoseconds() / int64(r.N)
+}
+
+func (r BenchmarkResult) mbPerSec() float64 {
+	if r.Bytes <= 0 || r.T <= 0 || r.N <= 0 {
+		return 0
+	}
+	return (float64(r.Bytes) * float64(r.N) / 1e6) / r.T.Seconds()
+}
+
+func (r BenchmarkResult) String() string {
+	mbs := r.mbPerSec()
+	mb := ""
+	if mbs != 0 {
+		mb = fmt.Sprintf("\t%7.2f MB/s", mbs)
+	}
+	nsop := r.NsPerOp()
+	ns := fmt.Sprintf("%10d ns/op", nsop)
+	if r.N > 0 && nsop < 100 {
+		// The format specifiers here make sure that
+		// the ones digits line up for all three possible formats.
+		if nsop < 10 {
+			ns = fmt.Sprintf("%13.2f ns/op", float64(r.T.Nanoseconds())/float64(r.N))
+		} else {
+			ns = fmt.Sprintf("%12.1f ns/op", float64(r.T.Nanoseconds())/float64(r.N))
+		}
+	}
+	return fmt.Sprintf("%8d\t%s%s", r.N, ns, mb)
+}
+
+// An internal function but exported because it is cross-package; part of the implementation
+// of go test.
+func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark) {
+	// If no flag was specified, don't run benchmarks.
+	if len(*matchBenchmarks) == 0 {
+		return
+	}
+	for _, Benchmark := range benchmarks {
+		matched, err := matchString(*matchBenchmarks, Benchmark.Name)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "testing: invalid regexp for -test.bench: %s\n", err)
+			os.Exit(1)
+		}
+		if !matched {
+			continue
+		}
+		for _, procs := range cpuList {
+			runtime.GOMAXPROCS(procs)
+			b := &B{
+				common: common{
+					signal: make(chan interface{}),
+				},
+				benchmark: Benchmark,
+			}
+			benchName := Benchmark.Name
+			if procs != 1 {
+				benchName = fmt.Sprintf("%s-%d", Benchmark.Name, procs)
+			}
+			fmt.Printf("%s\t", benchName)
+			r := b.run()
+			if b.failed {
+				// The output could be very long here, but probably isn't.
+				// We print it all, regardless, because we don't want to trim the reason
+				// the benchmark failed.
+				fmt.Printf("--- FAIL: %s\n%s", benchName, b.output)
+				continue
+			}
+			fmt.Printf("%v\n", r)
+			// Unlike with tests, we ignore the -chatty flag and always print output for
+			// benchmarks since the output generation time will skew the results.
+			if len(b.output) > 0 {
+				b.trimOutput()
+				fmt.Printf("--- BENCH: %s\n%s", benchName, b.output)
+			}
+			if p := runtime.GOMAXPROCS(-1); p != procs {
+				fmt.Fprintf(os.Stderr, "testing: %s left GOMAXPROCS set to %d\n", benchName, p)
+			}
+		}
+	}
+}
+
+// trimOutput shortens the output from a benchmark, which can be very long.
+func (b *B) trimOutput() {
+	// The output is likely to appear multiple times because the benchmark
+	// is run multiple times, but at least it will be seen. This is not a big deal
+	// because benchmarks rarely print, but just in case, we trim it if it's too long.
+	const maxNewlines = 10
+	for nlCount, j := 0, 0; j < len(b.output); j++ {
+		if b.output[j] == '\n' {
+			nlCount++
+			if nlCount >= maxNewlines {
+				b.output = append(b.output[:j], "\n\t... [output truncated]\n"...)
+				break
+			}
+		}
+	}
+}
+
+// Benchmark benchmarks a single function. Useful for creating
+// custom benchmarks that do not use go test.
+func Benchmark(f func(b *B)) BenchmarkResult {
+	b := &B{
+		common: common{
+			signal: make(chan interface{}),
+		},
+		benchmark: InternalBenchmark{"", f},
+	}
+	return b.run()
+}
diff --git a/src/go/doc/testdata/blank.0.golden b/src/go/doc/testdata/blank.0.golden
new file mode 100644
index 0000000..70f2929
--- /dev/null
+++ b/src/go/doc/testdata/blank.0.golden
@@ -0,0 +1,62 @@
+// Package blank is a go/doc test for the handling of _. See issue ...
+PACKAGE blank
+
+IMPORTPATH
+	testdata/blank
+
+IMPORTS
+	os
+
+FILENAMES
+	testdata/blank.go
+
+CONSTANTS
+	// T constants counting from unexported constants. 
+	const (
+		C1	T
+		C2
+	
+		C3
+	
+		C4	int
+	)
+
+	// Constants with a single type that is not propagated. 
+	const (
+		Default		= 0644
+		Useless		= 0312
+		WideOpen	= 0777
+	)
+
+	// Constants with an imported type that is propagated. 
+	const (
+		M1	os.FileMode
+		M2
+		M3
+	)
+
+	// Package constants. 
+	const (
+		I1	int
+		I2
+	)
+
+
+TYPES
+	// S has a padding field. 
+	type S struct {
+		H	uint32
+	
+		A	uint8
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T int
+
+	// T constants counting from a blank constant. 
+	const (
+		T1	T
+		T2
+	)
+
diff --git a/src/go/doc/testdata/blank.1.golden b/src/go/doc/testdata/blank.1.golden
new file mode 100644
index 0000000..8098cb6
--- /dev/null
+++ b/src/go/doc/testdata/blank.1.golden
@@ -0,0 +1,83 @@
+// Package blank is a go/doc test for the handling of _. See issue ...
+PACKAGE blank
+
+IMPORTPATH
+	testdata/blank
+
+IMPORTS
+	os
+
+FILENAMES
+	testdata/blank.go
+
+CONSTANTS
+	// T constants counting from unexported constants. 
+	const (
+		tweedledee	T	= iota
+		tweedledum
+		C1
+		C2
+		alice
+		C3
+		redQueen	int	= iota
+		C4
+	)
+
+	// Constants with a single type that is not propagated. 
+	const (
+		zero		os.FileMode	= 0
+		Default				= 0644
+		Useless				= 0312
+		WideOpen			= 0777
+	)
+
+	// Constants with an imported type that is propagated. 
+	const (
+		zero	os.FileMode	= 0
+		M1
+		M2
+		M3
+	)
+
+	// Package constants. 
+	const (
+		_	int	= iota
+		I1
+		I2
+	)
+
+	// Unexported constants counting from blank iota. See issue 9615. 
+	const (
+		_	= iota
+		one	= iota + 1
+	)
+
+
+VARIABLES
+	// 
+	var _ = T(55)
+
+
+FUNCTIONS
+	// 
+	func _()
+
+
+TYPES
+	// S has a padding field. 
+	type S struct {
+		H	uint32
+		_	uint8
+		A	uint8
+	}
+
+	// 
+	type T int
+
+	// T constants counting from a blank constant. 
+	const (
+		_	T	= iota
+		T1
+		T2
+	)
+
diff --git a/src/go/doc/testdata/blank.2.golden b/src/go/doc/testdata/blank.2.golden
new file mode 100644
index 0000000..70f2929
--- /dev/null
+++ b/src/go/doc/testdata/blank.2.golden
@@ -0,0 +1,62 @@
+// Package blank is a go/doc test for the handling of _. See issue ...
+PACKAGE blank
+
+IMPORTPATH
+	testdata/blank
+
+IMPORTS
+	os
+
+FILENAMES
+	testdata/blank.go
+
+CONSTANTS
+	// T constants counting from unexported constants. 
+	const (
+		C1	T
+		C2
+	
+		C3
+	
+		C4	int
+	)
+
+	// Constants with a single type that is not propagated. 
+	const (
+		Default		= 0644
+		Useless		= 0312
+		WideOpen	= 0777
+	)
+
+	// Constants with an imported type that is propagated. 
+	const (
+		M1	os.FileMode
+		M2
+		M3
+	)
+
+	// Package constants. 
+	const (
+		I1	int
+		I2
+	)
+
+
+TYPES
+	// S has a padding field. 
+	type S struct {
+		H	uint32
+	
+		A	uint8
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T int
+
+	// T constants counting from a blank constant. 
+	const (
+		T1	T
+		T2
+	)
+
diff --git a/src/go/doc/testdata/blank.go b/src/go/doc/testdata/blank.go
new file mode 100644
index 0000000..5ea6186
--- /dev/null
+++ b/src/go/doc/testdata/blank.go
@@ -0,0 +1,75 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package blank is a go/doc test for the handling of _.
+// See issue 5397.
+package blank
+
+import "os"
+
+type T int
+
+// T constants counting from a blank constant.
+const (
+	_ T = iota
+	T1
+	T2
+)
+
+// T constants counting from unexported constants.
+const (
+	tweedledee T = iota
+	tweedledum
+	C1
+	C2
+	alice
+	C3
+	redQueen int = iota
+	C4
+)
+
+// Constants with a single type that is not propagated.
+const (
+	zero     os.FileMode = 0
+	Default              = 0644
+	Useless              = 0312
+	WideOpen             = 0777
+)
+
+// Constants with an imported type that is propagated.
+const (
+	zero os.FileMode = 0
+	M1
+	M2
+	M3
+)
+
+// Package constants.
+const (
+	_ int = iota
+	I1
+	I2
+)
+
+// Unexported constants counting from blank iota.
+// See issue 9615.
+const (
+	_   = iota
+	one = iota + 1
+)
+
+// Blanks not in doc output:
+
+// S has a padding field.
+type S struct {
+	H uint32
+	_ uint8
+	A uint8
+}
+
+func _() {}
+
+type _ T
+
+var _ = T(55)
diff --git a/src/go/doc/testdata/bugpara.0.golden b/src/go/doc/testdata/bugpara.0.golden
new file mode 100644
index 0000000..5804859
--- /dev/null
+++ b/src/go/doc/testdata/bugpara.0.golden
@@ -0,0 +1,20 @@
+// 
+PACKAGE bugpara
+
+IMPORTPATH
+	testdata/bugpara
+
+FILENAMES
+	testdata/bugpara.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
+
+BUGS
+BUG(rsc)	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
diff --git a/src/go/doc/testdata/bugpara.1.golden b/src/go/doc/testdata/bugpara.1.golden
new file mode 100644
index 0000000..5804859
--- /dev/null
+++ b/src/go/doc/testdata/bugpara.1.golden
@@ -0,0 +1,20 @@
+// 
+PACKAGE bugpara
+
+IMPORTPATH
+	testdata/bugpara
+
+FILENAMES
+	testdata/bugpara.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
+
+BUGS
+BUG(rsc)	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
diff --git a/src/go/doc/testdata/bugpara.2.golden b/src/go/doc/testdata/bugpara.2.golden
new file mode 100644
index 0000000..5804859
--- /dev/null
+++ b/src/go/doc/testdata/bugpara.2.golden
@@ -0,0 +1,20 @@
+// 
+PACKAGE bugpara
+
+IMPORTPATH
+	testdata/bugpara
+
+FILENAMES
+	testdata/bugpara.go
+
+BUGS .Bugs is now deprecated, please use .Notes instead
+	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
+
+BUGS
+BUG(rsc)	Sometimes bugs have multiple paragraphs.
+	
+	Like this one.
+
diff --git a/src/go/doc/testdata/bugpara.go b/src/go/doc/testdata/bugpara.go
new file mode 100644
index 0000000..0360a6f
--- /dev/null
+++ b/src/go/doc/testdata/bugpara.go
@@ -0,0 +1,9 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bugpara
+
+// BUG(rsc): Sometimes bugs have multiple paragraphs.
+//
+// Like this one.
diff --git a/src/go/doc/testdata/c.0.golden b/src/go/doc/testdata/c.0.golden
new file mode 100644
index 0000000..e21959b
--- /dev/null
+++ b/src/go/doc/testdata/c.0.golden
@@ -0,0 +1,48 @@
+// 
+PACKAGE c
+
+IMPORTPATH
+	testdata/c
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/c.go
+
+TYPES
+	// A (should see this) 
+	type A struct{}
+
+	// B (should see this) 
+	type B struct{}
+
+	// C (should see this) 
+	type C struct{}
+
+	// D (should see this) 
+	type D struct{}
+
+	// E1 (should see this) 
+	type E1 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E2 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E3 struct{}
+
+	// E4 (should see this) 
+	type E4 struct{}
+
+	// 
+	type T1 struct{}
+
+	// 
+	func (t1 *T1) M()
+
+	// T2 must not show methods of local T1 
+	type T2 struct {
+		a.T1	// not the same as locally declared T1
+	}
+
diff --git a/src/go/doc/testdata/c.1.golden b/src/go/doc/testdata/c.1.golden
new file mode 100644
index 0000000..e21959b
--- /dev/null
+++ b/src/go/doc/testdata/c.1.golden
@@ -0,0 +1,48 @@
+// 
+PACKAGE c
+
+IMPORTPATH
+	testdata/c
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/c.go
+
+TYPES
+	// A (should see this) 
+	type A struct{}
+
+	// B (should see this) 
+	type B struct{}
+
+	// C (should see this) 
+	type C struct{}
+
+	// D (should see this) 
+	type D struct{}
+
+	// E1 (should see this) 
+	type E1 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E2 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E3 struct{}
+
+	// E4 (should see this) 
+	type E4 struct{}
+
+	// 
+	type T1 struct{}
+
+	// 
+	func (t1 *T1) M()
+
+	// T2 must not show methods of local T1 
+	type T2 struct {
+		a.T1	// not the same as locally declared T1
+	}
+
diff --git a/src/go/doc/testdata/c.2.golden b/src/go/doc/testdata/c.2.golden
new file mode 100644
index 0000000..e21959b
--- /dev/null
+++ b/src/go/doc/testdata/c.2.golden
@@ -0,0 +1,48 @@
+// 
+PACKAGE c
+
+IMPORTPATH
+	testdata/c
+
+IMPORTS
+	a
+
+FILENAMES
+	testdata/c.go
+
+TYPES
+	// A (should see this) 
+	type A struct{}
+
+	// B (should see this) 
+	type B struct{}
+
+	// C (should see this) 
+	type C struct{}
+
+	// D (should see this) 
+	type D struct{}
+
+	// E1 (should see this) 
+	type E1 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E2 struct{}
+
+	// E (should see this for E2 and E3) 
+	type E3 struct{}
+
+	// E4 (should see this) 
+	type E4 struct{}
+
+	// 
+	type T1 struct{}
+
+	// 
+	func (t1 *T1) M()
+
+	// T2 must not show methods of local T1 
+	type T2 struct {
+		a.T1	// not the same as locally declared T1
+	}
+
diff --git a/src/go/doc/testdata/c.go b/src/go/doc/testdata/c.go
new file mode 100644
index 0000000..e0f3919
--- /dev/null
+++ b/src/go/doc/testdata/c.go
@@ -0,0 +1,62 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package c
+
+import "a"
+
+// ----------------------------------------------------------------------------
+// Test that empty declarations don't cause problems
+
+const ()
+
+type ()
+
+var ()
+
+// ----------------------------------------------------------------------------
+// Test that types with documentation on both, the Decl and the Spec node
+// are handled correctly.
+
+// A (should see this)
+type A struct{}
+
+// B (should see this)
+type (
+	B struct{}
+)
+
+type (
+	// C (should see this)
+	C struct{}
+)
+
+// D (should not see this)
+type (
+	// D (should see this)
+	D struct{}
+)
+
+// E (should see this for E2 and E3)
+type (
+	// E1 (should see this)
+	E1 struct{}
+	E2 struct{}
+	E3 struct{}
+	// E4 (should see this)
+	E4 struct{}
+)
+
+// ----------------------------------------------------------------------------
+// Test that local and imported types are different when
+// handling anonymous fields.
+
+type T1 struct{}
+
+func (t1 *T1) M() {}
+
+// T2 must not show methods of local T1
+type T2 struct {
+	a.T1 // not the same as locally declared T1
+}
diff --git a/src/go/doc/testdata/d.0.golden b/src/go/doc/testdata/d.0.golden
new file mode 100644
index 0000000..c005199
--- /dev/null
+++ b/src/go/doc/testdata/d.0.golden
@@ -0,0 +1,104 @@
+// 
+PACKAGE d
+
+IMPORTPATH
+	testdata/d
+
+FILENAMES
+	testdata/d1.go
+	testdata/d2.go
+
+CONSTANTS
+	// CBx constants should appear before CAx constants. 
+	const (
+		CB2	= iota	// before CB1
+		CB1		// before CB0
+		CB0		// at end
+	)
+
+	// CAx constants should appear after CBx constants. 
+	const (
+		CA2	= iota	// before CA1
+		CA1		// before CA0
+		CA0		// at end
+	)
+
+	// C0 should be first. 
+	const C0 = 0
+
+	// C1 should be second. 
+	const C1 = 1
+
+	// C2 should be third. 
+	const C2 = 2
+
+	// 
+	const (
+		// Single const declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Cungrouped = 0
+	)
+
+
+VARIABLES
+	// VBx variables should appear before VAx variables. 
+	var (
+		VB2	int	// before VB1
+		VB1	int	// before VB0
+		VB0	int	// at end
+	)
+
+	// VAx variables should appear after VBx variables. 
+	var (
+		VA2	int	// before VA1
+		VA1	int	// before VA0
+		VA0	int	// at end
+	)
+
+	// V0 should be first. 
+	var V0 uintptr
+
+	// V1 should be second. 
+	var V1 uint
+
+	// V2 should be third. 
+	var V2 int
+
+	// 
+	var (
+		// Single var declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Vungrouped = 0
+	)
+
+
+FUNCTIONS
+	// F0 should be first. 
+	func F0()
+
+	// F1 should be second. 
+	func F1()
+
+	// F2 should be third. 
+	func F2()
+
+
+TYPES
+	// T0 should be first. 
+	type T0 struct{}
+
+	// T1 should be second. 
+	type T1 struct{}
+
+	// T2 should be third. 
+	type T2 struct{}
+
+	// TG0 should be first. 
+	type TG0 struct{}
+
+	// TG1 should be second. 
+	type TG1 struct{}
+
+	// TG2 should be third. 
+	type TG2 struct{}
+
diff --git a/src/go/doc/testdata/d.1.golden b/src/go/doc/testdata/d.1.golden
new file mode 100644
index 0000000..c005199
--- /dev/null
+++ b/src/go/doc/testdata/d.1.golden
@@ -0,0 +1,104 @@
+// 
+PACKAGE d
+
+IMPORTPATH
+	testdata/d
+
+FILENAMES
+	testdata/d1.go
+	testdata/d2.go
+
+CONSTANTS
+	// CBx constants should appear before CAx constants. 
+	const (
+		CB2	= iota	// before CB1
+		CB1		// before CB0
+		CB0		// at end
+	)
+
+	// CAx constants should appear after CBx constants. 
+	const (
+		CA2	= iota	// before CA1
+		CA1		// before CA0
+		CA0		// at end
+	)
+
+	// C0 should be first. 
+	const C0 = 0
+
+	// C1 should be second. 
+	const C1 = 1
+
+	// C2 should be third. 
+	const C2 = 2
+
+	// 
+	const (
+		// Single const declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Cungrouped = 0
+	)
+
+
+VARIABLES
+	// VBx variables should appear before VAx variables. 
+	var (
+		VB2	int	// before VB1
+		VB1	int	// before VB0
+		VB0	int	// at end
+	)
+
+	// VAx variables should appear after VBx variables. 
+	var (
+		VA2	int	// before VA1
+		VA1	int	// before VA0
+		VA0	int	// at end
+	)
+
+	// V0 should be first. 
+	var V0 uintptr
+
+	// V1 should be second. 
+	var V1 uint
+
+	// V2 should be third. 
+	var V2 int
+
+	// 
+	var (
+		// Single var declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Vungrouped = 0
+	)
+
+
+FUNCTIONS
+	// F0 should be first. 
+	func F0()
+
+	// F1 should be second. 
+	func F1()
+
+	// F2 should be third. 
+	func F2()
+
+
+TYPES
+	// T0 should be first. 
+	type T0 struct{}
+
+	// T1 should be second. 
+	type T1 struct{}
+
+	// T2 should be third. 
+	type T2 struct{}
+
+	// TG0 should be first. 
+	type TG0 struct{}
+
+	// TG1 should be second. 
+	type TG1 struct{}
+
+	// TG2 should be third. 
+	type TG2 struct{}
+
diff --git a/src/go/doc/testdata/d.2.golden b/src/go/doc/testdata/d.2.golden
new file mode 100644
index 0000000..c005199
--- /dev/null
+++ b/src/go/doc/testdata/d.2.golden
@@ -0,0 +1,104 @@
+// 
+PACKAGE d
+
+IMPORTPATH
+	testdata/d
+
+FILENAMES
+	testdata/d1.go
+	testdata/d2.go
+
+CONSTANTS
+	// CBx constants should appear before CAx constants. 
+	const (
+		CB2	= iota	// before CB1
+		CB1		// before CB0
+		CB0		// at end
+	)
+
+	// CAx constants should appear after CBx constants. 
+	const (
+		CA2	= iota	// before CA1
+		CA1		// before CA0
+		CA0		// at end
+	)
+
+	// C0 should be first. 
+	const C0 = 0
+
+	// C1 should be second. 
+	const C1 = 1
+
+	// C2 should be third. 
+	const C2 = 2
+
+	// 
+	const (
+		// Single const declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Cungrouped = 0
+	)
+
+
+VARIABLES
+	// VBx variables should appear before VAx variables. 
+	var (
+		VB2	int	// before VB1
+		VB1	int	// before VB0
+		VB0	int	// at end
+	)
+
+	// VAx variables should appear after VBx variables. 
+	var (
+		VA2	int	// before VA1
+		VA1	int	// before VA0
+		VA0	int	// at end
+	)
+
+	// V0 should be first. 
+	var V0 uintptr
+
+	// V1 should be second. 
+	var V1 uint
+
+	// V2 should be third. 
+	var V2 int
+
+	// 
+	var (
+		// Single var declarations inside ()'s are considered ungrouped
+		// and show up in sorted order.
+		Vungrouped = 0
+	)
+
+
+FUNCTIONS
+	// F0 should be first. 
+	func F0()
+
+	// F1 should be second. 
+	func F1()
+
+	// F2 should be third. 
+	func F2()
+
+
+TYPES
+	// T0 should be first. 
+	type T0 struct{}
+
+	// T1 should be second. 
+	type T1 struct{}
+
+	// T2 should be third. 
+	type T2 struct{}
+
+	// TG0 should be first. 
+	type TG0 struct{}
+
+	// TG1 should be second. 
+	type TG1 struct{}
+
+	// TG2 should be third. 
+	type TG2 struct{}
+
diff --git a/src/go/doc/testdata/d1.go b/src/go/doc/testdata/d1.go
new file mode 100644
index 0000000..ebd6941
--- /dev/null
+++ b/src/go/doc/testdata/d1.go
@@ -0,0 +1,57 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test cases for sort order of declarations.
+
+package d
+
+// C2 should be third.
+const C2 = 2
+
+// V2 should be third.
+var V2 int
+
+// CBx constants should appear before CAx constants.
+const (
+	CB2 = iota // before CB1
+	CB1        // before CB0
+	CB0        // at end
+)
+
+// VBx variables should appear before VAx variables.
+var (
+	VB2 int // before VB1
+	VB1 int // before VB0
+	VB0 int // at end
+)
+
+const (
+	// Single const declarations inside ()'s are considered ungrouped
+	// and show up in sorted order.
+	Cungrouped = 0
+)
+
+var (
+	// Single var declarations inside ()'s are considered ungrouped
+	// and show up in sorted order.
+	Vungrouped = 0
+)
+
+// T2 should be third.
+type T2 struct{}
+
+// Grouped types are sorted nevertheless.
+type (
+	// TG2 should be third.
+	TG2 struct{}
+
+	// TG1 should be second.
+	TG1 struct{}
+
+	// TG0 should be first.
+	TG0 struct{}
+)
+
+// F2 should be third.
+func F2() {}
diff --git a/src/go/doc/testdata/d2.go b/src/go/doc/testdata/d2.go
new file mode 100644
index 0000000..2f56f4f
--- /dev/null
+++ b/src/go/doc/testdata/d2.go
@@ -0,0 +1,45 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test cases for sort order of declarations.
+
+package d
+
+// C1 should be second.
+const C1 = 1
+
+// C0 should be first.
+const C0 = 0
+
+// V1 should be second.
+var V1 uint
+
+// V0 should be first.
+var V0 uintptr
+
+// CAx constants should appear after CBx constants.
+const (
+	CA2 = iota // before CA1
+	CA1        // before CA0
+	CA0        // at end
+)
+
+// VAx variables should appear after VBx variables.
+var (
+	VA2 int // before VA1
+	VA1 int // before VA0
+	VA0 int // at end
+)
+
+// T1 should be second.
+type T1 struct{}
+
+// T0 should be first.
+type T0 struct{}
+
+// F1 should be second.
+func F1() {}
+
+// F0 should be first.
+func F0() {}
diff --git a/src/go/doc/testdata/e.0.golden b/src/go/doc/testdata/e.0.golden
new file mode 100644
index 0000000..6987e58
--- /dev/null
+++ b/src/go/doc/testdata/e.0.golden
@@ -0,0 +1,109 @@
+// The package e is a go/doc test for embedded methods. 
+PACKAGE e
+
+IMPORTPATH
+	testdata/e
+
+FILENAMES
+	testdata/e.go
+
+TYPES
+	// T1 has no embedded (level 1) M method due to conflict. 
+	type T1 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T2 has only M as top-level method. 
+	type T2 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T2.M should appear as method of T2. 
+	func (T2) M()
+
+	// T3 has only M as top-level method. 
+	type T3 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T3.M should appear as method of T3. 
+	func (T3) M()
+
+	// 
+	type T4 struct{}
+
+	// T4.M should appear as method of T5 only if AllMethods is set. 
+	func (*T4) M()
+
+	// 
+	type T5 struct {
+		T4
+	}
+
+	// 
+	type U1 struct {
+		*U1
+	}
+
+	// U1.M should appear as method of U1. 
+	func (*U1) M()
+
+	// 
+	type U2 struct {
+		*U3
+	}
+
+	// U2.M should appear as method of U2 and as method of U3 only if ...
+	func (*U2) M()
+
+	// 
+	type U3 struct {
+		*U2
+	}
+
+	// U3.N should appear as method of U3 and as method of U2 only if ...
+	func (*U3) N()
+
+	// 
+	type U4 struct {
+		// contains filtered or unexported fields
+	}
+
+	// U4.M should appear as method of U4. 
+	func (*U4) M()
+
+	// 
+	type V1 struct {
+		*V2
+		*V5
+	}
+
+	// 
+	type V2 struct {
+		*V3
+	}
+
+	// 
+	type V3 struct {
+		*V4
+	}
+
+	// 
+	type V4 struct {
+		*V5
+	}
+
+	// V4.M should appear as method of V2 and V3 if AllMethods is set. 
+	func (*V4) M()
+
+	// 
+	type V5 struct {
+		*V6
+	}
+
+	// 
+	type V6 struct{}
+
+	// V6.M should appear as method of V1 and V5 if AllMethods is set. 
+	func (*V6) M()
+
diff --git a/src/go/doc/testdata/e.1.golden b/src/go/doc/testdata/e.1.golden
new file mode 100644
index 0000000..cbe22e0
--- /dev/null
+++ b/src/go/doc/testdata/e.1.golden
@@ -0,0 +1,144 @@
+// The package e is a go/doc test for embedded methods. 
+PACKAGE e
+
+IMPORTPATH
+	testdata/e
+
+FILENAMES
+	testdata/e.go
+
+TYPES
+	// T1 has no embedded (level 1) M method due to conflict. 
+	type T1 struct {
+		t1
+		t2
+	}
+
+	// T2 has only M as top-level method. 
+	type T2 struct {
+		t1
+	}
+
+	// T2.M should appear as method of T2. 
+	func (T2) M()
+
+	// T3 has only M as top-level method. 
+	type T3 struct {
+		t1e
+		t2e
+	}
+
+	// T3.M should appear as method of T3. 
+	func (T3) M()
+
+	// 
+	type T4 struct{}
+
+	// T4.M should appear as method of T5 only if AllMethods is set. 
+	func (*T4) M()
+
+	// 
+	type T5 struct {
+		T4
+	}
+
+	// 
+	type U1 struct {
+		*U1
+	}
+
+	// U1.M should appear as method of U1. 
+	func (*U1) M()
+
+	// 
+	type U2 struct {
+		*U3
+	}
+
+	// U2.M should appear as method of U2 and as method of U3 only if ...
+	func (*U2) M()
+
+	// 
+	type U3 struct {
+		*U2
+	}
+
+	// U3.N should appear as method of U3 and as method of U2 only if ...
+	func (*U3) N()
+
+	// 
+	type U4 struct {
+		*u5
+	}
+
+	// U4.M should appear as method of U4. 
+	func (*U4) M()
+
+	// 
+	type V1 struct {
+		*V2
+		*V5
+	}
+
+	// 
+	type V2 struct {
+		*V3
+	}
+
+	// 
+	type V3 struct {
+		*V4
+	}
+
+	// 
+	type V4 struct {
+		*V5
+	}
+
+	// V4.M should appear as method of V2 and V3 if AllMethods is set. 
+	func (*V4) M()
+
+	// 
+	type V5 struct {
+		*V6
+	}
+
+	// 
+	type V6 struct{}
+
+	// V6.M should appear as method of V1 and V5 if AllMethods is set. 
+	func (*V6) M()
+
+	// 
+	type t1 struct{}
+
+	// t1.M should not appear as method in a Tx type. 
+	func (t1) M()
+
+	// 
+	type t1e struct {
+		t1
+	}
+
+	// t1.M should not appear as method in a Tx type. 
+	func (t1e) M()
+
+	// 
+	type t2 struct{}
+
+	// t2.M should not appear as method in a Tx type. 
+	func (t2) M()
+
+	// 
+	type t2e struct {
+		t2
+	}
+
+	// t2.M should not appear as method in a Tx type. 
+	func (t2e) M()
+
+	// 
+	type u5 struct {
+		*U4
+	}
+
diff --git a/src/go/doc/testdata/e.2.golden b/src/go/doc/testdata/e.2.golden
new file mode 100644
index 0000000..e7b05e8
--- /dev/null
+++ b/src/go/doc/testdata/e.2.golden
@@ -0,0 +1,130 @@
+// The package e is a go/doc test for embedded methods. 
+PACKAGE e
+
+IMPORTPATH
+	testdata/e
+
+FILENAMES
+	testdata/e.go
+
+TYPES
+	// T1 has no embedded (level 1) M method due to conflict. 
+	type T1 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T2 has only M as top-level method. 
+	type T2 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T2.M should appear as method of T2. 
+	func (T2) M()
+
+	// T3 has only M as top-level method. 
+	type T3 struct {
+		// contains filtered or unexported fields
+	}
+
+	// T3.M should appear as method of T3. 
+	func (T3) M()
+
+	// 
+	type T4 struct{}
+
+	// T4.M should appear as method of T5 only if AllMethods is set. 
+	func (*T4) M()
+
+	// 
+	type T5 struct {
+		T4
+	}
+
+	// T4.M should appear as method of T5 only if AllMethods is set. 
+	func (*T5) M()
+
+	// 
+	type U1 struct {
+		*U1
+	}
+
+	// U1.M should appear as method of U1. 
+	func (*U1) M()
+
+	// 
+	type U2 struct {
+		*U3
+	}
+
+	// U2.M should appear as method of U2 and as method of U3 only if ...
+	func (*U2) M()
+
+	// U3.N should appear as method of U3 and as method of U2 only if ...
+	func (U2) N()
+
+	// 
+	type U3 struct {
+		*U2
+	}
+
+	// U2.M should appear as method of U2 and as method of U3 only if ...
+	func (U3) M()
+
+	// U3.N should appear as method of U3 and as method of U2 only if ...
+	func (*U3) N()
+
+	// 
+	type U4 struct {
+		// contains filtered or unexported fields
+	}
+
+	// U4.M should appear as method of U4. 
+	func (*U4) M()
+
+	// 
+	type V1 struct {
+		*V2
+		*V5
+	}
+
+	// V6.M should appear as method of V1 and V5 if AllMethods is set. 
+	func (V1) M()
+
+	// 
+	type V2 struct {
+		*V3
+	}
+
+	// V4.M should appear as method of V2 and V3 if AllMethods is set. 
+	func (V2) M()
+
+	// 
+	type V3 struct {
+		*V4
+	}
+
+	// V4.M should appear as method of V2 and V3 if AllMethods is set. 
+	func (V3) M()
+
+	// 
+	type V4 struct {
+		*V5
+	}
+
+	// V4.M should appear as method of V2 and V3 if AllMethods is set. 
+	func (*V4) M()
+
+	// 
+	type V5 struct {
+		*V6
+	}
+
+	// V6.M should appear as method of V1 and V5 if AllMethods is set. 
+	func (V5) M()
+
+	// 
+	type V6 struct{}
+
+	// V6.M should appear as method of V1 and V5 if AllMethods is set. 
+	func (*V6) M()
+
diff --git a/src/go/doc/testdata/e.go b/src/go/doc/testdata/e.go
new file mode 100644
index 0000000..ec432e3
--- /dev/null
+++ b/src/go/doc/testdata/e.go
@@ -0,0 +1,147 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The package e is a go/doc test for embedded methods.
+package e
+
+// ----------------------------------------------------------------------------
+// Conflicting methods M must not show up.
+
+type t1 struct{}
+
+// t1.M should not appear as method in a Tx type.
+func (t1) M() {}
+
+type t2 struct{}
+
+// t2.M should not appear as method in a Tx type.
+func (t2) M() {}
+
+// T1 has no embedded (level 1) M method due to conflict.
+type T1 struct {
+	t1
+	t2
+}
+
+// ----------------------------------------------------------------------------
+// Higher-level method M wins over lower-level method M.
+
+// T2 has only M as top-level method.
+type T2 struct {
+	t1
+}
+
+// T2.M should appear as method of T2.
+func (T2) M() {}
+
+// ----------------------------------------------------------------------------
+// Higher-level method M wins over lower-level conflicting methods M.
+
+type t1e struct {
+	t1
+}
+
+type t2e struct {
+	t2
+}
+
+// T3 has only M as top-level method.
+type T3 struct {
+	t1e
+	t2e
+}
+
+// T3.M should appear as method of T3.
+func (T3) M() {}
+
+// ----------------------------------------------------------------------------
+// Don't show conflicting methods M embedded via an exported and non-exported
+// type.
+
+// T1 has no embedded (level 1) M method due to conflict.
+type T4 struct {
+	t2
+	T2
+}
+
+// ----------------------------------------------------------------------------
+// Don't show embedded methods of exported anonymous fields unless AllMethods
+// is set.
+
+type T4 struct{}
+
+// T4.M should appear as method of T5 only if AllMethods is set.
+func (*T4) M() {}
+
+type T5 struct {
+	T4
+}
+
+// ----------------------------------------------------------------------------
+// Recursive type declarations must not lead to endless recursion.
+
+type U1 struct {
+	*U1
+}
+
+// U1.M should appear as method of U1.
+func (*U1) M() {}
+
+type U2 struct {
+	*U3
+}
+
+// U2.M should appear as method of U2 and as method of U3 only if AllMethods is set.
+func (*U2) M() {}
+
+type U3 struct {
+	*U2
+}
+
+// U3.N should appear as method of U3 and as method of U2 only if AllMethods is set.
+func (*U3) N() {}
+
+type U4 struct {
+	*u5
+}
+
+// U4.M should appear as method of U4.
+func (*U4) M() {}
+
+type u5 struct {
+	*U4
+}
+
+// ----------------------------------------------------------------------------
+// A higher-level embedded type (and its methods) wins over the same type (and
+// its methods) embedded at a lower level.
+
+type V1 struct {
+	*V2
+	*V5
+}
+
+type V2 struct {
+	*V3
+}
+
+type V3 struct {
+	*V4
+}
+
+type V4 struct {
+	*V5
+}
+
+type V5 struct {
+	*V6
+}
+
+type V6 struct{}
+
+// V4.M should appear as method of V2 and V3 if AllMethods is set.
+func (*V4) M() {}
+
+// V6.M should appear as method of V1 and V5 if AllMethods is set.
+func (*V6) M() {}
diff --git a/src/go/doc/testdata/error1.0.golden b/src/go/doc/testdata/error1.0.golden
new file mode 100644
index 0000000..6c6fe5d
--- /dev/null
+++ b/src/go/doc/testdata/error1.0.golden
@@ -0,0 +1,30 @@
+// 
+PACKAGE error1
+
+IMPORTPATH
+	testdata/error1
+
+FILENAMES
+	testdata/error1.go
+
+TYPES
+	// 
+	type I0 interface {
+		// When embedded, the predeclared error interface
+		// must remain visible in interface types.
+		error
+	}
+
+	// 
+	type S0 struct {
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// error should be visible
+			error
+		}
+	}
+
diff --git a/src/go/doc/testdata/error1.1.golden b/src/go/doc/testdata/error1.1.golden
new file mode 100644
index 0000000..a8dc2e7
--- /dev/null
+++ b/src/go/doc/testdata/error1.1.golden
@@ -0,0 +1,32 @@
+// 
+PACKAGE error1
+
+IMPORTPATH
+	testdata/error1
+
+FILENAMES
+	testdata/error1.go
+
+TYPES
+	// 
+	type I0 interface {
+		// When embedded, the predeclared error interface
+		// must remain visible in interface types.
+		error
+	}
+
+	// 
+	type S0 struct {
+		// In struct types, an embedded error must only be visible
+		// if AllDecls is set.
+		error
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// error should be visible
+			error
+		}
+	}
+
diff --git a/src/go/doc/testdata/error1.2.golden b/src/go/doc/testdata/error1.2.golden
new file mode 100644
index 0000000..6c6fe5d
--- /dev/null
+++ b/src/go/doc/testdata/error1.2.golden
@@ -0,0 +1,30 @@
+// 
+PACKAGE error1
+
+IMPORTPATH
+	testdata/error1
+
+FILENAMES
+	testdata/error1.go
+
+TYPES
+	// 
+	type I0 interface {
+		// When embedded, the predeclared error interface
+		// must remain visible in interface types.
+		error
+	}
+
+	// 
+	type S0 struct {
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// error should be visible
+			error
+		}
+	}
+
diff --git a/src/go/doc/testdata/error1.go b/src/go/doc/testdata/error1.go
new file mode 100644
index 0000000..3c777a7
--- /dev/null
+++ b/src/go/doc/testdata/error1.go
@@ -0,0 +1,24 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package error1
+
+type I0 interface {
+	// When embedded, the predeclared error interface
+	// must remain visible in interface types.
+	error
+}
+
+type T0 struct {
+	ExportedField interface {
+		// error should be visible
+		error
+	}
+}
+
+type S0 struct {
+	// In struct types, an embedded error must only be visible
+	// if AllDecls is set.
+	error
+}
diff --git a/src/go/doc/testdata/error2.0.golden b/src/go/doc/testdata/error2.0.golden
new file mode 100644
index 0000000..dedfe41
--- /dev/null
+++ b/src/go/doc/testdata/error2.0.golden
@@ -0,0 +1,27 @@
+// 
+PACKAGE error2
+
+IMPORTPATH
+	testdata/error2
+
+FILENAMES
+	testdata/error2.go
+
+TYPES
+	// 
+	type I0 interface {
+		// contains filtered or unexported methods
+	}
+
+	// 
+	type S0 struct {
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// contains filtered or unexported methods
+		}
+	}
+
diff --git a/src/go/doc/testdata/error2.1.golden b/src/go/doc/testdata/error2.1.golden
new file mode 100644
index 0000000..dbcc1b0
--- /dev/null
+++ b/src/go/doc/testdata/error2.1.golden
@@ -0,0 +1,37 @@
+// 
+PACKAGE error2
+
+IMPORTPATH
+	testdata/error2
+
+FILENAMES
+	testdata/error2.go
+
+TYPES
+	// 
+	type I0 interface {
+		// When embedded, the locally-declared error interface
+		// is only visible if all declarations are shown.
+		error
+	}
+
+	// 
+	type S0 struct {
+		// In struct types, an embedded error must only be visible
+		// if AllDecls is set.
+		error
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// error should not be visible
+			error
+		}
+	}
+
+	// This error declaration shadows the predeclared error type. 
+	type error interface {
+		Error() string
+	}
+
diff --git a/src/go/doc/testdata/error2.2.golden b/src/go/doc/testdata/error2.2.golden
new file mode 100644
index 0000000..dedfe41
--- /dev/null
+++ b/src/go/doc/testdata/error2.2.golden
@@ -0,0 +1,27 @@
+// 
+PACKAGE error2
+
+IMPORTPATH
+	testdata/error2
+
+FILENAMES
+	testdata/error2.go
+
+TYPES
+	// 
+	type I0 interface {
+		// contains filtered or unexported methods
+	}
+
+	// 
+	type S0 struct {
+		// contains filtered or unexported fields
+	}
+
+	// 
+	type T0 struct {
+		ExportedField interface {
+			// contains filtered or unexported methods
+		}
+	}
+
diff --git a/src/go/doc/testdata/error2.go b/src/go/doc/testdata/error2.go
new file mode 100644
index 0000000..6ee96c2
--- /dev/null
+++ b/src/go/doc/testdata/error2.go
@@ -0,0 +1,29 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package error2
+
+type I0 interface {
+	// When embedded, the locally-declared error interface
+	// is only visible if all declarations are shown.
+	error
+}
+
+type T0 struct {
+	ExportedField interface {
+		// error should not be visible
+		error
+	}
+}
+
+type S0 struct {
+	// In struct types, an embedded error must only be visible
+	// if AllDecls is set.
+	error
+}
+
+// This error declaration shadows the predeclared error type.
+type error interface {
+	Error() string
+}
diff --git a/src/go/doc/testdata/example.go b/src/go/doc/testdata/example.go
new file mode 100644
index 0000000..fdeda13
--- /dev/null
+++ b/src/go/doc/testdata/example.go
@@ -0,0 +1,81 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package testing
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+	"time"
+)
+
+type InternalExample struct {
+	Name   string
+	F      func()
+	Output string
+}
+
+func RunExamples(examples []InternalExample) (ok bool) {
+	ok = true
+
+	var eg InternalExample
+
+	stdout, stderr := os.Stdout, os.Stderr
+	defer func() {
+		os.Stdout, os.Stderr = stdout, stderr
+		if e := recover(); e != nil {
+			fmt.Printf("--- FAIL: %s\npanic: %v\n", eg.Name, e)
+			os.Exit(1)
+		}
+	}()
+
+	for _, eg = range examples {
+		if *chatty {
+			fmt.Printf("=== RUN: %s\n", eg.Name)
+		}
+
+		// capture stdout and stderr
+		r, w, err := os.Pipe()
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(1)
+		}
+		os.Stdout, os.Stderr = w, w
+		outC := make(chan string)
+		go func() {
+			buf := new(bytes.Buffer)
+			_, err := io.Copy(buf, r)
+			if err != nil {
+				fmt.Fprintf(stderr, "testing: copying pipe: %v\n", err)
+				os.Exit(1)
+			}
+			outC <- buf.String()
+		}()
+
+		// run example
+		t0 := time.Now()
+		eg.F()
+		dt := time.Now().Sub(t0)
+
+		// close pipe, restore stdout/stderr, get output
+		w.Close()
+		os.Stdout, os.Stderr = stdout, stderr
+		out := <-outC
+
+		// report any errors
+		tstr := fmt.Sprintf("(%.2f seconds)", dt.Seconds())
+		if g, e := strings.TrimSpace(out), strings.TrimSpace(eg.Output); g != e {
+			fmt.Printf("--- FAIL: %s %s\ngot:\n%s\nwant:\n%s\n",
+				eg.Name, tstr, g, e)
+			ok = false
+		} else if *chatty {
+			fmt.Printf("--- PASS: %s %s\n", eg.Name, tstr)
+		}
+	}
+
+	return
+}
diff --git a/src/go/doc/testdata/f.0.golden b/src/go/doc/testdata/f.0.golden
new file mode 100644
index 0000000..8175901
--- /dev/null
+++ b/src/go/doc/testdata/f.0.golden
@@ -0,0 +1,13 @@
+// The package f is a go/doc test for functions and factory ...
+PACKAGE f
+
+IMPORTPATH
+	testdata/f
+
+FILENAMES
+	testdata/f.go
+
+FUNCTIONS
+	// Exported must always be visible. Was issue 2824. 
+	func Exported() private
+
diff --git a/src/go/doc/testdata/f.1.golden b/src/go/doc/testdata/f.1.golden
new file mode 100644
index 0000000..ba68e88
--- /dev/null
+++ b/src/go/doc/testdata/f.1.golden
@@ -0,0 +1,16 @@
+// The package f is a go/doc test for functions and factory ...
+PACKAGE f
+
+IMPORTPATH
+	testdata/f
+
+FILENAMES
+	testdata/f.go
+
+TYPES
+	// 
+	type private struct{}
+
+	// Exported must always be visible. Was issue 2824. 
+	func Exported() private
+
diff --git a/src/go/doc/testdata/f.2.golden b/src/go/doc/testdata/f.2.golden
new file mode 100644
index 0000000..8175901
--- /dev/null
+++ b/src/go/doc/testdata/f.2.golden
@@ -0,0 +1,13 @@
+// The package f is a go/doc test for functions and factory ...
+PACKAGE f
+
+IMPORTPATH
+	testdata/f
+
+FILENAMES
+	testdata/f.go
+
+FUNCTIONS
+	// Exported must always be visible. Was issue 2824. 
+	func Exported() private
+
diff --git a/src/go/doc/testdata/f.go b/src/go/doc/testdata/f.go
new file mode 100644
index 0000000..7e9add9
--- /dev/null
+++ b/src/go/doc/testdata/f.go
@@ -0,0 +1,14 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The package f is a go/doc test for functions and factory methods.
+package f
+
+// ----------------------------------------------------------------------------
+// Factory functions for non-exported types must not get lost.
+
+type private struct{}
+
+// Exported must always be visible. Was issue 2824.
+func Exported() private {}
diff --git a/src/go/doc/testdata/g.0.golden b/src/go/doc/testdata/g.0.golden
new file mode 100644
index 0000000..487cf06
--- /dev/null
+++ b/src/go/doc/testdata/g.0.golden
@@ -0,0 +1,32 @@
+// The package g is a go/doc test for mixed exported/unexported ...
+PACKAGE g
+
+IMPORTPATH
+	testdata/g
+
+FILENAMES
+	testdata/g.go
+
+CONSTANTS
+	// 
+	const (
+		A, _	= iota, iota
+		_, D
+		E, _
+		G, H
+	)
+
+
+VARIABLES
+	// 
+	var (
+		_, C2, _	= 1, 2, 3
+		C4, _, C6	= 4, 5, 6
+		_, C8, _	= 7, 8, 9
+	)
+
+	// 
+	var (
+		_, X = f()
+	)
+
diff --git a/src/go/doc/testdata/g.1.golden b/src/go/doc/testdata/g.1.golden
new file mode 100644
index 0000000..438441a
--- /dev/null
+++ b/src/go/doc/testdata/g.1.golden
@@ -0,0 +1,34 @@
+// The package g is a go/doc test for mixed exported/unexported ...
+PACKAGE g
+
+IMPORTPATH
+	testdata/g
+
+FILENAMES
+	testdata/g.go
+
+CONSTANTS
+	// 
+	const (
+		A, b	= iota, iota
+		c, D
+		E, f
+		G, H
+	)
+
+
+VARIABLES
+	// 
+	var (
+		c1, C2, c3	= 1, 2, 3
+		C4, c5, C6	= 4, 5, 6
+		c7, C8, c9	= 7, 8, 9
+		xx, yy, zz	= 0, 0, 0	// all unexported and hidden
+	)
+
+	// 
+	var (
+		x, X	= f()
+		y, z	= f()
+	)
+
diff --git a/src/go/doc/testdata/g.2.golden b/src/go/doc/testdata/g.2.golden
new file mode 100644
index 0000000..487cf06
--- /dev/null
+++ b/src/go/doc/testdata/g.2.golden
@@ -0,0 +1,32 @@
+// The package g is a go/doc test for mixed exported/unexported ...
+PACKAGE g
+
+IMPORTPATH
+	testdata/g
+
+FILENAMES
+	testdata/g.go
+
+CONSTANTS
+	// 
+	const (
+		A, _	= iota, iota
+		_, D
+		E, _
+		G, H
+	)
+
+
+VARIABLES
+	// 
+	var (
+		_, C2, _	= 1, 2, 3
+		C4, _, C6	= 4, 5, 6
+		_, C8, _	= 7, 8, 9
+	)
+
+	// 
+	var (
+		_, X = f()
+	)
+
diff --git a/src/go/doc/testdata/g.go b/src/go/doc/testdata/g.go
new file mode 100644
index 0000000..ceeb417
--- /dev/null
+++ b/src/go/doc/testdata/g.go
@@ -0,0 +1,25 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The package g is a go/doc test for mixed exported/unexported values.
+package g
+
+const (
+	A, b = iota, iota
+	c, D
+	E, f
+	G, H
+)
+
+var (
+	c1, C2, c3 = 1, 2, 3
+	C4, c5, C6 = 4, 5, 6
+	c7, C8, c9 = 7, 8, 9
+	xx, yy, zz = 0, 0, 0 // all unexported and hidden
+)
+
+var (
+	x, X = f()
+	y, z = f()
+)
diff --git a/src/go/doc/testdata/issue12839.0.golden b/src/go/doc/testdata/issue12839.0.golden
new file mode 100644
index 0000000..6b59774
--- /dev/null
+++ b/src/go/doc/testdata/issue12839.0.golden
@@ -0,0 +1,51 @@
+// Package issue12839 is a go/doc test to test association of a ...
+PACKAGE issue12839
+
+IMPORTPATH
+	testdata/issue12839
+
+IMPORTS
+	p
+
+FILENAMES
+	testdata/issue12839.go
+
+FUNCTIONS
+	// F1 should not be associated with T1 
+	func F1() (*T1, *T2)
+
+	// F10 should not be associated with T1. 
+	func F10() (T1, T2, error)
+
+	// F4 should not be associated with a type (same as F1) 
+	func F4() (a T1, b T2)
+
+	// F9 should not be associated with T1. 
+	func F9() (int, T1, T2)
+
+
+TYPES
+	// 
+	type T1 struct{}
+
+	// F2 should be associated with T1 
+	func F2() (a, b, c T1)
+
+	// F3 should be associated with T1 because b.T3 is from a ...
+	func F3() (a T1, b p.T3)
+
+	// F5 should be associated with T1. 
+	func F5() (T1, error)
+
+	// F6 should be associated with T1. 
+	func F6() (*T1, error)
+
+	// F7 should be associated with T1. 
+	func F7() (T1, string)
+
+	// F8 should be associated with T1. 
+	func F8() (int, T1, string)
+
+	// 
+	type T2 struct{}
+
diff --git a/src/go/doc/testdata/issue12839.1.golden b/src/go/doc/testdata/issue12839.1.golden
new file mode 100644
index 0000000..4b9b9f6
--- /dev/null
+++ b/src/go/doc/testdata/issue12839.1.golden
@@ -0,0 +1,54 @@
+// Package issue12839 is a go/doc test to test association of a ...
+PACKAGE issue12839
+
+IMPORTPATH
+	testdata/issue12839
+
+IMPORTS
+	p
+
+FILENAMES
+	testdata/issue12839.go
+
+FUNCTIONS
+	// F1 should not be associated with T1 
+	func F1() (*T1, *T2)
+
+	// F10 should not be associated with T1. 
+	func F10() (T1, T2, error)
+
+	// F4 should not be associated with a type (same as F1) 
+	func F4() (a T1, b T2)
+
+	// F9 should not be associated with T1. 
+	func F9() (int, T1, T2)
+
+
+TYPES
+	// 
+	type T1 struct{}
+
+	// F2 should be associated with T1 
+	func F2() (a, b, c T1)
+
+	// F3 should be associated with T1 because b.T3 is from a ...
+	func F3() (a T1, b p.T3)
+
+	// F5 should be associated with T1. 
+	func F5() (T1, error)
+
+	// F6 should be associated with T1. 
+	func F6() (*T1, error)
+
+	// F7 should be associated with T1. 
+	func F7() (T1, string)
+
+	// F8 should be associated with T1. 
+	func F8() (int, T1, string)
+
+	// 
+	func (t T1) hello() string
+
+	// 
+	type T2 struct{}
+
diff --git a/src/go/doc/testdata/issue12839.2.golden b/src/go/doc/testdata/issue12839.2.golden
new file mode 100644
index 0000000..6b59774
--- /dev/null
+++ b/src/go/doc/testdata/issue12839.2.golden
@@ -0,0 +1,51 @@
+// Package issue12839 is a go/doc test to test association of a ...
+PACKAGE issue12839
+
+IMPORTPATH
+	testdata/issue12839
+
+IMPORTS
+	p
+
+FILENAMES
+	testdata/issue12839.go
+
+FUNCTIONS
+	// F1 should not be associated with T1 
+	func F1() (*T1, *T2)
+
+	// F10 should not be associated with T1. 
+	func F10() (T1, T2, error)
+
+	// F4 should not be associated with a type (same as F1) 
+	func F4() (a T1, b T2)
+
+	// F9 should not be associated with T1. 
+	func F9() (int, T1, T2)
+
+
+TYPES
+	// 
+	type T1 struct{}
+
+	// F2 should be associated with T1 
+	func F2() (a, b, c T1)
+
+	// F3 should be associated with T1 because b.T3 is from a ...
+	func F3() (a T1, b p.T3)
+
+	// F5 should be associated with T1. 
+	func F5() (T1, error)
+
+	// F6 should be associated with T1. 
+	func F6() (*T1, error)
+
+	// F7 should be associated with T1. 
+	func F7() (T1, string)
+
+	// F8 should be associated with T1. 
+	func F8() (int, T1, string)
+
+	// 
+	type T2 struct{}
+
diff --git a/src/go/doc/testdata/issue12839.go b/src/go/doc/testdata/issue12839.go
new file mode 100644
index 0000000..51c7ac1
--- /dev/null
+++ b/src/go/doc/testdata/issue12839.go
@@ -0,0 +1,69 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package issue12839 is a go/doc test to test association of a function
+// that returns multiple types.
+// See golang.org/issue/12839.
+// (See also golang.org/issue/27928.)
+package issue12839
+
+import "p"
+
+type T1 struct{}
+
+type T2 struct{}
+
+func (t T1) hello() string {
+	return "hello"
+}
+
+// F1 should not be associated with T1
+func F1() (*T1, *T2) {
+	return &T1{}, &T2{}
+}
+
+// F2 should be associated with T1
+func F2() (a, b, c T1) {
+	return T1{}, T1{}, T1{}
+}
+
+// F3 should be associated with T1 because b.T3 is from a different package
+func F3() (a T1, b p.T3) {
+	return T1{}, p.T3{}
+}
+
+// F4 should not be associated with a type (same as F1)
+func F4() (a T1, b T2) {
+	return T1{}, T2{}
+}
+
+// F5 should be associated with T1.
+func F5() (T1, error) {
+	return T1{}, nil
+}
+
+// F6 should be associated with T1.
+func F6() (*T1, error) {
+	return &T1{}, nil
+}
+
+// F7 should be associated with T1.
+func F7() (T1, string) {
+	return T1{}, nil
+}
+
+// F8 should be associated with T1.
+func F8() (int, T1, string) {
+	return 0, T1{}, nil
+}
+
+// F9 should not be associated with T1.
+func F9() (int, T1, T2) {
+	return 0, T1{}, T2{}
+}
+
+// F10 should not be associated with T1.
+func F10() (T1, T2, error) {
+	return T1{}, T2{}, nil
+}
diff --git a/src/go/doc/testdata/issue13742.0.golden b/src/go/doc/testdata/issue13742.0.golden
new file mode 100644
index 0000000..8dee9aa
--- /dev/null
+++ b/src/go/doc/testdata/issue13742.0.golden
@@ -0,0 +1,25 @@
+// 
+PACKAGE issue13742
+
+IMPORTPATH
+	testdata/issue13742
+
+IMPORTS
+	go/ast
+
+FILENAMES
+	testdata/issue13742.go
+
+FUNCTIONS
+	// Both F0 and G0 should appear as functions. 
+	func F0(Node)
+
+	// Both F1 and G1 should appear as functions. 
+	func F1(ast.Node)
+
+	// 
+	func G0() Node
+
+	// 
+	func G1() ast.Node
+
diff --git a/src/go/doc/testdata/issue13742.1.golden b/src/go/doc/testdata/issue13742.1.golden
new file mode 100644
index 0000000..8dee9aa
--- /dev/null
+++ b/src/go/doc/testdata/issue13742.1.golden
@@ -0,0 +1,25 @@
+// 
+PACKAGE issue13742
+
+IMPORTPATH
+	testdata/issue13742
+
+IMPORTS
+	go/ast
+
+FILENAMES
+	testdata/issue13742.go
+
+FUNCTIONS
+	// Both F0 and G0 should appear as functions. 
+	func F0(Node)
+
+	// Both F1 and G1 should appear as functions. 
+	func F1(ast.Node)
+
+	// 
+	func G0() Node
+
+	// 
+	func G1() ast.Node
+
diff --git a/src/go/doc/testdata/issue13742.2.golden b/src/go/doc/testdata/issue13742.2.golden
new file mode 100644
index 0000000..8dee9aa
--- /dev/null
+++ b/src/go/doc/testdata/issue13742.2.golden
@@ -0,0 +1,25 @@
+// 
+PACKAGE issue13742
+
+IMPORTPATH
+	testdata/issue13742
+
+IMPORTS
+	go/ast
+
+FILENAMES
+	testdata/issue13742.go
+
+FUNCTIONS
+	// Both F0 and G0 should appear as functions. 
+	func F0(Node)
+
+	// Both F1 and G1 should appear as functions. 
+	func F1(ast.Node)
+
+	// 
+	func G0() Node
+
+	// 
+	func G1() ast.Node
+
diff --git a/src/go/doc/testdata/issue13742.go b/src/go/doc/testdata/issue13742.go
new file mode 100644
index 0000000..dbc1941
--- /dev/null
+++ b/src/go/doc/testdata/issue13742.go
@@ -0,0 +1,18 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue13742
+
+import (
+	"go/ast"
+	. "go/ast"
+)
+
+// Both F0 and G0 should appear as functions.
+func F0(Node)  {}
+func G0() Node { return nil }
+
+// Both F1 and G1 should appear as functions.
+func F1(ast.Node)  {}
+func G1() ast.Node { return nil }
diff --git a/src/go/doc/testdata/issue16153.0.golden b/src/go/doc/testdata/issue16153.0.golden
new file mode 100644
index 0000000..189260b
--- /dev/null
+++ b/src/go/doc/testdata/issue16153.0.golden
@@ -0,0 +1,32 @@
+// 
+PACKAGE issue16153
+
+IMPORTPATH
+	testdata/issue16153
+
+FILENAMES
+	testdata/issue16153.go
+
+CONSTANTS
+	// 
+	const (
+		X3	int64	= iota
+		Y3		= 1
+	)
+
+	// 
+	const (
+		X4	int64	= iota
+		Y4
+	)
+
+	// original test case 
+	const (
+		Y1 = 256
+	)
+
+	// variations 
+	const (
+		Y2 uint8
+	)
+
diff --git a/src/go/doc/testdata/issue16153.1.golden b/src/go/doc/testdata/issue16153.1.golden
new file mode 100644
index 0000000..803df3e
--- /dev/null
+++ b/src/go/doc/testdata/issue16153.1.golden
@@ -0,0 +1,34 @@
+// 
+PACKAGE issue16153
+
+IMPORTPATH
+	testdata/issue16153
+
+FILENAMES
+	testdata/issue16153.go
+
+CONSTANTS
+	// original test case 
+	const (
+		x1	uint8	= 255
+		Y1		= 256
+	)
+
+	// variations 
+	const (
+		x2	uint8	= 255
+		Y2
+	)
+
+	// 
+	const (
+		X3	int64	= iota
+		Y3		= 1
+	)
+
+	// 
+	const (
+		X4	int64	= iota
+		Y4
+	)
+
diff --git a/src/go/doc/testdata/issue16153.2.golden b/src/go/doc/testdata/issue16153.2.golden
new file mode 100644
index 0000000..189260b
--- /dev/null
+++ b/src/go/doc/testdata/issue16153.2.golden
@@ -0,0 +1,32 @@
+// 
+PACKAGE issue16153
+
+IMPORTPATH
+	testdata/issue16153
+
+FILENAMES
+	testdata/issue16153.go
+
+CONSTANTS
+	// 
+	const (
+		X3	int64	= iota
+		Y3		= 1
+	)
+
+	// 
+	const (
+		X4	int64	= iota
+		Y4
+	)
+
+	// original test case 
+	const (
+		Y1 = 256
+	)
+
+	// variations 
+	const (
+		Y2 uint8
+	)
+
diff --git a/src/go/doc/testdata/issue16153.go b/src/go/doc/testdata/issue16153.go
new file mode 100644
index 0000000..528be42
--- /dev/null
+++ b/src/go/doc/testdata/issue16153.go
@@ -0,0 +1,27 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue16153
+
+// original test case
+const (
+	x1 uint8 = 255
+	Y1       = 256
+)
+
+// variations
+const (
+	x2 uint8 = 255
+	Y2
+)
+
+const (
+	X3 int64 = iota
+	Y3       = 1
+)
+
+const (
+	X4 int64 = iota
+	Y4
+)
diff --git a/src/go/doc/testdata/issue17788.0.golden b/src/go/doc/testdata/issue17788.0.golden
new file mode 100644
index 0000000..42c00da
--- /dev/null
+++ b/src/go/doc/testdata/issue17788.0.golden
@@ -0,0 +1,8 @@
+// 
+PACKAGE issue17788
+
+IMPORTPATH
+	testdata/issue17788
+
+FILENAMES
+	testdata/issue17788.go
diff --git a/src/go/doc/testdata/issue17788.1.golden b/src/go/doc/testdata/issue17788.1.golden
new file mode 100644
index 0000000..42c00da
--- /dev/null
+++ b/src/go/doc/testdata/issue17788.1.golden
@@ -0,0 +1,8 @@
+// 
+PACKAGE issue17788
+
+IMPORTPATH
+	testdata/issue17788
+
+FILENAMES
+	testdata/issue17788.go
diff --git a/src/go/doc/testdata/issue17788.2.golden b/src/go/doc/testdata/issue17788.2.golden
new file mode 100644
index 0000000..42c00da
--- /dev/null
+++ b/src/go/doc/testdata/issue17788.2.golden
@@ -0,0 +1,8 @@
+// 
+PACKAGE issue17788
+
+IMPORTPATH
+	testdata/issue17788
+
+FILENAMES
+	testdata/issue17788.go
diff --git a/src/go/doc/testdata/issue17788.go b/src/go/doc/testdata/issue17788.go
new file mode 100644
index 0000000..883ad5f
--- /dev/null
+++ b/src/go/doc/testdata/issue17788.go
@@ -0,0 +1,8 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue17788
+
+func ( /* receiver type */ ) f0() {
+}
diff --git a/src/go/doc/testdata/issue22856.0.golden b/src/go/doc/testdata/issue22856.0.golden
new file mode 100644
index 0000000..a88f43f
--- /dev/null
+++ b/src/go/doc/testdata/issue22856.0.golden
@@ -0,0 +1,45 @@
+// 
+PACKAGE issue22856
+
+IMPORTPATH
+	testdata/issue22856
+
+FILENAMES
+	testdata/issue22856.go
+
+FUNCTIONS
+	// NewPointerSliceOfSlice is not a factory function because slices ...
+	func NewPointerSliceOfSlice() [][]*T
+
+	// NewSlice3 is not a factory function because 3 nested slices of ...
+	func NewSlice3() [][][]T
+
+	// NewSliceOfSlice is not a factory function because slices of a ...
+	func NewSliceOfSlice() [][]T
+
+
+TYPES
+	// 
+	type T struct{}
+
+	// 
+	func New() T
+
+	// 
+	func NewArray() [1]T
+
+	// 
+	func NewPointer() *T
+
+	// 
+	func NewPointerArray() [1]*T
+
+	// 
+	func NewPointerOfPointer() **T
+
+	// 
+	func NewPointerSlice() []*T
+
+	// 
+	func NewSlice() []T
+
diff --git a/src/go/doc/testdata/issue22856.1.golden b/src/go/doc/testdata/issue22856.1.golden
new file mode 100644
index 0000000..a88f43f
--- /dev/null
+++ b/src/go/doc/testdata/issue22856.1.golden
@@ -0,0 +1,45 @@
+// 
+PACKAGE issue22856
+
+IMPORTPATH
+	testdata/issue22856
+
+FILENAMES
+	testdata/issue22856.go
+
+FUNCTIONS
+	// NewPointerSliceOfSlice is not a factory function because slices ...
+	func NewPointerSliceOfSlice() [][]*T
+
+	// NewSlice3 is not a factory function because 3 nested slices of ...
+	func NewSlice3() [][][]T
+
+	// NewSliceOfSlice is not a factory function because slices of a ...
+	func NewSliceOfSlice() [][]T
+
+
+TYPES
+	// 
+	type T struct{}
+
+	// 
+	func New() T
+
+	// 
+	func NewArray() [1]T
+
+	// 
+	func NewPointer() *T
+
+	// 
+	func NewPointerArray() [1]*T
+
+	// 
+	func NewPointerOfPointer() **T
+
+	// 
+	func NewPointerSlice() []*T
+
+	// 
+	func NewSlice() []T
+
diff --git a/src/go/doc/testdata/issue22856.2.golden b/src/go/doc/testdata/issue22856.2.golden
new file mode 100644
index 0000000..a88f43f
--- /dev/null
+++ b/src/go/doc/testdata/issue22856.2.golden
@@ -0,0 +1,45 @@
+// 
+PACKAGE issue22856
+
+IMPORTPATH
+	testdata/issue22856
+
+FILENAMES
+	testdata/issue22856.go
+
+FUNCTIONS
+	// NewPointerSliceOfSlice is not a factory function because slices ...
+	func NewPointerSliceOfSlice() [][]*T
+
+	// NewSlice3 is not a factory function because 3 nested slices of ...
+	func NewSlice3() [][][]T
+
+	// NewSliceOfSlice is not a factory function because slices of a ...
+	func NewSliceOfSlice() [][]T
+
+
+TYPES
+	// 
+	type T struct{}
+
+	// 
+	func New() T
+
+	// 
+	func NewArray() [1]T
+
+	// 
+	func NewPointer() *T
+
+	// 
+	func NewPointerArray() [1]*T
+
+	// 
+	func NewPointerOfPointer() **T
+
+	// 
+	func NewPointerSlice() []*T
+
+	// 
+	func NewSlice() []T
+
diff --git a/src/go/doc/testdata/issue22856.go b/src/go/doc/testdata/issue22856.go
new file mode 100644
index 0000000..f456998
--- /dev/null
+++ b/src/go/doc/testdata/issue22856.go
@@ -0,0 +1,27 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue22856
+
+type T struct{}
+
+func New() T                   { return T{} }
+func NewPointer() *T           { return &T{} }
+func NewPointerSlice() []*T    { return []*T{&T{}} }
+func NewSlice() []T            { return []T{T{}} }
+func NewPointerOfPointer() **T { x := &T{}; return &x }
+func NewArray() [1]T           { return [1]T{T{}} }
+func NewPointerArray() [1]*T   { return [1]*T{&T{}} }
+
+// NewSliceOfSlice is not a factory function because slices of a slice of
+// type *T are not factory functions of type T.
+func NewSliceOfSlice() [][]T { return []T{[]T{}} }
+
+// NewPointerSliceOfSlice is not a factory function because slices of a
+// slice of type *T are not factory functions of type T.
+func NewPointerSliceOfSlice() [][]*T { return []*T{[]*T{}} }
+
+// NewSlice3 is not a factory function because 3 nested slices of type T
+// are not factory functions of type T.
+func NewSlice3() [][][]T { return []T{[]T{[]T{}}} }
diff --git a/src/go/doc/testdata/predeclared.0.golden b/src/go/doc/testdata/predeclared.0.golden
new file mode 100644
index 0000000..9f37b06
--- /dev/null
+++ b/src/go/doc/testdata/predeclared.0.golden
@@ -0,0 +1,8 @@
+// Package predeclared is a go/doc test for handling of exported ...
+PACKAGE predeclared
+
+IMPORTPATH
+	testdata/predeclared
+
+FILENAMES
+	testdata/predeclared.go
diff --git a/src/go/doc/testdata/predeclared.1.golden b/src/go/doc/testdata/predeclared.1.golden
new file mode 100644
index 0000000..2ff8ee6
--- /dev/null
+++ b/src/go/doc/testdata/predeclared.1.golden
@@ -0,0 +1,22 @@
+// Package predeclared is a go/doc test for handling of exported ...
+PACKAGE predeclared
+
+IMPORTPATH
+	testdata/predeclared
+
+FILENAMES
+	testdata/predeclared.go
+
+TYPES
+	// 
+	type bool int
+
+	// Must not be visible. 
+	func (b bool) String() string
+
+	// 
+	type error struct{}
+
+	// Must not be visible. 
+	func (e error) Error() string
+
diff --git a/src/go/doc/testdata/predeclared.2.golden b/src/go/doc/testdata/predeclared.2.golden
new file mode 100644
index 0000000..9f37b06
--- /dev/null
+++ b/src/go/doc/testdata/predeclared.2.golden
@@ -0,0 +1,8 @@
+// Package predeclared is a go/doc test for handling of exported ...
+PACKAGE predeclared
+
+IMPORTPATH
+	testdata/predeclared
+
+FILENAMES
+	testdata/predeclared.go
diff --git a/src/go/doc/testdata/predeclared.go b/src/go/doc/testdata/predeclared.go
new file mode 100644
index 0000000..c6dd806
--- /dev/null
+++ b/src/go/doc/testdata/predeclared.go
@@ -0,0 +1,22 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package predeclared is a go/doc test for handling of
+// exported methods on locally-defined predeclared types.
+// See issue 9860.
+package predeclared
+
+type error struct{}
+
+// Must not be visible.
+func (e error) Error() string {
+	return ""
+}
+
+type bool int
+
+// Must not be visible.
+func (b bool) String() string {
+	return ""
+}
diff --git a/src/go/doc/testdata/template.txt b/src/go/doc/testdata/template.txt
new file mode 100644
index 0000000..1b07382
--- /dev/null
+++ b/src/go/doc/testdata/template.txt
@@ -0,0 +1,68 @@
+{{synopsis .Doc}}
+PACKAGE {{.Name}}
+
+IMPORTPATH
+	{{.ImportPath}}
+
+{{with .Imports}}IMPORTS
+{{range .}}	{{.}}
+{{end}}
+{{end}}{{/*
+
+*/}}FILENAMES
+{{range .Filenames}}	{{.}}
+{{end}}{{/*
+
+*/}}{{with .Consts}}
+CONSTANTS
+{{range .}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{end}}{{/*
+
+*/}}{{with .Vars}}
+VARIABLES
+{{range .}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{end}}{{/*
+
+*/}}{{with .Funcs}}
+FUNCTIONS
+{{range .}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{end}}{{/*
+
+*/}}{{with .Types}}
+TYPES
+{{range .}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{range .Consts}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{/*
+
+*/}}{{range .Vars}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{/*
+
+*/}}{{range .Funcs}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{/*
+
+*/}}{{range .Methods}}	{{synopsis .Doc}}
+	{{node .Decl $.FSet}}
+
+{{end}}{{end}}{{end}}{{/*
+
+*/}}{{with .Bugs}}
+BUGS .Bugs is now deprecated, please use .Notes instead
+{{range .}}{{indent "\t" .}}
+{{end}}{{end}}{{with .Notes}}{{range $marker, $content := .}}
+{{$marker}}S
+{{range $content}}{{$marker}}({{.UID}}){{indent "\t" .Body}}
+{{end}}{{end}}{{end}}
\ No newline at end of file
diff --git a/src/go/doc/testdata/testing.0.golden b/src/go/doc/testdata/testing.0.golden
new file mode 100644
index 0000000..83cf37c
--- /dev/null
+++ b/src/go/doc/testdata/testing.0.golden
@@ -0,0 +1,156 @@
+// Package testing provides support for automated testing of Go ...
+PACKAGE testing
+
+IMPORTPATH
+	testdata/testing
+
+IMPORTS
+	bytes
+	flag
+	fmt
+	io
+	os
+	runtime
+	runtime/pprof
+	strconv
+	strings
+	time
+
+FILENAMES
+	testdata/benchmark.go
+	testdata/example.go
+	testdata/testing.go
+
+FUNCTIONS
+	// An internal function but exported because it is cross-package; ...
+	func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample)
+
+	// An internal function but exported because it is cross-package; ...
+	func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark)
+
+	// 
+	func RunExamples(examples []InternalExample) (ok bool)
+
+	// 
+	func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool)
+
+	// Short reports whether the -test.short flag is set. 
+	func Short() bool
+
+
+TYPES
+	// B is a type passed to Benchmark functions to manage benchmark ...
+	type B struct {
+		N int
+		// contains filtered or unexported fields
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *B) Error(args ...interface{})
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *B) Errorf(format string, args ...interface{})
+
+	// Fail marks the function as having failed but continues ...
+	func (c *B) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *B) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *B) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *B) Fatal(args ...interface{})
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *B) Fatalf(format string, args ...interface{})
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *B) Log(args ...interface{})
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *B) Logf(format string, args ...interface{})
+
+	// ResetTimer sets the elapsed benchmark time to zero. It does not ...
+	func (b *B) ResetTimer()
+
+	// SetBytes records the number of bytes processed in a single ...
+	func (b *B) SetBytes(n int64)
+
+	// StartTimer starts timing a test. This function is called ...
+	func (b *B) StartTimer()
+
+	// StopTimer stops timing a test. This can be used to pause the ...
+	func (b *B) StopTimer()
+
+	// The results of a benchmark run. 
+	type BenchmarkResult struct {
+		N	int		// The number of iterations.
+		T	time.Duration	// The total time taken.
+		Bytes	int64		// Bytes processed in one iteration.
+	}
+
+	// Benchmark benchmarks a single function. Useful for creating ...
+	func Benchmark(f func(b *B)) BenchmarkResult
+
+	// 
+	func (r BenchmarkResult) NsPerOp() int64
+
+	// 
+	func (r BenchmarkResult) String() string
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalBenchmark struct {
+		Name	string
+		F	func(b *B)
+	}
+
+	// 
+	type InternalExample struct {
+		Name	string
+		F	func()
+		Output	string
+	}
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalTest struct {
+		Name	string
+		F	func(*T)
+	}
+
+	// T is a type passed to Test functions to manage test state and ...
+	type T struct {
+		// contains filtered or unexported fields
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *T) Error(args ...interface{})
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *T) Errorf(format string, args ...interface{})
+
+	// Fail marks the function as having failed but continues ...
+	func (c *T) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *T) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *T) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *T) Fatal(args ...interface{})
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *T) Fatalf(format string, args ...interface{})
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *T) Log(args ...interface{})
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *T) Logf(format string, args ...interface{})
+
+	// Parallel signals that this test is to be run in parallel with ...
+	func (t *T) Parallel()
+
diff --git a/src/go/doc/testdata/testing.1.golden b/src/go/doc/testdata/testing.1.golden
new file mode 100644
index 0000000..b9d1451
--- /dev/null
+++ b/src/go/doc/testdata/testing.1.golden
@@ -0,0 +1,298 @@
+// Package testing provides support for automated testing of Go ...
+PACKAGE testing
+
+IMPORTPATH
+	testdata/testing
+
+IMPORTS
+	bytes
+	flag
+	fmt
+	io
+	os
+	runtime
+	runtime/pprof
+	strconv
+	strings
+	time
+
+FILENAMES
+	testdata/benchmark.go
+	testdata/example.go
+	testdata/testing.go
+
+VARIABLES
+	// 
+	var (
+		// The short flag requests that tests run more quickly, but its functionality
+		// is provided by test writers themselves. The testing package is just its
+		// home. The all.bash installation script sets it to make installation more
+		// efficient, but by default the flag is off so a plain "go test" will do a
+		// full test of the package.
+		short	= flag.Bool("test.short", false, "run smaller test suite to save time")
+	
+		// Report as tests are run; default is silent for success.
+		chatty		= flag.Bool("test.v", false, "verbose: print additional output")
+		match		= flag.String("test.run", "", "regular expression to select tests to run")
+		memProfile	= flag.String("test.memprofile", "", "write a memory profile to the named file after execution")
+		memProfileRate	= flag.Int("test.memprofilerate", 0, "if >=0, sets runtime.MemProfileRate")
+		cpuProfile	= flag.String("test.cpuprofile", "", "write a cpu profile to the named file during execution")
+		timeout		= flag.Duration("test.timeout", 0, "if positive, sets an aggregate time limit for all tests")
+		cpuListStr	= flag.String("test.cpu", "", "comma-separated list of number of CPUs to use for each test")
+		parallel	= flag.Int("test.parallel", runtime.GOMAXPROCS(0), "maximum test parallelism")
+	
+		cpuList	[]int
+	)
+
+	// 
+	var benchTime = flag.Duration("test.benchtime", 1*time.Second, "approximate run time for each benchmark")
+
+	// 
+	var matchBenchmarks = flag.String("test.bench", "", "regular expression to select benchmarks to run")
+
+	// 
+	var timer *time.Timer
+
+
+FUNCTIONS
+	// An internal function but exported because it is cross-package; ...
+	func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample)
+
+	// An internal function but exported because it is cross-package; ...
+	func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark)
+
+	// 
+	func RunExamples(examples []InternalExample) (ok bool)
+
+	// 
+	func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool)
+
+	// Short reports whether the -test.short flag is set. 
+	func Short() bool
+
+	// after runs after all testing. 
+	func after()
+
+	// alarm is called if the timeout expires. 
+	func alarm()
+
+	// before runs before all testing. 
+	func before()
+
+	// decorate inserts the final newline if needed and indentation ...
+	func decorate(s string, addFileLine bool) string
+
+	// 
+	func max(x, y int) int
+
+	// 
+	func min(x, y int) int
+
+	// 
+	func parseCpuList()
+
+	// roundDown10 rounds a number down to the nearest power of 10. 
+	func roundDown10(n int) int
+
+	// roundUp rounds x up to a number of the form [1eX, 2eX, 5eX]. 
+	func roundUp(n int) int
+
+	// startAlarm starts an alarm if requested. 
+	func startAlarm()
+
+	// stopAlarm turns off the alarm. 
+	func stopAlarm()
+
+	// 
+	func tRunner(t *T, test *InternalTest)
+
+
+TYPES
+	// B is a type passed to Benchmark functions to manage benchmark ...
+	type B struct {
+		common
+		N		int
+		benchmark	InternalBenchmark
+		bytes		int64
+		timerOn		bool
+		result		BenchmarkResult
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *B) Error(args ...interface{})
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *B) Errorf(format string, args ...interface{})
+
+	// Fail marks the function as having failed but continues ...
+	func (c *B) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *B) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *B) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *B) Fatal(args ...interface{})
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *B) Fatalf(format string, args ...interface{})
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *B) Log(args ...interface{})
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *B) Logf(format string, args ...interface{})
+
+	// ResetTimer sets the elapsed benchmark time to zero. It does not ...
+	func (b *B) ResetTimer()
+
+	// SetBytes records the number of bytes processed in a single ...
+	func (b *B) SetBytes(n int64)
+
+	// StartTimer starts timing a test. This function is called ...
+	func (b *B) StartTimer()
+
+	// StopTimer stops timing a test. This can be used to pause the ...
+	func (b *B) StopTimer()
+
+	// launch launches the benchmark function. It gradually increases ...
+	func (b *B) launch()
+
+	// log generates the output. It's always at the same stack depth. 
+	func (c *B) log(s string)
+
+	// 
+	func (b *B) nsPerOp() int64
+
+	// run times the benchmark function in a separate goroutine. 
+	func (b *B) run() BenchmarkResult
+
+	// runN runs a single benchmark for the specified number of ...
+	func (b *B) runN(n int)
+
+	// trimOutput shortens the output from a benchmark, which can be ...
+	func (b *B) trimOutput()
+
+	// The results of a benchmark run. 
+	type BenchmarkResult struct {
+		N	int		// The number of iterations.
+		T	time.Duration	// The total time taken.
+		Bytes	int64		// Bytes processed in one iteration.
+	}
+
+	// Benchmark benchmarks a single function. Useful for creating ...
+	func Benchmark(f func(b *B)) BenchmarkResult
+
+	// 
+	func (r BenchmarkResult) NsPerOp() int64
+
+	// 
+	func (r BenchmarkResult) String() string
+
+	// 
+	func (r BenchmarkResult) mbPerSec() float64
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalBenchmark struct {
+		Name	string
+		F	func(b *B)
+	}
+
+	// 
+	type InternalExample struct {
+		Name	string
+		F	func()
+		Output	string
+	}
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalTest struct {
+		Name	string
+		F	func(*T)
+	}
+
+	// T is a type passed to Test functions to manage test state and ...
+	type T struct {
+		common
+		name		string		// Name of test.
+		startParallel	chan bool	// Parallel tests will wait on this.
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *T) Error(args ...interface{})
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *T) Errorf(format string, args ...interface{})
+
+	// Fail marks the function as having failed but continues ...
+	func (c *T) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *T) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *T) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *T) Fatal(args ...interface{})
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *T) Fatalf(format string, args ...interface{})
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *T) Log(args ...interface{})
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *T) Logf(format string, args ...interface{})
+
+	// Parallel signals that this test is to be run in parallel with ...
+	func (t *T) Parallel()
+
+	// log generates the output. It's always at the same stack depth. 
+	func (c *T) log(s string)
+
+	// 
+	func (t *T) report()
+
+	// common holds the elements common between T and B and captures ...
+	type common struct {
+		output		[]byte		// Output generated by test or benchmark.
+		failed		bool		// Test or benchmark has failed.
+		start		time.Time	// Time test or benchmark started
+		duration	time.Duration
+		self		interface{}		// To be sent on signal channel when done.
+		signal		chan interface{}	// Output for serial tests.
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *common) Error(args ...interface{})
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *common) Errorf(format string, args ...interface{})
+
+	// Fail marks the function as having failed but continues ...
+	func (c *common) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *common) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *common) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *common) Fatal(args ...interface{})
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *common) Fatalf(format string, args ...interface{})
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *common) Log(args ...interface{})
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *common) Logf(format string, args ...interface{})
+
+	// log generates the output. It's always at the same stack depth. 
+	func (c *common) log(s string)
+
diff --git a/src/go/doc/testdata/testing.2.golden b/src/go/doc/testdata/testing.2.golden
new file mode 100644
index 0000000..83cf37c
--- /dev/null
+++ b/src/go/doc/testdata/testing.2.golden
@@ -0,0 +1,156 @@
+// Package testing provides support for automated testing of Go ...
+PACKAGE testing
+
+IMPORTPATH
+	testdata/testing
+
+IMPORTS
+	bytes
+	flag
+	fmt
+	io
+	os
+	runtime
+	runtime/pprof
+	strconv
+	strings
+	time
+
+FILENAMES
+	testdata/benchmark.go
+	testdata/example.go
+	testdata/testing.go
+
+FUNCTIONS
+	// An internal function but exported because it is cross-package; ...
+	func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample)
+
+	// An internal function but exported because it is cross-package; ...
+	func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark)
+
+	// 
+	func RunExamples(examples []InternalExample) (ok bool)
+
+	// 
+	func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool)
+
+	// Short reports whether the -test.short flag is set. 
+	func Short() bool
+
+
+TYPES
+	// B is a type passed to Benchmark functions to manage benchmark ...
+	type B struct {
+		N int
+		// contains filtered or unexported fields
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *B) Error(args ...interface{})
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *B) Errorf(format string, args ...interface{})
+
+	// Fail marks the function as having failed but continues ...
+	func (c *B) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *B) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *B) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *B) Fatal(args ...interface{})
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *B) Fatalf(format string, args ...interface{})
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *B) Log(args ...interface{})
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *B) Logf(format string, args ...interface{})
+
+	// ResetTimer sets the elapsed benchmark time to zero. It does not ...
+	func (b *B) ResetTimer()
+
+	// SetBytes records the number of bytes processed in a single ...
+	func (b *B) SetBytes(n int64)
+
+	// StartTimer starts timing a test. This function is called ...
+	func (b *B) StartTimer()
+
+	// StopTimer stops timing a test. This can be used to pause the ...
+	func (b *B) StopTimer()
+
+	// The results of a benchmark run. 
+	type BenchmarkResult struct {
+		N	int		// The number of iterations.
+		T	time.Duration	// The total time taken.
+		Bytes	int64		// Bytes processed in one iteration.
+	}
+
+	// Benchmark benchmarks a single function. Useful for creating ...
+	func Benchmark(f func(b *B)) BenchmarkResult
+
+	// 
+	func (r BenchmarkResult) NsPerOp() int64
+
+	// 
+	func (r BenchmarkResult) String() string
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalBenchmark struct {
+		Name	string
+		F	func(b *B)
+	}
+
+	// 
+	type InternalExample struct {
+		Name	string
+		F	func()
+		Output	string
+	}
+
+	// An internal type but exported because it is cross-package; part ...
+	type InternalTest struct {
+		Name	string
+		F	func(*T)
+	}
+
+	// T is a type passed to Test functions to manage test state and ...
+	type T struct {
+		// contains filtered or unexported fields
+	}
+
+	// Error is equivalent to Log() followed by Fail(). 
+	func (c *T) Error(args ...interface{})
+
+	// Errorf is equivalent to Logf() followed by Fail(). 
+	func (c *T) Errorf(format string, args ...interface{})
+
+	// Fail marks the function as having failed but continues ...
+	func (c *T) Fail()
+
+	// FailNow marks the function as having failed and stops its ...
+	func (c *T) FailNow()
+
+	// Failed reports whether the function has failed. 
+	func (c *T) Failed() bool
+
+	// Fatal is equivalent to Log() followed by FailNow(). 
+	func (c *T) Fatal(args ...interface{})
+
+	// Fatalf is equivalent to Logf() followed by FailNow(). 
+	func (c *T) Fatalf(format string, args ...interface{})
+
+	// Log formats its arguments using default formatting, analogous ...
+	func (c *T) Log(args ...interface{})
+
+	// Logf formats its arguments according to the format, analogous ...
+	func (c *T) Logf(format string, args ...interface{})
+
+	// Parallel signals that this test is to be run in parallel with ...
+	func (t *T) Parallel()
+
diff --git a/src/go/doc/testdata/testing.go b/src/go/doc/testdata/testing.go
new file mode 100644
index 0000000..52810f7
--- /dev/null
+++ b/src/go/doc/testdata/testing.go
@@ -0,0 +1,404 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package testing provides support for automated testing of Go packages.
+// It is intended to be used in concert with the ``go test'' utility, which automates
+// execution of any function of the form
+//     func TestXxx(*testing.T)
+// where Xxx can be any alphanumeric string (but the first letter must not be in
+// [a-z]) and serves to identify the test routine.
+// These TestXxx routines should be declared within the package they are testing.
+//
+// Functions of the form
+//     func BenchmarkXxx(*testing.B)
+// are considered benchmarks, and are executed by go test when the -test.bench
+// flag is provided.
+//
+// A sample benchmark function looks like this:
+//     func BenchmarkHello(b *testing.B) {
+//         for i := 0; i < b.N; i++ {
+//             fmt.Sprintf("hello")
+//         }
+//     }
+// The benchmark package will vary b.N until the benchmark function lasts
+// long enough to be timed reliably. The output
+//     testing.BenchmarkHello    10000000    282 ns/op
+// means that the loop ran 10000000 times at a speed of 282 ns per loop.
+//
+// If a benchmark needs some expensive setup before running, the timer
+// may be stopped:
+//     func BenchmarkBigLen(b *testing.B) {
+//         b.StopTimer()
+//         big := NewBig()
+//         b.StartTimer()
+//         for i := 0; i < b.N; i++ {
+//             big.Len()
+//         }
+//     }
+package testing
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"runtime"
+	"runtime/pprof"
+	"strconv"
+	"strings"
+	"time"
+)
+
+var (
+	// The short flag requests that tests run more quickly, but its functionality
+	// is provided by test writers themselves. The testing package is just its
+	// home. The all.bash installation script sets it to make installation more
+	// efficient, but by default the flag is off so a plain "go test" will do a
+	// full test of the package.
+	short = flag.Bool("test.short", false, "run smaller test suite to save time")
+
+	// Report as tests are run; default is silent for success.
+	chatty         = flag.Bool("test.v", false, "verbose: print additional output")
+	match          = flag.String("test.run", "", "regular expression to select tests to run")
+	memProfile     = flag.String("test.memprofile", "", "write a memory profile to the named file after execution")
+	memProfileRate = flag.Int("test.memprofilerate", 0, "if >=0, sets runtime.MemProfileRate")
+	cpuProfile     = flag.String("test.cpuprofile", "", "write a cpu profile to the named file during execution")
+	timeout        = flag.Duration("test.timeout", 0, "if positive, sets an aggregate time limit for all tests")
+	cpuListStr     = flag.String("test.cpu", "", "comma-separated list of number of CPUs to use for each test")
+	parallel       = flag.Int("test.parallel", runtime.GOMAXPROCS(0), "maximum test parallelism")
+
+	cpuList []int
+)
+
+// common holds the elements common between T and B and
+// captures common methods such as Errorf.
+type common struct {
+	output   []byte    // Output generated by test or benchmark.
+	failed   bool      // Test or benchmark has failed.
+	start    time.Time // Time test or benchmark started
+	duration time.Duration
+	self     interface{}      // To be sent on signal channel when done.
+	signal   chan interface{} // Output for serial tests.
+}
+
+// Short reports whether the -test.short flag is set.
+func Short() bool {
+	return *short
+}
+
+// decorate inserts the final newline if needed and indentation tabs for formatting.
+// If addFileLine is true, it also prefixes the string with the file and line of the call site.
+func decorate(s string, addFileLine bool) string {
+	if addFileLine {
+		_, file, line, ok := runtime.Caller(3) // decorate + log + public function.
+		if ok {
+			// Truncate file name at last file name separator.
+			if index := strings.LastIndex(file, "/"); index >= 0 {
+				file = file[index+1:]
+			} else if index = strings.LastIndex(file, "\\"); index >= 0 {
+				file = file[index+1:]
+			}
+		} else {
+			file = "???"
+			line = 1
+		}
+		s = fmt.Sprintf("%s:%d: %s", file, line, s)
+	}
+	s = "\t" + s // Every line is indented at least one tab.
+	n := len(s)
+	if n > 0 && s[n-1] != '\n' {
+		s += "\n"
+		n++
+	}
+	for i := 0; i < n-1; i++ { // -1 to avoid final newline
+		if s[i] == '\n' {
+			// Second and subsequent lines are indented an extra tab.
+			return s[0:i+1] + "\t" + decorate(s[i+1:n], false)
+		}
+	}
+	return s
+}
+
+// T is a type passed to Test functions to manage test state and support formatted test logs.
+// Logs are accumulated during execution and dumped to standard error when done.
+type T struct {
+	common
+	name          string    // Name of test.
+	startParallel chan bool // Parallel tests will wait on this.
+}
+
+// Fail marks the function as having failed but continues execution.
+func (c *common) Fail() { c.failed = true }
+
+// Failed reports whether the function has failed.
+func (c *common) Failed() bool { return c.failed }
+
+// FailNow marks the function as having failed and stops its execution.
+// Execution will continue at the next Test.
+func (c *common) FailNow() {
+	c.Fail()
+
+	// Calling runtime.Goexit will exit the goroutine, which
+	// will run the deferred functions in this goroutine,
+	// which will eventually run the deferred lines in tRunner,
+	// which will signal to the test loop that this test is done.
+	//
+	// A previous version of this code said:
+	//
+	//	c.duration = ...
+	//	c.signal <- c.self
+	//	runtime.Goexit()
+	//
+	// This previous version duplicated code (those lines are in
+	// tRunner no matter what), but worse the goroutine teardown
+	// implicit in runtime.Goexit was not guaranteed to complete
+	// before the test exited. If a test deferred an important cleanup
+	// function (like removing temporary files), there was no guarantee
+	// it would run on a test failure. Because we send on c.signal during
+	// a top-of-stack deferred function now, we know that the send
+	// only happens after any other stacked defers have completed.
+	runtime.Goexit()
+}
+
+// log generates the output. It's always at the same stack depth.
+func (c *common) log(s string) {
+	c.output = append(c.output, decorate(s, true)...)
+}
+
+// Log formats its arguments using default formatting, analogous to Println(),
+// and records the text in the error log.
+func (c *common) Log(args ...interface{}) { c.log(fmt.Sprintln(args...)) }
+
+// Logf formats its arguments according to the format, analogous to Printf(),
+// and records the text in the error log.
+func (c *common) Logf(format string, args ...interface{}) { c.log(fmt.Sprintf(format, args...)) }
+
+// Error is equivalent to Log() followed by Fail().
+func (c *common) Error(args ...interface{}) {
+	c.log(fmt.Sprintln(args...))
+	c.Fail()
+}
+
+// Errorf is equivalent to Logf() followed by Fail().
+func (c *common) Errorf(format string, args ...interface{}) {
+	c.log(fmt.Sprintf(format, args...))
+	c.Fail()
+}
+
+// Fatal is equivalent to Log() followed by FailNow().
+func (c *common) Fatal(args ...interface{}) {
+	c.log(fmt.Sprintln(args...))
+	c.FailNow()
+}
+
+// Fatalf is equivalent to Logf() followed by FailNow().
+func (c *common) Fatalf(format string, args ...interface{}) {
+	c.log(fmt.Sprintf(format, args...))
+	c.FailNow()
+}
+
+// Parallel signals that this test is to be run in parallel with (and only with)
+// other parallel tests in this CPU group.
+func (t *T) Parallel() {
+	t.signal <- (*T)(nil) // Release main testing loop
+	<-t.startParallel     // Wait for serial tests to finish
+}
+
+// An internal type but exported because it is cross-package; part of the implementation
+// of go test.
+type InternalTest struct {
+	Name string
+	F    func(*T)
+}
+
+func tRunner(t *T, test *InternalTest) {
+	t.start = time.Now()
+
+	// When this goroutine is done, either because test.F(t)
+	// returned normally or because a test failure triggered
+	// a call to runtime.Goexit, record the duration and send
+	// a signal saying that the test is done.
+	defer func() {
+		t.duration = time.Now().Sub(t.start)
+		t.signal <- t
+	}()
+
+	test.F(t)
+}
+
+// An internal function but exported because it is cross-package; part of the implementation
+// of go test.
+func Main(matchString func(pat, str string) (bool, error), tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample) {
+	flag.Parse()
+	parseCpuList()
+
+	before()
+	startAlarm()
+	testOk := RunTests(matchString, tests)
+	exampleOk := RunExamples(examples)
+	if !testOk || !exampleOk {
+		fmt.Println("FAIL")
+		os.Exit(1)
+	}
+	fmt.Println("PASS")
+	stopAlarm()
+	RunBenchmarks(matchString, benchmarks)
+	after()
+}
+
+func (t *T) report() {
+	tstr := fmt.Sprintf("(%.2f seconds)", t.duration.Seconds())
+	format := "--- %s: %s %s\n%s"
+	if t.failed {
+		fmt.Printf(format, "FAIL", t.name, tstr, t.output)
+	} else if *chatty {
+		fmt.Printf(format, "PASS", t.name, tstr, t.output)
+	}
+}
+
+func RunTests(matchString func(pat, str string) (bool, error), tests []InternalTest) (ok bool) {
+	ok = true
+	if len(tests) == 0 {
+		fmt.Fprintln(os.Stderr, "testing: warning: no tests to run")
+		return
+	}
+	for _, procs := range cpuList {
+		runtime.GOMAXPROCS(procs)
+		// We build a new channel tree for each run of the loop.
+		// collector merges in one channel all the upstream signals from parallel tests.
+		// If all tests pump to the same channel, a bug can occur where a test
+		// kicks off a goroutine that Fails, yet the test still delivers a completion signal,
+		// which skews the counting.
+		var collector = make(chan interface{})
+
+		numParallel := 0
+		startParallel := make(chan bool)
+
+		for i := 0; i < len(tests); i++ {
+			matched, err := matchString(*match, tests[i].Name)
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "testing: invalid regexp for -test.run: %s\n", err)
+				os.Exit(1)
+			}
+			if !matched {
+				continue
+			}
+			testName := tests[i].Name
+			if procs != 1 {
+				testName = fmt.Sprintf("%s-%d", tests[i].Name, procs)
+			}
+			t := &T{
+				common: common{
+					signal: make(chan interface{}),
+				},
+				name:          testName,
+				startParallel: startParallel,
+			}
+			t.self = t
+			if *chatty {
+				fmt.Printf("=== RUN %s\n", t.name)
+			}
+			go tRunner(t, &tests[i])
+			out := (<-t.signal).(*T)
+			if out == nil { // Parallel run.
+				go func() {
+					collector <- <-t.signal
+				}()
+				numParallel++
+				continue
+			}
+			t.report()
+			ok = ok && !out.failed
+		}
+
+		running := 0
+		for numParallel+running > 0 {
+			if running < *parallel && numParallel > 0 {
+				startParallel <- true
+				running++
+				numParallel--
+				continue
+			}
+			t := (<-collector).(*T)
+			t.report()
+			ok = ok && !t.failed
+			running--
+		}
+	}
+	return
+}
+
+// before runs before all testing.
+func before() {
+	if *memProfileRate > 0 {
+		runtime.MemProfileRate = *memProfileRate
+	}
+	if *cpuProfile != "" {
+		f, err := os.Create(*cpuProfile)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "testing: %s", err)
+			return
+		}
+		if err := pprof.StartCPUProfile(f); err != nil {
+			fmt.Fprintf(os.Stderr, "testing: can't start cpu profile: %s", err)
+			f.Close()
+			return
+		}
+		// Could save f so after can call f.Close; not worth the effort.
+	}
+
+}
+
+// after runs after all testing.
+func after() {
+	if *cpuProfile != "" {
+		pprof.StopCPUProfile() // flushes profile to disk
+	}
+	if *memProfile != "" {
+		f, err := os.Create(*memProfile)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "testing: %s", err)
+			return
+		}
+		if err = pprof.WriteHeapProfile(f); err != nil {
+			fmt.Fprintf(os.Stderr, "testing: can't write %s: %s", *memProfile, err)
+		}
+		f.Close()
+	}
+}
+
+var timer *time.Timer
+
+// startAlarm starts an alarm if requested.
+func startAlarm() {
+	if *timeout > 0 {
+		timer = time.AfterFunc(*timeout, alarm)
+	}
+}
+
+// stopAlarm turns off the alarm.
+func stopAlarm() {
+	if *timeout > 0 {
+		timer.Stop()
+	}
+}
+
+// alarm is called if the timeout expires.
+func alarm() {
+	panic("test timed out")
+}
+
+func parseCpuList() {
+	if len(*cpuListStr) == 0 {
+		cpuList = append(cpuList, runtime.GOMAXPROCS(-1))
+	} else {
+		for _, val := range strings.Split(*cpuListStr, ",") {
+			cpu, err := strconv.Atoi(val)
+			if err != nil || cpu <= 0 {
+				fmt.Fprintf(os.Stderr, "testing: invalid value %q for -test.cpu", val)
+				os.Exit(1)
+			}
+			cpuList = append(cpuList, cpu)
+		}
+	}
+}
diff --git a/src/go/format/benchmark_test.go b/src/go/format/benchmark_test.go
new file mode 100644
index 0000000..ac19aa3
--- /dev/null
+++ b/src/go/format/benchmark_test.go
@@ -0,0 +1,91 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file provides a simple framework to add benchmarks
+// based on generated input (source) files.
+
+package format_test
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"go/format"
+	"os"
+	"testing"
+)
+
+var debug = flag.Bool("debug", false, "write .src files containing formatting input; for debugging")
+
+// array1 generates an array literal with n elements of the form:
+//
+// var _ = [...]byte{
+// 	// 0
+// 	0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
+// 	0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
+// 	0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
+// 	0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
+// 	0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27,
+// 	// 40
+// 	0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f,
+// 	0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37,
+// 	...
+//
+func array1(buf *bytes.Buffer, n int) {
+	buf.WriteString("var _ = [...]byte{\n")
+	for i := 0; i < n; {
+		if i%10 == 0 {
+			fmt.Fprintf(buf, "\t// %d\n", i)
+		}
+		buf.WriteByte('\t')
+		for j := 0; j < 8; j++ {
+			fmt.Fprintf(buf, "0x%02x, ", byte(i))
+			i++
+		}
+		buf.WriteString("\n")
+	}
+	buf.WriteString("}\n")
+}
+
+var tests = []struct {
+	name string
+	gen  func(*bytes.Buffer, int)
+	n    int
+}{
+	{"array1", array1, 10000},
+	// add new test cases here as needed
+}
+
+func BenchmarkFormat(b *testing.B) {
+	var src bytes.Buffer
+	for _, t := range tests {
+		src.Reset()
+		src.WriteString("package p\n")
+		t.gen(&src, t.n)
+		data := src.Bytes()
+
+		if *debug {
+			filename := t.name + ".src"
+			err := os.WriteFile(filename, data, 0660)
+			if err != nil {
+				b.Fatalf("couldn't write %s: %v", filename, err)
+			}
+		}
+
+		b.Run(fmt.Sprintf("%s-%d", t.name, t.n), func(b *testing.B) {
+			b.SetBytes(int64(len(data)))
+			b.ReportAllocs()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				var err error
+				sink, err = format.Source(data)
+				if err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}
+
+var sink []byte
diff --git a/src/go/format/example_test.go b/src/go/format/example_test.go
new file mode 100644
index 0000000..5b6789a
--- /dev/null
+++ b/src/go/format/example_test.go
@@ -0,0 +1,39 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package format_test
+
+import (
+	"bytes"
+	"fmt"
+	"go/format"
+	"go/parser"
+	"go/token"
+	"log"
+)
+
+func ExampleNode() {
+	const expr = "(6+2*3)/4"
+
+	// parser.ParseExpr parses the argument and returns the
+	// corresponding ast.Node.
+	node, err := parser.ParseExpr(expr)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Create a FileSet for node. Since the node does not come
+	// from a real source file, fset will be empty.
+	fset := token.NewFileSet()
+
+	var buf bytes.Buffer
+	err = format.Node(&buf, fset, node)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Println(buf.String())
+
+	// Output: (6 + 2*3) / 4
+}
diff --git a/src/go/format/format.go b/src/go/format/format.go
new file mode 100644
index 0000000..a603d96
--- /dev/null
+++ b/src/go/format/format.go
@@ -0,0 +1,135 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package format implements standard formatting of Go source.
+//
+// Note that formatting of Go source code changes over time, so tools relying on
+// consistent formatting should execute a specific version of the gofmt binary
+// instead of using this package. That way, the formatting will be stable, and
+// the tools won't need to be recompiled each time gofmt changes.
+//
+// For example, pre-submit checks that use this package directly would behave
+// differently depending on what Go version each developer uses, causing the
+// check to be inherently fragile.
+package format
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/printer"
+	"go/token"
+	"io"
+)
+
+// Keep these in sync with cmd/gofmt/gofmt.go.
+const (
+	tabWidth    = 8
+	printerMode = printer.UseSpaces | printer.TabIndent | printerNormalizeNumbers
+
+	// printerNormalizeNumbers means to canonicalize number literal prefixes
+	// and exponents while printing. See https://golang.org/doc/go1.13#gofmt.
+	//
+	// This value is defined in go/printer specifically for go/format and cmd/gofmt.
+	printerNormalizeNumbers = 1 << 30
+)
+
+var config = printer.Config{Mode: printerMode, Tabwidth: tabWidth}
+
+const parserMode = parser.ParseComments
+
+// Node formats node in canonical gofmt style and writes the result to dst.
+//
+// The node type must be *ast.File, *printer.CommentedNode, []ast.Decl,
+// []ast.Stmt, or assignment-compatible to ast.Expr, ast.Decl, ast.Spec,
+// or ast.Stmt. Node does not modify node. Imports are not sorted for
+// nodes representing partial source files (for instance, if the node is
+// not an *ast.File or a *printer.CommentedNode not wrapping an *ast.File).
+//
+// The function may return early (before the entire result is written)
+// and return a formatting error, for instance due to an incorrect AST.
+//
+func Node(dst io.Writer, fset *token.FileSet, node interface{}) error {
+	// Determine if we have a complete source file (file != nil).
+	var file *ast.File
+	var cnode *printer.CommentedNode
+	switch n := node.(type) {
+	case *ast.File:
+		file = n
+	case *printer.CommentedNode:
+		if f, ok := n.Node.(*ast.File); ok {
+			file = f
+			cnode = n
+		}
+	}
+
+	// Sort imports if necessary.
+	if file != nil && hasUnsortedImports(file) {
+		// Make a copy of the AST because ast.SortImports is destructive.
+		// TODO(gri) Do this more efficiently.
+		var buf bytes.Buffer
+		err := config.Fprint(&buf, fset, file)
+		if err != nil {
+			return err
+		}
+		file, err = parser.ParseFile(fset, "", buf.Bytes(), parserMode)
+		if err != nil {
+			// We should never get here. If we do, provide good diagnostic.
+			return fmt.Errorf("format.Node internal error (%s)", err)
+		}
+		ast.SortImports(fset, file)
+
+		// Use new file with sorted imports.
+		node = file
+		if cnode != nil {
+			node = &printer.CommentedNode{Node: file, Comments: cnode.Comments}
+		}
+	}
+
+	return config.Fprint(dst, fset, node)
+}
+
+// Source formats src in canonical gofmt style and returns the result
+// or an (I/O or syntax) error. src is expected to be a syntactically
+// correct Go source file, or a list of Go declarations or statements.
+//
+// If src is a partial source file, the leading and trailing space of src
+// is applied to the result (such that it has the same leading and trailing
+// space as src), and the result is indented by the same amount as the first
+// line of src containing code. Imports are not sorted for partial source files.
+//
+func Source(src []byte) ([]byte, error) {
+	fset := token.NewFileSet()
+	file, sourceAdj, indentAdj, err := parse(fset, "", src, true)
+	if err != nil {
+		return nil, err
+	}
+
+	if sourceAdj == nil {
+		// Complete source file.
+		// TODO(gri) consider doing this always.
+		ast.SortImports(fset, file)
+	}
+
+	return format(fset, file, sourceAdj, indentAdj, src, config)
+}
+
+func hasUnsortedImports(file *ast.File) bool {
+	for _, d := range file.Decls {
+		d, ok := d.(*ast.GenDecl)
+		if !ok || d.Tok != token.IMPORT {
+			// Not an import declaration, so we're done.
+			// Imports are always first.
+			return false
+		}
+		if d.Lparen.IsValid() {
+			// For now assume all grouped imports are unsorted.
+			// TODO(gri) Should check if they are sorted already.
+			return true
+		}
+		// Ungrouped imports are sorted by default.
+	}
+	return false
+}
diff --git a/src/go/format/format_test.go b/src/go/format/format_test.go
new file mode 100644
index 0000000..27f4c74
--- /dev/null
+++ b/src/go/format/format_test.go
@@ -0,0 +1,183 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package format
+
+import (
+	"bytes"
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"os"
+	"strings"
+	"testing"
+)
+
+const testfile = "format_test.go"
+
+func diff(t *testing.T, dst, src []byte) {
+	line := 1
+	offs := 0 // line offset
+	for i := 0; i < len(dst) && i < len(src); i++ {
+		d := dst[i]
+		s := src[i]
+		if d != s {
+			t.Errorf("dst:%d: %s\n", line, dst[offs:i+1])
+			t.Errorf("src:%d: %s\n", line, src[offs:i+1])
+			return
+		}
+		if s == '\n' {
+			line++
+			offs = i + 1
+		}
+	}
+	if len(dst) != len(src) {
+		t.Errorf("len(dst) = %d, len(src) = %d\nsrc = %q", len(dst), len(src), src)
+	}
+}
+
+func TestNode(t *testing.T) {
+	src, err := os.ReadFile(testfile)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, testfile, src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf bytes.Buffer
+
+	if err = Node(&buf, fset, file); err != nil {
+		t.Fatal("Node failed:", err)
+	}
+
+	diff(t, buf.Bytes(), src)
+}
+
+// Node is documented to not modify the AST.
+// Test that it is so even when numbers are normalized.
+func TestNodeNoModify(t *testing.T) {
+	const (
+		src    = "package p\n\nconst _ = 0000000123i\n"
+		golden = "package p\n\nconst _ = 123i\n"
+	)
+
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Capture original address and value of a BasicLit node
+	// which will undergo formatting changes during printing.
+	wantLit := file.Decls[0].(*ast.GenDecl).Specs[0].(*ast.ValueSpec).Values[0].(*ast.BasicLit)
+	wantVal := wantLit.Value
+
+	var buf bytes.Buffer
+	if err = Node(&buf, fset, file); err != nil {
+		t.Fatal("Node failed:", err)
+	}
+	diff(t, buf.Bytes(), []byte(golden))
+
+	// Check if anything changed after Node returned.
+	gotLit := file.Decls[0].(*ast.GenDecl).Specs[0].(*ast.ValueSpec).Values[0].(*ast.BasicLit)
+	gotVal := gotLit.Value
+
+	if gotLit != wantLit {
+		t.Errorf("got *ast.BasicLit address %p, want %p", gotLit, wantLit)
+	}
+	if gotVal != wantVal {
+		t.Errorf("got *ast.BasicLit value %q, want %q", gotVal, wantVal)
+	}
+}
+
+func TestSource(t *testing.T) {
+	src, err := os.ReadFile(testfile)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	res, err := Source(src)
+	if err != nil {
+		t.Fatal("Source failed:", err)
+	}
+
+	diff(t, res, src)
+}
+
+// Test cases that are expected to fail are marked by the prefix "ERROR".
+// The formatted result must look the same as the input for successful tests.
+var tests = []string{
+	// declaration lists
+	`import "go/format"`,
+	"var x int",
+	"var x int\n\ntype T struct{}",
+
+	// statement lists
+	"x := 0",
+	"f(a, b, c)\nvar x int = f(1, 2, 3)",
+
+	// indentation, leading and trailing space
+	"\tx := 0\n\tgo f()",
+	"\tx := 0\n\tgo f()\n\n\n",
+	"\n\t\t\n\n\tx := 0\n\tgo f()\n\n\n",
+	"\n\t\t\n\n\t\t\tx := 0\n\t\t\tgo f()\n\n\n",
+	"\n\t\t\n\n\t\t\tx := 0\n\t\t\tconst s = `\nfoo\n`\n\n\n",     // no indentation added inside raw strings
+	"\n\t\t\n\n\t\t\tx := 0\n\t\t\tconst s = `\n\t\tfoo\n`\n\n\n", // no indentation removed inside raw strings
+
+	// comments
+	"/* Comment */",
+	"\t/* Comment */ ",
+	"\n/* Comment */ ",
+	"i := 5 /* Comment */",         // issue #5551
+	"\ta()\n//line :1",             // issue #11276
+	"\t//xxx\n\ta()\n//line :2",    // issue #11276
+	"\ta() //line :1\n\tb()\n",     // issue #11276
+	"x := 0\n//line :1\n//line :2", // issue #11276
+
+	// whitespace
+	"",     // issue #11275
+	" ",    // issue #11275
+	"\t",   // issue #11275
+	"\t\t", // issue #11275
+	"\n",   // issue #11275
+	"\n\n", // issue #11275
+	"\t\n", // issue #11275
+
+	// erroneous programs
+	"ERROR1 + 2 +",
+	"ERRORx :=  0",
+}
+
+func String(s string) (string, error) {
+	res, err := Source([]byte(s))
+	if err != nil {
+		return "", err
+	}
+	return string(res), nil
+}
+
+func TestPartial(t *testing.T) {
+	for _, src := range tests {
+		if strings.HasPrefix(src, "ERROR") {
+			// test expected to fail
+			src = src[5:] // remove ERROR prefix
+			res, err := String(src)
+			if err == nil && res == src {
+				t.Errorf("formatting succeeded but was expected to fail:\n%q", src)
+			}
+		} else {
+			// test expected to succeed
+			res, err := String(src)
+			if err != nil {
+				t.Errorf("formatting failed (%s):\n%q", err, src)
+			} else if res != src {
+				t.Errorf("formatting incorrect:\nsource: %q\nresult: %q", src, res)
+			}
+		}
+	}
+}
diff --git a/src/go/format/internal.go b/src/go/format/internal.go
new file mode 100644
index 0000000..2f3b0e4
--- /dev/null
+++ b/src/go/format/internal.go
@@ -0,0 +1,176 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// TODO(gri): This file and the file src/cmd/gofmt/internal.go are
+// the same (but for this comment and the package name). Do not modify
+// one without the other. Determine if we can factor out functionality
+// in a public API. See also #11844 for context.
+
+package format
+
+import (
+	"bytes"
+	"go/ast"
+	"go/parser"
+	"go/printer"
+	"go/token"
+	"strings"
+)
+
+// parse parses src, which was read from the named file,
+// as a Go source file, declaration, or statement list.
+func parse(fset *token.FileSet, filename string, src []byte, fragmentOk bool) (
+	file *ast.File,
+	sourceAdj func(src []byte, indent int) []byte,
+	indentAdj int,
+	err error,
+) {
+	// Try as whole source file.
+	file, err = parser.ParseFile(fset, filename, src, parserMode)
+	// If there's no error, return. If the error is that the source file didn't begin with a
+	// package line and source fragments are ok, fall through to
+	// try as a source fragment. Stop and return on any other error.
+	if err == nil || !fragmentOk || !strings.Contains(err.Error(), "expected 'package'") {
+		return
+	}
+
+	// If this is a declaration list, make it a source file
+	// by inserting a package clause.
+	// Insert using a ';', not a newline, so that the line numbers
+	// in psrc match the ones in src.
+	psrc := append([]byte("package p;"), src...)
+	file, err = parser.ParseFile(fset, filename, psrc, parserMode)
+	if err == nil {
+		sourceAdj = func(src []byte, indent int) []byte {
+			// Remove the package clause.
+			// Gofmt has turned the ';' into a '\n'.
+			src = src[indent+len("package p\n"):]
+			return bytes.TrimSpace(src)
+		}
+		return
+	}
+	// If the error is that the source file didn't begin with a
+	// declaration, fall through to try as a statement list.
+	// Stop and return on any other error.
+	if !strings.Contains(err.Error(), "expected declaration") {
+		return
+	}
+
+	// If this is a statement list, make it a source file
+	// by inserting a package clause and turning the list
+	// into a function body. This handles expressions too.
+	// Insert using a ';', not a newline, so that the line numbers
+	// in fsrc match the ones in src. Add an extra '\n' before the '}'
+	// to make sure comments are flushed before the '}'.
+	fsrc := append(append([]byte("package p; func _() {"), src...), '\n', '\n', '}')
+	file, err = parser.ParseFile(fset, filename, fsrc, parserMode)
+	if err == nil {
+		sourceAdj = func(src []byte, indent int) []byte {
+			// Cap adjusted indent to zero.
+			if indent < 0 {
+				indent = 0
+			}
+			// Remove the wrapping.
+			// Gofmt has turned the "; " into a "\n\n".
+			// There will be two non-blank lines with indent, hence 2*indent.
+			src = src[2*indent+len("package p\n\nfunc _() {"):]
+			// Remove only the "}\n" suffix: remaining whitespaces will be trimmed anyway
+			src = src[:len(src)-len("}\n")]
+			return bytes.TrimSpace(src)
+		}
+		// Gofmt has also indented the function body one level.
+		// Adjust that with indentAdj.
+		indentAdj = -1
+	}
+
+	// Succeeded, or out of options.
+	return
+}
+
+// format formats the given package file originally obtained from src
+// and adjusts the result based on the original source via sourceAdj
+// and indentAdj.
+func format(
+	fset *token.FileSet,
+	file *ast.File,
+	sourceAdj func(src []byte, indent int) []byte,
+	indentAdj int,
+	src []byte,
+	cfg printer.Config,
+) ([]byte, error) {
+	if sourceAdj == nil {
+		// Complete source file.
+		var buf bytes.Buffer
+		err := cfg.Fprint(&buf, fset, file)
+		if err != nil {
+			return nil, err
+		}
+		return buf.Bytes(), nil
+	}
+
+	// Partial source file.
+	// Determine and prepend leading space.
+	i, j := 0, 0
+	for j < len(src) && isSpace(src[j]) {
+		if src[j] == '\n' {
+			i = j + 1 // byte offset of last line in leading space
+		}
+		j++
+	}
+	var res []byte
+	res = append(res, src[:i]...)
+
+	// Determine and prepend indentation of first code line.
+	// Spaces are ignored unless there are no tabs,
+	// in which case spaces count as one tab.
+	indent := 0
+	hasSpace := false
+	for _, b := range src[i:j] {
+		switch b {
+		case ' ':
+			hasSpace = true
+		case '\t':
+			indent++
+		}
+	}
+	if indent == 0 && hasSpace {
+		indent = 1
+	}
+	for i := 0; i < indent; i++ {
+		res = append(res, '\t')
+	}
+
+	// Format the source.
+	// Write it without any leading and trailing space.
+	cfg.Indent = indent + indentAdj
+	var buf bytes.Buffer
+	err := cfg.Fprint(&buf, fset, file)
+	if err != nil {
+		return nil, err
+	}
+	out := sourceAdj(buf.Bytes(), cfg.Indent)
+
+	// If the adjusted output is empty, the source
+	// was empty but (possibly) for white space.
+	// The result is the incoming source.
+	if len(out) == 0 {
+		return src, nil
+	}
+
+	// Otherwise, append output to leading space.
+	res = append(res, out...)
+
+	// Determine and append trailing space.
+	i = len(src)
+	for i > 0 && isSpace(src[i-1]) {
+		i--
+	}
+	return append(res, src[i:]...), nil
+}
+
+// isSpace reports whether the byte is a space character.
+// isSpace defines a space as being among the following bytes: ' ', '\t', '\n' and '\r'.
+func isSpace(b byte) bool {
+	return b == ' ' || b == '\t' || b == '\n' || b == '\r'
+}
diff --git a/src/go/importer/importer.go b/src/go/importer/importer.go
new file mode 100644
index 0000000..23118d3
--- /dev/null
+++ b/src/go/importer/importer.go
@@ -0,0 +1,122 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package importer provides access to export data importers.
+package importer
+
+import (
+	"go/build"
+	"go/internal/gccgoimporter"
+	"go/internal/gcimporter"
+	"go/internal/srcimporter"
+	"go/token"
+	"go/types"
+	"io"
+	"runtime"
+)
+
+// A Lookup function returns a reader to access package data for
+// a given import path, or an error if no matching package is found.
+type Lookup func(path string) (io.ReadCloser, error)
+
+// ForCompiler returns an Importer for importing from installed packages
+// for the compilers "gc" and "gccgo", or for importing directly
+// from the source if the compiler argument is "source". In this
+// latter case, importing may fail under circumstances where the
+// exported API is not entirely defined in pure Go source code
+// (if the package API depends on cgo-defined entities, the type
+// checker won't have access to those).
+//
+// The lookup function is called each time the resulting importer needs
+// to resolve an import path. In this mode the importer can only be
+// invoked with canonical import paths (not relative or absolute ones);
+// it is assumed that the translation to canonical import paths is being
+// done by the client of the importer.
+//
+// A lookup function must be provided for correct module-aware operation.
+// Deprecated: If lookup is nil, for backwards-compatibility, the importer
+// will attempt to resolve imports in the $GOPATH workspace.
+func ForCompiler(fset *token.FileSet, compiler string, lookup Lookup) types.Importer {
+	switch compiler {
+	case "gc":
+		return &gcimports{
+			fset:     fset,
+			packages: make(map[string]*types.Package),
+			lookup:   lookup,
+		}
+
+	case "gccgo":
+		var inst gccgoimporter.GccgoInstallation
+		if err := inst.InitFromDriver("gccgo"); err != nil {
+			return nil
+		}
+		return &gccgoimports{
+			packages: make(map[string]*types.Package),
+			importer: inst.GetImporter(nil, nil),
+			lookup:   lookup,
+		}
+
+	case "source":
+		if lookup != nil {
+			panic("source importer for custom import path lookup not supported (issue #13847).")
+		}
+
+		return srcimporter.New(&build.Default, fset, make(map[string]*types.Package))
+	}
+
+	// compiler not supported
+	return nil
+}
+
+// For calls ForCompiler with a new FileSet.
+//
+// Deprecated: Use ForCompiler, which populates a FileSet
+// with the positions of objects created by the importer.
+func For(compiler string, lookup Lookup) types.Importer {
+	return ForCompiler(token.NewFileSet(), compiler, lookup)
+}
+
+// Default returns an Importer for the compiler that built the running binary.
+// If available, the result implements types.ImporterFrom.
+func Default() types.Importer {
+	return For(runtime.Compiler, nil)
+}
+
+// gc importer
+
+type gcimports struct {
+	fset     *token.FileSet
+	packages map[string]*types.Package
+	lookup   Lookup
+}
+
+func (m *gcimports) Import(path string) (*types.Package, error) {
+	return m.ImportFrom(path, "" /* no vendoring */, 0)
+}
+
+func (m *gcimports) ImportFrom(path, srcDir string, mode types.ImportMode) (*types.Package, error) {
+	if mode != 0 {
+		panic("mode must be 0")
+	}
+	return gcimporter.Import(m.fset, m.packages, path, srcDir, m.lookup)
+}
+
+// gccgo importer
+
+type gccgoimports struct {
+	packages map[string]*types.Package
+	importer gccgoimporter.Importer
+	lookup   Lookup
+}
+
+func (m *gccgoimports) Import(path string) (*types.Package, error) {
+	return m.ImportFrom(path, "" /* no vendoring */, 0)
+}
+
+func (m *gccgoimports) ImportFrom(path, srcDir string, mode types.ImportMode) (*types.Package, error) {
+	if mode != 0 {
+		panic("mode must be 0")
+	}
+	return m.importer(m.packages, path, srcDir, m.lookup)
+}
diff --git a/src/go/importer/importer_test.go b/src/go/importer/importer_test.go
new file mode 100644
index 0000000..0f5121d
--- /dev/null
+++ b/src/go/importer/importer_test.go
@@ -0,0 +1,87 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package importer
+
+import (
+	"go/token"
+	"internal/testenv"
+	"io"
+	"os"
+	"os/exec"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+func TestForCompiler(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	const thePackage = "math/big"
+	out, err := exec.Command(testenv.GoToolPath(t), "list", "-f={{context.Compiler}}:{{.Target}}", thePackage).CombinedOutput()
+	if err != nil {
+		t.Fatalf("go list %s: %v\n%s", thePackage, err, out)
+	}
+	target := strings.TrimSpace(string(out))
+	i := strings.Index(target, ":")
+	compiler, target := target[:i], target[i+1:]
+	if !strings.HasSuffix(target, ".a") {
+		t.Fatalf("unexpected package %s target %q (not *.a)", thePackage, target)
+	}
+
+	if compiler == "gccgo" {
+		t.Skip("golang.org/issue/22500")
+	}
+
+	fset := token.NewFileSet()
+
+	t.Run("LookupDefault", func(t *testing.T) {
+		imp := ForCompiler(fset, compiler, nil)
+		pkg, err := imp.Import(thePackage)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if pkg.Path() != thePackage {
+			t.Fatalf("Path() = %q, want %q", pkg.Path(), thePackage)
+		}
+
+		// Check that the fileset positions are accurate.
+		// https://github.com/golang/go#28995
+		mathBigInt := pkg.Scope().Lookup("Int")
+		posn := fset.Position(mathBigInt.Pos()) // "$GOROOT/src/math/big/int.go:25:1"
+		filename := strings.Replace(posn.Filename, "$GOROOT", runtime.GOROOT(), 1)
+		data, err := os.ReadFile(filename)
+		if err != nil {
+			t.Fatalf("can't read file containing declaration of math/big.Int: %v", err)
+		}
+		lines := strings.Split(string(data), "\n")
+		if posn.Line > len(lines) || !strings.HasPrefix(lines[posn.Line-1], "type Int") {
+			t.Fatalf("Object %v position %s does not contain its declaration",
+				mathBigInt, posn)
+		}
+	})
+
+	t.Run("LookupCustom", func(t *testing.T) {
+		lookup := func(path string) (io.ReadCloser, error) {
+			if path != "math/bigger" {
+				t.Fatalf("lookup called with unexpected path %q", path)
+			}
+			f, err := os.Open(target)
+			if err != nil {
+				t.Fatal(err)
+			}
+			return f, nil
+		}
+		imp := ForCompiler(fset, compiler, lookup)
+		pkg, err := imp.Import("math/bigger")
+		if err != nil {
+			t.Fatal(err)
+		}
+		// Even though we open math/big.a, the import request was for math/bigger
+		// and that should be recorded in pkg.Path(), at least for the gc toolchain.
+		if pkg.Path() != "math/bigger" {
+			t.Fatalf("Path() = %q, want %q", pkg.Path(), "math/bigger")
+		}
+	})
+}
diff --git a/src/go/internal/gccgoimporter/ar.go b/src/go/internal/gccgoimporter/ar.go
new file mode 100644
index 0000000..443aa26
--- /dev/null
+++ b/src/go/internal/gccgoimporter/ar.go
@@ -0,0 +1,171 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gccgoimporter
+
+import (
+	"bytes"
+	"debug/elf"
+	"errors"
+	"fmt"
+	"internal/xcoff"
+	"io"
+	"strconv"
+	"strings"
+)
+
+// Magic strings for different archive file formats.
+const (
+	armag  = "!<arch>\n"
+	armagt = "!<thin>\n"
+	armagb = "<bigaf>\n"
+)
+
+// Offsets and sizes for fields in a standard archive header.
+const (
+	arNameOff  = 0
+	arNameSize = 16
+	arDateOff  = arNameOff + arNameSize
+	arDateSize = 12
+	arUIDOff   = arDateOff + arDateSize
+	arUIDSize  = 6
+	arGIDOff   = arUIDOff + arUIDSize
+	arGIDSize  = 6
+	arModeOff  = arGIDOff + arGIDSize
+	arModeSize = 8
+	arSizeOff  = arModeOff + arModeSize
+	arSizeSize = 10
+	arFmagOff  = arSizeOff + arSizeSize
+	arFmagSize = 2
+
+	arHdrSize = arFmagOff + arFmagSize
+)
+
+// The contents of the fmag field of a standard archive header.
+const arfmag = "`\n"
+
+// arExportData takes an archive file and returns a ReadSeeker for the
+// export data in that file. This assumes that there is only one
+// object in the archive containing export data, which is not quite
+// what gccgo does; gccgo concatenates together all the export data
+// for all the objects in the file.  In practice that case does not arise.
+func arExportData(archive io.ReadSeeker) (io.ReadSeeker, error) {
+	if _, err := archive.Seek(0, io.SeekStart); err != nil {
+		return nil, err
+	}
+
+	var buf [len(armag)]byte
+	if _, err := archive.Read(buf[:]); err != nil {
+		return nil, err
+	}
+
+	switch string(buf[:]) {
+	case armag:
+		return standardArExportData(archive)
+	case armagt:
+		return nil, errors.New("unsupported thin archive")
+	case armagb:
+		return aixBigArExportData(archive)
+	default:
+		return nil, fmt.Errorf("unrecognized archive file format %q", buf[:])
+	}
+}
+
+// standardArExportData returns export data from a standard archive.
+func standardArExportData(archive io.ReadSeeker) (io.ReadSeeker, error) {
+	off := int64(len(armag))
+	for {
+		var hdrBuf [arHdrSize]byte
+		if _, err := archive.Read(hdrBuf[:]); err != nil {
+			return nil, err
+		}
+		off += arHdrSize
+
+		if bytes.Compare(hdrBuf[arFmagOff:arFmagOff+arFmagSize], []byte(arfmag)) != 0 {
+			return nil, fmt.Errorf("archive header format header (%q)", hdrBuf[:])
+		}
+
+		size, err := strconv.ParseInt(strings.TrimSpace(string(hdrBuf[arSizeOff:arSizeOff+arSizeSize])), 10, 64)
+		if err != nil {
+			return nil, fmt.Errorf("error parsing size in archive header (%q): %v", hdrBuf[:], err)
+		}
+
+		fn := hdrBuf[arNameOff : arNameOff+arNameSize]
+		if fn[0] == '/' && (fn[1] == ' ' || fn[1] == '/' || bytes.Compare(fn[:8], []byte("/SYM64/ ")) == 0) {
+			// Archive symbol table or extended name table,
+			// which we don't care about.
+		} else {
+			archiveAt := readerAtFromSeeker(archive)
+			ret, err := elfFromAr(io.NewSectionReader(archiveAt, off, size))
+			if ret != nil || err != nil {
+				return ret, err
+			}
+		}
+
+		if size&1 != 0 {
+			size++
+		}
+		off += size
+		if _, err := archive.Seek(off, io.SeekStart); err != nil {
+			return nil, err
+		}
+	}
+}
+
+// elfFromAr tries to get export data from an archive member as an ELF file.
+// If there is no export data, this returns nil, nil.
+func elfFromAr(member *io.SectionReader) (io.ReadSeeker, error) {
+	ef, err := elf.NewFile(member)
+	if err != nil {
+		return nil, err
+	}
+	sec := ef.Section(".go_export")
+	if sec == nil {
+		return nil, nil
+	}
+	return sec.Open(), nil
+}
+
+// aixBigArExportData returns export data from an AIX big archive.
+func aixBigArExportData(archive io.ReadSeeker) (io.ReadSeeker, error) {
+	archiveAt := readerAtFromSeeker(archive)
+	arch, err := xcoff.NewArchive(archiveAt)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, mem := range arch.Members {
+		f, err := arch.GetFile(mem.Name)
+		if err != nil {
+			return nil, err
+		}
+		sdat := f.CSect(".go_export")
+		if sdat != nil {
+			return bytes.NewReader(sdat), nil
+		}
+	}
+
+	return nil, fmt.Errorf(".go_export not found in this archive")
+}
+
+// readerAtFromSeeker turns an io.ReadSeeker into an io.ReaderAt.
+// This is only safe because there won't be any concurrent seeks
+// while this code is executing.
+func readerAtFromSeeker(rs io.ReadSeeker) io.ReaderAt {
+	if ret, ok := rs.(io.ReaderAt); ok {
+		return ret
+	}
+	return seekerReadAt{rs}
+}
+
+type seekerReadAt struct {
+	seeker io.ReadSeeker
+}
+
+func (sra seekerReadAt) ReadAt(p []byte, off int64) (int, error) {
+	if _, err := sra.seeker.Seek(off, io.SeekStart); err != nil {
+		return 0, err
+	}
+	return sra.seeker.Read(p)
+}
diff --git a/src/go/internal/gccgoimporter/gccgoinstallation.go b/src/go/internal/gccgoimporter/gccgoinstallation.go
new file mode 100644
index 0000000..e90a3cc
--- /dev/null
+++ b/src/go/internal/gccgoimporter/gccgoinstallation.go
@@ -0,0 +1,97 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gccgoimporter
+
+import (
+	"bufio"
+	"go/types"
+	exec "internal/execabs"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// Information about a specific installation of gccgo.
+type GccgoInstallation struct {
+	// Version of gcc (e.g. 4.8.0).
+	GccVersion string
+
+	// Target triple (e.g. x86_64-unknown-linux-gnu).
+	TargetTriple string
+
+	// Built-in library paths used by this installation.
+	LibPaths []string
+}
+
+// Ask the driver at the given path for information for this GccgoInstallation.
+// The given arguments are passed directly to the call of the driver.
+func (inst *GccgoInstallation) InitFromDriver(gccgoPath string, args ...string) (err error) {
+	argv := append([]string{"-###", "-S", "-x", "go", "-"}, args...)
+	cmd := exec.Command(gccgoPath, argv...)
+	stderr, err := cmd.StderrPipe()
+	if err != nil {
+		return
+	}
+
+	err = cmd.Start()
+	if err != nil {
+		return
+	}
+
+	scanner := bufio.NewScanner(stderr)
+	for scanner.Scan() {
+		line := scanner.Text()
+		switch {
+		case strings.HasPrefix(line, "Target: "):
+			inst.TargetTriple = line[8:]
+
+		case line[0] == ' ':
+			args := strings.Fields(line)
+			for _, arg := range args[1:] {
+				if strings.HasPrefix(arg, "-L") {
+					inst.LibPaths = append(inst.LibPaths, arg[2:])
+				}
+			}
+		}
+	}
+
+	argv = append([]string{"-dumpversion"}, args...)
+	stdout, err := exec.Command(gccgoPath, argv...).Output()
+	if err != nil {
+		return
+	}
+	inst.GccVersion = strings.TrimSpace(string(stdout))
+
+	return
+}
+
+// Return the list of export search paths for this GccgoInstallation.
+func (inst *GccgoInstallation) SearchPaths() (paths []string) {
+	for _, lpath := range inst.LibPaths {
+		spath := filepath.Join(lpath, "go", inst.GccVersion)
+		fi, err := os.Stat(spath)
+		if err != nil || !fi.IsDir() {
+			continue
+		}
+		paths = append(paths, spath)
+
+		spath = filepath.Join(spath, inst.TargetTriple)
+		fi, err = os.Stat(spath)
+		if err != nil || !fi.IsDir() {
+			continue
+		}
+		paths = append(paths, spath)
+	}
+
+	paths = append(paths, inst.LibPaths...)
+
+	return
+}
+
+// Return an importer that searches incpaths followed by the gcc installation's
+// built-in search paths and the current directory.
+func (inst *GccgoInstallation) GetImporter(incpaths []string, initmap map[*types.Package]InitData) Importer {
+	return GetImporter(append(append(incpaths, inst.SearchPaths()...), "."), initmap)
+}
diff --git a/src/go/internal/gccgoimporter/gccgoinstallation_test.go b/src/go/internal/gccgoimporter/gccgoinstallation_test.go
new file mode 100644
index 0000000..df0188a
--- /dev/null
+++ b/src/go/internal/gccgoimporter/gccgoinstallation_test.go
@@ -0,0 +1,192 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gccgoimporter
+
+import (
+	"go/types"
+	"testing"
+)
+
+// importablePackages is a list of packages that we verify that we can
+// import. This should be all standard library packages in all relevant
+// versions of gccgo. Note that since gccgo follows a different release
+// cycle, and since different systems have different versions installed,
+// we can't use the last-two-versions rule of the gc toolchain.
+var importablePackages = [...]string{
+	"archive/tar",
+	"archive/zip",
+	"bufio",
+	"bytes",
+	"compress/bzip2",
+	"compress/flate",
+	"compress/gzip",
+	"compress/lzw",
+	"compress/zlib",
+	"container/heap",
+	"container/list",
+	"container/ring",
+	"crypto/aes",
+	"crypto/cipher",
+	"crypto/des",
+	"crypto/dsa",
+	"crypto/ecdsa",
+	"crypto/elliptic",
+	"crypto",
+	"crypto/hmac",
+	"crypto/md5",
+	"crypto/rand",
+	"crypto/rc4",
+	"crypto/rsa",
+	"crypto/sha1",
+	"crypto/sha256",
+	"crypto/sha512",
+	"crypto/subtle",
+	"crypto/tls",
+	"crypto/x509",
+	"crypto/x509/pkix",
+	"database/sql/driver",
+	"database/sql",
+	"debug/dwarf",
+	"debug/elf",
+	"debug/gosym",
+	"debug/macho",
+	"debug/pe",
+	"encoding/ascii85",
+	"encoding/asn1",
+	"encoding/base32",
+	"encoding/base64",
+	"encoding/binary",
+	"encoding/csv",
+	"encoding/gob",
+	// "encoding", // Added in GCC 4.9.
+	"encoding/hex",
+	"encoding/json",
+	"encoding/pem",
+	"encoding/xml",
+	"errors",
+	"expvar",
+	"flag",
+	"fmt",
+	"go/ast",
+	"go/build",
+	"go/doc",
+	// "go/format", // Added in GCC 4.8.
+	"go/parser",
+	"go/printer",
+	"go/scanner",
+	"go/token",
+	"hash/adler32",
+	"hash/crc32",
+	"hash/crc64",
+	"hash/fnv",
+	"hash",
+	"html",
+	"html/template",
+	"image/color",
+	// "image/color/palette", // Added in GCC 4.9.
+	"image/draw",
+	"image/gif",
+	"image",
+	"image/jpeg",
+	"image/png",
+	"index/suffixarray",
+	"io",
+	"io/ioutil",
+	"log",
+	"log/syslog",
+	"math/big",
+	"math/cmplx",
+	"math",
+	"math/rand",
+	"mime",
+	"mime/multipart",
+	"net",
+	"net/http/cgi",
+	// "net/http/cookiejar", // Added in GCC 4.8.
+	"net/http/fcgi",
+	"net/http",
+	"net/http/httptest",
+	"net/http/httputil",
+	"net/http/pprof",
+	"net/mail",
+	"net/rpc",
+	"net/rpc/jsonrpc",
+	"net/smtp",
+	"net/textproto",
+	"net/url",
+	"os/exec",
+	"os",
+	"os/signal",
+	"os/user",
+	"path/filepath",
+	"path",
+	"reflect",
+	"regexp",
+	"regexp/syntax",
+	"runtime/debug",
+	"runtime",
+	"runtime/pprof",
+	"sort",
+	"strconv",
+	"strings",
+	"sync/atomic",
+	"sync",
+	"syscall",
+	"testing",
+	"testing/iotest",
+	"testing/quick",
+	"text/scanner",
+	"text/tabwriter",
+	"text/template",
+	"text/template/parse",
+	"time",
+	"unicode",
+	"unicode/utf16",
+	"unicode/utf8",
+}
+
+func TestInstallationImporter(t *testing.T) {
+	// This test relies on gccgo being around.
+	gpath := gccgoPath()
+	if gpath == "" {
+		t.Skip("This test needs gccgo")
+	}
+
+	var inst GccgoInstallation
+	err := inst.InitFromDriver(gpath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	imp := inst.GetImporter(nil, nil)
+
+	// Ensure we don't regress the number of packages we can parse. First import
+	// all packages into the same map and then each individually.
+	pkgMap := make(map[string]*types.Package)
+	for _, pkg := range importablePackages {
+		_, err = imp(pkgMap, pkg, ".", nil)
+		if err != nil {
+			t.Error(err)
+		}
+	}
+
+	for _, pkg := range importablePackages {
+		_, err = imp(make(map[string]*types.Package), pkg, ".", nil)
+		if err != nil {
+			t.Error(err)
+		}
+	}
+
+	// Test for certain specific entities in the imported data.
+	for _, test := range [...]importerTest{
+		{pkgpath: "io", name: "Reader", want: "type Reader interface{Read(p []byte) (n int, err error)}"},
+		{pkgpath: "io", name: "ReadWriter", want: "type ReadWriter interface{Reader; Writer}"},
+		{pkgpath: "math", name: "Pi", want: "const Pi untyped float"},
+		{pkgpath: "math", name: "Sin", want: "func Sin(x float64) float64"},
+		{pkgpath: "sort", name: "Search", want: "func Search(n int, f func(int) bool) int"},
+		{pkgpath: "unsafe", name: "Pointer", want: "type Pointer"},
+	} {
+		runImporterTest(t, imp, nil, &test)
+	}
+}
diff --git a/src/go/internal/gccgoimporter/importer.go b/src/go/internal/gccgoimporter/importer.go
new file mode 100644
index 0000000..94f2def
--- /dev/null
+++ b/src/go/internal/gccgoimporter/importer.go
@@ -0,0 +1,261 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package gccgoimporter implements Import for gccgo-generated object files.
+package gccgoimporter // import "go/internal/gccgoimporter"
+
+import (
+	"bytes"
+	"debug/elf"
+	"fmt"
+	"go/types"
+	"internal/xcoff"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// A PackageInit describes an imported package that needs initialization.
+type PackageInit struct {
+	Name     string // short package name
+	InitFunc string // name of init function
+	Priority int    // priority of init function, see InitData.Priority
+}
+
+// The gccgo-specific init data for a package.
+type InitData struct {
+	// Initialization priority of this package relative to other packages.
+	// This is based on the maximum depth of the package's dependency graph;
+	// it is guaranteed to be greater than that of its dependencies.
+	Priority int
+
+	// The list of packages which this package depends on to be initialized,
+	// including itself if needed. This is the subset of the transitive closure of
+	// the package's dependencies that need initialization.
+	Inits []PackageInit
+}
+
+// Locate the file from which to read export data.
+// This is intended to replicate the logic in gofrontend.
+func findExportFile(searchpaths []string, pkgpath string) (string, error) {
+	for _, spath := range searchpaths {
+		pkgfullpath := filepath.Join(spath, pkgpath)
+		pkgdir, name := filepath.Split(pkgfullpath)
+
+		for _, filepath := range [...]string{
+			pkgfullpath,
+			pkgfullpath + ".gox",
+			pkgdir + "lib" + name + ".so",
+			pkgdir + "lib" + name + ".a",
+			pkgfullpath + ".o",
+		} {
+			fi, err := os.Stat(filepath)
+			if err == nil && !fi.IsDir() {
+				return filepath, nil
+			}
+		}
+	}
+
+	return "", fmt.Errorf("%s: could not find export data (tried %s)", pkgpath, strings.Join(searchpaths, ":"))
+}
+
+const (
+	gccgov1Magic    = "v1;\n"
+	gccgov2Magic    = "v2;\n"
+	gccgov3Magic    = "v3;\n"
+	goimporterMagic = "\n$$ "
+	archiveMagic    = "!<ar"
+	aixbigafMagic   = "<big"
+)
+
+// Opens the export data file at the given path. If this is an ELF file,
+// searches for and opens the .go_export section. If this is an archive,
+// reads the export data from the first member, which is assumed to be an ELF file.
+// This is intended to replicate the logic in gofrontend.
+func openExportFile(fpath string) (reader io.ReadSeeker, closer io.Closer, err error) {
+	f, err := os.Open(fpath)
+	if err != nil {
+		return
+	}
+	closer = f
+	defer func() {
+		if err != nil && closer != nil {
+			f.Close()
+		}
+	}()
+
+	var magic [4]byte
+	_, err = f.ReadAt(magic[:], 0)
+	if err != nil {
+		return
+	}
+
+	var objreader io.ReaderAt
+	switch string(magic[:]) {
+	case gccgov1Magic, gccgov2Magic, gccgov3Magic, goimporterMagic:
+		// Raw export data.
+		reader = f
+		return
+
+	case archiveMagic, aixbigafMagic:
+		reader, err = arExportData(f)
+		return
+
+	default:
+		objreader = f
+	}
+
+	ef, err := elf.NewFile(objreader)
+	if err == nil {
+		sec := ef.Section(".go_export")
+		if sec == nil {
+			err = fmt.Errorf("%s: .go_export section not found", fpath)
+			return
+		}
+		reader = sec.Open()
+		return
+	}
+
+	xf, err := xcoff.NewFile(objreader)
+	if err == nil {
+		sdat := xf.CSect(".go_export")
+		if sdat == nil {
+			err = fmt.Errorf("%s: .go_export section not found", fpath)
+			return
+		}
+		reader = bytes.NewReader(sdat)
+		return
+	}
+
+	err = fmt.Errorf("%s: unrecognized file format", fpath)
+	return
+}
+
+// An Importer resolves import paths to Packages. The imports map records
+// packages already known, indexed by package path.
+// An importer must determine the canonical package path and check imports
+// to see if it is already present in the map. If so, the Importer can return
+// the map entry. Otherwise, the importer must load the package data for the
+// given path into a new *Package, record it in imports map, and return the
+// package.
+type Importer func(imports map[string]*types.Package, path, srcDir string, lookup func(string) (io.ReadCloser, error)) (*types.Package, error)
+
+func GetImporter(searchpaths []string, initmap map[*types.Package]InitData) Importer {
+	return func(imports map[string]*types.Package, pkgpath, srcDir string, lookup func(string) (io.ReadCloser, error)) (pkg *types.Package, err error) {
+		// TODO(gri): Use srcDir.
+		// Or not. It's possible that srcDir will fade in importance as
+		// the go command and other tools provide a translation table
+		// for relative imports (like ./foo or vendored imports).
+		if pkgpath == "unsafe" {
+			return types.Unsafe, nil
+		}
+
+		var reader io.ReadSeeker
+		var fpath string
+		var rc io.ReadCloser
+		if lookup != nil {
+			if p := imports[pkgpath]; p != nil && p.Complete() {
+				return p, nil
+			}
+			rc, err = lookup(pkgpath)
+			if err != nil {
+				return nil, err
+			}
+		}
+		if rc != nil {
+			defer rc.Close()
+			rs, ok := rc.(io.ReadSeeker)
+			if !ok {
+				return nil, fmt.Errorf("gccgo importer requires lookup to return an io.ReadSeeker, have %T", rc)
+			}
+			reader = rs
+			fpath = "<lookup " + pkgpath + ">"
+			// Take name from Name method (like on os.File) if present.
+			if n, ok := rc.(interface{ Name() string }); ok {
+				fpath = n.Name()
+			}
+		} else {
+			fpath, err = findExportFile(searchpaths, pkgpath)
+			if err != nil {
+				return nil, err
+			}
+
+			r, closer, err := openExportFile(fpath)
+			if err != nil {
+				return nil, err
+			}
+			if closer != nil {
+				defer closer.Close()
+			}
+			reader = r
+		}
+
+		var magics string
+		magics, err = readMagic(reader)
+		if err != nil {
+			return
+		}
+
+		if magics == archiveMagic || magics == aixbigafMagic {
+			reader, err = arExportData(reader)
+			if err != nil {
+				return
+			}
+			magics, err = readMagic(reader)
+			if err != nil {
+				return
+			}
+		}
+
+		switch magics {
+		case gccgov1Magic, gccgov2Magic, gccgov3Magic:
+			var p parser
+			p.init(fpath, reader, imports)
+			pkg = p.parsePackage()
+			if initmap != nil {
+				initmap[pkg] = p.initdata
+			}
+
+		// Excluded for now: Standard gccgo doesn't support this import format currently.
+		// case goimporterMagic:
+		// 	var data []byte
+		// 	data, err = io.ReadAll(reader)
+		// 	if err != nil {
+		// 		return
+		// 	}
+		// 	var n int
+		// 	n, pkg, err = importer.ImportData(imports, data)
+		// 	if err != nil {
+		// 		return
+		// 	}
+
+		// 	if initmap != nil {
+		// 		suffixreader := bytes.NewReader(data[n:])
+		// 		var p parser
+		// 		p.init(fpath, suffixreader, nil)
+		// 		p.parseInitData()
+		// 		initmap[pkg] = p.initdata
+		// 	}
+
+		default:
+			err = fmt.Errorf("unrecognized magic string: %q", magics)
+		}
+
+		return
+	}
+}
+
+// readMagic reads the four bytes at the start of a ReadSeeker and
+// returns them as a string.
+func readMagic(reader io.ReadSeeker) (string, error) {
+	var magic [4]byte
+	if _, err := reader.Read(magic[:]); err != nil {
+		return "", err
+	}
+	if _, err := reader.Seek(0, io.SeekStart); err != nil {
+		return "", err
+	}
+	return string(magic[:]), nil
+}
diff --git a/src/go/internal/gccgoimporter/importer_test.go b/src/go/internal/gccgoimporter/importer_test.go
new file mode 100644
index 0000000..35240c8
--- /dev/null
+++ b/src/go/internal/gccgoimporter/importer_test.go
@@ -0,0 +1,209 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gccgoimporter
+
+import (
+	"go/types"
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"regexp"
+	"strconv"
+	"testing"
+)
+
+type importerTest struct {
+	pkgpath, name, want, wantval string
+	wantinits                    []string
+	gccgoVersion                 int // minimum gccgo version (0 => any)
+}
+
+func runImporterTest(t *testing.T, imp Importer, initmap map[*types.Package]InitData, test *importerTest) {
+	pkg, err := imp(make(map[string]*types.Package), test.pkgpath, ".", nil)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	if test.name != "" {
+		obj := pkg.Scope().Lookup(test.name)
+		if obj == nil {
+			t.Errorf("%s: object not found", test.name)
+			return
+		}
+
+		got := types.ObjectString(obj, types.RelativeTo(pkg))
+		if got != test.want {
+			t.Errorf("%s: got %q; want %q", test.name, got, test.want)
+		}
+
+		if test.wantval != "" {
+			gotval := obj.(*types.Const).Val().String()
+			if gotval != test.wantval {
+				t.Errorf("%s: got val %q; want val %q", test.name, gotval, test.wantval)
+			}
+		}
+	}
+
+	if len(test.wantinits) > 0 {
+		initdata := initmap[pkg]
+		found := false
+		// Check that the package's own init function has the package's priority
+		for _, pkginit := range initdata.Inits {
+			if pkginit.InitFunc == test.wantinits[0] {
+				found = true
+				break
+			}
+		}
+
+		if !found {
+			t.Errorf("%s: could not find expected function %q", test.pkgpath, test.wantinits[0])
+		}
+
+		// FIXME: the original version of this test was written against
+		// the v1 export data scheme for capturing init functions, so it
+		// verified the priority values. We moved away from the priority
+		// scheme some time ago; it is not clear how much work it would be
+		// to validate the new init export data.
+	}
+}
+
+// When adding tests to this list, be sure to set the 'gccgoVersion'
+// field if the testcases uses a "recent" Go addition (ex: aliases).
+var importerTests = [...]importerTest{
+	{pkgpath: "pointer", name: "Int8Ptr", want: "type Int8Ptr *int8"},
+	{pkgpath: "complexnums", name: "NN", want: "const NN untyped complex", wantval: "(-1 + -1i)"},
+	{pkgpath: "complexnums", name: "NP", want: "const NP untyped complex", wantval: "(-1 + 1i)"},
+	{pkgpath: "complexnums", name: "PN", want: "const PN untyped complex", wantval: "(1 + -1i)"},
+	{pkgpath: "complexnums", name: "PP", want: "const PP untyped complex", wantval: "(1 + 1i)"},
+	{pkgpath: "conversions", name: "Bits", want: "const Bits Units", wantval: `"bits"`},
+	{pkgpath: "time", name: "Duration", want: "type Duration int64"},
+	{pkgpath: "time", name: "Nanosecond", want: "const Nanosecond Duration", wantval: "1"},
+	{pkgpath: "unicode", name: "IsUpper", want: "func IsUpper(r rune) bool"},
+	{pkgpath: "unicode", name: "MaxRune", want: "const MaxRune untyped rune", wantval: "1114111"},
+	{pkgpath: "imports", wantinits: []string{"imports..import", "fmt..import"}},
+	{pkgpath: "importsar", name: "Hello", want: "var Hello string"},
+	{pkgpath: "aliases", name: "A14", gccgoVersion: 7, want: "type A14 = func(int, T0) chan T2"},
+	{pkgpath: "aliases", name: "C0", gccgoVersion: 7, want: "type C0 struct{f1 C1; f2 C1}"},
+	{pkgpath: "escapeinfo", name: "NewT", want: "func NewT(data []byte) *T"},
+	{pkgpath: "issue27856", name: "M", gccgoVersion: 7, want: "type M struct{E F}"},
+	{pkgpath: "v1reflect", name: "Type", want: "type Type interface{Align() int; AssignableTo(u Type) bool; Bits() int; ChanDir() ChanDir; Elem() Type; Field(i int) StructField; FieldAlign() int; FieldByIndex(index []int) StructField; FieldByName(name string) (StructField, bool); FieldByNameFunc(match func(string) bool) (StructField, bool); Implements(u Type) bool; In(i int) Type; IsVariadic() bool; Key() Type; Kind() Kind; Len() int; Method(int) Method; MethodByName(string) (Method, bool); Name() string; NumField() int; NumIn() int; NumMethod() int; NumOut() int; Out(i int) Type; PkgPath() string; Size() uintptr; String() string; common() *commonType; rawString() string; runtimeType() *runtimeType; uncommon() *uncommonType}"},
+	{pkgpath: "nointerface", name: "I", want: "type I int"},
+	{pkgpath: "issue29198", name: "FooServer", gccgoVersion: 7, want: "type FooServer struct{FooServer *FooServer; user string; ctx context.Context}"},
+	{pkgpath: "issue30628", name: "Apple", want: "type Apple struct{hey sync.RWMutex; x int; RQ [517]struct{Count uintptr; NumBytes uintptr; Last uintptr}}"},
+	{pkgpath: "issue31540", name: "S", gccgoVersion: 7, want: "type S struct{b int; map[Y]Z}"},
+	{pkgpath: "issue34182", name: "T1", want: "type T1 struct{f *T2}"},
+	{pkgpath: "notinheap", name: "S", want: "type S struct{}"},
+}
+
+func TestGoxImporter(t *testing.T) {
+	testenv.MustHaveExec(t)
+	initmap := make(map[*types.Package]InitData)
+	imp := GetImporter([]string{"testdata"}, initmap)
+
+	for _, test := range importerTests {
+		runImporterTest(t, imp, initmap, &test)
+	}
+}
+
+// gccgoPath returns a path to gccgo if it is present (either in
+// path or specified via GCCGO environment variable), or an
+// empty string if no gccgo is available.
+func gccgoPath() string {
+	gccgoname := os.Getenv("GCCGO")
+	if gccgoname == "" {
+		gccgoname = "gccgo"
+	}
+	if gpath, gerr := exec.LookPath(gccgoname); gerr == nil {
+		return gpath
+	}
+	return ""
+}
+
+func TestObjImporter(t *testing.T) {
+	// This test relies on gccgo being around.
+	gpath := gccgoPath()
+	if gpath == "" {
+		t.Skip("This test needs gccgo")
+	}
+
+	verout, err := exec.Command(gpath, "--version").CombinedOutput()
+	if err != nil {
+		t.Logf("%s", verout)
+		t.Fatal(err)
+	}
+	vers := regexp.MustCompile(`([0-9]+)\.([0-9]+)`).FindSubmatch(verout)
+	if len(vers) == 0 {
+		t.Fatalf("could not find version number in %s", verout)
+	}
+	major, err := strconv.Atoi(string(vers[1]))
+	if err != nil {
+		t.Fatal(err)
+	}
+	minor, err := strconv.Atoi(string(vers[2]))
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("gccgo version %d.%d", major, minor)
+
+	tmpdir, err := os.MkdirTemp("", "TestObjImporter")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	initmap := make(map[*types.Package]InitData)
+	imp := GetImporter([]string{tmpdir}, initmap)
+
+	artmpdir, err := os.MkdirTemp("", "TestObjImporter")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(artmpdir)
+
+	arinitmap := make(map[*types.Package]InitData)
+	arimp := GetImporter([]string{artmpdir}, arinitmap)
+
+	for _, test := range importerTests {
+		if major < test.gccgoVersion {
+			// Support for type aliases was added in GCC 7.
+			t.Logf("skipping %q: not supported before gccgo version %d", test.pkgpath, test.gccgoVersion)
+			continue
+		}
+
+		gofile := filepath.Join("testdata", test.pkgpath+".go")
+		if _, err := os.Stat(gofile); os.IsNotExist(err) {
+			continue
+		}
+		ofile := filepath.Join(tmpdir, test.pkgpath+".o")
+		afile := filepath.Join(artmpdir, "lib"+test.pkgpath+".a")
+
+		cmd := exec.Command(gpath, "-fgo-pkgpath="+test.pkgpath, "-c", "-o", ofile, gofile)
+		out, err := cmd.CombinedOutput()
+		if err != nil {
+			t.Logf("%s", out)
+			t.Fatalf("gccgo %s failed: %s", gofile, err)
+		}
+
+		runImporterTest(t, imp, initmap, &test)
+
+		cmd = exec.Command("ar", "cr", afile, ofile)
+		out, err = cmd.CombinedOutput()
+		if err != nil {
+			t.Logf("%s", out)
+			t.Fatalf("ar cr %s %s failed: %s", afile, ofile, err)
+		}
+
+		runImporterTest(t, arimp, arinitmap, &test)
+
+		if err = os.Remove(ofile); err != nil {
+			t.Fatal(err)
+		}
+		if err = os.Remove(afile); err != nil {
+			t.Fatal(err)
+		}
+	}
+}
diff --git a/src/go/internal/gccgoimporter/parser.go b/src/go/internal/gccgoimporter/parser.go
new file mode 100644
index 0000000..1b1d07d
--- /dev/null
+++ b/src/go/internal/gccgoimporter/parser.go
@@ -0,0 +1,1284 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gccgoimporter
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"go/constant"
+	"go/token"
+	"go/types"
+	"io"
+	"strconv"
+	"strings"
+	"text/scanner"
+	"unicode/utf8"
+)
+
+type parser struct {
+	scanner  *scanner.Scanner
+	version  string                    // format version
+	tok      rune                      // current token
+	lit      string                    // literal string; only valid for Ident, Int, String tokens
+	pkgpath  string                    // package path of imported package
+	pkgname  string                    // name of imported package
+	pkg      *types.Package            // reference to imported package
+	imports  map[string]*types.Package // package path -> package object
+	typeList []types.Type              // type number -> type
+	typeData []string                  // unparsed type data (v3 and later)
+	fixups   []fixupRecord             // fixups to apply at end of parsing
+	initdata InitData                  // package init priority data
+	aliases  map[int]string            // maps saved type number to alias name
+}
+
+// When reading export data it's possible to encounter a defined type
+// N1 with an underlying defined type N2 while we are still reading in
+// that defined type N2; see issues #29006 and #29198 for instances
+// of this. Example:
+//
+//   type N1 N2
+//   type N2 struct {
+//      ...
+//      p *N1
+//   }
+//
+// To handle such cases, the parser generates a fixup record (below) and
+// delays setting of N1's underlying type until parsing is complete, at
+// which point fixups are applied.
+
+type fixupRecord struct {
+	toUpdate *types.Named // type to modify when fixup is processed
+	target   types.Type   // type that was incomplete when fixup was created
+}
+
+func (p *parser) init(filename string, src io.Reader, imports map[string]*types.Package) {
+	p.scanner = new(scanner.Scanner)
+	p.initScanner(filename, src)
+	p.imports = imports
+	p.aliases = make(map[int]string)
+	p.typeList = make([]types.Type, 1 /* type numbers start at 1 */, 16)
+}
+
+func (p *parser) initScanner(filename string, src io.Reader) {
+	p.scanner.Init(src)
+	p.scanner.Error = func(_ *scanner.Scanner, msg string) { p.error(msg) }
+	p.scanner.Mode = scanner.ScanIdents | scanner.ScanInts | scanner.ScanFloats | scanner.ScanStrings
+	p.scanner.Whitespace = 1<<'\t' | 1<<' '
+	p.scanner.Filename = filename // for good error messages
+	p.next()
+}
+
+type importError struct {
+	pos scanner.Position
+	err error
+}
+
+func (e importError) Error() string {
+	return fmt.Sprintf("import error %s (byte offset = %d): %s", e.pos, e.pos.Offset, e.err)
+}
+
+func (p *parser) error(err interface{}) {
+	if s, ok := err.(string); ok {
+		err = errors.New(s)
+	}
+	// panic with a runtime.Error if err is not an error
+	panic(importError{p.scanner.Pos(), err.(error)})
+}
+
+func (p *parser) errorf(format string, args ...interface{}) {
+	p.error(fmt.Errorf(format, args...))
+}
+
+func (p *parser) expect(tok rune) string {
+	lit := p.lit
+	if p.tok != tok {
+		p.errorf("expected %s, got %s (%s)", scanner.TokenString(tok), scanner.TokenString(p.tok), lit)
+	}
+	p.next()
+	return lit
+}
+
+func (p *parser) expectEOL() {
+	if p.version == "v1" || p.version == "v2" {
+		p.expect(';')
+	}
+	p.expect('\n')
+}
+
+func (p *parser) expectKeyword(keyword string) {
+	lit := p.expect(scanner.Ident)
+	if lit != keyword {
+		p.errorf("expected keyword %s, got %q", keyword, lit)
+	}
+}
+
+func (p *parser) parseString() string {
+	str, err := strconv.Unquote(p.expect(scanner.String))
+	if err != nil {
+		p.error(err)
+	}
+	return str
+}
+
+// unquotedString     = { unquotedStringChar } .
+// unquotedStringChar = <neither a whitespace nor a ';' char> .
+func (p *parser) parseUnquotedString() string {
+	if p.tok == scanner.EOF {
+		p.error("unexpected EOF")
+	}
+	var buf bytes.Buffer
+	buf.WriteString(p.scanner.TokenText())
+	// This loop needs to examine each character before deciding whether to consume it. If we see a semicolon,
+	// we need to let it be consumed by p.next().
+	for ch := p.scanner.Peek(); ch != '\n' && ch != ';' && ch != scanner.EOF && p.scanner.Whitespace&(1<<uint(ch)) == 0; ch = p.scanner.Peek() {
+		buf.WriteRune(ch)
+		p.scanner.Next()
+	}
+	p.next()
+	return buf.String()
+}
+
+func (p *parser) next() {
+	p.tok = p.scanner.Scan()
+	switch p.tok {
+	case scanner.Ident, scanner.Int, scanner.Float, scanner.String, '·':
+		p.lit = p.scanner.TokenText()
+	default:
+		p.lit = ""
+	}
+}
+
+func (p *parser) parseQualifiedName() (path, name string) {
+	return p.parseQualifiedNameStr(p.parseString())
+}
+
+func (p *parser) parseUnquotedQualifiedName() (path, name string) {
+	return p.parseQualifiedNameStr(p.parseUnquotedString())
+}
+
+// qualifiedName = [ ["."] unquotedString "." ] unquotedString .
+//
+// The above production uses greedy matching.
+func (p *parser) parseQualifiedNameStr(unquotedName string) (pkgpath, name string) {
+	parts := strings.Split(unquotedName, ".")
+	if parts[0] == "" {
+		parts = parts[1:]
+	}
+
+	switch len(parts) {
+	case 0:
+		p.errorf("malformed qualified name: %q", unquotedName)
+	case 1:
+		// unqualified name
+		pkgpath = p.pkgpath
+		name = parts[0]
+	default:
+		// qualified name, which may contain periods
+		pkgpath = strings.Join(parts[0:len(parts)-1], ".")
+		name = parts[len(parts)-1]
+	}
+
+	return
+}
+
+// getPkg returns the package for a given path. If the package is
+// not found but we have a package name, create the package and
+// add it to the p.imports map.
+//
+func (p *parser) getPkg(pkgpath, name string) *types.Package {
+	// package unsafe is not in the imports map - handle explicitly
+	if pkgpath == "unsafe" {
+		return types.Unsafe
+	}
+	pkg := p.imports[pkgpath]
+	if pkg == nil && name != "" {
+		pkg = types.NewPackage(pkgpath, name)
+		p.imports[pkgpath] = pkg
+	}
+	return pkg
+}
+
+// parseExportedName is like parseQualifiedName, but
+// the package path is resolved to an imported *types.Package.
+//
+// ExportedName = string [string] .
+func (p *parser) parseExportedName() (pkg *types.Package, name string) {
+	path, name := p.parseQualifiedName()
+	var pkgname string
+	if p.tok == scanner.String {
+		pkgname = p.parseString()
+	}
+	pkg = p.getPkg(path, pkgname)
+	if pkg == nil {
+		p.errorf("package %s (path = %q) not found", name, path)
+	}
+	return
+}
+
+// Name = QualifiedName | "?" .
+func (p *parser) parseName() string {
+	if p.tok == '?' {
+		// Anonymous.
+		p.next()
+		return ""
+	}
+	// The package path is redundant for us. Don't try to parse it.
+	_, name := p.parseUnquotedQualifiedName()
+	return name
+}
+
+func deref(typ types.Type) types.Type {
+	if p, _ := typ.(*types.Pointer); p != nil {
+		typ = p.Elem()
+	}
+	return typ
+}
+
+// Field = Name Type [string] .
+func (p *parser) parseField(pkg *types.Package) (field *types.Var, tag string) {
+	name := p.parseName()
+	typ, n := p.parseTypeExtended(pkg)
+	anon := false
+	if name == "" {
+		anon = true
+		// Alias?
+		if aname, ok := p.aliases[n]; ok {
+			name = aname
+		} else {
+			switch typ := deref(typ).(type) {
+			case *types.Basic:
+				name = typ.Name()
+			case *types.Named:
+				name = typ.Obj().Name()
+			default:
+				p.error("embedded field expected")
+			}
+		}
+	}
+	field = types.NewField(token.NoPos, pkg, name, typ, anon)
+	if p.tok == scanner.String {
+		tag = p.parseString()
+	}
+	return
+}
+
+// Param = Name ["..."] Type .
+func (p *parser) parseParam(pkg *types.Package) (param *types.Var, isVariadic bool) {
+	name := p.parseName()
+	// Ignore names invented for inlinable functions.
+	if strings.HasPrefix(name, "p.") || strings.HasPrefix(name, "r.") || strings.HasPrefix(name, "$ret") {
+		name = ""
+	}
+	if p.tok == '<' && p.scanner.Peek() == 'e' {
+		// EscInfo = "<esc:" int ">" . (optional and ignored)
+		p.next()
+		p.expectKeyword("esc")
+		p.expect(':')
+		p.expect(scanner.Int)
+		p.expect('>')
+	}
+	if p.tok == '.' {
+		p.next()
+		p.expect('.')
+		p.expect('.')
+		isVariadic = true
+	}
+	typ := p.parseType(pkg)
+	if isVariadic {
+		typ = types.NewSlice(typ)
+	}
+	param = types.NewParam(token.NoPos, pkg, name, typ)
+	return
+}
+
+// Var = Name Type .
+func (p *parser) parseVar(pkg *types.Package) *types.Var {
+	name := p.parseName()
+	v := types.NewVar(token.NoPos, pkg, name, p.parseType(pkg))
+	if name[0] == '.' || name[0] == '<' {
+		// This is an unexported variable,
+		// or a variable defined in a different package.
+		// We only want to record exported variables.
+		return nil
+	}
+	return v
+}
+
+// Conversion = "convert" "(" Type "," ConstValue ")" .
+func (p *parser) parseConversion(pkg *types.Package) (val constant.Value, typ types.Type) {
+	p.expectKeyword("convert")
+	p.expect('(')
+	typ = p.parseType(pkg)
+	p.expect(',')
+	val, _ = p.parseConstValue(pkg)
+	p.expect(')')
+	return
+}
+
+// ConstValue     = string | "false" | "true" | ["-"] (int ["'"] | FloatOrComplex) | Conversion .
+// FloatOrComplex = float ["i" | ("+"|"-") float "i"] .
+func (p *parser) parseConstValue(pkg *types.Package) (val constant.Value, typ types.Type) {
+	// v3 changed to $false, $true, $convert, to avoid confusion
+	// with variable names in inline function bodies.
+	if p.tok == '$' {
+		p.next()
+		if p.tok != scanner.Ident {
+			p.errorf("expected identifier after '$', got %s (%q)", scanner.TokenString(p.tok), p.lit)
+		}
+	}
+
+	switch p.tok {
+	case scanner.String:
+		str := p.parseString()
+		val = constant.MakeString(str)
+		typ = types.Typ[types.UntypedString]
+		return
+
+	case scanner.Ident:
+		b := false
+		switch p.lit {
+		case "false":
+		case "true":
+			b = true
+
+		case "convert":
+			return p.parseConversion(pkg)
+
+		default:
+			p.errorf("expected const value, got %s (%q)", scanner.TokenString(p.tok), p.lit)
+		}
+
+		p.next()
+		val = constant.MakeBool(b)
+		typ = types.Typ[types.UntypedBool]
+		return
+	}
+
+	sign := ""
+	if p.tok == '-' {
+		p.next()
+		sign = "-"
+	}
+
+	switch p.tok {
+	case scanner.Int:
+		val = constant.MakeFromLiteral(sign+p.lit, token.INT, 0)
+		if val == nil {
+			p.error("could not parse integer literal")
+		}
+
+		p.next()
+		if p.tok == '\'' {
+			p.next()
+			typ = types.Typ[types.UntypedRune]
+		} else {
+			typ = types.Typ[types.UntypedInt]
+		}
+
+	case scanner.Float:
+		re := sign + p.lit
+		p.next()
+
+		var im string
+		switch p.tok {
+		case '+':
+			p.next()
+			im = p.expect(scanner.Float)
+
+		case '-':
+			p.next()
+			im = "-" + p.expect(scanner.Float)
+
+		case scanner.Ident:
+			// re is in fact the imaginary component. Expect "i" below.
+			im = re
+			re = "0"
+
+		default:
+			val = constant.MakeFromLiteral(re, token.FLOAT, 0)
+			if val == nil {
+				p.error("could not parse float literal")
+			}
+			typ = types.Typ[types.UntypedFloat]
+			return
+		}
+
+		p.expectKeyword("i")
+		reval := constant.MakeFromLiteral(re, token.FLOAT, 0)
+		if reval == nil {
+			p.error("could not parse real component of complex literal")
+		}
+		imval := constant.MakeFromLiteral(im+"i", token.IMAG, 0)
+		if imval == nil {
+			p.error("could not parse imag component of complex literal")
+		}
+		val = constant.BinaryOp(reval, token.ADD, imval)
+		typ = types.Typ[types.UntypedComplex]
+
+	default:
+		p.errorf("expected const value, got %s (%q)", scanner.TokenString(p.tok), p.lit)
+	}
+
+	return
+}
+
+// Const = Name [Type] "=" ConstValue .
+func (p *parser) parseConst(pkg *types.Package) *types.Const {
+	name := p.parseName()
+	var typ types.Type
+	if p.tok == '<' {
+		typ = p.parseType(pkg)
+	}
+	p.expect('=')
+	val, vtyp := p.parseConstValue(pkg)
+	if typ == nil {
+		typ = vtyp
+	}
+	return types.NewConst(token.NoPos, pkg, name, typ, val)
+}
+
+// reserved is a singleton type used to fill type map slots that have
+// been reserved (i.e., for which a type number has been parsed) but
+// which don't have their actual type yet. When the type map is updated,
+// the actual type must replace a reserved entry (or we have an internal
+// error). Used for self-verification only - not required for correctness.
+var reserved = new(struct{ types.Type })
+
+// reserve reserves the type map entry n for future use.
+func (p *parser) reserve(n int) {
+	// Notes:
+	// - for pre-V3 export data, the type numbers we see are
+	//   guaranteed to be in increasing order, so we append a
+	//   reserved entry onto the list.
+	// - for V3+ export data, type numbers can appear in
+	//   any order, however the 'types' section tells us the
+	//   total number of types, hence typeList is pre-allocated.
+	if len(p.typeData) == 0 {
+		if n != len(p.typeList) {
+			p.errorf("invalid type number %d (out of sync)", n)
+		}
+		p.typeList = append(p.typeList, reserved)
+	} else {
+		if p.typeList[n] != nil {
+			p.errorf("previously visited type number %d", n)
+		}
+		p.typeList[n] = reserved
+	}
+}
+
+// update sets the type map entries for the entries in nlist to t.
+// An entry in nlist can be a type number in p.typeList,
+// used to resolve named types, or it can be a *types.Pointer,
+// used to resolve pointers to named types in case they are referenced
+// by embedded fields.
+func (p *parser) update(t types.Type, nlist []interface{}) {
+	if t == reserved {
+		p.errorf("internal error: update(%v) invoked on reserved", nlist)
+	}
+	if t == nil {
+		p.errorf("internal error: update(%v) invoked on nil", nlist)
+	}
+	for _, n := range nlist {
+		switch n := n.(type) {
+		case int:
+			if p.typeList[n] == t {
+				continue
+			}
+			if p.typeList[n] != reserved {
+				p.errorf("internal error: update(%v): %d not reserved", nlist, n)
+			}
+			p.typeList[n] = t
+		case *types.Pointer:
+			if *n != (types.Pointer{}) {
+				elem := n.Elem()
+				if elem == t {
+					continue
+				}
+				p.errorf("internal error: update: pointer already set to %v, expected %v", elem, t)
+			}
+			*n = *types.NewPointer(t)
+		default:
+			p.errorf("internal error: %T on nlist", n)
+		}
+	}
+}
+
+// NamedType = TypeName [ "=" ] Type { Method } .
+// TypeName  = ExportedName .
+// Method    = "func" "(" Param ")" Name ParamList ResultList [InlineBody] ";" .
+func (p *parser) parseNamedType(nlist []interface{}) types.Type {
+	pkg, name := p.parseExportedName()
+	scope := pkg.Scope()
+	obj := scope.Lookup(name)
+	if obj != nil && obj.Type() == nil {
+		p.errorf("%v has nil type", obj)
+	}
+
+	if p.tok == scanner.Ident && p.lit == "notinheap" {
+		p.next()
+		// The go/types package has no way of recording that
+		// this type is marked notinheap. Presumably no user
+		// of this package actually cares.
+	}
+
+	// type alias
+	if p.tok == '=' {
+		p.next()
+		p.aliases[nlist[len(nlist)-1].(int)] = name
+		if obj != nil {
+			// use the previously imported (canonical) type
+			t := obj.Type()
+			p.update(t, nlist)
+			p.parseType(pkg) // discard
+			return t
+		}
+		t := p.parseType(pkg, nlist...)
+		obj = types.NewTypeName(token.NoPos, pkg, name, t)
+		scope.Insert(obj)
+		return t
+	}
+
+	// defined type
+	if obj == nil {
+		// A named type may be referred to before the underlying type
+		// is known - set it up.
+		tname := types.NewTypeName(token.NoPos, pkg, name, nil)
+		types.NewNamed(tname, nil, nil)
+		scope.Insert(tname)
+		obj = tname
+	}
+
+	// use the previously imported (canonical), or newly created type
+	t := obj.Type()
+	p.update(t, nlist)
+
+	nt, ok := t.(*types.Named)
+	if !ok {
+		// This can happen for unsafe.Pointer, which is a TypeName holding a Basic type.
+		pt := p.parseType(pkg)
+		if pt != t {
+			p.error("unexpected underlying type for non-named TypeName")
+		}
+		return t
+	}
+
+	underlying := p.parseType(pkg)
+	if nt.Underlying() == nil {
+		if underlying.Underlying() == nil {
+			fix := fixupRecord{toUpdate: nt, target: underlying}
+			p.fixups = append(p.fixups, fix)
+		} else {
+			nt.SetUnderlying(underlying.Underlying())
+		}
+	}
+
+	if p.tok == '\n' {
+		p.next()
+		// collect associated methods
+		for p.tok == scanner.Ident {
+			p.expectKeyword("func")
+			if p.tok == '/' {
+				// Skip a /*nointerface*/ or /*asm ID */ comment.
+				p.expect('/')
+				p.expect('*')
+				if p.expect(scanner.Ident) == "asm" {
+					p.parseUnquotedString()
+				}
+				p.expect('*')
+				p.expect('/')
+			}
+			p.expect('(')
+			receiver, _ := p.parseParam(pkg)
+			p.expect(')')
+			name := p.parseName()
+			params, isVariadic := p.parseParamList(pkg)
+			results := p.parseResultList(pkg)
+			p.skipInlineBody()
+			p.expectEOL()
+
+			sig := types.NewSignature(receiver, params, results, isVariadic)
+			nt.AddMethod(types.NewFunc(token.NoPos, pkg, name, sig))
+		}
+	}
+
+	return nt
+}
+
+func (p *parser) parseInt64() int64 {
+	lit := p.expect(scanner.Int)
+	n, err := strconv.ParseInt(lit, 10, 64)
+	if err != nil {
+		p.error(err)
+	}
+	return n
+}
+
+func (p *parser) parseInt() int {
+	lit := p.expect(scanner.Int)
+	n, err := strconv.ParseInt(lit, 10, 0 /* int */)
+	if err != nil {
+		p.error(err)
+	}
+	return int(n)
+}
+
+// ArrayOrSliceType = "[" [ int ] "]" Type .
+func (p *parser) parseArrayOrSliceType(pkg *types.Package, nlist []interface{}) types.Type {
+	p.expect('[')
+	if p.tok == ']' {
+		p.next()
+
+		t := new(types.Slice)
+		p.update(t, nlist)
+
+		*t = *types.NewSlice(p.parseType(pkg))
+		return t
+	}
+
+	t := new(types.Array)
+	p.update(t, nlist)
+
+	len := p.parseInt64()
+	p.expect(']')
+
+	*t = *types.NewArray(p.parseType(pkg), len)
+	return t
+}
+
+// MapType = "map" "[" Type "]" Type .
+func (p *parser) parseMapType(pkg *types.Package, nlist []interface{}) types.Type {
+	p.expectKeyword("map")
+
+	t := new(types.Map)
+	p.update(t, nlist)
+
+	p.expect('[')
+	key := p.parseType(pkg)
+	p.expect(']')
+	elem := p.parseType(pkg)
+
+	*t = *types.NewMap(key, elem)
+	return t
+}
+
+// ChanType = "chan" ["<-" | "-<"] Type .
+func (p *parser) parseChanType(pkg *types.Package, nlist []interface{}) types.Type {
+	p.expectKeyword("chan")
+
+	t := new(types.Chan)
+	p.update(t, nlist)
+
+	dir := types.SendRecv
+	switch p.tok {
+	case '-':
+		p.next()
+		p.expect('<')
+		dir = types.SendOnly
+
+	case '<':
+		// don't consume '<' if it belongs to Type
+		if p.scanner.Peek() == '-' {
+			p.next()
+			p.expect('-')
+			dir = types.RecvOnly
+		}
+	}
+
+	*t = *types.NewChan(dir, p.parseType(pkg))
+	return t
+}
+
+// StructType = "struct" "{" { Field } "}" .
+func (p *parser) parseStructType(pkg *types.Package, nlist []interface{}) types.Type {
+	p.expectKeyword("struct")
+
+	t := new(types.Struct)
+	p.update(t, nlist)
+
+	var fields []*types.Var
+	var tags []string
+
+	p.expect('{')
+	for p.tok != '}' && p.tok != scanner.EOF {
+		field, tag := p.parseField(pkg)
+		p.expect(';')
+		fields = append(fields, field)
+		tags = append(tags, tag)
+	}
+	p.expect('}')
+
+	*t = *types.NewStruct(fields, tags)
+	return t
+}
+
+// ParamList = "(" [ { Parameter "," } Parameter ] ")" .
+func (p *parser) parseParamList(pkg *types.Package) (*types.Tuple, bool) {
+	var list []*types.Var
+	isVariadic := false
+
+	p.expect('(')
+	for p.tok != ')' && p.tok != scanner.EOF {
+		if len(list) > 0 {
+			p.expect(',')
+		}
+		par, variadic := p.parseParam(pkg)
+		list = append(list, par)
+		if variadic {
+			if isVariadic {
+				p.error("... not on final argument")
+			}
+			isVariadic = true
+		}
+	}
+	p.expect(')')
+
+	return types.NewTuple(list...), isVariadic
+}
+
+// ResultList = Type | ParamList .
+func (p *parser) parseResultList(pkg *types.Package) *types.Tuple {
+	switch p.tok {
+	case '<':
+		p.next()
+		if p.tok == scanner.Ident && p.lit == "inl" {
+			return nil
+		}
+		taa, _ := p.parseTypeAfterAngle(pkg)
+		return types.NewTuple(types.NewParam(token.NoPos, pkg, "", taa))
+
+	case '(':
+		params, _ := p.parseParamList(pkg)
+		return params
+
+	default:
+		return nil
+	}
+}
+
+// FunctionType = ParamList ResultList .
+func (p *parser) parseFunctionType(pkg *types.Package, nlist []interface{}) *types.Signature {
+	t := new(types.Signature)
+	p.update(t, nlist)
+
+	params, isVariadic := p.parseParamList(pkg)
+	results := p.parseResultList(pkg)
+
+	*t = *types.NewSignature(nil, params, results, isVariadic)
+	return t
+}
+
+// Func = Name FunctionType [InlineBody] .
+func (p *parser) parseFunc(pkg *types.Package) *types.Func {
+	if p.tok == '/' {
+		// Skip an /*asm ID */ comment.
+		p.expect('/')
+		p.expect('*')
+		if p.expect(scanner.Ident) == "asm" {
+			p.parseUnquotedString()
+		}
+		p.expect('*')
+		p.expect('/')
+	}
+
+	name := p.parseName()
+	f := types.NewFunc(token.NoPos, pkg, name, p.parseFunctionType(pkg, nil))
+	p.skipInlineBody()
+
+	if name[0] == '.' || name[0] == '<' || strings.ContainsRune(name, '$') {
+		// This is an unexported function,
+		// or a function defined in a different package,
+		// or a type$equal or type$hash function.
+		// We only want to record exported functions.
+		return nil
+	}
+
+	return f
+}
+
+// InterfaceType = "interface" "{" { ("?" Type | Func) ";" } "}" .
+func (p *parser) parseInterfaceType(pkg *types.Package, nlist []interface{}) types.Type {
+	p.expectKeyword("interface")
+
+	t := new(types.Interface)
+	p.update(t, nlist)
+
+	var methods []*types.Func
+	var embeddeds []types.Type
+
+	p.expect('{')
+	for p.tok != '}' && p.tok != scanner.EOF {
+		if p.tok == '?' {
+			p.next()
+			embeddeds = append(embeddeds, p.parseType(pkg))
+		} else {
+			method := p.parseFunc(pkg)
+			if method != nil {
+				methods = append(methods, method)
+			}
+		}
+		p.expect(';')
+	}
+	p.expect('}')
+
+	*t = *types.NewInterfaceType(methods, embeddeds)
+	return t
+}
+
+// PointerType = "*" ("any" | Type) .
+func (p *parser) parsePointerType(pkg *types.Package, nlist []interface{}) types.Type {
+	p.expect('*')
+	if p.tok == scanner.Ident {
+		p.expectKeyword("any")
+		t := types.Typ[types.UnsafePointer]
+		p.update(t, nlist)
+		return t
+	}
+
+	t := new(types.Pointer)
+	p.update(t, nlist)
+
+	*t = *types.NewPointer(p.parseType(pkg, t))
+
+	return t
+}
+
+// TypeSpec = NamedType | MapType | ChanType | StructType | InterfaceType | PointerType | ArrayOrSliceType | FunctionType .
+func (p *parser) parseTypeSpec(pkg *types.Package, nlist []interface{}) types.Type {
+	switch p.tok {
+	case scanner.String:
+		return p.parseNamedType(nlist)
+
+	case scanner.Ident:
+		switch p.lit {
+		case "map":
+			return p.parseMapType(pkg, nlist)
+
+		case "chan":
+			return p.parseChanType(pkg, nlist)
+
+		case "struct":
+			return p.parseStructType(pkg, nlist)
+
+		case "interface":
+			return p.parseInterfaceType(pkg, nlist)
+		}
+
+	case '*':
+		return p.parsePointerType(pkg, nlist)
+
+	case '[':
+		return p.parseArrayOrSliceType(pkg, nlist)
+
+	case '(':
+		return p.parseFunctionType(pkg, nlist)
+	}
+
+	p.errorf("expected type name or literal, got %s", scanner.TokenString(p.tok))
+	return nil
+}
+
+const (
+	// From gofrontend/go/export.h
+	// Note that these values are negative in the gofrontend and have been made positive
+	// in the gccgoimporter.
+	gccgoBuiltinINT8       = 1
+	gccgoBuiltinINT16      = 2
+	gccgoBuiltinINT32      = 3
+	gccgoBuiltinINT64      = 4
+	gccgoBuiltinUINT8      = 5
+	gccgoBuiltinUINT16     = 6
+	gccgoBuiltinUINT32     = 7
+	gccgoBuiltinUINT64     = 8
+	gccgoBuiltinFLOAT32    = 9
+	gccgoBuiltinFLOAT64    = 10
+	gccgoBuiltinINT        = 11
+	gccgoBuiltinUINT       = 12
+	gccgoBuiltinUINTPTR    = 13
+	gccgoBuiltinBOOL       = 15
+	gccgoBuiltinSTRING     = 16
+	gccgoBuiltinCOMPLEX64  = 17
+	gccgoBuiltinCOMPLEX128 = 18
+	gccgoBuiltinERROR      = 19
+	gccgoBuiltinBYTE       = 20
+	gccgoBuiltinRUNE       = 21
+)
+
+func lookupBuiltinType(typ int) types.Type {
+	return [...]types.Type{
+		gccgoBuiltinINT8:       types.Typ[types.Int8],
+		gccgoBuiltinINT16:      types.Typ[types.Int16],
+		gccgoBuiltinINT32:      types.Typ[types.Int32],
+		gccgoBuiltinINT64:      types.Typ[types.Int64],
+		gccgoBuiltinUINT8:      types.Typ[types.Uint8],
+		gccgoBuiltinUINT16:     types.Typ[types.Uint16],
+		gccgoBuiltinUINT32:     types.Typ[types.Uint32],
+		gccgoBuiltinUINT64:     types.Typ[types.Uint64],
+		gccgoBuiltinFLOAT32:    types.Typ[types.Float32],
+		gccgoBuiltinFLOAT64:    types.Typ[types.Float64],
+		gccgoBuiltinINT:        types.Typ[types.Int],
+		gccgoBuiltinUINT:       types.Typ[types.Uint],
+		gccgoBuiltinUINTPTR:    types.Typ[types.Uintptr],
+		gccgoBuiltinBOOL:       types.Typ[types.Bool],
+		gccgoBuiltinSTRING:     types.Typ[types.String],
+		gccgoBuiltinCOMPLEX64:  types.Typ[types.Complex64],
+		gccgoBuiltinCOMPLEX128: types.Typ[types.Complex128],
+		gccgoBuiltinERROR:      types.Universe.Lookup("error").Type(),
+		gccgoBuiltinBYTE:       types.Universe.Lookup("byte").Type(),
+		gccgoBuiltinRUNE:       types.Universe.Lookup("rune").Type(),
+	}[typ]
+}
+
+// Type = "<" "type" ( "-" int | int [ TypeSpec ] ) ">" .
+//
+// parseType updates the type map to t for all type numbers n.
+//
+func (p *parser) parseType(pkg *types.Package, n ...interface{}) types.Type {
+	p.expect('<')
+	t, _ := p.parseTypeAfterAngle(pkg, n...)
+	return t
+}
+
+// (*parser).Type after reading the "<".
+func (p *parser) parseTypeAfterAngle(pkg *types.Package, n ...interface{}) (t types.Type, n1 int) {
+	p.expectKeyword("type")
+
+	n1 = 0
+	switch p.tok {
+	case scanner.Int:
+		n1 = p.parseInt()
+		if p.tok == '>' {
+			if len(p.typeData) > 0 && p.typeList[n1] == nil {
+				p.parseSavedType(pkg, n1, n)
+			}
+			t = p.typeList[n1]
+			if len(p.typeData) == 0 && t == reserved {
+				p.errorf("invalid type cycle, type %d not yet defined (nlist=%v)", n1, n)
+			}
+			p.update(t, n)
+		} else {
+			p.reserve(n1)
+			t = p.parseTypeSpec(pkg, append(n, n1))
+		}
+
+	case '-':
+		p.next()
+		n1 := p.parseInt()
+		t = lookupBuiltinType(n1)
+		p.update(t, n)
+
+	default:
+		p.errorf("expected type number, got %s (%q)", scanner.TokenString(p.tok), p.lit)
+		return nil, 0
+	}
+
+	if t == nil || t == reserved {
+		p.errorf("internal error: bad return from parseType(%v)", n)
+	}
+
+	p.expect('>')
+	return
+}
+
+// parseTypeExtended is identical to parseType, but if the type in
+// question is a saved type, returns the index as well as the type
+// pointer (index returned is zero if we parsed a builtin).
+func (p *parser) parseTypeExtended(pkg *types.Package, n ...interface{}) (t types.Type, n1 int) {
+	p.expect('<')
+	t, n1 = p.parseTypeAfterAngle(pkg, n...)
+	return
+}
+
+// InlineBody = "<inl:NN>" .{NN}
+// Reports whether a body was skipped.
+func (p *parser) skipInlineBody() {
+	// We may or may not have seen the '<' already, depending on
+	// whether the function had a result type or not.
+	if p.tok == '<' {
+		p.next()
+		p.expectKeyword("inl")
+	} else if p.tok != scanner.Ident || p.lit != "inl" {
+		return
+	} else {
+		p.next()
+	}
+
+	p.expect(':')
+	want := p.parseInt()
+	p.expect('>')
+
+	defer func(w uint64) {
+		p.scanner.Whitespace = w
+	}(p.scanner.Whitespace)
+	p.scanner.Whitespace = 0
+
+	got := 0
+	for got < want {
+		r := p.scanner.Next()
+		if r == scanner.EOF {
+			p.error("unexpected EOF")
+		}
+		got += utf8.RuneLen(r)
+	}
+}
+
+// Types = "types" maxp1 exportedp1 (offset length)* .
+func (p *parser) parseTypes(pkg *types.Package) {
+	maxp1 := p.parseInt()
+	exportedp1 := p.parseInt()
+	p.typeList = make([]types.Type, maxp1, maxp1)
+
+	type typeOffset struct {
+		offset int
+		length int
+	}
+	var typeOffsets []typeOffset
+
+	total := 0
+	for i := 1; i < maxp1; i++ {
+		len := p.parseInt()
+		typeOffsets = append(typeOffsets, typeOffset{total, len})
+		total += len
+	}
+
+	defer func(w uint64) {
+		p.scanner.Whitespace = w
+	}(p.scanner.Whitespace)
+	p.scanner.Whitespace = 0
+
+	// We should now have p.tok pointing to the final newline.
+	// The next runes from the scanner should be the type data.
+
+	var sb strings.Builder
+	for sb.Len() < total {
+		r := p.scanner.Next()
+		if r == scanner.EOF {
+			p.error("unexpected EOF")
+		}
+		sb.WriteRune(r)
+	}
+	allTypeData := sb.String()
+
+	p.typeData = []string{""} // type 0, unused
+	for _, to := range typeOffsets {
+		p.typeData = append(p.typeData, allTypeData[to.offset:to.offset+to.length])
+	}
+
+	for i := 1; i < int(exportedp1); i++ {
+		p.parseSavedType(pkg, i, nil)
+	}
+}
+
+// parseSavedType parses one saved type definition.
+func (p *parser) parseSavedType(pkg *types.Package, i int, nlist []interface{}) {
+	defer func(s *scanner.Scanner, tok rune, lit string) {
+		p.scanner = s
+		p.tok = tok
+		p.lit = lit
+	}(p.scanner, p.tok, p.lit)
+
+	p.scanner = new(scanner.Scanner)
+	p.initScanner(p.scanner.Filename, strings.NewReader(p.typeData[i]))
+	p.expectKeyword("type")
+	id := p.parseInt()
+	if id != i {
+		p.errorf("type ID mismatch: got %d, want %d", id, i)
+	}
+	if p.typeList[i] == reserved {
+		p.errorf("internal error: %d already reserved in parseSavedType", i)
+	}
+	if p.typeList[i] == nil {
+		p.reserve(i)
+		p.parseTypeSpec(pkg, append(nlist, i))
+	}
+	if p.typeList[i] == nil || p.typeList[i] == reserved {
+		p.errorf("internal error: parseSavedType(%d,%v) reserved/nil", i, nlist)
+	}
+}
+
+// PackageInit = unquotedString unquotedString int .
+func (p *parser) parsePackageInit() PackageInit {
+	name := p.parseUnquotedString()
+	initfunc := p.parseUnquotedString()
+	priority := -1
+	if p.version == "v1" {
+		priority = p.parseInt()
+	}
+	return PackageInit{Name: name, InitFunc: initfunc, Priority: priority}
+}
+
+// Create the package if we have parsed both the package path and package name.
+func (p *parser) maybeCreatePackage() {
+	if p.pkgname != "" && p.pkgpath != "" {
+		p.pkg = p.getPkg(p.pkgpath, p.pkgname)
+	}
+}
+
+// InitDataDirective = ( "v1" | "v2" | "v3" ) ";" |
+//                     "priority" int ";" |
+//                     "init" { PackageInit } ";" |
+//                     "checksum" unquotedString ";" .
+func (p *parser) parseInitDataDirective() {
+	if p.tok != scanner.Ident {
+		// unexpected token kind; panic
+		p.expect(scanner.Ident)
+	}
+
+	switch p.lit {
+	case "v1", "v2", "v3":
+		p.version = p.lit
+		p.next()
+		p.expect(';')
+		p.expect('\n')
+
+	case "priority":
+		p.next()
+		p.initdata.Priority = p.parseInt()
+		p.expectEOL()
+
+	case "init":
+		p.next()
+		for p.tok != '\n' && p.tok != ';' && p.tok != scanner.EOF {
+			p.initdata.Inits = append(p.initdata.Inits, p.parsePackageInit())
+		}
+		p.expectEOL()
+
+	case "init_graph":
+		p.next()
+		// The graph data is thrown away for now.
+		for p.tok != '\n' && p.tok != ';' && p.tok != scanner.EOF {
+			p.parseInt64()
+			p.parseInt64()
+		}
+		p.expectEOL()
+
+	case "checksum":
+		// Don't let the scanner try to parse the checksum as a number.
+		defer func(mode uint) {
+			p.scanner.Mode = mode
+		}(p.scanner.Mode)
+		p.scanner.Mode &^= scanner.ScanInts | scanner.ScanFloats
+		p.next()
+		p.parseUnquotedString()
+		p.expectEOL()
+
+	default:
+		p.errorf("unexpected identifier: %q", p.lit)
+	}
+}
+
+// Directive = InitDataDirective |
+//             "package" unquotedString [ unquotedString ] [ unquotedString ] ";" |
+//             "pkgpath" unquotedString ";" |
+//             "prefix" unquotedString ";" |
+//             "import" unquotedString unquotedString string ";" |
+//             "indirectimport" unquotedString unquotedstring ";" |
+//             "func" Func ";" |
+//             "type" Type ";" |
+//             "var" Var ";" |
+//             "const" Const ";" .
+func (p *parser) parseDirective() {
+	if p.tok != scanner.Ident {
+		// unexpected token kind; panic
+		p.expect(scanner.Ident)
+	}
+
+	switch p.lit {
+	case "v1", "v2", "v3", "priority", "init", "init_graph", "checksum":
+		p.parseInitDataDirective()
+
+	case "package":
+		p.next()
+		p.pkgname = p.parseUnquotedString()
+		p.maybeCreatePackage()
+		if p.version != "v1" && p.tok != '\n' && p.tok != ';' {
+			p.parseUnquotedString()
+			p.parseUnquotedString()
+		}
+		p.expectEOL()
+
+	case "pkgpath":
+		p.next()
+		p.pkgpath = p.parseUnquotedString()
+		p.maybeCreatePackage()
+		p.expectEOL()
+
+	case "prefix":
+		p.next()
+		p.pkgpath = p.parseUnquotedString()
+		p.expectEOL()
+
+	case "import":
+		p.next()
+		pkgname := p.parseUnquotedString()
+		pkgpath := p.parseUnquotedString()
+		p.getPkg(pkgpath, pkgname)
+		p.parseString()
+		p.expectEOL()
+
+	case "indirectimport":
+		p.next()
+		pkgname := p.parseUnquotedString()
+		pkgpath := p.parseUnquotedString()
+		p.getPkg(pkgpath, pkgname)
+		p.expectEOL()
+
+	case "types":
+		p.next()
+		p.parseTypes(p.pkg)
+		p.expectEOL()
+
+	case "func":
+		p.next()
+		fun := p.parseFunc(p.pkg)
+		if fun != nil {
+			p.pkg.Scope().Insert(fun)
+		}
+		p.expectEOL()
+
+	case "type":
+		p.next()
+		p.parseType(p.pkg)
+		p.expectEOL()
+
+	case "var":
+		p.next()
+		v := p.parseVar(p.pkg)
+		if v != nil {
+			p.pkg.Scope().Insert(v)
+		}
+		p.expectEOL()
+
+	case "const":
+		p.next()
+		c := p.parseConst(p.pkg)
+		p.pkg.Scope().Insert(c)
+		p.expectEOL()
+
+	default:
+		p.errorf("unexpected identifier: %q", p.lit)
+	}
+}
+
+// Package = { Directive } .
+func (p *parser) parsePackage() *types.Package {
+	for p.tok != scanner.EOF {
+		p.parseDirective()
+	}
+	for _, f := range p.fixups {
+		if f.target.Underlying() == nil {
+			p.errorf("internal error: fixup can't be applied, loop required")
+		}
+		f.toUpdate.SetUnderlying(f.target.Underlying())
+	}
+	p.fixups = nil
+	for _, typ := range p.typeList {
+		if it, ok := typ.(*types.Interface); ok {
+			it.Complete()
+		}
+	}
+	p.pkg.MarkComplete()
+	return p.pkg
+}
diff --git a/src/go/internal/gccgoimporter/parser_test.go b/src/go/internal/gccgoimporter/parser_test.go
new file mode 100644
index 0000000..00128b4
--- /dev/null
+++ b/src/go/internal/gccgoimporter/parser_test.go
@@ -0,0 +1,78 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gccgoimporter
+
+import (
+	"bytes"
+	"go/types"
+	"strings"
+	"testing"
+	"text/scanner"
+)
+
+var typeParserTests = []struct {
+	id, typ, want, underlying, methods string
+}{
+	{id: "foo", typ: "<type -1>", want: "int8"},
+	{id: "foo", typ: "<type 1 *<type -19>>", want: "*error"},
+	{id: "foo", typ: "<type 1 *any>", want: "unsafe.Pointer"},
+	{id: "foo", typ: "<type 1 \"Bar\" <type 2 *<type 1>>>", want: "foo.Bar", underlying: "*foo.Bar"},
+	{id: "foo", typ: "<type 1 \"bar.Foo\" \"bar\" <type -1>\nfunc (? <type 1>) M ();\n>", want: "bar.Foo", underlying: "int8", methods: "func (bar.Foo).M()"},
+	{id: "foo", typ: "<type 1 \".bar.foo\" \"bar\" <type -1>>", want: "bar.foo", underlying: "int8"},
+	{id: "foo", typ: "<type 1 []<type -1>>", want: "[]int8"},
+	{id: "foo", typ: "<type 1 [42]<type -1>>", want: "[42]int8"},
+	{id: "foo", typ: "<type 1 map [<type -1>] <type -2>>", want: "map[int8]int16"},
+	{id: "foo", typ: "<type 1 chan <type -1>>", want: "chan int8"},
+	{id: "foo", typ: "<type 1 chan <- <type -1>>", want: "<-chan int8"},
+	{id: "foo", typ: "<type 1 chan -< <type -1>>", want: "chan<- int8"},
+	{id: "foo", typ: "<type 1 struct { I8 <type -1>; I16 <type -2> \"i16\"; }>", want: "struct{I8 int8; I16 int16 \"i16\"}"},
+	{id: "foo", typ: "<type 1 interface { Foo (a <type -1>, b <type -2>) <type -1>; Bar (? <type -2>, ? ...<type -1>) (? <type -2>, ? <type -1>); Baz (); }>", want: "interface{Bar(int16, ...int8) (int16, int8); Baz(); Foo(a int8, b int16) int8}"},
+	{id: "foo", typ: "<type 1 (? <type -1>) <type -2>>", want: "func(int8) int16"},
+}
+
+func TestTypeParser(t *testing.T) {
+	for _, test := range typeParserTests {
+		var p parser
+		p.init("test.gox", strings.NewReader(test.typ), make(map[string]*types.Package))
+		p.version = "v2"
+		p.pkgname = test.id
+		p.pkgpath = test.id
+		p.maybeCreatePackage()
+		typ := p.parseType(p.pkg)
+
+		if p.tok != scanner.EOF {
+			t.Errorf("expected full parse, stopped at %q", p.lit)
+		}
+
+		// interfaces must be explicitly completed
+		if ityp, _ := typ.(*types.Interface); ityp != nil {
+			ityp.Complete()
+		}
+
+		got := typ.String()
+		if got != test.want {
+			t.Errorf("got type %q, expected %q", got, test.want)
+		}
+
+		if test.underlying != "" {
+			underlying := typ.Underlying().String()
+			if underlying != test.underlying {
+				t.Errorf("got underlying type %q, expected %q", underlying, test.underlying)
+			}
+		}
+
+		if test.methods != "" {
+			nt := typ.(*types.Named)
+			var buf bytes.Buffer
+			for i := 0; i != nt.NumMethods(); i++ {
+				buf.WriteString(nt.Method(i).String())
+			}
+			methods := buf.String()
+			if methods != test.methods {
+				t.Errorf("got methods %q, expected %q", methods, test.methods)
+			}
+		}
+	}
+}
diff --git a/src/go/internal/gccgoimporter/testdata/aliases.go b/src/go/internal/gccgoimporter/testdata/aliases.go
new file mode 100644
index 0000000..cfb59b3
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/aliases.go
@@ -0,0 +1,65 @@
+package aliases
+
+type (
+	T0 [10]int
+	T1 []byte
+	T2 struct {
+		x int
+	}
+	T3 interface {
+		m() T2
+	}
+	T4 func(int, T0) chan T2
+)
+
+// basic aliases
+type (
+	Ai = int
+	A0 = T0
+	A1 = T1
+	A2 = T2
+	A3 = T3
+	A4 = T4
+
+	A10 = [10]int
+	A11 = []byte
+	A12 = struct {
+		x int
+	}
+	A13 = interface {
+		m() A2
+	}
+	A14 = func(int, A0) chan A2
+)
+
+// alias receiver types
+func (T0) m1() {}
+func (A0) m2() {}
+
+// alias receiver types (long type declaration chains)
+type (
+	V0 = V1
+	V1 = (V2)
+	V2 = (V3)
+	V3 = T0
+)
+
+func (V1) n() {}
+
+// cycles
+type C0 struct {
+	f1 C1
+	f2 C2
+}
+
+type (
+	C1 *C0
+	C2 = C1
+)
+
+type (
+	C5 struct {
+		f *C6
+	}
+	C6 = C5
+)
diff --git a/src/go/internal/gccgoimporter/testdata/aliases.gox b/src/go/internal/gccgoimporter/testdata/aliases.gox
new file mode 100644
index 0000000..2428c06
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/aliases.gox
@@ -0,0 +1,33 @@
+v2;
+package aliases;
+prefix go;
+package aliases go.aliases go.aliases;
+type <type 1 "A0" = <type 2 "T0" <type 3 [10 ] <type -11>>
+ func (? <esc:0x1> <type 2>) .go.aliases.m1 ();
+ func (? <esc:0x1> <type 1>) .go.aliases.m2 ();
+ func (? <esc:0x1> <type 4 "V1" = <type 5 "V2" = <type 6 "V3" = <type 2>>>>) .go.aliases.n ();
+>>;
+type <type 7 "A1" = <type 8 "T1" <type 9 [] <type -20>>>>;
+type <type 10 "A10" = <type 11 [10 ] <type -11>>>;
+type <type 12 "A11" = <type 13 [] <type -20>>>;
+type <type 14 "A12" = <type 15 struct { .go.aliases.x <type -11>; }>>;
+type <type 16 "A13" = <type 17 interface { .go.aliases.m () <type 18 "A2" = <type 19 "T2" <type 20 struct { .go.aliases.x <type -11>; }>>>; }>>;
+type <type 21 "A14" = <type 22 (? <type -11>, ? <type 1>) <type 23 chan <type 18>>>>;
+type <type 18>;
+type <type 24 "A3" = <type 25 "T3" <type 26 interface { .go.aliases.m () <type 19>; }>>>;
+type <type 27 "A4" = <type 28 "T4" <type 29 (? <type -11>, ? <type 2>) <type 30 chan <type 19>>>>>;
+type <type 31 "Ai" = <type -11>>;
+type <type 32 "C0" <type 33 struct { .go.aliases.f1 <type 34 "C1" <type 35 *<type 32>>>; .go.aliases.f2 <type 36 "C2" = <type 34>>; }>>;
+type <type 34>;
+type <type 36>;
+type <type 37 "C5" <type 38 struct { .go.aliases.f <type 39 *<type 40 "C6" = <type 37>>>; }>>;
+type <type 40>;
+type <type 2>;
+type <type 8>;
+type <type 19>;
+type <type 25>;
+type <type 28>;
+type <type 41 "V0" = <type 4>>;
+type <type 4>;
+type <type 5>;
+type <type 6>;
diff --git a/src/go/internal/gccgoimporter/testdata/complexnums.go b/src/go/internal/gccgoimporter/testdata/complexnums.go
new file mode 100644
index 0000000..a51b6b0
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/complexnums.go
@@ -0,0 +1,6 @@
+package complexnums
+
+const NN = -1 - 1i
+const NP = -1 + 1i
+const PN = 1 - 1i
+const PP = 1 + 1i
diff --git a/src/go/internal/gccgoimporter/testdata/complexnums.gox b/src/go/internal/gccgoimporter/testdata/complexnums.gox
new file mode 100644
index 0000000..b66524f
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/complexnums.gox
@@ -0,0 +1,8 @@
+v1;
+package complexnums;
+pkgpath complexnums;
+priority 1;
+const NN = -0.1E1-0.1E1i ;
+const NP = -0.1E1+0.1E1i ;
+const PN = 0.1E1-0.1E1i ;
+const PP = 0.1E1+0.1E1i ;
diff --git a/src/go/internal/gccgoimporter/testdata/conversions.go b/src/go/internal/gccgoimporter/testdata/conversions.go
new file mode 100644
index 0000000..653927a
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/conversions.go
@@ -0,0 +1,5 @@
+package conversions
+
+type Units string
+
+const Bits = Units("bits")
diff --git a/src/go/internal/gccgoimporter/testdata/conversions.gox b/src/go/internal/gccgoimporter/testdata/conversions.gox
new file mode 100644
index 0000000..7de6cda
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/conversions.gox
@@ -0,0 +1,6 @@
+v2;
+package conversions;
+prefix go;
+package conversions go.conversions go.conversions;
+const Bits <type 1 "Units" <type -16>> = convert(<type 1>, "bits");
+type <type 1>;
diff --git a/src/go/internal/gccgoimporter/testdata/escapeinfo.go b/src/go/internal/gccgoimporter/testdata/escapeinfo.go
new file mode 100644
index 0000000..103ad95
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/escapeinfo.go
@@ -0,0 +1,13 @@
+// Test case for escape info in export data. To compile and extract .gox file:
+// gccgo -fgo-optimize-allocs -c escapeinfo.go
+// objcopy -j .go_export escapeinfo.o escapeinfo.gox
+
+package escapeinfo
+
+type T struct{ data []byte }
+
+func NewT(data []byte) *T {
+	return &T{data}
+}
+
+func (*T) Read(p []byte) {}
diff --git a/src/go/internal/gccgoimporter/testdata/escapeinfo.gox b/src/go/internal/gccgoimporter/testdata/escapeinfo.gox
new file mode 100644
index 0000000..1db8156
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/escapeinfo.gox
diff --git a/src/go/internal/gccgoimporter/testdata/imports.go b/src/go/internal/gccgoimporter/testdata/imports.go
new file mode 100644
index 0000000..7907316
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/imports.go
@@ -0,0 +1,5 @@
+package imports
+
+import "fmt"
+
+var Hello = fmt.Sprintf("Hello, world")
diff --git a/src/go/internal/gccgoimporter/testdata/imports.gox b/src/go/internal/gccgoimporter/testdata/imports.gox
new file mode 100644
index 0000000..958a4f5
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/imports.gox
@@ -0,0 +1,7 @@
+v1;
+package imports;
+pkgpath imports;
+priority 7;
+import fmt fmt "fmt";
+init imports imports..import 7 math math..import 1 runtime runtime..import 1 strconv strconv..import 2 io io..import 3 reflect reflect..import 3 syscall syscall..import 3 time time..import 4 os os..import 5 fmt fmt..import 6;
+var Hello <type -16>;
diff --git a/src/go/internal/gccgoimporter/testdata/issue27856.go b/src/go/internal/gccgoimporter/testdata/issue27856.go
new file mode 100644
index 0000000..bf361e1
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/issue27856.go
@@ -0,0 +1,9 @@
+package lib
+
+type M struct {
+	E E
+}
+type F struct {
+	_ *M
+}
+type E = F
diff --git a/src/go/internal/gccgoimporter/testdata/issue27856.gox b/src/go/internal/gccgoimporter/testdata/issue27856.gox
new file mode 100644
index 0000000..6665e64
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/issue27856.gox
@@ -0,0 +1,9 @@
+v2;
+package main;
+pkgpath main;
+import runtime runtime "runtime";
+init runtime runtime..import sys runtime_internal_sys..import;
+init_graph 0 1;
+type <type 1 "E" = <type 2 "F" <type 3 struct { .main._ <type 4 *<type 5 "M" <type 6 struct { E <type 1>; }>>>; }>>>;
+type <type 2>;
+type <type 5>;
diff --git a/src/go/internal/gccgoimporter/testdata/issue29198.go b/src/go/internal/gccgoimporter/testdata/issue29198.go
new file mode 100644
index 0000000..75c2162
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/issue29198.go
@@ -0,0 +1,37 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package server
+
+import (
+	"context"
+	"errors"
+)
+
+type A struct {
+	x int
+}
+
+func (a *A) AMethod(y int) *Server {
+	return nil
+}
+
+// FooServer is a server that provides Foo services
+type FooServer Server
+
+func (f *FooServer) WriteEvents(ctx context.Context, x int) error {
+	return errors.New("hey!")
+}
+
+type Server struct {
+	FooServer *FooServer
+	user      string
+	ctx       context.Context
+}
+
+func New(sctx context.Context, u string) (*Server, error) {
+	s := &Server{user: u, ctx: sctx}
+	s.FooServer = (*FooServer)(s)
+	return s, nil
+}
diff --git a/src/go/internal/gccgoimporter/testdata/issue29198.gox b/src/go/internal/gccgoimporter/testdata/issue29198.gox
new file mode 100644
index 0000000..905c866
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/issue29198.gox
@@ -0,0 +1,86 @@
+v2;
+package server;
+pkgpath issue29198;
+import context context "context";
+import errors errors "errors";
+init context context..import fmt fmt..import poll internal_poll..import testlog internal_testlog..import io io..import os os..import reflect reflect..import runtime runtime..import sys runtime_internal_sys..import strconv strconv..import sync sync..import syscall syscall..import time time..import unicode unicode..import;
+init_graph 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 10 0 11 0 12 0 13 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 1 10 1 11 1 12 1 13 2 4 2 7 2 8 2 10 2 11 2 12 4 7 4 8 4 10 5 2 5 3 5 4 5 7 5 8 5 10 5 11 5 12 6 7 6 8 6 9 6 10 6 13 7 8 9 7 9 8 10 7 10 8 11 7 11 8 11 10 12 7 12 8 12 10 12 11;
+type <type 1 "A" <type 2 struct { .issue29198.x <type -11>; }>
+ func (a <esc:0x1> <type 3 *<type 1>>) AMethod (y <type -11>) <type 4 *<type 5 "Server" <type 6 struct { FooServer <type 7 *<type 8 "FooServer" <type 5>
+ func (f <esc:0x1> <type 9 *<type 8>>) WriteEvents (ctx <esc:0x1> <type 10 "context.Context" <type 11 interface { Deadline () (deadline <type 12 "time.Time" "time" <type 13 struct { .time.wall <type -8>; .time.ext <type -4>; .time.loc <type 14 *<type 15 "time.Location" <type 16 struct { .time.name <type -16>; .time.zone <type 17 [] <type 18 ".time.zone" <type 19 struct { .time.name <type -16>; .time.offset <type -11>; .time.isDST <type -15>; }>>>; .time.tx <type 20 [] <type 21 ".time.zoneTrans" <type 22 struct { .time.when <type -4>; .time.index <type -5>; .time.isstd <type -15>; .time.isutc <type -15>; }>>>; .time.cacheStart <type -4>; .time.cacheEnd <type -4>; .time.cacheZone <type 23 *<type 18>>; }>
+ func (l <esc:0x22> <type 24 *<type 15>>) String () <type -16>;
+ func (l <esc:0x1> <type 24>) .time.lookupFirstZone () <type -11>;
+ func (l <esc:0x12> <type 24>) .time.get () <type 24>;
+ func (l <esc:0x32> <type 24>) .time.lookup (sec <type -4>) (name <type -16>, offset <type -11>, isDST <type -15>, start <type -4>, end <type -4>);
+ func (l <esc:0x1> <type 24>) .time.lookupName (name <esc:0x1> <type -16>, unix <type -4>) (offset <type -11>, ok <type -15>);
+ func (l <esc:0x1> <type 24>) .time.firstZoneUsed () <type -15>;
+>>; }>
+ func (t <esc:0x12> <type 12>) In (loc <type 14>) <type 12>;
+ func (t <esc:0x1> <type 12>) .time.date (full <type -15>) (year <type -11>, month <type 25 "time.Month" <type -11>
+ func (m <type 25>) String () <type -16>;
+>, day <type -11>, yday <type -11>);
+ func (t <esc:0x1> <type 12>) Sub (u <esc:0x1> <type 12>) <type 26 "time.Duration" <type -4>
+ func (d <type 26>) Truncate (m <type 26>) <type 26>;
+ func (d <type 26>) String () <type -16>;
+ func (d <type 26>) Round (m <type 26>) <type 26>;
+ func (d <type 26>) Seconds () <type -10>;
+ func (d <type 26>) Nanoseconds () <type -4>;
+ func (d <type 26>) Minutes () <type -10>;
+ func (d <type 26>) Hours () <type -10>;
+>;
+ func (t <esc:0x12> <type 12>) Add (d <type 26>) <type 12>;
+ func (t <esc:0x12> <type 12>) UTC () <type 12>;
+ func (t <type 12>) AddDate (years <type -11>, months <type -11>, days <type -11>) <type 12>;
+ func (t <esc:0x1> <type 12>) MarshalBinary () (? <type 27 [] <type -20>>, ? <type -19>);
+ func (t <esc:0x1> <type 12>) Nanosecond () <type -11>;
+ func (t <esc:0x12> <type 12>) Round (d <type 26>) <type 12>;
+ func (t <esc:0x1> <type 12>) Minute () <type -11>;
+ func (t <esc:0x1> <type 12>) Clock () (hour <type -11>, min <type -11>, sec <type -11>);
+ func (t <esc:0x1> <type 12>) ISOWeek () (year <type -11>, week <type -11>);
+ func (t <esc:0x1> <type 12>) Day () <type -11>;
+ func (t <esc:0x1> <type 28 *<type 12>>) .time.mono () <type -4>;
+ func (t <esc:0x1> <type 12>) UnixNano () <type -4>;
+ func (t <esc:0x1> <type 28>) .time.sec () <type -4>;
+ func (t <esc:0x1> <type 12>) Second () <type -11>;
+ func (t <esc:0x1> <type 12>) Before (u <esc:0x1> <type 12>) <type -15>;
+ func (t <esc:0x1> <type 28>) UnmarshalBinary (data <esc:0x1> <type 29 [] <type -20>>) <type -19>;
+ func (t <esc:0x1> <type 12>) Month () <type 25>;
+ func (t <esc:0x1> <type 12>) YearDay () <type -11>;
+ func (t <esc:0x12> <type 12>) Location () <type 14>;
+ func (t <esc:0x32> <type 12>) Zone () (name <type -16>, offset <type -11>);
+ func (t <esc:0x12> <type 12>) Local () <type 12>;
+ func (t <esc:0x1> <type 28>) .time.setLoc (loc <type 14>);
+ func (t <esc:0x12> <type 12>) Truncate (d <type 26>) <type 12>;
+ func (t <esc:0x1> <type 12>) MarshalJSON () (? <type 30 [] <type -20>>, ? <type -19>);
+ func (t <esc:0x1> <type 12>) AppendFormat (b <esc:0x12> <type 31 [] <type -20>>, layout <esc:0x1> <type -16>) <type 32 [] <type -20>>;
+ func (t <esc:0x1> <type 28>) GobDecode (data <esc:0x1> <type 33 [] <type -20>>) <type -19>;
+ func (t <esc:0x1> <type 28>) UnmarshalJSON (data <esc:0x1> <type 34 [] <type -20>>) <type -19>;
+ func (t <esc:0x1> <type 12>) MarshalText () (? <type 35 [] <type -20>>, ? <type -19>);
+ func (t <esc:0x1> <type 12>) GobEncode () (? <type 36 [] <type -20>>, ? <type -19>);
+ func (t <esc:0x1> <type 28>) .time.stripMono ();
+ func (t <esc:0x1> <type 12>) After (u <esc:0x1> <type 12>) <type -15>;
+ func (t <esc:0x1> <type 12>) Hour () <type -11>;
+ func (t <esc:0x1> <type 28>) UnmarshalText (data <esc:0x1> <type 37 [] <type -20>>) <type -19>;
+ func (t <esc:0x1> <type 12>) Equal (u <esc:0x1> <type 12>) <type -15>;
+ func (t <esc:0x1> <type 28>) .time.setMono (m <type -4>);
+ func (t <esc:0x1> <type 12>) Year () <type -11>;
+ func (t <esc:0x1> <type 12>) IsZero () <type -15>;
+ func (t <esc:0x1> <type 28>) .time.addSec (d <type -4>);
+ func (t <esc:0x1> <type 12>) Weekday () <type 38 "time.Weekday" <type -11>
+ func (d <type 38>) String () <type -16>;
+>;
+ func (t <esc:0x1> <type 12>) String () <type -16>;
+ func (t <esc:0x1> <type 28>) .time.nsec () <type -3>;
+ func (t <esc:0x1> <type 12>) Format (layout <esc:0x1> <type -16>) <type -16>;
+ func (t <esc:0x1> <type 28>) .time.unixSec () <type -4>;
+ func (t <esc:0x1> <type 12>) Unix () <type -4>;
+ func (t <esc:0x1> <type 12>) .time.abs () <type -8>;
+ func (t <esc:0x32> <type 12>) .time.locabs () (name <type -16>, offset <type -11>, abs <type -8>);
+ func (t <esc:0x1> <type 12>) Date () (year <type -11>, month <type 25>, day <type -11>);
+>, ok <type -15>); Done () <type 39 chan <- <type 40 struct { }>>; Err () <type -19>; Value (key <type 41 interface { }>) <type 42 interface { }>; }>>, x <type -11>) <type -19>;
+>>; .issue29198.user <type -16>; .issue29198.ctx <type 10>; }>>>;
+>;
+type <type 8>;
+func New (sctx <type 10>, u <type -16>) (? <type 43 *<type 5>>, ? <type -19>);
+type <type 5>;
+checksum 86C8D76B2582F55A8BD2CA9E00060358EC1CE214;
diff --git a/src/go/internal/gccgoimporter/testdata/issue30628.go b/src/go/internal/gccgoimporter/testdata/issue30628.go
new file mode 100644
index 0000000..8fd7c13
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/issue30628.go
@@ -0,0 +1,18 @@
+package issue30628
+
+import (
+	"os"
+	"sync"
+)
+
+const numR = int32(os.O_TRUNC + 5)
+
+type Apple struct {
+	hey sync.RWMutex
+	x   int
+	RQ  [numR]struct {
+		Count    uintptr
+		NumBytes uintptr
+		Last     uintptr
+	}
+}
diff --git a/src/go/internal/gccgoimporter/testdata/issue30628.gox b/src/go/internal/gccgoimporter/testdata/issue30628.gox
new file mode 100644
index 0000000..0ff6259
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/issue30628.gox
@@ -0,0 +1,28 @@
+v3;
+package issue30628
+pkgpath issue30628
+import os os "os"
+import sync sync "sync"
+init cpu internal..z2fcpu..import poll internal..z2fpoll..import testlog internal..z2ftestlog..import io io..import os os..import runtime runtime..import sys runtime..z2finternal..z2fsys..import sync sync..import syscall syscall..import time time..import
+init_graph 1 0 1 3 1 5 1 6 1 7 1 8 1 9 3 0 3 5 3 6 3 7 4 0 4 1 4 2 4 3 4 5 4 6 4 7 4 8 4 9 5 0 5 6 7 0 7 5 7 6 8 0 8 5 8 6 8 7 9 0 9 5 9 6 9 7 9 8
+types 13 2 24 84 208 17 30 41 147 86 17 64 25 75
+type 1 "Apple" <type 2>
+type 2 struct { .issue30628.hey <type 3>; .issue30628.x <type -11>; RQ <type 11>; }
+type 3 "sync.RWMutex" <type 7>
+ func (rw <type 4>) Lock ()
+ func (rw <esc:0x12> <type 4>) RLocker () ($ret8 <type 5>)
+ func (rw <type 4>) RUnlock ()
+ func (rw <type 4>) Unlock ()
+ func (rw <type 4>) RLock ()
+type 4 *<type 3>
+type 5 "sync.Locker" <type 6>
+type 6 interface { Lock (); Unlock (); }
+type 7 struct { .sync.w <type 8>; .sync.writerSem <type -7>; .sync.readerSem <type -7>; .sync.readerCount <type -3>; .sync.readerWait <type -3>; }
+type 8 "sync.Mutex" <type 10>
+ func (m <type 9>) Unlock ()
+ func (m <type 9>) Lock ()
+type 9 *<type 8>
+type 10 struct { .sync.state <type -3>; .sync.sema <type -7>; }
+type 11 [517 ] <type 12>
+type 12 struct { Count <type -13>; NumBytes <type -13>; Last <type -13>; }
+checksum 199DCF6D3EE2FCF39F715B4E42B5F87F5B15D3AF
diff --git a/src/go/internal/gccgoimporter/testdata/issue31540.go b/src/go/internal/gccgoimporter/testdata/issue31540.go
new file mode 100644
index 0000000..2c6799e
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/issue31540.go
@@ -0,0 +1,26 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue31540
+
+type Y struct {
+	q int
+}
+
+type Z map[int]int
+
+type X = map[Y]Z
+
+type A1 = X
+
+type A2 = A1
+
+type S struct {
+	b int
+	A2
+}
+
+func Hallo() S {
+	return S{}
+}
diff --git a/src/go/internal/gccgoimporter/testdata/issue31540.gox b/src/go/internal/gccgoimporter/testdata/issue31540.gox
new file mode 100644
index 0000000..abdc696
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/issue31540.gox
@@ -0,0 +1,16 @@
+v3;
+package issue31540
+pkgpath issue31540
+types 11 7 23 23 20 22 20 21 57 31 45 36
+type 1 "A1" = <type 4>
+type 2 "A2" = <type 1>
+type 3 "S" <type 7>
+type 4 "X" = <type 8>
+type 5 "Y" <type 9>
+type 6 "Z" <type 10>
+type 7 struct { .go.mapalias.b <type -11>; ? <type 2>; }
+type 8 map [<type 5>] <type 6>
+type 9 struct { .go.mapalias.q <type -11>; }
+type 10 map [<type -11>] <type -11>
+func Hallo () <type 3>
+checksum C3FAF2524A90BC11225EE65D059BF27DFB69134B
diff --git a/src/go/internal/gccgoimporter/testdata/issue34182.go b/src/go/internal/gccgoimporter/testdata/issue34182.go
new file mode 100644
index 0000000..2a5c333
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/issue34182.go
@@ -0,0 +1,17 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue34182
+
+type T1 struct {
+	f *T2
+}
+
+type T2 struct {
+	f T3
+}
+
+type T3 struct {
+	*T2
+}
diff --git a/src/go/internal/gccgoimporter/testdata/issue34182.gox b/src/go/internal/gccgoimporter/testdata/issue34182.gox
new file mode 100644
index 0000000..671a7d6
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/issue34182.gox
@@ -0,0 +1,13 @@
+v3;
+package issue34182
+pkgpath issue34182
+init issue34182 ~go.issue34182
+types 8 4 21 21 21 17 30 45 45
+type 1 "T1" <type 6>
+type 2 "T2" <type 7>
+type 3 "T3" <type 5>
+type 4 *<type 2>
+type 5 struct { ? <type 4>; }
+type 6 struct { .go.issue34182.f <type 4>; }
+type 7 struct { .go.issue34182.f <type 3>; }
+checksum FF02C49BAF44B06C087ED4E573F7CC880C79C208
diff --git a/src/go/internal/gccgoimporter/testdata/libimportsar.a b/src/go/internal/gccgoimporter/testdata/libimportsar.a
new file mode 100644
index 0000000..6f30758
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/libimportsar.a
diff --git a/src/go/internal/gccgoimporter/testdata/nointerface.go b/src/go/internal/gccgoimporter/testdata/nointerface.go
new file mode 100644
index 0000000..6a545f2
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/nointerface.go
@@ -0,0 +1,12 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package nointerface
+
+type I int
+
+//go:nointerface
+func (p *I) Get() int { return int(*p) }
+
+func (p *I) Set(v int) { *p = I(v) }
diff --git a/src/go/internal/gccgoimporter/testdata/nointerface.gox b/src/go/internal/gccgoimporter/testdata/nointerface.gox
new file mode 100644
index 0000000..7b73d17
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/nointerface.gox
@@ -0,0 +1,8 @@
+v3;
+package nointerface
+pkgpath nointerface
+types 3 2 133 17
+type 1 "I" <type -11>
+ func /*nointerface*/ (p <esc:0x1> <type 2>) Get () <type -11>
+ func (p <esc:0x1> <type 2>) Set (v <type -11>)
+type 2 *<type 1>
diff --git a/src/go/internal/gccgoimporter/testdata/notinheap.go b/src/go/internal/gccgoimporter/testdata/notinheap.go
new file mode 100644
index 0000000..b1ac967
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/notinheap.go
@@ -0,0 +1,4 @@
+package notinheap
+
+//go:notinheap
+type S struct{}
diff --git a/src/go/internal/gccgoimporter/testdata/notinheap.gox b/src/go/internal/gccgoimporter/testdata/notinheap.gox
new file mode 100644
index 0000000..cc438e7
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/notinheap.gox
@@ -0,0 +1,7 @@
+v3;
+package notinheap
+pkgpath notinheap
+init notinheap ~notinheap
+types 3 2 30 18
+type 1 "S" notinheap <type 2>
+type 2 struct { }
diff --git a/src/go/internal/gccgoimporter/testdata/pointer.go b/src/go/internal/gccgoimporter/testdata/pointer.go
new file mode 100644
index 0000000..4ebc671
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/pointer.go
@@ -0,0 +1,3 @@
+package pointer
+
+type Int8Ptr *int8
diff --git a/src/go/internal/gccgoimporter/testdata/pointer.gox b/src/go/internal/gccgoimporter/testdata/pointer.gox
new file mode 100644
index 0000000..d96ebbd
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/pointer.gox
@@ -0,0 +1,4 @@
+v1;
+package pointer;
+pkgpath pointer;
+type <type 1 "Int8Ptr" <type 2 *<type -1>>>;
diff --git a/src/go/internal/gccgoimporter/testdata/time.gox b/src/go/internal/gccgoimporter/testdata/time.gox
new file mode 100644
index 0000000..80c2dbc
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/time.gox
diff --git a/src/go/internal/gccgoimporter/testdata/unicode.gox b/src/go/internal/gccgoimporter/testdata/unicode.gox
new file mode 100644
index 0000000..e70e539
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/unicode.gox
diff --git a/src/go/internal/gccgoimporter/testdata/v1reflect.gox b/src/go/internal/gccgoimporter/testdata/v1reflect.gox
new file mode 100644
index 0000000..ea46841
--- /dev/null
+++ b/src/go/internal/gccgoimporter/testdata/v1reflect.gox
diff --git a/src/go/internal/gcimporter/exportdata.go b/src/go/internal/gcimporter/exportdata.go
new file mode 100644
index 0000000..c12e459
--- /dev/null
+++ b/src/go/internal/gcimporter/exportdata.go
@@ -0,0 +1,91 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements FindExportData.
+
+package gcimporter
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"strconv"
+	"strings"
+)
+
+func readGopackHeader(r *bufio.Reader) (name string, size int, err error) {
+	// See $GOROOT/include/ar.h.
+	hdr := make([]byte, 16+12+6+6+8+10+2)
+	_, err = io.ReadFull(r, hdr)
+	if err != nil {
+		return
+	}
+	// leave for debugging
+	if false {
+		fmt.Printf("header: %s", hdr)
+	}
+	s := strings.TrimSpace(string(hdr[16+12+6+6+8:][:10]))
+	size, err = strconv.Atoi(s)
+	if err != nil || hdr[len(hdr)-2] != '`' || hdr[len(hdr)-1] != '\n' {
+		err = fmt.Errorf("invalid archive header")
+		return
+	}
+	name = strings.TrimSpace(string(hdr[:16]))
+	return
+}
+
+// FindExportData positions the reader r at the beginning of the
+// export data section of an underlying GC-created object/archive
+// file by reading from it. The reader must be positioned at the
+// start of the file before calling this function. The hdr result
+// is the string before the export data, either "$$" or "$$B".
+//
+func FindExportData(r *bufio.Reader) (hdr string, err error) {
+	// Read first line to make sure this is an object file.
+	line, err := r.ReadSlice('\n')
+	if err != nil {
+		err = fmt.Errorf("can't find export data (%v)", err)
+		return
+	}
+
+	if string(line) == "!<arch>\n" {
+		// Archive file. Scan to __.PKGDEF.
+		var name string
+		if name, _, err = readGopackHeader(r); err != nil {
+			return
+		}
+
+		// First entry should be __.PKGDEF.
+		if name != "__.PKGDEF" {
+			err = fmt.Errorf("go archive is missing __.PKGDEF")
+			return
+		}
+
+		// Read first line of __.PKGDEF data, so that line
+		// is once again the first line of the input.
+		if line, err = r.ReadSlice('\n'); err != nil {
+			err = fmt.Errorf("can't find export data (%v)", err)
+			return
+		}
+	}
+
+	// Now at __.PKGDEF in archive or still at beginning of file.
+	// Either way, line should begin with "go object ".
+	if !strings.HasPrefix(string(line), "go object ") {
+		err = fmt.Errorf("not a Go object file")
+		return
+	}
+
+	// Skip over object header to export data.
+	// Begins after first line starting with $$.
+	for line[0] != '$' {
+		if line, err = r.ReadSlice('\n'); err != nil {
+			err = fmt.Errorf("can't find export data (%v)", err)
+			return
+		}
+	}
+	hdr = string(line)
+
+	return
+}
diff --git a/src/go/internal/gcimporter/gcimporter.go b/src/go/internal/gcimporter/gcimporter.go
new file mode 100644
index 0000000..b74daca
--- /dev/null
+++ b/src/go/internal/gcimporter/gcimporter.go
@@ -0,0 +1,174 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package gcimporter implements Import for gc-generated object files.
+package gcimporter // import "go/internal/gcimporter"
+
+import (
+	"bufio"
+	"fmt"
+	"go/build"
+	"go/token"
+	"go/types"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// debugging/development support
+const debug = false
+
+var pkgExts = [...]string{".a", ".o"}
+
+// FindPkg returns the filename and unique package id for an import
+// path based on package information provided by build.Import (using
+// the build.Default build.Context). A relative srcDir is interpreted
+// relative to the current working directory.
+// If no file was found, an empty filename is returned.
+//
+func FindPkg(path, srcDir string) (filename, id string) {
+	if path == "" {
+		return
+	}
+
+	var noext string
+	switch {
+	default:
+		// "x" -> "$GOPATH/pkg/$GOOS_$GOARCH/x.ext", "x"
+		// Don't require the source files to be present.
+		if abs, err := filepath.Abs(srcDir); err == nil { // see issue 14282
+			srcDir = abs
+		}
+		bp, _ := build.Import(path, srcDir, build.FindOnly|build.AllowBinary)
+		if bp.PkgObj == "" {
+			id = path // make sure we have an id to print in error message
+			return
+		}
+		noext = strings.TrimSuffix(bp.PkgObj, ".a")
+		id = bp.ImportPath
+
+	case build.IsLocalImport(path):
+		// "./x" -> "/this/directory/x.ext", "/this/directory/x"
+		noext = filepath.Join(srcDir, path)
+		id = noext
+
+	case filepath.IsAbs(path):
+		// for completeness only - go/build.Import
+		// does not support absolute imports
+		// "/x" -> "/x.ext", "/x"
+		noext = path
+		id = path
+	}
+
+	if false { // for debugging
+		if path != id {
+			fmt.Printf("%s -> %s\n", path, id)
+		}
+	}
+
+	// try extensions
+	for _, ext := range pkgExts {
+		filename = noext + ext
+		if f, err := os.Stat(filename); err == nil && !f.IsDir() {
+			return
+		}
+	}
+
+	filename = "" // not found
+	return
+}
+
+// Import imports a gc-generated package given its import path and srcDir, adds
+// the corresponding package object to the packages map, and returns the object.
+// The packages map must contain all packages already imported.
+//
+func Import(fset *token.FileSet, packages map[string]*types.Package, path, srcDir string, lookup func(path string) (io.ReadCloser, error)) (pkg *types.Package, err error) {
+	var rc io.ReadCloser
+	var id string
+	if lookup != nil {
+		// With custom lookup specified, assume that caller has
+		// converted path to a canonical import path for use in the map.
+		if path == "unsafe" {
+			return types.Unsafe, nil
+		}
+		id = path
+
+		// No need to re-import if the package was imported completely before.
+		if pkg = packages[id]; pkg != nil && pkg.Complete() {
+			return
+		}
+		f, err := lookup(path)
+		if err != nil {
+			return nil, err
+		}
+		rc = f
+	} else {
+		var filename string
+		filename, id = FindPkg(path, srcDir)
+		if filename == "" {
+			if path == "unsafe" {
+				return types.Unsafe, nil
+			}
+			return nil, fmt.Errorf("can't find import: %q", id)
+		}
+
+		// no need to re-import if the package was imported completely before
+		if pkg = packages[id]; pkg != nil && pkg.Complete() {
+			return
+		}
+
+		// open file
+		f, err := os.Open(filename)
+		if err != nil {
+			return nil, err
+		}
+		defer func() {
+			if err != nil {
+				// add file name to error
+				err = fmt.Errorf("%s: %v", filename, err)
+			}
+		}()
+		rc = f
+	}
+	defer rc.Close()
+
+	var hdr string
+	buf := bufio.NewReader(rc)
+	if hdr, err = FindExportData(buf); err != nil {
+		return
+	}
+
+	switch hdr {
+	case "$$\n":
+		err = fmt.Errorf("import %q: old textual export format no longer supported (recompile library)", path)
+
+	case "$$B\n":
+		var data []byte
+		data, err = io.ReadAll(buf)
+		if err != nil {
+			break
+		}
+
+		// The indexed export format starts with an 'i'; the older
+		// binary export format starts with a 'c', 'd', or 'v'
+		// (from "version"). Select appropriate importer.
+		if len(data) > 0 && data[0] == 'i' {
+			_, pkg, err = iImportData(fset, packages, data[1:], id)
+		} else {
+			err = fmt.Errorf("import %q: old binary export format no longer supported (recompile library)", path)
+		}
+
+	default:
+		err = fmt.Errorf("import %q: unknown export data header: %q", path, hdr)
+	}
+
+	return
+}
+
+type byPath []*types.Package
+
+func (a byPath) Len() int           { return len(a) }
+func (a byPath) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+func (a byPath) Less(i, j int) bool { return a[i].Path() < a[j].Path() }
diff --git a/src/go/internal/gcimporter/gcimporter_test.go b/src/go/internal/gcimporter/gcimporter_test.go
new file mode 100644
index 0000000..3c76aaf
--- /dev/null
+++ b/src/go/internal/gcimporter/gcimporter_test.go
@@ -0,0 +1,619 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gcimporter
+
+import (
+	"bytes"
+	"fmt"
+	"internal/testenv"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+
+	"go/token"
+	"go/types"
+)
+
+// skipSpecialPlatforms causes the test to be skipped for platforms where
+// builders (build.golang.org) don't have access to compiled packages for
+// import.
+func skipSpecialPlatforms(t *testing.T) {
+	switch platform := runtime.GOOS + "-" + runtime.GOARCH; platform {
+	case "darwin-arm64":
+		t.Skipf("no compiled packages available for import on %s", platform)
+	}
+}
+
+// compile runs the compiler on filename, with dirname as the working directory,
+// and writes the output file to outdirname.
+func compile(t *testing.T, dirname, filename, outdirname string) string {
+	// filename must end with ".go"
+	if !strings.HasSuffix(filename, ".go") {
+		t.Fatalf("filename doesn't end in .go: %s", filename)
+	}
+	basename := filepath.Base(filename)
+	outname := filepath.Join(outdirname, basename[:len(basename)-2]+"o")
+	cmd := exec.Command(testenv.GoToolPath(t), "tool", "compile", "-o", outname, filename)
+	cmd.Dir = dirname
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Logf("%s", out)
+		t.Fatalf("go tool compile %s failed: %s", filename, err)
+	}
+	return outname
+}
+
+func testPath(t *testing.T, path, srcDir string) *types.Package {
+	t0 := time.Now()
+	fset := token.NewFileSet()
+	pkg, err := Import(fset, make(map[string]*types.Package), path, srcDir, nil)
+	if err != nil {
+		t.Errorf("testPath(%s): %s", path, err)
+		return nil
+	}
+	t.Logf("testPath(%s): %v", path, time.Since(t0))
+	return pkg
+}
+
+const maxTime = 30 * time.Second
+
+func testDir(t *testing.T, dir string, endTime time.Time) (nimports int) {
+	dirname := filepath.Join(runtime.GOROOT(), "pkg", runtime.GOOS+"_"+runtime.GOARCH, dir)
+	list, err := os.ReadDir(dirname)
+	if err != nil {
+		t.Fatalf("testDir(%s): %s", dirname, err)
+	}
+	for _, f := range list {
+		if time.Now().After(endTime) {
+			t.Log("testing time used up")
+			return
+		}
+		switch {
+		case !f.IsDir():
+			// try extensions
+			for _, ext := range pkgExts {
+				if strings.HasSuffix(f.Name(), ext) {
+					name := f.Name()[0 : len(f.Name())-len(ext)] // remove extension
+					if testPath(t, filepath.Join(dir, name), dir) != nil {
+						nimports++
+					}
+				}
+			}
+		case f.IsDir():
+			nimports += testDir(t, filepath.Join(dir, f.Name()), endTime)
+		}
+	}
+	return
+}
+
+func mktmpdir(t *testing.T) string {
+	tmpdir, err := os.MkdirTemp("", "gcimporter_test")
+	if err != nil {
+		t.Fatal("mktmpdir:", err)
+	}
+	if err := os.Mkdir(filepath.Join(tmpdir, "testdata"), 0700); err != nil {
+		os.RemoveAll(tmpdir)
+		t.Fatal("mktmpdir:", err)
+	}
+	return tmpdir
+}
+
+func TestImportTestdata(t *testing.T) {
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	tmpdir := mktmpdir(t)
+	defer os.RemoveAll(tmpdir)
+
+	compile(t, "testdata", "exports.go", filepath.Join(tmpdir, "testdata"))
+
+	if pkg := testPath(t, "./testdata/exports", tmpdir); pkg != nil {
+		// The package's Imports list must include all packages
+		// explicitly imported by exports.go, plus all packages
+		// referenced indirectly via exported objects in exports.go.
+		// With the textual export format, the list may also include
+		// additional packages that are not strictly required for
+		// import processing alone (they are exported to err "on
+		// the safe side").
+		// TODO(gri) update the want list to be precise, now that
+		// the textual export data is gone.
+		got := fmt.Sprint(pkg.Imports())
+		for _, want := range []string{"go/ast", "go/token"} {
+			if !strings.Contains(got, want) {
+				t.Errorf(`Package("exports").Imports() = %s, does not contain %s`, got, want)
+			}
+		}
+	}
+}
+
+func TestVersionHandling(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	const dir = "./testdata/versions"
+	list, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tmpdir := mktmpdir(t)
+	defer os.RemoveAll(tmpdir)
+	corruptdir := filepath.Join(tmpdir, "testdata", "versions")
+	if err := os.Mkdir(corruptdir, 0700); err != nil {
+		t.Fatal(err)
+	}
+
+	fset := token.NewFileSet()
+
+	for _, f := range list {
+		name := f.Name()
+		if !strings.HasSuffix(name, ".a") {
+			continue // not a package file
+		}
+		if strings.Contains(name, "corrupted") {
+			continue // don't process a leftover corrupted file
+		}
+		pkgpath := "./" + name[:len(name)-2]
+
+		if testing.Verbose() {
+			t.Logf("importing %s", name)
+		}
+
+		// test that export data can be imported
+		_, err := Import(fset, make(map[string]*types.Package), pkgpath, dir, nil)
+		if err != nil {
+			// ok to fail if it fails with a no longer supported error for select files
+			if strings.Contains(err.Error(), "no longer supported") {
+				switch name {
+				case "test_go1.7_0.a", "test_go1.7_1.a",
+					"test_go1.8_4.a", "test_go1.8_5.a",
+					"test_go1.11_6b.a", "test_go1.11_999b.a":
+					continue
+				}
+				// fall through
+			}
+			// ok to fail if it fails with a newer version error for select files
+			if strings.Contains(err.Error(), "newer version") {
+				switch name {
+				case "test_go1.11_999i.a":
+					continue
+				}
+				// fall through
+			}
+			t.Errorf("import %q failed: %v", pkgpath, err)
+			continue
+		}
+
+		// create file with corrupted export data
+		// 1) read file
+		data, err := os.ReadFile(filepath.Join(dir, name))
+		if err != nil {
+			t.Fatal(err)
+		}
+		// 2) find export data
+		i := bytes.Index(data, []byte("\n$$B\n")) + 5
+		j := bytes.Index(data[i:], []byte("\n$$\n")) + i
+		if i < 0 || j < 0 || i > j {
+			t.Fatalf("export data section not found (i = %d, j = %d)", i, j)
+		}
+		// 3) corrupt the data (increment every 7th byte)
+		for k := j - 13; k >= i; k -= 7 {
+			data[k]++
+		}
+		// 4) write the file
+		pkgpath += "_corrupted"
+		filename := filepath.Join(corruptdir, pkgpath) + ".a"
+		os.WriteFile(filename, data, 0666)
+
+		// test that importing the corrupted file results in an error
+		_, err = Import(fset, make(map[string]*types.Package), pkgpath, corruptdir, nil)
+		if err == nil {
+			t.Errorf("import corrupted %q succeeded", pkgpath)
+		} else if msg := err.Error(); !strings.Contains(msg, "version skew") {
+			t.Errorf("import %q error incorrect (%s)", pkgpath, msg)
+		}
+	}
+}
+
+func TestImportStdLib(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	dt := maxTime
+	if testing.Short() && testenv.Builder() == "" {
+		dt = 10 * time.Millisecond
+	}
+	nimports := testDir(t, "", time.Now().Add(dt)) // installed packages
+	t.Logf("tested %d imports", nimports)
+}
+
+var importedObjectTests = []struct {
+	name string
+	want string
+}{
+	// non-interfaces
+	{"crypto.Hash", "type Hash uint"},
+	{"go/ast.ObjKind", "type ObjKind int"},
+	{"go/types.Qualifier", "type Qualifier func(*Package) string"},
+	{"go/types.Comparable", "func Comparable(T Type) bool"},
+	{"math.Pi", "const Pi untyped float"},
+	{"math.Sin", "func Sin(x float64) float64"},
+	{"go/ast.NotNilFilter", "func NotNilFilter(_ string, v reflect.Value) bool"},
+	{"go/internal/gcimporter.FindPkg", "func FindPkg(path string, srcDir string) (filename string, id string)"},
+
+	// interfaces
+	{"context.Context", "type Context interface{Deadline() (deadline time.Time, ok bool); Done() <-chan struct{}; Err() error; Value(key interface{}) interface{}}"},
+	{"crypto.Decrypter", "type Decrypter interface{Decrypt(rand io.Reader, msg []byte, opts DecrypterOpts) (plaintext []byte, err error); Public() PublicKey}"},
+	{"encoding.BinaryMarshaler", "type BinaryMarshaler interface{MarshalBinary() (data []byte, err error)}"},
+	{"io.Reader", "type Reader interface{Read(p []byte) (n int, err error)}"},
+	{"io.ReadWriter", "type ReadWriter interface{Reader; Writer}"},
+	{"go/ast.Node", "type Node interface{End() go/token.Pos; Pos() go/token.Pos}"},
+	{"go/types.Type", "type Type interface{String() string; Underlying() Type}"},
+}
+
+func TestImportedTypes(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	fset := token.NewFileSet()
+	for _, test := range importedObjectTests {
+		s := strings.Split(test.name, ".")
+		if len(s) != 2 {
+			t.Fatal("inconsistent test data")
+		}
+		importPath := s[0]
+		objName := s[1]
+
+		pkg, err := Import(fset, make(map[string]*types.Package), importPath, ".", nil)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+
+		obj := pkg.Scope().Lookup(objName)
+		if obj == nil {
+			t.Errorf("%s: object not found", test.name)
+			continue
+		}
+
+		got := types.ObjectString(obj, types.RelativeTo(pkg))
+		if got != test.want {
+			t.Errorf("%s: got %q; want %q", test.name, got, test.want)
+		}
+
+		if named, _ := obj.Type().(*types.Named); named != nil {
+			verifyInterfaceMethodRecvs(t, named, 0)
+		}
+	}
+}
+
+// verifyInterfaceMethodRecvs verifies that method receiver types
+// are named if the methods belong to a named interface type.
+func verifyInterfaceMethodRecvs(t *testing.T, named *types.Named, level int) {
+	// avoid endless recursion in case of an embedding bug that lead to a cycle
+	if level > 10 {
+		t.Errorf("%s: embeds itself", named)
+		return
+	}
+
+	iface, _ := named.Underlying().(*types.Interface)
+	if iface == nil {
+		return // not an interface
+	}
+
+	// check explicitly declared methods
+	for i := 0; i < iface.NumExplicitMethods(); i++ {
+		m := iface.ExplicitMethod(i)
+		recv := m.Type().(*types.Signature).Recv()
+		if recv == nil {
+			t.Errorf("%s: missing receiver type", m)
+			continue
+		}
+		if recv.Type() != named {
+			t.Errorf("%s: got recv type %s; want %s", m, recv.Type(), named)
+		}
+	}
+
+	// check embedded interfaces (if they are named, too)
+	for i := 0; i < iface.NumEmbeddeds(); i++ {
+		// embedding of interfaces cannot have cycles; recursion will terminate
+		if etype, _ := iface.EmbeddedType(i).(*types.Named); etype != nil {
+			verifyInterfaceMethodRecvs(t, etype, level+1)
+		}
+	}
+}
+
+func TestIssue5815(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	pkg := importPkg(t, "strings", ".")
+
+	scope := pkg.Scope()
+	for _, name := range scope.Names() {
+		obj := scope.Lookup(name)
+		if obj.Pkg() == nil {
+			t.Errorf("no pkg for %s", obj)
+		}
+		if tname, _ := obj.(*types.TypeName); tname != nil {
+			named := tname.Type().(*types.Named)
+			for i := 0; i < named.NumMethods(); i++ {
+				m := named.Method(i)
+				if m.Pkg() == nil {
+					t.Errorf("no pkg for %s", m)
+				}
+			}
+		}
+	}
+}
+
+// Smoke test to ensure that imported methods get the correct package.
+func TestCorrectMethodPackage(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	imports := make(map[string]*types.Package)
+	fset := token.NewFileSet()
+	_, err := Import(fset, imports, "net/http", ".", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	mutex := imports["sync"].Scope().Lookup("Mutex").(*types.TypeName).Type()
+	mset := types.NewMethodSet(types.NewPointer(mutex)) // methods of *sync.Mutex
+	sel := mset.Lookup(nil, "Lock")
+	lock := sel.Obj().(*types.Func)
+	if got, want := lock.Pkg().Path(), "sync"; got != want {
+		t.Errorf("got package path %q; want %q", got, want)
+	}
+}
+
+func TestIssue13566(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	tmpdir := mktmpdir(t)
+	defer os.RemoveAll(tmpdir)
+	testoutdir := filepath.Join(tmpdir, "testdata")
+
+	// b.go needs to be compiled from the output directory so that the compiler can
+	// find the compiled package a. We pass the full path to compile() so that we
+	// don't have to copy the file to that directory.
+	bpath, err := filepath.Abs(filepath.Join("testdata", "b.go"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	compile(t, "testdata", "a.go", testoutdir)
+	compile(t, testoutdir, bpath, testoutdir)
+
+	// import must succeed (test for issue at hand)
+	pkg := importPkg(t, "./testdata/b", tmpdir)
+
+	// make sure all indirectly imported packages have names
+	for _, imp := range pkg.Imports() {
+		if imp.Name() == "" {
+			t.Errorf("no name for %s package", imp.Path())
+		}
+	}
+}
+
+func TestIssue13898(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// import go/internal/gcimporter which imports go/types partially
+	fset := token.NewFileSet()
+	imports := make(map[string]*types.Package)
+	_, err := Import(fset, imports, "go/internal/gcimporter", ".", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// look for go/types package
+	var goTypesPkg *types.Package
+	for path, pkg := range imports {
+		if path == "go/types" {
+			goTypesPkg = pkg
+			break
+		}
+	}
+	if goTypesPkg == nil {
+		t.Fatal("go/types not found")
+	}
+
+	// look for go/types.Object type
+	obj := lookupObj(t, goTypesPkg.Scope(), "Object")
+	typ, ok := obj.Type().(*types.Named)
+	if !ok {
+		t.Fatalf("go/types.Object type is %v; wanted named type", typ)
+	}
+
+	// lookup go/types.Object.Pkg method
+	m, index, indirect := types.LookupFieldOrMethod(typ, false, nil, "Pkg")
+	if m == nil {
+		t.Fatalf("go/types.Object.Pkg not found (index = %v, indirect = %v)", index, indirect)
+	}
+
+	// the method must belong to go/types
+	if m.Pkg().Path() != "go/types" {
+		t.Fatalf("found %v; want go/types", m.Pkg())
+	}
+}
+
+func TestIssue15517(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	tmpdir := mktmpdir(t)
+	defer os.RemoveAll(tmpdir)
+
+	compile(t, "testdata", "p.go", filepath.Join(tmpdir, "testdata"))
+
+	// Multiple imports of p must succeed without redeclaration errors.
+	// We use an import path that's not cleaned up so that the eventual
+	// file path for the package is different from the package path; this
+	// will expose the error if it is present.
+	//
+	// (Issue: Both the textual and the binary importer used the file path
+	// of the package to be imported as key into the shared packages map.
+	// However, the binary importer then used the package path to identify
+	// the imported package to mark it as complete; effectively marking the
+	// wrong package as complete. By using an "unclean" package path, the
+	// file and package path are different, exposing the problem if present.
+	// The same issue occurs with vendoring.)
+	imports := make(map[string]*types.Package)
+	fset := token.NewFileSet()
+	for i := 0; i < 3; i++ {
+		if _, err := Import(fset, imports, "./././testdata/p", tmpdir, nil); err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+func TestIssue15920(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	compileAndImportPkg(t, "issue15920")
+}
+
+func TestIssue20046(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	// "./issue20046".V.M must exist
+	pkg := compileAndImportPkg(t, "issue20046")
+	obj := lookupObj(t, pkg.Scope(), "V")
+	if m, index, indirect := types.LookupFieldOrMethod(obj.Type(), false, nil, "M"); m == nil {
+		t.Fatalf("V.M not found (index = %v, indirect = %v)", index, indirect)
+	}
+}
+func TestIssue25301(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	compileAndImportPkg(t, "issue25301")
+}
+
+func TestIssue25596(t *testing.T) {
+	skipSpecialPlatforms(t)
+
+	// This package only handles gc export data.
+	if runtime.Compiler != "gc" {
+		t.Skipf("gc-built packages not available (compiler = %s)", runtime.Compiler)
+	}
+
+	// On windows, we have to set the -D option for the compiler to avoid having a drive
+	// letter and an illegal ':' in the import path - just skip it (see also issue #3483).
+	if runtime.GOOS == "windows" {
+		t.Skip("avoid dealing with relative paths/drive letters on windows")
+	}
+
+	compileAndImportPkg(t, "issue25596")
+}
+
+func importPkg(t *testing.T, path, srcDir string) *types.Package {
+	fset := token.NewFileSet()
+	pkg, err := Import(fset, make(map[string]*types.Package), path, srcDir, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return pkg
+}
+
+func compileAndImportPkg(t *testing.T, name string) *types.Package {
+	tmpdir := mktmpdir(t)
+	defer os.RemoveAll(tmpdir)
+	compile(t, "testdata", name+".go", filepath.Join(tmpdir, "testdata"))
+	return importPkg(t, "./testdata/"+name, tmpdir)
+}
+
+func lookupObj(t *testing.T, scope *types.Scope, name string) types.Object {
+	if obj := scope.Lookup(name); obj != nil {
+		return obj
+	}
+	t.Fatalf("%s not found", name)
+	return nil
+}
diff --git a/src/go/internal/gcimporter/iimport.go b/src/go/internal/gcimporter/iimport.go
new file mode 100644
index 0000000..c59dd16
--- /dev/null
+++ b/src/go/internal/gcimporter/iimport.go
@@ -0,0 +1,618 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Indexed package import.
+// See cmd/compile/internal/gc/iexport.go for the export data format.
+
+package gcimporter
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"go/constant"
+	"go/token"
+	"go/types"
+	"io"
+	"sort"
+)
+
+type intReader struct {
+	*bytes.Reader
+	path string
+}
+
+func (r *intReader) int64() int64 {
+	i, err := binary.ReadVarint(r.Reader)
+	if err != nil {
+		errorf("import %q: read varint error: %v", r.path, err)
+	}
+	return i
+}
+
+func (r *intReader) uint64() uint64 {
+	i, err := binary.ReadUvarint(r.Reader)
+	if err != nil {
+		errorf("import %q: read varint error: %v", r.path, err)
+	}
+	return i
+}
+
+const predeclReserved = 32
+
+type itag uint64
+
+const (
+	// Types
+	definedType itag = iota
+	pointerType
+	sliceType
+	arrayType
+	chanType
+	mapType
+	signatureType
+	structType
+	interfaceType
+)
+
+// iImportData imports a package from the serialized package data
+// and returns the number of bytes consumed and a reference to the package.
+// If the export data version is not recognized or the format is otherwise
+// compromised, an error is returned.
+func iImportData(fset *token.FileSet, imports map[string]*types.Package, data []byte, path string) (_ int, pkg *types.Package, err error) {
+	const currentVersion = 1
+	version := int64(-1)
+	defer func() {
+		if e := recover(); e != nil {
+			if version > currentVersion {
+				err = fmt.Errorf("cannot import %q (%v), export data is newer version - update tool", path, e)
+			} else {
+				err = fmt.Errorf("cannot import %q (%v), possibly version skew - reinstall package", path, e)
+			}
+		}
+	}()
+
+	r := &intReader{bytes.NewReader(data), path}
+
+	version = int64(r.uint64())
+	switch version {
+	case currentVersion, 0:
+	default:
+		errorf("unknown iexport format version %d", version)
+	}
+
+	sLen := int64(r.uint64())
+	dLen := int64(r.uint64())
+
+	whence, _ := r.Seek(0, io.SeekCurrent)
+	stringData := data[whence : whence+sLen]
+	declData := data[whence+sLen : whence+sLen+dLen]
+	r.Seek(sLen+dLen, io.SeekCurrent)
+
+	p := iimporter{
+		ipath:   path,
+		version: int(version),
+
+		stringData:  stringData,
+		stringCache: make(map[uint64]string),
+		pkgCache:    make(map[uint64]*types.Package),
+
+		declData: declData,
+		pkgIndex: make(map[*types.Package]map[string]uint64),
+		typCache: make(map[uint64]types.Type),
+
+		fake: fakeFileSet{
+			fset:  fset,
+			files: make(map[string]*token.File),
+		},
+	}
+
+	for i, pt := range predeclared {
+		p.typCache[uint64(i)] = pt
+	}
+
+	pkgList := make([]*types.Package, r.uint64())
+	for i := range pkgList {
+		pkgPathOff := r.uint64()
+		pkgPath := p.stringAt(pkgPathOff)
+		pkgName := p.stringAt(r.uint64())
+		_ = r.uint64() // package height; unused by go/types
+
+		if pkgPath == "" {
+			pkgPath = path
+		}
+		pkg := imports[pkgPath]
+		if pkg == nil {
+			pkg = types.NewPackage(pkgPath, pkgName)
+			imports[pkgPath] = pkg
+		} else if pkg.Name() != pkgName {
+			errorf("conflicting names %s and %s for package %q", pkg.Name(), pkgName, path)
+		}
+
+		p.pkgCache[pkgPathOff] = pkg
+
+		nameIndex := make(map[string]uint64)
+		for nSyms := r.uint64(); nSyms > 0; nSyms-- {
+			name := p.stringAt(r.uint64())
+			nameIndex[name] = r.uint64()
+		}
+
+		p.pkgIndex[pkg] = nameIndex
+		pkgList[i] = pkg
+	}
+
+	localpkg := pkgList[0]
+
+	names := make([]string, 0, len(p.pkgIndex[localpkg]))
+	for name := range p.pkgIndex[localpkg] {
+		names = append(names, name)
+	}
+	sort.Strings(names)
+	for _, name := range names {
+		p.doDecl(localpkg, name)
+	}
+
+	for _, typ := range p.interfaceList {
+		typ.Complete()
+	}
+
+	// record all referenced packages as imports
+	list := append(([]*types.Package)(nil), pkgList[1:]...)
+	sort.Sort(byPath(list))
+	localpkg.SetImports(list)
+
+	// package was imported completely and without errors
+	localpkg.MarkComplete()
+
+	consumed, _ := r.Seek(0, io.SeekCurrent)
+	return int(consumed), localpkg, nil
+}
+
+type iimporter struct {
+	ipath   string
+	version int
+
+	stringData  []byte
+	stringCache map[uint64]string
+	pkgCache    map[uint64]*types.Package
+
+	declData []byte
+	pkgIndex map[*types.Package]map[string]uint64
+	typCache map[uint64]types.Type
+
+	fake          fakeFileSet
+	interfaceList []*types.Interface
+}
+
+func (p *iimporter) doDecl(pkg *types.Package, name string) {
+	// See if we've already imported this declaration.
+	if obj := pkg.Scope().Lookup(name); obj != nil {
+		return
+	}
+
+	off, ok := p.pkgIndex[pkg][name]
+	if !ok {
+		errorf("%v.%v not in index", pkg, name)
+	}
+
+	r := &importReader{p: p, currPkg: pkg}
+	r.declReader.Reset(p.declData[off:])
+
+	r.obj(name)
+}
+
+func (p *iimporter) stringAt(off uint64) string {
+	if s, ok := p.stringCache[off]; ok {
+		return s
+	}
+
+	slen, n := binary.Uvarint(p.stringData[off:])
+	if n <= 0 {
+		errorf("varint failed")
+	}
+	spos := off + uint64(n)
+	s := string(p.stringData[spos : spos+slen])
+	p.stringCache[off] = s
+	return s
+}
+
+func (p *iimporter) pkgAt(off uint64) *types.Package {
+	if pkg, ok := p.pkgCache[off]; ok {
+		return pkg
+	}
+	path := p.stringAt(off)
+	errorf("missing package %q in %q", path, p.ipath)
+	return nil
+}
+
+func (p *iimporter) typAt(off uint64, base *types.Named) types.Type {
+	if t, ok := p.typCache[off]; ok && (base == nil || !isInterface(t)) {
+		return t
+	}
+
+	if off < predeclReserved {
+		errorf("predeclared type missing from cache: %v", off)
+	}
+
+	r := &importReader{p: p}
+	r.declReader.Reset(p.declData[off-predeclReserved:])
+	t := r.doType(base)
+
+	if base == nil || !isInterface(t) {
+		p.typCache[off] = t
+	}
+	return t
+}
+
+type importReader struct {
+	p          *iimporter
+	declReader bytes.Reader
+	currPkg    *types.Package
+	prevFile   string
+	prevLine   int64
+	prevColumn int64
+}
+
+func (r *importReader) obj(name string) {
+	tag := r.byte()
+	pos := r.pos()
+
+	switch tag {
+	case 'A':
+		typ := r.typ()
+
+		r.declare(types.NewTypeName(pos, r.currPkg, name, typ))
+
+	case 'C':
+		typ, val := r.value()
+
+		r.declare(types.NewConst(pos, r.currPkg, name, typ, val))
+
+	case 'F':
+		sig := r.signature(nil)
+
+		r.declare(types.NewFunc(pos, r.currPkg, name, sig))
+
+	case 'T':
+		// Types can be recursive. We need to setup a stub
+		// declaration before recursing.
+		obj := types.NewTypeName(pos, r.currPkg, name, nil)
+		named := types.NewNamed(obj, nil, nil)
+		r.declare(obj)
+
+		underlying := r.p.typAt(r.uint64(), named).Underlying()
+		named.SetUnderlying(underlying)
+
+		if !isInterface(underlying) {
+			for n := r.uint64(); n > 0; n-- {
+				mpos := r.pos()
+				mname := r.ident()
+				recv := r.param()
+				msig := r.signature(recv)
+
+				named.AddMethod(types.NewFunc(mpos, r.currPkg, mname, msig))
+			}
+		}
+
+	case 'V':
+		typ := r.typ()
+
+		r.declare(types.NewVar(pos, r.currPkg, name, typ))
+
+	default:
+		errorf("unexpected tag: %v", tag)
+	}
+}
+
+func (r *importReader) declare(obj types.Object) {
+	obj.Pkg().Scope().Insert(obj)
+}
+
+func (r *importReader) value() (typ types.Type, val constant.Value) {
+	typ = r.typ()
+
+	switch b := typ.Underlying().(*types.Basic); b.Info() & types.IsConstType {
+	case types.IsBoolean:
+		val = constant.MakeBool(r.bool())
+
+	case types.IsString:
+		val = constant.MakeString(r.string())
+
+	case types.IsInteger:
+		val = r.mpint(b)
+
+	case types.IsFloat:
+		val = r.mpfloat(b)
+
+	case types.IsComplex:
+		re := r.mpfloat(b)
+		im := r.mpfloat(b)
+		val = constant.BinaryOp(re, token.ADD, constant.MakeImag(im))
+
+	default:
+		errorf("unexpected type %v", typ) // panics
+		panic("unreachable")
+	}
+
+	return
+}
+
+func intSize(b *types.Basic) (signed bool, maxBytes uint) {
+	if (b.Info() & types.IsUntyped) != 0 {
+		return true, 64
+	}
+
+	switch b.Kind() {
+	case types.Float32, types.Complex64:
+		return true, 3
+	case types.Float64, types.Complex128:
+		return true, 7
+	}
+
+	signed = (b.Info() & types.IsUnsigned) == 0
+	switch b.Kind() {
+	case types.Int8, types.Uint8:
+		maxBytes = 1
+	case types.Int16, types.Uint16:
+		maxBytes = 2
+	case types.Int32, types.Uint32:
+		maxBytes = 4
+	default:
+		maxBytes = 8
+	}
+
+	return
+}
+
+func (r *importReader) mpint(b *types.Basic) constant.Value {
+	signed, maxBytes := intSize(b)
+
+	maxSmall := 256 - maxBytes
+	if signed {
+		maxSmall = 256 - 2*maxBytes
+	}
+	if maxBytes == 1 {
+		maxSmall = 256
+	}
+
+	n, _ := r.declReader.ReadByte()
+	if uint(n) < maxSmall {
+		v := int64(n)
+		if signed {
+			v >>= 1
+			if n&1 != 0 {
+				v = ^v
+			}
+		}
+		return constant.MakeInt64(v)
+	}
+
+	v := -n
+	if signed {
+		v = -(n &^ 1) >> 1
+	}
+	if v < 1 || uint(v) > maxBytes {
+		errorf("weird decoding: %v, %v => %v", n, signed, v)
+	}
+
+	buf := make([]byte, v)
+	io.ReadFull(&r.declReader, buf)
+
+	// convert to little endian
+	// TODO(gri) go/constant should have a more direct conversion function
+	//           (e.g., once it supports a big.Float based implementation)
+	for i, j := 0, len(buf)-1; i < j; i, j = i+1, j-1 {
+		buf[i], buf[j] = buf[j], buf[i]
+	}
+
+	x := constant.MakeFromBytes(buf)
+	if signed && n&1 != 0 {
+		x = constant.UnaryOp(token.SUB, x, 0)
+	}
+	return x
+}
+
+func (r *importReader) mpfloat(b *types.Basic) constant.Value {
+	x := r.mpint(b)
+	if constant.Sign(x) == 0 {
+		return x
+	}
+
+	exp := r.int64()
+	switch {
+	case exp > 0:
+		x = constant.Shift(x, token.SHL, uint(exp))
+	case exp < 0:
+		d := constant.Shift(constant.MakeInt64(1), token.SHL, uint(-exp))
+		x = constant.BinaryOp(x, token.QUO, d)
+	}
+	return x
+}
+
+func (r *importReader) ident() string {
+	return r.string()
+}
+
+func (r *importReader) qualifiedIdent() (*types.Package, string) {
+	name := r.string()
+	pkg := r.pkg()
+	return pkg, name
+}
+
+func (r *importReader) pos() token.Pos {
+	if r.p.version >= 1 {
+		r.posv1()
+	} else {
+		r.posv0()
+	}
+
+	if r.prevFile == "" && r.prevLine == 0 && r.prevColumn == 0 {
+		return token.NoPos
+	}
+	return r.p.fake.pos(r.prevFile, int(r.prevLine), int(r.prevColumn))
+}
+
+func (r *importReader) posv0() {
+	delta := r.int64()
+	if delta != deltaNewFile {
+		r.prevLine += delta
+	} else if l := r.int64(); l == -1 {
+		r.prevLine += deltaNewFile
+	} else {
+		r.prevFile = r.string()
+		r.prevLine = l
+	}
+}
+
+func (r *importReader) posv1() {
+	delta := r.int64()
+	r.prevColumn += delta >> 1
+	if delta&1 != 0 {
+		delta = r.int64()
+		r.prevLine += delta >> 1
+		if delta&1 != 0 {
+			r.prevFile = r.string()
+		}
+	}
+}
+
+func (r *importReader) typ() types.Type {
+	return r.p.typAt(r.uint64(), nil)
+}
+
+func isInterface(t types.Type) bool {
+	_, ok := t.(*types.Interface)
+	return ok
+}
+
+func (r *importReader) pkg() *types.Package { return r.p.pkgAt(r.uint64()) }
+func (r *importReader) string() string      { return r.p.stringAt(r.uint64()) }
+
+func (r *importReader) doType(base *types.Named) types.Type {
+	switch k := r.kind(); k {
+	default:
+		errorf("unexpected kind tag in %q: %v", r.p.ipath, k)
+		return nil
+
+	case definedType:
+		pkg, name := r.qualifiedIdent()
+		r.p.doDecl(pkg, name)
+		return pkg.Scope().Lookup(name).(*types.TypeName).Type()
+	case pointerType:
+		return types.NewPointer(r.typ())
+	case sliceType:
+		return types.NewSlice(r.typ())
+	case arrayType:
+		n := r.uint64()
+		return types.NewArray(r.typ(), int64(n))
+	case chanType:
+		dir := chanDir(int(r.uint64()))
+		return types.NewChan(dir, r.typ())
+	case mapType:
+		return types.NewMap(r.typ(), r.typ())
+	case signatureType:
+		r.currPkg = r.pkg()
+		return r.signature(nil)
+
+	case structType:
+		r.currPkg = r.pkg()
+
+		fields := make([]*types.Var, r.uint64())
+		tags := make([]string, len(fields))
+		for i := range fields {
+			fpos := r.pos()
+			fname := r.ident()
+			ftyp := r.typ()
+			emb := r.bool()
+			tag := r.string()
+
+			fields[i] = types.NewField(fpos, r.currPkg, fname, ftyp, emb)
+			tags[i] = tag
+		}
+		return types.NewStruct(fields, tags)
+
+	case interfaceType:
+		r.currPkg = r.pkg()
+
+		embeddeds := make([]types.Type, r.uint64())
+		for i := range embeddeds {
+			_ = r.pos()
+			embeddeds[i] = r.typ()
+		}
+
+		methods := make([]*types.Func, r.uint64())
+		for i := range methods {
+			mpos := r.pos()
+			mname := r.ident()
+
+			// TODO(mdempsky): Matches bimport.go, but I
+			// don't agree with this.
+			var recv *types.Var
+			if base != nil {
+				recv = types.NewVar(token.NoPos, r.currPkg, "", base)
+			}
+
+			msig := r.signature(recv)
+			methods[i] = types.NewFunc(mpos, r.currPkg, mname, msig)
+		}
+
+		typ := types.NewInterfaceType(methods, embeddeds)
+		r.p.interfaceList = append(r.p.interfaceList, typ)
+		return typ
+	}
+}
+
+func (r *importReader) kind() itag {
+	return itag(r.uint64())
+}
+
+func (r *importReader) signature(recv *types.Var) *types.Signature {
+	params := r.paramList()
+	results := r.paramList()
+	variadic := params.Len() > 0 && r.bool()
+	return types.NewSignature(recv, params, results, variadic)
+}
+
+func (r *importReader) paramList() *types.Tuple {
+	xs := make([]*types.Var, r.uint64())
+	for i := range xs {
+		xs[i] = r.param()
+	}
+	return types.NewTuple(xs...)
+}
+
+func (r *importReader) param() *types.Var {
+	pos := r.pos()
+	name := r.ident()
+	typ := r.typ()
+	return types.NewParam(pos, r.currPkg, name, typ)
+}
+
+func (r *importReader) bool() bool {
+	return r.uint64() != 0
+}
+
+func (r *importReader) int64() int64 {
+	n, err := binary.ReadVarint(&r.declReader)
+	if err != nil {
+		errorf("readVarint: %v", err)
+	}
+	return n
+}
+
+func (r *importReader) uint64() uint64 {
+	n, err := binary.ReadUvarint(&r.declReader)
+	if err != nil {
+		errorf("readUvarint: %v", err)
+	}
+	return n
+}
+
+func (r *importReader) byte() byte {
+	x, err := r.declReader.ReadByte()
+	if err != nil {
+		errorf("declReader.ReadByte: %v", err)
+	}
+	return x
+}
diff --git a/src/go/internal/gcimporter/support.go b/src/go/internal/gcimporter/support.go
new file mode 100644
index 0000000..b8bb14d
--- /dev/null
+++ b/src/go/internal/gcimporter/support.go
@@ -0,0 +1,130 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements support functionality for iimport.go.
+
+package gcimporter
+
+import (
+	"fmt"
+	"go/token"
+	"go/types"
+	"sync"
+)
+
+func errorf(format string, args ...interface{}) {
+	panic(fmt.Sprintf(format, args...))
+}
+
+// deltaNewFile is a magic line delta offset indicating a new file.
+// We use -64 because it is rare; see issue 20080 and CL 41619.
+// -64 is the smallest int that fits in a single byte as a varint.
+const deltaNewFile = -64
+
+// Synthesize a token.Pos
+type fakeFileSet struct {
+	fset  *token.FileSet
+	files map[string]*token.File
+}
+
+func (s *fakeFileSet) pos(file string, line, column int) token.Pos {
+	// TODO(mdempsky): Make use of column.
+
+	// Since we don't know the set of needed file positions, we
+	// reserve maxlines positions per file.
+	const maxlines = 64 * 1024
+	f := s.files[file]
+	if f == nil {
+		f = s.fset.AddFile(file, -1, maxlines)
+		s.files[file] = f
+		// Allocate the fake linebreak indices on first use.
+		// TODO(adonovan): opt: save ~512KB using a more complex scheme?
+		fakeLinesOnce.Do(func() {
+			fakeLines = make([]int, maxlines)
+			for i := range fakeLines {
+				fakeLines[i] = i
+			}
+		})
+		f.SetLines(fakeLines)
+	}
+
+	if line > maxlines {
+		line = 1
+	}
+
+	// Treat the file as if it contained only newlines
+	// and column=1: use the line number as the offset.
+	return f.Pos(line - 1)
+}
+
+var (
+	fakeLines     []int
+	fakeLinesOnce sync.Once
+)
+
+func chanDir(d int) types.ChanDir {
+	// tag values must match the constants in cmd/compile/internal/gc/go.go
+	switch d {
+	case 1 /* Crecv */ :
+		return types.RecvOnly
+	case 2 /* Csend */ :
+		return types.SendOnly
+	case 3 /* Cboth */ :
+		return types.SendRecv
+	default:
+		errorf("unexpected channel dir %d", d)
+		return 0
+	}
+}
+
+var predeclared = []types.Type{
+	// basic types
+	types.Typ[types.Bool],
+	types.Typ[types.Int],
+	types.Typ[types.Int8],
+	types.Typ[types.Int16],
+	types.Typ[types.Int32],
+	types.Typ[types.Int64],
+	types.Typ[types.Uint],
+	types.Typ[types.Uint8],
+	types.Typ[types.Uint16],
+	types.Typ[types.Uint32],
+	types.Typ[types.Uint64],
+	types.Typ[types.Uintptr],
+	types.Typ[types.Float32],
+	types.Typ[types.Float64],
+	types.Typ[types.Complex64],
+	types.Typ[types.Complex128],
+	types.Typ[types.String],
+
+	// basic type aliases
+	types.Universe.Lookup("byte").Type(),
+	types.Universe.Lookup("rune").Type(),
+
+	// error
+	types.Universe.Lookup("error").Type(),
+
+	// untyped types
+	types.Typ[types.UntypedBool],
+	types.Typ[types.UntypedInt],
+	types.Typ[types.UntypedRune],
+	types.Typ[types.UntypedFloat],
+	types.Typ[types.UntypedComplex],
+	types.Typ[types.UntypedString],
+	types.Typ[types.UntypedNil],
+
+	// package unsafe
+	types.Typ[types.UnsafePointer],
+
+	// invalid type
+	types.Typ[types.Invalid], // only appears in packages with errors
+
+	// used internally by gc; never used by this package or in .a files
+	anyType{},
+}
+
+type anyType struct{}
+
+func (t anyType) Underlying() types.Type { return t }
+func (t anyType) String() string         { return "any" }
diff --git a/src/go/internal/gcimporter/testdata/a.go b/src/go/internal/gcimporter/testdata/a.go
new file mode 100644
index 0000000..56e4292
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/a.go
@@ -0,0 +1,14 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Input for TestIssue13566
+
+package a
+
+import "encoding/json"
+
+type A struct {
+	a    *A
+	json json.RawMessage
+}
diff --git a/src/go/internal/gcimporter/testdata/b.go b/src/go/internal/gcimporter/testdata/b.go
new file mode 100644
index 0000000..4196678
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/b.go
@@ -0,0 +1,11 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Input for TestIssue13566
+
+package b
+
+import "./a"
+
+type A a.A
diff --git a/src/go/internal/gcimporter/testdata/exports.go b/src/go/internal/gcimporter/testdata/exports.go
new file mode 100644
index 0000000..8ba3242
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/exports.go
@@ -0,0 +1,88 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is used to generate an object file which
+// serves as test file for gcimporter_test.go.
+
+package exports
+
+import "go/ast"
+
+// Issue 3682: Correctly read dotted identifiers from export data.
+const init1 = 0
+
+func init() {}
+
+const (
+	C0 int = 0
+	C1     = 3.14159265
+	C2     = 2.718281828i
+	C3     = -123.456e-789
+	C4     = +123.456e+789
+	C5     = 1234i
+	C6     = "foo\n"
+	C7     = `bar\n`
+)
+
+type (
+	T1  int
+	T2  [10]int
+	T3  []int
+	T4  *int
+	T5  chan int
+	T6a chan<- int
+	T6b chan (<-chan int)
+	T6c chan<- (chan int)
+	T7  <-chan *ast.File
+	T8  struct{}
+	T9  struct {
+		a    int
+		b, c float32
+		d    []string `go:"tag"`
+	}
+	T10 struct {
+		T8
+		T9
+		_ *T10
+	}
+	T11 map[int]string
+	T12 interface{}
+	T13 interface {
+		m1()
+		m2(int) float32
+	}
+	T14 interface {
+		T12
+		T13
+		m3(x ...struct{}) []T9
+	}
+	T15 func()
+	T16 func(int)
+	T17 func(x int)
+	T18 func() float32
+	T19 func() (x float32)
+	T20 func(...interface{})
+	T21 struct{ next *T21 }
+	T22 struct{ link *T23 }
+	T23 struct{ link *T22 }
+	T24 *T24
+	T25 *T26
+	T26 *T27
+	T27 *T25
+	T28 func(T28) T28
+)
+
+var (
+	V0 int
+	V1         = -991.0
+	V2 float32 = 1.2
+)
+
+func F1()         {}
+func F2(x int)    {}
+func F3() int     { return 0 }
+func F4() float32 { return 0 }
+func F5(a, b, c int, u, v, w struct{ x, y T1 }, more ...interface{}) (p, q, r chan<- T10)
+
+func (p *T1) M1()
diff --git a/src/go/internal/gcimporter/testdata/issue15920.go b/src/go/internal/gcimporter/testdata/issue15920.go
new file mode 100644
index 0000000..c70f7d8
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/issue15920.go
@@ -0,0 +1,11 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// The underlying type of Error is the underlying type of error.
+// Make sure we can import this again without problems.
+type Error error
+
+func F() Error { return nil }
diff --git a/src/go/internal/gcimporter/testdata/issue20046.go b/src/go/internal/gcimporter/testdata/issue20046.go
new file mode 100644
index 0000000..c63ee82
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/issue20046.go
@@ -0,0 +1,9 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+var V interface {
+	M()
+}
diff --git a/src/go/internal/gcimporter/testdata/issue25301.go b/src/go/internal/gcimporter/testdata/issue25301.go
new file mode 100644
index 0000000..e3dc98b
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/issue25301.go
@@ -0,0 +1,17 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue25301
+
+type (
+	A = interface {
+		M()
+	}
+	T interface {
+		A
+	}
+	S struct{}
+)
+
+func (S) M() { println("m") }
diff --git a/src/go/internal/gcimporter/testdata/issue25596.go b/src/go/internal/gcimporter/testdata/issue25596.go
new file mode 100644
index 0000000..8923373
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/issue25596.go
@@ -0,0 +1,13 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue25596
+
+type E interface {
+	M() T
+}
+
+type T interface {
+	E
+}
diff --git a/src/go/internal/gcimporter/testdata/p.go b/src/go/internal/gcimporter/testdata/p.go
new file mode 100644
index 0000000..9e2e705
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/p.go
@@ -0,0 +1,13 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Input for TestIssue15517
+
+package p
+
+const C = 0
+
+var V int
+
+func F() {}
diff --git a/src/go/internal/gcimporter/testdata/versions/test.go b/src/go/internal/gcimporter/testdata/versions/test.go
new file mode 100644
index 0000000..227fc09
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/versions/test.go
@@ -0,0 +1,28 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// To create a test case for a new export format version,
+// build this package with the latest compiler and store
+// the resulting .a file appropriately named in the versions
+// directory. The VersionHandling test will pick it up.
+//
+// In the testdata/versions:
+//
+// go build -o test_go1.$X_$Y.a test.go
+//
+// with $X = Go version and $Y = export format version
+// (add 'b' or 'i' to distinguish between binary and
+// indexed format starting with 1.11 as long as both
+// formats are supported).
+//
+// Make sure this source is extended such that it exercises
+// whatever export format change has taken place.
+
+package test
+
+// Any release before and including Go 1.7 didn't encode
+// the package for a blank struct field.
+type BlankField struct {
+	_ int
+}
diff --git a/src/go/internal/gcimporter/testdata/versions/test_go1.11_0i.a b/src/go/internal/gcimporter/testdata/versions/test_go1.11_0i.a
new file mode 100644
index 0000000..b00fefe
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/versions/test_go1.11_0i.a
diff --git a/src/go/internal/gcimporter/testdata/versions/test_go1.11_6b.a b/src/go/internal/gcimporter/testdata/versions/test_go1.11_6b.a
new file mode 100644
index 0000000..c0a211e
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/versions/test_go1.11_6b.a
diff --git a/src/go/internal/gcimporter/testdata/versions/test_go1.11_999b.a b/src/go/internal/gcimporter/testdata/versions/test_go1.11_999b.a
new file mode 100644
index 0000000..c35d22d
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/versions/test_go1.11_999b.a
diff --git a/src/go/internal/gcimporter/testdata/versions/test_go1.11_999i.a b/src/go/internal/gcimporter/testdata/versions/test_go1.11_999i.a
new file mode 100644
index 0000000..99401d7
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/versions/test_go1.11_999i.a
diff --git a/src/go/internal/gcimporter/testdata/versions/test_go1.7_0.a b/src/go/internal/gcimporter/testdata/versions/test_go1.7_0.a
new file mode 100644
index 0000000..edb6c3f
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/versions/test_go1.7_0.a
diff --git a/src/go/internal/gcimporter/testdata/versions/test_go1.7_1.a b/src/go/internal/gcimporter/testdata/versions/test_go1.7_1.a
new file mode 100644
index 0000000..554d04a
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/versions/test_go1.7_1.a
diff --git a/src/go/internal/gcimporter/testdata/versions/test_go1.8_4.a b/src/go/internal/gcimporter/testdata/versions/test_go1.8_4.a
new file mode 100644
index 0000000..26b8531
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/versions/test_go1.8_4.a
diff --git a/src/go/internal/gcimporter/testdata/versions/test_go1.8_5.a b/src/go/internal/gcimporter/testdata/versions/test_go1.8_5.a
new file mode 100644
index 0000000..60e52ef
--- /dev/null
+++ b/src/go/internal/gcimporter/testdata/versions/test_go1.8_5.a
diff --git a/src/go/internal/srcimporter/srcimporter.go b/src/go/internal/srcimporter/srcimporter.go
new file mode 100644
index 0000000..438ae0f
--- /dev/null
+++ b/src/go/internal/srcimporter/srcimporter.go
@@ -0,0 +1,265 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package srcimporter implements importing directly
+// from source files rather than installed packages.
+package srcimporter // import "go/internal/srcimporter"
+
+import (
+	"fmt"
+	"go/ast"
+	"go/build"
+	"go/parser"
+	"go/token"
+	"go/types"
+	exec "internal/execabs"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+	_ "unsafe" // for go:linkname
+)
+
+// An Importer provides the context for importing packages from source code.
+type Importer struct {
+	ctxt     *build.Context
+	fset     *token.FileSet
+	sizes    types.Sizes
+	packages map[string]*types.Package
+}
+
+// NewImporter returns a new Importer for the given context, file set, and map
+// of packages. The context is used to resolve import paths to package paths,
+// and identifying the files belonging to the package. If the context provides
+// non-nil file system functions, they are used instead of the regular package
+// os functions. The file set is used to track position information of package
+// files; and imported packages are added to the packages map.
+func New(ctxt *build.Context, fset *token.FileSet, packages map[string]*types.Package) *Importer {
+	return &Importer{
+		ctxt:     ctxt,
+		fset:     fset,
+		sizes:    types.SizesFor(ctxt.Compiler, ctxt.GOARCH), // uses go/types default if GOARCH not found
+		packages: packages,
+	}
+}
+
+// Importing is a sentinel taking the place in Importer.packages
+// for a package that is in the process of being imported.
+var importing types.Package
+
+// Import(path) is a shortcut for ImportFrom(path, ".", 0).
+func (p *Importer) Import(path string) (*types.Package, error) {
+	return p.ImportFrom(path, ".", 0) // use "." rather than "" (see issue #24441)
+}
+
+// ImportFrom imports the package with the given import path resolved from the given srcDir,
+// adds the new package to the set of packages maintained by the importer, and returns the
+// package. Package path resolution and file system operations are controlled by the context
+// maintained with the importer. The import mode must be zero but is otherwise ignored.
+// Packages that are not comprised entirely of pure Go files may fail to import because the
+// type checker may not be able to determine all exported entities (e.g. due to cgo dependencies).
+func (p *Importer) ImportFrom(path, srcDir string, mode types.ImportMode) (*types.Package, error) {
+	if mode != 0 {
+		panic("non-zero import mode")
+	}
+
+	if abs, err := p.absPath(srcDir); err == nil { // see issue #14282
+		srcDir = abs
+	}
+	bp, err := p.ctxt.Import(path, srcDir, 0)
+	if err != nil {
+		return nil, err // err may be *build.NoGoError - return as is
+	}
+
+	// package unsafe is known to the type checker
+	if bp.ImportPath == "unsafe" {
+		return types.Unsafe, nil
+	}
+
+	// no need to re-import if the package was imported completely before
+	pkg := p.packages[bp.ImportPath]
+	if pkg != nil {
+		if pkg == &importing {
+			return nil, fmt.Errorf("import cycle through package %q", bp.ImportPath)
+		}
+		if !pkg.Complete() {
+			// Package exists but is not complete - we cannot handle this
+			// at the moment since the source importer replaces the package
+			// wholesale rather than augmenting it (see #19337 for details).
+			// Return incomplete package with error (see #16088).
+			return pkg, fmt.Errorf("reimported partially imported package %q", bp.ImportPath)
+		}
+		return pkg, nil
+	}
+
+	p.packages[bp.ImportPath] = &importing
+	defer func() {
+		// clean up in case of error
+		// TODO(gri) Eventually we may want to leave a (possibly empty)
+		// package in the map in all cases (and use that package to
+		// identify cycles). See also issue 16088.
+		if p.packages[bp.ImportPath] == &importing {
+			p.packages[bp.ImportPath] = nil
+		}
+	}()
+
+	var filenames []string
+	filenames = append(filenames, bp.GoFiles...)
+	filenames = append(filenames, bp.CgoFiles...)
+
+	files, err := p.parseFiles(bp.Dir, filenames)
+	if err != nil {
+		return nil, err
+	}
+
+	// type-check package files
+	var firstHardErr error
+	conf := types.Config{
+		IgnoreFuncBodies: true,
+		// continue type-checking after the first error
+		Error: func(err error) {
+			if firstHardErr == nil && !err.(types.Error).Soft {
+				firstHardErr = err
+			}
+		},
+		Importer: p,
+		Sizes:    p.sizes,
+	}
+	if len(bp.CgoFiles) > 0 {
+		if p.ctxt.OpenFile != nil {
+			// cgo, gcc, pkg-config, etc. do not support
+			// build.Context's VFS.
+			conf.FakeImportC = true
+		} else {
+			setUsesCgo(&conf)
+			file, err := p.cgo(bp)
+			if err != nil {
+				return nil, err
+			}
+			files = append(files, file)
+		}
+	}
+
+	pkg, err = conf.Check(bp.ImportPath, p.fset, files, nil)
+	if err != nil {
+		// If there was a hard error it is possibly unsafe
+		// to use the package as it may not be fully populated.
+		// Do not return it (see also #20837, #20855).
+		if firstHardErr != nil {
+			pkg = nil
+			err = firstHardErr // give preference to first hard error over any soft error
+		}
+		return pkg, fmt.Errorf("type-checking package %q failed (%v)", bp.ImportPath, err)
+	}
+	if firstHardErr != nil {
+		// this can only happen if we have a bug in go/types
+		panic("package is not safe yet no error was returned")
+	}
+
+	p.packages[bp.ImportPath] = pkg
+	return pkg, nil
+}
+
+func (p *Importer) parseFiles(dir string, filenames []string) ([]*ast.File, error) {
+	// use build.Context's OpenFile if there is one
+	open := p.ctxt.OpenFile
+	if open == nil {
+		open = func(name string) (io.ReadCloser, error) { return os.Open(name) }
+	}
+
+	files := make([]*ast.File, len(filenames))
+	errors := make([]error, len(filenames))
+
+	var wg sync.WaitGroup
+	wg.Add(len(filenames))
+	for i, filename := range filenames {
+		go func(i int, filepath string) {
+			defer wg.Done()
+			src, err := open(filepath)
+			if err != nil {
+				errors[i] = err // open provides operation and filename in error
+				return
+			}
+			files[i], errors[i] = parser.ParseFile(p.fset, filepath, src, 0)
+			src.Close() // ignore Close error - parsing may have succeeded which is all we need
+		}(i, p.joinPath(dir, filename))
+	}
+	wg.Wait()
+
+	// if there are errors, return the first one for deterministic results
+	for _, err := range errors {
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return files, nil
+}
+
+func (p *Importer) cgo(bp *build.Package) (*ast.File, error) {
+	tmpdir, err := os.MkdirTemp("", "srcimporter")
+	if err != nil {
+		return nil, err
+	}
+	defer os.RemoveAll(tmpdir)
+
+	args := []string{"go", "tool", "cgo", "-objdir", tmpdir}
+	if bp.Goroot {
+		switch bp.ImportPath {
+		case "runtime/cgo":
+			args = append(args, "-import_runtime_cgo=false", "-import_syscall=false")
+		case "runtime/race":
+			args = append(args, "-import_syscall=false")
+		}
+	}
+	args = append(args, "--")
+	args = append(args, strings.Fields(os.Getenv("CGO_CPPFLAGS"))...)
+	args = append(args, bp.CgoCPPFLAGS...)
+	if len(bp.CgoPkgConfig) > 0 {
+		cmd := exec.Command("pkg-config", append([]string{"--cflags"}, bp.CgoPkgConfig...)...)
+		out, err := cmd.CombinedOutput()
+		if err != nil {
+			return nil, err
+		}
+		args = append(args, strings.Fields(string(out))...)
+	}
+	args = append(args, "-I", tmpdir)
+	args = append(args, strings.Fields(os.Getenv("CGO_CFLAGS"))...)
+	args = append(args, bp.CgoCFLAGS...)
+	args = append(args, bp.CgoFiles...)
+
+	cmd := exec.Command(args[0], args[1:]...)
+	cmd.Dir = bp.Dir
+	if err := cmd.Run(); err != nil {
+		return nil, err
+	}
+
+	return parser.ParseFile(p.fset, filepath.Join(tmpdir, "_cgo_gotypes.go"), nil, 0)
+}
+
+// context-controlled file system operations
+
+func (p *Importer) absPath(path string) (string, error) {
+	// TODO(gri) This should be using p.ctxt.AbsPath which doesn't
+	// exist but probably should. See also issue #14282.
+	return filepath.Abs(path)
+}
+
+func (p *Importer) isAbsPath(path string) bool {
+	if f := p.ctxt.IsAbsPath; f != nil {
+		return f(path)
+	}
+	return filepath.IsAbs(path)
+}
+
+func (p *Importer) joinPath(elem ...string) string {
+	if f := p.ctxt.JoinPath; f != nil {
+		return f(elem...)
+	}
+	return filepath.Join(elem...)
+}
+
+//go:linkname setUsesCgo go/types.srcimporter_setUsesCgo
+func setUsesCgo(conf *types.Config)
diff --git a/src/go/internal/srcimporter/srcimporter_test.go b/src/go/internal/srcimporter/srcimporter_test.go
new file mode 100644
index 0000000..05b12f1
--- /dev/null
+++ b/src/go/internal/srcimporter/srcimporter_test.go
@@ -0,0 +1,254 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package srcimporter
+
+import (
+	"flag"
+	"go/build"
+	"go/token"
+	"go/types"
+	"internal/testenv"
+	"os"
+	"path"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestMain(m *testing.M) {
+	flag.Parse()
+	if goTool, err := testenv.GoTool(); err == nil {
+		os.Setenv("PATH", filepath.Dir(goTool)+string(os.PathListSeparator)+os.Getenv("PATH"))
+	}
+	os.Exit(m.Run())
+}
+
+const maxTime = 2 * time.Second
+
+var importer = New(&build.Default, token.NewFileSet(), make(map[string]*types.Package))
+
+func doImport(t *testing.T, path, srcDir string) {
+	t0 := time.Now()
+	if _, err := importer.ImportFrom(path, srcDir, 0); err != nil {
+		// don't report an error if there's no buildable Go files
+		if _, nogo := err.(*build.NoGoError); !nogo {
+			t.Errorf("import %q failed (%v)", path, err)
+		}
+		return
+	}
+	t.Logf("import %q: %v", path, time.Since(t0))
+}
+
+// walkDir imports the all the packages with the given path
+// prefix recursively. It returns the number of packages
+// imported and whether importing was aborted because time
+// has passed endTime.
+func walkDir(t *testing.T, path string, endTime time.Time) (int, bool) {
+	if time.Now().After(endTime) {
+		t.Log("testing time used up")
+		return 0, true
+	}
+
+	// ignore fake packages and testdata directories
+	if path == "builtin" || path == "unsafe" || strings.HasSuffix(path, "testdata") {
+		return 0, false
+	}
+
+	list, err := os.ReadDir(filepath.Join(runtime.GOROOT(), "src", path))
+	if err != nil {
+		t.Fatalf("walkDir %s failed (%v)", path, err)
+	}
+
+	nimports := 0
+	hasGoFiles := false
+	for _, f := range list {
+		if f.IsDir() {
+			n, abort := walkDir(t, filepath.Join(path, f.Name()), endTime)
+			nimports += n
+			if abort {
+				return nimports, true
+			}
+		} else if strings.HasSuffix(f.Name(), ".go") {
+			hasGoFiles = true
+		}
+	}
+
+	if hasGoFiles {
+		doImport(t, path, "")
+		nimports++
+	}
+
+	return nimports, false
+}
+
+func TestImportStdLib(t *testing.T) {
+	if !testenv.HasSrc() {
+		t.Skip("no source code available")
+	}
+
+	if testing.Short() && testenv.Builder() == "" {
+		t.Skip("skipping in -short mode")
+	}
+	dt := maxTime
+	nimports, _ := walkDir(t, "", time.Now().Add(dt)) // installed packages
+	t.Logf("tested %d imports", nimports)
+}
+
+var importedObjectTests = []struct {
+	name string
+	want string
+}{
+	{"flag.Bool", "func Bool(name string, value bool, usage string) *bool"},
+	{"io.Reader", "type Reader interface{Read(p []byte) (n int, err error)}"},
+	{"io.ReadWriter", "type ReadWriter interface{Reader; Writer}"}, // go/types.gcCompatibilityMode is off => interface not flattened
+	{"math.Pi", "const Pi untyped float"},
+	{"math.Sin", "func Sin(x float64) float64"},
+	{"math/big.Int", "type Int struct{neg bool; abs nat}"},
+	{"golang.org/x/text/unicode/norm.MaxSegmentSize", "const MaxSegmentSize untyped int"},
+}
+
+func TestImportedTypes(t *testing.T) {
+	if !testenv.HasSrc() {
+		t.Skip("no source code available")
+	}
+
+	for _, test := range importedObjectTests {
+		i := strings.LastIndex(test.name, ".")
+		if i < 0 {
+			t.Fatal("invalid test data format")
+		}
+		importPath := test.name[:i]
+		objName := test.name[i+1:]
+
+		pkg, err := importer.ImportFrom(importPath, ".", 0)
+		if err != nil {
+			t.Error(err)
+			continue
+		}
+
+		obj := pkg.Scope().Lookup(objName)
+		if obj == nil {
+			t.Errorf("%s: object not found", test.name)
+			continue
+		}
+
+		got := types.ObjectString(obj, types.RelativeTo(pkg))
+		if got != test.want {
+			t.Errorf("%s: got %q; want %q", test.name, got, test.want)
+		}
+
+		if named, _ := obj.Type().(*types.Named); named != nil {
+			verifyInterfaceMethodRecvs(t, named, 0)
+		}
+	}
+}
+
+// verifyInterfaceMethodRecvs verifies that method receiver types
+// are named if the methods belong to a named interface type.
+func verifyInterfaceMethodRecvs(t *testing.T, named *types.Named, level int) {
+	// avoid endless recursion in case of an embedding bug that lead to a cycle
+	if level > 10 {
+		t.Errorf("%s: embeds itself", named)
+		return
+	}
+
+	iface, _ := named.Underlying().(*types.Interface)
+	if iface == nil {
+		return // not an interface
+	}
+
+	// check explicitly declared methods
+	for i := 0; i < iface.NumExplicitMethods(); i++ {
+		m := iface.ExplicitMethod(i)
+		recv := m.Type().(*types.Signature).Recv()
+		if recv == nil {
+			t.Errorf("%s: missing receiver type", m)
+			continue
+		}
+		if recv.Type() != named {
+			t.Errorf("%s: got recv type %s; want %s", m, recv.Type(), named)
+		}
+	}
+
+	// check embedded interfaces (they are named, too)
+	for i := 0; i < iface.NumEmbeddeds(); i++ {
+		// embedding of interfaces cannot have cycles; recursion will terminate
+		verifyInterfaceMethodRecvs(t, iface.Embedded(i), level+1)
+	}
+}
+
+func TestReimport(t *testing.T) {
+	if !testenv.HasSrc() {
+		t.Skip("no source code available")
+	}
+
+	// Reimporting a partially imported (incomplete) package is not supported (see issue #19337).
+	// Make sure we recognize the situation and report an error.
+
+	mathPkg := types.NewPackage("math", "math") // incomplete package
+	importer := New(&build.Default, token.NewFileSet(), map[string]*types.Package{mathPkg.Path(): mathPkg})
+	_, err := importer.ImportFrom("math", ".", 0)
+	if err == nil || !strings.HasPrefix(err.Error(), "reimport") {
+		t.Errorf("got %v; want reimport error", err)
+	}
+}
+
+func TestIssue20855(t *testing.T) {
+	if !testenv.HasSrc() {
+		t.Skip("no source code available")
+	}
+
+	pkg, err := importer.ImportFrom("go/internal/srcimporter/testdata/issue20855", ".", 0)
+	if err == nil || !strings.Contains(err.Error(), "missing function body") {
+		t.Fatalf("got unexpected or no error: %v", err)
+	}
+	if pkg == nil {
+		t.Error("got no package despite no hard errors")
+	}
+}
+
+func testImportPath(t *testing.T, pkgPath string) {
+	if !testenv.HasSrc() {
+		t.Skip("no source code available")
+	}
+
+	pkgName := path.Base(pkgPath)
+
+	pkg, err := importer.Import(pkgPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if pkg.Name() != pkgName {
+		t.Errorf("got %q; want %q", pkg.Name(), pkgName)
+	}
+
+	if pkg.Path() != pkgPath {
+		t.Errorf("got %q; want %q", pkg.Path(), pkgPath)
+	}
+}
+
+// TestIssue23092 tests relative imports.
+func TestIssue23092(t *testing.T) {
+	testImportPath(t, "./testdata/issue23092")
+}
+
+// TestIssue24392 tests imports against a path containing 'testdata'.
+func TestIssue24392(t *testing.T) {
+	testImportPath(t, "go/internal/srcimporter/testdata/issue24392")
+}
+
+func TestCgo(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	testenv.MustHaveCGO(t)
+
+	importer := New(&build.Default, token.NewFileSet(), make(map[string]*types.Package))
+	_, err := importer.ImportFrom("./misc/cgo/test", runtime.GOROOT(), 0)
+	if err != nil {
+		t.Fatalf("Import failed: %v", err)
+	}
+}
diff --git a/src/go/internal/srcimporter/testdata/issue20855/issue20855.go b/src/go/internal/srcimporter/testdata/issue20855/issue20855.go
new file mode 100644
index 0000000..d55448b
--- /dev/null
+++ b/src/go/internal/srcimporter/testdata/issue20855/issue20855.go
@@ -0,0 +1,7 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue20855
+
+func init() // "missing function body" is a soft error
diff --git a/src/go/internal/srcimporter/testdata/issue23092/issue23092.go b/src/go/internal/srcimporter/testdata/issue23092/issue23092.go
new file mode 100644
index 0000000..608698b
--- /dev/null
+++ b/src/go/internal/srcimporter/testdata/issue23092/issue23092.go
@@ -0,0 +1,5 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue23092
diff --git a/src/go/internal/srcimporter/testdata/issue24392/issue24392.go b/src/go/internal/srcimporter/testdata/issue24392/issue24392.go
new file mode 100644
index 0000000..8ad5221
--- /dev/null
+++ b/src/go/internal/srcimporter/testdata/issue24392/issue24392.go
@@ -0,0 +1,5 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue24392
diff --git a/src/go/parser/error_test.go b/src/go/parser/error_test.go
new file mode 100644
index 0000000..358a844
--- /dev/null
+++ b/src/go/parser/error_test.go
@@ -0,0 +1,187 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements a parser test harness. The files in the testdata
+// directory are parsed and the errors reported are compared against the
+// error messages expected in the test files. The test files must end in
+// .src rather than .go so that they are not disturbed by gofmt runs.
+//
+// Expected errors are indicated in the test files by putting a comment
+// of the form /* ERROR "rx" */ immediately following an offending token.
+// The harness will verify that an error matching the regular expression
+// rx is reported at that source position.
+//
+// For instance, the following test file indicates that a "not declared"
+// error should be reported for the undeclared variable x:
+//
+//	package p
+//	func f() {
+//		_ = x /* ERROR "not declared" */ + 1
+//	}
+
+package parser
+
+import (
+	"go/scanner"
+	"go/token"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+const testdata = "testdata"
+
+// getFile assumes that each filename occurs at most once
+func getFile(fset *token.FileSet, filename string) (file *token.File) {
+	fset.Iterate(func(f *token.File) bool {
+		if f.Name() == filename {
+			if file != nil {
+				panic(filename + " used multiple times")
+			}
+			file = f
+		}
+		return true
+	})
+	return file
+}
+
+func getPos(fset *token.FileSet, filename string, offset int) token.Pos {
+	if f := getFile(fset, filename); f != nil {
+		return f.Pos(offset)
+	}
+	return token.NoPos
+}
+
+// ERROR comments must be of the form /* ERROR "rx" */ and rx is
+// a regular expression that matches the expected error message.
+// The special form /* ERROR HERE "rx" */ must be used for error
+// messages that appear immediately after a token, rather than at
+// a token's position.
+//
+var errRx = regexp.MustCompile(`^/\* *ERROR *(HERE)? *"([^"]*)" *\*/$`)
+
+// expectedErrors collects the regular expressions of ERROR comments found
+// in files and returns them as a map of error positions to error messages.
+//
+func expectedErrors(fset *token.FileSet, filename string, src []byte) map[token.Pos]string {
+	errors := make(map[token.Pos]string)
+
+	var s scanner.Scanner
+	// file was parsed already - do not add it again to the file
+	// set otherwise the position information returned here will
+	// not match the position information collected by the parser
+	s.Init(getFile(fset, filename), src, nil, scanner.ScanComments)
+	var prev token.Pos // position of last non-comment, non-semicolon token
+	var here token.Pos // position immediately after the token at position prev
+
+	for {
+		pos, tok, lit := s.Scan()
+		switch tok {
+		case token.EOF:
+			return errors
+		case token.COMMENT:
+			s := errRx.FindStringSubmatch(lit)
+			if len(s) == 3 {
+				pos := prev
+				if s[1] == "HERE" {
+					pos = here
+				}
+				errors[pos] = string(s[2])
+			}
+		case token.SEMICOLON:
+			// don't use the position of auto-inserted (invisible) semicolons
+			if lit != ";" {
+				break
+			}
+			fallthrough
+		default:
+			prev = pos
+			var l int // token length
+			if tok.IsLiteral() {
+				l = len(lit)
+			} else {
+				l = len(tok.String())
+			}
+			here = prev + token.Pos(l)
+		}
+	}
+}
+
+// compareErrors compares the map of expected error messages with the list
+// of found errors and reports discrepancies.
+//
+func compareErrors(t *testing.T, fset *token.FileSet, expected map[token.Pos]string, found scanner.ErrorList) {
+	for _, error := range found {
+		// error.Pos is a token.Position, but we want
+		// a token.Pos so we can do a map lookup
+		pos := getPos(fset, error.Pos.Filename, error.Pos.Offset)
+		if msg, found := expected[pos]; found {
+			// we expect a message at pos; check if it matches
+			rx, err := regexp.Compile(msg)
+			if err != nil {
+				t.Errorf("%s: %v", error.Pos, err)
+				continue
+			}
+			if match := rx.MatchString(error.Msg); !match {
+				t.Errorf("%s: %q does not match %q", error.Pos, error.Msg, msg)
+				continue
+			}
+			// we have a match - eliminate this error
+			delete(expected, pos)
+		} else {
+			// To keep in mind when analyzing failed test output:
+			// If the same error position occurs multiple times in errors,
+			// this message will be triggered (because the first error at
+			// the position removes this position from the expected errors).
+			t.Errorf("%s: unexpected error: %s", error.Pos, error.Msg)
+		}
+	}
+
+	// there should be no expected errors left
+	if len(expected) > 0 {
+		t.Errorf("%d errors not reported:", len(expected))
+		for pos, msg := range expected {
+			t.Errorf("%s: %s\n", fset.Position(pos), msg)
+		}
+	}
+}
+
+func checkErrors(t *testing.T, filename string, input interface{}) {
+	src, err := readSource(filename, input)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	fset := token.NewFileSet()
+	_, err = ParseFile(fset, filename, src, DeclarationErrors|AllErrors)
+	found, ok := err.(scanner.ErrorList)
+	if err != nil && !ok {
+		t.Error(err)
+		return
+	}
+	found.RemoveMultiples()
+
+	// we are expecting the following errors
+	// (collect these after parsing a file so that it is found in the file set)
+	expected := expectedErrors(fset, filename, src)
+
+	// verify errors returned by the parser
+	compareErrors(t, fset, expected, found)
+}
+
+func TestErrors(t *testing.T) {
+	list, err := os.ReadDir(testdata)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, d := range list {
+		name := d.Name()
+		if !d.IsDir() && !strings.HasPrefix(name, ".") && strings.HasSuffix(name, ".src") {
+			checkErrors(t, filepath.Join(testdata, name), nil)
+		}
+	}
+}
diff --git a/src/go/parser/example_test.go b/src/go/parser/example_test.go
new file mode 100644
index 0000000..c2f7f29
--- /dev/null
+++ b/src/go/parser/example_test.go
@@ -0,0 +1,43 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package parser_test
+
+import (
+	"fmt"
+	"go/parser"
+	"go/token"
+)
+
+func ExampleParseFile() {
+	fset := token.NewFileSet() // positions are relative to fset
+
+	src := `package foo
+
+import (
+	"fmt"
+	"time"
+)
+
+func bar() {
+	fmt.Println(time.Now())
+}`
+
+	// Parse src but stop after processing the imports.
+	f, err := parser.ParseFile(fset, "", src, parser.ImportsOnly)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	// Print the imports from the file's AST.
+	for _, s := range f.Imports {
+		fmt.Println(s.Path.Value)
+	}
+
+	// output:
+	//
+	// "fmt"
+	// "time"
+}
diff --git a/src/go/parser/interface.go b/src/go/parser/interface.go
new file mode 100644
index 0000000..56ff5fe
--- /dev/null
+++ b/src/go/parser/interface.go
@@ -0,0 +1,241 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains the exported entry points for invoking the parser.
+
+package parser
+
+import (
+	"bytes"
+	"errors"
+	"go/ast"
+	"go/token"
+	"io"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// If src != nil, readSource converts src to a []byte if possible;
+// otherwise it returns an error. If src == nil, readSource returns
+// the result of reading the file specified by filename.
+//
+func readSource(filename string, src interface{}) ([]byte, error) {
+	if src != nil {
+		switch s := src.(type) {
+		case string:
+			return []byte(s), nil
+		case []byte:
+			return s, nil
+		case *bytes.Buffer:
+			// is io.Reader, but src is already available in []byte form
+			if s != nil {
+				return s.Bytes(), nil
+			}
+		case io.Reader:
+			return io.ReadAll(s)
+		}
+		return nil, errors.New("invalid source")
+	}
+	return os.ReadFile(filename)
+}
+
+// A Mode value is a set of flags (or 0).
+// They control the amount of source code parsed and other optional
+// parser functionality.
+//
+type Mode uint
+
+const (
+	PackageClauseOnly Mode             = 1 << iota // stop parsing after package clause
+	ImportsOnly                                    // stop parsing after import declarations
+	ParseComments                                  // parse comments and add them to AST
+	Trace                                          // print a trace of parsed productions
+	DeclarationErrors                              // report declaration errors
+	SpuriousErrors                                 // same as AllErrors, for backward-compatibility
+	AllErrors         = SpuriousErrors             // report all errors (not just the first 10 on different lines)
+)
+
+// ParseFile parses the source code of a single Go source file and returns
+// the corresponding ast.File node. The source code may be provided via
+// the filename of the source file, or via the src parameter.
+//
+// If src != nil, ParseFile parses the source from src and the filename is
+// only used when recording position information. The type of the argument
+// for the src parameter must be string, []byte, or io.Reader.
+// If src == nil, ParseFile parses the file specified by filename.
+//
+// The mode parameter controls the amount of source text parsed and other
+// optional parser functionality. Position information is recorded in the
+// file set fset, which must not be nil.
+//
+// If the source couldn't be read, the returned AST is nil and the error
+// indicates the specific failure. If the source was read but syntax
+// errors were found, the result is a partial AST (with ast.Bad* nodes
+// representing the fragments of erroneous source code). Multiple errors
+// are returned via a scanner.ErrorList which is sorted by source position.
+//
+func ParseFile(fset *token.FileSet, filename string, src interface{}, mode Mode) (f *ast.File, err error) {
+	if fset == nil {
+		panic("parser.ParseFile: no token.FileSet provided (fset == nil)")
+	}
+
+	// get source
+	text, err := readSource(filename, src)
+	if err != nil {
+		return nil, err
+	}
+
+	var p parser
+	defer func() {
+		if e := recover(); e != nil {
+			// resume same panic if it's not a bailout
+			if _, ok := e.(bailout); !ok {
+				panic(e)
+			}
+		}
+
+		// set result values
+		if f == nil {
+			// source is not a valid Go source file - satisfy
+			// ParseFile API and return a valid (but) empty
+			// *ast.File
+			f = &ast.File{
+				Name:  new(ast.Ident),
+				Scope: ast.NewScope(nil),
+			}
+		}
+
+		p.errors.Sort()
+		err = p.errors.Err()
+	}()
+
+	// parse source
+	p.init(fset, filename, text, mode)
+	f = p.parseFile()
+
+	return
+}
+
+// ParseDir calls ParseFile for all files with names ending in ".go" in the
+// directory specified by path and returns a map of package name -> package
+// AST with all the packages found.
+//
+// If filter != nil, only the files with fs.FileInfo entries passing through
+// the filter (and ending in ".go") are considered. The mode bits are passed
+// to ParseFile unchanged. Position information is recorded in fset, which
+// must not be nil.
+//
+// If the directory couldn't be read, a nil map and the respective error are
+// returned. If a parse error occurred, a non-nil but incomplete map and the
+// first error encountered are returned.
+//
+func ParseDir(fset *token.FileSet, path string, filter func(fs.FileInfo) bool, mode Mode) (pkgs map[string]*ast.Package, first error) {
+	list, err := os.ReadDir(path)
+	if err != nil {
+		return nil, err
+	}
+
+	pkgs = make(map[string]*ast.Package)
+	for _, d := range list {
+		if d.IsDir() || !strings.HasSuffix(d.Name(), ".go") {
+			continue
+		}
+		if filter != nil {
+			info, err := d.Info()
+			if err != nil {
+				return nil, err
+			}
+			if !filter(info) {
+				continue
+			}
+		}
+		filename := filepath.Join(path, d.Name())
+		if src, err := ParseFile(fset, filename, nil, mode); err == nil {
+			name := src.Name.Name
+			pkg, found := pkgs[name]
+			if !found {
+				pkg = &ast.Package{
+					Name:  name,
+					Files: make(map[string]*ast.File),
+				}
+				pkgs[name] = pkg
+			}
+			pkg.Files[filename] = src
+		} else if first == nil {
+			first = err
+		}
+	}
+
+	return
+}
+
+// ParseExprFrom is a convenience function for parsing an expression.
+// The arguments have the same meaning as for ParseFile, but the source must
+// be a valid Go (type or value) expression. Specifically, fset must not
+// be nil.
+//
+// If the source couldn't be read, the returned AST is nil and the error
+// indicates the specific failure. If the source was read but syntax
+// errors were found, the result is a partial AST (with ast.Bad* nodes
+// representing the fragments of erroneous source code). Multiple errors
+// are returned via a scanner.ErrorList which is sorted by source position.
+//
+func ParseExprFrom(fset *token.FileSet, filename string, src interface{}, mode Mode) (expr ast.Expr, err error) {
+	if fset == nil {
+		panic("parser.ParseExprFrom: no token.FileSet provided (fset == nil)")
+	}
+
+	// get source
+	text, err := readSource(filename, src)
+	if err != nil {
+		return nil, err
+	}
+
+	var p parser
+	defer func() {
+		if e := recover(); e != nil {
+			// resume same panic if it's not a bailout
+			if _, ok := e.(bailout); !ok {
+				panic(e)
+			}
+		}
+		p.errors.Sort()
+		err = p.errors.Err()
+	}()
+
+	// parse expr
+	p.init(fset, filename, text, mode)
+	// Set up pkg-level scopes to avoid nil-pointer errors.
+	// This is not needed for a correct expression x as the
+	// parser will be ok with a nil topScope, but be cautious
+	// in case of an erroneous x.
+	p.openScope()
+	p.pkgScope = p.topScope
+	expr = p.parseRhsOrType()
+	p.closeScope()
+	assert(p.topScope == nil, "unbalanced scopes")
+
+	// If a semicolon was inserted, consume it;
+	// report an error if there's more tokens.
+	if p.tok == token.SEMICOLON && p.lit == "\n" {
+		p.next()
+	}
+	p.expect(token.EOF)
+
+	return
+}
+
+// ParseExpr is a convenience function for obtaining the AST of an expression x.
+// The position information recorded in the AST is undefined. The filename used
+// in error messages is the empty string.
+//
+// If syntax errors were found, the result is a partial AST (with ast.Bad* nodes
+// representing the fragments of erroneous source code). Multiple errors are
+// returned via a scanner.ErrorList which is sorted by source position.
+//
+func ParseExpr(x string) (ast.Expr, error) {
+	return ParseExprFrom(token.NewFileSet(), "", []byte(x), 0)
+}
diff --git a/src/go/parser/parser.go b/src/go/parser/parser.go
new file mode 100644
index 0000000..31a7398
--- /dev/null
+++ b/src/go/parser/parser.go
@@ -0,0 +1,2591 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package parser implements a parser for Go source files. Input may be
+// provided in a variety of forms (see the various Parse* functions); the
+// output is an abstract syntax tree (AST) representing the Go source. The
+// parser is invoked through one of the Parse* functions.
+//
+// The parser accepts a larger language than is syntactically permitted by
+// the Go spec, for simplicity, and for improved robustness in the presence
+// of syntax errors. For instance, in method declarations, the receiver is
+// treated like an ordinary parameter list and thus may contain multiple
+// entries where the spec permits exactly one. Consequently, the corresponding
+// field in the AST (ast.FuncDecl.Recv) field is not restricted to one entry.
+//
+package parser
+
+import (
+	"fmt"
+	"go/ast"
+	"go/scanner"
+	"go/token"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+// The parser structure holds the parser's internal state.
+type parser struct {
+	file    *token.File
+	errors  scanner.ErrorList
+	scanner scanner.Scanner
+
+	// Tracing/debugging
+	mode   Mode // parsing mode
+	trace  bool // == (mode & Trace != 0)
+	indent int  // indentation used for tracing output
+
+	// Comments
+	comments    []*ast.CommentGroup
+	leadComment *ast.CommentGroup // last lead comment
+	lineComment *ast.CommentGroup // last line comment
+
+	// Next token
+	pos token.Pos   // token position
+	tok token.Token // one token look-ahead
+	lit string      // token literal
+
+	// Error recovery
+	// (used to limit the number of calls to parser.advance
+	// w/o making scanning progress - avoids potential endless
+	// loops across multiple parser functions during error recovery)
+	syncPos token.Pos // last synchronization position
+	syncCnt int       // number of parser.advance calls without progress
+
+	// Non-syntactic parser control
+	exprLev int  // < 0: in control clause, >= 0: in expression
+	inRhs   bool // if set, the parser is parsing a rhs expression
+
+	// Ordinary identifier scopes
+	pkgScope   *ast.Scope        // pkgScope.Outer == nil
+	topScope   *ast.Scope        // top-most scope; may be pkgScope
+	unresolved []*ast.Ident      // unresolved identifiers
+	imports    []*ast.ImportSpec // list of imports
+
+	// Label scopes
+	// (maintained by open/close LabelScope)
+	labelScope  *ast.Scope     // label scope for current function
+	targetStack [][]*ast.Ident // stack of unresolved labels
+}
+
+func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode Mode) {
+	p.file = fset.AddFile(filename, -1, len(src))
+	var m scanner.Mode
+	if mode&ParseComments != 0 {
+		m = scanner.ScanComments
+	}
+	eh := func(pos token.Position, msg string) { p.errors.Add(pos, msg) }
+	p.scanner.Init(p.file, src, eh, m)
+
+	p.mode = mode
+	p.trace = mode&Trace != 0 // for convenience (p.trace is used frequently)
+
+	p.next()
+}
+
+// ----------------------------------------------------------------------------
+// Scoping support
+
+func (p *parser) openScope() {
+	p.topScope = ast.NewScope(p.topScope)
+}
+
+func (p *parser) closeScope() {
+	p.topScope = p.topScope.Outer
+}
+
+func (p *parser) openLabelScope() {
+	p.labelScope = ast.NewScope(p.labelScope)
+	p.targetStack = append(p.targetStack, nil)
+}
+
+func (p *parser) closeLabelScope() {
+	// resolve labels
+	n := len(p.targetStack) - 1
+	scope := p.labelScope
+	for _, ident := range p.targetStack[n] {
+		ident.Obj = scope.Lookup(ident.Name)
+		if ident.Obj == nil && p.mode&DeclarationErrors != 0 {
+			p.error(ident.Pos(), fmt.Sprintf("label %s undefined", ident.Name))
+		}
+	}
+	// pop label scope
+	p.targetStack = p.targetStack[0:n]
+	p.labelScope = p.labelScope.Outer
+}
+
+func (p *parser) declare(decl, data interface{}, scope *ast.Scope, kind ast.ObjKind, idents ...*ast.Ident) {
+	for _, ident := range idents {
+		assert(ident.Obj == nil, "identifier already declared or resolved")
+		obj := ast.NewObj(kind, ident.Name)
+		// remember the corresponding declaration for redeclaration
+		// errors and global variable resolution/typechecking phase
+		obj.Decl = decl
+		obj.Data = data
+		ident.Obj = obj
+		if ident.Name != "_" {
+			if alt := scope.Insert(obj); alt != nil && p.mode&DeclarationErrors != 0 {
+				prevDecl := ""
+				if pos := alt.Pos(); pos.IsValid() {
+					prevDecl = fmt.Sprintf("\n\tprevious declaration at %s", p.file.Position(pos))
+				}
+				p.error(ident.Pos(), fmt.Sprintf("%s redeclared in this block%s", ident.Name, prevDecl))
+			}
+		}
+	}
+}
+
+func (p *parser) shortVarDecl(decl *ast.AssignStmt, list []ast.Expr) {
+	// Go spec: A short variable declaration may redeclare variables
+	// provided they were originally declared in the same block with
+	// the same type, and at least one of the non-blank variables is new.
+	n := 0 // number of new variables
+	for _, x := range list {
+		if ident, isIdent := x.(*ast.Ident); isIdent {
+			assert(ident.Obj == nil, "identifier already declared or resolved")
+			obj := ast.NewObj(ast.Var, ident.Name)
+			// remember corresponding assignment for other tools
+			obj.Decl = decl
+			ident.Obj = obj
+			if ident.Name != "_" {
+				if alt := p.topScope.Insert(obj); alt != nil {
+					ident.Obj = alt // redeclaration
+				} else {
+					n++ // new declaration
+				}
+			}
+		} else {
+			p.errorExpected(x.Pos(), "identifier on left side of :=")
+		}
+	}
+	if n == 0 && p.mode&DeclarationErrors != 0 {
+		p.error(list[0].Pos(), "no new variables on left side of :=")
+	}
+}
+
+// The unresolved object is a sentinel to mark identifiers that have been added
+// to the list of unresolved identifiers. The sentinel is only used for verifying
+// internal consistency.
+var unresolved = new(ast.Object)
+
+// If x is an identifier, tryResolve attempts to resolve x by looking up
+// the object it denotes. If no object is found and collectUnresolved is
+// set, x is marked as unresolved and collected in the list of unresolved
+// identifiers.
+//
+func (p *parser) tryResolve(x ast.Expr, collectUnresolved bool) {
+	// nothing to do if x is not an identifier or the blank identifier
+	ident, _ := x.(*ast.Ident)
+	if ident == nil {
+		return
+	}
+	assert(ident.Obj == nil, "identifier already declared or resolved")
+	if ident.Name == "_" {
+		return
+	}
+	// try to resolve the identifier
+	for s := p.topScope; s != nil; s = s.Outer {
+		if obj := s.Lookup(ident.Name); obj != nil {
+			ident.Obj = obj
+			return
+		}
+	}
+	// all local scopes are known, so any unresolved identifier
+	// must be found either in the file scope, package scope
+	// (perhaps in another file), or universe scope --- collect
+	// them so that they can be resolved later
+	if collectUnresolved {
+		ident.Obj = unresolved
+		p.unresolved = append(p.unresolved, ident)
+	}
+}
+
+func (p *parser) resolve(x ast.Expr) {
+	p.tryResolve(x, true)
+}
+
+// ----------------------------------------------------------------------------
+// Parsing support
+
+func (p *parser) printTrace(a ...interface{}) {
+	const dots = ". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "
+	const n = len(dots)
+	pos := p.file.Position(p.pos)
+	fmt.Printf("%5d:%3d: ", pos.Line, pos.Column)
+	i := 2 * p.indent
+	for i > n {
+		fmt.Print(dots)
+		i -= n
+	}
+	// i <= n
+	fmt.Print(dots[0:i])
+	fmt.Println(a...)
+}
+
+func trace(p *parser, msg string) *parser {
+	p.printTrace(msg, "(")
+	p.indent++
+	return p
+}
+
+// Usage pattern: defer un(trace(p, "..."))
+func un(p *parser) {
+	p.indent--
+	p.printTrace(")")
+}
+
+// Advance to the next token.
+func (p *parser) next0() {
+	// Because of one-token look-ahead, print the previous token
+	// when tracing as it provides a more readable output. The
+	// very first token (!p.pos.IsValid()) is not initialized
+	// (it is token.ILLEGAL), so don't print it .
+	if p.trace && p.pos.IsValid() {
+		s := p.tok.String()
+		switch {
+		case p.tok.IsLiteral():
+			p.printTrace(s, p.lit)
+		case p.tok.IsOperator(), p.tok.IsKeyword():
+			p.printTrace("\"" + s + "\"")
+		default:
+			p.printTrace(s)
+		}
+	}
+
+	p.pos, p.tok, p.lit = p.scanner.Scan()
+}
+
+// Consume a comment and return it and the line on which it ends.
+func (p *parser) consumeComment() (comment *ast.Comment, endline int) {
+	// /*-style comments may end on a different line than where they start.
+	// Scan the comment for '\n' chars and adjust endline accordingly.
+	endline = p.file.Line(p.pos)
+	if p.lit[1] == '*' {
+		// don't use range here - no need to decode Unicode code points
+		for i := 0; i < len(p.lit); i++ {
+			if p.lit[i] == '\n' {
+				endline++
+			}
+		}
+	}
+
+	comment = &ast.Comment{Slash: p.pos, Text: p.lit}
+	p.next0()
+
+	return
+}
+
+// Consume a group of adjacent comments, add it to the parser's
+// comments list, and return it together with the line at which
+// the last comment in the group ends. A non-comment token or n
+// empty lines terminate a comment group.
+//
+func (p *parser) consumeCommentGroup(n int) (comments *ast.CommentGroup, endline int) {
+	var list []*ast.Comment
+	endline = p.file.Line(p.pos)
+	for p.tok == token.COMMENT && p.file.Line(p.pos) <= endline+n {
+		var comment *ast.Comment
+		comment, endline = p.consumeComment()
+		list = append(list, comment)
+	}
+
+	// add comment group to the comments list
+	comments = &ast.CommentGroup{List: list}
+	p.comments = append(p.comments, comments)
+
+	return
+}
+
+// Advance to the next non-comment token. In the process, collect
+// any comment groups encountered, and remember the last lead and
+// line comments.
+//
+// A lead comment is a comment group that starts and ends in a
+// line without any other tokens and that is followed by a non-comment
+// token on the line immediately after the comment group.
+//
+// A line comment is a comment group that follows a non-comment
+// token on the same line, and that has no tokens after it on the line
+// where it ends.
+//
+// Lead and line comments may be considered documentation that is
+// stored in the AST.
+//
+func (p *parser) next() {
+	p.leadComment = nil
+	p.lineComment = nil
+	prev := p.pos
+	p.next0()
+
+	if p.tok == token.COMMENT {
+		var comment *ast.CommentGroup
+		var endline int
+
+		if p.file.Line(p.pos) == p.file.Line(prev) {
+			// The comment is on same line as the previous token; it
+			// cannot be a lead comment but may be a line comment.
+			comment, endline = p.consumeCommentGroup(0)
+			if p.file.Line(p.pos) != endline || p.tok == token.EOF {
+				// The next token is on a different line, thus
+				// the last comment group is a line comment.
+				p.lineComment = comment
+			}
+		}
+
+		// consume successor comments, if any
+		endline = -1
+		for p.tok == token.COMMENT {
+			comment, endline = p.consumeCommentGroup(1)
+		}
+
+		if endline+1 == p.file.Line(p.pos) {
+			// The next token is following on the line immediately after the
+			// comment group, thus the last comment group is a lead comment.
+			p.leadComment = comment
+		}
+	}
+}
+
+// A bailout panic is raised to indicate early termination.
+type bailout struct{}
+
+func (p *parser) error(pos token.Pos, msg string) {
+	epos := p.file.Position(pos)
+
+	// If AllErrors is not set, discard errors reported on the same line
+	// as the last recorded error and stop parsing if there are more than
+	// 10 errors.
+	if p.mode&AllErrors == 0 {
+		n := len(p.errors)
+		if n > 0 && p.errors[n-1].Pos.Line == epos.Line {
+			return // discard - likely a spurious error
+		}
+		if n > 10 {
+			panic(bailout{})
+		}
+	}
+
+	p.errors.Add(epos, msg)
+}
+
+func (p *parser) errorExpected(pos token.Pos, msg string) {
+	msg = "expected " + msg
+	if pos == p.pos {
+		// the error happened at the current position;
+		// make the error message more specific
+		switch {
+		case p.tok == token.SEMICOLON && p.lit == "\n":
+			msg += ", found newline"
+		case p.tok.IsLiteral():
+			// print 123 rather than 'INT', etc.
+			msg += ", found " + p.lit
+		default:
+			msg += ", found '" + p.tok.String() + "'"
+		}
+	}
+	p.error(pos, msg)
+}
+
+func (p *parser) expect(tok token.Token) token.Pos {
+	pos := p.pos
+	if p.tok != tok {
+		p.errorExpected(pos, "'"+tok.String()+"'")
+	}
+	p.next() // make progress
+	return pos
+}
+
+// expect2 is like expect, but it returns an invalid position
+// if the expected token is not found.
+func (p *parser) expect2(tok token.Token) (pos token.Pos) {
+	if p.tok == tok {
+		pos = p.pos
+	} else {
+		p.errorExpected(p.pos, "'"+tok.String()+"'")
+	}
+	p.next() // make progress
+	return
+}
+
+// expectClosing is like expect but provides a better error message
+// for the common case of a missing comma before a newline.
+//
+func (p *parser) expectClosing(tok token.Token, context string) token.Pos {
+	if p.tok != tok && p.tok == token.SEMICOLON && p.lit == "\n" {
+		p.error(p.pos, "missing ',' before newline in "+context)
+		p.next()
+	}
+	return p.expect(tok)
+}
+
+func (p *parser) expectSemi() {
+	// semicolon is optional before a closing ')' or '}'
+	if p.tok != token.RPAREN && p.tok != token.RBRACE {
+		switch p.tok {
+		case token.COMMA:
+			// permit a ',' instead of a ';' but complain
+			p.errorExpected(p.pos, "';'")
+			fallthrough
+		case token.SEMICOLON:
+			p.next()
+		default:
+			p.errorExpected(p.pos, "';'")
+			p.advance(stmtStart)
+		}
+	}
+}
+
+func (p *parser) atComma(context string, follow token.Token) bool {
+	if p.tok == token.COMMA {
+		return true
+	}
+	if p.tok != follow {
+		msg := "missing ','"
+		if p.tok == token.SEMICOLON && p.lit == "\n" {
+			msg += " before newline"
+		}
+		p.error(p.pos, msg+" in "+context)
+		return true // "insert" comma and continue
+	}
+	return false
+}
+
+func assert(cond bool, msg string) {
+	if !cond {
+		panic("go/parser internal error: " + msg)
+	}
+}
+
+// advance consumes tokens until the current token p.tok
+// is in the 'to' set, or token.EOF. For error recovery.
+func (p *parser) advance(to map[token.Token]bool) {
+	for ; p.tok != token.EOF; p.next() {
+		if to[p.tok] {
+			// Return only if parser made some progress since last
+			// sync or if it has not reached 10 advance calls without
+			// progress. Otherwise consume at least one token to
+			// avoid an endless parser loop (it is possible that
+			// both parseOperand and parseStmt call advance and
+			// correctly do not advance, thus the need for the
+			// invocation limit p.syncCnt).
+			if p.pos == p.syncPos && p.syncCnt < 10 {
+				p.syncCnt++
+				return
+			}
+			if p.pos > p.syncPos {
+				p.syncPos = p.pos
+				p.syncCnt = 0
+				return
+			}
+			// Reaching here indicates a parser bug, likely an
+			// incorrect token list in this function, but it only
+			// leads to skipping of possibly correct code if a
+			// previous error is present, and thus is preferred
+			// over a non-terminating parse.
+		}
+	}
+}
+
+var stmtStart = map[token.Token]bool{
+	token.BREAK:       true,
+	token.CONST:       true,
+	token.CONTINUE:    true,
+	token.DEFER:       true,
+	token.FALLTHROUGH: true,
+	token.FOR:         true,
+	token.GO:          true,
+	token.GOTO:        true,
+	token.IF:          true,
+	token.RETURN:      true,
+	token.SELECT:      true,
+	token.SWITCH:      true,
+	token.TYPE:        true,
+	token.VAR:         true,
+}
+
+var declStart = map[token.Token]bool{
+	token.CONST: true,
+	token.TYPE:  true,
+	token.VAR:   true,
+}
+
+var exprEnd = map[token.Token]bool{
+	token.COMMA:     true,
+	token.COLON:     true,
+	token.SEMICOLON: true,
+	token.RPAREN:    true,
+	token.RBRACK:    true,
+	token.RBRACE:    true,
+}
+
+// safePos returns a valid file position for a given position: If pos
+// is valid to begin with, safePos returns pos. If pos is out-of-range,
+// safePos returns the EOF position.
+//
+// This is hack to work around "artificial" end positions in the AST which
+// are computed by adding 1 to (presumably valid) token positions. If the
+// token positions are invalid due to parse errors, the resulting end position
+// may be past the file's EOF position, which would lead to panics if used
+// later on.
+//
+func (p *parser) safePos(pos token.Pos) (res token.Pos) {
+	defer func() {
+		if recover() != nil {
+			res = token.Pos(p.file.Base() + p.file.Size()) // EOF position
+		}
+	}()
+	_ = p.file.Offset(pos) // trigger a panic if position is out-of-range
+	return pos
+}
+
+// ----------------------------------------------------------------------------
+// Identifiers
+
+func (p *parser) parseIdent() *ast.Ident {
+	pos := p.pos
+	name := "_"
+	if p.tok == token.IDENT {
+		name = p.lit
+		p.next()
+	} else {
+		p.expect(token.IDENT) // use expect() error handling
+	}
+	return &ast.Ident{NamePos: pos, Name: name}
+}
+
+func (p *parser) parseIdentList() (list []*ast.Ident) {
+	if p.trace {
+		defer un(trace(p, "IdentList"))
+	}
+
+	list = append(list, p.parseIdent())
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.parseIdent())
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Common productions
+
+// If lhs is set, result list elements which are identifiers are not resolved.
+func (p *parser) parseExprList(lhs bool) (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "ExpressionList"))
+	}
+
+	list = append(list, p.checkExpr(p.parseExpr(lhs)))
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.checkExpr(p.parseExpr(lhs)))
+	}
+
+	return
+}
+
+func (p *parser) parseLhsList() []ast.Expr {
+	old := p.inRhs
+	p.inRhs = false
+	list := p.parseExprList(true)
+	switch p.tok {
+	case token.DEFINE:
+		// lhs of a short variable declaration
+		// but doesn't enter scope until later:
+		// caller must call p.shortVarDecl(p.makeIdentList(list))
+		// at appropriate time.
+	case token.COLON:
+		// lhs of a label declaration or a communication clause of a select
+		// statement (parseLhsList is not called when parsing the case clause
+		// of a switch statement):
+		// - labels are declared by the caller of parseLhsList
+		// - for communication clauses, if there is a stand-alone identifier
+		//   followed by a colon, we have a syntax error; there is no need
+		//   to resolve the identifier in that case
+	default:
+		// identifiers must be declared elsewhere
+		for _, x := range list {
+			p.resolve(x)
+		}
+	}
+	p.inRhs = old
+	return list
+}
+
+func (p *parser) parseRhsList() []ast.Expr {
+	old := p.inRhs
+	p.inRhs = true
+	list := p.parseExprList(false)
+	p.inRhs = old
+	return list
+}
+
+// ----------------------------------------------------------------------------
+// Types
+
+func (p *parser) parseType() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Type"))
+	}
+
+	typ := p.tryType()
+
+	if typ == nil {
+		pos := p.pos
+		p.errorExpected(pos, "type")
+		p.advance(exprEnd)
+		return &ast.BadExpr{From: pos, To: p.pos}
+	}
+
+	return typ
+}
+
+// If the result is an identifier, it is not resolved.
+func (p *parser) parseTypeName() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "TypeName"))
+	}
+
+	ident := p.parseIdent()
+	// don't resolve ident yet - it may be a parameter or field name
+
+	if p.tok == token.PERIOD {
+		// ident is a package name
+		p.next()
+		p.resolve(ident)
+		sel := p.parseIdent()
+		return &ast.SelectorExpr{X: ident, Sel: sel}
+	}
+
+	return ident
+}
+
+func (p *parser) parseArrayType() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "ArrayType"))
+	}
+
+	lbrack := p.expect(token.LBRACK)
+	p.exprLev++
+	var len ast.Expr
+	// always permit ellipsis for more fault-tolerant parsing
+	if p.tok == token.ELLIPSIS {
+		len = &ast.Ellipsis{Ellipsis: p.pos}
+		p.next()
+	} else if p.tok != token.RBRACK {
+		len = p.parseRhs()
+	}
+	p.exprLev--
+	p.expect(token.RBRACK)
+	elt := p.parseType()
+
+	return &ast.ArrayType{Lbrack: lbrack, Len: len, Elt: elt}
+}
+
+func (p *parser) makeIdentList(list []ast.Expr) []*ast.Ident {
+	idents := make([]*ast.Ident, len(list))
+	for i, x := range list {
+		ident, isIdent := x.(*ast.Ident)
+		if !isIdent {
+			if _, isBad := x.(*ast.BadExpr); !isBad {
+				// only report error if it's a new one
+				p.errorExpected(x.Pos(), "identifier")
+			}
+			ident = &ast.Ident{NamePos: x.Pos(), Name: "_"}
+		}
+		idents[i] = ident
+	}
+	return idents
+}
+
+func (p *parser) parseFieldDecl(scope *ast.Scope) *ast.Field {
+	if p.trace {
+		defer un(trace(p, "FieldDecl"))
+	}
+
+	doc := p.leadComment
+
+	// 1st FieldDecl
+	// A type name used as an anonymous field looks like a field identifier.
+	var list []ast.Expr
+	for {
+		list = append(list, p.parseVarType(false))
+		if p.tok != token.COMMA {
+			break
+		}
+		p.next()
+	}
+
+	typ := p.tryVarType(false)
+
+	// analyze case
+	var idents []*ast.Ident
+	if typ != nil {
+		// IdentifierList Type
+		idents = p.makeIdentList(list)
+	} else {
+		// ["*"] TypeName (AnonymousField)
+		typ = list[0] // we always have at least one element
+		if n := len(list); n > 1 {
+			p.errorExpected(p.pos, "type")
+			typ = &ast.BadExpr{From: p.pos, To: p.pos}
+		} else if !isTypeName(deref(typ)) {
+			p.errorExpected(typ.Pos(), "anonymous field")
+			typ = &ast.BadExpr{From: typ.Pos(), To: p.safePos(typ.End())}
+		}
+	}
+
+	// Tag
+	var tag *ast.BasicLit
+	if p.tok == token.STRING {
+		tag = &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
+		p.next()
+	}
+
+	p.expectSemi() // call before accessing p.linecomment
+
+	field := &ast.Field{Doc: doc, Names: idents, Type: typ, Tag: tag, Comment: p.lineComment}
+	p.declare(field, nil, scope, ast.Var, idents...)
+	p.resolve(typ)
+
+	return field
+}
+
+func (p *parser) parseStructType() *ast.StructType {
+	if p.trace {
+		defer un(trace(p, "StructType"))
+	}
+
+	pos := p.expect(token.STRUCT)
+	lbrace := p.expect(token.LBRACE)
+	scope := ast.NewScope(nil) // struct scope
+	var list []*ast.Field
+	for p.tok == token.IDENT || p.tok == token.MUL || p.tok == token.LPAREN {
+		// a field declaration cannot start with a '(' but we accept
+		// it here for more robust parsing and better error messages
+		// (parseFieldDecl will check and complain if necessary)
+		list = append(list, p.parseFieldDecl(scope))
+	}
+	rbrace := p.expect(token.RBRACE)
+
+	return &ast.StructType{
+		Struct: pos,
+		Fields: &ast.FieldList{
+			Opening: lbrace,
+			List:    list,
+			Closing: rbrace,
+		},
+	}
+}
+
+func (p *parser) parsePointerType() *ast.StarExpr {
+	if p.trace {
+		defer un(trace(p, "PointerType"))
+	}
+
+	star := p.expect(token.MUL)
+	base := p.parseType()
+
+	return &ast.StarExpr{Star: star, X: base}
+}
+
+// If the result is an identifier, it is not resolved.
+func (p *parser) tryVarType(isParam bool) ast.Expr {
+	if isParam && p.tok == token.ELLIPSIS {
+		pos := p.pos
+		p.next()
+		typ := p.tryIdentOrType() // don't use parseType so we can provide better error message
+		if typ != nil {
+			p.resolve(typ)
+		} else {
+			p.error(pos, "'...' parameter is missing type")
+			typ = &ast.BadExpr{From: pos, To: p.pos}
+		}
+		return &ast.Ellipsis{Ellipsis: pos, Elt: typ}
+	}
+	return p.tryIdentOrType()
+}
+
+// If the result is an identifier, it is not resolved.
+func (p *parser) parseVarType(isParam bool) ast.Expr {
+	typ := p.tryVarType(isParam)
+	if typ == nil {
+		pos := p.pos
+		p.errorExpected(pos, "type")
+		p.next() // make progress
+		typ = &ast.BadExpr{From: pos, To: p.pos}
+	}
+	return typ
+}
+
+func (p *parser) parseParameterList(scope *ast.Scope, ellipsisOk bool) (params []*ast.Field) {
+	if p.trace {
+		defer un(trace(p, "ParameterList"))
+	}
+
+	// 1st ParameterDecl
+	// A list of identifiers looks like a list of type names.
+	var list []ast.Expr
+	for {
+		list = append(list, p.parseVarType(ellipsisOk))
+		if p.tok != token.COMMA {
+			break
+		}
+		p.next()
+		if p.tok == token.RPAREN {
+			break
+		}
+	}
+
+	// analyze case
+	if typ := p.tryVarType(ellipsisOk); typ != nil {
+		// IdentifierList Type
+		idents := p.makeIdentList(list)
+		field := &ast.Field{Names: idents, Type: typ}
+		params = append(params, field)
+		// Go spec: The scope of an identifier denoting a function
+		// parameter or result variable is the function body.
+		p.declare(field, nil, scope, ast.Var, idents...)
+		p.resolve(typ)
+		if !p.atComma("parameter list", token.RPAREN) {
+			return
+		}
+		p.next()
+		for p.tok != token.RPAREN && p.tok != token.EOF {
+			idents := p.parseIdentList()
+			typ := p.parseVarType(ellipsisOk)
+			field := &ast.Field{Names: idents, Type: typ}
+			params = append(params, field)
+			// Go spec: The scope of an identifier denoting a function
+			// parameter or result variable is the function body.
+			p.declare(field, nil, scope, ast.Var, idents...)
+			p.resolve(typ)
+			if !p.atComma("parameter list", token.RPAREN) {
+				break
+			}
+			p.next()
+		}
+		return
+	}
+
+	// Type { "," Type } (anonymous parameters)
+	params = make([]*ast.Field, len(list))
+	for i, typ := range list {
+		p.resolve(typ)
+		params[i] = &ast.Field{Type: typ}
+	}
+	return
+}
+
+func (p *parser) parseParameters(scope *ast.Scope, ellipsisOk bool) *ast.FieldList {
+	if p.trace {
+		defer un(trace(p, "Parameters"))
+	}
+
+	var params []*ast.Field
+	lparen := p.expect(token.LPAREN)
+	if p.tok != token.RPAREN {
+		params = p.parseParameterList(scope, ellipsisOk)
+	}
+	rparen := p.expect(token.RPAREN)
+
+	return &ast.FieldList{Opening: lparen, List: params, Closing: rparen}
+}
+
+func (p *parser) parseResult(scope *ast.Scope) *ast.FieldList {
+	if p.trace {
+		defer un(trace(p, "Result"))
+	}
+
+	if p.tok == token.LPAREN {
+		return p.parseParameters(scope, false)
+	}
+
+	typ := p.tryType()
+	if typ != nil {
+		list := make([]*ast.Field, 1)
+		list[0] = &ast.Field{Type: typ}
+		return &ast.FieldList{List: list}
+	}
+
+	return nil
+}
+
+func (p *parser) parseSignature(scope *ast.Scope) (params, results *ast.FieldList) {
+	if p.trace {
+		defer un(trace(p, "Signature"))
+	}
+
+	params = p.parseParameters(scope, true)
+	results = p.parseResult(scope)
+
+	return
+}
+
+func (p *parser) parseFuncType() (*ast.FuncType, *ast.Scope) {
+	if p.trace {
+		defer un(trace(p, "FuncType"))
+	}
+
+	pos := p.expect(token.FUNC)
+	scope := ast.NewScope(p.topScope) // function scope
+	params, results := p.parseSignature(scope)
+
+	return &ast.FuncType{Func: pos, Params: params, Results: results}, scope
+}
+
+func (p *parser) parseMethodSpec(scope *ast.Scope) *ast.Field {
+	if p.trace {
+		defer un(trace(p, "MethodSpec"))
+	}
+
+	doc := p.leadComment
+	var idents []*ast.Ident
+	var typ ast.Expr
+	x := p.parseTypeName()
+	if ident, isIdent := x.(*ast.Ident); isIdent && p.tok == token.LPAREN {
+		// method
+		idents = []*ast.Ident{ident}
+		scope := ast.NewScope(nil) // method scope
+		params, results := p.parseSignature(scope)
+		typ = &ast.FuncType{Func: token.NoPos, Params: params, Results: results}
+	} else {
+		// embedded interface
+		typ = x
+		p.resolve(typ)
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	spec := &ast.Field{Doc: doc, Names: idents, Type: typ, Comment: p.lineComment}
+	p.declare(spec, nil, scope, ast.Fun, idents...)
+
+	return spec
+}
+
+func (p *parser) parseInterfaceType() *ast.InterfaceType {
+	if p.trace {
+		defer un(trace(p, "InterfaceType"))
+	}
+
+	pos := p.expect(token.INTERFACE)
+	lbrace := p.expect(token.LBRACE)
+	scope := ast.NewScope(nil) // interface scope
+	var list []*ast.Field
+	for p.tok == token.IDENT {
+		list = append(list, p.parseMethodSpec(scope))
+	}
+	rbrace := p.expect(token.RBRACE)
+
+	return &ast.InterfaceType{
+		Interface: pos,
+		Methods: &ast.FieldList{
+			Opening: lbrace,
+			List:    list,
+			Closing: rbrace,
+		},
+	}
+}
+
+func (p *parser) parseMapType() *ast.MapType {
+	if p.trace {
+		defer un(trace(p, "MapType"))
+	}
+
+	pos := p.expect(token.MAP)
+	p.expect(token.LBRACK)
+	key := p.parseType()
+	p.expect(token.RBRACK)
+	value := p.parseType()
+
+	return &ast.MapType{Map: pos, Key: key, Value: value}
+}
+
+func (p *parser) parseChanType() *ast.ChanType {
+	if p.trace {
+		defer un(trace(p, "ChanType"))
+	}
+
+	pos := p.pos
+	dir := ast.SEND | ast.RECV
+	var arrow token.Pos
+	if p.tok == token.CHAN {
+		p.next()
+		if p.tok == token.ARROW {
+			arrow = p.pos
+			p.next()
+			dir = ast.SEND
+		}
+	} else {
+		arrow = p.expect(token.ARROW)
+		p.expect(token.CHAN)
+		dir = ast.RECV
+	}
+	value := p.parseType()
+
+	return &ast.ChanType{Begin: pos, Arrow: arrow, Dir: dir, Value: value}
+}
+
+// If the result is an identifier, it is not resolved.
+func (p *parser) tryIdentOrType() ast.Expr {
+	switch p.tok {
+	case token.IDENT:
+		return p.parseTypeName()
+	case token.LBRACK:
+		return p.parseArrayType()
+	case token.STRUCT:
+		return p.parseStructType()
+	case token.MUL:
+		return p.parsePointerType()
+	case token.FUNC:
+		typ, _ := p.parseFuncType()
+		return typ
+	case token.INTERFACE:
+		return p.parseInterfaceType()
+	case token.MAP:
+		return p.parseMapType()
+	case token.CHAN, token.ARROW:
+		return p.parseChanType()
+	case token.LPAREN:
+		lparen := p.pos
+		p.next()
+		typ := p.parseType()
+		rparen := p.expect(token.RPAREN)
+		return &ast.ParenExpr{Lparen: lparen, X: typ, Rparen: rparen}
+	}
+
+	// no type found
+	return nil
+}
+
+func (p *parser) tryType() ast.Expr {
+	typ := p.tryIdentOrType()
+	if typ != nil {
+		p.resolve(typ)
+	}
+	return typ
+}
+
+// ----------------------------------------------------------------------------
+// Blocks
+
+func (p *parser) parseStmtList() (list []ast.Stmt) {
+	if p.trace {
+		defer un(trace(p, "StatementList"))
+	}
+
+	for p.tok != token.CASE && p.tok != token.DEFAULT && p.tok != token.RBRACE && p.tok != token.EOF {
+		list = append(list, p.parseStmt())
+	}
+
+	return
+}
+
+func (p *parser) parseBody(scope *ast.Scope) *ast.BlockStmt {
+	if p.trace {
+		defer un(trace(p, "Body"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	p.topScope = scope // open function scope
+	p.openLabelScope()
+	list := p.parseStmtList()
+	p.closeLabelScope()
+	p.closeScope()
+	rbrace := p.expect2(token.RBRACE)
+
+	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
+}
+
+func (p *parser) parseBlockStmt() *ast.BlockStmt {
+	if p.trace {
+		defer un(trace(p, "BlockStmt"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	p.openScope()
+	list := p.parseStmtList()
+	p.closeScope()
+	rbrace := p.expect2(token.RBRACE)
+
+	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
+}
+
+// ----------------------------------------------------------------------------
+// Expressions
+
+func (p *parser) parseFuncTypeOrLit() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "FuncTypeOrLit"))
+	}
+
+	typ, scope := p.parseFuncType()
+	if p.tok != token.LBRACE {
+		// function type only
+		return typ
+	}
+
+	p.exprLev++
+	body := p.parseBody(scope)
+	p.exprLev--
+
+	return &ast.FuncLit{Type: typ, Body: body}
+}
+
+// parseOperand may return an expression or a raw type (incl. array
+// types of the form [...]T. Callers must verify the result.
+// If lhs is set and the result is an identifier, it is not resolved.
+//
+func (p *parser) parseOperand(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Operand"))
+	}
+
+	switch p.tok {
+	case token.IDENT:
+		x := p.parseIdent()
+		if !lhs {
+			p.resolve(x)
+		}
+		return x
+
+	case token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING:
+		x := &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
+		p.next()
+		return x
+
+	case token.LPAREN:
+		lparen := p.pos
+		p.next()
+		p.exprLev++
+		x := p.parseRhsOrType() // types may be parenthesized: (some type)
+		p.exprLev--
+		rparen := p.expect(token.RPAREN)
+		return &ast.ParenExpr{Lparen: lparen, X: x, Rparen: rparen}
+
+	case token.FUNC:
+		return p.parseFuncTypeOrLit()
+	}
+
+	if typ := p.tryIdentOrType(); typ != nil {
+		// could be type for composite literal or conversion
+		_, isIdent := typ.(*ast.Ident)
+		assert(!isIdent, "type cannot be identifier")
+		return typ
+	}
+
+	// we have an error
+	pos := p.pos
+	p.errorExpected(pos, "operand")
+	p.advance(stmtStart)
+	return &ast.BadExpr{From: pos, To: p.pos}
+}
+
+func (p *parser) parseSelector(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Selector"))
+	}
+
+	sel := p.parseIdent()
+
+	return &ast.SelectorExpr{X: x, Sel: sel}
+}
+
+func (p *parser) parseTypeAssertion(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "TypeAssertion"))
+	}
+
+	lparen := p.expect(token.LPAREN)
+	var typ ast.Expr
+	if p.tok == token.TYPE {
+		// type switch: typ == nil
+		p.next()
+	} else {
+		typ = p.parseType()
+	}
+	rparen := p.expect(token.RPAREN)
+
+	return &ast.TypeAssertExpr{X: x, Type: typ, Lparen: lparen, Rparen: rparen}
+}
+
+func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "IndexOrSlice"))
+	}
+
+	const N = 3 // change the 3 to 2 to disable 3-index slices
+	lbrack := p.expect(token.LBRACK)
+	p.exprLev++
+	var index [N]ast.Expr
+	var colons [N - 1]token.Pos
+	if p.tok != token.COLON {
+		index[0] = p.parseRhs()
+	}
+	ncolons := 0
+	for p.tok == token.COLON && ncolons < len(colons) {
+		colons[ncolons] = p.pos
+		ncolons++
+		p.next()
+		if p.tok != token.COLON && p.tok != token.RBRACK && p.tok != token.EOF {
+			index[ncolons] = p.parseRhs()
+		}
+	}
+	p.exprLev--
+	rbrack := p.expect(token.RBRACK)
+
+	if ncolons > 0 {
+		// slice expression
+		slice3 := false
+		if ncolons == 2 {
+			slice3 = true
+			// Check presence of 2nd and 3rd index here rather than during type-checking
+			// to prevent erroneous programs from passing through gofmt (was issue 7305).
+			if index[1] == nil {
+				p.error(colons[0], "2nd index required in 3-index slice")
+				index[1] = &ast.BadExpr{From: colons[0] + 1, To: colons[1]}
+			}
+			if index[2] == nil {
+				p.error(colons[1], "3rd index required in 3-index slice")
+				index[2] = &ast.BadExpr{From: colons[1] + 1, To: rbrack}
+			}
+		}
+		return &ast.SliceExpr{X: x, Lbrack: lbrack, Low: index[0], High: index[1], Max: index[2], Slice3: slice3, Rbrack: rbrack}
+	}
+
+	return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: index[0], Rbrack: rbrack}
+}
+
+func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
+	if p.trace {
+		defer un(trace(p, "CallOrConversion"))
+	}
+
+	lparen := p.expect(token.LPAREN)
+	p.exprLev++
+	var list []ast.Expr
+	var ellipsis token.Pos
+	for p.tok != token.RPAREN && p.tok != token.EOF && !ellipsis.IsValid() {
+		list = append(list, p.parseRhsOrType()) // builtins may expect a type: make(some type, ...)
+		if p.tok == token.ELLIPSIS {
+			ellipsis = p.pos
+			p.next()
+		}
+		if !p.atComma("argument list", token.RPAREN) {
+			break
+		}
+		p.next()
+	}
+	p.exprLev--
+	rparen := p.expectClosing(token.RPAREN, "argument list")
+
+	return &ast.CallExpr{Fun: fun, Lparen: lparen, Args: list, Ellipsis: ellipsis, Rparen: rparen}
+}
+
+func (p *parser) parseValue(keyOk bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Element"))
+	}
+
+	if p.tok == token.LBRACE {
+		return p.parseLiteralValue(nil)
+	}
+
+	// Because the parser doesn't know the composite literal type, it cannot
+	// know if a key that's an identifier is a struct field name or a name
+	// denoting a value. The former is not resolved by the parser or the
+	// resolver.
+	//
+	// Instead, _try_ to resolve such a key if possible. If it resolves,
+	// it a) has correctly resolved, or b) incorrectly resolved because
+	// the key is a struct field with a name matching another identifier.
+	// In the former case we are done, and in the latter case we don't
+	// care because the type checker will do a separate field lookup.
+	//
+	// If the key does not resolve, it a) must be defined at the top
+	// level in another file of the same package, the universe scope, or be
+	// undeclared; or b) it is a struct field. In the former case, the type
+	// checker can do a top-level lookup, and in the latter case it will do
+	// a separate field lookup.
+	x := p.checkExpr(p.parseExpr(keyOk))
+	if keyOk {
+		if p.tok == token.COLON {
+			// Try to resolve the key but don't collect it
+			// as unresolved identifier if it fails so that
+			// we don't get (possibly false) errors about
+			// undeclared names.
+			p.tryResolve(x, false)
+		} else {
+			// not a key
+			p.resolve(x)
+		}
+	}
+
+	return x
+}
+
+func (p *parser) parseElement() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Element"))
+	}
+
+	x := p.parseValue(true)
+	if p.tok == token.COLON {
+		colon := p.pos
+		p.next()
+		x = &ast.KeyValueExpr{Key: x, Colon: colon, Value: p.parseValue(false)}
+	}
+
+	return x
+}
+
+func (p *parser) parseElementList() (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "ElementList"))
+	}
+
+	for p.tok != token.RBRACE && p.tok != token.EOF {
+		list = append(list, p.parseElement())
+		if !p.atComma("composite literal", token.RBRACE) {
+			break
+		}
+		p.next()
+	}
+
+	return
+}
+
+func (p *parser) parseLiteralValue(typ ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "LiteralValue"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	var elts []ast.Expr
+	p.exprLev++
+	if p.tok != token.RBRACE {
+		elts = p.parseElementList()
+	}
+	p.exprLev--
+	rbrace := p.expectClosing(token.RBRACE, "composite literal")
+	return &ast.CompositeLit{Type: typ, Lbrace: lbrace, Elts: elts, Rbrace: rbrace}
+}
+
+// checkExpr checks that x is an expression (and not a type).
+func (p *parser) checkExpr(x ast.Expr) ast.Expr {
+	switch unparen(x).(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.BasicLit:
+	case *ast.FuncLit:
+	case *ast.CompositeLit:
+	case *ast.ParenExpr:
+		panic("unreachable")
+	case *ast.SelectorExpr:
+	case *ast.IndexExpr:
+	case *ast.SliceExpr:
+	case *ast.TypeAssertExpr:
+		// If t.Type == nil we have a type assertion of the form
+		// y.(type), which is only allowed in type switch expressions.
+		// It's hard to exclude those but for the case where we are in
+		// a type switch. Instead be lenient and test this in the type
+		// checker.
+	case *ast.CallExpr:
+	case *ast.StarExpr:
+	case *ast.UnaryExpr:
+	case *ast.BinaryExpr:
+	default:
+		// all other nodes are not proper expressions
+		p.errorExpected(x.Pos(), "expression")
+		x = &ast.BadExpr{From: x.Pos(), To: p.safePos(x.End())}
+	}
+	return x
+}
+
+// isTypeName reports whether x is a (qualified) TypeName.
+func isTypeName(x ast.Expr) bool {
+	switch t := x.(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.SelectorExpr:
+		_, isIdent := t.X.(*ast.Ident)
+		return isIdent
+	default:
+		return false // all other nodes are not type names
+	}
+	return true
+}
+
+// isLiteralType reports whether x is a legal composite literal type.
+func isLiteralType(x ast.Expr) bool {
+	switch t := x.(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.SelectorExpr:
+		_, isIdent := t.X.(*ast.Ident)
+		return isIdent
+	case *ast.ArrayType:
+	case *ast.StructType:
+	case *ast.MapType:
+	default:
+		return false // all other nodes are not legal composite literal types
+	}
+	return true
+}
+
+// If x is of the form *T, deref returns T, otherwise it returns x.
+func deref(x ast.Expr) ast.Expr {
+	if p, isPtr := x.(*ast.StarExpr); isPtr {
+		x = p.X
+	}
+	return x
+}
+
+// If x is of the form (T), unparen returns unparen(T), otherwise it returns x.
+func unparen(x ast.Expr) ast.Expr {
+	if p, isParen := x.(*ast.ParenExpr); isParen {
+		x = unparen(p.X)
+	}
+	return x
+}
+
+// checkExprOrType checks that x is an expression or a type
+// (and not a raw type such as [...]T).
+//
+func (p *parser) checkExprOrType(x ast.Expr) ast.Expr {
+	switch t := unparen(x).(type) {
+	case *ast.ParenExpr:
+		panic("unreachable")
+	case *ast.ArrayType:
+		if len, isEllipsis := t.Len.(*ast.Ellipsis); isEllipsis {
+			p.error(len.Pos(), "expected array length, found '...'")
+			x = &ast.BadExpr{From: x.Pos(), To: p.safePos(x.End())}
+		}
+	}
+
+	// all other nodes are expressions or types
+	return x
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parsePrimaryExpr(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "PrimaryExpr"))
+	}
+
+	x := p.parseOperand(lhs)
+L:
+	for {
+		switch p.tok {
+		case token.PERIOD:
+			p.next()
+			if lhs {
+				p.resolve(x)
+			}
+			switch p.tok {
+			case token.IDENT:
+				x = p.parseSelector(p.checkExprOrType(x))
+			case token.LPAREN:
+				x = p.parseTypeAssertion(p.checkExpr(x))
+			default:
+				pos := p.pos
+				p.errorExpected(pos, "selector or type assertion")
+				p.next() // make progress
+				sel := &ast.Ident{NamePos: pos, Name: "_"}
+				x = &ast.SelectorExpr{X: x, Sel: sel}
+			}
+		case token.LBRACK:
+			if lhs {
+				p.resolve(x)
+			}
+			x = p.parseIndexOrSlice(p.checkExpr(x))
+		case token.LPAREN:
+			if lhs {
+				p.resolve(x)
+			}
+			x = p.parseCallOrConversion(p.checkExprOrType(x))
+		case token.LBRACE:
+			if isLiteralType(x) && (p.exprLev >= 0 || !isTypeName(x)) {
+				if lhs {
+					p.resolve(x)
+				}
+				x = p.parseLiteralValue(x)
+			} else {
+				break L
+			}
+		default:
+			break L
+		}
+		lhs = false // no need to try to resolve again
+	}
+
+	return x
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parseUnaryExpr(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "UnaryExpr"))
+	}
+
+	switch p.tok {
+	case token.ADD, token.SUB, token.NOT, token.XOR, token.AND:
+		pos, op := p.pos, p.tok
+		p.next()
+		x := p.parseUnaryExpr(false)
+		return &ast.UnaryExpr{OpPos: pos, Op: op, X: p.checkExpr(x)}
+
+	case token.ARROW:
+		// channel type or receive expression
+		arrow := p.pos
+		p.next()
+
+		// If the next token is token.CHAN we still don't know if it
+		// is a channel type or a receive operation - we only know
+		// once we have found the end of the unary expression. There
+		// are two cases:
+		//
+		//   <- type  => (<-type) must be channel type
+		//   <- expr  => <-(expr) is a receive from an expression
+		//
+		// In the first case, the arrow must be re-associated with
+		// the channel type parsed already:
+		//
+		//   <- (chan type)    =>  (<-chan type)
+		//   <- (chan<- type)  =>  (<-chan (<-type))
+
+		x := p.parseUnaryExpr(false)
+
+		// determine which case we have
+		if typ, ok := x.(*ast.ChanType); ok {
+			// (<-type)
+
+			// re-associate position info and <-
+			dir := ast.SEND
+			for ok && dir == ast.SEND {
+				if typ.Dir == ast.RECV {
+					// error: (<-type) is (<-(<-chan T))
+					p.errorExpected(typ.Arrow, "'chan'")
+				}
+				arrow, typ.Begin, typ.Arrow = typ.Arrow, arrow, arrow
+				dir, typ.Dir = typ.Dir, ast.RECV
+				typ, ok = typ.Value.(*ast.ChanType)
+			}
+			if dir == ast.SEND {
+				p.errorExpected(arrow, "channel type")
+			}
+
+			return x
+		}
+
+		// <-(expr)
+		return &ast.UnaryExpr{OpPos: arrow, Op: token.ARROW, X: p.checkExpr(x)}
+
+	case token.MUL:
+		// pointer type or unary "*" expression
+		pos := p.pos
+		p.next()
+		x := p.parseUnaryExpr(false)
+		return &ast.StarExpr{Star: pos, X: p.checkExprOrType(x)}
+	}
+
+	return p.parsePrimaryExpr(lhs)
+}
+
+func (p *parser) tokPrec() (token.Token, int) {
+	tok := p.tok
+	if p.inRhs && tok == token.ASSIGN {
+		tok = token.EQL
+	}
+	return tok, tok.Precedence()
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parseBinaryExpr(lhs bool, prec1 int) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "BinaryExpr"))
+	}
+
+	x := p.parseUnaryExpr(lhs)
+	for {
+		op, oprec := p.tokPrec()
+		if oprec < prec1 {
+			return x
+		}
+		pos := p.expect(op)
+		if lhs {
+			p.resolve(x)
+			lhs = false
+		}
+		y := p.parseBinaryExpr(false, oprec+1)
+		x = &ast.BinaryExpr{X: p.checkExpr(x), OpPos: pos, Op: op, Y: p.checkExpr(y)}
+	}
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+// The result may be a type or even a raw type ([...]int). Callers must
+// check the result (using checkExpr or checkExprOrType), depending on
+// context.
+func (p *parser) parseExpr(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Expression"))
+	}
+
+	return p.parseBinaryExpr(lhs, token.LowestPrec+1)
+}
+
+func (p *parser) parseRhs() ast.Expr {
+	old := p.inRhs
+	p.inRhs = true
+	x := p.checkExpr(p.parseExpr(false))
+	p.inRhs = old
+	return x
+}
+
+func (p *parser) parseRhsOrType() ast.Expr {
+	old := p.inRhs
+	p.inRhs = true
+	x := p.checkExprOrType(p.parseExpr(false))
+	p.inRhs = old
+	return x
+}
+
+// ----------------------------------------------------------------------------
+// Statements
+
+// Parsing modes for parseSimpleStmt.
+const (
+	basic = iota
+	labelOk
+	rangeOk
+)
+
+// parseSimpleStmt returns true as 2nd result if it parsed the assignment
+// of a range clause (with mode == rangeOk). The returned statement is an
+// assignment with a right-hand side that is a single unary expression of
+// the form "range x". No guarantees are given for the left-hand side.
+func (p *parser) parseSimpleStmt(mode int) (ast.Stmt, bool) {
+	if p.trace {
+		defer un(trace(p, "SimpleStmt"))
+	}
+
+	x := p.parseLhsList()
+
+	switch p.tok {
+	case
+		token.DEFINE, token.ASSIGN, token.ADD_ASSIGN,
+		token.SUB_ASSIGN, token.MUL_ASSIGN, token.QUO_ASSIGN,
+		token.REM_ASSIGN, token.AND_ASSIGN, token.OR_ASSIGN,
+		token.XOR_ASSIGN, token.SHL_ASSIGN, token.SHR_ASSIGN, token.AND_NOT_ASSIGN:
+		// assignment statement, possibly part of a range clause
+		pos, tok := p.pos, p.tok
+		p.next()
+		var y []ast.Expr
+		isRange := false
+		if mode == rangeOk && p.tok == token.RANGE && (tok == token.DEFINE || tok == token.ASSIGN) {
+			pos := p.pos
+			p.next()
+			y = []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
+			isRange = true
+		} else {
+			y = p.parseRhsList()
+		}
+		as := &ast.AssignStmt{Lhs: x, TokPos: pos, Tok: tok, Rhs: y}
+		if tok == token.DEFINE {
+			p.shortVarDecl(as, x)
+		}
+		return as, isRange
+	}
+
+	if len(x) > 1 {
+		p.errorExpected(x[0].Pos(), "1 expression")
+		// continue with first expression
+	}
+
+	switch p.tok {
+	case token.COLON:
+		// labeled statement
+		colon := p.pos
+		p.next()
+		if label, isIdent := x[0].(*ast.Ident); mode == labelOk && isIdent {
+			// Go spec: The scope of a label is the body of the function
+			// in which it is declared and excludes the body of any nested
+			// function.
+			stmt := &ast.LabeledStmt{Label: label, Colon: colon, Stmt: p.parseStmt()}
+			p.declare(stmt, nil, p.labelScope, ast.Lbl, label)
+			return stmt, false
+		}
+		// The label declaration typically starts at x[0].Pos(), but the label
+		// declaration may be erroneous due to a token after that position (and
+		// before the ':'). If SpuriousErrors is not set, the (only) error
+		// reported for the line is the illegal label error instead of the token
+		// before the ':' that caused the problem. Thus, use the (latest) colon
+		// position for error reporting.
+		p.error(colon, "illegal label declaration")
+		return &ast.BadStmt{From: x[0].Pos(), To: colon + 1}, false
+
+	case token.ARROW:
+		// send statement
+		arrow := p.pos
+		p.next()
+		y := p.parseRhs()
+		return &ast.SendStmt{Chan: x[0], Arrow: arrow, Value: y}, false
+
+	case token.INC, token.DEC:
+		// increment or decrement
+		s := &ast.IncDecStmt{X: x[0], TokPos: p.pos, Tok: p.tok}
+		p.next()
+		return s, false
+	}
+
+	// expression
+	return &ast.ExprStmt{X: x[0]}, false
+}
+
+func (p *parser) parseCallExpr(callType string) *ast.CallExpr {
+	x := p.parseRhsOrType() // could be a conversion: (some type)(x)
+	if call, isCall := x.(*ast.CallExpr); isCall {
+		return call
+	}
+	if _, isBad := x.(*ast.BadExpr); !isBad {
+		// only report error if it's a new one
+		p.error(p.safePos(x.End()), fmt.Sprintf("function must be invoked in %s statement", callType))
+	}
+	return nil
+}
+
+func (p *parser) parseGoStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "GoStmt"))
+	}
+
+	pos := p.expect(token.GO)
+	call := p.parseCallExpr("go")
+	p.expectSemi()
+	if call == nil {
+		return &ast.BadStmt{From: pos, To: pos + 2} // len("go")
+	}
+
+	return &ast.GoStmt{Go: pos, Call: call}
+}
+
+func (p *parser) parseDeferStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "DeferStmt"))
+	}
+
+	pos := p.expect(token.DEFER)
+	call := p.parseCallExpr("defer")
+	p.expectSemi()
+	if call == nil {
+		return &ast.BadStmt{From: pos, To: pos + 5} // len("defer")
+	}
+
+	return &ast.DeferStmt{Defer: pos, Call: call}
+}
+
+func (p *parser) parseReturnStmt() *ast.ReturnStmt {
+	if p.trace {
+		defer un(trace(p, "ReturnStmt"))
+	}
+
+	pos := p.pos
+	p.expect(token.RETURN)
+	var x []ast.Expr
+	if p.tok != token.SEMICOLON && p.tok != token.RBRACE {
+		x = p.parseRhsList()
+	}
+	p.expectSemi()
+
+	return &ast.ReturnStmt{Return: pos, Results: x}
+}
+
+func (p *parser) parseBranchStmt(tok token.Token) *ast.BranchStmt {
+	if p.trace {
+		defer un(trace(p, "BranchStmt"))
+	}
+
+	pos := p.expect(tok)
+	var label *ast.Ident
+	if tok != token.FALLTHROUGH && p.tok == token.IDENT {
+		label = p.parseIdent()
+		// add to list of unresolved targets
+		n := len(p.targetStack) - 1
+		p.targetStack[n] = append(p.targetStack[n], label)
+	}
+	p.expectSemi()
+
+	return &ast.BranchStmt{TokPos: pos, Tok: tok, Label: label}
+}
+
+func (p *parser) makeExpr(s ast.Stmt, want string) ast.Expr {
+	if s == nil {
+		return nil
+	}
+	if es, isExpr := s.(*ast.ExprStmt); isExpr {
+		return p.checkExpr(es.X)
+	}
+	found := "simple statement"
+	if _, isAss := s.(*ast.AssignStmt); isAss {
+		found = "assignment"
+	}
+	p.error(s.Pos(), fmt.Sprintf("expected %s, found %s (missing parentheses around composite literal?)", want, found))
+	return &ast.BadExpr{From: s.Pos(), To: p.safePos(s.End())}
+}
+
+// parseIfHeader is an adjusted version of parser.header
+// in cmd/compile/internal/syntax/parser.go, which has
+// been tuned for better error handling.
+func (p *parser) parseIfHeader() (init ast.Stmt, cond ast.Expr) {
+	if p.tok == token.LBRACE {
+		p.error(p.pos, "missing condition in if statement")
+		cond = &ast.BadExpr{From: p.pos, To: p.pos}
+		return
+	}
+	// p.tok != token.LBRACE
+
+	outer := p.exprLev
+	p.exprLev = -1
+
+	if p.tok != token.SEMICOLON {
+		// accept potential variable declaration but complain
+		if p.tok == token.VAR {
+			p.next()
+			p.error(p.pos, fmt.Sprintf("var declaration not allowed in 'IF' initializer"))
+		}
+		init, _ = p.parseSimpleStmt(basic)
+	}
+
+	var condStmt ast.Stmt
+	var semi struct {
+		pos token.Pos
+		lit string // ";" or "\n"; valid if pos.IsValid()
+	}
+	if p.tok != token.LBRACE {
+		if p.tok == token.SEMICOLON {
+			semi.pos = p.pos
+			semi.lit = p.lit
+			p.next()
+		} else {
+			p.expect(token.SEMICOLON)
+		}
+		if p.tok != token.LBRACE {
+			condStmt, _ = p.parseSimpleStmt(basic)
+		}
+	} else {
+		condStmt = init
+		init = nil
+	}
+
+	if condStmt != nil {
+		cond = p.makeExpr(condStmt, "boolean expression")
+	} else if semi.pos.IsValid() {
+		if semi.lit == "\n" {
+			p.error(semi.pos, "unexpected newline, expecting { after if clause")
+		} else {
+			p.error(semi.pos, "missing condition in if statement")
+		}
+	}
+
+	// make sure we have a valid AST
+	if cond == nil {
+		cond = &ast.BadExpr{From: p.pos, To: p.pos}
+	}
+
+	p.exprLev = outer
+	return
+}
+
+func (p *parser) parseIfStmt() *ast.IfStmt {
+	if p.trace {
+		defer un(trace(p, "IfStmt"))
+	}
+
+	pos := p.expect(token.IF)
+	p.openScope()
+	defer p.closeScope()
+
+	init, cond := p.parseIfHeader()
+	body := p.parseBlockStmt()
+
+	var else_ ast.Stmt
+	if p.tok == token.ELSE {
+		p.next()
+		switch p.tok {
+		case token.IF:
+			else_ = p.parseIfStmt()
+		case token.LBRACE:
+			else_ = p.parseBlockStmt()
+			p.expectSemi()
+		default:
+			p.errorExpected(p.pos, "if statement or block")
+			else_ = &ast.BadStmt{From: p.pos, To: p.pos}
+		}
+	} else {
+		p.expectSemi()
+	}
+
+	return &ast.IfStmt{If: pos, Init: init, Cond: cond, Body: body, Else: else_}
+}
+
+func (p *parser) parseTypeList() (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "TypeList"))
+	}
+
+	list = append(list, p.parseType())
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.parseType())
+	}
+
+	return
+}
+
+func (p *parser) parseCaseClause(typeSwitch bool) *ast.CaseClause {
+	if p.trace {
+		defer un(trace(p, "CaseClause"))
+	}
+
+	pos := p.pos
+	var list []ast.Expr
+	if p.tok == token.CASE {
+		p.next()
+		if typeSwitch {
+			list = p.parseTypeList()
+		} else {
+			list = p.parseRhsList()
+		}
+	} else {
+		p.expect(token.DEFAULT)
+	}
+
+	colon := p.expect(token.COLON)
+	p.openScope()
+	body := p.parseStmtList()
+	p.closeScope()
+
+	return &ast.CaseClause{Case: pos, List: list, Colon: colon, Body: body}
+}
+
+func isTypeSwitchAssert(x ast.Expr) bool {
+	a, ok := x.(*ast.TypeAssertExpr)
+	return ok && a.Type == nil
+}
+
+func (p *parser) isTypeSwitchGuard(s ast.Stmt) bool {
+	switch t := s.(type) {
+	case *ast.ExprStmt:
+		// x.(type)
+		return isTypeSwitchAssert(t.X)
+	case *ast.AssignStmt:
+		// v := x.(type)
+		if len(t.Lhs) == 1 && len(t.Rhs) == 1 && isTypeSwitchAssert(t.Rhs[0]) {
+			switch t.Tok {
+			case token.ASSIGN:
+				// permit v = x.(type) but complain
+				p.error(t.TokPos, "expected ':=', found '='")
+				fallthrough
+			case token.DEFINE:
+				return true
+			}
+		}
+	}
+	return false
+}
+
+func (p *parser) parseSwitchStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "SwitchStmt"))
+	}
+
+	pos := p.expect(token.SWITCH)
+	p.openScope()
+	defer p.closeScope()
+
+	var s1, s2 ast.Stmt
+	if p.tok != token.LBRACE {
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok != token.SEMICOLON {
+			s2, _ = p.parseSimpleStmt(basic)
+		}
+		if p.tok == token.SEMICOLON {
+			p.next()
+			s1 = s2
+			s2 = nil
+			if p.tok != token.LBRACE {
+				// A TypeSwitchGuard may declare a variable in addition
+				// to the variable declared in the initial SimpleStmt.
+				// Introduce extra scope to avoid redeclaration errors:
+				//
+				//	switch t := 0; t := x.(T) { ... }
+				//
+				// (this code is not valid Go because the first t
+				// cannot be accessed and thus is never used, the extra
+				// scope is needed for the correct error message).
+				//
+				// If we don't have a type switch, s2 must be an expression.
+				// Having the extra nested but empty scope won't affect it.
+				p.openScope()
+				defer p.closeScope()
+				s2, _ = p.parseSimpleStmt(basic)
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	typeSwitch := p.isTypeSwitchGuard(s2)
+	lbrace := p.expect(token.LBRACE)
+	var list []ast.Stmt
+	for p.tok == token.CASE || p.tok == token.DEFAULT {
+		list = append(list, p.parseCaseClause(typeSwitch))
+	}
+	rbrace := p.expect(token.RBRACE)
+	p.expectSemi()
+	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
+
+	if typeSwitch {
+		return &ast.TypeSwitchStmt{Switch: pos, Init: s1, Assign: s2, Body: body}
+	}
+
+	return &ast.SwitchStmt{Switch: pos, Init: s1, Tag: p.makeExpr(s2, "switch expression"), Body: body}
+}
+
+func (p *parser) parseCommClause() *ast.CommClause {
+	if p.trace {
+		defer un(trace(p, "CommClause"))
+	}
+
+	p.openScope()
+	pos := p.pos
+	var comm ast.Stmt
+	if p.tok == token.CASE {
+		p.next()
+		lhs := p.parseLhsList()
+		if p.tok == token.ARROW {
+			// SendStmt
+			if len(lhs) > 1 {
+				p.errorExpected(lhs[0].Pos(), "1 expression")
+				// continue with first expression
+			}
+			arrow := p.pos
+			p.next()
+			rhs := p.parseRhs()
+			comm = &ast.SendStmt{Chan: lhs[0], Arrow: arrow, Value: rhs}
+		} else {
+			// RecvStmt
+			if tok := p.tok; tok == token.ASSIGN || tok == token.DEFINE {
+				// RecvStmt with assignment
+				if len(lhs) > 2 {
+					p.errorExpected(lhs[0].Pos(), "1 or 2 expressions")
+					// continue with first two expressions
+					lhs = lhs[0:2]
+				}
+				pos := p.pos
+				p.next()
+				rhs := p.parseRhs()
+				as := &ast.AssignStmt{Lhs: lhs, TokPos: pos, Tok: tok, Rhs: []ast.Expr{rhs}}
+				if tok == token.DEFINE {
+					p.shortVarDecl(as, lhs)
+				}
+				comm = as
+			} else {
+				// lhs must be single receive operation
+				if len(lhs) > 1 {
+					p.errorExpected(lhs[0].Pos(), "1 expression")
+					// continue with first expression
+				}
+				comm = &ast.ExprStmt{X: lhs[0]}
+			}
+		}
+	} else {
+		p.expect(token.DEFAULT)
+	}
+
+	colon := p.expect(token.COLON)
+	body := p.parseStmtList()
+	p.closeScope()
+
+	return &ast.CommClause{Case: pos, Comm: comm, Colon: colon, Body: body}
+}
+
+func (p *parser) parseSelectStmt() *ast.SelectStmt {
+	if p.trace {
+		defer un(trace(p, "SelectStmt"))
+	}
+
+	pos := p.expect(token.SELECT)
+	lbrace := p.expect(token.LBRACE)
+	var list []ast.Stmt
+	for p.tok == token.CASE || p.tok == token.DEFAULT {
+		list = append(list, p.parseCommClause())
+	}
+	rbrace := p.expect(token.RBRACE)
+	p.expectSemi()
+	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
+
+	return &ast.SelectStmt{Select: pos, Body: body}
+}
+
+func (p *parser) parseForStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "ForStmt"))
+	}
+
+	pos := p.expect(token.FOR)
+	p.openScope()
+	defer p.closeScope()
+
+	var s1, s2, s3 ast.Stmt
+	var isRange bool
+	if p.tok != token.LBRACE {
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok != token.SEMICOLON {
+			if p.tok == token.RANGE {
+				// "for range x" (nil lhs in assignment)
+				pos := p.pos
+				p.next()
+				y := []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
+				s2 = &ast.AssignStmt{Rhs: y}
+				isRange = true
+			} else {
+				s2, isRange = p.parseSimpleStmt(rangeOk)
+			}
+		}
+		if !isRange && p.tok == token.SEMICOLON {
+			p.next()
+			s1 = s2
+			s2 = nil
+			if p.tok != token.SEMICOLON {
+				s2, _ = p.parseSimpleStmt(basic)
+			}
+			p.expectSemi()
+			if p.tok != token.LBRACE {
+				s3, _ = p.parseSimpleStmt(basic)
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	body := p.parseBlockStmt()
+	p.expectSemi()
+
+	if isRange {
+		as := s2.(*ast.AssignStmt)
+		// check lhs
+		var key, value ast.Expr
+		switch len(as.Lhs) {
+		case 0:
+			// nothing to do
+		case 1:
+			key = as.Lhs[0]
+		case 2:
+			key, value = as.Lhs[0], as.Lhs[1]
+		default:
+			p.errorExpected(as.Lhs[len(as.Lhs)-1].Pos(), "at most 2 expressions")
+			return &ast.BadStmt{From: pos, To: p.safePos(body.End())}
+		}
+		// parseSimpleStmt returned a right-hand side that
+		// is a single unary expression of the form "range x"
+		x := as.Rhs[0].(*ast.UnaryExpr).X
+		return &ast.RangeStmt{
+			For:    pos,
+			Key:    key,
+			Value:  value,
+			TokPos: as.TokPos,
+			Tok:    as.Tok,
+			X:      x,
+			Body:   body,
+		}
+	}
+
+	// regular for statement
+	return &ast.ForStmt{
+		For:  pos,
+		Init: s1,
+		Cond: p.makeExpr(s2, "boolean or range expression"),
+		Post: s3,
+		Body: body,
+	}
+}
+
+func (p *parser) parseStmt() (s ast.Stmt) {
+	if p.trace {
+		defer un(trace(p, "Statement"))
+	}
+
+	switch p.tok {
+	case token.CONST, token.TYPE, token.VAR:
+		s = &ast.DeclStmt{Decl: p.parseDecl(stmtStart)}
+	case
+		// tokens that may start an expression
+		token.IDENT, token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operands
+		token.LBRACK, token.STRUCT, token.MAP, token.CHAN, token.INTERFACE, // composite types
+		token.ADD, token.SUB, token.MUL, token.AND, token.XOR, token.ARROW, token.NOT: // unary operators
+		s, _ = p.parseSimpleStmt(labelOk)
+		// because of the required look-ahead, labeled statements are
+		// parsed by parseSimpleStmt - don't expect a semicolon after
+		// them
+		if _, isLabeledStmt := s.(*ast.LabeledStmt); !isLabeledStmt {
+			p.expectSemi()
+		}
+	case token.GO:
+		s = p.parseGoStmt()
+	case token.DEFER:
+		s = p.parseDeferStmt()
+	case token.RETURN:
+		s = p.parseReturnStmt()
+	case token.BREAK, token.CONTINUE, token.GOTO, token.FALLTHROUGH:
+		s = p.parseBranchStmt(p.tok)
+	case token.LBRACE:
+		s = p.parseBlockStmt()
+		p.expectSemi()
+	case token.IF:
+		s = p.parseIfStmt()
+	case token.SWITCH:
+		s = p.parseSwitchStmt()
+	case token.SELECT:
+		s = p.parseSelectStmt()
+	case token.FOR:
+		s = p.parseForStmt()
+	case token.SEMICOLON:
+		// Is it ever possible to have an implicit semicolon
+		// producing an empty statement in a valid program?
+		// (handle correctly anyway)
+		s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: p.lit == "\n"}
+		p.next()
+	case token.RBRACE:
+		// a semicolon may be omitted before a closing "}"
+		s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: true}
+	default:
+		// no statement found
+		pos := p.pos
+		p.errorExpected(pos, "statement")
+		p.advance(stmtStart)
+		s = &ast.BadStmt{From: pos, To: p.pos}
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Declarations
+
+type parseSpecFunction func(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec
+
+func isValidImport(lit string) bool {
+	const illegalChars = `!"#$%&'()*,:;<=>?[\]^{|}` + "`\uFFFD"
+	s, _ := strconv.Unquote(lit) // go/scanner returns a legal string literal
+	for _, r := range s {
+		if !unicode.IsGraphic(r) || unicode.IsSpace(r) || strings.ContainsRune(illegalChars, r) {
+			return false
+		}
+	}
+	return s != ""
+}
+
+func (p *parser) parseImportSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "ImportSpec"))
+	}
+
+	var ident *ast.Ident
+	switch p.tok {
+	case token.PERIOD:
+		ident = &ast.Ident{NamePos: p.pos, Name: "."}
+		p.next()
+	case token.IDENT:
+		ident = p.parseIdent()
+	}
+
+	pos := p.pos
+	var path string
+	if p.tok == token.STRING {
+		path = p.lit
+		if !isValidImport(path) {
+			p.error(pos, "invalid import path: "+path)
+		}
+		p.next()
+	} else {
+		p.expect(token.STRING) // use expect() error handling
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	// collect imports
+	spec := &ast.ImportSpec{
+		Doc:     doc,
+		Name:    ident,
+		Path:    &ast.BasicLit{ValuePos: pos, Kind: token.STRING, Value: path},
+		Comment: p.lineComment,
+	}
+	p.imports = append(p.imports, spec)
+
+	return spec
+}
+
+func (p *parser) parseValueSpec(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, keyword.String()+"Spec"))
+	}
+
+	pos := p.pos
+	idents := p.parseIdentList()
+	typ := p.tryType()
+	var values []ast.Expr
+	// always permit optional initialization for more tolerant parsing
+	if p.tok == token.ASSIGN {
+		p.next()
+		values = p.parseRhsList()
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	switch keyword {
+	case token.VAR:
+		if typ == nil && values == nil {
+			p.error(pos, "missing variable type or initialization")
+		}
+	case token.CONST:
+		if values == nil && (iota == 0 || typ != nil) {
+			p.error(pos, "missing constant value")
+		}
+	}
+
+	// Go spec: The scope of a constant or variable identifier declared inside
+	// a function begins at the end of the ConstSpec or VarSpec and ends at
+	// the end of the innermost containing block.
+	// (Global identifiers are resolved in a separate phase after parsing.)
+	spec := &ast.ValueSpec{
+		Doc:     doc,
+		Names:   idents,
+		Type:    typ,
+		Values:  values,
+		Comment: p.lineComment,
+	}
+	kind := ast.Con
+	if keyword == token.VAR {
+		kind = ast.Var
+	}
+	p.declare(spec, iota, p.topScope, kind, idents...)
+
+	return spec
+}
+
+func (p *parser) parseTypeSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "TypeSpec"))
+	}
+
+	ident := p.parseIdent()
+
+	// Go spec: The scope of a type identifier declared inside a function begins
+	// at the identifier in the TypeSpec and ends at the end of the innermost
+	// containing block.
+	// (Global identifiers are resolved in a separate phase after parsing.)
+	spec := &ast.TypeSpec{Doc: doc, Name: ident}
+	p.declare(spec, nil, p.topScope, ast.Typ, ident)
+	if p.tok == token.ASSIGN {
+		spec.Assign = p.pos
+		p.next()
+	}
+	spec.Type = p.parseType()
+	p.expectSemi() // call before accessing p.linecomment
+	spec.Comment = p.lineComment
+
+	return spec
+}
+
+func (p *parser) parseGenDecl(keyword token.Token, f parseSpecFunction) *ast.GenDecl {
+	if p.trace {
+		defer un(trace(p, "GenDecl("+keyword.String()+")"))
+	}
+
+	doc := p.leadComment
+	pos := p.expect(keyword)
+	var lparen, rparen token.Pos
+	var list []ast.Spec
+	if p.tok == token.LPAREN {
+		lparen = p.pos
+		p.next()
+		for iota := 0; p.tok != token.RPAREN && p.tok != token.EOF; iota++ {
+			list = append(list, f(p.leadComment, keyword, iota))
+		}
+		rparen = p.expect(token.RPAREN)
+		p.expectSemi()
+	} else {
+		list = append(list, f(nil, keyword, 0))
+	}
+
+	return &ast.GenDecl{
+		Doc:    doc,
+		TokPos: pos,
+		Tok:    keyword,
+		Lparen: lparen,
+		Specs:  list,
+		Rparen: rparen,
+	}
+}
+
+func (p *parser) parseFuncDecl() *ast.FuncDecl {
+	if p.trace {
+		defer un(trace(p, "FunctionDecl"))
+	}
+
+	doc := p.leadComment
+	pos := p.expect(token.FUNC)
+	scope := ast.NewScope(p.topScope) // function scope
+
+	var recv *ast.FieldList
+	if p.tok == token.LPAREN {
+		recv = p.parseParameters(scope, false)
+	}
+
+	ident := p.parseIdent()
+
+	params, results := p.parseSignature(scope)
+
+	var body *ast.BlockStmt
+	if p.tok == token.LBRACE {
+		body = p.parseBody(scope)
+		p.expectSemi()
+	} else if p.tok == token.SEMICOLON {
+		p.next()
+		if p.tok == token.LBRACE {
+			// opening { of function declaration on next line
+			p.error(p.pos, "unexpected semicolon or newline before {")
+			body = p.parseBody(scope)
+			p.expectSemi()
+		}
+	} else {
+		p.expectSemi()
+	}
+
+	decl := &ast.FuncDecl{
+		Doc:  doc,
+		Recv: recv,
+		Name: ident,
+		Type: &ast.FuncType{
+			Func:    pos,
+			Params:  params,
+			Results: results,
+		},
+		Body: body,
+	}
+	if recv == nil {
+		// Go spec: The scope of an identifier denoting a constant, type,
+		// variable, or function (but not method) declared at top level
+		// (outside any function) is the package block.
+		//
+		// init() functions cannot be referred to and there may
+		// be more than one - don't put them in the pkgScope
+		if ident.Name != "init" {
+			p.declare(decl, nil, p.pkgScope, ast.Fun, ident)
+		}
+	}
+
+	return decl
+}
+
+func (p *parser) parseDecl(sync map[token.Token]bool) ast.Decl {
+	if p.trace {
+		defer un(trace(p, "Declaration"))
+	}
+
+	var f parseSpecFunction
+	switch p.tok {
+	case token.CONST, token.VAR:
+		f = p.parseValueSpec
+
+	case token.TYPE:
+		f = p.parseTypeSpec
+
+	case token.FUNC:
+		return p.parseFuncDecl()
+
+	default:
+		pos := p.pos
+		p.errorExpected(pos, "declaration")
+		p.advance(sync)
+		return &ast.BadDecl{From: pos, To: p.pos}
+	}
+
+	return p.parseGenDecl(p.tok, f)
+}
+
+// ----------------------------------------------------------------------------
+// Source files
+
+func (p *parser) parseFile() *ast.File {
+	if p.trace {
+		defer un(trace(p, "File"))
+	}
+
+	// Don't bother parsing the rest if we had errors scanning the first token.
+	// Likely not a Go source file at all.
+	if p.errors.Len() != 0 {
+		return nil
+	}
+
+	// package clause
+	doc := p.leadComment
+	pos := p.expect(token.PACKAGE)
+	// Go spec: The package clause is not a declaration;
+	// the package name does not appear in any scope.
+	ident := p.parseIdent()
+	if ident.Name == "_" && p.mode&DeclarationErrors != 0 {
+		p.error(p.pos, "invalid package name _")
+	}
+	p.expectSemi()
+
+	// Don't bother parsing the rest if we had errors parsing the package clause.
+	// Likely not a Go source file at all.
+	if p.errors.Len() != 0 {
+		return nil
+	}
+
+	p.openScope()
+	p.pkgScope = p.topScope
+	var decls []ast.Decl
+	if p.mode&PackageClauseOnly == 0 {
+		// import decls
+		for p.tok == token.IMPORT {
+			decls = append(decls, p.parseGenDecl(token.IMPORT, p.parseImportSpec))
+		}
+
+		if p.mode&ImportsOnly == 0 {
+			// rest of package body
+			for p.tok != token.EOF {
+				decls = append(decls, p.parseDecl(declStart))
+			}
+		}
+	}
+	p.closeScope()
+	assert(p.topScope == nil, "unbalanced scopes")
+	assert(p.labelScope == nil, "unbalanced label scopes")
+
+	// resolve global identifiers within the same file
+	i := 0
+	for _, ident := range p.unresolved {
+		// i <= index for current ident
+		assert(ident.Obj == unresolved, "object already resolved")
+		ident.Obj = p.pkgScope.Lookup(ident.Name) // also removes unresolved sentinel
+		if ident.Obj == nil {
+			p.unresolved[i] = ident
+			i++
+		}
+	}
+
+	return &ast.File{
+		Doc:        doc,
+		Package:    pos,
+		Name:       ident,
+		Decls:      decls,
+		Scope:      p.pkgScope,
+		Imports:    p.imports,
+		Unresolved: p.unresolved[0:i],
+		Comments:   p.comments,
+	}
+}
diff --git a/src/go/parser/parser_test.go b/src/go/parser/parser_test.go
new file mode 100644
index 0000000..a4f882d
--- /dev/null
+++ b/src/go/parser/parser_test.go
@@ -0,0 +1,579 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package parser
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/token"
+	"io/fs"
+	"strings"
+	"testing"
+)
+
+var validFiles = []string{
+	"parser.go",
+	"parser_test.go",
+	"error_test.go",
+	"short_test.go",
+}
+
+func TestParse(t *testing.T) {
+	for _, filename := range validFiles {
+		_, err := ParseFile(token.NewFileSet(), filename, nil, DeclarationErrors)
+		if err != nil {
+			t.Fatalf("ParseFile(%s): %v", filename, err)
+		}
+	}
+}
+
+func nameFilter(filename string) bool {
+	switch filename {
+	case "parser.go", "interface.go", "parser_test.go":
+		return true
+	case "parser.go.orig":
+		return true // permit but should be ignored by ParseDir
+	}
+	return false
+}
+
+func dirFilter(f fs.FileInfo) bool { return nameFilter(f.Name()) }
+
+func TestParseFile(t *testing.T) {
+	src := "package p\nvar _=s[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]"
+	_, err := ParseFile(token.NewFileSet(), "", src, 0)
+	if err == nil {
+		t.Errorf("ParseFile(%s) succeeded unexpectedly", src)
+	}
+}
+
+func TestParseExprFrom(t *testing.T) {
+	src := "s[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]+\ns[::]"
+	_, err := ParseExprFrom(token.NewFileSet(), "", src, 0)
+	if err == nil {
+		t.Errorf("ParseExprFrom(%s) succeeded unexpectedly", src)
+	}
+}
+
+func TestParseDir(t *testing.T) {
+	path := "."
+	pkgs, err := ParseDir(token.NewFileSet(), path, dirFilter, 0)
+	if err != nil {
+		t.Fatalf("ParseDir(%s): %v", path, err)
+	}
+	if n := len(pkgs); n != 1 {
+		t.Errorf("got %d packages; want 1", n)
+	}
+	pkg := pkgs["parser"]
+	if pkg == nil {
+		t.Errorf(`package "parser" not found`)
+		return
+	}
+	if n := len(pkg.Files); n != 3 {
+		t.Errorf("got %d package files; want 3", n)
+	}
+	for filename := range pkg.Files {
+		if !nameFilter(filename) {
+			t.Errorf("unexpected package file: %s", filename)
+		}
+	}
+}
+
+func TestIssue42951(t *testing.T) {
+	path := "./testdata/issue42951"
+	_, err := ParseDir(token.NewFileSet(), path, nil, 0)
+	if err != nil {
+		t.Errorf("ParseDir(%s): %v", path, err)
+	}
+}
+
+func TestParseExpr(t *testing.T) {
+	// just kicking the tires:
+	// a valid arithmetic expression
+	src := "a + b"
+	x, err := ParseExpr(src)
+	if err != nil {
+		t.Errorf("ParseExpr(%q): %v", src, err)
+	}
+	// sanity check
+	if _, ok := x.(*ast.BinaryExpr); !ok {
+		t.Errorf("ParseExpr(%q): got %T, want *ast.BinaryExpr", src, x)
+	}
+
+	// a valid type expression
+	src = "struct{x *int}"
+	x, err = ParseExpr(src)
+	if err != nil {
+		t.Errorf("ParseExpr(%q): %v", src, err)
+	}
+	// sanity check
+	if _, ok := x.(*ast.StructType); !ok {
+		t.Errorf("ParseExpr(%q): got %T, want *ast.StructType", src, x)
+	}
+
+	// an invalid expression
+	src = "a + *"
+	x, err = ParseExpr(src)
+	if err == nil {
+		t.Errorf("ParseExpr(%q): got no error", src)
+	}
+	if x == nil {
+		t.Errorf("ParseExpr(%q): got no (partial) result", src)
+	}
+	if _, ok := x.(*ast.BinaryExpr); !ok {
+		t.Errorf("ParseExpr(%q): got %T, want *ast.BinaryExpr", src, x)
+	}
+
+	// a valid expression followed by extra tokens is invalid
+	src = "a[i] := x"
+	if _, err := ParseExpr(src); err == nil {
+		t.Errorf("ParseExpr(%q): got no error", src)
+	}
+
+	// a semicolon is not permitted unless automatically inserted
+	src = "a + b\n"
+	if _, err := ParseExpr(src); err != nil {
+		t.Errorf("ParseExpr(%q): got error %s", src, err)
+	}
+	src = "a + b;"
+	if _, err := ParseExpr(src); err == nil {
+		t.Errorf("ParseExpr(%q): got no error", src)
+	}
+
+	// various other stuff following a valid expression
+	const validExpr = "a + b"
+	const anything = "dh3*#D)#_"
+	for _, c := range "!)]};," {
+		src := validExpr + string(c) + anything
+		if _, err := ParseExpr(src); err == nil {
+			t.Errorf("ParseExpr(%q): got no error", src)
+		}
+	}
+
+	// ParseExpr must not crash
+	for _, src := range valids {
+		ParseExpr(src)
+	}
+}
+
+func TestColonEqualsScope(t *testing.T) {
+	f, err := ParseFile(token.NewFileSet(), "", `package p; func f() { x, y, z := x, y, z }`, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// RHS refers to undefined globals; LHS does not.
+	as := f.Decls[0].(*ast.FuncDecl).Body.List[0].(*ast.AssignStmt)
+	for _, v := range as.Rhs {
+		id := v.(*ast.Ident)
+		if id.Obj != nil {
+			t.Errorf("rhs %s has Obj, should not", id.Name)
+		}
+	}
+	for _, v := range as.Lhs {
+		id := v.(*ast.Ident)
+		if id.Obj == nil {
+			t.Errorf("lhs %s does not have Obj, should", id.Name)
+		}
+	}
+}
+
+func TestVarScope(t *testing.T) {
+	f, err := ParseFile(token.NewFileSet(), "", `package p; func f() { var x, y, z = x, y, z }`, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// RHS refers to undefined globals; LHS does not.
+	as := f.Decls[0].(*ast.FuncDecl).Body.List[0].(*ast.DeclStmt).Decl.(*ast.GenDecl).Specs[0].(*ast.ValueSpec)
+	for _, v := range as.Values {
+		id := v.(*ast.Ident)
+		if id.Obj != nil {
+			t.Errorf("rhs %s has Obj, should not", id.Name)
+		}
+	}
+	for _, id := range as.Names {
+		if id.Obj == nil {
+			t.Errorf("lhs %s does not have Obj, should", id.Name)
+		}
+	}
+}
+
+func TestObjects(t *testing.T) {
+	const src = `
+package p
+import fmt "fmt"
+const pi = 3.14
+type T struct{}
+var x int
+func f() { L: }
+`
+
+	f, err := ParseFile(token.NewFileSet(), "", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	objects := map[string]ast.ObjKind{
+		"p":   ast.Bad, // not in a scope
+		"fmt": ast.Bad, // not resolved yet
+		"pi":  ast.Con,
+		"T":   ast.Typ,
+		"x":   ast.Var,
+		"int": ast.Bad, // not resolved yet
+		"f":   ast.Fun,
+		"L":   ast.Lbl,
+	}
+
+	ast.Inspect(f, func(n ast.Node) bool {
+		if ident, ok := n.(*ast.Ident); ok {
+			obj := ident.Obj
+			if obj == nil {
+				if objects[ident.Name] != ast.Bad {
+					t.Errorf("no object for %s", ident.Name)
+				}
+				return true
+			}
+			if obj.Name != ident.Name {
+				t.Errorf("names don't match: obj.Name = %s, ident.Name = %s", obj.Name, ident.Name)
+			}
+			kind := objects[ident.Name]
+			if obj.Kind != kind {
+				t.Errorf("%s: obj.Kind = %s; want %s", ident.Name, obj.Kind, kind)
+			}
+		}
+		return true
+	})
+}
+
+func TestUnresolved(t *testing.T) {
+	f, err := ParseFile(token.NewFileSet(), "", `
+package p
+//
+func f1a(int)
+func f2a(byte, int, float)
+func f3a(a, b int, c float)
+func f4a(...complex)
+func f5a(a s1a, b ...complex)
+//
+func f1b(*int)
+func f2b([]byte, (int), *float)
+func f3b(a, b *int, c []float)
+func f4b(...*complex)
+func f5b(a s1a, b ...[]complex)
+//
+type s1a struct { int }
+type s2a struct { byte; int; s1a }
+type s3a struct { a, b int; c float }
+//
+type s1b struct { *int }
+type s2b struct { byte; int; *float }
+type s3b struct { a, b *s3b; c []float }
+`, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	want := "int " + // f1a
+		"byte int float " + // f2a
+		"int float " + // f3a
+		"complex " + // f4a
+		"complex " + // f5a
+		//
+		"int " + // f1b
+		"byte int float " + // f2b
+		"int float " + // f3b
+		"complex " + // f4b
+		"complex " + // f5b
+		//
+		"int " + // s1a
+		"byte int " + // s2a
+		"int float " + // s3a
+		//
+		"int " + // s1a
+		"byte int float " + // s2a
+		"float " // s3a
+
+	// collect unresolved identifiers
+	var buf bytes.Buffer
+	for _, u := range f.Unresolved {
+		buf.WriteString(u.Name)
+		buf.WriteByte(' ')
+	}
+	got := buf.String()
+
+	if got != want {
+		t.Errorf("\ngot:  %s\nwant: %s", got, want)
+	}
+}
+
+var imports = map[string]bool{
+	`"a"`:        true,
+	"`a`":        true,
+	`"a/b"`:      true,
+	`"a.b"`:      true,
+	`"m\x61th"`:  true,
+	`"greek/αβ"`: true,
+	`""`:         false,
+
+	// Each of these pairs tests both `` vs "" strings
+	// and also use of invalid characters spelled out as
+	// escape sequences and written directly.
+	// For example `"\x00"` tests import "\x00"
+	// while "`\x00`" tests import `<actual-NUL-byte>`.
+	`"\x00"`:     false,
+	"`\x00`":     false,
+	`"\x7f"`:     false,
+	"`\x7f`":     false,
+	`"a!"`:       false,
+	"`a!`":       false,
+	`"a b"`:      false,
+	"`a b`":      false,
+	`"a\\b"`:     false,
+	"`a\\b`":     false,
+	"\"`a`\"":    false,
+	"`\"a\"`":    false,
+	`"\x80\x80"`: false,
+	"`\x80\x80`": false,
+	`"\xFFFD"`:   false,
+	"`\xFFFD`":   false,
+}
+
+func TestImports(t *testing.T) {
+	for path, isValid := range imports {
+		src := fmt.Sprintf("package p; import %s", path)
+		_, err := ParseFile(token.NewFileSet(), "", src, 0)
+		switch {
+		case err != nil && isValid:
+			t.Errorf("ParseFile(%s): got %v; expected no error", src, err)
+		case err == nil && !isValid:
+			t.Errorf("ParseFile(%s): got no error; expected one", src)
+		}
+	}
+}
+
+func TestCommentGroups(t *testing.T) {
+	f, err := ParseFile(token.NewFileSet(), "", `
+package p /* 1a */ /* 1b */      /* 1c */ // 1d
+/* 2a
+*/
+// 2b
+const pi = 3.1415
+/* 3a */ // 3b
+/* 3c */ const e = 2.7182
+
+// Example from issue 3139
+func ExampleCount() {
+	fmt.Println(strings.Count("cheese", "e"))
+	fmt.Println(strings.Count("five", "")) // before & after each rune
+	// Output:
+	// 3
+	// 5
+}
+`, ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expected := [][]string{
+		{"/* 1a */", "/* 1b */", "/* 1c */", "// 1d"},
+		{"/* 2a\n*/", "// 2b"},
+		{"/* 3a */", "// 3b", "/* 3c */"},
+		{"// Example from issue 3139"},
+		{"// before & after each rune"},
+		{"// Output:", "// 3", "// 5"},
+	}
+	if len(f.Comments) != len(expected) {
+		t.Fatalf("got %d comment groups; expected %d", len(f.Comments), len(expected))
+	}
+	for i, exp := range expected {
+		got := f.Comments[i].List
+		if len(got) != len(exp) {
+			t.Errorf("got %d comments in group %d; expected %d", len(got), i, len(exp))
+			continue
+		}
+		for j, exp := range exp {
+			got := got[j].Text
+			if got != exp {
+				t.Errorf("got %q in group %d; expected %q", got, i, exp)
+			}
+		}
+	}
+}
+
+func getField(file *ast.File, fieldname string) *ast.Field {
+	parts := strings.Split(fieldname, ".")
+	for _, d := range file.Decls {
+		if d, ok := d.(*ast.GenDecl); ok && d.Tok == token.TYPE {
+			for _, s := range d.Specs {
+				if s, ok := s.(*ast.TypeSpec); ok && s.Name.Name == parts[0] {
+					if s, ok := s.Type.(*ast.StructType); ok {
+						for _, f := range s.Fields.List {
+							for _, name := range f.Names {
+								if name.Name == parts[1] {
+									return f
+								}
+							}
+						}
+					}
+				}
+			}
+		}
+	}
+	return nil
+}
+
+// Don't use ast.CommentGroup.Text() - we want to see exact comment text.
+func commentText(c *ast.CommentGroup) string {
+	var buf bytes.Buffer
+	if c != nil {
+		for _, c := range c.List {
+			buf.WriteString(c.Text)
+		}
+	}
+	return buf.String()
+}
+
+func checkFieldComments(t *testing.T, file *ast.File, fieldname, lead, line string) {
+	f := getField(file, fieldname)
+	if f == nil {
+		t.Fatalf("field not found: %s", fieldname)
+	}
+	if got := commentText(f.Doc); got != lead {
+		t.Errorf("got lead comment %q; expected %q", got, lead)
+	}
+	if got := commentText(f.Comment); got != line {
+		t.Errorf("got line comment %q; expected %q", got, line)
+	}
+}
+
+func TestLeadAndLineComments(t *testing.T) {
+	f, err := ParseFile(token.NewFileSet(), "", `
+package p
+type T struct {
+	/* F1 lead comment */
+	//
+	F1 int  /* F1 */ // line comment
+	// F2 lead
+	// comment
+	F2 int  // F2 line comment
+	// f3 lead comment
+	f3 int  // f3 line comment
+}
+`, ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	checkFieldComments(t, f, "T.F1", "/* F1 lead comment *///", "/* F1 */// line comment")
+	checkFieldComments(t, f, "T.F2", "// F2 lead// comment", "// F2 line comment")
+	checkFieldComments(t, f, "T.f3", "// f3 lead comment", "// f3 line comment")
+	ast.FileExports(f)
+	checkFieldComments(t, f, "T.F1", "/* F1 lead comment *///", "/* F1 */// line comment")
+	checkFieldComments(t, f, "T.F2", "// F2 lead// comment", "// F2 line comment")
+	if getField(f, "T.f3") != nil {
+		t.Error("not expected to find T.f3")
+	}
+}
+
+// TestIssue9979 verifies that empty statements are contained within their enclosing blocks.
+func TestIssue9979(t *testing.T) {
+	for _, src := range []string{
+		"package p; func f() {;}",
+		"package p; func f() {L:}",
+		"package p; func f() {L:;}",
+		"package p; func f() {L:\n}",
+		"package p; func f() {L:\n;}",
+		"package p; func f() { ; }",
+		"package p; func f() { L: }",
+		"package p; func f() { L: ; }",
+		"package p; func f() { L: \n}",
+		"package p; func f() { L: \n; }",
+	} {
+		fset := token.NewFileSet()
+		f, err := ParseFile(fset, "", src, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		var pos, end token.Pos
+		ast.Inspect(f, func(x ast.Node) bool {
+			switch s := x.(type) {
+			case *ast.BlockStmt:
+				pos, end = s.Pos()+1, s.End()-1 // exclude "{", "}"
+			case *ast.LabeledStmt:
+				pos, end = s.Pos()+2, s.End() // exclude "L:"
+			case *ast.EmptyStmt:
+				// check containment
+				if s.Pos() < pos || s.End() > end {
+					t.Errorf("%s: %T[%d, %d] not inside [%d, %d]", src, s, s.Pos(), s.End(), pos, end)
+				}
+				// check semicolon
+				offs := fset.Position(s.Pos()).Offset
+				if ch := src[offs]; ch != ';' != s.Implicit {
+					want := "want ';'"
+					if s.Implicit {
+						want = "but ';' is implicit"
+					}
+					t.Errorf("%s: found %q at offset %d; %s", src, ch, offs, want)
+				}
+			}
+			return true
+		})
+	}
+}
+
+// TestIncompleteSelection ensures that an incomplete selector
+// expression is parsed as a (blank) *ast.SelectorExpr, not a
+// *ast.BadExpr.
+func TestIncompleteSelection(t *testing.T) {
+	for _, src := range []string{
+		"package p; var _ = fmt.",             // at EOF
+		"package p; var _ = fmt.\ntype X int", // not at EOF
+	} {
+		fset := token.NewFileSet()
+		f, err := ParseFile(fset, "", src, 0)
+		if err == nil {
+			t.Errorf("ParseFile(%s) succeeded unexpectedly", src)
+			continue
+		}
+
+		const wantErr = "expected selector or type assertion"
+		if !strings.Contains(err.Error(), wantErr) {
+			t.Errorf("ParseFile returned wrong error %q, want %q", err, wantErr)
+		}
+
+		var sel *ast.SelectorExpr
+		ast.Inspect(f, func(n ast.Node) bool {
+			if n, ok := n.(*ast.SelectorExpr); ok {
+				sel = n
+			}
+			return true
+		})
+		if sel == nil {
+			t.Error("found no *ast.SelectorExpr")
+			continue
+		}
+		const wantSel = "&{fmt _}"
+		if fmt.Sprint(sel) != wantSel {
+			t.Errorf("found selector %s, want %s", sel, wantSel)
+			continue
+		}
+	}
+}
+
+func TestLastLineComment(t *testing.T) {
+	const src = `package main
+type x int // comment
+`
+	fset := token.NewFileSet()
+	f, err := ParseFile(fset, "", src, ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+	comment := f.Decls[0].(*ast.GenDecl).Specs[0].(*ast.TypeSpec).Comment.List[0].Text
+	if comment != "// comment" {
+		t.Errorf("got %q, want %q", comment, "// comment")
+	}
+}
diff --git a/src/go/parser/performance_test.go b/src/go/parser/performance_test.go
new file mode 100644
index 0000000..f81bcee
--- /dev/null
+++ b/src/go/parser/performance_test.go
@@ -0,0 +1,30 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package parser
+
+import (
+	"go/token"
+	"os"
+	"testing"
+)
+
+var src = readFile("parser.go")
+
+func readFile(filename string) []byte {
+	data, err := os.ReadFile(filename)
+	if err != nil {
+		panic(err)
+	}
+	return data
+}
+
+func BenchmarkParse(b *testing.B) {
+	b.SetBytes(int64(len(src)))
+	for i := 0; i < b.N; i++ {
+		if _, err := ParseFile(token.NewFileSet(), "", src, ParseComments); err != nil {
+			b.Fatalf("benchmark failed due to parse error: %s", err)
+		}
+	}
+}
diff --git a/src/go/parser/short_test.go b/src/go/parser/short_test.go
new file mode 100644
index 0000000..49bb681
--- /dev/null
+++ b/src/go/parser/short_test.go
@@ -0,0 +1,136 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains test cases for short valid and invalid programs.
+
+package parser
+
+import "testing"
+
+var valids = []string{
+	"package p\n",
+	`package p;`,
+	`package p; import "fmt"; func f() { fmt.Println("Hello, World!") };`,
+	`package p; func f() { if f(T{}) {} };`,
+	`package p; func f() { _ = <-chan int(nil) };`,
+	`package p; func f() { _ = (<-chan int)(nil) };`,
+	`package p; func f() { _ = (<-chan <-chan int)(nil) };`,
+	`package p; func f() { _ = <-chan <-chan <-chan <-chan <-int(nil) };`,
+	`package p; func f(func() func() func());`,
+	`package p; func f(...T);`,
+	`package p; func f(float, ...int);`,
+	`package p; func f(x int, a ...int) { f(0, a...); f(1, a...,) };`,
+	`package p; func f(int,) {};`,
+	`package p; func f(...int,) {};`,
+	`package p; func f(x ...int,) {};`,
+	`package p; type T []int; var a []bool; func f() { if a[T{42}[0]] {} };`,
+	`package p; type T []int; func g(int) bool { return true }; func f() { if g(T{42}[0]) {} };`,
+	`package p; type T []int; func f() { for _ = range []int{T{42}[0]} {} };`,
+	`package p; var a = T{{1, 2}, {3, 4}}`,
+	`package p; func f() { select { case <- c: case c <- d: case c <- <- d: case <-c <- d: } };`,
+	`package p; func f() { select { case x := (<-c): } };`,
+	`package p; func f() { if ; true {} };`,
+	`package p; func f() { switch ; {} };`,
+	`package p; func f() { for _ = range "foo" + "bar" {} };`,
+	`package p; func f() { var s []int; g(s[:], s[i:], s[:j], s[i:j], s[i:j:k], s[:j:k]) };`,
+	`package p; var ( _ = (struct {*T}).m; _ = (interface {T}).m )`,
+	`package p; func ((T),) m() {}`,
+	`package p; func ((*T),) m() {}`,
+	`package p; func (*(T),) m() {}`,
+	`package p; func _(x []int) { for range x {} }`,
+	`package p; func _() { if [T{}.n]int{} {} }`,
+	`package p; func _() { map[int]int{}[0]++; map[int]int{}[0] += 1 }`,
+	`package p; func _(x interface{f()}) { interface{f()}(x).f() }`,
+	`package p; func _(x chan int) { chan int(x) <- 0 }`,
+	`package p; const (x = 0; y; z)`, // issue 9639
+	`package p; var _ = map[P]int{P{}:0, {}:1}`,
+	`package p; var _ = map[*P]int{&P{}:0, {}:1}`,
+	`package p; type T = int`,
+	`package p; type (T = p.T; _ = struct{}; x = *T)`,
+}
+
+func TestValid(t *testing.T) {
+	for _, src := range valids {
+		checkErrors(t, src, src)
+	}
+}
+
+var invalids = []string{
+	`foo /* ERROR "expected 'package'" */ !`,
+	`package p; func f() { if { /* ERROR "missing condition" */ } };`,
+	`package p; func f() { if ; /* ERROR "missing condition" */ {} };`,
+	`package p; func f() { if f(); /* ERROR "missing condition" */ {} };`,
+	`package p; func f() { if _ = range /* ERROR "expected operand" */ x; true {} };`,
+	`package p; func f() { switch _ /* ERROR "expected switch expression" */ = range x; true {} };`,
+	`package p; func f() { for _ = range x ; /* ERROR "expected '{'" */ ; {} };`,
+	`package p; func f() { for ; ; _ = range /* ERROR "expected operand" */ x {} };`,
+	`package p; func f() { for ; _ /* ERROR "expected boolean or range expression" */ = range x ; {} };`,
+	`package p; func f() { switch t = /* ERROR "expected ':=', found '='" */ t.(type) {} };`,
+	`package p; func f() { switch t /* ERROR "expected switch expression" */ , t = t.(type) {} };`,
+	`package p; func f() { switch t /* ERROR "expected switch expression" */ = t.(type), t {} };`,
+	`package p; var a = [ /* ERROR "expected expression" */ 1]int;`,
+	`package p; var a = [ /* ERROR "expected expression" */ ...]int;`,
+	`package p; var a = struct /* ERROR "expected expression" */ {}`,
+	`package p; var a = func /* ERROR "expected expression" */ ();`,
+	`package p; var a = interface /* ERROR "expected expression" */ {}`,
+	`package p; var a = [ /* ERROR "expected expression" */ ]int`,
+	`package p; var a = map /* ERROR "expected expression" */ [int]int`,
+	`package p; var a = chan /* ERROR "expected expression" */ int;`,
+	`package p; var a = []int{[ /* ERROR "expected expression" */ ]int};`,
+	`package p; var a = ( /* ERROR "expected expression" */ []int);`,
+	`package p; var a = a[[ /* ERROR "expected expression" */ ]int:[]int];`,
+	`package p; var a = <- /* ERROR "expected expression" */ chan int;`,
+	`package p; func f() { select { case _ <- chan /* ERROR "expected expression" */ int: } };`,
+	`package p; func f() { _ = (<-<- /* ERROR "expected 'chan'" */ chan int)(nil) };`,
+	`package p; func f() { _ = (<-chan<-chan<-chan<-chan<-chan<- /* ERROR "expected channel type" */ int)(nil) };`,
+	`package p; func f() { var t []int; t /* ERROR "expected identifier on left side of :=" */ [0] := 0 };`,
+	`package p; func f() { if x := g(); x /* ERROR "expected boolean expression" */ = 0 {}};`,
+	`package p; func f() { _ = x = /* ERROR "expected '=='" */ 0 {}};`,
+	`package p; func f() { _ = 1 == func()int { var x bool; x = x = /* ERROR "expected '=='" */ true; return x }() };`,
+	`package p; func f() { var s []int; _ = s[] /* ERROR "expected operand" */ };`,
+	`package p; func f() { var s []int; _ = s[i:j: /* ERROR "3rd index required" */ ] };`,
+	`package p; func f() { var s []int; _ = s[i: /* ERROR "2nd index required" */ :k] };`,
+	`package p; func f() { var s []int; _ = s[i: /* ERROR "2nd index required" */ :] };`,
+	`package p; func f() { var s []int; _ = s[: /* ERROR "2nd index required" */ :] };`,
+	`package p; func f() { var s []int; _ = s[: /* ERROR "2nd index required" */ ::] };`,
+	`package p; func f() { var s []int; _ = s[i:j:k: /* ERROR "expected ']'" */ l] };`,
+	`package p; func f() { for x /* ERROR "boolean or range expression" */ = []string {} }`,
+	`package p; func f() { for x /* ERROR "boolean or range expression" */ := []string {} }`,
+	`package p; func f() { for i /* ERROR "boolean or range expression" */ , x = []string {} }`,
+	`package p; func f() { for i /* ERROR "boolean or range expression" */ , x := []string {} }`,
+	`package p; func f() { go f /* ERROR HERE "function must be invoked" */ }`,
+	`package p; func f() { defer func() {} /* ERROR HERE "function must be invoked" */ }`,
+	`package p; func f() { go func() { func() { f(x func /* ERROR "missing ','" */ (){}) } } }`,
+	`package p; func f(x func(), u v func /* ERROR "missing ','" */ ()){}`,
+
+	// issue 8656
+	`package p; func f() (a b string /* ERROR "missing ','" */ , ok bool)`,
+
+	// issue 9639
+	`package p; var x /* ERROR "missing variable type or initialization" */ , y, z;`,
+	`package p; const x /* ERROR "missing constant value" */ ;`,
+	`package p; const x /* ERROR "missing constant value" */ int;`,
+	`package p; const (x = 0; y; z /* ERROR "missing constant value" */ int);`,
+
+	// issue 12437
+	`package p; var _ = struct { x int, /* ERROR "expected ';', found ','" */ }{};`,
+	`package p; var _ = struct { x int, /* ERROR "expected ';', found ','" */ y float }{};`,
+
+	// issue 11611
+	`package p; type _ struct { int, } /* ERROR "expected type, found '}'" */ ;`,
+	`package p; type _ struct { int, float } /* ERROR "expected type, found '}'" */ ;`,
+	`package p; type _ struct { ( /* ERROR "expected anonymous field" */ int) };`,
+	`package p; func _()(x, y, z ... /* ERROR "expected '\)', found '...'" */ int){}`,
+	`package p; func _()(... /* ERROR "expected type, found '...'" */ int){}`,
+
+	// issue 13475
+	`package p; func f() { if true {} else ; /* ERROR "expected if statement or block" */ }`,
+	`package p; func f() { if true {} else defer /* ERROR "expected if statement or block" */ f() }`,
+}
+
+func TestInvalid(t *testing.T) {
+	for _, src := range invalids {
+		checkErrors(t, src, src)
+	}
+}
diff --git a/src/go/parser/testdata/commas.src b/src/go/parser/testdata/commas.src
new file mode 100644
index 0000000..e0603cf
--- /dev/null
+++ b/src/go/parser/testdata/commas.src
@@ -0,0 +1,19 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for error messages/parser synchronization
+// after missing commas.
+
+package p
+
+var _ = []int{
+	0/* ERROR HERE "missing ','" */
+}
+
+var _ = []int{
+	0,
+	1,
+	2,
+	3/* ERROR HERE "missing ','" */
+}
diff --git a/src/go/parser/testdata/issue11377.src b/src/go/parser/testdata/issue11377.src
new file mode 100644
index 0000000..1c43800
--- /dev/null
+++ b/src/go/parser/testdata/issue11377.src
@@ -0,0 +1,27 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for issue 11377: Better synchronization of
+// parser after certain syntax errors.
+
+package p
+
+func bad1() {
+    if f()) /* ERROR "expected ';', found '\)'" */ {
+        return
+    }
+}
+
+// There shouldn't be any errors down below.
+
+func F1() {}
+func F2() {}
+func F3() {}
+func F4() {}
+func F5() {}
+func F6() {}
+func F7() {}
+func F8() {}
+func F9() {}
+func F10() {}
diff --git a/src/go/parser/testdata/issue23434.src b/src/go/parser/testdata/issue23434.src
new file mode 100644
index 0000000..24a0832
--- /dev/null
+++ b/src/go/parser/testdata/issue23434.src
@@ -0,0 +1,25 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for issue 23434: Better synchronization of
+// parser after missing type. There should be exactly
+// one error each time, with now follow errors.
+
+package p
+
+func g() {
+	m := make(map[string]! /* ERROR "expected type, found '!'" */ )
+	for {
+		x := 1
+		print(x)
+	}
+}
+
+func f() {
+	m := make(map[string]) /* ERROR "expected type, found '\)'" */
+	for {
+		x := 1
+		print(x)
+	}
+}
diff --git a/src/go/parser/testdata/issue3106.src b/src/go/parser/testdata/issue3106.src
new file mode 100644
index 0000000..2db10be
--- /dev/null
+++ b/src/go/parser/testdata/issue3106.src
@@ -0,0 +1,46 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for issue 3106: Better synchronization of
+// parser after certain syntax errors.
+
+package main
+
+func f() {
+	var m Mutex
+	c := MakeCond(&m)
+	percent := 0
+	const step = 10
+	for i := 0; i < 5; i++ {
+		go func() {
+			for {
+				// Emulates some useful work.
+				time.Sleep(1e8)
+				m.Lock()
+				defer
+				if /* ERROR "expected ';', found 'if'" */ percent == 100 {
+					m.Unlock()
+					break
+				}
+				percent++
+				if percent % step == 0 {
+					//c.Signal()
+				}
+				m.Unlock()
+			}
+		}()
+	}
+	for {
+		m.Lock()
+		if percent == 0 || percent % step != 0 {
+			c.Wait()
+		}
+		fmt.Print(",")
+		if percent == 100 {
+			m.Unlock()
+			break
+		}
+		m.Unlock()
+	}
+}
diff --git a/src/go/parser/testdata/issue34946.src b/src/go/parser/testdata/issue34946.src
new file mode 100644
index 0000000..6bb15e1
--- /dev/null
+++ b/src/go/parser/testdata/issue34946.src
@@ -0,0 +1,22 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for issue 34946: Better synchronization of
+// parser for function declarations that start their
+// body's opening { on a new line.
+
+package p
+
+// accept Allman/BSD-style declaration but complain
+// (implicit semicolon between signature and body)
+func _() int
+{ /* ERROR "unexpected semicolon or newline before {" */
+	{ return 0 }
+}
+
+func _() {}
+
+func _(); { /* ERROR "unexpected semicolon or newline before {" */ }
+
+func _() {}
diff --git a/src/go/parser/testdata/issue42951/not_a_file.go/invalid.go b/src/go/parser/testdata/issue42951/not_a_file.go/invalid.go
new file mode 100644
index 0000000..bb698be
--- /dev/null
+++ b/src/go/parser/testdata/issue42951/not_a_file.go/invalid.go
@@ -0,0 +1 @@
+This file should not be parsed by ParseDir.
diff --git a/src/go/printer/example_test.go b/src/go/printer/example_test.go
new file mode 100644
index 0000000..3081693
--- /dev/null
+++ b/src/go/printer/example_test.go
@@ -0,0 +1,67 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package printer_test
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/printer"
+	"go/token"
+	"strings"
+	"testing"
+)
+
+// Dummy test function so that godoc does not use the entire file as example.
+func Test(*testing.T) {}
+
+func parseFunc(filename, functionname string) (fun *ast.FuncDecl, fset *token.FileSet) {
+	fset = token.NewFileSet()
+	if file, err := parser.ParseFile(fset, filename, nil, 0); err == nil {
+		for _, d := range file.Decls {
+			if f, ok := d.(*ast.FuncDecl); ok && f.Name.Name == functionname {
+				fun = f
+				return
+			}
+		}
+	}
+	panic("function not found")
+}
+
+func ExampleFprint() {
+	// Parse source file and extract the AST without comments for
+	// this function, with position information referring to the
+	// file set fset.
+	funcAST, fset := parseFunc("example_test.go", "ExampleFprint")
+
+	// Print the function body into buffer buf.
+	// The file set is provided to the printer so that it knows
+	// about the original source formatting and can add additional
+	// line breaks where they were present in the source.
+	var buf bytes.Buffer
+	printer.Fprint(&buf, fset, funcAST.Body)
+
+	// Remove braces {} enclosing the function body, unindent,
+	// and trim leading and trailing white space.
+	s := buf.String()
+	s = s[1 : len(s)-1]
+	s = strings.TrimSpace(strings.ReplaceAll(s, "\n\t", "\n"))
+
+	// Print the cleaned-up body text to stdout.
+	fmt.Println(s)
+
+	// output:
+	// funcAST, fset := parseFunc("example_test.go", "ExampleFprint")
+	//
+	// var buf bytes.Buffer
+	// printer.Fprint(&buf, fset, funcAST.Body)
+	//
+	// s := buf.String()
+	// s = s[1 : len(s)-1]
+	// s = strings.TrimSpace(strings.ReplaceAll(s, "\n\t", "\n"))
+	//
+	// fmt.Println(s)
+}
diff --git a/src/go/printer/nodes.go b/src/go/printer/nodes.go
new file mode 100644
index 0000000..95b9e91
--- /dev/null
+++ b/src/go/printer/nodes.go
@@ -0,0 +1,1844 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements printing of AST nodes; specifically
+// expressions, statements, declarations, and files. It uses
+// the print functionality implemented in printer.go.
+
+package printer
+
+import (
+	"bytes"
+	"go/ast"
+	"go/token"
+	"math"
+	"strconv"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// Formatting issues:
+// - better comment formatting for /*-style comments at the end of a line (e.g. a declaration)
+//   when the comment spans multiple lines; if such a comment is just two lines, formatting is
+//   not idempotent
+// - formatting of expression lists
+// - should use blank instead of tab to separate one-line function bodies from
+//   the function header unless there is a group of consecutive one-liners
+
+// ----------------------------------------------------------------------------
+// Common AST nodes.
+
+// Print as many newlines as necessary (but at least min newlines) to get to
+// the current line. ws is printed before the first line break. If newSection
+// is set, the first line break is printed as formfeed. Returns 0 if no line
+// breaks were printed, returns 1 if there was exactly one newline printed,
+// and returns a value > 1 if there was a formfeed or more than one newline
+// printed.
+//
+// TODO(gri): linebreak may add too many lines if the next statement at "line"
+//            is preceded by comments because the computation of n assumes
+//            the current position before the comment and the target position
+//            after the comment. Thus, after interspersing such comments, the
+//            space taken up by them is not considered to reduce the number of
+//            linebreaks. At the moment there is no easy way to know about
+//            future (not yet interspersed) comments in this function.
+//
+func (p *printer) linebreak(line, min int, ws whiteSpace, newSection bool) (nbreaks int) {
+	n := nlimit(line - p.pos.Line)
+	if n < min {
+		n = min
+	}
+	if n > 0 {
+		p.print(ws)
+		if newSection {
+			p.print(formfeed)
+			n--
+			nbreaks = 2
+		}
+		nbreaks += n
+		for ; n > 0; n-- {
+			p.print(newline)
+		}
+	}
+	return
+}
+
+// setComment sets g as the next comment if g != nil and if node comments
+// are enabled - this mode is used when printing source code fragments such
+// as exports only. It assumes that there is no pending comment in p.comments
+// and at most one pending comment in the p.comment cache.
+func (p *printer) setComment(g *ast.CommentGroup) {
+	if g == nil || !p.useNodeComments {
+		return
+	}
+	if p.comments == nil {
+		// initialize p.comments lazily
+		p.comments = make([]*ast.CommentGroup, 1)
+	} else if p.cindex < len(p.comments) {
+		// for some reason there are pending comments; this
+		// should never happen - handle gracefully and flush
+		// all comments up to g, ignore anything after that
+		p.flush(p.posFor(g.List[0].Pos()), token.ILLEGAL)
+		p.comments = p.comments[0:1]
+		// in debug mode, report error
+		p.internalError("setComment found pending comments")
+	}
+	p.comments[0] = g
+	p.cindex = 0
+	// don't overwrite any pending comment in the p.comment cache
+	// (there may be a pending comment when a line comment is
+	// immediately followed by a lead comment with no other
+	// tokens between)
+	if p.commentOffset == infinity {
+		p.nextComment() // get comment ready for use
+	}
+}
+
+type exprListMode uint
+
+const (
+	commaTerm exprListMode = 1 << iota // list is optionally terminated by a comma
+	noIndent                           // no extra indentation in multi-line lists
+)
+
+// If indent is set, a multi-line identifier list is indented after the
+// first linebreak encountered.
+func (p *printer) identList(list []*ast.Ident, indent bool) {
+	// convert into an expression list so we can re-use exprList formatting
+	xlist := make([]ast.Expr, len(list))
+	for i, x := range list {
+		xlist[i] = x
+	}
+	var mode exprListMode
+	if !indent {
+		mode = noIndent
+	}
+	p.exprList(token.NoPos, xlist, 1, mode, token.NoPos, false)
+}
+
+const filteredMsg = "contains filtered or unexported fields"
+
+// Print a list of expressions. If the list spans multiple
+// source lines, the original line breaks are respected between
+// expressions.
+//
+// TODO(gri) Consider rewriting this to be independent of []ast.Expr
+//           so that we can use the algorithm for any kind of list
+//           (e.g., pass list via a channel over which to range).
+func (p *printer) exprList(prev0 token.Pos, list []ast.Expr, depth int, mode exprListMode, next0 token.Pos, isIncomplete bool) {
+	if len(list) == 0 {
+		if isIncomplete {
+			prev := p.posFor(prev0)
+			next := p.posFor(next0)
+			if prev.IsValid() && prev.Line == next.Line {
+				p.print("/* " + filteredMsg + " */")
+			} else {
+				p.print(newline)
+				p.print(indent, "// "+filteredMsg, unindent, newline)
+			}
+		}
+		return
+	}
+
+	prev := p.posFor(prev0)
+	next := p.posFor(next0)
+	line := p.lineFor(list[0].Pos())
+	endLine := p.lineFor(list[len(list)-1].End())
+
+	if prev.IsValid() && prev.Line == line && line == endLine {
+		// all list entries on a single line
+		for i, x := range list {
+			if i > 0 {
+				// use position of expression following the comma as
+				// comma position for correct comment placement
+				p.print(x.Pos(), token.COMMA, blank)
+			}
+			p.expr0(x, depth)
+		}
+		if isIncomplete {
+			p.print(token.COMMA, blank, "/* "+filteredMsg+" */")
+		}
+		return
+	}
+
+	// list entries span multiple lines;
+	// use source code positions to guide line breaks
+
+	// Don't add extra indentation if noIndent is set;
+	// i.e., pretend that the first line is already indented.
+	ws := ignore
+	if mode&noIndent == 0 {
+		ws = indent
+	}
+
+	// The first linebreak is always a formfeed since this section must not
+	// depend on any previous formatting.
+	prevBreak := -1 // index of last expression that was followed by a linebreak
+	if prev.IsValid() && prev.Line < line && p.linebreak(line, 0, ws, true) > 0 {
+		ws = ignore
+		prevBreak = 0
+	}
+
+	// initialize expression/key size: a zero value indicates expr/key doesn't fit on a single line
+	size := 0
+
+	// We use the ratio between the geometric mean of the previous key sizes and
+	// the current size to determine if there should be a break in the alignment.
+	// To compute the geometric mean we accumulate the ln(size) values (lnsum)
+	// and the number of sizes included (count).
+	lnsum := 0.0
+	count := 0
+
+	// print all list elements
+	prevLine := prev.Line
+	for i, x := range list {
+		line = p.lineFor(x.Pos())
+
+		// Determine if the next linebreak, if any, needs to use formfeed:
+		// in general, use the entire node size to make the decision; for
+		// key:value expressions, use the key size.
+		// TODO(gri) for a better result, should probably incorporate both
+		//           the key and the node size into the decision process
+		useFF := true
+
+		// Determine element size: All bets are off if we don't have
+		// position information for the previous and next token (likely
+		// generated code - simply ignore the size in this case by setting
+		// it to 0).
+		prevSize := size
+		const infinity = 1e6 // larger than any source line
+		size = p.nodeSize(x, infinity)
+		pair, isPair := x.(*ast.KeyValueExpr)
+		if size <= infinity && prev.IsValid() && next.IsValid() {
+			// x fits on a single line
+			if isPair {
+				size = p.nodeSize(pair.Key, infinity) // size <= infinity
+			}
+		} else {
+			// size too large or we don't have good layout information
+			size = 0
+		}
+
+		// If the previous line and the current line had single-
+		// line-expressions and the key sizes are small or the
+		// ratio between the current key and the geometric mean
+		// if the previous key sizes does not exceed a threshold,
+		// align columns and do not use formfeed.
+		if prevSize > 0 && size > 0 {
+			const smallSize = 40
+			if count == 0 || prevSize <= smallSize && size <= smallSize {
+				useFF = false
+			} else {
+				const r = 2.5                               // threshold
+				geomean := math.Exp(lnsum / float64(count)) // count > 0
+				ratio := float64(size) / geomean
+				useFF = r*ratio <= 1 || r <= ratio
+			}
+		}
+
+		needsLinebreak := 0 < prevLine && prevLine < line
+		if i > 0 {
+			// Use position of expression following the comma as
+			// comma position for correct comment placement, but
+			// only if the expression is on the same line.
+			if !needsLinebreak {
+				p.print(x.Pos())
+			}
+			p.print(token.COMMA)
+			needsBlank := true
+			if needsLinebreak {
+				// Lines are broken using newlines so comments remain aligned
+				// unless useFF is set or there are multiple expressions on
+				// the same line in which case formfeed is used.
+				nbreaks := p.linebreak(line, 0, ws, useFF || prevBreak+1 < i)
+				if nbreaks > 0 {
+					ws = ignore
+					prevBreak = i
+					needsBlank = false // we got a line break instead
+				}
+				// If there was a new section or more than one new line
+				// (which means that the tabwriter will implicitly break
+				// the section), reset the geomean variables since we are
+				// starting a new group of elements with the next element.
+				if nbreaks > 1 {
+					lnsum = 0
+					count = 0
+				}
+			}
+			if needsBlank {
+				p.print(blank)
+			}
+		}
+
+		if len(list) > 1 && isPair && size > 0 && needsLinebreak {
+			// We have a key:value expression that fits onto one line
+			// and it's not on the same line as the prior expression:
+			// Use a column for the key such that consecutive entries
+			// can align if possible.
+			// (needsLinebreak is set if we started a new line before)
+			p.expr(pair.Key)
+			p.print(pair.Colon, token.COLON, vtab)
+			p.expr(pair.Value)
+		} else {
+			p.expr0(x, depth)
+		}
+
+		if size > 0 {
+			lnsum += math.Log(float64(size))
+			count++
+		}
+
+		prevLine = line
+	}
+
+	if mode&commaTerm != 0 && next.IsValid() && p.pos.Line < next.Line {
+		// Print a terminating comma if the next token is on a new line.
+		p.print(token.COMMA)
+		if isIncomplete {
+			p.print(newline)
+			p.print("// " + filteredMsg)
+		}
+		if ws == ignore && mode&noIndent == 0 {
+			// unindent if we indented
+			p.print(unindent)
+		}
+		p.print(formfeed) // terminating comma needs a line break to look good
+		return
+	}
+
+	if isIncomplete {
+		p.print(token.COMMA, newline)
+		p.print("// "+filteredMsg, newline)
+	}
+
+	if ws == ignore && mode&noIndent == 0 {
+		// unindent if we indented
+		p.print(unindent)
+	}
+}
+
+func (p *printer) parameters(fields *ast.FieldList) {
+	p.print(fields.Opening, token.LPAREN)
+	if len(fields.List) > 0 {
+		prevLine := p.lineFor(fields.Opening)
+		ws := indent
+		for i, par := range fields.List {
+			// determine par begin and end line (may be different
+			// if there are multiple parameter names for this par
+			// or the type is on a separate line)
+			var parLineBeg int
+			if len(par.Names) > 0 {
+				parLineBeg = p.lineFor(par.Names[0].Pos())
+			} else {
+				parLineBeg = p.lineFor(par.Type.Pos())
+			}
+			var parLineEnd = p.lineFor(par.Type.End())
+			// separating "," if needed
+			needsLinebreak := 0 < prevLine && prevLine < parLineBeg
+			if i > 0 {
+				// use position of parameter following the comma as
+				// comma position for correct comma placement, but
+				// only if the next parameter is on the same line
+				if !needsLinebreak {
+					p.print(par.Pos())
+				}
+				p.print(token.COMMA)
+			}
+			// separator if needed (linebreak or blank)
+			if needsLinebreak && p.linebreak(parLineBeg, 0, ws, true) > 0 {
+				// break line if the opening "(" or previous parameter ended on a different line
+				ws = ignore
+			} else if i > 0 {
+				p.print(blank)
+			}
+			// parameter names
+			if len(par.Names) > 0 {
+				// Very subtle: If we indented before (ws == ignore), identList
+				// won't indent again. If we didn't (ws == indent), identList will
+				// indent if the identList spans multiple lines, and it will outdent
+				// again at the end (and still ws == indent). Thus, a subsequent indent
+				// by a linebreak call after a type, or in the next multi-line identList
+				// will do the right thing.
+				p.identList(par.Names, ws == indent)
+				p.print(blank)
+			}
+			// parameter type
+			p.expr(stripParensAlways(par.Type))
+			prevLine = parLineEnd
+		}
+		// if the closing ")" is on a separate line from the last parameter,
+		// print an additional "," and line break
+		if closing := p.lineFor(fields.Closing); 0 < prevLine && prevLine < closing {
+			p.print(token.COMMA)
+			p.linebreak(closing, 0, ignore, true)
+		}
+		// unindent if we indented
+		if ws == ignore {
+			p.print(unindent)
+		}
+	}
+	p.print(fields.Closing, token.RPAREN)
+}
+
+func (p *printer) signature(params, result *ast.FieldList) {
+	if params != nil {
+		p.parameters(params)
+	} else {
+		p.print(token.LPAREN, token.RPAREN)
+	}
+	n := result.NumFields()
+	if n > 0 {
+		// result != nil
+		p.print(blank)
+		if n == 1 && result.List[0].Names == nil {
+			// single anonymous result; no ()'s
+			p.expr(stripParensAlways(result.List[0].Type))
+			return
+		}
+		p.parameters(result)
+	}
+}
+
+func identListSize(list []*ast.Ident, maxSize int) (size int) {
+	for i, x := range list {
+		if i > 0 {
+			size += len(", ")
+		}
+		size += utf8.RuneCountInString(x.Name)
+		if size >= maxSize {
+			break
+		}
+	}
+	return
+}
+
+func (p *printer) isOneLineFieldList(list []*ast.Field) bool {
+	if len(list) != 1 {
+		return false // allow only one field
+	}
+	f := list[0]
+	if f.Tag != nil || f.Comment != nil {
+		return false // don't allow tags or comments
+	}
+	// only name(s) and type
+	const maxSize = 30 // adjust as appropriate, this is an approximate value
+	namesSize := identListSize(f.Names, maxSize)
+	if namesSize > 0 {
+		namesSize = 1 // blank between names and types
+	}
+	typeSize := p.nodeSize(f.Type, maxSize)
+	return namesSize+typeSize <= maxSize
+}
+
+func (p *printer) setLineComment(text string) {
+	p.setComment(&ast.CommentGroup{List: []*ast.Comment{{Slash: token.NoPos, Text: text}}})
+}
+
+func (p *printer) fieldList(fields *ast.FieldList, isStruct, isIncomplete bool) {
+	lbrace := fields.Opening
+	list := fields.List
+	rbrace := fields.Closing
+	hasComments := isIncomplete || p.commentBefore(p.posFor(rbrace))
+	srcIsOneLine := lbrace.IsValid() && rbrace.IsValid() && p.lineFor(lbrace) == p.lineFor(rbrace)
+
+	if !hasComments && srcIsOneLine {
+		// possibly a one-line struct/interface
+		if len(list) == 0 {
+			// no blank between keyword and {} in this case
+			p.print(lbrace, token.LBRACE, rbrace, token.RBRACE)
+			return
+		} else if p.isOneLineFieldList(list) {
+			// small enough - print on one line
+			// (don't use identList and ignore source line breaks)
+			p.print(lbrace, token.LBRACE, blank)
+			f := list[0]
+			if isStruct {
+				for i, x := range f.Names {
+					if i > 0 {
+						// no comments so no need for comma position
+						p.print(token.COMMA, blank)
+					}
+					p.expr(x)
+				}
+				if len(f.Names) > 0 {
+					p.print(blank)
+				}
+				p.expr(f.Type)
+			} else { // interface
+				if ftyp, isFtyp := f.Type.(*ast.FuncType); isFtyp {
+					// method
+					p.expr(f.Names[0])
+					p.signature(ftyp.Params, ftyp.Results)
+				} else {
+					// embedded interface
+					p.expr(f.Type)
+				}
+			}
+			p.print(blank, rbrace, token.RBRACE)
+			return
+		}
+	}
+	// hasComments || !srcIsOneLine
+
+	p.print(blank, lbrace, token.LBRACE, indent)
+	if hasComments || len(list) > 0 {
+		p.print(formfeed)
+	}
+
+	if isStruct {
+
+		sep := vtab
+		if len(list) == 1 {
+			sep = blank
+		}
+		var line int
+		for i, f := range list {
+			if i > 0 {
+				p.linebreak(p.lineFor(f.Pos()), 1, ignore, p.linesFrom(line) > 0)
+			}
+			extraTabs := 0
+			p.setComment(f.Doc)
+			p.recordLine(&line)
+			if len(f.Names) > 0 {
+				// named fields
+				p.identList(f.Names, false)
+				p.print(sep)
+				p.expr(f.Type)
+				extraTabs = 1
+			} else {
+				// anonymous field
+				p.expr(f.Type)
+				extraTabs = 2
+			}
+			if f.Tag != nil {
+				if len(f.Names) > 0 && sep == vtab {
+					p.print(sep)
+				}
+				p.print(sep)
+				p.expr(f.Tag)
+				extraTabs = 0
+			}
+			if f.Comment != nil {
+				for ; extraTabs > 0; extraTabs-- {
+					p.print(sep)
+				}
+				p.setComment(f.Comment)
+			}
+		}
+		if isIncomplete {
+			if len(list) > 0 {
+				p.print(formfeed)
+			}
+			p.flush(p.posFor(rbrace), token.RBRACE) // make sure we don't lose the last line comment
+			p.setLineComment("// " + filteredMsg)
+		}
+
+	} else { // interface
+
+		var line int
+		for i, f := range list {
+			if i > 0 {
+				p.linebreak(p.lineFor(f.Pos()), 1, ignore, p.linesFrom(line) > 0)
+			}
+			p.setComment(f.Doc)
+			p.recordLine(&line)
+			if ftyp, isFtyp := f.Type.(*ast.FuncType); isFtyp {
+				// method
+				p.expr(f.Names[0])
+				p.signature(ftyp.Params, ftyp.Results)
+			} else {
+				// embedded interface
+				p.expr(f.Type)
+			}
+			p.setComment(f.Comment)
+		}
+		if isIncomplete {
+			if len(list) > 0 {
+				p.print(formfeed)
+			}
+			p.flush(p.posFor(rbrace), token.RBRACE) // make sure we don't lose the last line comment
+			p.setLineComment("// contains filtered or unexported methods")
+		}
+
+	}
+	p.print(unindent, formfeed, rbrace, token.RBRACE)
+}
+
+// ----------------------------------------------------------------------------
+// Expressions
+
+func walkBinary(e *ast.BinaryExpr) (has4, has5 bool, maxProblem int) {
+	switch e.Op.Precedence() {
+	case 4:
+		has4 = true
+	case 5:
+		has5 = true
+	}
+
+	switch l := e.X.(type) {
+	case *ast.BinaryExpr:
+		if l.Op.Precedence() < e.Op.Precedence() {
+			// parens will be inserted.
+			// pretend this is an *ast.ParenExpr and do nothing.
+			break
+		}
+		h4, h5, mp := walkBinary(l)
+		has4 = has4 || h4
+		has5 = has5 || h5
+		if maxProblem < mp {
+			maxProblem = mp
+		}
+	}
+
+	switch r := e.Y.(type) {
+	case *ast.BinaryExpr:
+		if r.Op.Precedence() <= e.Op.Precedence() {
+			// parens will be inserted.
+			// pretend this is an *ast.ParenExpr and do nothing.
+			break
+		}
+		h4, h5, mp := walkBinary(r)
+		has4 = has4 || h4
+		has5 = has5 || h5
+		if maxProblem < mp {
+			maxProblem = mp
+		}
+
+	case *ast.StarExpr:
+		if e.Op == token.QUO { // `*/`
+			maxProblem = 5
+		}
+
+	case *ast.UnaryExpr:
+		switch e.Op.String() + r.Op.String() {
+		case "/*", "&&", "&^":
+			maxProblem = 5
+		case "++", "--":
+			if maxProblem < 4 {
+				maxProblem = 4
+			}
+		}
+	}
+	return
+}
+
+func cutoff(e *ast.BinaryExpr, depth int) int {
+	has4, has5, maxProblem := walkBinary(e)
+	if maxProblem > 0 {
+		return maxProblem + 1
+	}
+	if has4 && has5 {
+		if depth == 1 {
+			return 5
+		}
+		return 4
+	}
+	if depth == 1 {
+		return 6
+	}
+	return 4
+}
+
+func diffPrec(expr ast.Expr, prec int) int {
+	x, ok := expr.(*ast.BinaryExpr)
+	if !ok || prec != x.Op.Precedence() {
+		return 1
+	}
+	return 0
+}
+
+func reduceDepth(depth int) int {
+	depth--
+	if depth < 1 {
+		depth = 1
+	}
+	return depth
+}
+
+// Format the binary expression: decide the cutoff and then format.
+// Let's call depth == 1 Normal mode, and depth > 1 Compact mode.
+// (Algorithm suggestion by Russ Cox.)
+//
+// The precedences are:
+//	5             *  /  %  <<  >>  &  &^
+//	4             +  -  |  ^
+//	3             ==  !=  <  <=  >  >=
+//	2             &&
+//	1             ||
+//
+// The only decision is whether there will be spaces around levels 4 and 5.
+// There are never spaces at level 6 (unary), and always spaces at levels 3 and below.
+//
+// To choose the cutoff, look at the whole expression but excluding primary
+// expressions (function calls, parenthesized exprs), and apply these rules:
+//
+//	1) If there is a binary operator with a right side unary operand
+//	   that would clash without a space, the cutoff must be (in order):
+//
+//		/*	6
+//		&&	6
+//		&^	6
+//		++	5
+//		--	5
+//
+//         (Comparison operators always have spaces around them.)
+//
+//	2) If there is a mix of level 5 and level 4 operators, then the cutoff
+//	   is 5 (use spaces to distinguish precedence) in Normal mode
+//	   and 4 (never use spaces) in Compact mode.
+//
+//	3) If there are no level 4 operators or no level 5 operators, then the
+//	   cutoff is 6 (always use spaces) in Normal mode
+//	   and 4 (never use spaces) in Compact mode.
+//
+func (p *printer) binaryExpr(x *ast.BinaryExpr, prec1, cutoff, depth int) {
+	prec := x.Op.Precedence()
+	if prec < prec1 {
+		// parenthesis needed
+		// Note: The parser inserts an ast.ParenExpr node; thus this case
+		//       can only occur if the AST is created in a different way.
+		p.print(token.LPAREN)
+		p.expr0(x, reduceDepth(depth)) // parentheses undo one level of depth
+		p.print(token.RPAREN)
+		return
+	}
+
+	printBlank := prec < cutoff
+
+	ws := indent
+	p.expr1(x.X, prec, depth+diffPrec(x.X, prec))
+	if printBlank {
+		p.print(blank)
+	}
+	xline := p.pos.Line // before the operator (it may be on the next line!)
+	yline := p.lineFor(x.Y.Pos())
+	p.print(x.OpPos, x.Op)
+	if xline != yline && xline > 0 && yline > 0 {
+		// at least one line break, but respect an extra empty line
+		// in the source
+		if p.linebreak(yline, 1, ws, true) > 0 {
+			ws = ignore
+			printBlank = false // no blank after line break
+		}
+	}
+	if printBlank {
+		p.print(blank)
+	}
+	p.expr1(x.Y, prec+1, depth+1)
+	if ws == ignore {
+		p.print(unindent)
+	}
+}
+
+func isBinary(expr ast.Expr) bool {
+	_, ok := expr.(*ast.BinaryExpr)
+	return ok
+}
+
+func (p *printer) expr1(expr ast.Expr, prec1, depth int) {
+	p.print(expr.Pos())
+
+	switch x := expr.(type) {
+	case *ast.BadExpr:
+		p.print("BadExpr")
+
+	case *ast.Ident:
+		p.print(x)
+
+	case *ast.BinaryExpr:
+		if depth < 1 {
+			p.internalError("depth < 1:", depth)
+			depth = 1
+		}
+		p.binaryExpr(x, prec1, cutoff(x, depth), depth)
+
+	case *ast.KeyValueExpr:
+		p.expr(x.Key)
+		p.print(x.Colon, token.COLON, blank)
+		p.expr(x.Value)
+
+	case *ast.StarExpr:
+		const prec = token.UnaryPrec
+		if prec < prec1 {
+			// parenthesis needed
+			p.print(token.LPAREN)
+			p.print(token.MUL)
+			p.expr(x.X)
+			p.print(token.RPAREN)
+		} else {
+			// no parenthesis needed
+			p.print(token.MUL)
+			p.expr(x.X)
+		}
+
+	case *ast.UnaryExpr:
+		const prec = token.UnaryPrec
+		if prec < prec1 {
+			// parenthesis needed
+			p.print(token.LPAREN)
+			p.expr(x)
+			p.print(token.RPAREN)
+		} else {
+			// no parenthesis needed
+			p.print(x.Op)
+			if x.Op == token.RANGE {
+				// TODO(gri) Remove this code if it cannot be reached.
+				p.print(blank)
+			}
+			p.expr1(x.X, prec, depth)
+		}
+
+	case *ast.BasicLit:
+		if p.Config.Mode&normalizeNumbers != 0 {
+			x = normalizedNumber(x)
+		}
+		p.print(x)
+
+	case *ast.FuncLit:
+		p.print(x.Type.Pos(), token.FUNC)
+		// See the comment in funcDecl about how the header size is computed.
+		startCol := p.out.Column - len("func")
+		p.signature(x.Type.Params, x.Type.Results)
+		p.funcBody(p.distanceFrom(x.Type.Pos(), startCol), blank, x.Body)
+
+	case *ast.ParenExpr:
+		if _, hasParens := x.X.(*ast.ParenExpr); hasParens {
+			// don't print parentheses around an already parenthesized expression
+			// TODO(gri) consider making this more general and incorporate precedence levels
+			p.expr0(x.X, depth)
+		} else {
+			p.print(token.LPAREN)
+			p.expr0(x.X, reduceDepth(depth)) // parentheses undo one level of depth
+			p.print(x.Rparen, token.RPAREN)
+		}
+
+	case *ast.SelectorExpr:
+		p.selectorExpr(x, depth, false)
+
+	case *ast.TypeAssertExpr:
+		p.expr1(x.X, token.HighestPrec, depth)
+		p.print(token.PERIOD, x.Lparen, token.LPAREN)
+		if x.Type != nil {
+			p.expr(x.Type)
+		} else {
+			p.print(token.TYPE)
+		}
+		p.print(x.Rparen, token.RPAREN)
+
+	case *ast.IndexExpr:
+		// TODO(gri): should treat[] like parentheses and undo one level of depth
+		p.expr1(x.X, token.HighestPrec, 1)
+		p.print(x.Lbrack, token.LBRACK)
+		p.expr0(x.Index, depth+1)
+		p.print(x.Rbrack, token.RBRACK)
+
+	case *ast.SliceExpr:
+		// TODO(gri): should treat[] like parentheses and undo one level of depth
+		p.expr1(x.X, token.HighestPrec, 1)
+		p.print(x.Lbrack, token.LBRACK)
+		indices := []ast.Expr{x.Low, x.High}
+		if x.Max != nil {
+			indices = append(indices, x.Max)
+		}
+		// determine if we need extra blanks around ':'
+		var needsBlanks bool
+		if depth <= 1 {
+			var indexCount int
+			var hasBinaries bool
+			for _, x := range indices {
+				if x != nil {
+					indexCount++
+					if isBinary(x) {
+						hasBinaries = true
+					}
+				}
+			}
+			if indexCount > 1 && hasBinaries {
+				needsBlanks = true
+			}
+		}
+		for i, x := range indices {
+			if i > 0 {
+				if indices[i-1] != nil && needsBlanks {
+					p.print(blank)
+				}
+				p.print(token.COLON)
+				if x != nil && needsBlanks {
+					p.print(blank)
+				}
+			}
+			if x != nil {
+				p.expr0(x, depth+1)
+			}
+		}
+		p.print(x.Rbrack, token.RBRACK)
+
+	case *ast.CallExpr:
+		if len(x.Args) > 1 {
+			depth++
+		}
+		var wasIndented bool
+		if _, ok := x.Fun.(*ast.FuncType); ok {
+			// conversions to literal function types require parentheses around the type
+			p.print(token.LPAREN)
+			wasIndented = p.possibleSelectorExpr(x.Fun, token.HighestPrec, depth)
+			p.print(token.RPAREN)
+		} else {
+			wasIndented = p.possibleSelectorExpr(x.Fun, token.HighestPrec, depth)
+		}
+		p.print(x.Lparen, token.LPAREN)
+		if x.Ellipsis.IsValid() {
+			p.exprList(x.Lparen, x.Args, depth, 0, x.Ellipsis, false)
+			p.print(x.Ellipsis, token.ELLIPSIS)
+			if x.Rparen.IsValid() && p.lineFor(x.Ellipsis) < p.lineFor(x.Rparen) {
+				p.print(token.COMMA, formfeed)
+			}
+		} else {
+			p.exprList(x.Lparen, x.Args, depth, commaTerm, x.Rparen, false)
+		}
+		p.print(x.Rparen, token.RPAREN)
+		if wasIndented {
+			p.print(unindent)
+		}
+
+	case *ast.CompositeLit:
+		// composite literal elements that are composite literals themselves may have the type omitted
+		if x.Type != nil {
+			p.expr1(x.Type, token.HighestPrec, depth)
+		}
+		p.level++
+		p.print(x.Lbrace, token.LBRACE)
+		p.exprList(x.Lbrace, x.Elts, 1, commaTerm, x.Rbrace, x.Incomplete)
+		// do not insert extra line break following a /*-style comment
+		// before the closing '}' as it might break the code if there
+		// is no trailing ','
+		mode := noExtraLinebreak
+		// do not insert extra blank following a /*-style comment
+		// before the closing '}' unless the literal is empty
+		if len(x.Elts) > 0 {
+			mode |= noExtraBlank
+		}
+		// need the initial indent to print lone comments with
+		// the proper level of indentation
+		p.print(indent, unindent, mode, x.Rbrace, token.RBRACE, mode)
+		p.level--
+
+	case *ast.Ellipsis:
+		p.print(token.ELLIPSIS)
+		if x.Elt != nil {
+			p.expr(x.Elt)
+		}
+
+	case *ast.ArrayType:
+		p.print(token.LBRACK)
+		if x.Len != nil {
+			p.expr(x.Len)
+		}
+		p.print(token.RBRACK)
+		p.expr(x.Elt)
+
+	case *ast.StructType:
+		p.print(token.STRUCT)
+		p.fieldList(x.Fields, true, x.Incomplete)
+
+	case *ast.FuncType:
+		p.print(token.FUNC)
+		p.signature(x.Params, x.Results)
+
+	case *ast.InterfaceType:
+		p.print(token.INTERFACE)
+		p.fieldList(x.Methods, false, x.Incomplete)
+
+	case *ast.MapType:
+		p.print(token.MAP, token.LBRACK)
+		p.expr(x.Key)
+		p.print(token.RBRACK)
+		p.expr(x.Value)
+
+	case *ast.ChanType:
+		switch x.Dir {
+		case ast.SEND | ast.RECV:
+			p.print(token.CHAN)
+		case ast.RECV:
+			p.print(token.ARROW, token.CHAN) // x.Arrow and x.Pos() are the same
+		case ast.SEND:
+			p.print(token.CHAN, x.Arrow, token.ARROW)
+		}
+		p.print(blank)
+		p.expr(x.Value)
+
+	default:
+		panic("unreachable")
+	}
+}
+
+// normalizedNumber rewrites base prefixes and exponents
+// of numbers to use lower-case letters (0X123 to 0x123 and 1.2E3 to 1.2e3),
+// and removes leading 0's from integer imaginary literals (0765i to 765i).
+// It leaves hexadecimal digits alone.
+//
+// normalizedNumber doesn't modify the ast.BasicLit value lit points to.
+// If lit is not a number or a number in canonical format already,
+// lit is returned as is. Otherwise a new ast.BasicLit is created.
+func normalizedNumber(lit *ast.BasicLit) *ast.BasicLit {
+	if lit.Kind != token.INT && lit.Kind != token.FLOAT && lit.Kind != token.IMAG {
+		return lit // not a number - nothing to do
+	}
+	if len(lit.Value) < 2 {
+		return lit // only one digit (common case) - nothing to do
+	}
+	// len(lit.Value) >= 2
+
+	// We ignore lit.Kind because for lit.Kind == token.IMAG the literal may be an integer
+	// or floating-point value, decimal or not. Instead, just consider the literal pattern.
+	x := lit.Value
+	switch x[:2] {
+	default:
+		// 0-prefix octal, decimal int, or float (possibly with 'i' suffix)
+		if i := strings.LastIndexByte(x, 'E'); i >= 0 {
+			x = x[:i] + "e" + x[i+1:]
+			break
+		}
+		// remove leading 0's from integer (but not floating-point) imaginary literals
+		if x[len(x)-1] == 'i' && strings.IndexByte(x, '.') < 0 && strings.IndexByte(x, 'e') < 0 {
+			x = strings.TrimLeft(x, "0_")
+			if x == "i" {
+				x = "0i"
+			}
+		}
+	case "0X":
+		x = "0x" + x[2:]
+		// possibly a hexadecimal float
+		if i := strings.LastIndexByte(x, 'P'); i >= 0 {
+			x = x[:i] + "p" + x[i+1:]
+		}
+	case "0x":
+		// possibly a hexadecimal float
+		i := strings.LastIndexByte(x, 'P')
+		if i == -1 {
+			return lit // nothing to do
+		}
+		x = x[:i] + "p" + x[i+1:]
+	case "0O":
+		x = "0o" + x[2:]
+	case "0o":
+		return lit // nothing to do
+	case "0B":
+		x = "0b" + x[2:]
+	case "0b":
+		return lit // nothing to do
+	}
+
+	return &ast.BasicLit{ValuePos: lit.ValuePos, Kind: lit.Kind, Value: x}
+}
+
+func (p *printer) possibleSelectorExpr(expr ast.Expr, prec1, depth int) bool {
+	if x, ok := expr.(*ast.SelectorExpr); ok {
+		return p.selectorExpr(x, depth, true)
+	}
+	p.expr1(expr, prec1, depth)
+	return false
+}
+
+// selectorExpr handles an *ast.SelectorExpr node and reports whether x spans
+// multiple lines.
+func (p *printer) selectorExpr(x *ast.SelectorExpr, depth int, isMethod bool) bool {
+	p.expr1(x.X, token.HighestPrec, depth)
+	p.print(token.PERIOD)
+	if line := p.lineFor(x.Sel.Pos()); p.pos.IsValid() && p.pos.Line < line {
+		p.print(indent, newline, x.Sel.Pos(), x.Sel)
+		if !isMethod {
+			p.print(unindent)
+		}
+		return true
+	}
+	p.print(x.Sel.Pos(), x.Sel)
+	return false
+}
+
+func (p *printer) expr0(x ast.Expr, depth int) {
+	p.expr1(x, token.LowestPrec, depth)
+}
+
+func (p *printer) expr(x ast.Expr) {
+	const depth = 1
+	p.expr1(x, token.LowestPrec, depth)
+}
+
+// ----------------------------------------------------------------------------
+// Statements
+
+// Print the statement list indented, but without a newline after the last statement.
+// Extra line breaks between statements in the source are respected but at most one
+// empty line is printed between statements.
+func (p *printer) stmtList(list []ast.Stmt, nindent int, nextIsRBrace bool) {
+	if nindent > 0 {
+		p.print(indent)
+	}
+	var line int
+	i := 0
+	for _, s := range list {
+		// ignore empty statements (was issue 3466)
+		if _, isEmpty := s.(*ast.EmptyStmt); !isEmpty {
+			// nindent == 0 only for lists of switch/select case clauses;
+			// in those cases each clause is a new section
+			if len(p.output) > 0 {
+				// only print line break if we are not at the beginning of the output
+				// (i.e., we are not printing only a partial program)
+				p.linebreak(p.lineFor(s.Pos()), 1, ignore, i == 0 || nindent == 0 || p.linesFrom(line) > 0)
+			}
+			p.recordLine(&line)
+			p.stmt(s, nextIsRBrace && i == len(list)-1)
+			// labeled statements put labels on a separate line, but here
+			// we only care about the start line of the actual statement
+			// without label - correct line for each label
+			for t := s; ; {
+				lt, _ := t.(*ast.LabeledStmt)
+				if lt == nil {
+					break
+				}
+				line++
+				t = lt.Stmt
+			}
+			i++
+		}
+	}
+	if nindent > 0 {
+		p.print(unindent)
+	}
+}
+
+// block prints an *ast.BlockStmt; it always spans at least two lines.
+func (p *printer) block(b *ast.BlockStmt, nindent int) {
+	p.print(b.Lbrace, token.LBRACE)
+	p.stmtList(b.List, nindent, true)
+	p.linebreak(p.lineFor(b.Rbrace), 1, ignore, true)
+	p.print(b.Rbrace, token.RBRACE)
+}
+
+func isTypeName(x ast.Expr) bool {
+	switch t := x.(type) {
+	case *ast.Ident:
+		return true
+	case *ast.SelectorExpr:
+		return isTypeName(t.X)
+	}
+	return false
+}
+
+func stripParens(x ast.Expr) ast.Expr {
+	if px, strip := x.(*ast.ParenExpr); strip {
+		// parentheses must not be stripped if there are any
+		// unparenthesized composite literals starting with
+		// a type name
+		ast.Inspect(px.X, func(node ast.Node) bool {
+			switch x := node.(type) {
+			case *ast.ParenExpr:
+				// parentheses protect enclosed composite literals
+				return false
+			case *ast.CompositeLit:
+				if isTypeName(x.Type) {
+					strip = false // do not strip parentheses
+				}
+				return false
+			}
+			// in all other cases, keep inspecting
+			return true
+		})
+		if strip {
+			return stripParens(px.X)
+		}
+	}
+	return x
+}
+
+func stripParensAlways(x ast.Expr) ast.Expr {
+	if x, ok := x.(*ast.ParenExpr); ok {
+		return stripParensAlways(x.X)
+	}
+	return x
+}
+
+func (p *printer) controlClause(isForStmt bool, init ast.Stmt, expr ast.Expr, post ast.Stmt) {
+	p.print(blank)
+	needsBlank := false
+	if init == nil && post == nil {
+		// no semicolons required
+		if expr != nil {
+			p.expr(stripParens(expr))
+			needsBlank = true
+		}
+	} else {
+		// all semicolons required
+		// (they are not separators, print them explicitly)
+		if init != nil {
+			p.stmt(init, false)
+		}
+		p.print(token.SEMICOLON, blank)
+		if expr != nil {
+			p.expr(stripParens(expr))
+			needsBlank = true
+		}
+		if isForStmt {
+			p.print(token.SEMICOLON, blank)
+			needsBlank = false
+			if post != nil {
+				p.stmt(post, false)
+				needsBlank = true
+			}
+		}
+	}
+	if needsBlank {
+		p.print(blank)
+	}
+}
+
+// indentList reports whether an expression list would look better if it
+// were indented wholesale (starting with the very first element, rather
+// than starting at the first line break).
+//
+func (p *printer) indentList(list []ast.Expr) bool {
+	// Heuristic: indentList reports whether there are more than one multi-
+	// line element in the list, or if there is any element that is not
+	// starting on the same line as the previous one ends.
+	if len(list) >= 2 {
+		var b = p.lineFor(list[0].Pos())
+		var e = p.lineFor(list[len(list)-1].End())
+		if 0 < b && b < e {
+			// list spans multiple lines
+			n := 0 // multi-line element count
+			line := b
+			for _, x := range list {
+				xb := p.lineFor(x.Pos())
+				xe := p.lineFor(x.End())
+				if line < xb {
+					// x is not starting on the same
+					// line as the previous one ended
+					return true
+				}
+				if xb < xe {
+					// x is a multi-line element
+					n++
+				}
+				line = xe
+			}
+			return n > 1
+		}
+	}
+	return false
+}
+
+func (p *printer) stmt(stmt ast.Stmt, nextIsRBrace bool) {
+	p.print(stmt.Pos())
+
+	switch s := stmt.(type) {
+	case *ast.BadStmt:
+		p.print("BadStmt")
+
+	case *ast.DeclStmt:
+		p.decl(s.Decl)
+
+	case *ast.EmptyStmt:
+		// nothing to do
+
+	case *ast.LabeledStmt:
+		// a "correcting" unindent immediately following a line break
+		// is applied before the line break if there is no comment
+		// between (see writeWhitespace)
+		p.print(unindent)
+		p.expr(s.Label)
+		p.print(s.Colon, token.COLON, indent)
+		if e, isEmpty := s.Stmt.(*ast.EmptyStmt); isEmpty {
+			if !nextIsRBrace {
+				p.print(newline, e.Pos(), token.SEMICOLON)
+				break
+			}
+		} else {
+			p.linebreak(p.lineFor(s.Stmt.Pos()), 1, ignore, true)
+		}
+		p.stmt(s.Stmt, nextIsRBrace)
+
+	case *ast.ExprStmt:
+		const depth = 1
+		p.expr0(s.X, depth)
+
+	case *ast.SendStmt:
+		const depth = 1
+		p.expr0(s.Chan, depth)
+		p.print(blank, s.Arrow, token.ARROW, blank)
+		p.expr0(s.Value, depth)
+
+	case *ast.IncDecStmt:
+		const depth = 1
+		p.expr0(s.X, depth+1)
+		p.print(s.TokPos, s.Tok)
+
+	case *ast.AssignStmt:
+		var depth = 1
+		if len(s.Lhs) > 1 && len(s.Rhs) > 1 {
+			depth++
+		}
+		p.exprList(s.Pos(), s.Lhs, depth, 0, s.TokPos, false)
+		p.print(blank, s.TokPos, s.Tok, blank)
+		p.exprList(s.TokPos, s.Rhs, depth, 0, token.NoPos, false)
+
+	case *ast.GoStmt:
+		p.print(token.GO, blank)
+		p.expr(s.Call)
+
+	case *ast.DeferStmt:
+		p.print(token.DEFER, blank)
+		p.expr(s.Call)
+
+	case *ast.ReturnStmt:
+		p.print(token.RETURN)
+		if s.Results != nil {
+			p.print(blank)
+			// Use indentList heuristic to make corner cases look
+			// better (issue 1207). A more systematic approach would
+			// always indent, but this would cause significant
+			// reformatting of the code base and not necessarily
+			// lead to more nicely formatted code in general.
+			if p.indentList(s.Results) {
+				p.print(indent)
+				// Use NoPos so that a newline never goes before
+				// the results (see issue #32854).
+				p.exprList(token.NoPos, s.Results, 1, noIndent, token.NoPos, false)
+				p.print(unindent)
+			} else {
+				p.exprList(token.NoPos, s.Results, 1, 0, token.NoPos, false)
+			}
+		}
+
+	case *ast.BranchStmt:
+		p.print(s.Tok)
+		if s.Label != nil {
+			p.print(blank)
+			p.expr(s.Label)
+		}
+
+	case *ast.BlockStmt:
+		p.block(s, 1)
+
+	case *ast.IfStmt:
+		p.print(token.IF)
+		p.controlClause(false, s.Init, s.Cond, nil)
+		p.block(s.Body, 1)
+		if s.Else != nil {
+			p.print(blank, token.ELSE, blank)
+			switch s.Else.(type) {
+			case *ast.BlockStmt, *ast.IfStmt:
+				p.stmt(s.Else, nextIsRBrace)
+			default:
+				// This can only happen with an incorrectly
+				// constructed AST. Permit it but print so
+				// that it can be parsed without errors.
+				p.print(token.LBRACE, indent, formfeed)
+				p.stmt(s.Else, true)
+				p.print(unindent, formfeed, token.RBRACE)
+			}
+		}
+
+	case *ast.CaseClause:
+		if s.List != nil {
+			p.print(token.CASE, blank)
+			p.exprList(s.Pos(), s.List, 1, 0, s.Colon, false)
+		} else {
+			p.print(token.DEFAULT)
+		}
+		p.print(s.Colon, token.COLON)
+		p.stmtList(s.Body, 1, nextIsRBrace)
+
+	case *ast.SwitchStmt:
+		p.print(token.SWITCH)
+		p.controlClause(false, s.Init, s.Tag, nil)
+		p.block(s.Body, 0)
+
+	case *ast.TypeSwitchStmt:
+		p.print(token.SWITCH)
+		if s.Init != nil {
+			p.print(blank)
+			p.stmt(s.Init, false)
+			p.print(token.SEMICOLON)
+		}
+		p.print(blank)
+		p.stmt(s.Assign, false)
+		p.print(blank)
+		p.block(s.Body, 0)
+
+	case *ast.CommClause:
+		if s.Comm != nil {
+			p.print(token.CASE, blank)
+			p.stmt(s.Comm, false)
+		} else {
+			p.print(token.DEFAULT)
+		}
+		p.print(s.Colon, token.COLON)
+		p.stmtList(s.Body, 1, nextIsRBrace)
+
+	case *ast.SelectStmt:
+		p.print(token.SELECT, blank)
+		body := s.Body
+		if len(body.List) == 0 && !p.commentBefore(p.posFor(body.Rbrace)) {
+			// print empty select statement w/o comments on one line
+			p.print(body.Lbrace, token.LBRACE, body.Rbrace, token.RBRACE)
+		} else {
+			p.block(body, 0)
+		}
+
+	case *ast.ForStmt:
+		p.print(token.FOR)
+		p.controlClause(true, s.Init, s.Cond, s.Post)
+		p.block(s.Body, 1)
+
+	case *ast.RangeStmt:
+		p.print(token.FOR, blank)
+		if s.Key != nil {
+			p.expr(s.Key)
+			if s.Value != nil {
+				// use position of value following the comma as
+				// comma position for correct comment placement
+				p.print(s.Value.Pos(), token.COMMA, blank)
+				p.expr(s.Value)
+			}
+			p.print(blank, s.TokPos, s.Tok, blank)
+		}
+		p.print(token.RANGE, blank)
+		p.expr(stripParens(s.X))
+		p.print(blank)
+		p.block(s.Body, 1)
+
+	default:
+		panic("unreachable")
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Declarations
+
+// The keepTypeColumn function determines if the type column of a series of
+// consecutive const or var declarations must be kept, or if initialization
+// values (V) can be placed in the type column (T) instead. The i'th entry
+// in the result slice is true if the type column in spec[i] must be kept.
+//
+// For example, the declaration:
+//
+//	const (
+//		foobar int = 42 // comment
+//		x          = 7  // comment
+//		foo
+//              bar = 991
+//	)
+//
+// leads to the type/values matrix below. A run of value columns (V) can
+// be moved into the type column if there is no type for any of the values
+// in that column (we only move entire columns so that they align properly).
+//
+//	matrix        formatted     result
+//                    matrix
+//	T  V    ->    T  V     ->   true      there is a T and so the type
+//	-  V          -  V          true      column must be kept
+//	-  -          -  -          false
+//	-  V          V  -          false     V is moved into T column
+//
+func keepTypeColumn(specs []ast.Spec) []bool {
+	m := make([]bool, len(specs))
+
+	populate := func(i, j int, keepType bool) {
+		if keepType {
+			for ; i < j; i++ {
+				m[i] = true
+			}
+		}
+	}
+
+	i0 := -1 // if i0 >= 0 we are in a run and i0 is the start of the run
+	var keepType bool
+	for i, s := range specs {
+		t := s.(*ast.ValueSpec)
+		if t.Values != nil {
+			if i0 < 0 {
+				// start of a run of ValueSpecs with non-nil Values
+				i0 = i
+				keepType = false
+			}
+		} else {
+			if i0 >= 0 {
+				// end of a run
+				populate(i0, i, keepType)
+				i0 = -1
+			}
+		}
+		if t.Type != nil {
+			keepType = true
+		}
+	}
+	if i0 >= 0 {
+		// end of a run
+		populate(i0, len(specs), keepType)
+	}
+
+	return m
+}
+
+func (p *printer) valueSpec(s *ast.ValueSpec, keepType bool) {
+	p.setComment(s.Doc)
+	p.identList(s.Names, false) // always present
+	extraTabs := 3
+	if s.Type != nil || keepType {
+		p.print(vtab)
+		extraTabs--
+	}
+	if s.Type != nil {
+		p.expr(s.Type)
+	}
+	if s.Values != nil {
+		p.print(vtab, token.ASSIGN, blank)
+		p.exprList(token.NoPos, s.Values, 1, 0, token.NoPos, false)
+		extraTabs--
+	}
+	if s.Comment != nil {
+		for ; extraTabs > 0; extraTabs-- {
+			p.print(vtab)
+		}
+		p.setComment(s.Comment)
+	}
+}
+
+func sanitizeImportPath(lit *ast.BasicLit) *ast.BasicLit {
+	// Note: An unmodified AST generated by go/parser will already
+	// contain a backward- or double-quoted path string that does
+	// not contain any invalid characters, and most of the work
+	// here is not needed. However, a modified or generated AST
+	// may possibly contain non-canonical paths. Do the work in
+	// all cases since it's not too hard and not speed-critical.
+
+	// if we don't have a proper string, be conservative and return whatever we have
+	if lit.Kind != token.STRING {
+		return lit
+	}
+	s, err := strconv.Unquote(lit.Value)
+	if err != nil {
+		return lit
+	}
+
+	// if the string is an invalid path, return whatever we have
+	//
+	// spec: "Implementation restriction: A compiler may restrict
+	// ImportPaths to non-empty strings using only characters belonging
+	// to Unicode's L, M, N, P, and S general categories (the Graphic
+	// characters without spaces) and may also exclude the characters
+	// !"#$%&'()*,:;<=>?[\]^`{|} and the Unicode replacement character
+	// U+FFFD."
+	if s == "" {
+		return lit
+	}
+	const illegalChars = `!"#$%&'()*,:;<=>?[\]^{|}` + "`\uFFFD"
+	for _, r := range s {
+		if !unicode.IsGraphic(r) || unicode.IsSpace(r) || strings.ContainsRune(illegalChars, r) {
+			return lit
+		}
+	}
+
+	// otherwise, return the double-quoted path
+	s = strconv.Quote(s)
+	if s == lit.Value {
+		return lit // nothing wrong with lit
+	}
+	return &ast.BasicLit{ValuePos: lit.ValuePos, Kind: token.STRING, Value: s}
+}
+
+// The parameter n is the number of specs in the group. If doIndent is set,
+// multi-line identifier lists in the spec are indented when the first
+// linebreak is encountered.
+//
+func (p *printer) spec(spec ast.Spec, n int, doIndent bool) {
+	switch s := spec.(type) {
+	case *ast.ImportSpec:
+		p.setComment(s.Doc)
+		if s.Name != nil {
+			p.expr(s.Name)
+			p.print(blank)
+		}
+		p.expr(sanitizeImportPath(s.Path))
+		p.setComment(s.Comment)
+		p.print(s.EndPos)
+
+	case *ast.ValueSpec:
+		if n != 1 {
+			p.internalError("expected n = 1; got", n)
+		}
+		p.setComment(s.Doc)
+		p.identList(s.Names, doIndent) // always present
+		if s.Type != nil {
+			p.print(blank)
+			p.expr(s.Type)
+		}
+		if s.Values != nil {
+			p.print(blank, token.ASSIGN, blank)
+			p.exprList(token.NoPos, s.Values, 1, 0, token.NoPos, false)
+		}
+		p.setComment(s.Comment)
+
+	case *ast.TypeSpec:
+		p.setComment(s.Doc)
+		p.expr(s.Name)
+		if n == 1 {
+			p.print(blank)
+		} else {
+			p.print(vtab)
+		}
+		if s.Assign.IsValid() {
+			p.print(token.ASSIGN, blank)
+		}
+		p.expr(s.Type)
+		p.setComment(s.Comment)
+
+	default:
+		panic("unreachable")
+	}
+}
+
+func (p *printer) genDecl(d *ast.GenDecl) {
+	p.setComment(d.Doc)
+	p.print(d.Pos(), d.Tok, blank)
+
+	if d.Lparen.IsValid() || len(d.Specs) > 1 {
+		// group of parenthesized declarations
+		p.print(d.Lparen, token.LPAREN)
+		if n := len(d.Specs); n > 0 {
+			p.print(indent, formfeed)
+			if n > 1 && (d.Tok == token.CONST || d.Tok == token.VAR) {
+				// two or more grouped const/var declarations:
+				// determine if the type column must be kept
+				keepType := keepTypeColumn(d.Specs)
+				var line int
+				for i, s := range d.Specs {
+					if i > 0 {
+						p.linebreak(p.lineFor(s.Pos()), 1, ignore, p.linesFrom(line) > 0)
+					}
+					p.recordLine(&line)
+					p.valueSpec(s.(*ast.ValueSpec), keepType[i])
+				}
+			} else {
+				var line int
+				for i, s := range d.Specs {
+					if i > 0 {
+						p.linebreak(p.lineFor(s.Pos()), 1, ignore, p.linesFrom(line) > 0)
+					}
+					p.recordLine(&line)
+					p.spec(s, n, false)
+				}
+			}
+			p.print(unindent, formfeed)
+		}
+		p.print(d.Rparen, token.RPAREN)
+
+	} else if len(d.Specs) > 0 {
+		// single declaration
+		p.spec(d.Specs[0], 1, true)
+	}
+}
+
+// nodeSize determines the size of n in chars after formatting.
+// The result is <= maxSize if the node fits on one line with at
+// most maxSize chars and the formatted output doesn't contain
+// any control chars. Otherwise, the result is > maxSize.
+//
+func (p *printer) nodeSize(n ast.Node, maxSize int) (size int) {
+	// nodeSize invokes the printer, which may invoke nodeSize
+	// recursively. For deep composite literal nests, this can
+	// lead to an exponential algorithm. Remember previous
+	// results to prune the recursion (was issue 1628).
+	if size, found := p.nodeSizes[n]; found {
+		return size
+	}
+
+	size = maxSize + 1 // assume n doesn't fit
+	p.nodeSizes[n] = size
+
+	// nodeSize computation must be independent of particular
+	// style so that we always get the same decision; print
+	// in RawFormat
+	cfg := Config{Mode: RawFormat}
+	var buf bytes.Buffer
+	if err := cfg.fprint(&buf, p.fset, n, p.nodeSizes); err != nil {
+		return
+	}
+	if buf.Len() <= maxSize {
+		for _, ch := range buf.Bytes() {
+			if ch < ' ' {
+				return
+			}
+		}
+		size = buf.Len() // n fits
+		p.nodeSizes[n] = size
+	}
+	return
+}
+
+// numLines returns the number of lines spanned by node n in the original source.
+func (p *printer) numLines(n ast.Node) int {
+	if from := n.Pos(); from.IsValid() {
+		if to := n.End(); to.IsValid() {
+			return p.lineFor(to) - p.lineFor(from) + 1
+		}
+	}
+	return infinity
+}
+
+// bodySize is like nodeSize but it is specialized for *ast.BlockStmt's.
+func (p *printer) bodySize(b *ast.BlockStmt, maxSize int) int {
+	pos1 := b.Pos()
+	pos2 := b.Rbrace
+	if pos1.IsValid() && pos2.IsValid() && p.lineFor(pos1) != p.lineFor(pos2) {
+		// opening and closing brace are on different lines - don't make it a one-liner
+		return maxSize + 1
+	}
+	if len(b.List) > 5 {
+		// too many statements - don't make it a one-liner
+		return maxSize + 1
+	}
+	// otherwise, estimate body size
+	bodySize := p.commentSizeBefore(p.posFor(pos2))
+	for i, s := range b.List {
+		if bodySize > maxSize {
+			break // no need to continue
+		}
+		if i > 0 {
+			bodySize += 2 // space for a semicolon and blank
+		}
+		bodySize += p.nodeSize(s, maxSize)
+	}
+	return bodySize
+}
+
+// funcBody prints a function body following a function header of given headerSize.
+// If the header's and block's size are "small enough" and the block is "simple enough",
+// the block is printed on the current line, without line breaks, spaced from the header
+// by sep. Otherwise the block's opening "{" is printed on the current line, followed by
+// lines for the block's statements and its closing "}".
+//
+func (p *printer) funcBody(headerSize int, sep whiteSpace, b *ast.BlockStmt) {
+	if b == nil {
+		return
+	}
+
+	// save/restore composite literal nesting level
+	defer func(level int) {
+		p.level = level
+	}(p.level)
+	p.level = 0
+
+	const maxSize = 100
+	if headerSize+p.bodySize(b, maxSize) <= maxSize {
+		p.print(sep, b.Lbrace, token.LBRACE)
+		if len(b.List) > 0 {
+			p.print(blank)
+			for i, s := range b.List {
+				if i > 0 {
+					p.print(token.SEMICOLON, blank)
+				}
+				p.stmt(s, i == len(b.List)-1)
+			}
+			p.print(blank)
+		}
+		p.print(noExtraLinebreak, b.Rbrace, token.RBRACE, noExtraLinebreak)
+		return
+	}
+
+	if sep != ignore {
+		p.print(blank) // always use blank
+	}
+	p.block(b, 1)
+}
+
+// distanceFrom returns the column difference between p.out (the current output
+// position) and startOutCol. If the start position is on a different line from
+// the current position (or either is unknown), the result is infinity.
+func (p *printer) distanceFrom(startPos token.Pos, startOutCol int) int {
+	if startPos.IsValid() && p.pos.IsValid() && p.posFor(startPos).Line == p.pos.Line {
+		return p.out.Column - startOutCol
+	}
+	return infinity
+}
+
+func (p *printer) funcDecl(d *ast.FuncDecl) {
+	p.setComment(d.Doc)
+	p.print(d.Pos(), token.FUNC, blank)
+	// We have to save startCol only after emitting FUNC; otherwise it can be on a
+	// different line (all whitespace preceding the FUNC is emitted only when the
+	// FUNC is emitted).
+	startCol := p.out.Column - len("func ")
+	if d.Recv != nil {
+		p.parameters(d.Recv) // method: print receiver
+		p.print(blank)
+	}
+	p.expr(d.Name)
+	p.signature(d.Type.Params, d.Type.Results)
+	p.funcBody(p.distanceFrom(d.Pos(), startCol), vtab, d.Body)
+}
+
+func (p *printer) decl(decl ast.Decl) {
+	switch d := decl.(type) {
+	case *ast.BadDecl:
+		p.print(d.Pos(), "BadDecl")
+	case *ast.GenDecl:
+		p.genDecl(d)
+	case *ast.FuncDecl:
+		p.funcDecl(d)
+	default:
+		panic("unreachable")
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Files
+
+func declToken(decl ast.Decl) (tok token.Token) {
+	tok = token.ILLEGAL
+	switch d := decl.(type) {
+	case *ast.GenDecl:
+		tok = d.Tok
+	case *ast.FuncDecl:
+		tok = token.FUNC
+	}
+	return
+}
+
+func (p *printer) declList(list []ast.Decl) {
+	tok := token.ILLEGAL
+	for _, d := range list {
+		prev := tok
+		tok = declToken(d)
+		// If the declaration token changed (e.g., from CONST to TYPE)
+		// or the next declaration has documentation associated with it,
+		// print an empty line between top-level declarations.
+		// (because p.linebreak is called with the position of d, which
+		// is past any documentation, the minimum requirement is satisfied
+		// even w/o the extra getDoc(d) nil-check - leave it in case the
+		// linebreak logic improves - there's already a TODO).
+		if len(p.output) > 0 {
+			// only print line break if we are not at the beginning of the output
+			// (i.e., we are not printing only a partial program)
+			min := 1
+			if prev != tok || getDoc(d) != nil {
+				min = 2
+			}
+			// start a new section if the next declaration is a function
+			// that spans multiple lines (see also issue #19544)
+			p.linebreak(p.lineFor(d.Pos()), min, ignore, tok == token.FUNC && p.numLines(d) > 1)
+		}
+		p.decl(d)
+	}
+}
+
+func (p *printer) file(src *ast.File) {
+	p.setComment(src.Doc)
+	p.print(src.Pos(), token.PACKAGE, blank)
+	p.expr(src.Name)
+	p.declList(src.Decls)
+	p.print(newline)
+}
diff --git a/src/go/printer/performance_test.go b/src/go/printer/performance_test.go
new file mode 100644
index 0000000..e655fa1
--- /dev/null
+++ b/src/go/printer/performance_test.go
@@ -0,0 +1,58 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements a simple printer performance benchmark:
+// go test -bench=BenchmarkPrint
+
+package printer
+
+import (
+	"bytes"
+	"go/ast"
+	"go/parser"
+	"io"
+	"log"
+	"os"
+	"testing"
+)
+
+var testfile *ast.File
+
+func testprint(out io.Writer, file *ast.File) {
+	if err := (&Config{TabIndent | UseSpaces | normalizeNumbers, 8, 0}).Fprint(out, fset, file); err != nil {
+		log.Fatalf("print error: %s", err)
+	}
+}
+
+// cannot initialize in init because (printer) Fprint launches goroutines.
+func initialize() {
+	const filename = "testdata/parser.go"
+
+	src, err := os.ReadFile(filename)
+	if err != nil {
+		log.Fatalf("%s", err)
+	}
+
+	file, err := parser.ParseFile(fset, filename, src, parser.ParseComments)
+	if err != nil {
+		log.Fatalf("%s", err)
+	}
+
+	var buf bytes.Buffer
+	testprint(&buf, file)
+	if !bytes.Equal(buf.Bytes(), src) {
+		log.Fatalf("print error: %s not idempotent", filename)
+	}
+
+	testfile = file
+}
+
+func BenchmarkPrint(b *testing.B) {
+	if testfile == nil {
+		initialize()
+	}
+	for i := 0; i < b.N; i++ {
+		testprint(io.Discard, testfile)
+	}
+}
diff --git a/src/go/printer/printer.go b/src/go/printer/printer.go
new file mode 100644
index 0000000..0077afe
--- /dev/null
+++ b/src/go/printer/printer.go
@@ -0,0 +1,1377 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package printer implements printing of AST nodes.
+package printer
+
+import (
+	"fmt"
+	"go/ast"
+	"go/token"
+	"io"
+	"os"
+	"strings"
+	"text/tabwriter"
+	"unicode"
+)
+
+const (
+	maxNewlines = 2     // max. number of newlines between source text
+	debug       = false // enable for debugging
+	infinity    = 1 << 30
+)
+
+type whiteSpace byte
+
+const (
+	ignore   = whiteSpace(0)
+	blank    = whiteSpace(' ')
+	vtab     = whiteSpace('\v')
+	newline  = whiteSpace('\n')
+	formfeed = whiteSpace('\f')
+	indent   = whiteSpace('>')
+	unindent = whiteSpace('<')
+)
+
+// A pmode value represents the current printer mode.
+type pmode int
+
+const (
+	noExtraBlank     pmode = 1 << iota // disables extra blank after /*-style comment
+	noExtraLinebreak                   // disables extra line break after /*-style comment
+)
+
+type commentInfo struct {
+	cindex         int               // current comment index
+	comment        *ast.CommentGroup // = printer.comments[cindex]; or nil
+	commentOffset  int               // = printer.posFor(printer.comments[cindex].List[0].Pos()).Offset; or infinity
+	commentNewline bool              // true if the comment group contains newlines
+}
+
+type printer struct {
+	// Configuration (does not change after initialization)
+	Config
+	fset *token.FileSet
+
+	// Current state
+	output       []byte       // raw printer result
+	indent       int          // current indentation
+	level        int          // level == 0: outside composite literal; level > 0: inside composite literal
+	mode         pmode        // current printer mode
+	endAlignment bool         // if set, terminate alignment immediately
+	impliedSemi  bool         // if set, a linebreak implies a semicolon
+	lastTok      token.Token  // last token printed (token.ILLEGAL if it's whitespace)
+	prevOpen     token.Token  // previous non-brace "open" token (, [, or token.ILLEGAL
+	wsbuf        []whiteSpace // delayed white space
+
+	// Positions
+	// The out position differs from the pos position when the result
+	// formatting differs from the source formatting (in the amount of
+	// white space). If there's a difference and SourcePos is set in
+	// ConfigMode, //line directives are used in the output to restore
+	// original source positions for a reader.
+	pos     token.Position // current position in AST (source) space
+	out     token.Position // current position in output space
+	last    token.Position // value of pos after calling writeString
+	linePtr *int           // if set, record out.Line for the next token in *linePtr
+
+	// The list of all source comments, in order of appearance.
+	comments        []*ast.CommentGroup // may be nil
+	useNodeComments bool                // if not set, ignore lead and line comments of nodes
+
+	// Information about p.comments[p.cindex]; set up by nextComment.
+	commentInfo
+
+	// Cache of already computed node sizes.
+	nodeSizes map[ast.Node]int
+
+	// Cache of most recently computed line position.
+	cachedPos  token.Pos
+	cachedLine int // line corresponding to cachedPos
+}
+
+func (p *printer) init(cfg *Config, fset *token.FileSet, nodeSizes map[ast.Node]int) {
+	p.Config = *cfg
+	p.fset = fset
+	p.pos = token.Position{Line: 1, Column: 1}
+	p.out = token.Position{Line: 1, Column: 1}
+	p.wsbuf = make([]whiteSpace, 0, 16) // whitespace sequences are short
+	p.nodeSizes = nodeSizes
+	p.cachedPos = -1
+}
+
+func (p *printer) internalError(msg ...interface{}) {
+	if debug {
+		fmt.Print(p.pos.String() + ": ")
+		fmt.Println(msg...)
+		panic("go/printer")
+	}
+}
+
+// commentsHaveNewline reports whether a list of comments belonging to
+// an *ast.CommentGroup contains newlines. Because the position information
+// may only be partially correct, we also have to read the comment text.
+func (p *printer) commentsHaveNewline(list []*ast.Comment) bool {
+	// len(list) > 0
+	line := p.lineFor(list[0].Pos())
+	for i, c := range list {
+		if i > 0 && p.lineFor(list[i].Pos()) != line {
+			// not all comments on the same line
+			return true
+		}
+		if t := c.Text; len(t) >= 2 && (t[1] == '/' || strings.Contains(t, "\n")) {
+			return true
+		}
+	}
+	_ = line
+	return false
+}
+
+func (p *printer) nextComment() {
+	for p.cindex < len(p.comments) {
+		c := p.comments[p.cindex]
+		p.cindex++
+		if list := c.List; len(list) > 0 {
+			p.comment = c
+			p.commentOffset = p.posFor(list[0].Pos()).Offset
+			p.commentNewline = p.commentsHaveNewline(list)
+			return
+		}
+		// we should not reach here (correct ASTs don't have empty
+		// ast.CommentGroup nodes), but be conservative and try again
+	}
+	// no more comments
+	p.commentOffset = infinity
+}
+
+// commentBefore reports whether the current comment group occurs
+// before the next position in the source code and printing it does
+// not introduce implicit semicolons.
+//
+func (p *printer) commentBefore(next token.Position) bool {
+	return p.commentOffset < next.Offset && (!p.impliedSemi || !p.commentNewline)
+}
+
+// commentSizeBefore returns the estimated size of the
+// comments on the same line before the next position.
+//
+func (p *printer) commentSizeBefore(next token.Position) int {
+	// save/restore current p.commentInfo (p.nextComment() modifies it)
+	defer func(info commentInfo) {
+		p.commentInfo = info
+	}(p.commentInfo)
+
+	size := 0
+	for p.commentBefore(next) {
+		for _, c := range p.comment.List {
+			size += len(c.Text)
+		}
+		p.nextComment()
+	}
+	return size
+}
+
+// recordLine records the output line number for the next non-whitespace
+// token in *linePtr. It is used to compute an accurate line number for a
+// formatted construct, independent of pending (not yet emitted) whitespace
+// or comments.
+//
+func (p *printer) recordLine(linePtr *int) {
+	p.linePtr = linePtr
+}
+
+// linesFrom returns the number of output lines between the current
+// output line and the line argument, ignoring any pending (not yet
+// emitted) whitespace or comments. It is used to compute an accurate
+// size (in number of lines) for a formatted construct.
+//
+func (p *printer) linesFrom(line int) int {
+	return p.out.Line - line
+}
+
+func (p *printer) posFor(pos token.Pos) token.Position {
+	// not used frequently enough to cache entire token.Position
+	return p.fset.PositionFor(pos, false /* absolute position */)
+}
+
+func (p *printer) lineFor(pos token.Pos) int {
+	if pos != p.cachedPos {
+		p.cachedPos = pos
+		p.cachedLine = p.fset.PositionFor(pos, false /* absolute position */).Line
+	}
+	return p.cachedLine
+}
+
+// writeLineDirective writes a //line directive if necessary.
+func (p *printer) writeLineDirective(pos token.Position) {
+	if pos.IsValid() && (p.out.Line != pos.Line || p.out.Filename != pos.Filename) {
+		p.output = append(p.output, tabwriter.Escape) // protect '\n' in //line from tabwriter interpretation
+		p.output = append(p.output, fmt.Sprintf("//line %s:%d\n", pos.Filename, pos.Line)...)
+		p.output = append(p.output, tabwriter.Escape)
+		// p.out must match the //line directive
+		p.out.Filename = pos.Filename
+		p.out.Line = pos.Line
+	}
+}
+
+// writeIndent writes indentation.
+func (p *printer) writeIndent() {
+	// use "hard" htabs - indentation columns
+	// must not be discarded by the tabwriter
+	n := p.Config.Indent + p.indent // include base indentation
+	for i := 0; i < n; i++ {
+		p.output = append(p.output, '\t')
+	}
+
+	// update positions
+	p.pos.Offset += n
+	p.pos.Column += n
+	p.out.Column += n
+}
+
+// writeByte writes ch n times to p.output and updates p.pos.
+// Only used to write formatting (white space) characters.
+func (p *printer) writeByte(ch byte, n int) {
+	if p.endAlignment {
+		// Ignore any alignment control character;
+		// and at the end of the line, break with
+		// a formfeed to indicate termination of
+		// existing columns.
+		switch ch {
+		case '\t', '\v':
+			ch = ' '
+		case '\n', '\f':
+			ch = '\f'
+			p.endAlignment = false
+		}
+	}
+
+	if p.out.Column == 1 {
+		// no need to write line directives before white space
+		p.writeIndent()
+	}
+
+	for i := 0; i < n; i++ {
+		p.output = append(p.output, ch)
+	}
+
+	// update positions
+	p.pos.Offset += n
+	if ch == '\n' || ch == '\f' {
+		p.pos.Line += n
+		p.out.Line += n
+		p.pos.Column = 1
+		p.out.Column = 1
+		return
+	}
+	p.pos.Column += n
+	p.out.Column += n
+}
+
+// writeString writes the string s to p.output and updates p.pos, p.out,
+// and p.last. If isLit is set, s is escaped w/ tabwriter.Escape characters
+// to protect s from being interpreted by the tabwriter.
+//
+// Note: writeString is only used to write Go tokens, literals, and
+// comments, all of which must be written literally. Thus, it is correct
+// to always set isLit = true. However, setting it explicitly only when
+// needed (i.e., when we don't know that s contains no tabs or line breaks)
+// avoids processing extra escape characters and reduces run time of the
+// printer benchmark by up to 10%.
+//
+func (p *printer) writeString(pos token.Position, s string, isLit bool) {
+	if p.out.Column == 1 {
+		if p.Config.Mode&SourcePos != 0 {
+			p.writeLineDirective(pos)
+		}
+		p.writeIndent()
+	}
+
+	if pos.IsValid() {
+		// update p.pos (if pos is invalid, continue with existing p.pos)
+		// Note: Must do this after handling line beginnings because
+		// writeIndent updates p.pos if there's indentation, but p.pos
+		// is the position of s.
+		p.pos = pos
+	}
+
+	if isLit {
+		// Protect s such that is passes through the tabwriter
+		// unchanged. Note that valid Go programs cannot contain
+		// tabwriter.Escape bytes since they do not appear in legal
+		// UTF-8 sequences.
+		p.output = append(p.output, tabwriter.Escape)
+	}
+
+	if debug {
+		p.output = append(p.output, fmt.Sprintf("/*%s*/", pos)...) // do not update p.pos!
+	}
+	p.output = append(p.output, s...)
+
+	// update positions
+	nlines := 0
+	var li int // index of last newline; valid if nlines > 0
+	for i := 0; i < len(s); i++ {
+		// Raw string literals may contain any character except back quote (`).
+		if ch := s[i]; ch == '\n' || ch == '\f' {
+			// account for line break
+			nlines++
+			li = i
+			// A line break inside a literal will break whatever column
+			// formatting is in place; ignore any further alignment through
+			// the end of the line.
+			p.endAlignment = true
+		}
+	}
+	p.pos.Offset += len(s)
+	if nlines > 0 {
+		p.pos.Line += nlines
+		p.out.Line += nlines
+		c := len(s) - li
+		p.pos.Column = c
+		p.out.Column = c
+	} else {
+		p.pos.Column += len(s)
+		p.out.Column += len(s)
+	}
+
+	if isLit {
+		p.output = append(p.output, tabwriter.Escape)
+	}
+
+	p.last = p.pos
+}
+
+// writeCommentPrefix writes the whitespace before a comment.
+// If there is any pending whitespace, it consumes as much of
+// it as is likely to help position the comment nicely.
+// pos is the comment position, next the position of the item
+// after all pending comments, prev is the previous comment in
+// a group of comments (or nil), and tok is the next token.
+//
+func (p *printer) writeCommentPrefix(pos, next token.Position, prev *ast.Comment, tok token.Token) {
+	if len(p.output) == 0 {
+		// the comment is the first item to be printed - don't write any whitespace
+		return
+	}
+
+	if pos.IsValid() && pos.Filename != p.last.Filename {
+		// comment in a different file - separate with newlines
+		p.writeByte('\f', maxNewlines)
+		return
+	}
+
+	if pos.Line == p.last.Line && (prev == nil || prev.Text[1] != '/') {
+		// comment on the same line as last item:
+		// separate with at least one separator
+		hasSep := false
+		if prev == nil {
+			// first comment of a comment group
+			j := 0
+			for i, ch := range p.wsbuf {
+				switch ch {
+				case blank:
+					// ignore any blanks before a comment
+					p.wsbuf[i] = ignore
+					continue
+				case vtab:
+					// respect existing tabs - important
+					// for proper formatting of commented structs
+					hasSep = true
+					continue
+				case indent:
+					// apply pending indentation
+					continue
+				}
+				j = i
+				break
+			}
+			p.writeWhitespace(j)
+		}
+		// make sure there is at least one separator
+		if !hasSep {
+			sep := byte('\t')
+			if pos.Line == next.Line {
+				// next item is on the same line as the comment
+				// (which must be a /*-style comment): separate
+				// with a blank instead of a tab
+				sep = ' '
+			}
+			p.writeByte(sep, 1)
+		}
+
+	} else {
+		// comment on a different line:
+		// separate with at least one line break
+		droppedLinebreak := false
+		j := 0
+		for i, ch := range p.wsbuf {
+			switch ch {
+			case blank, vtab:
+				// ignore any horizontal whitespace before line breaks
+				p.wsbuf[i] = ignore
+				continue
+			case indent:
+				// apply pending indentation
+				continue
+			case unindent:
+				// if this is not the last unindent, apply it
+				// as it is (likely) belonging to the last
+				// construct (e.g., a multi-line expression list)
+				// and is not part of closing a block
+				if i+1 < len(p.wsbuf) && p.wsbuf[i+1] == unindent {
+					continue
+				}
+				// if the next token is not a closing }, apply the unindent
+				// if it appears that the comment is aligned with the
+				// token; otherwise assume the unindent is part of a
+				// closing block and stop (this scenario appears with
+				// comments before a case label where the comments
+				// apply to the next case instead of the current one)
+				if tok != token.RBRACE && pos.Column == next.Column {
+					continue
+				}
+			case newline, formfeed:
+				p.wsbuf[i] = ignore
+				droppedLinebreak = prev == nil // record only if first comment of a group
+			}
+			j = i
+			break
+		}
+		p.writeWhitespace(j)
+
+		// determine number of linebreaks before the comment
+		n := 0
+		if pos.IsValid() && p.last.IsValid() {
+			n = pos.Line - p.last.Line
+			if n < 0 { // should never happen
+				n = 0
+			}
+		}
+
+		// at the package scope level only (p.indent == 0),
+		// add an extra newline if we dropped one before:
+		// this preserves a blank line before documentation
+		// comments at the package scope level (issue 2570)
+		if p.indent == 0 && droppedLinebreak {
+			n++
+		}
+
+		// make sure there is at least one line break
+		// if the previous comment was a line comment
+		if n == 0 && prev != nil && prev.Text[1] == '/' {
+			n = 1
+		}
+
+		if n > 0 {
+			// use formfeeds to break columns before a comment;
+			// this is analogous to using formfeeds to separate
+			// individual lines of /*-style comments
+			p.writeByte('\f', nlimit(n))
+		}
+	}
+}
+
+// Returns true if s contains only white space
+// (only tabs and blanks can appear in the printer's context).
+//
+func isBlank(s string) bool {
+	for i := 0; i < len(s); i++ {
+		if s[i] > ' ' {
+			return false
+		}
+	}
+	return true
+}
+
+// commonPrefix returns the common prefix of a and b.
+func commonPrefix(a, b string) string {
+	i := 0
+	for i < len(a) && i < len(b) && a[i] == b[i] && (a[i] <= ' ' || a[i] == '*') {
+		i++
+	}
+	return a[0:i]
+}
+
+// trimRight returns s with trailing whitespace removed.
+func trimRight(s string) string {
+	return strings.TrimRightFunc(s, unicode.IsSpace)
+}
+
+// stripCommonPrefix removes a common prefix from /*-style comment lines (unless no
+// comment line is indented, all but the first line have some form of space prefix).
+// The prefix is computed using heuristics such that is likely that the comment
+// contents are nicely laid out after re-printing each line using the printer's
+// current indentation.
+//
+func stripCommonPrefix(lines []string) {
+	if len(lines) <= 1 {
+		return // at most one line - nothing to do
+	}
+	// len(lines) > 1
+
+	// The heuristic in this function tries to handle a few
+	// common patterns of /*-style comments: Comments where
+	// the opening /* and closing */ are aligned and the
+	// rest of the comment text is aligned and indented with
+	// blanks or tabs, cases with a vertical "line of stars"
+	// on the left, and cases where the closing */ is on the
+	// same line as the last comment text.
+
+	// Compute maximum common white prefix of all but the first,
+	// last, and blank lines, and replace blank lines with empty
+	// lines (the first line starts with /* and has no prefix).
+	// In cases where only the first and last lines are not blank,
+	// such as two-line comments, or comments where all inner lines
+	// are blank, consider the last line for the prefix computation
+	// since otherwise the prefix would be empty.
+	//
+	// Note that the first and last line are never empty (they
+	// contain the opening /* and closing */ respectively) and
+	// thus they can be ignored by the blank line check.
+	prefix := ""
+	prefixSet := false
+	if len(lines) > 2 {
+		for i, line := range lines[1 : len(lines)-1] {
+			if isBlank(line) {
+				lines[1+i] = "" // range starts with lines[1]
+			} else {
+				if !prefixSet {
+					prefix = line
+					prefixSet = true
+				}
+				prefix = commonPrefix(prefix, line)
+			}
+
+		}
+	}
+	// If we don't have a prefix yet, consider the last line.
+	if !prefixSet {
+		line := lines[len(lines)-1]
+		prefix = commonPrefix(line, line)
+	}
+
+	/*
+	 * Check for vertical "line of stars" and correct prefix accordingly.
+	 */
+	lineOfStars := false
+	if i := strings.Index(prefix, "*"); i >= 0 {
+		// Line of stars present.
+		if i > 0 && prefix[i-1] == ' ' {
+			i-- // remove trailing blank from prefix so stars remain aligned
+		}
+		prefix = prefix[0:i]
+		lineOfStars = true
+	} else {
+		// No line of stars present.
+		// Determine the white space on the first line after the /*
+		// and before the beginning of the comment text, assume two
+		// blanks instead of the /* unless the first character after
+		// the /* is a tab. If the first comment line is empty but
+		// for the opening /*, assume up to 3 blanks or a tab. This
+		// whitespace may be found as suffix in the common prefix.
+		first := lines[0]
+		if isBlank(first[2:]) {
+			// no comment text on the first line:
+			// reduce prefix by up to 3 blanks or a tab
+			// if present - this keeps comment text indented
+			// relative to the /* and */'s if it was indented
+			// in the first place
+			i := len(prefix)
+			for n := 0; n < 3 && i > 0 && prefix[i-1] == ' '; n++ {
+				i--
+			}
+			if i == len(prefix) && i > 0 && prefix[i-1] == '\t' {
+				i--
+			}
+			prefix = prefix[0:i]
+		} else {
+			// comment text on the first line
+			suffix := make([]byte, len(first))
+			n := 2 // start after opening /*
+			for n < len(first) && first[n] <= ' ' {
+				suffix[n] = first[n]
+				n++
+			}
+			if n > 2 && suffix[2] == '\t' {
+				// assume the '\t' compensates for the /*
+				suffix = suffix[2:n]
+			} else {
+				// otherwise assume two blanks
+				suffix[0], suffix[1] = ' ', ' '
+				suffix = suffix[0:n]
+			}
+			// Shorten the computed common prefix by the length of
+			// suffix, if it is found as suffix of the prefix.
+			prefix = strings.TrimSuffix(prefix, string(suffix))
+		}
+	}
+
+	// Handle last line: If it only contains a closing */, align it
+	// with the opening /*, otherwise align the text with the other
+	// lines.
+	last := lines[len(lines)-1]
+	closing := "*/"
+	i := strings.Index(last, closing) // i >= 0 (closing is always present)
+	if isBlank(last[0:i]) {
+		// last line only contains closing */
+		if lineOfStars {
+			closing = " */" // add blank to align final star
+		}
+		lines[len(lines)-1] = prefix + closing
+	} else {
+		// last line contains more comment text - assume
+		// it is aligned like the other lines and include
+		// in prefix computation
+		prefix = commonPrefix(prefix, last)
+	}
+
+	// Remove the common prefix from all but the first and empty lines.
+	for i, line := range lines {
+		if i > 0 && line != "" {
+			lines[i] = line[len(prefix):]
+		}
+	}
+}
+
+func (p *printer) writeComment(comment *ast.Comment) {
+	text := comment.Text
+	pos := p.posFor(comment.Pos())
+
+	const linePrefix = "//line "
+	if strings.HasPrefix(text, linePrefix) && (!pos.IsValid() || pos.Column == 1) {
+		// Possibly a //-style line directive.
+		// Suspend indentation temporarily to keep line directive valid.
+		defer func(indent int) { p.indent = indent }(p.indent)
+		p.indent = 0
+	}
+
+	// shortcut common case of //-style comments
+	if text[1] == '/' {
+		p.writeString(pos, trimRight(text), true)
+		return
+	}
+
+	// for /*-style comments, print line by line and let the
+	// write function take care of the proper indentation
+	lines := strings.Split(text, "\n")
+
+	// The comment started in the first column but is going
+	// to be indented. For an idempotent result, add indentation
+	// to all lines such that they look like they were indented
+	// before - this will make sure the common prefix computation
+	// is the same independent of how many times formatting is
+	// applied (was issue 1835).
+	if pos.IsValid() && pos.Column == 1 && p.indent > 0 {
+		for i, line := range lines[1:] {
+			lines[1+i] = "   " + line
+		}
+	}
+
+	stripCommonPrefix(lines)
+
+	// write comment lines, separated by formfeed,
+	// without a line break after the last line
+	for i, line := range lines {
+		if i > 0 {
+			p.writeByte('\f', 1)
+			pos = p.pos
+		}
+		if len(line) > 0 {
+			p.writeString(pos, trimRight(line), true)
+		}
+	}
+}
+
+// writeCommentSuffix writes a line break after a comment if indicated
+// and processes any leftover indentation information. If a line break
+// is needed, the kind of break (newline vs formfeed) depends on the
+// pending whitespace. The writeCommentSuffix result indicates if a
+// newline was written or if a formfeed was dropped from the whitespace
+// buffer.
+//
+func (p *printer) writeCommentSuffix(needsLinebreak bool) (wroteNewline, droppedFF bool) {
+	for i, ch := range p.wsbuf {
+		switch ch {
+		case blank, vtab:
+			// ignore trailing whitespace
+			p.wsbuf[i] = ignore
+		case indent, unindent:
+			// don't lose indentation information
+		case newline, formfeed:
+			// if we need a line break, keep exactly one
+			// but remember if we dropped any formfeeds
+			if needsLinebreak {
+				needsLinebreak = false
+				wroteNewline = true
+			} else {
+				if ch == formfeed {
+					droppedFF = true
+				}
+				p.wsbuf[i] = ignore
+			}
+		}
+	}
+	p.writeWhitespace(len(p.wsbuf))
+
+	// make sure we have a line break
+	if needsLinebreak {
+		p.writeByte('\n', 1)
+		wroteNewline = true
+	}
+
+	return
+}
+
+// containsLinebreak reports whether the whitespace buffer contains any line breaks.
+func (p *printer) containsLinebreak() bool {
+	for _, ch := range p.wsbuf {
+		if ch == newline || ch == formfeed {
+			return true
+		}
+	}
+	return false
+}
+
+// intersperseComments consumes all comments that appear before the next token
+// tok and prints it together with the buffered whitespace (i.e., the whitespace
+// that needs to be written before the next token). A heuristic is used to mix
+// the comments and whitespace. The intersperseComments result indicates if a
+// newline was written or if a formfeed was dropped from the whitespace buffer.
+//
+func (p *printer) intersperseComments(next token.Position, tok token.Token) (wroteNewline, droppedFF bool) {
+	var last *ast.Comment
+	for p.commentBefore(next) {
+		for _, c := range p.comment.List {
+			p.writeCommentPrefix(p.posFor(c.Pos()), next, last, tok)
+			p.writeComment(c)
+			last = c
+		}
+		p.nextComment()
+	}
+
+	if last != nil {
+		// If the last comment is a /*-style comment and the next item
+		// follows on the same line but is not a comma, and not a "closing"
+		// token immediately following its corresponding "opening" token,
+		// add an extra separator unless explicitly disabled. Use a blank
+		// as separator unless we have pending linebreaks, they are not
+		// disabled, and we are outside a composite literal, in which case
+		// we want a linebreak (issue 15137).
+		// TODO(gri) This has become overly complicated. We should be able
+		// to track whether we're inside an expression or statement and
+		// use that information to decide more directly.
+		needsLinebreak := false
+		if p.mode&noExtraBlank == 0 &&
+			last.Text[1] == '*' && p.lineFor(last.Pos()) == next.Line &&
+			tok != token.COMMA &&
+			(tok != token.RPAREN || p.prevOpen == token.LPAREN) &&
+			(tok != token.RBRACK || p.prevOpen == token.LBRACK) {
+			if p.containsLinebreak() && p.mode&noExtraLinebreak == 0 && p.level == 0 {
+				needsLinebreak = true
+			} else {
+				p.writeByte(' ', 1)
+			}
+		}
+		// Ensure that there is a line break after a //-style comment,
+		// before EOF, and before a closing '}' unless explicitly disabled.
+		if last.Text[1] == '/' ||
+			tok == token.EOF ||
+			tok == token.RBRACE && p.mode&noExtraLinebreak == 0 {
+			needsLinebreak = true
+		}
+		return p.writeCommentSuffix(needsLinebreak)
+	}
+
+	// no comment was written - we should never reach here since
+	// intersperseComments should not be called in that case
+	p.internalError("intersperseComments called without pending comments")
+	return
+}
+
+// whiteWhitespace writes the first n whitespace entries.
+func (p *printer) writeWhitespace(n int) {
+	// write entries
+	for i := 0; i < n; i++ {
+		switch ch := p.wsbuf[i]; ch {
+		case ignore:
+			// ignore!
+		case indent:
+			p.indent++
+		case unindent:
+			p.indent--
+			if p.indent < 0 {
+				p.internalError("negative indentation:", p.indent)
+				p.indent = 0
+			}
+		case newline, formfeed:
+			// A line break immediately followed by a "correcting"
+			// unindent is swapped with the unindent - this permits
+			// proper label positioning. If a comment is between
+			// the line break and the label, the unindent is not
+			// part of the comment whitespace prefix and the comment
+			// will be positioned correctly indented.
+			if i+1 < n && p.wsbuf[i+1] == unindent {
+				// Use a formfeed to terminate the current section.
+				// Otherwise, a long label name on the next line leading
+				// to a wide column may increase the indentation column
+				// of lines before the label; effectively leading to wrong
+				// indentation.
+				p.wsbuf[i], p.wsbuf[i+1] = unindent, formfeed
+				i-- // do it again
+				continue
+			}
+			fallthrough
+		default:
+			p.writeByte(byte(ch), 1)
+		}
+	}
+
+	// shift remaining entries down
+	l := copy(p.wsbuf, p.wsbuf[n:])
+	p.wsbuf = p.wsbuf[:l]
+}
+
+// ----------------------------------------------------------------------------
+// Printing interface
+
+// nlines limits n to maxNewlines.
+func nlimit(n int) int {
+	if n > maxNewlines {
+		n = maxNewlines
+	}
+	return n
+}
+
+func mayCombine(prev token.Token, next byte) (b bool) {
+	switch prev {
+	case token.INT:
+		b = next == '.' // 1.
+	case token.ADD:
+		b = next == '+' // ++
+	case token.SUB:
+		b = next == '-' // --
+	case token.QUO:
+		b = next == '*' // /*
+	case token.LSS:
+		b = next == '-' || next == '<' // <- or <<
+	case token.AND:
+		b = next == '&' || next == '^' // && or &^
+	}
+	return
+}
+
+// print prints a list of "items" (roughly corresponding to syntactic
+// tokens, but also including whitespace and formatting information).
+// It is the only print function that should be called directly from
+// any of the AST printing functions in nodes.go.
+//
+// Whitespace is accumulated until a non-whitespace token appears. Any
+// comments that need to appear before that token are printed first,
+// taking into account the amount and structure of any pending white-
+// space for best comment placement. Then, any leftover whitespace is
+// printed, followed by the actual token.
+//
+func (p *printer) print(args ...interface{}) {
+	for _, arg := range args {
+		// information about the current arg
+		var data string
+		var isLit bool
+		var impliedSemi bool // value for p.impliedSemi after this arg
+
+		// record previous opening token, if any
+		switch p.lastTok {
+		case token.ILLEGAL:
+			// ignore (white space)
+		case token.LPAREN, token.LBRACK:
+			p.prevOpen = p.lastTok
+		default:
+			// other tokens followed any opening token
+			p.prevOpen = token.ILLEGAL
+		}
+
+		switch x := arg.(type) {
+		case pmode:
+			// toggle printer mode
+			p.mode ^= x
+			continue
+
+		case whiteSpace:
+			if x == ignore {
+				// don't add ignore's to the buffer; they
+				// may screw up "correcting" unindents (see
+				// LabeledStmt)
+				continue
+			}
+			i := len(p.wsbuf)
+			if i == cap(p.wsbuf) {
+				// Whitespace sequences are very short so this should
+				// never happen. Handle gracefully (but possibly with
+				// bad comment placement) if it does happen.
+				p.writeWhitespace(i)
+				i = 0
+			}
+			p.wsbuf = p.wsbuf[0 : i+1]
+			p.wsbuf[i] = x
+			if x == newline || x == formfeed {
+				// newlines affect the current state (p.impliedSemi)
+				// and not the state after printing arg (impliedSemi)
+				// because comments can be interspersed before the arg
+				// in this case
+				p.impliedSemi = false
+			}
+			p.lastTok = token.ILLEGAL
+			continue
+
+		case *ast.Ident:
+			data = x.Name
+			impliedSemi = true
+			p.lastTok = token.IDENT
+
+		case *ast.BasicLit:
+			data = x.Value
+			isLit = true
+			impliedSemi = true
+			p.lastTok = x.Kind
+
+		case token.Token:
+			s := x.String()
+			if mayCombine(p.lastTok, s[0]) {
+				// the previous and the current token must be
+				// separated by a blank otherwise they combine
+				// into a different incorrect token sequence
+				// (except for token.INT followed by a '.' this
+				// should never happen because it is taken care
+				// of via binary expression formatting)
+				if len(p.wsbuf) != 0 {
+					p.internalError("whitespace buffer not empty")
+				}
+				p.wsbuf = p.wsbuf[0:1]
+				p.wsbuf[0] = ' '
+			}
+			data = s
+			// some keywords followed by a newline imply a semicolon
+			switch x {
+			case token.BREAK, token.CONTINUE, token.FALLTHROUGH, token.RETURN,
+				token.INC, token.DEC, token.RPAREN, token.RBRACK, token.RBRACE:
+				impliedSemi = true
+			}
+			p.lastTok = x
+
+		case token.Pos:
+			if x.IsValid() {
+				p.pos = p.posFor(x) // accurate position of next item
+			}
+			continue
+
+		case string:
+			// incorrect AST - print error message
+			data = x
+			isLit = true
+			impliedSemi = true
+			p.lastTok = token.STRING
+
+		default:
+			fmt.Fprintf(os.Stderr, "print: unsupported argument %v (%T)\n", arg, arg)
+			panic("go/printer type")
+		}
+		// data != ""
+
+		next := p.pos // estimated/accurate position of next item
+		wroteNewline, droppedFF := p.flush(next, p.lastTok)
+
+		// intersperse extra newlines if present in the source and
+		// if they don't cause extra semicolons (don't do this in
+		// flush as it will cause extra newlines at the end of a file)
+		if !p.impliedSemi {
+			n := nlimit(next.Line - p.pos.Line)
+			// don't exceed maxNewlines if we already wrote one
+			if wroteNewline && n == maxNewlines {
+				n = maxNewlines - 1
+			}
+			if n > 0 {
+				ch := byte('\n')
+				if droppedFF {
+					ch = '\f' // use formfeed since we dropped one before
+				}
+				p.writeByte(ch, n)
+				impliedSemi = false
+			}
+		}
+
+		// the next token starts now - record its line number if requested
+		if p.linePtr != nil {
+			*p.linePtr = p.out.Line
+			p.linePtr = nil
+		}
+
+		p.writeString(next, data, isLit)
+		p.impliedSemi = impliedSemi
+	}
+}
+
+// flush prints any pending comments and whitespace occurring textually
+// before the position of the next token tok. The flush result indicates
+// if a newline was written or if a formfeed was dropped from the whitespace
+// buffer.
+//
+func (p *printer) flush(next token.Position, tok token.Token) (wroteNewline, droppedFF bool) {
+	if p.commentBefore(next) {
+		// if there are comments before the next item, intersperse them
+		wroteNewline, droppedFF = p.intersperseComments(next, tok)
+	} else {
+		// otherwise, write any leftover whitespace
+		p.writeWhitespace(len(p.wsbuf))
+	}
+	return
+}
+
+// getNode returns the ast.CommentGroup associated with n, if any.
+func getDoc(n ast.Node) *ast.CommentGroup {
+	switch n := n.(type) {
+	case *ast.Field:
+		return n.Doc
+	case *ast.ImportSpec:
+		return n.Doc
+	case *ast.ValueSpec:
+		return n.Doc
+	case *ast.TypeSpec:
+		return n.Doc
+	case *ast.GenDecl:
+		return n.Doc
+	case *ast.FuncDecl:
+		return n.Doc
+	case *ast.File:
+		return n.Doc
+	}
+	return nil
+}
+
+func getLastComment(n ast.Node) *ast.CommentGroup {
+	switch n := n.(type) {
+	case *ast.Field:
+		return n.Comment
+	case *ast.ImportSpec:
+		return n.Comment
+	case *ast.ValueSpec:
+		return n.Comment
+	case *ast.TypeSpec:
+		return n.Comment
+	case *ast.GenDecl:
+		if len(n.Specs) > 0 {
+			return getLastComment(n.Specs[len(n.Specs)-1])
+		}
+	case *ast.File:
+		if len(n.Comments) > 0 {
+			return n.Comments[len(n.Comments)-1]
+		}
+	}
+	return nil
+}
+
+func (p *printer) printNode(node interface{}) error {
+	// unpack *CommentedNode, if any
+	var comments []*ast.CommentGroup
+	if cnode, ok := node.(*CommentedNode); ok {
+		node = cnode.Node
+		comments = cnode.Comments
+	}
+
+	if comments != nil {
+		// commented node - restrict comment list to relevant range
+		n, ok := node.(ast.Node)
+		if !ok {
+			goto unsupported
+		}
+		beg := n.Pos()
+		end := n.End()
+		// if the node has associated documentation,
+		// include that commentgroup in the range
+		// (the comment list is sorted in the order
+		// of the comment appearance in the source code)
+		if doc := getDoc(n); doc != nil {
+			beg = doc.Pos()
+		}
+		if com := getLastComment(n); com != nil {
+			if e := com.End(); e > end {
+				end = e
+			}
+		}
+		// token.Pos values are global offsets, we can
+		// compare them directly
+		i := 0
+		for i < len(comments) && comments[i].End() < beg {
+			i++
+		}
+		j := i
+		for j < len(comments) && comments[j].Pos() < end {
+			j++
+		}
+		if i < j {
+			p.comments = comments[i:j]
+		}
+	} else if n, ok := node.(*ast.File); ok {
+		// use ast.File comments, if any
+		p.comments = n.Comments
+	}
+
+	// if there are no comments, use node comments
+	p.useNodeComments = p.comments == nil
+
+	// get comments ready for use
+	p.nextComment()
+
+	// format node
+	switch n := node.(type) {
+	case ast.Expr:
+		p.expr(n)
+	case ast.Stmt:
+		// A labeled statement will un-indent to position the label.
+		// Set p.indent to 1 so we don't get indent "underflow".
+		if _, ok := n.(*ast.LabeledStmt); ok {
+			p.indent = 1
+		}
+		p.stmt(n, false)
+	case ast.Decl:
+		p.decl(n)
+	case ast.Spec:
+		p.spec(n, 1, false)
+	case []ast.Stmt:
+		// A labeled statement will un-indent to position the label.
+		// Set p.indent to 1 so we don't get indent "underflow".
+		for _, s := range n {
+			if _, ok := s.(*ast.LabeledStmt); ok {
+				p.indent = 1
+			}
+		}
+		p.stmtList(n, 0, false)
+	case []ast.Decl:
+		p.declList(n)
+	case *ast.File:
+		p.file(n)
+	default:
+		goto unsupported
+	}
+
+	return nil
+
+unsupported:
+	return fmt.Errorf("go/printer: unsupported node type %T", node)
+}
+
+// ----------------------------------------------------------------------------
+// Trimmer
+
+// A trimmer is an io.Writer filter for stripping tabwriter.Escape
+// characters, trailing blanks and tabs, and for converting formfeed
+// and vtab characters into newlines and htabs (in case no tabwriter
+// is used). Text bracketed by tabwriter.Escape characters is passed
+// through unchanged.
+//
+type trimmer struct {
+	output io.Writer
+	state  int
+	space  []byte
+}
+
+// trimmer is implemented as a state machine.
+// It can be in one of the following states:
+const (
+	inSpace  = iota // inside space
+	inEscape        // inside text bracketed by tabwriter.Escapes
+	inText          // inside text
+)
+
+func (p *trimmer) resetSpace() {
+	p.state = inSpace
+	p.space = p.space[0:0]
+}
+
+// Design note: It is tempting to eliminate extra blanks occurring in
+//              whitespace in this function as it could simplify some
+//              of the blanks logic in the node printing functions.
+//              However, this would mess up any formatting done by
+//              the tabwriter.
+
+var aNewline = []byte("\n")
+
+func (p *trimmer) Write(data []byte) (n int, err error) {
+	// invariants:
+	// p.state == inSpace:
+	//	p.space is unwritten
+	// p.state == inEscape, inText:
+	//	data[m:n] is unwritten
+	m := 0
+	var b byte
+	for n, b = range data {
+		if b == '\v' {
+			b = '\t' // convert to htab
+		}
+		switch p.state {
+		case inSpace:
+			switch b {
+			case '\t', ' ':
+				p.space = append(p.space, b)
+			case '\n', '\f':
+				p.resetSpace() // discard trailing space
+				_, err = p.output.Write(aNewline)
+			case tabwriter.Escape:
+				_, err = p.output.Write(p.space)
+				p.state = inEscape
+				m = n + 1 // +1: skip tabwriter.Escape
+			default:
+				_, err = p.output.Write(p.space)
+				p.state = inText
+				m = n
+			}
+		case inEscape:
+			if b == tabwriter.Escape {
+				_, err = p.output.Write(data[m:n])
+				p.resetSpace()
+			}
+		case inText:
+			switch b {
+			case '\t', ' ':
+				_, err = p.output.Write(data[m:n])
+				p.resetSpace()
+				p.space = append(p.space, b)
+			case '\n', '\f':
+				_, err = p.output.Write(data[m:n])
+				p.resetSpace()
+				if err == nil {
+					_, err = p.output.Write(aNewline)
+				}
+			case tabwriter.Escape:
+				_, err = p.output.Write(data[m:n])
+				p.state = inEscape
+				m = n + 1 // +1: skip tabwriter.Escape
+			}
+		default:
+			panic("unreachable")
+		}
+		if err != nil {
+			return
+		}
+	}
+	n = len(data)
+
+	switch p.state {
+	case inEscape, inText:
+		_, err = p.output.Write(data[m:n])
+		p.resetSpace()
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Public interface
+
+// A Mode value is a set of flags (or 0). They control printing.
+type Mode uint
+
+const (
+	RawFormat Mode = 1 << iota // do not use a tabwriter; if set, UseSpaces is ignored
+	TabIndent                  // use tabs for indentation independent of UseSpaces
+	UseSpaces                  // use spaces instead of tabs for alignment
+	SourcePos                  // emit //line directives to preserve original source positions
+)
+
+// The mode below is not included in printer's public API because
+// editing code text is deemed out of scope. Because this mode is
+// unexported, it's also possible to modify or remove it based on
+// the evolving needs of go/format and cmd/gofmt without breaking
+// users. See discussion in CL 240683.
+const (
+	// normalizeNumbers means to canonicalize number
+	// literal prefixes and exponents while printing.
+	//
+	// This value is known in and used by go/format and cmd/gofmt.
+	// It is currently more convenient and performant for those
+	// packages to apply number normalization during printing,
+	// rather than by modifying the AST in advance.
+	normalizeNumbers Mode = 1 << 30
+)
+
+// A Config node controls the output of Fprint.
+type Config struct {
+	Mode     Mode // default: 0
+	Tabwidth int  // default: 8
+	Indent   int  // default: 0 (all code is indented at least by this much)
+}
+
+// fprint implements Fprint and takes a nodesSizes map for setting up the printer state.
+func (cfg *Config) fprint(output io.Writer, fset *token.FileSet, node interface{}, nodeSizes map[ast.Node]int) (err error) {
+	// print node
+	var p printer
+	p.init(cfg, fset, nodeSizes)
+	if err = p.printNode(node); err != nil {
+		return
+	}
+	// print outstanding comments
+	p.impliedSemi = false // EOF acts like a newline
+	p.flush(token.Position{Offset: infinity, Line: infinity}, token.EOF)
+
+	// redirect output through a trimmer to eliminate trailing whitespace
+	// (Input to a tabwriter must be untrimmed since trailing tabs provide
+	// formatting information. The tabwriter could provide trimming
+	// functionality but no tabwriter is used when RawFormat is set.)
+	output = &trimmer{output: output}
+
+	// redirect output through a tabwriter if necessary
+	if cfg.Mode&RawFormat == 0 {
+		minwidth := cfg.Tabwidth
+
+		padchar := byte('\t')
+		if cfg.Mode&UseSpaces != 0 {
+			padchar = ' '
+		}
+
+		twmode := tabwriter.DiscardEmptyColumns
+		if cfg.Mode&TabIndent != 0 {
+			minwidth = 0
+			twmode |= tabwriter.TabIndent
+		}
+
+		output = tabwriter.NewWriter(output, minwidth, cfg.Tabwidth, 1, padchar, twmode)
+	}
+
+	// write printer result via tabwriter/trimmer to output
+	if _, err = output.Write(p.output); err != nil {
+		return
+	}
+
+	// flush tabwriter, if any
+	if tw, _ := output.(*tabwriter.Writer); tw != nil {
+		err = tw.Flush()
+	}
+
+	return
+}
+
+// A CommentedNode bundles an AST node and corresponding comments.
+// It may be provided as argument to any of the Fprint functions.
+//
+type CommentedNode struct {
+	Node     interface{} // *ast.File, or ast.Expr, ast.Decl, ast.Spec, or ast.Stmt
+	Comments []*ast.CommentGroup
+}
+
+// Fprint "pretty-prints" an AST node to output for a given configuration cfg.
+// Position information is interpreted relative to the file set fset.
+// The node type must be *ast.File, *CommentedNode, []ast.Decl, []ast.Stmt,
+// or assignment-compatible to ast.Expr, ast.Decl, ast.Spec, or ast.Stmt.
+//
+func (cfg *Config) Fprint(output io.Writer, fset *token.FileSet, node interface{}) error {
+	return cfg.fprint(output, fset, node, make(map[ast.Node]int))
+}
+
+// Fprint "pretty-prints" an AST node to output.
+// It calls Config.Fprint with default settings.
+// Note that gofmt uses tabs for indentation but spaces for alignment;
+// use format.Node (package go/format) for output that matches gofmt.
+//
+func Fprint(output io.Writer, fset *token.FileSet, node interface{}) error {
+	return (&Config{Tabwidth: 8}).Fprint(output, fset, node)
+}
diff --git a/src/go/printer/printer_test.go b/src/go/printer/printer_test.go
new file mode 100644
index 0000000..45e5011
--- /dev/null
+++ b/src/go/printer/printer_test.go
@@ -0,0 +1,814 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package printer
+
+import (
+	"bytes"
+	"errors"
+	"flag"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"io"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+const (
+	dataDir  = "testdata"
+	tabwidth = 8
+)
+
+var update = flag.Bool("update", false, "update golden files")
+
+var fset = token.NewFileSet()
+
+type checkMode uint
+
+const (
+	export checkMode = 1 << iota
+	rawFormat
+	normNumber
+	idempotent
+)
+
+// format parses src, prints the corresponding AST, verifies the resulting
+// src is syntactically correct, and returns the resulting src or an error
+// if any.
+func format(src []byte, mode checkMode) ([]byte, error) {
+	// parse src
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		return nil, fmt.Errorf("parse: %s\n%s", err, src)
+	}
+
+	// filter exports if necessary
+	if mode&export != 0 {
+		ast.FileExports(f) // ignore result
+		f.Comments = nil   // don't print comments that are not in AST
+	}
+
+	// determine printer configuration
+	cfg := Config{Tabwidth: tabwidth}
+	if mode&rawFormat != 0 {
+		cfg.Mode |= RawFormat
+	}
+	if mode&normNumber != 0 {
+		cfg.Mode |= normalizeNumbers
+	}
+
+	// print AST
+	var buf bytes.Buffer
+	if err := cfg.Fprint(&buf, fset, f); err != nil {
+		return nil, fmt.Errorf("print: %s", err)
+	}
+
+	// make sure formatted output is syntactically correct
+	res := buf.Bytes()
+	if _, err := parser.ParseFile(fset, "", res, 0); err != nil {
+		return nil, fmt.Errorf("re-parse: %s\n%s", err, buf.Bytes())
+	}
+
+	return res, nil
+}
+
+// lineAt returns the line in text starting at offset offs.
+func lineAt(text []byte, offs int) []byte {
+	i := offs
+	for i < len(text) && text[i] != '\n' {
+		i++
+	}
+	return text[offs:i]
+}
+
+// diff compares a and b.
+func diff(aname, bname string, a, b []byte) error {
+	var buf bytes.Buffer // holding long error message
+
+	// compare lengths
+	if len(a) != len(b) {
+		fmt.Fprintf(&buf, "\nlength changed: len(%s) = %d, len(%s) = %d", aname, len(a), bname, len(b))
+	}
+
+	// compare contents
+	line := 1
+	offs := 1
+	for i := 0; i < len(a) && i < len(b); i++ {
+		ch := a[i]
+		if ch != b[i] {
+			fmt.Fprintf(&buf, "\n%s:%d:%d: %s", aname, line, i-offs+1, lineAt(a, offs))
+			fmt.Fprintf(&buf, "\n%s:%d:%d: %s", bname, line, i-offs+1, lineAt(b, offs))
+			fmt.Fprintf(&buf, "\n\n")
+			break
+		}
+		if ch == '\n' {
+			line++
+			offs = i + 1
+		}
+	}
+
+	if buf.Len() > 0 {
+		return errors.New(buf.String())
+	}
+	return nil
+}
+
+func runcheck(t *testing.T, source, golden string, mode checkMode) {
+	src, err := os.ReadFile(source)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	res, err := format(src, mode)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	// update golden files if necessary
+	if *update {
+		if err := os.WriteFile(golden, res, 0644); err != nil {
+			t.Error(err)
+		}
+		return
+	}
+
+	// get golden
+	gld, err := os.ReadFile(golden)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	// formatted source and golden must be the same
+	if err := diff(source, golden, res, gld); err != nil {
+		t.Error(err)
+		return
+	}
+
+	if mode&idempotent != 0 {
+		// formatting golden must be idempotent
+		// (This is very difficult to achieve in general and for now
+		// it is only checked for files explicitly marked as such.)
+		res, err = format(gld, mode)
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		if err := diff(golden, fmt.Sprintf("format(%s)", golden), gld, res); err != nil {
+			t.Errorf("golden is not idempotent: %s", err)
+		}
+	}
+}
+
+func check(t *testing.T, source, golden string, mode checkMode) {
+	// run the test
+	cc := make(chan int, 1)
+	go func() {
+		runcheck(t, source, golden, mode)
+		cc <- 0
+	}()
+
+	// wait with timeout
+	select {
+	case <-time.After(10 * time.Second): // plenty of a safety margin, even for very slow machines
+		// test running past time out
+		t.Errorf("%s: running too slowly", source)
+	case <-cc:
+		// test finished within allotted time margin
+	}
+}
+
+type entry struct {
+	source, golden string
+	mode           checkMode
+}
+
+// Use go test -update to create/update the respective golden files.
+var data = []entry{
+	{"empty.input", "empty.golden", idempotent},
+	{"comments.input", "comments.golden", 0},
+	{"comments.input", "comments.x", export},
+	{"comments2.input", "comments2.golden", idempotent},
+	{"alignment.input", "alignment.golden", idempotent},
+	{"linebreaks.input", "linebreaks.golden", idempotent},
+	{"expressions.input", "expressions.golden", idempotent},
+	{"expressions.input", "expressions.raw", rawFormat | idempotent},
+	{"declarations.input", "declarations.golden", 0},
+	{"statements.input", "statements.golden", 0},
+	{"slow.input", "slow.golden", idempotent},
+	{"complit.input", "complit.x", export},
+	{"go2numbers.input", "go2numbers.golden", idempotent},
+	{"go2numbers.input", "go2numbers.norm", normNumber | idempotent},
+}
+
+func TestFiles(t *testing.T) {
+	t.Parallel()
+	for _, e := range data {
+		source := filepath.Join(dataDir, e.source)
+		golden := filepath.Join(dataDir, e.golden)
+		mode := e.mode
+		t.Run(e.source, func(t *testing.T) {
+			t.Parallel()
+			check(t, source, golden, mode)
+			// TODO(gri) check that golden is idempotent
+			//check(t, golden, golden, e.mode)
+		})
+	}
+}
+
+// TestLineComments, using a simple test case, checks that consecutive line
+// comments are properly terminated with a newline even if the AST position
+// information is incorrect.
+//
+func TestLineComments(t *testing.T) {
+	const src = `// comment 1
+	// comment 2
+	// comment 3
+	package main
+	`
+
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		panic(err) // error in test
+	}
+
+	var buf bytes.Buffer
+	fset = token.NewFileSet() // use the wrong file set
+	Fprint(&buf, fset, f)
+
+	nlines := 0
+	for _, ch := range buf.Bytes() {
+		if ch == '\n' {
+			nlines++
+		}
+	}
+
+	const expected = 3
+	if nlines < expected {
+		t.Errorf("got %d, expected %d\n", nlines, expected)
+		t.Errorf("result:\n%s", buf.Bytes())
+	}
+}
+
+// Verify that the printer can be invoked during initialization.
+func init() {
+	const name = "foobar"
+	var buf bytes.Buffer
+	if err := Fprint(&buf, fset, &ast.Ident{Name: name}); err != nil {
+		panic(err) // error in test
+	}
+	// in debug mode, the result contains additional information;
+	// ignore it
+	if s := buf.String(); !debug && s != name {
+		panic("got " + s + ", want " + name)
+	}
+}
+
+// Verify that the printer doesn't crash if the AST contains BadXXX nodes.
+func TestBadNodes(t *testing.T) {
+	const src = "package p\n("
+	const res = "package p\nBadDecl\n"
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err == nil {
+		t.Error("expected illegal program") // error in test
+	}
+	var buf bytes.Buffer
+	Fprint(&buf, fset, f)
+	if buf.String() != res {
+		t.Errorf("got %q, expected %q", buf.String(), res)
+	}
+}
+
+// testComment verifies that f can be parsed again after printing it
+// with its first comment set to comment at any possible source offset.
+func testComment(t *testing.T, f *ast.File, srclen int, comment *ast.Comment) {
+	f.Comments[0].List[0] = comment
+	var buf bytes.Buffer
+	for offs := 0; offs <= srclen; offs++ {
+		buf.Reset()
+		// Printing f should result in a correct program no
+		// matter what the (incorrect) comment position is.
+		if err := Fprint(&buf, fset, f); err != nil {
+			t.Error(err)
+		}
+		if _, err := parser.ParseFile(fset, "", buf.Bytes(), 0); err != nil {
+			t.Fatalf("incorrect program for pos = %d:\n%s", comment.Slash, buf.String())
+		}
+		// Position information is just an offset.
+		// Move comment one byte down in the source.
+		comment.Slash++
+	}
+}
+
+// Verify that the printer produces a correct program
+// even if the position information of comments introducing newlines
+// is incorrect.
+func TestBadComments(t *testing.T) {
+	t.Parallel()
+	const src = `
+// first comment - text and position changed by test
+package p
+import "fmt"
+const pi = 3.14 // rough circle
+var (
+	x, y, z int = 1, 2, 3
+	u, v float64
+)
+func fibo(n int) {
+	if n < 2 {
+		return n /* seed values */
+	}
+	return fibo(n-1) + fibo(n-2)
+}
+`
+
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		t.Error(err) // error in test
+	}
+
+	comment := f.Comments[0].List[0]
+	pos := comment.Pos()
+	if fset.PositionFor(pos, false /* absolute position */).Offset != 1 {
+		t.Error("expected offset 1") // error in test
+	}
+
+	testComment(t, f, len(src), &ast.Comment{Slash: pos, Text: "//-style comment"})
+	testComment(t, f, len(src), &ast.Comment{Slash: pos, Text: "/*-style comment */"})
+	testComment(t, f, len(src), &ast.Comment{Slash: pos, Text: "/*-style \n comment */"})
+	testComment(t, f, len(src), &ast.Comment{Slash: pos, Text: "/*-style comment \n\n\n */"})
+}
+
+type visitor chan *ast.Ident
+
+func (v visitor) Visit(n ast.Node) (w ast.Visitor) {
+	if ident, ok := n.(*ast.Ident); ok {
+		v <- ident
+	}
+	return v
+}
+
+// idents is an iterator that returns all idents in f via the result channel.
+func idents(f *ast.File) <-chan *ast.Ident {
+	v := make(visitor)
+	go func() {
+		ast.Walk(v, f)
+		close(v)
+	}()
+	return v
+}
+
+// identCount returns the number of identifiers found in f.
+func identCount(f *ast.File) int {
+	n := 0
+	for range idents(f) {
+		n++
+	}
+	return n
+}
+
+// Verify that the SourcePos mode emits correct //line directives
+// by testing that position information for matching identifiers
+// is maintained.
+func TestSourcePos(t *testing.T) {
+	const src = `
+package p
+import ( "go/printer"; "math" )
+const pi = 3.14; var x = 0
+type t struct{ x, y, z int; u, v, w float32 }
+func (t *t) foo(a, b, c int) int {
+	return a*t.x + b*t.y +
+		// two extra lines here
+		// ...
+		c*t.z
+}
+`
+
+	// parse original
+	f1, err := parser.ParseFile(fset, "src", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// pretty-print original
+	var buf bytes.Buffer
+	err = (&Config{Mode: UseSpaces | SourcePos, Tabwidth: 8}).Fprint(&buf, fset, f1)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// parse pretty printed original
+	// (//line directives must be interpreted even w/o parser.ParseComments set)
+	f2, err := parser.ParseFile(fset, "", buf.Bytes(), 0)
+	if err != nil {
+		t.Fatalf("%s\n%s", err, buf.Bytes())
+	}
+
+	// At this point the position information of identifiers in f2 should
+	// match the position information of corresponding identifiers in f1.
+
+	// number of identifiers must be > 0 (test should run) and must match
+	n1 := identCount(f1)
+	n2 := identCount(f2)
+	if n1 == 0 {
+		t.Fatal("got no idents")
+	}
+	if n2 != n1 {
+		t.Errorf("got %d idents; want %d", n2, n1)
+	}
+
+	// verify that all identifiers have correct line information
+	i2range := idents(f2)
+	for i1 := range idents(f1) {
+		i2 := <-i2range
+
+		if i2.Name != i1.Name {
+			t.Errorf("got ident %s; want %s", i2.Name, i1.Name)
+		}
+
+		// here we care about the relative (line-directive adjusted) positions
+		l1 := fset.Position(i1.Pos()).Line
+		l2 := fset.Position(i2.Pos()).Line
+		if l2 != l1 {
+			t.Errorf("got line %d; want %d for %s", l2, l1, i1.Name)
+		}
+	}
+
+	if t.Failed() {
+		t.Logf("\n%s", buf.Bytes())
+	}
+}
+
+// Verify that the SourcePos mode doesn't emit unnecessary //line directives
+// before empty lines.
+func TestIssue5945(t *testing.T) {
+	const orig = `
+package p   // line 2
+func f() {} // line 3
+
+var x, y, z int
+
+
+func g() { // line 8
+}
+`
+
+	const want = `//line src.go:2
+package p
+
+//line src.go:3
+func f() {}
+
+var x, y, z int
+
+//line src.go:8
+func g() {
+}
+`
+
+	// parse original
+	f1, err := parser.ParseFile(fset, "src.go", orig, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// pretty-print original
+	var buf bytes.Buffer
+	err = (&Config{Mode: UseSpaces | SourcePos, Tabwidth: 8}).Fprint(&buf, fset, f1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := buf.String()
+
+	// compare original with desired output
+	if got != want {
+		t.Errorf("got:\n%s\nwant:\n%s\n", got, want)
+	}
+}
+
+var decls = []string{
+	`import "fmt"`,
+	"const pi = 3.1415\nconst e = 2.71828\n\nvar x = pi",
+	"func sum(x, y int) int\t{ return x + y }",
+}
+
+func TestDeclLists(t *testing.T) {
+	for _, src := range decls {
+		file, err := parser.ParseFile(fset, "", "package p;"+src, parser.ParseComments)
+		if err != nil {
+			panic(err) // error in test
+		}
+
+		var buf bytes.Buffer
+		err = Fprint(&buf, fset, file.Decls) // only print declarations
+		if err != nil {
+			panic(err) // error in test
+		}
+
+		out := buf.String()
+		if out != src {
+			t.Errorf("\ngot : %q\nwant: %q\n", out, src)
+		}
+	}
+}
+
+var stmts = []string{
+	"i := 0",
+	"select {}\nvar a, b = 1, 2\nreturn a + b",
+	"go f()\ndefer func() {}()",
+}
+
+func TestStmtLists(t *testing.T) {
+	for _, src := range stmts {
+		file, err := parser.ParseFile(fset, "", "package p; func _() {"+src+"}", parser.ParseComments)
+		if err != nil {
+			panic(err) // error in test
+		}
+
+		var buf bytes.Buffer
+		err = Fprint(&buf, fset, file.Decls[0].(*ast.FuncDecl).Body.List) // only print statements
+		if err != nil {
+			panic(err) // error in test
+		}
+
+		out := buf.String()
+		if out != src {
+			t.Errorf("\ngot : %q\nwant: %q\n", out, src)
+		}
+	}
+}
+
+func TestBaseIndent(t *testing.T) {
+	t.Parallel()
+	// The testfile must not contain multi-line raw strings since those
+	// are not indented (because their values must not change) and make
+	// this test fail.
+	const filename = "printer.go"
+	src, err := os.ReadFile(filename)
+	if err != nil {
+		panic(err) // error in test
+	}
+
+	file, err := parser.ParseFile(fset, filename, src, 0)
+	if err != nil {
+		panic(err) // error in test
+	}
+
+	for indent := 0; indent < 4; indent++ {
+		indent := indent
+		t.Run(fmt.Sprint(indent), func(t *testing.T) {
+			t.Parallel()
+			var buf bytes.Buffer
+			(&Config{Tabwidth: tabwidth, Indent: indent}).Fprint(&buf, fset, file)
+			// all code must be indented by at least 'indent' tabs
+			lines := bytes.Split(buf.Bytes(), []byte{'\n'})
+			for i, line := range lines {
+				if len(line) == 0 {
+					continue // empty lines don't have indentation
+				}
+				n := 0
+				for j, b := range line {
+					if b != '\t' {
+						// end of indentation
+						n = j
+						break
+					}
+				}
+				if n < indent {
+					t.Errorf("line %d: got only %d tabs; want at least %d: %q", i, n, indent, line)
+				}
+			}
+		})
+	}
+}
+
+// TestFuncType tests that an ast.FuncType with a nil Params field
+// can be printed (per go/ast specification). Test case for issue 3870.
+func TestFuncType(t *testing.T) {
+	src := &ast.File{
+		Name: &ast.Ident{Name: "p"},
+		Decls: []ast.Decl{
+			&ast.FuncDecl{
+				Name: &ast.Ident{Name: "f"},
+				Type: &ast.FuncType{},
+			},
+		},
+	}
+
+	var buf bytes.Buffer
+	if err := Fprint(&buf, fset, src); err != nil {
+		t.Fatal(err)
+	}
+	got := buf.String()
+
+	const want = `package p
+
+func f()
+`
+
+	if got != want {
+		t.Fatalf("got:\n%s\nwant:\n%s\n", got, want)
+	}
+}
+
+type limitWriter struct {
+	remaining int
+	errCount  int
+}
+
+func (l *limitWriter) Write(buf []byte) (n int, err error) {
+	n = len(buf)
+	if n >= l.remaining {
+		n = l.remaining
+		err = io.EOF
+		l.errCount++
+	}
+	l.remaining -= n
+	return n, err
+}
+
+// Test whether the printer stops writing after the first error
+func TestWriteErrors(t *testing.T) {
+	t.Parallel()
+	const filename = "printer.go"
+	src, err := os.ReadFile(filename)
+	if err != nil {
+		panic(err) // error in test
+	}
+	file, err := parser.ParseFile(fset, filename, src, 0)
+	if err != nil {
+		panic(err) // error in test
+	}
+	for i := 0; i < 20; i++ {
+		lw := &limitWriter{remaining: i}
+		err := (&Config{Mode: RawFormat}).Fprint(lw, fset, file)
+		if lw.errCount > 1 {
+			t.Fatal("Writes continued after first error returned")
+		}
+		// We expect errCount be 1 iff err is set
+		if (lw.errCount != 0) != (err != nil) {
+			t.Fatal("Expected err when errCount != 0")
+		}
+	}
+}
+
+// TextX is a skeleton test that can be filled in for debugging one-off cases.
+// Do not remove.
+func TestX(t *testing.T) {
+	const src = `
+package p
+func _() {}
+`
+	_, err := format([]byte(src), 0)
+	if err != nil {
+		t.Error(err)
+	}
+}
+
+func TestCommentedNode(t *testing.T) {
+	const (
+		input = `package main
+
+func foo() {
+	// comment inside func
+}
+
+// leading comment
+type bar int // comment2
+
+`
+
+		foo = `func foo() {
+	// comment inside func
+}`
+
+		bar = `// leading comment
+type bar int	// comment2
+`
+	)
+
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "input.go", input, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf bytes.Buffer
+
+	err = Fprint(&buf, fset, &CommentedNode{Node: f.Decls[0], Comments: f.Comments})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if buf.String() != foo {
+		t.Errorf("got %q, want %q", buf.String(), foo)
+	}
+
+	buf.Reset()
+
+	err = Fprint(&buf, fset, &CommentedNode{Node: f.Decls[1], Comments: f.Comments})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if buf.String() != bar {
+		t.Errorf("got %q, want %q", buf.String(), bar)
+	}
+}
+
+func TestIssue11151(t *testing.T) {
+	const src = "package p\t/*\r/1\r*\r/2*\r\r\r\r/3*\r\r+\r\r/4*/\n"
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf bytes.Buffer
+	Fprint(&buf, fset, f)
+	got := buf.String()
+	const want = "package p\t/*/1*\r/2*\r/3*+/4*/\n" // \r following opening /* should be stripped
+	if got != want {
+		t.Errorf("\ngot : %q\nwant: %q", got, want)
+	}
+
+	// the resulting program must be valid
+	_, err = parser.ParseFile(fset, "", got, 0)
+	if err != nil {
+		t.Errorf("%v\norig: %q\ngot : %q", err, src, got)
+	}
+}
+
+// If a declaration has multiple specifications, a parenthesized
+// declaration must be printed even if Lparen is token.NoPos.
+func TestParenthesizedDecl(t *testing.T) {
+	// a package with multiple specs in a single declaration
+	const src = "package p; var ( a float64; b int )"
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// print the original package
+	var buf bytes.Buffer
+	err = Fprint(&buf, fset, f)
+	if err != nil {
+		t.Fatal(err)
+	}
+	original := buf.String()
+
+	// now remove parentheses from the declaration
+	for i := 0; i != len(f.Decls); i++ {
+		f.Decls[i].(*ast.GenDecl).Lparen = token.NoPos
+	}
+	buf.Reset()
+	err = Fprint(&buf, fset, f)
+	if err != nil {
+		t.Fatal(err)
+	}
+	noparen := buf.String()
+
+	if noparen != original {
+		t.Errorf("got %q, want %q", noparen, original)
+	}
+}
+
+// Verify that we don't print a newline between "return" and its results, as
+// that would incorrectly cause a naked return.
+func TestIssue32854(t *testing.T) {
+	src := `package foo
+
+func f() {
+        return Composite{
+                call(),
+        }
+}`
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, "", src, 0)
+	if err != nil {
+		panic(err)
+	}
+
+	// Replace the result with call(), which is on the next line.
+	fd := file.Decls[0].(*ast.FuncDecl)
+	ret := fd.Body.List[0].(*ast.ReturnStmt)
+	ret.Results[0] = ret.Results[0].(*ast.CompositeLit).Elts[0]
+
+	var buf bytes.Buffer
+	if err := Fprint(&buf, fset, ret); err != nil {
+		t.Fatal(err)
+	}
+	want := "return call()"
+	if got := buf.String(); got != want {
+		t.Fatalf("got %q, want %q", got, want)
+	}
+}
diff --git a/src/go/printer/testdata/alignment.golden b/src/go/printer/testdata/alignment.golden
new file mode 100644
index 0000000..96086ed
--- /dev/null
+++ b/src/go/printer/testdata/alignment.golden
@@ -0,0 +1,172 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package alignment
+
+// ----------------------------------------------------------------------------
+// Examples from issue #7335.
+
+func main() {
+	z := MyStruct{
+		Foo:		"foo",
+		Bar:		"bar",
+		Name:		"name",
+		LongName:	"longname",
+		Baz:		"baz",
+	}
+	y := MyStruct{
+		Foo:			"foo",
+		Bar:			"bar",
+		NameXX:			"name",
+		LongNameXXXXXXXXXXXXX:	"longname",
+		Baz:			"baz",
+	}
+	z := MyStruct{
+		Foo:			"foo",
+		Bar:			"bar",
+		Name:			"name",
+		LongNameXXXXXXXXXXXXX:	"longname",
+		Baz:			"baz",
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #10392.
+
+var kcfg = KubeletConfig{
+	Address:			s.Address,
+	AllowPrivileged:		s.AllowPrivileged,
+	HostNetworkSources:		hostNetworkSources,
+	HostnameOverride:		s.HostnameOverride,
+	RootDirectory:			s.RootDirectory,
+	ConfigFile:			s.Config,
+	ManifestURL:			s.ManifestURL,
+	FileCheckFrequency:		s.FileCheckFrequency,
+	HTTPCheckFrequency:		s.HTTPCheckFrequency,
+	PodInfraContainerImage:		s.PodInfraContainerImage,
+	SyncFrequency:			s.SyncFrequency,
+	RegistryPullQPS:		s.RegistryPullQPS,
+	RegistryBurst:			s.RegistryBurst,
+	MinimumGCAge:			s.MinimumGCAge,
+	MaxPerPodContainerCount:	s.MaxPerPodContainerCount,
+	MaxContainerCount:		s.MaxContainerCount,
+	ClusterDomain:			s.ClusterDomain,
+	ClusterDNS:			s.ClusterDNS,
+	Runonce:			s.RunOnce,
+	Port:				s.Port,
+	ReadOnlyPort:			s.ReadOnlyPort,
+	CadvisorInterface:		cadvisorInterface,
+	EnableServer:			s.EnableServer,
+	EnableDebuggingHandlers:	s.EnableDebuggingHandlers,
+	DockerClient:			dockertools.ConnectToDockerOrDie(s.DockerEndpoint),
+	KubeClient:			client,
+	MasterServiceNamespace:		s.MasterServiceNamespace,
+	VolumePlugins:			ProbeVolumePlugins(),
+	NetworkPlugins:			ProbeNetworkPlugins(),
+	NetworkPluginName:		s.NetworkPluginName,
+	StreamingConnectionIdleTimeout:	s.StreamingConnectionIdleTimeout,
+	TLSOptions:			tlsOptions,
+	ImageGCPolicy:			imageGCPolicy, imageGCPolicy,
+	Cloud:				cloud,
+	NodeStatusUpdateFrequency:	s.NodeStatusUpdateFrequency,
+}
+
+var a = A{
+	Long:					1,
+	LongLong:				1,
+	LongLongLong:				1,
+	LongLongLongLong:			1,
+	LongLongLongLongLong:			1,
+	LongLongLongLongLongLong:		1,
+	LongLongLongLongLongLongLong:		1,
+	LongLongLongLongLongLongLongLong:	1,
+	Short:					1,
+	LongLongLongLongLongLongLongLongLong:	3,
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #22852.
+
+var fmtMap = map[string]string{
+	"1":					"123",
+	"12":					"123",
+	"123":					"123",
+	"1234":					"123",
+	"12345":				"123",
+	"123456":				"123",
+	"12345678901234567890123456789":	"123",
+	"abcde":				"123",
+	"123456789012345678901234567890":	"123",
+	"1234567":				"123",
+	"abcdefghijklmnopqrstuvwxyzabcd":	"123",
+	"abcd":					"123",
+}
+
+type Fmt struct {
+	abcdefghijklmnopqrstuvwx	string
+	abcdefghijklmnopqrstuvwxy	string
+	abcdefghijklmnopqrstuvwxyz	string
+	abcdefghijklmnopqrstuvwxyza	string
+	abcdefghijklmnopqrstuvwxyzab	string
+	abcdefghijklmnopqrstuvwxyzabc	string
+	abcde				string
+	abcdefghijklmnopqrstuvwxyzabcde	string
+	abcdefg				string
+}
+
+func main() {
+	_ := Fmt{
+		abcdefghijklmnopqrstuvwx:		"foo",
+		abcdefghijklmnopqrstuvwxyza:		"foo",
+		abcdefghijklmnopqrstuvwxyzab:		"foo",
+		abcdefghijklmnopqrstuvwxyzabc:		"foo",
+		abcde:					"foo",
+		abcdefghijklmnopqrstuvwxyzabcde:	"foo",
+		abcdefg:				"foo",
+		abcdefghijklmnopqrstuvwxy:		"foo",
+		abcdefghijklmnopqrstuvwxyz:		"foo",
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #26352.
+
+var _ = map[int]string{
+	1:	"",
+
+	12345678901234567890123456789:		"",
+	12345678901234567890123456789012345678:	"",
+}
+
+func f() {
+	_ = map[int]string{
+		1:	"",
+
+		12345678901234567:				"",
+		12345678901234567890123456789012345678901:	"",
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #26930.
+
+var _ = S{
+	F1:	[]string{},
+	F2____:	[]string{},
+}
+
+var _ = S{
+	F1:	[]string{},
+	F2____:	[]string{},
+}
+
+var _ = S{
+	F1____:	[]string{},
+	F2:	[]string{},
+}
+
+var _ = S{
+	F1____:	[]string{},
+	F2:	[]string{},
+}
diff --git a/src/go/printer/testdata/alignment.input b/src/go/printer/testdata/alignment.input
new file mode 100644
index 0000000..323d268
--- /dev/null
+++ b/src/go/printer/testdata/alignment.input
@@ -0,0 +1,179 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package alignment
+
+// ----------------------------------------------------------------------------
+// Examples from issue #7335.
+
+func main() {
+    z := MyStruct{
+        Foo:      "foo",
+        Bar:      "bar",
+        Name:     "name",
+        LongName: "longname",
+        Baz:      "baz",
+    }
+    y := MyStruct{
+        Foo:                   "foo",
+        Bar:                   "bar",
+        NameXX:                "name",
+        LongNameXXXXXXXXXXXXX: "longname",
+        Baz: "baz",
+    }
+    z := MyStruct{
+        Foo:  "foo",
+        Bar:  "bar",
+        Name: "name",
+        LongNameXXXXXXXXXXXXX: "longname",
+        Baz: "baz",
+    }
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #10392.
+
+var kcfg = KubeletConfig{
+    Address:                        s.Address,
+    AllowPrivileged:                s.AllowPrivileged,
+    HostNetworkSources:             hostNetworkSources,
+    HostnameOverride:               s.HostnameOverride,
+    RootDirectory:                  s.RootDirectory,
+    ConfigFile:                     s.Config,
+    ManifestURL:                    s.ManifestURL,
+    FileCheckFrequency:             s.FileCheckFrequency,
+    HTTPCheckFrequency:             s.HTTPCheckFrequency,
+    PodInfraContainerImage:         s.PodInfraContainerImage,
+    SyncFrequency:                  s.SyncFrequency,
+    RegistryPullQPS:                s.RegistryPullQPS,
+    RegistryBurst:                  s.RegistryBurst,
+    MinimumGCAge:                   s.MinimumGCAge,
+    MaxPerPodContainerCount:        s.MaxPerPodContainerCount,
+    MaxContainerCount:              s.MaxContainerCount,
+    ClusterDomain:                  s.ClusterDomain,
+    ClusterDNS:                     s.ClusterDNS,
+    Runonce:                        s.RunOnce,
+    Port:                           s.Port,
+    ReadOnlyPort:                   s.ReadOnlyPort,
+    CadvisorInterface:              cadvisorInterface,
+    EnableServer:                   s.EnableServer,
+    EnableDebuggingHandlers:        s.EnableDebuggingHandlers,
+    DockerClient:                   dockertools.ConnectToDockerOrDie(s.DockerEndpoint),
+    KubeClient:                     client,
+    MasterServiceNamespace:         s.MasterServiceNamespace,
+    VolumePlugins:                  ProbeVolumePlugins(),
+    NetworkPlugins:                 ProbeNetworkPlugins(),
+    NetworkPluginName:              s.NetworkPluginName,
+    StreamingConnectionIdleTimeout: s.StreamingConnectionIdleTimeout,
+    TLSOptions:                     tlsOptions,
+    ImageGCPolicy:                  imageGCPolicy,imageGCPolicy,
+    Cloud:                          cloud,
+    NodeStatusUpdateFrequency: s.NodeStatusUpdateFrequency,
+}
+
+var a = A{
+    Long:                             1,
+    LongLong:                         1,
+    LongLongLong:                     1,
+    LongLongLongLong:                 1,
+    LongLongLongLongLong:             1,
+    LongLongLongLongLongLong:         1,
+    LongLongLongLongLongLongLong:     1,
+    LongLongLongLongLongLongLongLong: 1,
+    Short: 1,
+    LongLongLongLongLongLongLongLongLong: 3,
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #22852.
+
+var fmtMap = map[string]string{
+	"1": "123",
+	"12": "123",
+	"123": "123",
+	"1234": "123",
+	"12345": "123",
+	"123456": "123",
+	"12345678901234567890123456789": "123",
+	"abcde": "123",
+	"123456789012345678901234567890": "123",
+	"1234567": "123",
+	"abcdefghijklmnopqrstuvwxyzabcd": "123",
+	"abcd": "123",
+}
+
+type Fmt struct {
+	abcdefghijklmnopqrstuvwx string
+	abcdefghijklmnopqrstuvwxy string
+	abcdefghijklmnopqrstuvwxyz string
+	abcdefghijklmnopqrstuvwxyza string
+	abcdefghijklmnopqrstuvwxyzab string
+	abcdefghijklmnopqrstuvwxyzabc string
+	abcde string
+	abcdefghijklmnopqrstuvwxyzabcde string
+	abcdefg string
+}
+
+func main() {
+	_ := Fmt{
+		abcdefghijklmnopqrstuvwx: "foo",
+		abcdefghijklmnopqrstuvwxyza: "foo",
+		abcdefghijklmnopqrstuvwxyzab: "foo",
+		abcdefghijklmnopqrstuvwxyzabc: "foo",
+		abcde: "foo",
+		abcdefghijklmnopqrstuvwxyzabcde: "foo",
+		abcdefg: "foo",
+		abcdefghijklmnopqrstuvwxy: "foo",
+		abcdefghijklmnopqrstuvwxyz: "foo",
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #26352.
+
+var _ = map[int]string{
+	1: "",
+
+	12345678901234567890123456789: "",
+	12345678901234567890123456789012345678: "",
+}
+
+func f() {
+	_ = map[int]string{
+		1: "",
+
+		12345678901234567: "",
+		12345678901234567890123456789012345678901: "",
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Examples from issue #26930.
+
+var _ = S{
+	F1: []string{
+	},
+	F2____: []string{},
+}
+
+var _ = S{
+	F1: []string{
+
+
+	},
+	F2____: []string{},
+}
+
+var _ = S{
+	F1____: []string{
+	},
+	F2: []string{},
+}
+
+var _ = S{
+	F1____: []string{
+
+	},
+	F2: []string{},
+}
diff --git a/src/go/printer/testdata/comments.golden b/src/go/printer/testdata/comments.golden
new file mode 100644
index 0000000..1a21fff
--- /dev/null
+++ b/src/go/printer/testdata/comments.golden
@@ -0,0 +1,759 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+import "fmt"	// fmt
+
+const c0 = 0	// zero
+const (
+	c1	= iota	// c1
+	c2		// c2
+)
+
+// Alignment of comments in declarations>
+const (
+	_	T	= iota	// comment
+	_			// comment
+	_			// comment
+	_	= iota + 10
+	_	// comments
+
+	_		= 10	// comment
+	_	T	= 20	// comment
+)
+
+const (
+	_____	= iota	// foo
+	_		// bar
+	_	= 0	// bal
+	_		// bat
+)
+
+const (
+	_	T	= iota	// comment
+	_			// comment
+	_			// comment
+	_	= iota + 10
+	_		// comment
+	_		= 10
+	_		= 20	// comment
+	_	T	= 0	// comment
+)
+
+// The SZ struct; it is empty.
+type SZ struct{}
+
+// The S0 struct; no field is exported.
+type S0 struct {
+	int
+	x, y, z	int	// 3 unexported fields
+}
+
+// The S1 struct; some fields are not exported.
+type S1 struct {
+	S0
+	A, B, C	float	// 3 exported fields
+	D, b, c	int	// 2 unexported fields
+}
+
+// The S2 struct; all fields are exported.
+type S2 struct {
+	S1
+	A, B, C	float	// 3 exported fields
+}
+
+// The IZ interface; it is empty.
+type SZ interface{}
+
+// The I0 interface; no method is exported.
+type I0 interface {
+	f(x int) int	// unexported method
+}
+
+// The I1 interface; some methods are not exported.
+type I1 interface {
+	I0
+	F(x float) float	// exported methods
+	g(x int) int		// unexported method
+}
+
+// The I2 interface; all methods are exported.
+type I2 interface {
+	I0
+	F(x float) float	// exported method
+	G(x float) float	// exported method
+}
+
+// The S3 struct; all comments except for the last one must appear in the export.
+type S3 struct {
+	// lead comment for F1
+	F1	int	// line comment for F1
+	// lead comment for F2
+	F2	int	// line comment for F2
+	f3	int	// f3 is not exported
+}
+
+// This comment group should be separated
+// with a newline from the next comment
+// group.
+
+// This comment should NOT be associated with the next declaration.
+
+var x int	// x
+var ()
+
+// This comment SHOULD be associated with f0.
+func f0() {
+	const pi = 3.14	// pi
+	var s1 struct{}	/* an empty struct */	/* foo */
+	// a struct constructor
+	// --------------------
+	var s2 struct{} = struct{}{}
+	x := pi
+}
+
+//
+// This comment should be associated with f1, with one blank line before the comment.
+//
+func f1() {
+	f0()
+	/* 1 */
+	// 2
+	/* 3 */
+	/* 4 */
+	f0()
+}
+
+func _() {
+	// this comment should be properly indented
+}
+
+func _(x int) int {
+	if x < 0 {	// the tab printed before this comment's // must not affect the remaining lines
+		return -x	// this statement should be properly indented
+	}
+	if x < 0 {	/* the tab printed before this comment's /* must not affect the remaining lines */
+		return -x	// this statement should be properly indented
+	}
+	return x
+}
+
+func typeswitch(x interface{}) {
+	switch v := x.(type) {
+	case bool, int, float:
+	case string:
+	default:
+	}
+
+	switch x.(type) {
+	}
+
+	switch v0, ok := x.(int); v := x.(type) {
+	}
+
+	switch v0, ok := x.(int); x.(type) {
+	case byte:	// this comment should be on the same line as the keyword
+		// this comment should be normally indented
+		_ = 0
+	case bool, int, float:
+		// this comment should be indented
+	case string:
+	default:
+		// this comment should be indented
+	}
+	// this comment should not be indented
+}
+
+//
+// Indentation of comments after possibly indented multi-line constructs
+// (test cases for issue 3147).
+//
+
+func _() {
+	s := 1 +
+		2
+	// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2	// comment
+	// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2	// comment
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+
+	// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2	// comment
+
+	// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2	// comment
+
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+
+	// should be indented like s
+	_ = 0
+}
+
+// Test case from issue 3147.
+func f() {
+	templateText := "a" +	// A
+		"b" +	// B
+		"c"	// C
+
+	// should be aligned with f()
+	f()
+}
+
+// Modified test case from issue 3147.
+func f() {
+	templateText := "a" +	// A
+		"b" +	// B
+		"c"	// C
+
+		// may not be aligned with f() (source is not aligned)
+	f()
+}
+
+//
+// Test cases for alignment of lines in general comments.
+//
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line */
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line */
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line */
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line */
+}
+
+// Issue 9751.
+func _() {
+	/*a string
+
+	b string*/
+
+	/*A string
+
+
+
+	Z string*/
+
+	/*a string
+
+	b string
+
+	c string*/
+
+	{
+		/*a string
+		b string*/
+
+		/*a string
+
+		b string*/
+
+		/*a string
+
+		b string
+
+		c string*/
+	}
+
+	{
+		/*a string
+		b string*/
+
+		/*a string
+
+		b string*/
+
+		/*a string
+
+		b string
+
+		c string*/
+	}
+
+	/*
+	 */
+
+	/*
+
+	 */
+
+	/*
+
+	 * line
+
+	 */
+}
+
+/*
+ * line
+ * of
+ * stars
+ */
+
+/* another line
+ * of
+ * stars */
+
+/*	and another line
+ *	of
+ *	stars */
+
+/* a line of
+ * stars */
+
+/*	and another line of
+ *	stars */
+
+/* a line of stars
+ */
+
+/*	and another line of
+ */
+
+/* a line of stars
+ */
+
+/*	and another line of
+ */
+
+/*
+aligned in middle
+here
+        not here
+*/
+
+/*
+blank line in middle:
+
+with no leading spaces on blank line.
+*/
+
+/*
+   aligned in middle
+   here
+           not here
+*/
+
+/*
+	blank line in middle:
+
+	with no leading spaces on blank line.
+*/
+
+func _() {
+	/*
+	 * line
+	 * of
+	 * stars
+	 */
+
+	/*
+		aligned in middle
+		here
+			not here
+	*/
+
+	/*
+		blank line in middle:
+
+		with no leading spaces on blank line.
+	*/
+}
+
+// Some interesting interspersed comments.
+// See below for more common cases.
+func _( /* this */ x /* is */ /* an */ int) {
+}
+
+func _( /* no params - extra blank before and after comment */ )	{}
+func _(a, b int /* params - no extra blank after comment */)		{}
+
+func _()	{ f( /* no args - extra blank before and after comment */ ) }
+func _()	{ f(a, b /* args - no extra blank after comment */) }
+
+func _() {
+	f( /* no args - extra blank before and after comment */ )
+	f(a, b /* args - no extra blank after comment */)
+}
+
+func ( /* comment1 */ T /* comment2 */) _()	{}
+
+func _()	{ /* "short-ish one-line functions with comments are formatted as multi-line functions */ }
+func _()	{ x := 0; /* comment */ y = x /* comment */ }
+
+func _() {
+	_ = 0
+	/* closing curly brace should be on new line */
+}
+
+func _() {
+	_ = []int{0, 1 /* don't introduce a newline after this comment - was issue 1365 */}
+}
+
+// Test cases from issue 1542:
+// Comments must not be placed before commas and cause invalid programs.
+func _() {
+	var a = []int{1, 2	/*jasldf*/}
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2}/*jasldf
+	 */
+
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2}// jasldf
+
+	_ = a
+}
+
+// Test cases from issues 11274, 15137:
+// Semicolon must not be lost when multiple statements are on the same line with a comment.
+func _() {
+	x := 0 /**/
+	y := 1
+}
+
+func _() {
+	f()
+	f()
+	f() /* comment */
+	f()
+	f() /* comment */
+	f()
+	f() /* a */ /* b */
+	f()
+	f() /* a */ /* b */
+	f()
+	f() /* a */ /* b */
+	f()
+}
+
+func _() {
+	f() /* a */ /* b */
+}
+
+// Comments immediately adjacent to punctuation followed by a newline
+// remain after the punctuation (looks better and permits alignment of
+// comments).
+func _() {
+	_ = T{
+		1,	// comment after comma
+		2,	/* comment after comma */
+		3,	// comment after comma
+	}
+	_ = T{
+		1,	// comment after comma
+		2,	/* comment after comma */
+		3,	// comment after comma
+	}
+	_ = T{
+		/* comment before literal */ 1,
+		2,	/* comment before comma - ok to move after comma */
+		3,	/* comment before comma - ok to move after comma */
+	}
+
+	for i = 0;	// comment after semicolon
+	i < 9;		/* comment after semicolon */
+	i++ {		// comment after opening curly brace
+	}
+
+	// TODO(gri) the last comment in this example should be aligned */
+	for i = 0;	// comment after semicolon
+	i < 9;		/* comment before semicolon - ok to move after semicolon */
+	i++ /* comment before opening curly brace */ {
+	}
+}
+
+// If there is no newline following punctuation, commas move before the punctuation.
+// This way, commas interspersed in lists stay with the respective expression.
+func f(x /* comment */, y int, z int /* comment */, u, v, w int /* comment */) {
+	f(x /* comment */, y)
+	f(x,	/* comment */
+		y)
+	f(
+		x,	/* comment */
+	)
+}
+
+func g(
+	x int,	/* comment */
+) {
+}
+
+type _ struct {
+	a, b /* comment */, c int
+}
+
+type _ struct {
+	a, b /* comment */, c int
+}
+
+func _() {
+	for a /* comment */, b := range x {
+	}
+}
+
+// Print line directives correctly.
+
+// The following is a legal line directive.
+//line foo:1
+func _() {
+	_ = 0
+	// The following is a legal line directive. It must not be indented:
+//line foo:2
+	_ = 1
+
+	// The following is not a legal line directive (it doesn't start in column 1):
+	//line foo:2
+	_ = 2
+
+	// The following is not a legal line directive (missing colon):
+//line foo -3
+	_ = 3
+}
+
+// Line comments with tabs
+func _() {
+	var finput *bufio.Reader	// input file
+	var stderr *bufio.Writer
+	var ftable *bufio.Writer	// y.go file
+	var foutput *bufio.Writer	// y.output file
+
+	var oflag string	// -o [y.go]		- y.go file
+	var vflag string	// -v [y.output]	- y.output file
+	var lflag bool		// -l			- disable line directives
+}
+
+// Trailing white space in comments should be trimmed
+func _() {
+	// This comment has 4 blanks following that should be trimmed:
+	/* Each line of this comment has blanks or tabs following that should be trimmed:
+	   line 2:
+	   line 3:
+	*/
+}
+
+var _ = []T{ /* lone comment */ }
+
+var _ = []T{
+	/* lone comment */
+}
+
+var _ = []T{
+	// lone comments
+	// in composite lit
+}
+
+var _ = [][]T{
+	{
+		// lone comments
+		// in composite lit
+	},
+}
+
+// TODO: gofmt doesn't add these tabs; make it so that these golden
+// tests run the printer in a way that it's exactly like gofmt.
+
+var _ = []T{	// lone comment
+}
+
+var _ = []T{	// lone comments
+	// in composite lit
+}
+
+/* This comment is the last entry in this file. It must be printed and should be followed by a newline */
diff --git a/src/go/printer/testdata/comments.input b/src/go/printer/testdata/comments.input
new file mode 100644
index 0000000..aa428a2
--- /dev/null
+++ b/src/go/printer/testdata/comments.input
@@ -0,0 +1,756 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+import "fmt"  // fmt
+
+const c0 = 0  // zero
+const (
+	c1 = iota  // c1
+	c2  // c2
+)
+
+// Alignment of comments in declarations>
+const (
+	_ T = iota  // comment
+	_  // comment
+	_  // comment
+	_ = iota+10
+	_  // comments
+
+	_ = 10  // comment
+	_ T = 20  // comment
+)
+
+const (
+	_____ = iota // foo
+	_ // bar
+	_  = 0    // bal
+	_ // bat
+)
+
+const (
+	_ T = iota // comment
+	_ // comment
+	_ // comment
+	_ = iota + 10
+	_ // comment
+	_ = 10
+	_ = 20 // comment
+	_ T = 0 // comment
+)
+
+// The SZ struct; it is empty.
+type SZ struct {}
+
+// The S0 struct; no field is exported.
+type S0 struct {
+	int
+	x, y, z int  // 3 unexported fields
+}
+
+// The S1 struct; some fields are not exported.
+type S1 struct {
+	S0
+	A, B, C float  // 3 exported fields
+	D, b, c int  // 2 unexported fields
+}
+
+// The S2 struct; all fields are exported.
+type S2 struct {
+	S1
+	A, B, C float  // 3 exported fields
+}
+
+// The IZ interface; it is empty.
+type SZ interface {}
+
+// The I0 interface; no method is exported.
+type I0 interface {
+	f(x int) int  // unexported method
+}
+
+// The I1 interface; some methods are not exported.
+type I1 interface {
+	I0
+	F(x float) float  // exported methods
+	g(x int) int  // unexported method
+}
+
+// The I2 interface; all methods are exported.
+type I2 interface {
+	I0
+	F(x float) float  // exported method
+	G(x float) float  // exported method
+}
+
+// The S3 struct; all comments except for the last one must appear in the export.
+type S3 struct {
+	// lead comment for F1
+	F1 int // line comment for F1
+	// lead comment for F2
+	F2 int // line comment for F2
+	f3 int // f3 is not exported
+}
+
+// This comment group should be separated
+// with a newline from the next comment
+// group.
+
+// This comment should NOT be associated with the next declaration.
+
+var x int  // x
+var ()
+
+
+// This comment SHOULD be associated with f0.
+func f0() {
+	const pi = 3.14  // pi
+	var s1 struct {}  /* an empty struct */ /* foo */
+	// a struct constructor
+	// --------------------
+	var s2 struct {} = struct {}{}
+	x := pi
+}
+//
+// This comment should be associated with f1, with one blank line before the comment.
+//
+func f1() {
+	f0()
+	/* 1 */
+	// 2
+	/* 3 */
+	/* 4 */
+	f0()
+}
+
+
+func _() {
+// this comment should be properly indented
+}
+
+
+func _(x int) int {
+	if x < 0 {  // the tab printed before this comment's // must not affect the remaining lines
+		return -x  // this statement should be properly indented
+	}
+	if x < 0 {  /* the tab printed before this comment's /* must not affect the remaining lines */
+		return -x  // this statement should be properly indented
+	}
+	return x
+}
+
+
+func typeswitch(x interface{}) {
+	switch v := x.(type) {
+	case bool, int, float:
+	case string:
+	default:
+	}
+
+	switch x.(type) {
+	}
+
+	switch v0, ok := x.(int); v := x.(type) {
+	}
+
+	switch v0, ok := x.(int); x.(type) {
+	case byte:  // this comment should be on the same line as the keyword
+		// this comment should be normally indented
+		_ = 0
+	case bool, int, float:
+		// this comment should be indented
+	case string:
+	default:
+		// this comment should be indented
+	}
+	// this comment should not be indented
+}
+
+//
+// Indentation of comments after possibly indented multi-line constructs
+// (test cases for issue 3147).
+//
+
+func _() {
+	s := 1 +
+		2
+// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2 // comment
+		// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2 // comment
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+
+// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2 // comment
+
+		// should be indented like s
+}
+
+func _() {
+	s := 1 +
+		2 // comment
+
+	// should be indented like s
+	_ = 0
+}
+
+func _() {
+	s := 1 +
+		2
+
+	// should be indented like s
+	_ = 0
+}
+
+// Test case from issue 3147.
+func f() {
+	templateText := "a" + // A
+		"b" + // B
+		"c" // C
+
+	// should be aligned with f()
+	f()
+}
+
+// Modified test case from issue 3147.
+func f() {
+	templateText := "a" + // A
+		"b" + // B
+		"c" // C
+
+		// may not be aligned with f() (source is not aligned)
+	f()
+}
+
+//
+// Test cases for alignment of lines in general comments.
+//
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line
+	   */
+}
+
+func _() {
+	/* freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line
+		*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned		line
+		aligned line */
+}
+
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line
+	   */
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned		line
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line
+		*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned		line
+		aligned line */
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line
+	   */
+}
+
+func _() {
+	/* freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line
+		*/
+}
+
+func _() {
+	/*	freestanding comment
+		aligned line */
+}
+
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	*/
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line
+	   */
+}
+
+func _() {
+	/*
+	   freestanding comment
+	   aligned line */
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+	*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line
+		*/
+}
+
+func _() {
+	/*
+		freestanding comment
+		aligned line */
+}
+
+// Issue 9751.
+func _() {
+	/*a string
+
+	b string*/
+
+	/*A string
+
+
+
+	Z string*/
+
+	/*a string
+
+	b string
+
+	c string*/
+
+	{
+		/*a string
+b string*/
+
+		/*a string
+
+b string*/
+
+		/*a string
+
+b string
+
+c string*/
+	}
+
+	{
+		/*a string
+				b string*/
+
+		/*a string
+
+				b string*/
+
+		/*a string
+
+				b string
+
+				c string*/
+	}
+
+	/*
+	*/
+
+	/*
+
+	*/
+
+	/*
+
+	 * line
+
+	*/
+}
+
+/*
+ * line
+ * of
+ * stars
+ */
+
+/* another line
+ * of
+ * stars */
+
+/*	and another line
+ *	of
+ *	stars */
+
+/* a line of
+ * stars */
+
+/*	and another line of
+ *	stars */
+
+/* a line of stars
+*/
+
+/*	and another line of
+*/
+
+/* a line of stars
+ */
+
+/*	and another line of
+ */
+
+/*
+aligned in middle
+here
+        not here
+*/
+
+/*
+blank line in middle:
+
+with no leading spaces on blank line.
+*/
+
+/*
+   aligned in middle
+   here
+           not here
+*/
+
+/*
+	blank line in middle:
+
+	with no leading spaces on blank line.
+*/
+
+func _() {
+	/*
+	 * line
+	 * of
+	 * stars
+	 */
+
+	/*
+	aligned in middle
+	here
+		not here
+	*/
+
+	/*
+	blank line in middle:
+
+	with no leading spaces on blank line.
+*/
+}
+
+
+// Some interesting interspersed comments.
+// See below for more common cases.
+func _(/* this */x/* is *//* an */ int) {
+}
+
+func _(/* no params - extra blank before and after comment */) {}
+func _(a, b int /* params - no extra blank after comment */) {}
+
+func _() { f(/* no args - extra blank before and after comment */) }
+func _() { f(a, b /* args - no extra blank after comment */) }
+
+func _() {
+	f(/* no args - extra blank before and after comment */)
+	f(a, b /* args - no extra blank after comment */)
+}
+
+func (/* comment1 */ T /* comment2 */) _() {}
+
+func _() { /* "short-ish one-line functions with comments are formatted as multi-line functions */ }
+func _() { x := 0; /* comment */ y = x /* comment */ }
+
+func _() {
+	_ = 0
+	/* closing curly brace should be on new line */ }
+
+func _() {
+	_ = []int{0, 1 /* don't introduce a newline after this comment - was issue 1365 */}
+}
+
+// Test cases from issue 1542:
+// Comments must not be placed before commas and cause invalid programs.
+func _() {
+	var a = []int{1, 2, /*jasldf*/
+	}
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2, /*jasldf
+						*/
+	}
+	_ = a
+}
+
+func _() {
+	var a = []int{1, 2, // jasldf
+	}
+	_ = a
+}
+
+// Test cases from issues 11274, 15137:
+// Semicolon must not be lost when multiple statements are on the same line with a comment.
+func _() {
+    x := 0 /**/; y := 1
+}
+
+func _() {
+	f(); f()
+	f(); /* comment */ f()
+	f() /* comment */; f()	
+	f(); /* a */ /* b */ f()
+	f() /* a */ /* b */; f()
+	f() /* a */; /* b */ f()
+}
+
+func _() {
+	f() /* a */ /* b */ }
+
+// Comments immediately adjacent to punctuation followed by a newline
+// remain after the punctuation (looks better and permits alignment of
+// comments).
+func _() {
+	_ = T{
+		1,    // comment after comma
+		2,    /* comment after comma */
+		3  ,  // comment after comma
+	}
+	_ = T{
+		1  ,// comment after comma
+		2  ,/* comment after comma */
+		3,// comment after comma
+	}
+	_ = T{
+		/* comment before literal */1,
+		2/* comment before comma - ok to move after comma */,
+		3  /* comment before comma - ok to move after comma */  ,
+	}
+
+	for
+		i=0;// comment after semicolon
+		i<9;/* comment after semicolon */
+		i++{// comment after opening curly brace
+	}
+
+	// TODO(gri) the last comment in this example should be aligned */
+	for
+		i=0;// comment after semicolon
+		i<9/* comment before semicolon - ok to move after semicolon */;
+		i++ /* comment before opening curly brace */ {
+	}
+}
+
+// If there is no newline following punctuation, commas move before the punctuation.
+// This way, commas interspersed in lists stay with the respective expression.
+func f(x/* comment */, y int, z int /* comment */, u, v, w int /* comment */) {
+	f(x /* comment */, y)
+	f(x /* comment */, 
+	y)
+	f(
+		x /* comment */,
+	)
+}
+
+func g(
+	x int /* comment */,
+) {}
+
+type _ struct {
+	a, b /* comment */, c int
+}
+
+type _ struct { a, b /* comment */, c int }
+
+func _() {
+	for a /* comment */, b := range x {
+	}
+}
+
+// Print line directives correctly.
+
+// The following is a legal line directive.
+//line foo:1
+func _() {
+	_ = 0
+// The following is a legal line directive. It must not be indented:
+//line foo:2
+	_ = 1
+
+// The following is not a legal line directive (it doesn't start in column 1):
+	//line foo:2
+	_ = 2
+
+// The following is not a legal line directive (missing colon):
+//line foo -3
+	_ = 3
+}
+
+// Line comments with tabs
+func _() {
+var	finput		*bufio.Reader			// input file
+var	stderr		*bufio.Writer
+var	ftable		*bufio.Writer			// y.go file
+var	foutput		*bufio.Writer			// y.output file
+
+var	oflag		string				// -o [y.go]		- y.go file
+var	vflag		string				// -v [y.output]	- y.output file
+var	lflag		bool				// -l			- disable line directives
+}
+
+// Trailing white space in comments should be trimmed
+func _() {
+// This comment has 4 blanks following that should be trimmed:    
+/* Each line of this comment has blanks or tabs following that should be trimmed:	
+   line 2:    
+   line 3:    			
+*/
+}
+
+var _ = []T{/* lone comment */}
+
+var _ = []T{
+/* lone comment */
+}
+
+var _ = []T{
+// lone comments
+// in composite lit
+}
+
+var _ = [][]T{
+	{
+		// lone comments
+		// in composite lit
+	},
+}
+
+// TODO: gofmt doesn't add these tabs; make it so that these golden
+// tests run the printer in a way that it's exactly like gofmt.
+
+var _ = []T{// lone comment
+}
+
+var _ = []T{// lone comments
+// in composite lit
+}
+
+/* This comment is the last entry in this file. It must be printed and should be followed by a newline */
diff --git a/src/go/printer/testdata/comments.x b/src/go/printer/testdata/comments.x
new file mode 100644
index 0000000..ae77292
--- /dev/null
+++ b/src/go/printer/testdata/comments.x
@@ -0,0 +1,56 @@
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+// The SZ struct; it is empty.
+type SZ struct{}
+
+// The S0 struct; no field is exported.
+type S0 struct {
+	// contains filtered or unexported fields
+}
+
+// The S1 struct; some fields are not exported.
+type S1 struct {
+	S0
+	A, B, C	float	// 3 exported fields
+	D	int	// 2 unexported fields
+	// contains filtered or unexported fields
+}
+
+// The S2 struct; all fields are exported.
+type S2 struct {
+	S1
+	A, B, C	float	// 3 exported fields
+}
+
+// The IZ interface; it is empty.
+type SZ interface{}
+
+// The I0 interface; no method is exported.
+type I0 interface {
+	// contains filtered or unexported methods
+}
+
+// The I1 interface; some methods are not exported.
+type I1 interface {
+	I0
+	F(x float) float	// exported methods
+	// contains filtered or unexported methods
+}
+
+// The I2 interface; all methods are exported.
+type I2 interface {
+	I0
+	F(x float) float	// exported method
+	G(x float) float	// exported method
+}
+
+// The S3 struct; all comments except for the last one must appear in the export.
+type S3 struct {
+	// lead comment for F1
+	F1	int	// line comment for F1
+	// lead comment for F2
+	F2	int	// line comment for F2
+	// contains filtered or unexported fields
+}
diff --git a/src/go/printer/testdata/comments2.golden b/src/go/printer/testdata/comments2.golden
new file mode 100644
index 0000000..8b3a94d
--- /dev/null
+++ b/src/go/printer/testdata/comments2.golden
@@ -0,0 +1,164 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+// Test cases for idempotent comment formatting (was issue 1835).
+/*
+c1a
+*/
+/*
+   c1b
+*/
+/* foo
+c1c
+*/
+/* foo
+   c1d
+*/
+/*
+c1e
+foo */
+/*
+   c1f
+   foo */
+
+func f() {
+	/*
+	   c2a
+	*/
+	/*
+	   c2b
+	*/
+	/* foo
+	   c2c
+	*/
+	/* foo
+	   c2d
+	*/
+	/*
+	   c2e
+	   foo */
+	/*
+	   c2f
+	   foo */
+}
+
+func g() {
+	/*
+	   c3a
+	*/
+	/*
+	   c3b
+	*/
+	/* foo
+	   c3c
+	*/
+	/* foo
+	   c3d
+	*/
+	/*
+	   c3e
+	   foo */
+	/*
+	   c3f
+	   foo */
+}
+
+// Test case taken literally from issue 1835.
+func main() {
+	/*
+	   prints test 5 times
+	*/
+	for i := 0; i < 5; i++ {
+		println("test")
+	}
+}
+
+func issue5623() {
+L:
+	_ = yyyyyyyyyyyyyyyy			// comment - should be aligned
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	/* comment */
+
+	_ = yyyyyyyyyyyyyyyy			/* comment - should be aligned */
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	// comment
+
+LLLLLLL:
+	_ = yyyyyyyyyyyyyyyy			// comment - should be aligned
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	// comment
+
+LL:
+LLLLL:
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	/* comment */
+	_ = yyyyyyyyyyyyyyyy			/* comment - should be aligned */
+
+	_ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx	// comment
+	_ = yyyyyyyyyyyyyyyy			// comment - should be aligned
+
+	// test case from issue
+label:
+	mask := uint64(1)<<c - 1		// Allocation mask
+	used := atomic.LoadUint64(&h.used)	// Current allocations
+}
+
+// Test cases for issue 18782
+var _ = [][]int{
+	/*       a, b, c, d, e */
+	/* a */ {0, 0, 0, 0, 0},
+	/* b */ {0, 5, 4, 4, 4},
+	/* c */ {0, 4, 5, 4, 4},
+	/* d */ {0, 4, 4, 5, 4},
+	/* e */ {0, 4, 4, 4, 5},
+}
+
+var _ = T{ /* a */ 0}
+
+var _ = T{ /* a */ /* b */ 0}
+
+var _ = T{	/* a */	/* b */
+	/* c */ 0,
+}
+
+var _ = T{	/* a */	/* b */
+	/* c */
+	/* d */ 0,
+}
+
+var _ = T{
+	/* a */
+	/* b */ 0,
+}
+
+var _ = T{ /* a */ {}}
+
+var _ = T{ /* a */ /* b */ {}}
+
+var _ = T{	/* a */	/* b */
+	/* c */ {},
+}
+
+var _ = T{	/* a */	/* b */
+	/* c */
+	/* d */ {},
+}
+
+var _ = T{
+	/* a */
+	/* b */ {},
+}
+
+var _ = []T{
+	func() {
+		var _ = [][]int{
+			/*       a, b, c, d, e */
+			/* a */ {0, 0, 0, 0, 0},
+			/* b */ {0, 5, 4, 4, 4},
+			/* c */ {0, 4, 5, 4, 4},
+			/* d */ {0, 4, 4, 5, 4},
+			/* e */ {0, 4, 4, 4, 5},
+		}
+	},
+}
diff --git a/src/go/printer/testdata/comments2.input b/src/go/printer/testdata/comments2.input
new file mode 100644
index 0000000..8d38c41
--- /dev/null
+++ b/src/go/printer/testdata/comments2.input
@@ -0,0 +1,168 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a package for testing comment placement by go/printer.
+//
+package main
+
+// Test cases for idempotent comment formatting (was issue 1835).
+/*
+c1a
+*/
+/*
+   c1b
+*/
+/* foo
+c1c
+*/
+/* foo
+   c1d
+*/
+/*
+c1e
+foo */
+/*
+   c1f
+   foo */
+
+func f() {
+/*
+c2a
+*/
+/*
+   c2b
+*/
+/* foo
+c2c
+*/
+/* foo
+   c2d
+*/
+/*
+c2e
+foo */
+/*
+   c2f
+   foo */
+}
+
+func g() {
+/*
+c3a
+*/
+/*
+   c3b
+*/
+/* foo
+c3c
+*/
+/* foo
+   c3d
+*/
+/*
+c3e
+foo */
+/*
+   c3f
+   foo */
+}
+
+// Test case taken literally from issue 1835.
+func main() {
+/*
+prints test 5 times
+*/
+   for i := 0; i < 5; i++ {
+      println("test")
+   }
+}
+
+func issue5623() {
+L:
+   _ = yyyyyyyyyyyyyyyy // comment - should be aligned
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx /* comment */
+
+   _ = yyyyyyyyyyyyyyyy /* comment - should be aligned */
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx // comment
+
+LLLLLLL:
+   _ = yyyyyyyyyyyyyyyy // comment - should be aligned
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx // comment
+
+LL:
+LLLLL:
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx /* comment */
+   _ = yyyyyyyyyyyyyyyy /* comment - should be aligned */
+
+   _ = xxxxxxxxxxxxxxxxxxxxxxxxxxxx // comment
+   _ = yyyyyyyyyyyyyyyy // comment - should be aligned
+
+// test case from issue
+label:
+   mask := uint64(1)<<c - 1 // Allocation mask
+   used := atomic.LoadUint64(&h.used) // Current allocations
+}
+
+// Test cases for issue 18782
+var _ = [][]int{
+   /*       a, b, c, d, e */
+   /* a */ {0, 0, 0, 0, 0},
+   /* b */ {0, 5, 4, 4, 4},
+   /* c */ {0, 4, 5, 4, 4},
+   /* d */ {0, 4, 4, 5, 4},
+   /* e */ {0, 4, 4, 4, 5},
+}
+
+var _ = T{ /* a */ 0,
+}
+
+var _ = T{ /* a */ /* b */ 0,
+}
+
+var _ = T{ /* a */ /* b */
+   /* c */ 0,
+}
+
+var _ = T{ /* a */ /* b */
+   /* c */
+   /* d */ 0,
+}
+
+var _ = T{
+   /* a */
+   /* b */ 0,
+}
+
+var _ = T{ /* a */ {},
+}
+
+var _ = T{ /* a */ /* b */ {},
+}
+
+var _ = T{ /* a */ /* b */
+   /* c */ {},
+}
+
+var _ = T{ /* a */ /* b */
+   /* c */
+   /* d */ {},
+}
+
+var _ = T{
+   /* a */
+   /* b */ {},
+}
+
+var _ = []T{
+   func() {
+      var _ = [][]int{
+         /*       a, b, c, d, e */
+         /* a */ {0, 0, 0, 0, 0},
+         /* b */ {0, 5, 4, 4, 4},
+         /* c */ {0, 4, 5, 4, 4},
+         /* d */ {0, 4, 4, 5, 4},
+         /* e */ {0, 4, 4, 4, 5},
+      }
+   },
+}
diff --git a/src/go/printer/testdata/complit.input b/src/go/printer/testdata/complit.input
new file mode 100644
index 0000000..82806a4
--- /dev/null
+++ b/src/go/printer/testdata/complit.input
@@ -0,0 +1,65 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package complit
+
+var (
+	// Multi-line declarations
+	V1 = T{
+		F1: "hello",
+		f2: 1,
+	}
+	V2 = T{
+		f2: 1,
+		F1: "hello",
+	}
+	V3 = T{
+		F1: "hello",
+		F2: T2{
+			A: "world",
+			b: "hidden",
+		},
+		f3: T2{
+			A: "world",
+		},
+	}
+	V4 = T{
+		f2: 1,
+	}
+
+	// Single-line declarations
+	V5 = T{F1: "hello", f2: 1}
+	V6 = T{f2: 1, F1: "hello"}
+	V7 = T{f2: 1}
+
+	// Mixed-mode declarations
+	V8 = T{
+		F1: "hello", f2: 1,
+		F3: "world",
+		f4: 2}
+	V9 = T{
+	f2: 1, F1: "hello",}
+	V10 = T{
+		F1: "hello", f2: 1,
+		f3: 2,
+		F4: "world", f5: 3,
+	}
+
+	// Other miscellaneous declarations
+	V11 = T{
+		t{
+			A: "world",
+			b: "hidden",
+		},
+		f2: t{
+			A: "world",
+			b: "hidden",
+		},
+	}
+	V12 = T{
+		F1: make(chan int),
+		f2: []int{},
+		F3: make(map[int]string), f4: 1,
+	}
+)
\ No newline at end of file
diff --git a/src/go/printer/testdata/complit.x b/src/go/printer/testdata/complit.x
new file mode 100644
index 0000000..458ac61
--- /dev/null
+++ b/src/go/printer/testdata/complit.x
@@ -0,0 +1,62 @@
+package complit
+
+var (
+	// Multi-line declarations
+	V1	= T{
+		F1: "hello",
+		// contains filtered or unexported fields
+	}
+	V2	= T{
+
+		F1: "hello",
+		// contains filtered or unexported fields
+	}
+	V3	= T{
+		F1:	"hello",
+		F2: T2{
+			A: "world",
+			// contains filtered or unexported fields
+		},
+		// contains filtered or unexported fields
+	}
+	V4	= T{
+		// contains filtered or unexported fields
+	}
+
+	// Single-line declarations
+	V5	= T{F1: "hello", /* contains filtered or unexported fields */}
+	V6	= T{F1: "hello", /* contains filtered or unexported fields */}
+	V7	= T{/* contains filtered or unexported fields */}
+
+	// Mixed-mode declarations
+	V8	= T{
+		F1:	"hello",
+		F3:	"world",
+		// contains filtered or unexported fields
+	}
+	V9	= T{
+		F1: "hello",
+		// contains filtered or unexported fields
+	}
+	V10	= T{
+		F1:	"hello",
+
+		F4:	"world",
+		// contains filtered or unexported fields
+	}
+
+	// Other miscellaneous declarations
+	V11	= T{
+		t{
+			A: "world",
+			// contains filtered or unexported fields
+		},
+		// contains filtered or unexported fields
+	}
+	V12	= T{
+		F1:	make(chan int),
+
+		F3:	make(map[int]string),
+		// contains filtered or unexported fields
+	}
+)
diff --git a/src/go/printer/testdata/declarations.golden b/src/go/printer/testdata/declarations.golden
new file mode 100644
index 0000000..fe0f783
--- /dev/null
+++ b/src/go/printer/testdata/declarations.golden
@@ -0,0 +1,1008 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package imports
+
+import "io"
+
+import (
+	_ "io"
+)
+
+import _ "io"
+
+import (
+	"io"
+	"io"
+	"io"
+)
+
+import (
+	"io"
+	aLongRename "io"
+
+	b "io"
+)
+
+import (
+	"unrenamed"
+	renamed "renameMe"
+	. "io"
+	_ "io"
+	"io"
+	. "os"
+)
+
+// no newlines between consecutive single imports, but
+// respect extra line breaks in the source (at most one empty line)
+import _ "io"
+import _ "io"
+import _ "io"
+
+import _ "os"
+import _ "os"
+import _ "os"
+
+import _ "fmt"
+import _ "fmt"
+import _ "fmt"
+
+import "foo"	// a comment
+import "bar"	// a comment
+
+import (
+	_ "foo"
+	// a comment
+	"bar"
+	"foo"	// a comment
+	"bar"	// a comment
+)
+
+// comments + renames
+import (
+	"unrenamed"	// a comment
+	renamed "renameMe"
+	. "io"		/* a comment */
+	_ "io/ioutil"	// a comment
+	"io"		// testing alignment
+	. "os"
+	// a comment
+)
+
+// a case that caused problems in the past (comment placement)
+import (
+	. "fmt"
+	"io"
+	"malloc"	// for the malloc count test only
+	"math"
+	"strings"
+	"testing"
+)
+
+// more import examples
+import (
+	"xxx"
+	"much_longer_name"	// comment
+	"short_name"		// comment
+)
+
+import (
+	_ "xxx"
+	"much_longer_name"	// comment
+)
+
+import (
+	mymath "math"
+	"/foo/bar/long_package_path"	// a comment
+)
+
+import (
+	"package_a"	// comment
+	"package_b"
+	my_better_c "package_c"	// comment
+	"package_d"		// comment
+	my_e "package_e"	// comment
+
+	"package_a"	// comment
+	"package_bb"
+	"package_ccc"	// comment
+	"package_dddd"	// comment
+)
+
+// print import paths as double-quoted strings
+// (we would like more test cases but the go/parser
+// already excludes most incorrect paths, and we don't
+// bother setting up test-ASTs manually)
+import (
+	"fmt"
+	"math"
+)
+
+// at least one empty line between declarations of different kind
+import _ "io"
+
+var _ int
+
+// at least one empty line between declarations of the same kind
+// if there is associated documentation (was issue 2570)
+type T1 struct{}
+
+// T2 comment
+type T2 struct {
+}	// should be a two-line struct
+
+// T3 comment
+type T2 struct {
+}	// should be a two-line struct
+
+// printing of constant literals
+const (
+	_	= "foobar"
+	_	= "a۰۱۸"
+	_	= "foo६४"
+	_	= "bar９８７６"
+	_	= 0
+	_	= 1
+	_	= 123456789012345678890
+	_	= 01234567
+	_	= 0xcafebabe
+	_	= 0.
+	_	= .0
+	_	= 3.14159265
+	_	= 1e0
+	_	= 1e+100
+	_	= 1e-100
+	_	= 2.71828e-1000
+	_	= 0i
+	_	= 1i
+	_	= 012345678901234567889i
+	_	= 123456789012345678890i
+	_	= 0.i
+	_	= .0i
+	_	= 3.14159265i
+	_	= 1e0i
+	_	= 1e+100i
+	_	= 1e-100i
+	_	= 2.71828e-1000i
+	_	= 'a'
+	_	= '\000'
+	_	= '\xFF'
+	_	= '\uff16'
+	_	= '\U0000ff16'
+	_	= `foobar`
+	_	= `foo
+---
+---
+bar`
+)
+
+func _() {
+	type _ int
+	type _ *int
+	type _ []int
+	type _ map[string]int
+	type _ chan int
+	type _ func() int
+
+	var _ int
+	var _ *int
+	var _ []int
+	var _ map[string]int
+	var _ chan int
+	var _ func() int
+
+	type _ struct{}
+	type _ *struct{}
+	type _ []struct{}
+	type _ map[string]struct{}
+	type _ chan struct{}
+	type _ func() struct{}
+
+	type _ interface{}
+	type _ *interface{}
+	type _ []interface{}
+	type _ map[string]interface{}
+	type _ chan interface{}
+	type _ func() interface{}
+
+	var _ struct{}
+	var _ *struct{}
+	var _ []struct{}
+	var _ map[string]struct{}
+	var _ chan struct{}
+	var _ func() struct{}
+
+	var _ interface{}
+	var _ *interface{}
+	var _ []interface{}
+	var _ map[string]interface{}
+	var _ chan interface{}
+	var _ func() interface{}
+}
+
+// don't lose blank lines in grouped declarations
+const (
+	_	int	= 0
+	_	float	= 1
+
+	_	string	= "foo"
+
+	_	= iota
+	_
+
+	// a comment
+	_
+
+	_
+)
+
+type (
+	_	int
+	_	struct{}
+
+	_	interface{}
+
+	// a comment
+	_	map[string]int
+)
+
+var (
+	_	int	= 0
+	_	float	= 1
+
+	_	string	= "foo"
+
+	_	bool
+
+	// a comment
+	_	bool
+)
+
+// don't lose blank lines in this struct
+type _ struct {
+	String	struct {
+		Str, Len int
+	}
+	Slice	struct {
+		Array, Len, Cap int
+	}
+	Eface	struct {
+		Typ, Ptr int
+	}
+
+	UncommonType	struct {
+		Name, PkgPath int
+	}
+	CommonType	struct {
+		Size, Hash, Alg, Align, FieldAlign, String, UncommonType int
+	}
+	Type	struct {
+		Typ, Ptr int
+	}
+	StructField	struct {
+		Name, PkgPath, Typ, Tag, Offset int
+	}
+	StructType	struct {
+		Fields int
+	}
+	PtrType	struct {
+		Elem int
+	}
+	SliceType	struct {
+		Elem int
+	}
+	ArrayType	struct {
+		Elem, Len int
+	}
+
+	Stktop	struct {
+		Stackguard, Stackbase, Gobuf int
+	}
+	Gobuf	struct {
+		Sp, Pc, G int
+	}
+	G	struct {
+		Stackbase, Sched, Status, Alllink int
+	}
+}
+
+// no blank lines in empty structs and interfaces, but leave 1- or 2-line layout alone
+type _ struct{}
+type _ struct {
+}
+
+type _ interface{}
+type _ interface {
+}
+
+// no tabs for single or ungrouped decls
+func _() {
+	const xxxxxx = 0
+	type x int
+	var xxx int
+	var yyyy float = 3.14
+	var zzzzz = "bar"
+
+	const (
+		xxxxxx = 0
+	)
+	type (
+		x int
+	)
+	var (
+		xxx int
+	)
+	var (
+		yyyy float = 3.14
+	)
+	var (
+		zzzzz = "bar"
+	)
+}
+
+// tabs for multiple or grouped decls
+func _() {
+	// no entry has a type
+	const (
+		zzzzzz	= 1
+		z	= 2
+		zzz	= 3
+	)
+	// some entries have a type
+	const (
+		xxxxxx			= 1
+		x			= 2
+		xxx			= 3
+		yyyyyyyy	float	= iota
+		yyyy			= "bar"
+		yyy
+		yy	= 2
+	)
+}
+
+func _() {
+	// no entry has a type
+	var (
+		zzzzzz	= 1
+		z	= 2
+		zzz	= 3
+	)
+	// no entry has a value
+	var (
+		_	int
+		_	float
+		_	string
+
+		_	int	// comment
+		_	float	// comment
+		_	string	// comment
+	)
+	// some entries have a type
+	var (
+		xxxxxx		int
+		x		float
+		xxx		string
+		yyyyyyyy	int	= 1234
+		y		float	= 3.14
+		yyyy			= "bar"
+		yyy		string	= "foo"
+	)
+	// mixed entries - all comments should be aligned
+	var (
+		a, b, c			int
+		x			= 10
+		d			int			// comment
+		y				= 20		// comment
+		f, ff, fff, ffff	int	= 0, 1, 2, 3	// comment
+	)
+	// respect original line breaks
+	var _ = []T{
+		T{0x20, "Telugu"},
+	}
+	var _ = []T{
+		// respect original line breaks
+		T{0x20, "Telugu"},
+	}
+}
+
+// use the formatted output rather than the input to decide when to align
+// (was issue 4505)
+const (
+	short		= 2 * (1 + 2)
+	aMuchLongerName	= 3
+)
+
+var (
+	short		= X{}
+	aMuchLongerName	= X{}
+
+	x1	= X{}	// foo
+	x2	= X{}	// foo
+)
+
+func _() {
+	type (
+		xxxxxx	int
+		x	float
+		xxx	string
+		xxxxx	[]x
+		xx	struct{}
+		xxxxxxx	struct {
+			_, _	int
+			_	float
+		}
+		xxxx	chan<- string
+	)
+}
+
+// alignment of "=" in consecutive lines (extended example from issue 1414)
+const (
+	umax	uint	= ^uint(0)		// maximum value for a uint
+	bpu		= 1 << (5 + umax>>63)	// bits per uint
+	foo
+	bar	= -1
+)
+
+// typical enum
+const (
+	a	MyType	= iota
+	abcd
+	b
+	c
+	def
+)
+
+// excerpt from godoc.go
+var (
+	goroot		= flag.String("goroot", runtime.GOROOT(), "Go root directory")
+	testDir		= flag.String("testdir", "", "Go root subdirectory - for testing only (faster startups)")
+	pkgPath		= flag.String("path", "", "additional package directories (colon-separated)")
+	filter		= flag.String("filter", "", "filter file containing permitted package directory paths")
+	filterMin	= flag.Int("filter_minutes", 0, "filter file update interval in minutes; disabled if <= 0")
+	filterDelay	delayTime	// actual filter update interval in minutes; usually filterDelay == filterMin, but filterDelay may back off exponentially
+)
+
+// formatting of structs
+type _ struct{}
+
+type _ struct { /* this comment should be visible */
+}
+
+type _ struct {
+	// this comment should be visible and properly indented
+}
+
+type _ struct {	// this comment must not change indentation
+	f			int
+	f, ff, fff, ffff	int
+}
+
+type _ struct {
+	string
+}
+
+type _ struct {
+	string	// comment
+}
+
+type _ struct {
+	string "tag"
+}
+
+type _ struct {
+	string "tag"	// comment
+}
+
+type _ struct {
+	f int
+}
+
+type _ struct {
+	f int	// comment
+}
+
+type _ struct {
+	f int "tag"
+}
+
+type _ struct {
+	f int "tag"	// comment
+}
+
+type _ struct {
+	bool
+	a, b, c			int
+	int			"tag"
+	ES				// comment
+	float			"tag"	// comment
+	f			int	// comment
+	f, ff, fff, ffff	int	// comment
+	g			float	"tag"
+	h			float	"tag"	// comment
+}
+
+type _ struct {
+	a, b,
+	c, d	int	// this line should be indented
+	u, v, w, x	float	// this line should be indented
+	p, q,
+	r, s	float	// this line should be indented
+}
+
+// difficult cases
+type _ struct {
+	bool		// comment
+	text	[]byte	// comment
+}
+
+// formatting of interfaces
+type EI interface{}
+
+type _ interface {
+	EI
+}
+
+type _ interface {
+	f()
+	fffff()
+}
+
+type _ interface {
+	EI
+	f()
+	fffffg()
+}
+
+type _ interface {	// this comment must not change indentation
+	EI				// here's a comment
+	f()				// no blank between identifier and ()
+	fffff()				// no blank between identifier and ()
+	gggggggggggg(x, y, z int)	// hurray
+}
+
+// formatting of variable declarations
+func _() {
+	type day struct {
+		n		int
+		short, long	string
+	}
+	var (
+		Sunday		= day{0, "SUN", "Sunday"}
+		Monday		= day{1, "MON", "Monday"}
+		Tuesday		= day{2, "TUE", "Tuesday"}
+		Wednesday	= day{3, "WED", "Wednesday"}
+		Thursday	= day{4, "THU", "Thursday"}
+		Friday		= day{5, "FRI", "Friday"}
+		Saturday	= day{6, "SAT", "Saturday"}
+	)
+}
+
+// formatting of multi-line variable declarations
+var a1, b1, c1 int	// all on one line
+
+var a2, b2,
+	c2 int	// this line should be indented
+
+var (
+	a3, b3,
+	c3, d3	int	// this line should be indented
+	a4, b4, c4	int	// this line should be indented
+)
+
+// Test case from issue 3304: multi-line declarations must end
+// a formatting section and not influence indentation of the
+// next line.
+var (
+	minRefreshTimeSec	= flag.Int64("min_refresh_time_sec", 604800,
+		"minimum time window between two refreshes for a given user.")
+	x	= flag.Int64("refresh_user_rollout_percent", 100,
+		"temporary flag to ramp up the refresh user rpc")
+	aVeryLongVariableName	= stats.GetVarInt("refresh-user-count")
+)
+
+func _() {
+	var privateKey2 = &Block{Type: "RSA PRIVATE KEY",
+		Headers:	map[string]string{},
+		Bytes: []uint8{0x30, 0x82, 0x1, 0x3a, 0x2, 0x1, 0x0, 0x2,
+			0x41, 0x0, 0xb2, 0x99, 0xf, 0x49, 0xc4, 0x7d, 0xfa, 0x8c,
+			0xd4, 0x0, 0xae, 0x6a, 0x4d, 0x1b, 0x8a, 0x3b, 0x6a, 0x13,
+			0x64, 0x2b, 0x23, 0xf2, 0x8b, 0x0, 0x3b, 0xfb, 0x97, 0x79,
+		},
+	}
+}
+
+func _() {
+	var Universe = Scope{
+		Names: map[string]*Ident{
+			// basic types
+			"bool":		nil,
+			"byte":		nil,
+			"int8":		nil,
+			"int16":	nil,
+			"int32":	nil,
+			"int64":	nil,
+			"uint8":	nil,
+			"uint16":	nil,
+			"uint32":	nil,
+			"uint64":	nil,
+			"float32":	nil,
+			"float64":	nil,
+			"string":	nil,
+
+			// convenience types
+			"int":		nil,
+			"uint":		nil,
+			"uintptr":	nil,
+			"float":	nil,
+
+			// constants
+			"false":	nil,
+			"true":		nil,
+			"iota":		nil,
+			"nil":		nil,
+
+			// functions
+			"cap":		nil,
+			"len":		nil,
+			"new":		nil,
+			"make":		nil,
+			"panic":	nil,
+			"panicln":	nil,
+			"print":	nil,
+			"println":	nil,
+		},
+	}
+}
+
+// alignment of map composite entries
+var _ = map[int]int{
+	// small key sizes: always align even if size ratios are large
+	a:			a,
+	abcdefghabcdefgh:	a,
+	ab:			a,
+	abc:			a,
+	abcdefgabcdefg:		a,
+	abcd:			a,
+	abcde:			a,
+	abcdef:			a,
+
+	// mixed key sizes: align when key sizes change within accepted ratio
+	abcdefgh:		a,
+	abcdefghabcdefg:	a,
+	abcdefghij:		a,
+	abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij:	a,	// outlier - do not align with previous line
+	abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij:		a,	// align with previous line
+
+	ab:	a,	// do not align with previous line
+	abcde:	a,	// align with previous line
+}
+
+// alignment of map composite entries: test cases from issue 3965
+// aligned
+var _ = T1{
+	a:			x,
+	b:			y,
+	cccccccccccccccccccc:	z,
+}
+
+// not aligned
+var _ = T2{
+	a:			x,
+	b:			y,
+	ccccccccccccccccccccc:	z,
+}
+
+// aligned
+var _ = T3{
+	aaaaaaaaaaaaaaaaaaaa:	x,
+	b:			y,
+	c:			z,
+}
+
+// not aligned
+var _ = T4{
+	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa:	x,
+	b:						y,
+	c:						z,
+}
+
+// no alignment of map composite entries if they are not the first entry on a line
+var _ = T{0: 0}	// not aligned
+var _ = T{0: 0,	// not aligned
+	1:	1,				// aligned
+	22:	22,				// aligned
+	333:	333, 1234: 12, 12345: 0,	// first on line aligned
+}
+
+// test cases form issue 8685
+// not aligned
+var _ = map[int]string{1: "spring", 2: "summer",
+	3:	"autumn", 4: "winter"}
+
+// not aligned
+var _ = map[string]string{"a": "spring", "b": "summer",
+	"c":	"autumn", "d": "winter"}
+
+// aligned
+var _ = map[string]string{"a": "spring",
+	"b":	"summer",
+	"c":	"autumn",
+	"d":	"winter"}
+
+func _() {
+	var _ = T{
+		a,	// must introduce trailing comma
+	}
+}
+
+// formatting of function results
+func _() func()				{}
+func _() func(int)			{ return nil }
+func _() func(int) int			{ return nil }
+func _() func(int) func(int) func()	{ return nil }
+
+// formatting of consecutive single-line functions
+func _()	{}
+func _()	{}
+func _()	{}
+
+func _()	{}	// an empty line before this function
+func _()	{}
+func _()	{}
+
+func _()		{ f(1, 2, 3) }
+func _(x int) int	{ y := x; return y + 1 }
+func _() int		{ type T struct{}; var x T; return x }
+
+// these must remain multi-line since they are multi-line in the source
+func _() {
+	f(1, 2, 3)
+}
+func _(x int) int {
+	y := x
+	return y + 1
+}
+func _() int {
+	type T struct{}
+	var x T
+	return x
+}
+
+// making function declarations safe for new semicolon rules
+func _()	{ /* single-line function because of "short-ish" comment */ }
+func _() { /* multi-line function because of "long-ish" comment - much more comment text is following here */ /* and more */
+}
+
+func _() {
+	/* multi-line func because block is on multiple lines */
+}
+
+// test case for issue #19544
+func _()	{}
+func _longer_name_() {	// this comment must not force the {} from above to alignment
+	// multiple lines
+}
+
+// ellipsis parameters
+func _(...int)
+func _(...*int)
+func _(...[]int)
+func _(...struct{})
+func _(bool, ...interface{})
+func _(bool, ...func())
+func _(bool, ...func(...int))
+func _(bool, ...map[string]int)
+func _(bool, ...chan int)
+
+func _(b bool, x ...int)
+func _(b bool, x ...*int)
+func _(b bool, x ...[]int)
+func _(b bool, x ...struct{})
+func _(x ...interface{})
+func _(x ...func())
+func _(x ...func(...int))
+func _(x ...map[string]int)
+func _(x ...chan int)
+
+// these parameter lists must remain multi-line since they are multi-line in the source
+func _(bool,
+	int) {
+}
+func _(x bool,
+	y int) {
+}
+func _(x,
+	y bool) {
+}
+func _(bool,	// comment
+	int) {
+}
+func _(x bool,	// comment
+	y int) {
+}
+func _(x,	// comment
+	y bool) {
+}
+func _(bool,	// comment
+	// comment
+	int) {
+}
+func _(x bool,	// comment
+	// comment
+	y int) {
+}
+func _(x,	// comment
+	// comment
+	y bool) {
+}
+func _(bool,
+	// comment
+	int) {
+}
+func _(x bool,
+	// comment
+	y int) {
+}
+func _(x,
+	// comment
+	y bool) {
+}
+func _(x,	// comment
+	y,	// comment
+	z bool) {
+}
+func _(x,	// comment
+	y,	// comment
+	z bool) {
+}
+func _(x int,	// comment
+	y float,	// comment
+	z bool) {
+}
+
+// properly indent multi-line signatures
+func ManageStatus(in <-chan *Status, req <-chan Request,
+	stat chan<- *TargetInfo,
+	TargetHistorySize int) {
+}
+
+func MultiLineSignature0(
+	a, b, c int,
+) {
+}
+
+func MultiLineSignature1(
+	a, b, c int,
+	u, v, w float,
+) {
+}
+
+func MultiLineSignature2(
+	a, b,
+	c int,
+) {
+}
+
+func MultiLineSignature3(
+	a, b,
+	c int, u, v,
+	w float,
+	x ...int) {
+}
+
+func MultiLineSignature4(
+	a, b, c int,
+	u, v,
+	w float,
+	x ...int) {
+}
+
+func MultiLineSignature5(
+	a, b, c int,
+	u, v, w float,
+	p, q,
+	r string,
+	x ...int) {
+}
+
+// make sure it also works for methods in interfaces
+type _ interface {
+	MultiLineSignature0(
+		a, b, c int,
+	)
+
+	MultiLineSignature1(
+		a, b, c int,
+		u, v, w float,
+	)
+
+	MultiLineSignature2(
+		a, b,
+		c int,
+	)
+
+	MultiLineSignature3(
+		a, b,
+		c int, u, v,
+		w float,
+		x ...int)
+
+	MultiLineSignature4(
+		a, b, c int,
+		u, v,
+		w float,
+		x ...int)
+
+	MultiLineSignature5(
+		a, b, c int,
+		u, v, w float,
+		p, q,
+		r string,
+		x ...int)
+}
+
+// omit superfluous parentheses in parameter lists
+func _(int)
+func _(int)
+func _(x int)
+func _(x int)
+func _(x, y int)
+func _(x, y int)
+
+func _() int
+func _() int
+func _() int
+
+func _() (x int)
+func _() (x int)
+func _() (x int)
+
+// special cases: some channel types require parentheses
+func _(x chan (<-chan int))
+func _(x chan (<-chan int))
+func _(x chan (<-chan int))
+
+func _(x chan<- (chan int))
+func _(x chan<- (chan int))
+func _(x chan<- (chan int))
+
+// don't introduce comma after last parameter if the closing ) is on the same line
+// even if the parameter type itself is multi-line (test cases from issue 4533)
+func _(...interface{})
+func _(...interface {
+	m()
+	n()
+})	// no extra comma between } and )
+
+func (t *T) _(...interface{})
+func (t *T) _(...interface {
+	m()
+	n()
+})	// no extra comma between } and )
+
+func _(interface{})
+func _(interface {
+	m()
+})	// no extra comma between } and )
+
+func _(struct{})
+func _(struct {
+	x	int
+	y	int
+})	// no extra comma between } and )
+
+// alias declarations
+
+type c0 struct{}
+type c1 = C
+type c2 = struct{ x int }
+type c3 = p.C
+type (
+	s	struct{}
+	a	= A
+	b	= A
+	c	= foo
+	d	= interface{}
+	ddd	= p.Foo
+)
diff --git a/src/go/printer/testdata/declarations.input b/src/go/printer/testdata/declarations.input
new file mode 100644
index 0000000..a858051
--- /dev/null
+++ b/src/go/printer/testdata/declarations.input
@@ -0,0 +1,1021 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package imports
+
+import "io"
+
+import (
+	_ "io"
+)
+
+import _ "io"
+
+import (
+	"io"
+	"io"
+	"io"
+)
+
+import (
+	"io"
+	aLongRename "io"
+
+	b "io"
+)
+
+import (
+       "unrenamed"
+       renamed "renameMe"
+       . "io"
+       _ "io"
+       "io"
+       . "os"
+)
+
+// no newlines between consecutive single imports, but
+// respect extra line breaks in the source (at most one empty line)
+import _ "io"
+import _ "io"
+import _ "io"
+
+import _ "os"
+import _ "os"
+import _ "os"
+
+
+import _ "fmt"
+import _ "fmt"
+import _ "fmt"
+
+import "foo"  // a comment
+import "bar"  // a comment
+
+import (
+	_ "foo"
+	// a comment
+	"bar"
+	"foo"  // a comment
+	"bar"  // a comment
+)
+
+// comments + renames
+import (
+       "unrenamed" // a comment
+       renamed "renameMe"
+       . "io" /* a comment */
+       _ "io/ioutil" // a comment
+       "io" // testing alignment
+       . "os"
+       // a comment
+)
+
+// a case that caused problems in the past (comment placement)
+import (
+	. "fmt"
+	"io"
+	"malloc"	// for the malloc count test only
+	"math"
+	"strings"
+	"testing"
+)
+
+// more import examples
+import (
+	"xxx"
+	"much_longer_name" // comment
+	"short_name" // comment
+)
+
+import (
+	_ "xxx"
+	"much_longer_name" // comment
+)
+
+import (
+	mymath "math"
+	"/foo/bar/long_package_path" // a comment
+)
+
+import (
+	"package_a" // comment
+	"package_b"
+	my_better_c "package_c" // comment
+	"package_d" // comment
+	my_e "package_e" // comment
+
+	"package_a"    // comment
+	"package_bb"
+	"package_ccc"  // comment
+	"package_dddd" // comment
+)
+
+// print import paths as double-quoted strings
+// (we would like more test cases but the go/parser
+// already excludes most incorrect paths, and we don't
+// bother setting up test-ASTs manually)
+import (
+	`fmt`
+	"math"
+)
+
+// at least one empty line between declarations of different kind
+import _ "io"
+var _ int
+
+// at least one empty line between declarations of the same kind
+// if there is associated documentation (was issue 2570)
+type T1 struct{}
+// T2 comment
+type T2 struct {
+} // should be a two-line struct
+
+
+// T3 comment
+type T2 struct {
+
+
+} // should be a two-line struct
+
+
+// printing of constant literals
+const (
+	_ = "foobar"
+	_ = "a۰۱۸"
+	_ = "foo६४"
+	_ = "bar９８７６"
+	_ = 0
+	_ = 1
+	_ = 123456789012345678890
+	_ = 01234567
+	_ = 0xcafebabe
+	_ = 0.
+	_ = .0
+	_ = 3.14159265
+	_ = 1e0
+	_ = 1e+100
+	_ = 1e-100
+	_ = 2.71828e-1000
+	_ = 0i
+	_ = 1i
+	_ = 012345678901234567889i
+	_ = 123456789012345678890i
+	_ = 0.i
+	_ = .0i
+	_ = 3.14159265i
+	_ = 1e0i
+	_ = 1e+100i
+	_ = 1e-100i
+	_ = 2.71828e-1000i
+	_ = 'a'
+	_ = '\000'
+	_ = '\xFF'
+	_ = '\uff16'
+	_ = '\U0000ff16'
+	_ = `foobar`
+	_ = `foo
+---
+---
+bar`
+)
+
+
+func _() {
+	type _ int
+	type _ *int
+	type _ []int
+	type _ map[string]int
+	type _ chan int
+	type _ func() int
+
+	var _ int
+	var _ *int
+	var _ []int
+	var _ map[string]int
+	var _ chan int
+	var _ func() int
+
+	type _ struct{}
+	type _ *struct{}
+	type _ []struct{}
+	type _ map[string]struct{}
+	type _ chan struct{}
+	type _ func() struct{}
+
+	type _ interface{}
+	type _ *interface{}
+	type _ []interface{}
+	type _ map[string]interface{}
+	type _ chan interface{}
+	type _ func() interface{}
+
+	var _ struct{}
+	var _ *struct{}
+	var _ []struct{}
+	var _ map[string]struct{}
+	var _ chan struct{}
+	var _ func() struct{}
+
+	var _ interface{}
+	var _ *interface{}
+	var _ []interface{}
+	var _ map[string]interface{}
+	var _ chan interface{}
+	var _ func() interface{}
+}
+
+
+// don't lose blank lines in grouped declarations
+const (
+	_ int = 0
+	_ float = 1
+
+	_ string = "foo"
+
+	_ = iota
+	_
+	
+	// a comment
+	_
+
+	_
+)
+
+
+type (
+	_ int
+	_ struct {}
+	
+	_ interface{}
+	
+	// a comment
+	_ map[string]int
+)
+
+
+var (
+	_ int = 0
+	_ float = 1
+
+	_ string = "foo"
+
+	_ bool
+	
+	// a comment
+	_ bool
+)
+
+
+// don't lose blank lines in this struct
+type _ struct {
+	String struct {
+		Str, Len int
+	}
+	Slice struct {
+		Array, Len, Cap int
+	}
+	Eface struct {
+		Typ, Ptr int
+	}
+
+	UncommonType struct {
+		Name, PkgPath int
+	}
+	CommonType struct {
+		Size, Hash, Alg, Align, FieldAlign, String, UncommonType int
+	}
+	Type struct {
+		Typ, Ptr int
+	}
+	StructField struct {
+		Name, PkgPath, Typ, Tag, Offset int
+	}
+	StructType struct {
+		Fields int
+	}
+	PtrType struct {
+		Elem int
+	}
+	SliceType struct {
+		Elem int
+	}
+	ArrayType struct {
+		Elem, Len int
+	}
+
+	Stktop struct {
+		Stackguard, Stackbase, Gobuf int
+	}
+	Gobuf struct {
+		Sp, Pc, G int
+	}
+	G struct {
+		Stackbase, Sched, Status, Alllink int
+	}
+}
+
+
+// no blank lines in empty structs and interfaces, but leave 1- or 2-line layout alone
+type _ struct{            }
+type _ struct {
+
+}
+
+type _ interface{            }
+type _ interface {
+
+}
+
+
+// no tabs for single or ungrouped decls
+func _() {
+	const xxxxxx = 0
+	type x int
+	var xxx int
+	var yyyy float = 3.14
+	var zzzzz = "bar"
+
+	const (
+		xxxxxx = 0
+	)
+	type (
+		x int
+	)
+	var (
+		xxx int
+	)
+	var (
+		yyyy float = 3.14
+	)
+	var (
+		zzzzz = "bar"
+	)
+}
+
+// tabs for multiple or grouped decls
+func _() {
+	// no entry has a type
+	const (
+		zzzzzz = 1
+		z = 2
+		zzz = 3
+	)
+	// some entries have a type
+	const (
+		xxxxxx = 1
+		x = 2
+		xxx = 3
+		yyyyyyyy float = iota
+		yyyy = "bar"
+		yyy
+		yy = 2
+	)
+}
+
+func _() {
+	// no entry has a type
+	var (
+		zzzzzz = 1
+		z = 2
+		zzz = 3
+	)
+	// no entry has a value
+	var (
+		_ int
+		_ float
+		_ string
+
+		_ int  // comment
+		_ float  // comment
+		_ string  // comment
+	)
+	// some entries have a type
+	var (
+		xxxxxx int
+		x float
+		xxx string
+		yyyyyyyy int = 1234
+		y float = 3.14
+		yyyy = "bar"
+		yyy string = "foo"
+	)
+	// mixed entries - all comments should be aligned
+	var (
+		a, b, c int
+		x = 10
+		d int  // comment
+		y = 20  // comment
+		f, ff, fff, ffff int = 0, 1, 2, 3  // comment
+	)
+	// respect original line breaks
+	var _ = []T {
+		T{0x20,	"Telugu"},
+	}
+	var _ = []T {
+		// respect original line breaks
+		T{0x20,	"Telugu"},
+	}
+}
+
+// use the formatted output rather than the input to decide when to align
+// (was issue 4505)
+const (
+	short = 2 * (
+	1 + 2)
+	aMuchLongerName = 3
+)
+
+var (
+	short = X{
+	}
+	aMuchLongerName = X{}
+
+	x1 = X{} // foo
+	x2 = X{
+	} // foo
+)
+
+func _() {
+	type (
+		xxxxxx int
+		x float
+		xxx string
+		xxxxx []x
+		xx struct{}
+		xxxxxxx struct {
+			_, _ int
+			_ float
+		}
+		xxxx chan<- string
+	)
+}
+
+// alignment of "=" in consecutive lines (extended example from issue 1414)
+const (
+	umax uint                  = ^uint(0) // maximum value for a uint
+	bpu  = 1 << (5 + umax>>63)            // bits per uint
+	foo
+	bar  = -1
+)
+
+// typical enum
+const (
+	a MyType = iota
+	abcd
+	b
+	c
+	def
+)
+
+// excerpt from godoc.go
+var (
+	goroot = flag.String("goroot", runtime.GOROOT(), "Go root directory")
+	testDir = flag.String("testdir", "", "Go root subdirectory - for testing only (faster startups)")
+	pkgPath = flag.String("path", "", "additional package directories (colon-separated)")
+	filter = flag.String("filter", "", "filter file containing permitted package directory paths")
+	filterMin = flag.Int("filter_minutes", 0, "filter file update interval in minutes; disabled if <= 0")
+	filterDelay delayTime // actual filter update interval in minutes; usually filterDelay == filterMin, but filterDelay may back off exponentially
+)
+
+
+// formatting of structs
+type _ struct{}
+
+type _ struct{ /* this comment should be visible */ }
+
+type _ struct{
+	// this comment should be visible and properly indented
+}
+
+type _ struct {  // this comment must not change indentation
+	f int
+	f, ff, fff, ffff int
+}
+
+type _ struct {
+	string
+}
+
+type _ struct {
+	string  // comment
+}
+
+type _ struct {
+	string "tag"
+}
+
+type _ struct {
+	string "tag"  // comment
+}
+
+type _ struct {
+	f int
+}
+
+type _ struct {
+	f int  // comment
+}
+
+type _ struct {
+	f int "tag"
+}
+
+type _ struct {
+	f int "tag"  // comment
+}
+
+type _ struct {
+	bool
+	a, b, c int
+	int "tag"
+	ES // comment
+	float "tag"  // comment
+	f int  // comment
+	f, ff, fff, ffff int  // comment
+	g float "tag"
+	h float "tag"  // comment
+}
+
+type _ struct { a, b,
+c, d int  // this line should be indented
+u, v, w, x float // this line should be indented
+p, q,
+r, s float // this line should be indented
+}
+
+
+// difficult cases
+type _ struct {
+	bool  // comment
+	text []byte  // comment
+}
+
+
+// formatting of interfaces
+type EI interface{}
+
+type _ interface {
+	EI
+}
+
+type _ interface {
+	f()
+	fffff()
+}
+
+type _ interface {
+	EI
+	f()
+	fffffg()
+}
+
+type _ interface {  // this comment must not change indentation
+	EI  // here's a comment
+	f()  // no blank between identifier and ()
+	fffff()  // no blank between identifier and ()
+	gggggggggggg(x, y, z int) ()  // hurray
+}
+
+
+// formatting of variable declarations
+func _() {
+	type day struct { n int; short, long string }
+	var (
+		Sunday = day{ 0, "SUN", "Sunday" }
+		Monday = day{ 1, "MON", "Monday" }
+		Tuesday = day{ 2, "TUE", "Tuesday" }
+		Wednesday = day{ 3, "WED", "Wednesday" }
+		Thursday = day{ 4, "THU", "Thursday" }
+		Friday = day{ 5, "FRI", "Friday" }
+		Saturday = day{ 6, "SAT", "Saturday" }
+	)
+}
+
+
+// formatting of multi-line variable declarations
+var a1, b1, c1 int  // all on one line
+
+var a2, b2,
+c2 int  // this line should be indented
+
+var (a3, b3,
+c3, d3 int  // this line should be indented
+a4, b4, c4 int  // this line should be indented
+)
+
+// Test case from issue 3304: multi-line declarations must end
+// a formatting section and not influence indentation of the
+// next line.
+var (
+	minRefreshTimeSec = flag.Int64("min_refresh_time_sec", 604800,
+		"minimum time window between two refreshes for a given user.")
+	x = flag.Int64("refresh_user_rollout_percent", 100,
+		"temporary flag to ramp up the refresh user rpc")
+	aVeryLongVariableName = stats.GetVarInt("refresh-user-count")
+)
+
+func _() {
+	var privateKey2 = &Block{Type: "RSA PRIVATE KEY",
+					Headers: map[string]string{},
+					Bytes: []uint8{0x30, 0x82, 0x1, 0x3a, 0x2, 0x1, 0x0, 0x2,
+			0x41, 0x0, 0xb2, 0x99, 0xf, 0x49, 0xc4, 0x7d, 0xfa, 0x8c,
+			0xd4, 0x0, 0xae, 0x6a, 0x4d, 0x1b, 0x8a, 0x3b, 0x6a, 0x13,
+			0x64, 0x2b, 0x23, 0xf2, 0x8b, 0x0, 0x3b, 0xfb, 0x97, 0x79,
+		},
+	}
+}
+
+
+func _() {
+	var Universe = Scope {
+		Names: map[string]*Ident {
+			// basic types
+			"bool": nil,
+			"byte": nil,
+			"int8": nil,
+			"int16": nil,
+			"int32": nil,
+			"int64": nil,
+			"uint8": nil,
+			"uint16": nil,
+			"uint32": nil,
+			"uint64": nil,
+			"float32": nil,
+			"float64": nil,
+			"string": nil,
+
+			// convenience types
+			"int": nil,
+			"uint": nil,
+			"uintptr": nil,
+			"float": nil,
+
+			// constants
+			"false": nil,
+			"true": nil,
+			"iota": nil,
+			"nil": nil,
+
+			// functions
+			"cap": nil,
+			"len": nil,
+			"new": nil,
+			"make": nil,
+			"panic": nil,
+			"panicln": nil,
+			"print": nil,
+			"println": nil,
+		},
+	}
+}
+
+
+// alignment of map composite entries
+var _ = map[int]int{
+	// small key sizes: always align even if size ratios are large
+	a: a,
+	abcdefghabcdefgh: a,
+	ab: a,
+	abc: a,
+	abcdefgabcdefg: a,
+	abcd: a,
+	abcde: a,
+	abcdef: a,
+
+	// mixed key sizes: align when key sizes change within accepted ratio
+	abcdefgh: a,
+	abcdefghabcdefg: a,
+	abcdefghij: a,
+	abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij: a, // outlier - do not align with previous line
+	abcdefghijabcdefghijabcdefghijabcdefghijabcdefghijabcdefghij: a, // align with previous line
+
+	ab: a, // do not align with previous line
+	abcde: a, // align with previous line
+}
+
+// alignment of map composite entries: test cases from issue 3965
+// aligned
+var _ = T1{
+	a:                    x,
+	b:                    y,
+	cccccccccccccccccccc: z,
+}
+
+// not aligned
+var _ = T2{
+	a: x,
+	b: y,
+	ccccccccccccccccccccc: z,
+}
+
+// aligned
+var _ = T3{
+	aaaaaaaaaaaaaaaaaaaa: x,
+	b:                    y,
+	c:                    z,
+}
+
+// not aligned
+var _ = T4{
+	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa: x,
+	b:                                       y,
+	c:                                       z,
+}
+
+
+// no alignment of map composite entries if they are not the first entry on a line
+var _ = T{0: 0} // not aligned
+var _ = T{0: 0, // not aligned
+	1: 1, // aligned
+	22: 22, // aligned
+	333: 333, 1234: 12, 12345: 0, // first on line aligned
+}
+
+
+// test cases form issue 8685
+// not aligned
+var _ = map[int]string{1: "spring", 2: "summer",
+					3:             "autumn", 4: "winter"}
+
+// not aligned
+var _ = map[string]string{"a": "spring", "b": "summer",
+	"c": "autumn", "d": "winter"}
+
+// aligned
+var _ = map[string]string{"a": "spring",
+"b": "summer",
+	"c": "autumn",
+"d": "winter"}
+
+
+func _() {
+	var _ = T{
+		a,	// must introduce trailing comma
+	}
+}
+
+
+// formatting of function results
+func _() func() {}
+func _() func(int) { return nil }
+func _() func(int) int { return nil }
+func _() func(int) func(int) func() { return nil }
+
+
+// formatting of consecutive single-line functions
+func _() {}
+func _() {}
+func _() {}
+
+func _() {}  // an empty line before this function
+func _() {}
+func _() {}
+
+func _() { f(1, 2, 3) }
+func _(x int) int { y := x; return y+1 }
+func _() int { type T struct{}; var x T; return x }
+
+// these must remain multi-line since they are multi-line in the source
+func _() {
+	f(1, 2, 3)
+}
+func _(x int) int {
+	y := x; return y+1
+}
+func _() int {
+	type T struct{}; var x T; return x
+}
+
+
+// making function declarations safe for new semicolon rules
+func _() { /* single-line function because of "short-ish" comment */ }
+func _() { /* multi-line function because of "long-ish" comment - much more comment text is following here */ /* and more */ }
+
+func _() {
+/* multi-line func because block is on multiple lines */ }
+
+// test case for issue #19544
+func _() {}
+func _longer_name_() { // this comment must not force the {} from above to alignment
+	// multiple lines
+}
+
+// ellipsis parameters
+func _(...int)
+func _(...*int)
+func _(...[]int)
+func _(...struct{})
+func _(bool, ...interface{})
+func _(bool, ...func())
+func _(bool, ...func(...int))
+func _(bool, ...map[string]int)
+func _(bool, ...chan int)
+
+func _(b bool, x ...int)
+func _(b bool, x ...*int)
+func _(b bool, x ...[]int)
+func _(b bool, x ...struct{})
+func _(x ...interface{})
+func _(x ...func())
+func _(x ...func(...int))
+func _(x ...map[string]int)
+func _(x ...chan int)
+
+
+// these parameter lists must remain multi-line since they are multi-line in the source
+func _(bool,
+int) {
+}
+func _(x bool,
+y int) {
+}
+func _(x,
+y bool) {
+}
+func _(bool, // comment
+int) {
+}
+func _(x bool, // comment
+y int) {
+}
+func _(x, // comment
+y bool) {
+}
+func _(bool, // comment
+// comment
+int) {
+}
+func _(x bool, // comment
+// comment
+y int) {
+}
+func _(x, // comment
+// comment
+y bool) {
+}
+func _(bool,
+// comment
+int) {
+}
+func _(x bool,
+// comment
+y int) {
+}
+func _(x,
+// comment
+y bool) {
+}
+func _(x, // comment
+y,// comment
+z bool) {
+}
+func _(x, // comment
+	y,// comment
+	z bool) {
+}
+func _(x int,	// comment
+	y float,	// comment
+	z bool) {
+}
+
+
+// properly indent multi-line signatures
+func ManageStatus(in <-chan *Status, req <-chan Request,
+stat chan<- *TargetInfo,
+TargetHistorySize int) {
+}
+
+func MultiLineSignature0(
+a, b, c int,
+) {}
+
+func MultiLineSignature1(
+a, b, c int,
+u, v, w float,
+) {}
+
+func MultiLineSignature2(
+a, b,
+c int,
+) {}
+
+func MultiLineSignature3(
+a, b,
+c int, u, v,
+w float,
+		x ...int) {}
+
+func MultiLineSignature4(
+a, b, c int,
+u, v,
+w float,
+		x ...int) {}
+
+func MultiLineSignature5(
+a, b, c int,
+u, v, w float,
+p, q,
+r string,
+		x ...int) {}
+
+// make sure it also works for methods in interfaces
+type _ interface {
+MultiLineSignature0(
+a, b, c int,
+)
+
+MultiLineSignature1(
+a, b, c int,
+u, v, w float,
+)
+
+MultiLineSignature2(
+a, b,
+c int,
+)
+
+MultiLineSignature3(
+a, b,
+c int, u, v,
+w float,
+		x ...int)
+
+MultiLineSignature4(
+a, b, c int,
+u, v,
+w float,
+		x ...int)
+
+MultiLineSignature5(
+a, b, c int,
+u, v, w float,
+p, q,
+r string,
+		x ...int)
+}
+
+// omit superfluous parentheses in parameter lists
+func _((int))
+func _((((((int))))))
+func _(x (int))
+func _(x (((((int))))))
+func _(x, y (int))
+func _(x, y (((((int))))))
+
+func _() (int)
+func _() ((int))
+func _() ((((((int))))))
+
+func _() (x int)
+func _() (x (int))
+func _() (x (((((int))))))
+
+// special cases: some channel types require parentheses
+func _(x chan(<-chan int))
+func _(x (chan(<-chan int)))
+func _(x ((((chan(<-chan int))))))
+
+func _(x chan<-(chan int))
+func _(x (chan<-(chan int)))
+func _(x ((((chan<-(chan int))))))
+
+// don't introduce comma after last parameter if the closing ) is on the same line
+// even if the parameter type itself is multi-line (test cases from issue 4533)
+func _(...interface{})
+func _(...interface {
+	m()
+	n()
+}) // no extra comma between } and )
+
+func (t *T) _(...interface{})
+func (t *T) _(...interface {
+	m()
+	n()
+}) // no extra comma between } and )
+
+func _(interface{})
+func _(interface {
+	m()
+}) // no extra comma between } and )
+
+func _(struct{})
+func _(struct {
+	x int
+	y int
+}) // no extra comma between } and )
+
+// alias declarations
+
+type c0 struct{}
+type c1 = C
+type c2 = struct{ x int}
+type c3 = p.C
+type (
+	s struct{}
+	a = A
+	b = A
+	c = foo
+	d = interface{}
+	ddd = p.Foo
+)
\ No newline at end of file
diff --git a/src/go/printer/testdata/empty.golden b/src/go/printer/testdata/empty.golden
new file mode 100644
index 0000000..a055f47
--- /dev/null
+++ b/src/go/printer/testdata/empty.golden
@@ -0,0 +1,5 @@
+// a comment at the beginning of the file
+
+package empty
+
+// a comment at the end of the file
diff --git a/src/go/printer/testdata/empty.input b/src/go/printer/testdata/empty.input
new file mode 100644
index 0000000..a055f47
--- /dev/null
+++ b/src/go/printer/testdata/empty.input
@@ -0,0 +1,5 @@
+// a comment at the beginning of the file
+
+package empty
+
+// a comment at the end of the file
diff --git a/src/go/printer/testdata/expressions.golden b/src/go/printer/testdata/expressions.golden
new file mode 100644
index 0000000..16a68c7
--- /dev/null
+++ b/src/go/printer/testdata/expressions.golden
@@ -0,0 +1,743 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package expressions
+
+type T struct {
+	x, y, z int
+}
+
+var (
+	a, b, c, d, e						int
+	under_bar						int
+	longIdentifier1, longIdentifier2, longIdentifier3	int
+	t0, t1, t2						T
+	s							string
+	p							*int
+)
+
+func _() {
+	// no spaces around simple or parenthesized expressions
+	_ = (a + 0)
+	_ = a + b
+	_ = a + b + c
+	_ = a + b - c
+	_ = a - b - c
+	_ = a + (b * c)
+	_ = a + (b / c)
+	_ = a - (b % c)
+	_ = 1 + a
+	_ = a + 1
+	_ = a + b + 1
+	_ = s[a]
+	_ = s[a:]
+	_ = s[:b]
+	_ = s[1:2]
+	_ = s[a:b]
+	_ = s[0:len(s)]
+	_ = s[0] << 1
+	_ = (s[0] << 1) & 0xf
+	_ = s[0]<<2 | s[1]>>4
+	_ = "foo" + s
+	_ = s + "foo"
+	_ = 'a' + 'b'
+	_ = len(s) / 2
+	_ = len(t0.x) / a
+
+	// spaces around expressions of different precedence or expressions containing spaces
+	_ = a + -b
+	_ = a - ^b
+	_ = a / *p
+	_ = a + b*c
+	_ = 1 + b*c
+	_ = a + 2*c
+	_ = a + c*2
+	_ = 1 + 2*3
+	_ = s[1 : 2*3]
+	_ = s[a : b-c]
+	_ = s[0:]
+	_ = s[a+b]
+	_ = s[:b-c]
+	_ = s[a+b:]
+	_ = a[a<<b+1]
+	_ = a[a<<b+1:]
+	_ = s[a+b : len(s)]
+	_ = s[len(s):-a]
+	_ = s[a : len(s)+1]
+	_ = s[a:len(s)+1] + s
+
+	// spaces around operators with equal or lower precedence than comparisons
+	_ = a == b
+	_ = a != b
+	_ = a > b
+	_ = a >= b
+	_ = a < b
+	_ = a <= b
+	_ = a < b && c > d
+	_ = a < b || c > d
+
+	// spaces around "long" operands
+	_ = a + longIdentifier1
+	_ = longIdentifier1 + a
+	_ = longIdentifier1 + longIdentifier2*longIdentifier3
+	_ = s + "a longer string"
+
+	// some selected cases
+	_ = a + t0.x
+	_ = a + t0.x + t1.x*t2.x
+	_ = a + b + c + d + e + 2*3
+	_ = a + b + c + 2*3 + d + e
+	_ = (a + b + c) * 2
+	_ = a - b + c - d + (a + b + c) + d&e
+	_ = under_bar - 1
+	_ = Open(dpath+"/file", O_WRONLY|O_CREAT, 0666)
+	_ = int(c0&_Mask4)<<18 | int(c1&_Maskx)<<12 | int(c2&_Maskx)<<6 | int(c3&_Maskx)
+
+	// test case for issue 8021
+	// want:
+	//  ([]bool{})[([]int{})[((1)+(((1)+((((1)*(((1)+(1))+(1)))+(1))*(1)))+(1)))]]
+	_ = ([]bool{})[([]int{})[((1)+(((1)+((((1)*(((1)+(1))+(1)))+(1))*(1)))+(1)))]]
+
+	// the parser does not restrict expressions that may appear as statements
+	true
+	42
+	"foo"
+	x
+	(x)
+	a + b
+	a + b + c
+	a + (b * c)
+	a + (b / c)
+	1 + a
+	a + 1
+	s[a]
+	x << 1
+	(s[0] << 1) & 0xf
+	"foo" + s
+	x == y
+	x < y || z > 42
+}
+
+// slice expressions with cap
+func _() {
+	_ = x[a:b:c]
+	_ = x[a : b : c+d]
+	_ = x[a : b+d : c]
+	_ = x[a : b+d : c+d]
+	_ = x[a+d : b : c]
+	_ = x[a+d : b : c+d]
+	_ = x[a+d : b+d : c]
+	_ = x[a+d : b+d : c+d]
+
+	_ = x[:b:c]
+	_ = x[: b : c+d]
+	_ = x[: b+d : c]
+	_ = x[: b+d : c+d]
+}
+
+func issue22111() {
+	_ = x[:]
+
+	_ = x[:b]
+	_ = x[:b+1]
+
+	_ = x[a:]
+	_ = x[a+1:]
+
+	_ = x[a:b]
+	_ = x[a+1 : b]
+	_ = x[a : b+1]
+	_ = x[a+1 : b+1]
+
+	_ = x[:b:c]
+	_ = x[: b+1 : c]
+	_ = x[: b : c+1]
+	_ = x[: b+1 : c+1]
+
+	_ = x[a:b:c]
+	_ = x[a+1 : b : c]
+	_ = x[a : b+1 : c]
+	_ = x[a+1 : b+1 : c]
+	_ = x[a : b : c+1]
+	_ = x[a+1 : b : c+1]
+	_ = x[a : b+1 : c+1]
+	_ = x[a+1 : b+1 : c+1]
+}
+
+func _() {
+	_ = a + b
+	_ = a + b + c
+	_ = a + b*c
+	_ = a + (b * c)
+	_ = (a + b) * c
+	_ = a + (b * c * d)
+	_ = a + (b*c + d)
+
+	_ = 1 << x
+	_ = -1 << x
+	_ = 1<<x - 1
+	_ = -1<<x - 1
+
+	_ = f(a + b)
+	_ = f(a + b + c)
+	_ = f(a + b*c)
+	_ = f(a + (b * c))
+	_ = f(1<<x-1, 1<<x-2)
+
+	_ = 1<<d.logWindowSize - 1
+
+	buf = make(x, 2*cap(b.buf)+n)
+
+	dst[i*3+2] = dbuf[0] << 2
+	dst[i*3+2] = dbuf[0]<<2 | dbuf[1]>>4
+
+	b.buf = b.buf[0 : b.off+m+n]
+	b.buf = b.buf[0 : b.off+m*n]
+	f(b.buf[0 : b.off+m+n])
+
+	signed += ' ' * 8
+	tw.octal(header[148:155], chksum)
+
+	_ = x > 0 && i >= 0
+
+	x1, x0 := x>>w2, x&m2
+	z0 = t1<<w2 + t0
+	z1 = (t1 + t0>>w2) >> w2
+	q1, r1 := x1/d1, x1%d1
+	r1 = r1*b2 | x0>>w2
+	x1 = (x1 << z) | (x0 >> (uint(w) - z))
+	x1 = x1<<z | x0>>(uint(w)-z)
+
+	_ = buf[0 : len(buf)+1]
+	_ = buf[0 : n+1]
+
+	a, b = b, a
+	a = b + c
+	a = b*c + d
+	_ = a*b + c
+	_ = a - b - c
+	_ = a - (b - c)
+	_ = a - b*c
+	_ = a - (b * c)
+	_ = a * b / c
+	_ = a / *b
+	_ = x[a|^b]
+	_ = x[a / *b]
+	_ = a & ^b
+	_ = a + +b
+	_ = a - -b
+	_ = x[a*-b]
+	_ = x[a + +b]
+	_ = x ^ y ^ z
+	_ = b[a>>24] ^ b[(a>>16)&0xFF] ^ b[(a>>8)&0xFF] ^ b[a&0xFF]
+	_ = len(longVariableName) * 2
+
+	_ = token(matchType + xlength<<lengthShift + xoffset)
+}
+
+func f(x int, args ...int) {
+	f(0, args...)
+	f(1, args)
+	f(2, args[0])
+
+	// make sure syntactically legal code remains syntactically legal
+	f(3, 42 ...)	// a blank must remain between 42 and ...
+	f(4, 42....)
+	f(5, 42....)
+	f(6, 42.0...)
+	f(7, 42.0...)
+	f(8, .42...)
+	f(9, .42...)
+	f(10, 42e0...)
+	f(11, 42e0...)
+
+	_ = 42 .x	// a blank must remain between 42 and .x
+	_ = 42..x
+	_ = 42..x
+	_ = 42.0.x
+	_ = 42.0.x
+	_ = .42.x
+	_ = .42.x
+	_ = 42e0.x
+	_ = 42e0.x
+
+	// a blank must remain between the binary operator and the 2nd operand
+	_ = x / *y
+	_ = x < -1
+	_ = x < <-1
+	_ = x + +1
+	_ = x - -1
+	_ = x & &x
+	_ = x & ^x
+
+	_ = f(x / *y, x < -1, x < <-1, x + +1, x - -1, x & &x, x & ^x)
+}
+
+func _() {
+	_ = T{}
+	_ = struct{}{}
+	_ = [10]T{}
+	_ = [...]T{}
+	_ = []T{}
+	_ = map[int]T{}
+}
+
+// one-line structs/interfaces in composite literals (up to a threshold)
+func _() {
+	_ = struct{}{}
+	_ = struct{ x int }{0}
+	_ = struct{ x, y, z int }{0, 1, 2}
+	_ = struct{ int }{0}
+	_ = struct{ s struct{ int } }{struct{ int }{0}}
+
+	_ = (interface{})(nil)
+	_ = (interface{ String() string })(nil)
+	_ = (interface {
+		String() string
+	})(nil)
+	_ = (interface{ fmt.Stringer })(nil)
+	_ = (interface {
+		fmt.Stringer
+	})(nil)
+}
+
+func _() {
+	// do not modify literals
+	_ = "tab1	tab2	tab3	end"	// string contains 3 tabs
+	_ = "tab1 tab2 tab3 end"	// same string with 3 blanks - may be unaligned because editors see tabs in strings
+	_ = ""				// this comment should be aligned with the one on the previous line
+	_ = ``
+	_ = `
+`
+	_ = `foo
+		bar`
+	_ = `three spaces before the end of the line starting here:   
+they must not be removed`
+}
+
+func _() {
+	// smart handling of indentation for multi-line raw strings
+	var _ = ``
+	var _ = `foo`
+	var _ = `foo
+bar`
+
+	var _ = ``
+	var _ = `foo`
+	var _ =
+	// the next line should remain indented
+	`foo
+bar`
+
+	var _ =	// comment
+	``
+	var _ =	// comment
+	`foo`
+	var _ =	// comment
+	// the next line should remain indented
+	`foo
+bar`
+
+	var _ = /* comment */ ``
+	var _ = /* comment */ `foo`
+	var _ = /* comment */ `foo
+bar`
+
+	var _ =	/* comment */
+	``
+	var _ =	/* comment */
+	`foo`
+	var _ =	/* comment */
+	// the next line should remain indented
+	`foo
+bar`
+
+	var board = []int(
+		`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`)
+
+	var state = S{
+		"foo",
+		// the next line should remain indented
+		`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`,
+		"bar",
+	}
+}
+
+func _() {
+	// one-line function literals (body is on a single line)
+	_ = func() {}
+	_ = func() int { return 0 }
+	_ = func(x, y int) bool { m := (x + y) / 2; return m < 0 }
+
+	// multi-line function literals (body is not on one line)
+	_ = func() {
+	}
+	_ = func() int {
+		return 0
+	}
+	_ = func(x, y int) bool {
+		m := (x + y) / 2
+		return x < y
+	}
+
+	f(func() {
+	})
+	f(func() int {
+		return 0
+	})
+	f(func(x, y int) bool {
+		m := (x + y) / 2
+		return x < y
+	})
+}
+
+func _() {
+	_ = [][]int{
+		[]int{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int{
+		{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int{
+		{1},
+		{1, 2},
+		{1, 2, 3},
+	}
+	_ = [][]int{{1}, {1, 2}, {1, 2, 3}}
+}
+
+// various multi-line expressions
+func _() {
+	// do not add extra indentation to multi-line string lists
+	_ = "foo" + "bar"
+	_ = "foo" +
+		"bar" +
+		"bah"
+	_ = []string{
+		"abc" +
+			"def",
+		"foo" +
+			"bar",
+	}
+}
+
+const _ = F1 +
+	`string = "%s";` +
+	`ptr = *;` +
+	`datafmt.T2 = s ["-" p "-"];`
+
+const _ = `datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+
+const _ = `datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+
+func _() {
+	_ = F1 +
+		`string = "%s";` +
+		`ptr = *;` +
+		`datafmt.T2 = s ["-" p "-"];`
+
+	_ =
+		`datafmt "datafmt";` +
+			`default = "%v";` +
+			`array = *;` +
+			`datafmt.T3 = s  {" " a a / ","};`
+
+	_ = `datafmt "datafmt";` +
+		`default = "%v";` +
+		`array = *;` +
+		`datafmt.T3 = s  {" " a a / ","};`
+}
+
+func _() {
+	// respect source lines in multi-line expressions
+	_ = a +
+		b +
+		c
+	_ = a < b ||
+		b < a
+	_ = "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000"	// 100!
+	_ = "170141183460469231731687303715884105727"	// prime
+}
+
+// Alignment after overlong lines
+const (
+	_	= "991"
+	_	= "2432902008176640000"	// 20!
+	_	= "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000"	// 100!
+	_	= "170141183460469231731687303715884105727"	// prime
+)
+
+// Correct placement of operators and comments in multi-line expressions
+func _() {
+	_ = a +	// comment
+		b +	// comment
+		c
+	_ = "a" +
+		"b" +	// comment
+		"c"
+	_ = "ba0408" + "7265717569726564"	// field 71, encoding 2, string "required"
+}
+
+// Correct placement of terminating comma/closing parentheses in multi-line calls.
+func _() {
+	f(1,
+		2,
+		3)
+	f(1,
+		2,
+		3,
+	)
+	f(1,
+		2,
+		3)	// comment
+	f(1,
+		2,
+		3,	// comment
+	)
+	f(1,
+		2,
+		3)	// comment
+	f(1,
+		2,
+		3,	// comment
+	)
+}
+
+// Align comments in multi-line lists of single-line expressions.
+var txpix = [NCOL]draw.Color{
+	draw.Yellow,		// yellow
+	draw.Cyan,		// cyan
+	draw.Green,		// lime green
+	draw.GreyBlue,		// slate
+	draw.Red,		/* red */
+	draw.GreyGreen,		/* olive green */
+	draw.Blue,		/* blue */
+	draw.Color(0xFF55AAFF),	/* pink */
+	draw.Color(0xFFAAFFFF),	/* lavender */
+	draw.Color(0xBB005DFF),	/* maroon */
+}
+
+func same(t, u *Time) bool {
+	// respect source lines in multi-line expressions
+	return t.Year == u.Year &&
+		t.Month == u.Month &&
+		t.Day == u.Day &&
+		t.Hour == u.Hour &&
+		t.Minute == u.Minute &&
+		t.Second == u.Second &&
+		t.Weekday == u.Weekday &&
+		t.ZoneOffset == u.ZoneOffset &&
+		t.Zone == u.Zone
+}
+
+func (p *parser) charClass() {
+	// respect source lines in multi-line expressions
+	if cc.negate && len(cc.ranges) == 2 &&
+		cc.ranges[0] == '\n' && cc.ranges[1] == '\n' {
+		nl := new(_NotNl)
+		p.re.add(nl)
+	}
+}
+
+func addState(s []state, inst instr, match []int) {
+	// handle comments correctly in multi-line expressions
+	for i := 0; i < l; i++ {
+		if s[i].inst.index() == index &&	// same instruction
+			s[i].match[0] < pos {	// earlier match already going; leftmost wins
+			return s
+		}
+	}
+}
+
+func (self *T) foo(x int) *T	{ return self }
+
+func _()	{ module.Func1().Func2() }
+
+func _() {
+	_ = new(T).
+		foo(1).
+		foo(2).
+		foo(3)
+
+	_ = new(T).
+		foo(1).
+		foo(2).	// inline comments
+		foo(3)
+
+	_ = new(T).foo(1).foo(2).foo(3)
+
+	// handle multiline argument list correctly
+	_ = new(T).
+		foo(
+			1).
+		foo(2)
+
+	_ = new(T).foo(
+		1).foo(2)
+
+	_ = Array[3+
+		4]
+
+	_ = Method(1, 2,
+		3)
+
+	_ = new(T).
+		foo().
+		bar().(*Type)
+
+	_ = new(T).
+		foo().
+		bar().(*Type).
+		baz()
+
+	_ = new(T).
+		foo().
+		bar()["idx"]
+
+	_ = new(T).
+		foo().
+		bar()["idx"].
+		baz()
+
+	_ = new(T).
+		foo().
+		bar()[1:2]
+
+	_ = new(T).
+		foo().
+		bar()[1:2].
+		baz()
+
+	_ = new(T).
+		Field.
+		Array[3+
+		4].
+		Table["foo"].
+		Blob.(*Type).
+		Slices[1:4].
+		Method(1, 2,
+			3).
+		Thingy
+
+	_ = a.b.c
+	_ = a.
+		b.
+		c
+	_ = a.b().c
+	_ = a.
+		b().
+		c
+	_ = a.b[0].c
+	_ = a.
+		b[0].
+		c
+	_ = a.b[0:].c
+	_ = a.
+		b[0:].
+		c
+	_ = a.b.(T).c
+	_ = a.
+		b.(T).
+		c
+}
+
+// Don't introduce extra newlines in strangely formatted expression lists.
+func f() {
+	// os.Open parameters should remain on two lines
+	if writer, err = os.Open(outfile, s.O_WRONLY|os.O_CREATE|
+		os.O_TRUNC, 0666); err != nil {
+		log.Fatal(err)
+	}
+}
+
+// Handle multi-line argument lists ending in ... correctly.
+// Was issue 3130.
+func _() {
+	_ = append(s, a...)
+	_ = append(
+		s, a...)
+	_ = append(s,
+		a...)
+	_ = append(
+		s,
+		a...)
+	_ = append(s, a...,
+	)
+	_ = append(s,
+		a...,
+	)
+	_ = append(
+		s,
+		a...,
+	)
+}
+
+// Literal function types in conversions must be parenthesized;
+// for now go/parser accepts the unparenthesized form where it
+// is non-ambiguous.
+func _() {
+	// these conversions should be rewritten to look
+	// the same as the parenthesized conversions below
+	_ = (func())(nil)
+	_ = (func(x int) float)(nil)
+	_ = (func() func() func())(nil)
+
+	_ = (func())(nil)
+	_ = (func(x int) float)(nil)
+	_ = (func() func() func())(nil)
+}
+
+func _() {
+	_ = f().
+		f(func() {
+			f()
+		}).
+		f(map[int]int{
+			1:	2,
+			3:	4,
+		})
+
+	_ = f().
+		f(
+			func() {
+				f()
+			},
+		)
+}
diff --git a/src/go/printer/testdata/expressions.input b/src/go/printer/testdata/expressions.input
new file mode 100644
index 0000000..8c523b6
--- /dev/null
+++ b/src/go/printer/testdata/expressions.input
@@ -0,0 +1,771 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package expressions
+
+type T struct {
+	x, y, z int
+}
+
+var (
+	a, b, c, d, e int
+	under_bar int
+	longIdentifier1, longIdentifier2, longIdentifier3 int
+	t0, t1, t2 T
+	s string
+	p *int
+)
+
+
+func _() {
+	// no spaces around simple or parenthesized expressions
+	_ = (a+0)
+	_ = a+b
+	_ = a+b+c
+	_ = a+b-c
+	_ = a-b-c
+	_ = a+(b*c)
+	_ = a+(b/c)
+	_ = a-(b%c)
+	_ = 1+a
+	_ = a+1
+	_ = a+b+1
+	_ = s[a]
+	_ = s[a:]
+	_ = s[:b]
+	_ = s[1:2]
+	_ = s[a:b]
+	_ = s[0:len(s)]
+	_ = s[0]<<1
+	_ = (s[0]<<1)&0xf
+	_ = s[0] << 2 | s[1] >> 4
+	_ = "foo"+s
+	_ = s+"foo"
+	_ = 'a'+'b'
+	_ = len(s)/2
+	_ = len(t0.x)/a
+
+	// spaces around expressions of different precedence or expressions containing spaces
+	_ = a + -b
+	_ = a - ^b
+	_ = a / *p
+	_ = a + b*c
+	_ = 1 + b*c
+	_ = a + 2*c
+	_ = a + c*2
+	_ = 1 + 2*3
+	_ = s[1 : 2*3]
+	_ = s[a : b-c]
+	_ = s[0:]
+	_ = s[a+b]
+	_ = s[: b-c]
+	_ = s[a+b :]
+	_ = a[a<<b+1]
+	_ = a[a<<b+1 :]
+	_ = s[a+b : len(s)]
+	_ = s[len(s) : -a]
+	_ = s[a : len(s)+1]
+	_ = s[a : len(s)+1]+s
+
+	// spaces around operators with equal or lower precedence than comparisons
+	_ = a == b
+	_ = a != b
+	_ = a > b
+	_ = a >= b
+	_ = a < b
+	_ = a <= b
+	_ = a < b && c > d
+	_ = a < b || c > d
+
+	// spaces around "long" operands
+	_ = a + longIdentifier1
+	_ = longIdentifier1 + a
+	_ = longIdentifier1 + longIdentifier2 * longIdentifier3
+	_ = s + "a longer string"
+
+	// some selected cases
+	_ = a + t0.x
+	_ = a + t0.x + t1.x * t2.x
+	_ = a + b + c + d + e + 2*3
+	_ = a + b + c + 2*3 + d + e
+	_ = (a+b+c)*2
+	_ = a - b + c - d + (a+b+c) + d&e
+	_ = under_bar-1
+	_ = Open(dpath + "/file", O_WRONLY | O_CREAT, 0666)
+	_ = int(c0&_Mask4)<<18 | int(c1&_Maskx)<<12 | int(c2&_Maskx)<<6 | int(c3&_Maskx)
+
+	// test case for issue 8021
+	// want:
+	//  ([]bool{})[([]int{})[((1)+(((1)+((((1)*(((1)+(1))+(1)))+(1))*(1)))+(1)))]]
+	_ = ([]bool{})[([]int{})[((1) + (((((1) + (((((((1) * (((((1) + (1))) + (1))))) + (1))) * (1))))) + (1))))]]
+
+	// the parser does not restrict expressions that may appear as statements
+	true
+	42
+	"foo"
+	x
+	(x)
+	a+b
+	a+b+c
+	a+(b*c)
+	a+(b/c)
+	1+a
+	a+1
+	s[a]
+	x<<1
+	(s[0]<<1)&0xf
+	"foo"+s
+	x == y
+	x < y || z > 42
+}
+
+
+// slice expressions with cap
+func _() {
+	_ = x[a:b:c]
+	_ = x[a:b:c+d]
+	_ = x[a:b+d:c]
+	_ = x[a:b+d:c+d]
+	_ = x[a+d:b:c]
+	_ = x[a+d:b:c+d]
+	_ = x[a+d:b+d:c]
+	_ = x[a+d:b+d:c+d]
+
+	_ = x[:b:c]
+	_ = x[:b:c+d]
+	_ = x[:b+d:c]
+	_ = x[:b+d:c+d]
+}
+
+func issue22111() {
+	_ = x[:]
+
+	_ = x[:b]
+	_ = x[:b+1]
+
+	_ = x[a:]
+	_ = x[a+1:]
+
+	_ = x[a:b]
+	_ = x[a+1:b]
+	_ = x[a:b+1]
+	_ = x[a+1:b+1]
+
+	_ = x[:b:c]
+	_ = x[:b+1:c]
+	_ = x[:b:c+1]
+	_ = x[:b+1:c+1]
+
+	_ = x[a:b:c]
+	_ = x[a+1:b:c]
+	_ = x[a:b+1:c]
+	_ = x[a+1:b+1:c]
+	_ = x[a:b:c+1]
+	_ = x[a+1:b:c+1]
+	_ = x[a:b+1:c+1]
+	_ = x[a+1:b+1:c+1]
+}
+
+func _() {
+	_ = a+b
+	_ = a+b+c
+	_ = a+b*c
+	_ = a+(b*c)
+	_ = (a+b)*c
+	_ = a+(b*c*d)
+	_ = a+(b*c+d)
+
+	_ = 1<<x
+	_ = -1<<x
+	_ = 1<<x-1
+	_ = -1<<x-1
+
+	_ = f(a+b)
+	_ = f(a+b+c)
+	_ = f(a+b*c)
+	_ = f(a+(b*c))
+	_ = f(1<<x-1, 1<<x-2)
+
+	_ = 1<<d.logWindowSize-1
+
+	buf = make(x, 2*cap(b.buf) + n)
+
+	dst[i*3+2] = dbuf[0]<<2
+	dst[i*3+2] = dbuf[0]<<2 | dbuf[1]>>4
+
+	b.buf = b.buf[0:b.off+m+n]
+	b.buf = b.buf[0:b.off+m*n]
+	f(b.buf[0:b.off+m+n])
+
+	signed += ' '*8
+	tw.octal(header[148:155], chksum)
+
+	_ = x > 0 && i >= 0
+
+	x1, x0 := x>>w2, x&m2
+	z0 = t1<<w2+t0
+	z1 = (t1+t0>>w2)>>w2
+	q1, r1 := x1/d1, x1%d1
+	r1 = r1*b2 | x0>>w2
+	x1 = (x1<<z)|(x0>>(uint(w)-z))
+	x1 = x1<<z | x0>>(uint(w)-z)
+
+	_ = buf[0:len(buf)+1]
+	_ = buf[0:n+1]
+
+	a,b = b,a
+	a = b+c
+	a = b*c+d
+	_ = a*b+c
+	_ = a-b-c
+	_ = a-(b-c)
+	_ = a-b*c
+	_ = a-(b*c)
+	_ = a*b/c
+	_ = a/ *b
+	_ = x[a|^b]
+	_ = x[a/ *b]
+	_ = a& ^b
+	_ = a+ +b
+	_ = a- -b
+	_ = x[a*-b]
+	_ = x[a+ +b]
+	_ = x^y^z
+	_ = b[a>>24] ^ b[(a>>16)&0xFF] ^ b[(a>>8)&0xFF] ^ b[a&0xFF]
+	_ = len(longVariableName)*2
+
+	_ = token(matchType + xlength<<lengthShift + xoffset)
+}
+
+
+func f(x int, args ...int) {
+	f(0, args...)
+	f(1, args)
+	f(2, args[0])
+
+	// make sure syntactically legal code remains syntactically legal
+	f(3, 42 ...) // a blank must remain between 42 and ...
+	f(4, 42. ...)
+	f(5, 42....)
+	f(6, 42.0 ...)
+	f(7, 42.0...)
+	f(8, .42 ...)
+	f(9, .42...)
+	f(10, 42e0 ...)
+	f(11, 42e0...)
+
+	_ = 42 .x // a blank must remain between 42 and .x
+	_ = 42. .x
+	_ = 42..x
+	_ = 42.0 .x
+	_ = 42.0.x
+	_ = .42 .x
+	_ = .42.x
+	_ = 42e0 .x
+	_ = 42e0.x
+
+	// a blank must remain between the binary operator and the 2nd operand
+	_ = x/ *y
+	_ = x< -1
+	_ = x< <-1
+	_ = x+ +1
+	_ = x- -1
+	_ = x& &x
+	_ = x& ^x
+
+	_ = f(x/ *y, x< -1, x< <-1, x+ +1, x- -1, x& &x, x& ^x)
+}
+
+
+func _() {
+	_ = T{}
+	_ = struct{}{}
+	_ = [10]T{}
+	_ = [...]T{}
+	_ = []T{}
+	_ = map[int]T{}
+}
+
+
+// one-line structs/interfaces in composite literals (up to a threshold)
+func _() {
+	_ = struct{}{}
+	_ = struct{ x int }{0}
+	_ = struct{ x, y, z int }{0, 1, 2}
+	_ = struct{ int }{0}
+	_ = struct{ s struct { int } }{struct{ int}{0} }
+
+	_ = (interface{})(nil)
+	_ = (interface{String() string})(nil)
+	_ = (interface{
+		String()    string
+	})(nil)
+	_ = (interface{fmt.Stringer})(nil)
+	_ = (interface{
+		    fmt.Stringer
+	})(nil)
+}
+
+func _() {
+	// do not modify literals
+	_ = "tab1	tab2	tab3	end"  // string contains 3 tabs
+	_ = "tab1 tab2 tab3 end"  // same string with 3 blanks - may be unaligned because editors see tabs in strings
+	_ = ""  // this comment should be aligned with the one on the previous line
+	_ = ``
+	_ = `
+`
+_ = `foo
+		bar`
+	_ = `three spaces before the end of the line starting here:   
+they must not be removed`
+}
+
+
+func _() {
+	// smart handling of indentation for multi-line raw strings
+	var _ = ``
+	var _ = `foo`
+	var _ = `foo
+bar`
+
+
+var _ =
+	``
+var _ =
+	`foo`
+var _ =
+	// the next line should remain indented
+	`foo
+bar`
+
+
+	var _ = // comment
+		``
+	var _ = // comment
+		`foo`
+	var _ = // comment
+		// the next line should remain indented
+		`foo
+bar`
+
+
+var _ = /* comment */ ``
+var _ = /* comment */ `foo`
+var _ = /* comment */ `foo
+bar`
+
+
+	var _ = /* comment */
+		``
+	var _ = /* comment */
+		`foo`
+	var _ = /* comment */
+		// the next line should remain indented
+		`foo
+bar`
+
+
+var board = []int(
+	`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`)
+
+
+	var state = S{
+		"foo",
+		// the next line should remain indented
+		`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`,
+		"bar",
+	}
+}
+
+
+func _() {
+	// one-line function literals (body is on a single line)
+	_ = func() {}
+	_ = func() int { return 0 }
+	_ = func(x, y int) bool { m := (x+y)/2; return m < 0 }
+
+	// multi-line function literals (body is not on one line)
+	_ = func() {
+	}
+	_ = func() int {
+		return 0
+	}
+	_ = func(x, y int) bool {
+		m := (x+y)/2; return x < y }
+
+	f(func() {
+	})
+	f(func() int {
+		return 0
+	})
+	f(func(x, y int) bool {
+		m := (x+y)/2; return x < y })
+}
+
+
+func _() {
+	_ = [][]int {
+		[]int{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int {
+		{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int {
+		{1},
+		{1, 2},
+		{1, 2, 3},
+	}
+	_ = [][]int {{1}, {1, 2}, {1, 2, 3}}
+}
+
+
+// various multi-line expressions
+func _() {
+	// do not add extra indentation to multi-line string lists
+	_ = "foo" + "bar"
+	_ = "foo" +
+	"bar" +
+	"bah"
+	_ = []string {
+		"abc" +
+		"def",
+		"foo" +
+		"bar",
+	}
+}
+
+
+const _ = F1 +
+	`string = "%s";` +
+	`ptr = *;` +
+	`datafmt.T2 = s ["-" p "-"];`
+
+
+const _ =
+	`datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+
+
+const _ = `datafmt "datafmt";` +
+`default = "%v";` +
+`array = *;` +
+`datafmt.T3 = s  {" " a a / ","};`
+
+
+func _() {
+	_ = F1 +
+		`string = "%s";` +
+		`ptr = *;` +
+		`datafmt.T2 = s ["-" p "-"];`
+
+	_ =
+		`datafmt "datafmt";` +
+		`default = "%v";` +
+		`array = *;` +
+		`datafmt.T3 = s  {" " a a / ","};`
+
+	_ = `datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+}
+
+
+func _() {
+	// respect source lines in multi-line expressions
+	_ = a+
+	b+
+	c
+	_ = a < b ||
+		b < a
+	_ = "933262154439441526816992388562667004907159682643816214685929" +
+	"638952175999932299156089414639761565182862536979208272237582" +
+	"51185210916864000000000000000000000000"  // 100!
+	_ = "170141183460469231731687303715884105727"  // prime
+}
+
+
+// Alignment after overlong lines
+const (
+	_ = "991"
+	_ = "2432902008176640000"  // 20!
+	_ = "933262154439441526816992388562667004907159682643816214685929" +
+	"638952175999932299156089414639761565182862536979208272237582" +
+	"51185210916864000000000000000000000000"  // 100!
+	_ = "170141183460469231731687303715884105727"  // prime
+)
+
+
+// Correct placement of operators and comments in multi-line expressions
+func _() {
+	_ = a +  // comment
+		b +  // comment
+		c
+	_ = "a"	+
+		"b" +	// comment
+		"c"
+	_ = "ba0408" + "7265717569726564"     // field 71, encoding 2, string "required"
+}
+
+
+// Correct placement of terminating comma/closing parentheses in multi-line calls.
+func _() {
+	f(1,
+		2,
+		3)
+	f(1,
+		2,
+		3,
+	)
+	f(1,
+		2,
+		3)  // comment
+	f(1,
+		2,
+		3,  // comment
+	)
+	f(1,
+		2,
+		3)// comment
+	f(1,
+		2,
+		3,// comment
+	)
+}
+
+
+// Align comments in multi-line lists of single-line expressions.
+var txpix = [NCOL]draw.Color{
+	draw.Yellow, // yellow
+	draw.Cyan, // cyan
+	draw.Green, // lime green
+	draw.GreyBlue, // slate
+	draw.Red, /* red */
+	draw.GreyGreen, /* olive green */
+	draw.Blue, /* blue */
+	draw.Color(0xFF55AAFF), /* pink */
+	draw.Color(0xFFAAFFFF), /* lavender */
+	draw.Color(0xBB005DFF), /* maroon */
+}
+
+
+func same(t, u *Time) bool {
+	// respect source lines in multi-line expressions
+	return t.Year == u.Year &&
+		t.Month == u.Month &&
+		t.Day == u.Day &&
+		t.Hour == u.Hour &&
+		t.Minute == u.Minute &&
+		t.Second == u.Second &&
+		t.Weekday == u.Weekday &&
+		t.ZoneOffset == u.ZoneOffset &&
+		t.Zone == u.Zone
+}
+
+
+func (p *parser) charClass() {
+	// respect source lines in multi-line expressions
+	if cc.negate && len(cc.ranges) == 2 &&
+		cc.ranges[0] == '\n' && cc.ranges[1] == '\n' {
+		nl := new(_NotNl)
+		p.re.add(nl)
+	}
+}
+
+
+func addState(s []state, inst instr, match []int) {
+	// handle comments correctly in multi-line expressions
+	for i := 0; i < l; i++ {
+		if s[i].inst.index() == index && // same instruction
+		   s[i].match[0] < pos {	// earlier match already going; leftmost wins
+		   	return s
+		 }
+	}
+}
+
+func (self *T) foo(x int) *T { return self }
+
+func _() { module.Func1().Func2() }
+
+func _() {
+	_ = new(T).
+		foo(1).
+			foo(2).
+		foo(3)
+
+	_ = new(T).
+	foo(1).
+	foo(2). // inline comments
+	foo(3)
+
+	_ = new(T).foo(1).foo(2).foo(3)
+
+	// handle multiline argument list correctly
+	_ = new(T).
+	foo(
+		1).
+		foo(2)
+
+	_ = new(T).foo(
+		1).foo(2)
+
+	_ = Array[3 +
+4]
+
+	_ = Method(1, 2,
+		3)
+
+	_ = new(T).
+   foo().
+   bar() . (*Type)
+
+	_ = new(T).
+foo().
+bar().(*Type).
+baz()
+
+	_ = new(T).
+	foo().
+	bar()["idx"]
+
+	_ = new(T).
+	foo().
+	bar()["idx"]	.
+	baz()
+
+	_ = new(T).
+	foo().
+	bar()[1:2]
+
+	_ = new(T).
+	foo().
+	bar()[1:2].
+	baz()
+
+	_ = new(T).
+		Field.
+		Array[3+
+       		4].
+		Table ["foo"].
+		Blob. (*Type).
+	Slices[1:4].
+	Method(1, 2,
+	3).
+		Thingy
+
+	_ = a.b.c
+	_ = a.
+	b.
+	c
+	_ = a.b().c
+	_ = a.
+	b().
+	c
+	_ = a.b[0].c
+	_ = a.
+	b[0].
+	c
+	_ = a.b[0:].c
+	_ = a.
+	b[0:].
+	c
+	_ = a.b.(T).c
+	_ = a.
+	b.
+	(T).
+	c
+}
+
+
+// Don't introduce extra newlines in strangely formatted expression lists.
+func f() {
+	// os.Open parameters should remain on two lines
+	if writer, err = os.Open(outfile, s.O_WRONLY|os.O_CREATE|
+		os.O_TRUNC, 0666); err != nil {
+	    log.Fatal(err)
+	}
+}
+
+// Handle multi-line argument lists ending in ... correctly.
+// Was issue 3130.
+func _() {
+	_ = append(s, a...)
+	_ = append(
+		s, a...)
+	_ = append(s,
+		a...)
+	_ = append(
+		s,
+		a...)
+	_ = append(s, a...,
+	)
+	_ = append(s,
+		a...,
+	)
+	_ = append(
+		s,
+		a...,
+	)
+}
+
+// Literal function types in conversions must be parenthesized;
+// for now go/parser accepts the unparenthesized form where it
+// is non-ambiguous.
+func _() {
+	// these conversions should be rewritten to look
+	// the same as the parenthesized conversions below
+	_ = func()()(nil)
+	_ = func(x int)(float)(nil)
+	_ = func() func() func()()(nil)
+
+	_ = (func()())(nil)
+	_ = (func(x int)(float))(nil)
+	_ = (func() func() func()())(nil)
+}
+
+func _() {
+	_ = f().
+	f(func() {
+		f()
+	}).
+	f(map[int]int{
+	1: 2,
+	3: 4,
+})
+
+	_ = f().
+	f(
+	func() {
+		f()
+	},
+	)
+}
diff --git a/src/go/printer/testdata/expressions.raw b/src/go/printer/testdata/expressions.raw
new file mode 100644
index 0000000..058fded
--- /dev/null
+++ b/src/go/printer/testdata/expressions.raw
@@ -0,0 +1,743 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package expressions
+
+type T struct {
+	x, y, z int
+}
+
+var (
+	a, b, c, d, e	int
+	under_bar	int
+	longIdentifier1, longIdentifier2, longIdentifier3	int
+	t0, t1, t2	T
+	s	string
+	p	*int
+)
+
+func _() {
+	// no spaces around simple or parenthesized expressions
+	_ = (a + 0)
+	_ = a + b
+	_ = a + b + c
+	_ = a + b - c
+	_ = a - b - c
+	_ = a + (b * c)
+	_ = a + (b / c)
+	_ = a - (b % c)
+	_ = 1 + a
+	_ = a + 1
+	_ = a + b + 1
+	_ = s[a]
+	_ = s[a:]
+	_ = s[:b]
+	_ = s[1:2]
+	_ = s[a:b]
+	_ = s[0:len(s)]
+	_ = s[0] << 1
+	_ = (s[0] << 1) & 0xf
+	_ = s[0]<<2 | s[1]>>4
+	_ = "foo" + s
+	_ = s + "foo"
+	_ = 'a' + 'b'
+	_ = len(s) / 2
+	_ = len(t0.x) / a
+
+	// spaces around expressions of different precedence or expressions containing spaces
+	_ = a + -b
+	_ = a - ^b
+	_ = a / *p
+	_ = a + b*c
+	_ = 1 + b*c
+	_ = a + 2*c
+	_ = a + c*2
+	_ = 1 + 2*3
+	_ = s[1 : 2*3]
+	_ = s[a : b-c]
+	_ = s[0:]
+	_ = s[a+b]
+	_ = s[:b-c]
+	_ = s[a+b:]
+	_ = a[a<<b+1]
+	_ = a[a<<b+1:]
+	_ = s[a+b : len(s)]
+	_ = s[len(s):-a]
+	_ = s[a : len(s)+1]
+	_ = s[a:len(s)+1] + s
+
+	// spaces around operators with equal or lower precedence than comparisons
+	_ = a == b
+	_ = a != b
+	_ = a > b
+	_ = a >= b
+	_ = a < b
+	_ = a <= b
+	_ = a < b && c > d
+	_ = a < b || c > d
+
+	// spaces around "long" operands
+	_ = a + longIdentifier1
+	_ = longIdentifier1 + a
+	_ = longIdentifier1 + longIdentifier2*longIdentifier3
+	_ = s + "a longer string"
+
+	// some selected cases
+	_ = a + t0.x
+	_ = a + t0.x + t1.x*t2.x
+	_ = a + b + c + d + e + 2*3
+	_ = a + b + c + 2*3 + d + e
+	_ = (a + b + c) * 2
+	_ = a - b + c - d + (a + b + c) + d&e
+	_ = under_bar - 1
+	_ = Open(dpath+"/file", O_WRONLY|O_CREAT, 0666)
+	_ = int(c0&_Mask4)<<18 | int(c1&_Maskx)<<12 | int(c2&_Maskx)<<6 | int(c3&_Maskx)
+
+	// test case for issue 8021
+	// want:
+	//  ([]bool{})[([]int{})[((1)+(((1)+((((1)*(((1)+(1))+(1)))+(1))*(1)))+(1)))]]
+	_ = ([]bool{})[([]int{})[((1)+(((1)+((((1)*(((1)+(1))+(1)))+(1))*(1)))+(1)))]]
+
+	// the parser does not restrict expressions that may appear as statements
+	true
+	42
+	"foo"
+	x
+	(x)
+	a + b
+	a + b + c
+	a + (b * c)
+	a + (b / c)
+	1 + a
+	a + 1
+	s[a]
+	x << 1
+	(s[0] << 1) & 0xf
+	"foo" + s
+	x == y
+	x < y || z > 42
+}
+
+// slice expressions with cap
+func _() {
+	_ = x[a:b:c]
+	_ = x[a : b : c+d]
+	_ = x[a : b+d : c]
+	_ = x[a : b+d : c+d]
+	_ = x[a+d : b : c]
+	_ = x[a+d : b : c+d]
+	_ = x[a+d : b+d : c]
+	_ = x[a+d : b+d : c+d]
+
+	_ = x[:b:c]
+	_ = x[: b : c+d]
+	_ = x[: b+d : c]
+	_ = x[: b+d : c+d]
+}
+
+func issue22111() {
+	_ = x[:]
+
+	_ = x[:b]
+	_ = x[:b+1]
+
+	_ = x[a:]
+	_ = x[a+1:]
+
+	_ = x[a:b]
+	_ = x[a+1 : b]
+	_ = x[a : b+1]
+	_ = x[a+1 : b+1]
+
+	_ = x[:b:c]
+	_ = x[: b+1 : c]
+	_ = x[: b : c+1]
+	_ = x[: b+1 : c+1]
+
+	_ = x[a:b:c]
+	_ = x[a+1 : b : c]
+	_ = x[a : b+1 : c]
+	_ = x[a+1 : b+1 : c]
+	_ = x[a : b : c+1]
+	_ = x[a+1 : b : c+1]
+	_ = x[a : b+1 : c+1]
+	_ = x[a+1 : b+1 : c+1]
+}
+
+func _() {
+	_ = a + b
+	_ = a + b + c
+	_ = a + b*c
+	_ = a + (b * c)
+	_ = (a + b) * c
+	_ = a + (b * c * d)
+	_ = a + (b*c + d)
+
+	_ = 1 << x
+	_ = -1 << x
+	_ = 1<<x - 1
+	_ = -1<<x - 1
+
+	_ = f(a + b)
+	_ = f(a + b + c)
+	_ = f(a + b*c)
+	_ = f(a + (b * c))
+	_ = f(1<<x-1, 1<<x-2)
+
+	_ = 1<<d.logWindowSize - 1
+
+	buf = make(x, 2*cap(b.buf)+n)
+
+	dst[i*3+2] = dbuf[0] << 2
+	dst[i*3+2] = dbuf[0]<<2 | dbuf[1]>>4
+
+	b.buf = b.buf[0 : b.off+m+n]
+	b.buf = b.buf[0 : b.off+m*n]
+	f(b.buf[0 : b.off+m+n])
+
+	signed += ' ' * 8
+	tw.octal(header[148:155], chksum)
+
+	_ = x > 0 && i >= 0
+
+	x1, x0 := x>>w2, x&m2
+	z0 = t1<<w2 + t0
+	z1 = (t1 + t0>>w2) >> w2
+	q1, r1 := x1/d1, x1%d1
+	r1 = r1*b2 | x0>>w2
+	x1 = (x1 << z) | (x0 >> (uint(w) - z))
+	x1 = x1<<z | x0>>(uint(w)-z)
+
+	_ = buf[0 : len(buf)+1]
+	_ = buf[0 : n+1]
+
+	a, b = b, a
+	a = b + c
+	a = b*c + d
+	_ = a*b + c
+	_ = a - b - c
+	_ = a - (b - c)
+	_ = a - b*c
+	_ = a - (b * c)
+	_ = a * b / c
+	_ = a / *b
+	_ = x[a|^b]
+	_ = x[a / *b]
+	_ = a & ^b
+	_ = a + +b
+	_ = a - -b
+	_ = x[a*-b]
+	_ = x[a + +b]
+	_ = x ^ y ^ z
+	_ = b[a>>24] ^ b[(a>>16)&0xFF] ^ b[(a>>8)&0xFF] ^ b[a&0xFF]
+	_ = len(longVariableName) * 2
+
+	_ = token(matchType + xlength<<lengthShift + xoffset)
+}
+
+func f(x int, args ...int) {
+	f(0, args...)
+	f(1, args)
+	f(2, args[0])
+
+	// make sure syntactically legal code remains syntactically legal
+	f(3, 42 ...)	// a blank must remain between 42 and ...
+	f(4, 42....)
+	f(5, 42....)
+	f(6, 42.0...)
+	f(7, 42.0...)
+	f(8, .42...)
+	f(9, .42...)
+	f(10, 42e0...)
+	f(11, 42e0...)
+
+	_ = 42 .x	// a blank must remain between 42 and .x
+	_ = 42..x
+	_ = 42..x
+	_ = 42.0.x
+	_ = 42.0.x
+	_ = .42.x
+	_ = .42.x
+	_ = 42e0.x
+	_ = 42e0.x
+
+	// a blank must remain between the binary operator and the 2nd operand
+	_ = x / *y
+	_ = x < -1
+	_ = x < <-1
+	_ = x + +1
+	_ = x - -1
+	_ = x & &x
+	_ = x & ^x
+
+	_ = f(x / *y, x < -1, x < <-1, x + +1, x - -1, x & &x, x & ^x)
+}
+
+func _() {
+	_ = T{}
+	_ = struct{}{}
+	_ = [10]T{}
+	_ = [...]T{}
+	_ = []T{}
+	_ = map[int]T{}
+}
+
+// one-line structs/interfaces in composite literals (up to a threshold)
+func _() {
+	_ = struct{}{}
+	_ = struct{ x int }{0}
+	_ = struct{ x, y, z int }{0, 1, 2}
+	_ = struct{ int }{0}
+	_ = struct{ s struct{ int } }{struct{ int }{0}}
+
+	_ = (interface{})(nil)
+	_ = (interface{ String() string })(nil)
+	_ = (interface {
+		String() string
+	})(nil)
+	_ = (interface{ fmt.Stringer })(nil)
+	_ = (interface {
+		fmt.Stringer
+	})(nil)
+}
+
+func _() {
+	// do not modify literals
+	_ = "tab1	tab2	tab3	end"	// string contains 3 tabs
+	_ = "tab1 tab2 tab3 end"	// same string with 3 blanks - may be unaligned because editors see tabs in strings
+	_ = ""	// this comment should be aligned with the one on the previous line
+	_ = ``
+	_ = `
+`
+	_ = `foo
+		bar`
+	_ = `three spaces before the end of the line starting here:   
+they must not be removed`
+}
+
+func _() {
+	// smart handling of indentation for multi-line raw strings
+	var _ = ``
+	var _ = `foo`
+	var _ = `foo
+bar`
+
+	var _ = ``
+	var _ = `foo`
+	var _ =
+	// the next line should remain indented
+	`foo
+bar`
+
+	var _ =	// comment
+	``
+	var _ =	// comment
+	`foo`
+	var _ =	// comment
+	// the next line should remain indented
+	`foo
+bar`
+
+	var _ = /* comment */ ``
+	var _ = /* comment */ `foo`
+	var _ = /* comment */ `foo
+bar`
+
+	var _ =	/* comment */
+	``
+	var _ =	/* comment */
+	`foo`
+	var _ =	/* comment */
+	// the next line should remain indented
+	`foo
+bar`
+
+	var board = []int(
+		`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`)
+
+	var state = S{
+		"foo",
+		// the next line should remain indented
+		`...........
+...........
+....●●●....
+....●●●....
+..●●●●●●●..
+..●●●○●●●..
+..●●●●●●●..
+....●●●....
+....●●●....
+...........
+...........
+`,
+		"bar",
+	}
+}
+
+func _() {
+	// one-line function literals (body is on a single line)
+	_ = func() {}
+	_ = func() int { return 0 }
+	_ = func(x, y int) bool { m := (x + y) / 2; return m < 0 }
+
+	// multi-line function literals (body is not on one line)
+	_ = func() {
+	}
+	_ = func() int {
+		return 0
+	}
+	_ = func(x, y int) bool {
+		m := (x + y) / 2
+		return x < y
+	}
+
+	f(func() {
+	})
+	f(func() int {
+		return 0
+	})
+	f(func(x, y int) bool {
+		m := (x + y) / 2
+		return x < y
+	})
+}
+
+func _() {
+	_ = [][]int{
+		[]int{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int{
+		{1},
+		[]int{1, 2},
+		[]int{1, 2, 3},
+	}
+	_ = [][]int{
+		{1},
+		{1, 2},
+		{1, 2, 3},
+	}
+	_ = [][]int{{1}, {1, 2}, {1, 2, 3}}
+}
+
+// various multi-line expressions
+func _() {
+	// do not add extra indentation to multi-line string lists
+	_ = "foo" + "bar"
+	_ = "foo" +
+		"bar" +
+		"bah"
+	_ = []string{
+		"abc" +
+			"def",
+		"foo" +
+			"bar",
+	}
+}
+
+const _ = F1 +
+	`string = "%s";` +
+	`ptr = *;` +
+	`datafmt.T2 = s ["-" p "-"];`
+
+const _ = `datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+
+const _ = `datafmt "datafmt";` +
+	`default = "%v";` +
+	`array = *;` +
+	`datafmt.T3 = s  {" " a a / ","};`
+
+func _() {
+	_ = F1 +
+		`string = "%s";` +
+		`ptr = *;` +
+		`datafmt.T2 = s ["-" p "-"];`
+
+	_ =
+		`datafmt "datafmt";` +
+			`default = "%v";` +
+			`array = *;` +
+			`datafmt.T3 = s  {" " a a / ","};`
+
+	_ = `datafmt "datafmt";` +
+		`default = "%v";` +
+		`array = *;` +
+		`datafmt.T3 = s  {" " a a / ","};`
+}
+
+func _() {
+	// respect source lines in multi-line expressions
+	_ = a +
+		b +
+		c
+	_ = a < b ||
+		b < a
+	_ = "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000"	// 100!
+	_ = "170141183460469231731687303715884105727"	// prime
+}
+
+// Alignment after overlong lines
+const (
+	_	= "991"
+	_	= "2432902008176640000"		// 20!
+	_	= "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000"	// 100!
+	_	= "170141183460469231731687303715884105727"		// prime
+)
+
+// Correct placement of operators and comments in multi-line expressions
+func _() {
+	_ = a +	// comment
+		b +	// comment
+		c
+	_ = "a" +
+		"b" +	// comment
+		"c"
+	_ = "ba0408" + "7265717569726564"	// field 71, encoding 2, string "required"
+}
+
+// Correct placement of terminating comma/closing parentheses in multi-line calls.
+func _() {
+	f(1,
+		2,
+		3)
+	f(1,
+		2,
+		3,
+	)
+	f(1,
+		2,
+		3)	// comment
+	f(1,
+		2,
+		3,	// comment
+	)
+	f(1,
+		2,
+		3)	// comment
+	f(1,
+		2,
+		3,	// comment
+	)
+}
+
+// Align comments in multi-line lists of single-line expressions.
+var txpix = [NCOL]draw.Color{
+	draw.Yellow,	// yellow
+	draw.Cyan,	// cyan
+	draw.Green,	// lime green
+	draw.GreyBlue,	// slate
+	draw.Red,	/* red */
+	draw.GreyGreen,	/* olive green */
+	draw.Blue,	/* blue */
+	draw.Color(0xFF55AAFF),	/* pink */
+	draw.Color(0xFFAAFFFF),	/* lavender */
+	draw.Color(0xBB005DFF),	/* maroon */
+}
+
+func same(t, u *Time) bool {
+	// respect source lines in multi-line expressions
+	return t.Year == u.Year &&
+		t.Month == u.Month &&
+		t.Day == u.Day &&
+		t.Hour == u.Hour &&
+		t.Minute == u.Minute &&
+		t.Second == u.Second &&
+		t.Weekday == u.Weekday &&
+		t.ZoneOffset == u.ZoneOffset &&
+		t.Zone == u.Zone
+}
+
+func (p *parser) charClass() {
+	// respect source lines in multi-line expressions
+	if cc.negate && len(cc.ranges) == 2 &&
+		cc.ranges[0] == '\n' && cc.ranges[1] == '\n' {
+		nl := new(_NotNl)
+		p.re.add(nl)
+	}
+}
+
+func addState(s []state, inst instr, match []int) {
+	// handle comments correctly in multi-line expressions
+	for i := 0; i < l; i++ {
+		if s[i].inst.index() == index &&	// same instruction
+			s[i].match[0] < pos {	// earlier match already going; leftmost wins
+			return s
+		}
+	}
+}
+
+func (self *T) foo(x int) *T	{ return self }
+
+func _()	{ module.Func1().Func2() }
+
+func _() {
+	_ = new(T).
+		foo(1).
+		foo(2).
+		foo(3)
+
+	_ = new(T).
+		foo(1).
+		foo(2).	// inline comments
+		foo(3)
+
+	_ = new(T).foo(1).foo(2).foo(3)
+
+	// handle multiline argument list correctly
+	_ = new(T).
+		foo(
+			1).
+		foo(2)
+
+	_ = new(T).foo(
+		1).foo(2)
+
+	_ = Array[3+
+		4]
+
+	_ = Method(1, 2,
+		3)
+
+	_ = new(T).
+		foo().
+		bar().(*Type)
+
+	_ = new(T).
+		foo().
+		bar().(*Type).
+		baz()
+
+	_ = new(T).
+		foo().
+		bar()["idx"]
+
+	_ = new(T).
+		foo().
+		bar()["idx"].
+		baz()
+
+	_ = new(T).
+		foo().
+		bar()[1:2]
+
+	_ = new(T).
+		foo().
+		bar()[1:2].
+		baz()
+
+	_ = new(T).
+		Field.
+		Array[3+
+		4].
+		Table["foo"].
+		Blob.(*Type).
+		Slices[1:4].
+		Method(1, 2,
+			3).
+		Thingy
+
+	_ = a.b.c
+	_ = a.
+		b.
+		c
+	_ = a.b().c
+	_ = a.
+		b().
+		c
+	_ = a.b[0].c
+	_ = a.
+		b[0].
+		c
+	_ = a.b[0:].c
+	_ = a.
+		b[0:].
+		c
+	_ = a.b.(T).c
+	_ = a.
+		b.(T).
+		c
+}
+
+// Don't introduce extra newlines in strangely formatted expression lists.
+func f() {
+	// os.Open parameters should remain on two lines
+	if writer, err = os.Open(outfile, s.O_WRONLY|os.O_CREATE|
+		os.O_TRUNC, 0666); err != nil {
+		log.Fatal(err)
+	}
+}
+
+// Handle multi-line argument lists ending in ... correctly.
+// Was issue 3130.
+func _() {
+	_ = append(s, a...)
+	_ = append(
+		s, a...)
+	_ = append(s,
+		a...)
+	_ = append(
+		s,
+		a...)
+	_ = append(s, a...,
+	)
+	_ = append(s,
+		a...,
+	)
+	_ = append(
+		s,
+		a...,
+	)
+}
+
+// Literal function types in conversions must be parenthesized;
+// for now go/parser accepts the unparenthesized form where it
+// is non-ambiguous.
+func _() {
+	// these conversions should be rewritten to look
+	// the same as the parenthesized conversions below
+	_ = (func())(nil)
+	_ = (func(x int) float)(nil)
+	_ = (func() func() func())(nil)
+
+	_ = (func())(nil)
+	_ = (func(x int) float)(nil)
+	_ = (func() func() func())(nil)
+}
+
+func _() {
+	_ = f().
+		f(func() {
+			f()
+		}).
+		f(map[int]int{
+			1:	2,
+			3:	4,
+		})
+
+	_ = f().
+		f(
+			func() {
+				f()
+			},
+		)
+}
diff --git a/src/go/printer/testdata/go2numbers.golden b/src/go/printer/testdata/go2numbers.golden
new file mode 100644
index 0000000..3c12049
--- /dev/null
+++ b/src/go/printer/testdata/go2numbers.golden
@@ -0,0 +1,186 @@
+package p
+
+const (
+	// 0-octals
+	_	= 0
+	_	= 0123
+	_	= 0123456
+
+	_	= 0_123
+	_	= 0123_456
+
+	// decimals
+	_	= 1
+	_	= 1234
+	_	= 1234567
+
+	_	= 1_234
+	_	= 1_234_567
+
+	// hexadecimals
+	_	= 0x0
+	_	= 0x1234
+	_	= 0xcafef00d
+
+	_	= 0X0
+	_	= 0X1234
+	_	= 0XCAFEf00d
+
+	_	= 0X_0
+	_	= 0X_1234
+	_	= 0X_CAFE_f00d
+
+	// octals
+	_	= 0o0
+	_	= 0o1234
+	_	= 0o01234567
+
+	_	= 0O0
+	_	= 0O1234
+	_	= 0O01234567
+
+	_	= 0o_0
+	_	= 0o_1234
+	_	= 0o0123_4567
+
+	_	= 0O_0
+	_	= 0O_1234
+	_	= 0O0123_4567
+
+	// binaries
+	_	= 0b0
+	_	= 0b1011
+	_	= 0b00101101
+
+	_	= 0B0
+	_	= 0B1011
+	_	= 0B00101101
+
+	_	= 0b_0
+	_	= 0b10_11
+	_	= 0b_0010_1101
+
+	// decimal floats
+	_	= 0.
+	_	= 123.
+	_	= 0123.
+
+	_	= .0
+	_	= .123
+	_	= .0123
+
+	_	= 0e0
+	_	= 123e+0
+	_	= 0123E-1
+
+	_	= 0e-0
+	_	= 123E+0
+	_	= 0123E123
+
+	_	= 0.e+1
+	_	= 123.E-10
+	_	= 0123.e123
+
+	_	= .0e-1
+	_	= .123E+10
+	_	= .0123E123
+
+	_	= 0.0
+	_	= 123.123
+	_	= 0123.0123
+
+	_	= 0.0e1
+	_	= 123.123E-10
+	_	= 0123.0123e+456
+
+	_	= 1_2_3.
+	_	= 0_123.
+
+	_	= 0_0e0
+	_	= 1_2_3e0
+	_	= 0_123e0
+
+	_	= 0e-0_0
+	_	= 1_2_3E+0
+	_	= 0123E1_2_3
+
+	_	= 0.e+1
+	_	= 123.E-1_0
+	_	= 01_23.e123
+
+	_	= .0e-1
+	_	= .123E+10
+	_	= .0123E123
+
+	_	= 1_2_3.123
+	_	= 0123.01_23
+
+	// hexadecimal floats
+	_	= 0x0.p+0
+	_	= 0Xdeadcafe.p-10
+	_	= 0x1234.P123
+
+	_	= 0x.1p-0
+	_	= 0X.deadcafep2
+	_	= 0x.1234P+10
+
+	_	= 0x0p0
+	_	= 0Xdeadcafep+1
+	_	= 0x1234P-10
+
+	_	= 0x0.0p0
+	_	= 0Xdead.cafep+1
+	_	= 0x12.34P-10
+
+	_	= 0Xdead_cafep+1
+	_	= 0x_1234P-10
+
+	_	= 0X_dead_cafe.p-10
+	_	= 0x12_34.P1_2_3
+	_	= 0X1_2_3_4.P-1_2_3
+
+	// imaginaries
+	_	= 0i
+	_	= 00i
+	_	= 08i
+	_	= 0000000000i
+	_	= 0123i
+	_	= 0000000123i
+	_	= 0000056789i
+	_	= 1234i
+	_	= 1234567i
+
+	_	= 0i
+	_	= 0_0i
+	_	= 0_8i
+	_	= 0_000_000_000i
+	_	= 0_123i
+	_	= 0_000_000_123i
+	_	= 0_000_056_789i
+	_	= 1_234i
+	_	= 1_234_567i
+
+	_	= 0.i
+	_	= 123.i
+	_	= 0123.i
+	_	= 000123.i
+
+	_	= 0e0i
+	_	= 123e0i
+	_	= 0123E0i
+	_	= 000123E0i
+
+	_	= 0.e+1i
+	_	= 123.E-1_0i
+	_	= 01_23.e123i
+	_	= 00_01_23.e123i
+
+	_	= 0b1010i
+	_	= 0B1010i
+	_	= 0o660i
+	_	= 0O660i
+	_	= 0xabcDEFi
+	_	= 0XabcDEFi
+	_	= 0xabcDEFP0i
+	_	= 0XabcDEFp0i
+)
diff --git a/src/go/printer/testdata/go2numbers.input b/src/go/printer/testdata/go2numbers.input
new file mode 100644
index 0000000..f3e7828
--- /dev/null
+++ b/src/go/printer/testdata/go2numbers.input
@@ -0,0 +1,186 @@
+package p
+
+const (
+	// 0-octals
+	_ = 0
+	_ = 0123
+	_ = 0123456
+
+	_ = 0_123
+	_ = 0123_456
+
+	// decimals
+	_ = 1
+	_ = 1234
+	_ = 1234567
+
+	_ = 1_234
+	_ = 1_234_567
+
+	// hexadecimals
+	_ = 0x0
+	_ = 0x1234
+	_ = 0xcafef00d
+
+	_ = 0X0
+	_ = 0X1234
+	_ = 0XCAFEf00d
+
+	_ = 0X_0
+	_ = 0X_1234
+	_ = 0X_CAFE_f00d
+
+	// octals
+	_ = 0o0
+	_ = 0o1234
+	_ = 0o01234567
+
+	_ = 0O0
+	_ = 0O1234
+	_ = 0O01234567
+
+	_ = 0o_0
+	_ = 0o_1234
+	_ = 0o0123_4567
+
+	_ = 0O_0
+	_ = 0O_1234
+	_ = 0O0123_4567
+
+	// binaries
+	_ = 0b0
+	_ = 0b1011
+	_ = 0b00101101
+
+	_ = 0B0
+	_ = 0B1011
+	_ = 0B00101101
+
+	_ = 0b_0
+	_ = 0b10_11
+	_ = 0b_0010_1101
+
+	// decimal floats
+	_ = 0.
+	_ = 123.
+	_ = 0123.
+
+	_ = .0
+	_ = .123
+	_ = .0123
+
+	_ = 0e0
+	_ = 123e+0
+	_ = 0123E-1
+
+	_ = 0e-0
+	_ = 123E+0
+	_ = 0123E123
+
+	_ = 0.e+1
+	_ = 123.E-10
+	_ = 0123.e123
+
+	_ = .0e-1
+	_ = .123E+10
+	_ = .0123E123
+
+	_ = 0.0
+	_ = 123.123
+	_ = 0123.0123
+
+	_ = 0.0e1
+	_ = 123.123E-10
+	_ = 0123.0123e+456
+
+	_ = 1_2_3.
+	_ = 0_123.
+
+	_ = 0_0e0
+	_ = 1_2_3e0
+	_ = 0_123e0
+
+	_ = 0e-0_0
+	_ = 1_2_3E+0
+	_ = 0123E1_2_3
+
+	_ = 0.e+1
+	_ = 123.E-1_0
+	_ = 01_23.e123
+
+	_ = .0e-1
+	_ = .123E+10
+	_ = .0123E123
+
+	_ = 1_2_3.123
+	_ = 0123.01_23
+
+	// hexadecimal floats
+	_ = 0x0.p+0
+	_ = 0Xdeadcafe.p-10
+	_ = 0x1234.P123
+
+	_ = 0x.1p-0
+	_ = 0X.deadcafep2
+	_ = 0x.1234P+10
+
+	_ = 0x0p0
+	_ = 0Xdeadcafep+1
+	_ = 0x1234P-10
+
+	_ = 0x0.0p0
+	_ = 0Xdead.cafep+1
+	_ = 0x12.34P-10
+
+	_ = 0Xdead_cafep+1
+	_ = 0x_1234P-10
+
+	_ = 0X_dead_cafe.p-10
+	_ = 0x12_34.P1_2_3
+	_ = 0X1_2_3_4.P-1_2_3
+
+	// imaginaries
+	_ = 0i
+	_ = 00i
+	_ = 08i
+	_ = 0000000000i
+	_ = 0123i
+	_ = 0000000123i
+	_ = 0000056789i
+	_ = 1234i
+	_ = 1234567i
+
+	_ = 0i
+	_ = 0_0i
+	_ = 0_8i
+	_ = 0_000_000_000i
+	_ = 0_123i
+	_ = 0_000_000_123i
+	_ = 0_000_056_789i
+	_ = 1_234i
+	_ = 1_234_567i
+
+	_ = 0.i
+	_ = 123.i
+	_ = 0123.i
+	_ = 000123.i
+
+	_ = 0e0i
+	_ = 123e0i
+	_ = 0123E0i
+	_ = 000123E0i
+
+	_ = 0.e+1i
+	_ = 123.E-1_0i
+	_ = 01_23.e123i
+	_ = 00_01_23.e123i
+
+	_ = 0b1010i
+	_ = 0B1010i
+	_ = 0o660i
+	_ = 0O660i
+	_ = 0xabcDEFi
+	_ = 0XabcDEFi
+	_ = 0xabcDEFP0i
+	_ = 0XabcDEFp0i
+)
diff --git a/src/go/printer/testdata/go2numbers.norm b/src/go/printer/testdata/go2numbers.norm
new file mode 100644
index 0000000..855f0fc
--- /dev/null
+++ b/src/go/printer/testdata/go2numbers.norm
@@ -0,0 +1,186 @@
+package p
+
+const (
+	// 0-octals
+	_	= 0
+	_	= 0123
+	_	= 0123456
+
+	_	= 0_123
+	_	= 0123_456
+
+	// decimals
+	_	= 1
+	_	= 1234
+	_	= 1234567
+
+	_	= 1_234
+	_	= 1_234_567
+
+	// hexadecimals
+	_	= 0x0
+	_	= 0x1234
+	_	= 0xcafef00d
+
+	_	= 0x0
+	_	= 0x1234
+	_	= 0xCAFEf00d
+
+	_	= 0x_0
+	_	= 0x_1234
+	_	= 0x_CAFE_f00d
+
+	// octals
+	_	= 0o0
+	_	= 0o1234
+	_	= 0o01234567
+
+	_	= 0o0
+	_	= 0o1234
+	_	= 0o01234567
+
+	_	= 0o_0
+	_	= 0o_1234
+	_	= 0o0123_4567
+
+	_	= 0o_0
+	_	= 0o_1234
+	_	= 0o0123_4567
+
+	// binaries
+	_	= 0b0
+	_	= 0b1011
+	_	= 0b00101101
+
+	_	= 0b0
+	_	= 0b1011
+	_	= 0b00101101
+
+	_	= 0b_0
+	_	= 0b10_11
+	_	= 0b_0010_1101
+
+	// decimal floats
+	_	= 0.
+	_	= 123.
+	_	= 0123.
+
+	_	= .0
+	_	= .123
+	_	= .0123
+
+	_	= 0e0
+	_	= 123e+0
+	_	= 0123e-1
+
+	_	= 0e-0
+	_	= 123e+0
+	_	= 0123e123
+
+	_	= 0.e+1
+	_	= 123.e-10
+	_	= 0123.e123
+
+	_	= .0e-1
+	_	= .123e+10
+	_	= .0123e123
+
+	_	= 0.0
+	_	= 123.123
+	_	= 0123.0123
+
+	_	= 0.0e1
+	_	= 123.123e-10
+	_	= 0123.0123e+456
+
+	_	= 1_2_3.
+	_	= 0_123.
+
+	_	= 0_0e0
+	_	= 1_2_3e0
+	_	= 0_123e0
+
+	_	= 0e-0_0
+	_	= 1_2_3e+0
+	_	= 0123e1_2_3
+
+	_	= 0.e+1
+	_	= 123.e-1_0
+	_	= 01_23.e123
+
+	_	= .0e-1
+	_	= .123e+10
+	_	= .0123e123
+
+	_	= 1_2_3.123
+	_	= 0123.01_23
+
+	// hexadecimal floats
+	_	= 0x0.p+0
+	_	= 0xdeadcafe.p-10
+	_	= 0x1234.p123
+
+	_	= 0x.1p-0
+	_	= 0x.deadcafep2
+	_	= 0x.1234p+10
+
+	_	= 0x0p0
+	_	= 0xdeadcafep+1
+	_	= 0x1234p-10
+
+	_	= 0x0.0p0
+	_	= 0xdead.cafep+1
+	_	= 0x12.34p-10
+
+	_	= 0xdead_cafep+1
+	_	= 0x_1234p-10
+
+	_	= 0x_dead_cafe.p-10
+	_	= 0x12_34.p1_2_3
+	_	= 0x1_2_3_4.p-1_2_3
+
+	// imaginaries
+	_	= 0i
+	_	= 0i
+	_	= 8i
+	_	= 0i
+	_	= 123i
+	_	= 123i
+	_	= 56789i
+	_	= 1234i
+	_	= 1234567i
+
+	_	= 0i
+	_	= 0i
+	_	= 8i
+	_	= 0i
+	_	= 123i
+	_	= 123i
+	_	= 56_789i
+	_	= 1_234i
+	_	= 1_234_567i
+
+	_	= 0.i
+	_	= 123.i
+	_	= 0123.i
+	_	= 000123.i
+
+	_	= 0e0i
+	_	= 123e0i
+	_	= 0123e0i
+	_	= 000123e0i
+
+	_	= 0.e+1i
+	_	= 123.e-1_0i
+	_	= 01_23.e123i
+	_	= 00_01_23.e123i
+
+	_	= 0b1010i
+	_	= 0b1010i
+	_	= 0o660i
+	_	= 0o660i
+	_	= 0xabcDEFi
+	_	= 0xabcDEFi
+	_	= 0xabcDEFp0i
+	_	= 0xabcDEFp0i
+)
diff --git a/src/go/printer/testdata/linebreaks.golden b/src/go/printer/testdata/linebreaks.golden
new file mode 100644
index 0000000..17d2b5c
--- /dev/null
+++ b/src/go/printer/testdata/linebreaks.golden
@@ -0,0 +1,295 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package linebreaks
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+type writerTestEntry struct {
+	header		*Header
+	contents	string
+}
+
+type writerTest struct {
+	file	string	// filename of expected output
+	entries	[]*writerTestEntry
+}
+
+var writerTests = []*writerTest{
+	&writerTest{
+		file:	"testdata/writer.tar",
+		entries: []*writerTestEntry{
+			&writerTestEntry{
+				header: &Header{
+					Name:		"small.txt",
+					Mode:		0640,
+					Uid:		73025,
+					Gid:		5000,
+					Size:		5,
+					Mtime:		1246508266,
+					Typeflag:	'0',
+					Uname:		"dsymonds",
+					Gname:		"eng",
+				},
+				contents:	"Kilts",
+			},
+			&writerTestEntry{
+				header: &Header{
+					Name:		"small2.txt",
+					Mode:		0640,
+					Uid:		73025,
+					Gid:		5000,
+					Size:		11,
+					Mtime:		1245217492,
+					Typeflag:	'0',
+					Uname:		"dsymonds",
+					Gname:		"eng",
+				},
+				contents:	"Google.com\n",
+			},
+		},
+	},
+	// The truncated test file was produced using these commands:
+	//   dd if=/dev/zero bs=1048576 count=16384 > /tmp/16gig.txt
+	//   tar -b 1 -c -f- /tmp/16gig.txt | dd bs=512 count=8 > writer-big.tar
+	&writerTest{
+		file:	"testdata/writer-big.tar",
+		entries: []*writerTestEntry{
+			&writerTestEntry{
+				header: &Header{
+					Name:		"tmp/16gig.txt",
+					Mode:		0640,
+					Uid:		73025,
+					Gid:		5000,
+					Size:		16 << 30,
+					Mtime:		1254699560,
+					Typeflag:	'0',
+					Uname:		"dsymonds",
+					Gname:		"eng",
+				},
+				// no contents
+			},
+		},
+	},
+}
+
+type untarTest struct {
+	file	string
+	headers	[]*Header
+}
+
+var untarTests = []*untarTest{
+	&untarTest{
+		file:	"testdata/gnu.tar",
+		headers: []*Header{
+			&Header{
+				Name:		"small.txt",
+				Mode:		0640,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		5,
+				Mtime:		1244428340,
+				Typeflag:	'0',
+				Uname:		"dsymonds",
+				Gname:		"eng",
+			},
+			&Header{
+				Name:		"small2.txt",
+				Mode:		0640,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		11,
+				Mtime:		1244436044,
+				Typeflag:	'0',
+				Uname:		"dsymonds",
+				Gname:		"eng",
+			},
+		},
+	},
+	&untarTest{
+		file:	"testdata/star.tar",
+		headers: []*Header{
+			&Header{
+				Name:		"small.txt",
+				Mode:		0640,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		5,
+				Mtime:		1244592783,
+				Typeflag:	'0',
+				Uname:		"dsymonds",
+				Gname:		"eng",
+				Atime:		1244592783,
+				Ctime:		1244592783,
+			},
+			&Header{
+				Name:		"small2.txt",
+				Mode:		0640,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		11,
+				Mtime:		1244592783,
+				Typeflag:	'0',
+				Uname:		"dsymonds",
+				Gname:		"eng",
+				Atime:		1244592783,
+				Ctime:		1244592783,
+			},
+		},
+	},
+	&untarTest{
+		file:	"testdata/v7.tar",
+		headers: []*Header{
+			&Header{
+				Name:		"small.txt",
+				Mode:		0444,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		5,
+				Mtime:		1244593104,
+				Typeflag:	'\x00',
+			},
+			&Header{
+				Name:		"small2.txt",
+				Mode:		0444,
+				Uid:		73025,
+				Gid:		5000,
+				Size:		11,
+				Mtime:		1244593104,
+				Typeflag:	'\x00',
+			},
+		},
+	},
+}
+
+var facts = map[int]string{
+	0:	"1",
+	1:	"1",
+	2:	"2",
+	10:	"3628800",
+	20:	"2432902008176640000",
+	100: "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000",
+}
+
+func usage() {
+	fmt.Fprintf(os.Stderr,
+		// TODO(gri): the 2nd string of this string list should not be indented
+		"usage: godoc package [name ...]\n"+
+			"	godoc -http=:6060\n")
+	flag.PrintDefaults()
+	os.Exit(2)
+}
+
+func TestReader(t *testing.T) {
+testLoop:
+	for i, test := range untarTests {
+		f, err := os.Open(test.file, os.O_RDONLY, 0444)
+		if err != nil {
+			t.Errorf("test %d: Unexpected error: %v", i, err)
+			continue
+		}
+		tr := NewReader(f)
+		for j, header := range test.headers {
+			hdr, err := tr.Next()
+			if err != nil || hdr == nil {
+				t.Errorf("test %d, entry %d: Didn't get entry: %v", i, j, err)
+				f.Close()
+				continue testLoop
+			}
+			if !reflect.DeepEqual(hdr, header) {
+				t.Errorf("test %d, entry %d: Incorrect header:\nhave %+v\nwant %+v",
+					i, j, *hdr, *header)
+			}
+		}
+		hdr, err := tr.Next()
+		if hdr != nil || err != nil {
+			t.Errorf("test %d: Unexpected entry or error: hdr=%v err=%v", i, err)
+		}
+		f.Close()
+	}
+}
+
+// Respect line breaks in function calls.
+func _() {
+	f(x)
+	f(x,
+		x)
+	f(x,
+		x,
+	)
+	f(
+		x,
+		x)
+	f(
+		x,
+		x,
+	)
+}
+
+// Respect line breaks in function declarations.
+func _(x T)	{}
+func _(x T,
+	y T) {
+}
+func _(x T,
+	y T,
+) {
+}
+func _(
+	x T,
+	y T) {
+}
+func _(
+	x T,
+	y T,
+) {
+}
+
+// Example from issue #2597.
+func ManageStatus0(
+	in <-chan *Status,
+	req <-chan Request,
+	stat chan<- *TargetInfo,
+	TargetHistorySize int) {
+}
+
+func ManageStatus1(
+	in <-chan *Status,
+	req <-chan Request,
+	stat chan<- *TargetInfo,
+	TargetHistorySize int,
+) {
+}
+
+// Example from issue #9064.
+func (y *y) xerrors() error {
+	_ = "xerror.test"	//TODO-
+	_ = []byte(`
+foo bar foo bar foo bar
+`) //TODO-
+}
+
+func _() {
+	_ = "abc"		// foo
+	_ = `abc_0123456789_`	// foo
+}
+
+func _() {
+	_ = "abc"	// foo
+	_ = `abc
+0123456789
+` // foo
+}
+
+// There should be exactly one linebreak after this comment.
diff --git a/src/go/printer/testdata/linebreaks.input b/src/go/printer/testdata/linebreaks.input
new file mode 100644
index 0000000..9e714f3
--- /dev/null
+++ b/src/go/printer/testdata/linebreaks.input
@@ -0,0 +1,291 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package linebreaks
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+type writerTestEntry struct {
+	header *Header
+	contents string
+}
+
+type writerTest struct {
+	file string  // filename of expected output
+	entries []*writerTestEntry
+}
+
+var writerTests = []*writerTest{
+	&writerTest{
+		file: "testdata/writer.tar",
+		entries: []*writerTestEntry{
+			&writerTestEntry{
+				header: &Header{
+					Name: "small.txt",
+					Mode: 0640,
+					Uid: 73025,
+					Gid: 5000,
+					Size: 5,
+					Mtime: 1246508266,
+					Typeflag: '0',
+					Uname: "dsymonds",
+					Gname: "eng",
+				},
+				contents: "Kilts",
+			},
+			&writerTestEntry{
+				header: &Header{
+					Name: "small2.txt",
+					Mode: 0640,
+					Uid: 73025,
+					Gid: 5000,
+					Size: 11,
+					Mtime: 1245217492,
+					Typeflag: '0',
+					Uname: "dsymonds",
+					Gname: "eng",
+				},
+				contents: "Google.com\n",
+			},
+		},
+	},
+	// The truncated test file was produced using these commands:
+	//   dd if=/dev/zero bs=1048576 count=16384 > /tmp/16gig.txt
+	//   tar -b 1 -c -f- /tmp/16gig.txt | dd bs=512 count=8 > writer-big.tar
+	&writerTest{
+		file: "testdata/writer-big.tar",
+		entries: []*writerTestEntry{
+			&writerTestEntry{
+				header: &Header{
+					Name: "tmp/16gig.txt",
+					Mode: 0640,
+					Uid: 73025,
+					Gid: 5000,
+					Size: 16 << 30,
+					Mtime: 1254699560,
+					Typeflag: '0',
+					Uname: "dsymonds",
+					Gname: "eng",
+				},
+				// no contents
+			},
+		},
+	},
+}
+
+type untarTest struct {
+	file string
+	headers []*Header
+}
+
+var untarTests = []*untarTest{
+	&untarTest{
+		file: "testdata/gnu.tar",
+		headers: []*Header{
+			&Header{
+				Name: "small.txt",
+				Mode: 0640,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 5,
+				Mtime: 1244428340,
+				Typeflag: '0',
+				Uname: "dsymonds",
+				Gname: "eng",
+			},
+			&Header{
+				Name: "small2.txt",
+				Mode: 0640,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 11,
+				Mtime: 1244436044,
+				Typeflag: '0',
+				Uname: "dsymonds",
+				Gname: "eng",
+			},
+		},
+	},
+	&untarTest{
+		file: "testdata/star.tar",
+		headers: []*Header{
+			&Header{
+				Name: "small.txt",
+				Mode: 0640,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 5,
+				Mtime: 1244592783,
+				Typeflag: '0',
+				Uname: "dsymonds",
+				Gname: "eng",
+				Atime: 1244592783,
+				Ctime: 1244592783,
+			},
+			&Header{
+				Name: "small2.txt",
+				Mode: 0640,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 11,
+				Mtime: 1244592783,
+				Typeflag: '0',
+				Uname: "dsymonds",
+				Gname: "eng",
+				Atime: 1244592783,
+				Ctime: 1244592783,
+			},
+		},
+	},
+	&untarTest{
+		file: "testdata/v7.tar",
+		headers: []*Header{
+			&Header{
+				Name: "small.txt",
+				Mode: 0444,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 5,
+				Mtime: 1244593104,
+				Typeflag: '\x00',
+			},
+			&Header{
+				Name: "small2.txt",
+				Mode: 0444,
+				Uid: 73025,
+				Gid: 5000,
+				Size: 11,
+				Mtime: 1244593104,
+				Typeflag: '\x00',
+			},
+		},
+	},
+}
+
+var facts = map[int] string {
+	0: "1",
+	1: "1",
+	2: "2",
+	10: "3628800",
+	20: "2432902008176640000",
+	100: "933262154439441526816992388562667004907159682643816214685929" +
+		"638952175999932299156089414639761565182862536979208272237582" +
+		"51185210916864000000000000000000000000",
+}
+
+func usage() {
+	fmt.Fprintf(os.Stderr,
+		// TODO(gri): the 2nd string of this string list should not be indented
+		"usage: godoc package [name ...]\n" +
+		"	godoc -http=:6060\n")
+	flag.PrintDefaults()
+	os.Exit(2)
+}
+
+func TestReader(t *testing.T) {
+testLoop:
+	for i, test := range untarTests {
+		f, err := os.Open(test.file, os.O_RDONLY, 0444)
+		if err != nil {
+			t.Errorf("test %d: Unexpected error: %v", i, err)
+			continue
+		}
+		tr := NewReader(f)
+		for j, header := range test.headers {
+			hdr, err := tr.Next()
+			if err != nil || hdr == nil {
+				t.Errorf("test %d, entry %d: Didn't get entry: %v", i, j, err)
+				f.Close()
+				continue testLoop
+			}
+			if !reflect.DeepEqual(hdr, header) {
+				t.Errorf("test %d, entry %d: Incorrect header:\nhave %+v\nwant %+v",
+					 i, j, *hdr, *header)
+			}
+		}
+		hdr, err := tr.Next()
+		if hdr != nil || err != nil {
+			t.Errorf("test %d: Unexpected entry or error: hdr=%v err=%v", i, err)
+		}
+		f.Close()
+	}
+}
+
+// Respect line breaks in function calls.
+func _() {
+	f(x)
+	f(x,
+	  x)
+	f(x,
+	  x,
+	)
+	f(
+	  x,
+	  x)
+	f(
+	  x,
+	  x,
+	)
+}
+
+// Respect line breaks in function declarations.
+func _(x T) {}
+func _(x T,
+       y T) {}
+func _(x T,
+       y T,
+) {}
+func _(
+       x T,
+       y T) {}
+func _(
+       x T,
+       y T,
+) {}
+
+// Example from issue #2597.
+func ManageStatus0(
+	in <-chan *Status,
+	req <-chan Request,
+	stat chan<- *TargetInfo,
+	TargetHistorySize int) {
+}
+    
+func ManageStatus1(
+	in <-chan *Status,
+	req <-chan Request,
+	stat chan<- *TargetInfo,
+	TargetHistorySize int,
+) {
+}
+
+// Example from issue #9064.
+func (y *y) xerrors() error {
+	_ = "xerror.test" //TODO-
+	_ = []byte(`
+foo bar foo bar foo bar
+`) //TODO-
+}
+
+func _() {
+	_ = "abc" // foo
+	_ = `abc_0123456789_` // foo
+}
+
+func _() {
+	_ = "abc" // foo
+	_ = `abc
+0123456789
+` // foo
+}
+
+// There should be exactly one linebreak after this comment.
diff --git a/src/go/printer/testdata/parser.go b/src/go/printer/testdata/parser.go
new file mode 100644
index 0000000..80b476c
--- /dev/null
+++ b/src/go/printer/testdata/parser.go
@@ -0,0 +1,2153 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package parser implements a parser for Go source files. Input may be
+// provided in a variety of forms (see the various Parse* functions); the
+// output is an abstract syntax tree (AST) representing the Go source. The
+// parser is invoked through one of the Parse* functions.
+
+package parser
+
+import (
+	"fmt"
+	"go/ast"
+	"go/scanner"
+	"go/token"
+)
+
+// The mode parameter to the Parse* functions is a set of flags (or 0).
+// They control the amount of source code parsed and other optional
+// parser functionality.
+//
+const (
+	PackageClauseOnly uint = 1 << iota // parsing stops after package clause
+	ImportsOnly                        // parsing stops after import declarations
+	ParseComments                      // parse comments and add them to AST
+	Trace                              // print a trace of parsed productions
+	DeclarationErrors                  // report declaration errors
+)
+
+// The parser structure holds the parser's internal state.
+type parser struct {
+	file *token.File
+	scanner.ErrorVector
+	scanner scanner.Scanner
+
+	// Tracing/debugging
+	mode   uint // parsing mode
+	trace  bool // == (mode & Trace != 0)
+	indent uint // indentation used for tracing output
+
+	// Comments
+	comments    []*ast.CommentGroup
+	leadComment *ast.CommentGroup // last lead comment
+	lineComment *ast.CommentGroup // last line comment
+
+	// Next token
+	pos token.Pos   // token position
+	tok token.Token // one token look-ahead
+	lit string      // token literal
+
+	// Non-syntactic parser control
+	exprLev int // < 0: in control clause, >= 0: in expression
+
+	// Ordinary identifier scopes
+	pkgScope   *ast.Scope        // pkgScope.Outer == nil
+	topScope   *ast.Scope        // top-most scope; may be pkgScope
+	unresolved []*ast.Ident      // unresolved identifiers
+	imports    []*ast.ImportSpec // list of imports
+
+	// Label scope
+	// (maintained by open/close LabelScope)
+	labelScope  *ast.Scope     // label scope for current function
+	targetStack [][]*ast.Ident // stack of unresolved labels
+}
+
+// scannerMode returns the scanner mode bits given the parser's mode bits.
+func scannerMode(mode uint) uint {
+	var m uint = scanner.InsertSemis
+	if mode&ParseComments != 0 {
+		m |= scanner.ScanComments
+	}
+	return m
+}
+
+func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode uint) {
+	p.file = fset.AddFile(filename, fset.Base(), len(src))
+	p.scanner.Init(p.file, src, p, scannerMode(mode))
+
+	p.mode = mode
+	p.trace = mode&Trace != 0 // for convenience (p.trace is used frequently)
+
+	p.next()
+
+	// set up the pkgScope here (as opposed to in parseFile) because
+	// there are other parser entry points (ParseExpr, etc.)
+	p.openScope()
+	p.pkgScope = p.topScope
+
+	// for the same reason, set up a label scope
+	p.openLabelScope()
+}
+
+// ----------------------------------------------------------------------------
+// Scoping support
+
+func (p *parser) openScope() {
+	p.topScope = ast.NewScope(p.topScope)
+}
+
+func (p *parser) closeScope() {
+	p.topScope = p.topScope.Outer
+}
+
+func (p *parser) openLabelScope() {
+	p.labelScope = ast.NewScope(p.labelScope)
+	p.targetStack = append(p.targetStack, nil)
+}
+
+func (p *parser) closeLabelScope() {
+	// resolve labels
+	n := len(p.targetStack) - 1
+	scope := p.labelScope
+	for _, ident := range p.targetStack[n] {
+		ident.Obj = scope.Lookup(ident.Name)
+		if ident.Obj == nil && p.mode&DeclarationErrors != 0 {
+			p.error(ident.Pos(), fmt.Sprintf("label %s undefined", ident.Name))
+		}
+	}
+	// pop label scope
+	p.targetStack = p.targetStack[0:n]
+	p.labelScope = p.labelScope.Outer
+}
+
+func (p *parser) declare(decl interface{}, scope *ast.Scope, kind ast.ObjKind, idents ...*ast.Ident) {
+	for _, ident := range idents {
+		assert(ident.Obj == nil, "identifier already declared or resolved")
+		if ident.Name != "_" {
+			obj := ast.NewObj(kind, ident.Name)
+			// remember the corresponding declaration for redeclaration
+			// errors and global variable resolution/typechecking phase
+			obj.Decl = decl
+			if alt := scope.Insert(obj); alt != nil && p.mode&DeclarationErrors != 0 {
+				prevDecl := ""
+				if pos := alt.Pos(); pos.IsValid() {
+					prevDecl = fmt.Sprintf("\n\tprevious declaration at %s", p.file.Position(pos))
+				}
+				p.error(ident.Pos(), fmt.Sprintf("%s redeclared in this block%s", ident.Name, prevDecl))
+			}
+			ident.Obj = obj
+		}
+	}
+}
+
+func (p *parser) shortVarDecl(idents []*ast.Ident) {
+	// Go spec: A short variable declaration may redeclare variables
+	// provided they were originally declared in the same block with
+	// the same type, and at least one of the non-blank variables is new.
+	n := 0 // number of new variables
+	for _, ident := range idents {
+		assert(ident.Obj == nil, "identifier already declared or resolved")
+		if ident.Name != "_" {
+			obj := ast.NewObj(ast.Var, ident.Name)
+			// short var declarations cannot have redeclaration errors
+			// and are not global => no need to remember the respective
+			// declaration
+			alt := p.topScope.Insert(obj)
+			if alt == nil {
+				n++ // new declaration
+				alt = obj
+			}
+			ident.Obj = alt
+		}
+	}
+	if n == 0 && p.mode&DeclarationErrors != 0 {
+		p.error(idents[0].Pos(), "no new variables on left side of :=")
+	}
+}
+
+// The unresolved object is a sentinel to mark identifiers that have been added
+// to the list of unresolved identifiers. The sentinel is only used for verifying
+// internal consistency.
+var unresolved = new(ast.Object)
+
+func (p *parser) resolve(x ast.Expr) {
+	// nothing to do if x is not an identifier or the blank identifier
+	ident, _ := x.(*ast.Ident)
+	if ident == nil {
+		return
+	}
+	assert(ident.Obj == nil, "identifier already declared or resolved")
+	if ident.Name == "_" {
+		return
+	}
+	// try to resolve the identifier
+	for s := p.topScope; s != nil; s = s.Outer {
+		if obj := s.Lookup(ident.Name); obj != nil {
+			ident.Obj = obj
+			return
+		}
+	}
+	// all local scopes are known, so any unresolved identifier
+	// must be found either in the file scope, package scope
+	// (perhaps in another file), or universe scope --- collect
+	// them so that they can be resolved later
+	ident.Obj = unresolved
+	p.unresolved = append(p.unresolved, ident)
+}
+
+// ----------------------------------------------------------------------------
+// Parsing support
+
+func (p *parser) printTrace(a ...interface{}) {
+	const dots = ". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . " +
+		". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "
+	const n = uint(len(dots))
+	pos := p.file.Position(p.pos)
+	fmt.Printf("%5d:%3d: ", pos.Line, pos.Column)
+	i := 2 * p.indent
+	for ; i > n; i -= n {
+		fmt.Print(dots)
+	}
+	fmt.Print(dots[0:i])
+	fmt.Println(a...)
+}
+
+func trace(p *parser, msg string) *parser {
+	p.printTrace(msg, "(")
+	p.indent++
+	return p
+}
+
+// Usage pattern: defer un(trace(p, "..."));
+func un(p *parser) {
+	p.indent--
+	p.printTrace(")")
+}
+
+// Advance to the next token.
+func (p *parser) next0() {
+	// Because of one-token look-ahead, print the previous token
+	// when tracing as it provides a more readable output. The
+	// very first token (!p.pos.IsValid()) is not initialized
+	// (it is token.ILLEGAL), so don't print it .
+	if p.trace && p.pos.IsValid() {
+		s := p.tok.String()
+		switch {
+		case p.tok.IsLiteral():
+			p.printTrace(s, p.lit)
+		case p.tok.IsOperator(), p.tok.IsKeyword():
+			p.printTrace("\"" + s + "\"")
+		default:
+			p.printTrace(s)
+		}
+	}
+
+	p.pos, p.tok, p.lit = p.scanner.Scan()
+}
+
+// Consume a comment and return it and the line on which it ends.
+func (p *parser) consumeComment() (comment *ast.Comment, endline int) {
+	// /*-style comments may end on a different line than where they start.
+	// Scan the comment for '\n' chars and adjust endline accordingly.
+	endline = p.file.Line(p.pos)
+	if p.lit[1] == '*' {
+		// don't use range here - no need to decode Unicode code points
+		for i := 0; i < len(p.lit); i++ {
+			if p.lit[i] == '\n' {
+				endline++
+			}
+		}
+	}
+
+	comment = &ast.Comment{p.pos, p.lit}
+	p.next0()
+
+	return
+}
+
+// Consume a group of adjacent comments, add it to the parser's
+// comments list, and return it together with the line at which
+// the last comment in the group ends. An empty line or non-comment
+// token terminates a comment group.
+//
+func (p *parser) consumeCommentGroup() (comments *ast.CommentGroup, endline int) {
+	var list []*ast.Comment
+	endline = p.file.Line(p.pos)
+	for p.tok == token.COMMENT && endline+1 >= p.file.Line(p.pos) {
+		var comment *ast.Comment
+		comment, endline = p.consumeComment()
+		list = append(list, comment)
+	}
+
+	// add comment group to the comments list
+	comments = &ast.CommentGroup{list}
+	p.comments = append(p.comments, comments)
+
+	return
+}
+
+// Advance to the next non-comment token. In the process, collect
+// any comment groups encountered, and remember the last lead and
+// line comments.
+//
+// A lead comment is a comment group that starts and ends in a
+// line without any other tokens and that is followed by a non-comment
+// token on the line immediately after the comment group.
+//
+// A line comment is a comment group that follows a non-comment
+// token on the same line, and that has no tokens after it on the line
+// where it ends.
+//
+// Lead and line comments may be considered documentation that is
+// stored in the AST.
+//
+func (p *parser) next() {
+	p.leadComment = nil
+	p.lineComment = nil
+	line := p.file.Line(p.pos) // current line
+	p.next0()
+
+	if p.tok == token.COMMENT {
+		var comment *ast.CommentGroup
+		var endline int
+
+		if p.file.Line(p.pos) == line {
+			// The comment is on same line as the previous token; it
+			// cannot be a lead comment but may be a line comment.
+			comment, endline = p.consumeCommentGroup()
+			if p.file.Line(p.pos) != endline {
+				// The next token is on a different line, thus
+				// the last comment group is a line comment.
+				p.lineComment = comment
+			}
+		}
+
+		// consume successor comments, if any
+		endline = -1
+		for p.tok == token.COMMENT {
+			comment, endline = p.consumeCommentGroup()
+		}
+
+		if endline+1 == p.file.Line(p.pos) {
+			// The next token is following on the line immediately after the
+			// comment group, thus the last comment group is a lead comment.
+			p.leadComment = comment
+		}
+	}
+}
+
+func (p *parser) error(pos token.Pos, msg string) {
+	p.Error(p.file.Position(pos), msg)
+}
+
+func (p *parser) errorExpected(pos token.Pos, msg string) {
+	msg = "expected " + msg
+	if pos == p.pos {
+		// the error happened at the current position;
+		// make the error message more specific
+		if p.tok == token.SEMICOLON && p.lit[0] == '\n' {
+			msg += ", found newline"
+		} else {
+			msg += ", found '" + p.tok.String() + "'"
+			if p.tok.IsLiteral() {
+				msg += " " + p.lit
+			}
+		}
+	}
+	p.error(pos, msg)
+}
+
+func (p *parser) expect(tok token.Token) token.Pos {
+	pos := p.pos
+	if p.tok != tok {
+		p.errorExpected(pos, "'"+tok.String()+"'")
+	}
+	p.next() // make progress
+	return pos
+}
+
+func (p *parser) expectSemi() {
+	if p.tok != token.RPAREN && p.tok != token.RBRACE {
+		p.expect(token.SEMICOLON)
+	}
+}
+
+func assert(cond bool, msg string) {
+	if !cond {
+		panic("go/parser internal error: " + msg)
+	}
+}
+
+// ----------------------------------------------------------------------------
+// Identifiers
+
+func (p *parser) parseIdent() *ast.Ident {
+	pos := p.pos
+	name := "_"
+	if p.tok == token.IDENT {
+		name = p.lit
+		p.next()
+	} else {
+		p.expect(token.IDENT) // use expect() error handling
+	}
+	return &ast.Ident{pos, name, nil}
+}
+
+func (p *parser) parseIdentList() (list []*ast.Ident) {
+	if p.trace {
+		defer un(trace(p, "IdentList"))
+	}
+
+	list = append(list, p.parseIdent())
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.parseIdent())
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Common productions
+
+// If lhs is set, result list elements which are identifiers are not resolved.
+func (p *parser) parseExprList(lhs bool) (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "ExpressionList"))
+	}
+
+	list = append(list, p.parseExpr(lhs))
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.parseExpr(lhs))
+	}
+
+	return
+}
+
+func (p *parser) parseLhsList() []ast.Expr {
+	list := p.parseExprList(true)
+	switch p.tok {
+	case token.DEFINE:
+		// lhs of a short variable declaration
+		p.shortVarDecl(p.makeIdentList(list))
+	case token.COLON:
+		// lhs of a label declaration or a communication clause of a select
+		// statement (parseLhsList is not called when parsing the case clause
+		// of a switch statement):
+		// - labels are declared by the caller of parseLhsList
+		// - for communication clauses, if there is a stand-alone identifier
+		//   followed by a colon, we have a syntax error; there is no need
+		//   to resolve the identifier in that case
+	default:
+		// identifiers must be declared elsewhere
+		for _, x := range list {
+			p.resolve(x)
+		}
+	}
+	return list
+}
+
+func (p *parser) parseRhsList() []ast.Expr {
+	return p.parseExprList(false)
+}
+
+// ----------------------------------------------------------------------------
+// Types
+
+func (p *parser) parseType() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Type"))
+	}
+
+	typ := p.tryType()
+
+	if typ == nil {
+		pos := p.pos
+		p.errorExpected(pos, "type")
+		p.next() // make progress
+		return &ast.BadExpr{pos, p.pos}
+	}
+
+	return typ
+}
+
+// If the result is an identifier, it is not resolved.
+func (p *parser) parseTypeName() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "TypeName"))
+	}
+
+	ident := p.parseIdent()
+	// don't resolve ident yet - it may be a parameter or field name
+
+	if p.tok == token.PERIOD {
+		// ident is a package name
+		p.next()
+		p.resolve(ident)
+		sel := p.parseIdent()
+		return &ast.SelectorExpr{ident, sel}
+	}
+
+	return ident
+}
+
+func (p *parser) parseArrayType(ellipsisOk bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "ArrayType"))
+	}
+
+	lbrack := p.expect(token.LBRACK)
+	var len ast.Expr
+	if ellipsisOk && p.tok == token.ELLIPSIS {
+		len = &ast.Ellipsis{p.pos, nil}
+		p.next()
+	} else if p.tok != token.RBRACK {
+		len = p.parseRhs()
+	}
+	p.expect(token.RBRACK)
+	elt := p.parseType()
+
+	return &ast.ArrayType{lbrack, len, elt}
+}
+
+func (p *parser) makeIdentList(list []ast.Expr) []*ast.Ident {
+	idents := make([]*ast.Ident, len(list))
+	for i, x := range list {
+		ident, isIdent := x.(*ast.Ident)
+		if !isIdent {
+			pos := x.(ast.Expr).Pos()
+			p.errorExpected(pos, "identifier")
+			ident = &ast.Ident{pos, "_", nil}
+		}
+		idents[i] = ident
+	}
+	return idents
+}
+
+func (p *parser) parseFieldDecl(scope *ast.Scope) *ast.Field {
+	if p.trace {
+		defer un(trace(p, "FieldDecl"))
+	}
+
+	doc := p.leadComment
+
+	// fields
+	list, typ := p.parseVarList(false)
+
+	// optional tag
+	var tag *ast.BasicLit
+	if p.tok == token.STRING {
+		tag = &ast.BasicLit{p.pos, p.tok, p.lit}
+		p.next()
+	}
+
+	// analyze case
+	var idents []*ast.Ident
+	if typ != nil {
+		// IdentifierList Type
+		idents = p.makeIdentList(list)
+	} else {
+		// ["*"] TypeName (AnonymousField)
+		typ = list[0] // we always have at least one element
+		p.resolve(typ)
+		if n := len(list); n > 1 || !isTypeName(deref(typ)) {
+			pos := typ.Pos()
+			p.errorExpected(pos, "anonymous field")
+			typ = &ast.BadExpr{pos, list[n-1].End()}
+		}
+	}
+
+	p.expectSemi() // call before accessing p.linecomment
+
+	field := &ast.Field{doc, idents, typ, tag, p.lineComment}
+	p.declare(field, scope, ast.Var, idents...)
+
+	return field
+}
+
+func (p *parser) parseStructType() *ast.StructType {
+	if p.trace {
+		defer un(trace(p, "StructType"))
+	}
+
+	pos := p.expect(token.STRUCT)
+	lbrace := p.expect(token.LBRACE)
+	scope := ast.NewScope(nil) // struct scope
+	var list []*ast.Field
+	for p.tok == token.IDENT || p.tok == token.MUL || p.tok == token.LPAREN {
+		// a field declaration cannot start with a '(' but we accept
+		// it here for more robust parsing and better error messages
+		// (parseFieldDecl will check and complain if necessary)
+		list = append(list, p.parseFieldDecl(scope))
+	}
+	rbrace := p.expect(token.RBRACE)
+
+	// TODO(gri): store struct scope in AST
+	return &ast.StructType{pos, &ast.FieldList{lbrace, list, rbrace}, false}
+}
+
+func (p *parser) parsePointerType() *ast.StarExpr {
+	if p.trace {
+		defer un(trace(p, "PointerType"))
+	}
+
+	star := p.expect(token.MUL)
+	base := p.parseType()
+
+	return &ast.StarExpr{star, base}
+}
+
+func (p *parser) tryVarType(isParam bool) ast.Expr {
+	if isParam && p.tok == token.ELLIPSIS {
+		pos := p.pos
+		p.next()
+		typ := p.tryIdentOrType(isParam) // don't use parseType so we can provide better error message
+		if typ == nil {
+			p.error(pos, "'...' parameter is missing type")
+			typ = &ast.BadExpr{pos, p.pos}
+		}
+		if p.tok != token.RPAREN {
+			p.error(pos, "can use '...' with last parameter type only")
+		}
+		return &ast.Ellipsis{pos, typ}
+	}
+	return p.tryIdentOrType(false)
+}
+
+func (p *parser) parseVarType(isParam bool) ast.Expr {
+	typ := p.tryVarType(isParam)
+	if typ == nil {
+		pos := p.pos
+		p.errorExpected(pos, "type")
+		p.next() // make progress
+		typ = &ast.BadExpr{pos, p.pos}
+	}
+	return typ
+}
+
+func (p *parser) parseVarList(isParam bool) (list []ast.Expr, typ ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "VarList"))
+	}
+
+	// a list of identifiers looks like a list of type names
+	for {
+		// parseVarType accepts any type (including parenthesized ones)
+		// even though the syntax does not permit them here: we
+		// accept them all for more robust parsing and complain
+		// afterwards
+		list = append(list, p.parseVarType(isParam))
+		if p.tok != token.COMMA {
+			break
+		}
+		p.next()
+	}
+
+	// if we had a list of identifiers, it must be followed by a type
+	typ = p.tryVarType(isParam)
+	if typ != nil {
+		p.resolve(typ)
+	}
+
+	return
+}
+
+func (p *parser) parseParameterList(scope *ast.Scope, ellipsisOk bool) (params []*ast.Field) {
+	if p.trace {
+		defer un(trace(p, "ParameterList"))
+	}
+
+	list, typ := p.parseVarList(ellipsisOk)
+	if typ != nil {
+		// IdentifierList Type
+		idents := p.makeIdentList(list)
+		field := &ast.Field{nil, idents, typ, nil, nil}
+		params = append(params, field)
+		// Go spec: The scope of an identifier denoting a function
+		// parameter or result variable is the function body.
+		p.declare(field, scope, ast.Var, idents...)
+		if p.tok == token.COMMA {
+			p.next()
+		}
+
+		for p.tok != token.RPAREN && p.tok != token.EOF {
+			idents := p.parseIdentList()
+			typ := p.parseVarType(ellipsisOk)
+			field := &ast.Field{nil, idents, typ, nil, nil}
+			params = append(params, field)
+			// Go spec: The scope of an identifier denoting a function
+			// parameter or result variable is the function body.
+			p.declare(field, scope, ast.Var, idents...)
+			if p.tok != token.COMMA {
+				break
+			}
+			p.next()
+		}
+
+	} else {
+		// Type { "," Type } (anonymous parameters)
+		params = make([]*ast.Field, len(list))
+		for i, x := range list {
+			p.resolve(x)
+			params[i] = &ast.Field{Type: x}
+		}
+	}
+
+	return
+}
+
+func (p *parser) parseParameters(scope *ast.Scope, ellipsisOk bool) *ast.FieldList {
+	if p.trace {
+		defer un(trace(p, "Parameters"))
+	}
+
+	var params []*ast.Field
+	lparen := p.expect(token.LPAREN)
+	if p.tok != token.RPAREN {
+		params = p.parseParameterList(scope, ellipsisOk)
+	}
+	rparen := p.expect(token.RPAREN)
+
+	return &ast.FieldList{lparen, params, rparen}
+}
+
+func (p *parser) parseResult(scope *ast.Scope) *ast.FieldList {
+	if p.trace {
+		defer un(trace(p, "Result"))
+	}
+
+	if p.tok == token.LPAREN {
+		return p.parseParameters(scope, false)
+	}
+
+	typ := p.tryType()
+	if typ != nil {
+		list := make([]*ast.Field, 1)
+		list[0] = &ast.Field{Type: typ}
+		return &ast.FieldList{List: list}
+	}
+
+	return nil
+}
+
+func (p *parser) parseSignature(scope *ast.Scope) (params, results *ast.FieldList) {
+	if p.trace {
+		defer un(trace(p, "Signature"))
+	}
+
+	params = p.parseParameters(scope, true)
+	results = p.parseResult(scope)
+
+	return
+}
+
+func (p *parser) parseFuncType() (*ast.FuncType, *ast.Scope) {
+	if p.trace {
+		defer un(trace(p, "FuncType"))
+	}
+
+	pos := p.expect(token.FUNC)
+	scope := ast.NewScope(p.topScope) // function scope
+	params, results := p.parseSignature(scope)
+
+	return &ast.FuncType{pos, params, results}, scope
+}
+
+func (p *parser) parseMethodSpec(scope *ast.Scope) *ast.Field {
+	if p.trace {
+		defer un(trace(p, "MethodSpec"))
+	}
+
+	doc := p.leadComment
+	var idents []*ast.Ident
+	var typ ast.Expr
+	x := p.parseTypeName()
+	if ident, isIdent := x.(*ast.Ident); isIdent && p.tok == token.LPAREN {
+		// method
+		idents = []*ast.Ident{ident}
+		scope := ast.NewScope(nil) // method scope
+		params, results := p.parseSignature(scope)
+		typ = &ast.FuncType{token.NoPos, params, results}
+	} else {
+		// embedded interface
+		typ = x
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	spec := &ast.Field{doc, idents, typ, nil, p.lineComment}
+	p.declare(spec, scope, ast.Fun, idents...)
+
+	return spec
+}
+
+func (p *parser) parseInterfaceType() *ast.InterfaceType {
+	if p.trace {
+		defer un(trace(p, "InterfaceType"))
+	}
+
+	pos := p.expect(token.INTERFACE)
+	lbrace := p.expect(token.LBRACE)
+	scope := ast.NewScope(nil) // interface scope
+	var list []*ast.Field
+	for p.tok == token.IDENT {
+		list = append(list, p.parseMethodSpec(scope))
+	}
+	rbrace := p.expect(token.RBRACE)
+
+	// TODO(gri): store interface scope in AST
+	return &ast.InterfaceType{pos, &ast.FieldList{lbrace, list, rbrace}, false}
+}
+
+func (p *parser) parseMapType() *ast.MapType {
+	if p.trace {
+		defer un(trace(p, "MapType"))
+	}
+
+	pos := p.expect(token.MAP)
+	p.expect(token.LBRACK)
+	key := p.parseType()
+	p.expect(token.RBRACK)
+	value := p.parseType()
+
+	return &ast.MapType{pos, key, value}
+}
+
+func (p *parser) parseChanType() *ast.ChanType {
+	if p.trace {
+		defer un(trace(p, "ChanType"))
+	}
+
+	pos := p.pos
+	dir := ast.SEND | ast.RECV
+	if p.tok == token.CHAN {
+		p.next()
+		if p.tok == token.ARROW {
+			p.next()
+			dir = ast.SEND
+		}
+	} else {
+		p.expect(token.ARROW)
+		p.expect(token.CHAN)
+		dir = ast.RECV
+	}
+	value := p.parseType()
+
+	return &ast.ChanType{pos, dir, value}
+}
+
+// If the result is an identifier, it is not resolved.
+func (p *parser) tryIdentOrType(ellipsisOk bool) ast.Expr {
+	switch p.tok {
+	case token.IDENT:
+		return p.parseTypeName()
+	case token.LBRACK:
+		return p.parseArrayType(ellipsisOk)
+	case token.STRUCT:
+		return p.parseStructType()
+	case token.MUL:
+		return p.parsePointerType()
+	case token.FUNC:
+		typ, _ := p.parseFuncType()
+		return typ
+	case token.INTERFACE:
+		return p.parseInterfaceType()
+	case token.MAP:
+		return p.parseMapType()
+	case token.CHAN, token.ARROW:
+		return p.parseChanType()
+	case token.LPAREN:
+		lparen := p.pos
+		p.next()
+		typ := p.parseType()
+		rparen := p.expect(token.RPAREN)
+		return &ast.ParenExpr{lparen, typ, rparen}
+	}
+
+	// no type found
+	return nil
+}
+
+func (p *parser) tryType() ast.Expr {
+	typ := p.tryIdentOrType(false)
+	if typ != nil {
+		p.resolve(typ)
+	}
+	return typ
+}
+
+// ----------------------------------------------------------------------------
+// Blocks
+
+func (p *parser) parseStmtList() (list []ast.Stmt) {
+	if p.trace {
+		defer un(trace(p, "StatementList"))
+	}
+
+	for p.tok != token.CASE && p.tok != token.DEFAULT && p.tok != token.RBRACE && p.tok != token.EOF {
+		list = append(list, p.parseStmt())
+	}
+
+	return
+}
+
+func (p *parser) parseBody(scope *ast.Scope) *ast.BlockStmt {
+	if p.trace {
+		defer un(trace(p, "Body"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	p.topScope = scope // open function scope
+	p.openLabelScope()
+	list := p.parseStmtList()
+	p.closeLabelScope()
+	p.closeScope()
+	rbrace := p.expect(token.RBRACE)
+
+	return &ast.BlockStmt{lbrace, list, rbrace}
+}
+
+func (p *parser) parseBlockStmt() *ast.BlockStmt {
+	if p.trace {
+		defer un(trace(p, "BlockStmt"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	p.openScope()
+	list := p.parseStmtList()
+	p.closeScope()
+	rbrace := p.expect(token.RBRACE)
+
+	return &ast.BlockStmt{lbrace, list, rbrace}
+}
+
+// ----------------------------------------------------------------------------
+// Expressions
+
+func (p *parser) parseFuncTypeOrLit() ast.Expr {
+	if p.trace {
+		defer un(trace(p, "FuncTypeOrLit"))
+	}
+
+	typ, scope := p.parseFuncType()
+	if p.tok != token.LBRACE {
+		// function type only
+		return typ
+	}
+
+	p.exprLev++
+	body := p.parseBody(scope)
+	p.exprLev--
+
+	return &ast.FuncLit{typ, body}
+}
+
+// parseOperand may return an expression or a raw type (incl. array
+// types of the form [...]T. Callers must verify the result.
+// If lhs is set and the result is an identifier, it is not resolved.
+//
+func (p *parser) parseOperand(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Operand"))
+	}
+
+	switch p.tok {
+	case token.IDENT:
+		x := p.parseIdent()
+		if !lhs {
+			p.resolve(x)
+		}
+		return x
+
+	case token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING:
+		x := &ast.BasicLit{p.pos, p.tok, p.lit}
+		p.next()
+		return x
+
+	case token.LPAREN:
+		lparen := p.pos
+		p.next()
+		p.exprLev++
+		x := p.parseRhs()
+		p.exprLev--
+		rparen := p.expect(token.RPAREN)
+		return &ast.ParenExpr{lparen, x, rparen}
+
+	case token.FUNC:
+		return p.parseFuncTypeOrLit()
+
+	default:
+		if typ := p.tryIdentOrType(true); typ != nil {
+			// could be type for composite literal or conversion
+			_, isIdent := typ.(*ast.Ident)
+			assert(!isIdent, "type cannot be identifier")
+			return typ
+		}
+	}
+
+	pos := p.pos
+	p.errorExpected(pos, "operand")
+	p.next() // make progress
+	return &ast.BadExpr{pos, p.pos}
+}
+
+func (p *parser) parseSelector(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Selector"))
+	}
+
+	sel := p.parseIdent()
+
+	return &ast.SelectorExpr{x, sel}
+}
+
+func (p *parser) parseTypeAssertion(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "TypeAssertion"))
+	}
+
+	p.expect(token.LPAREN)
+	var typ ast.Expr
+	if p.tok == token.TYPE {
+		// type switch: typ == nil
+		p.next()
+	} else {
+		typ = p.parseType()
+	}
+	p.expect(token.RPAREN)
+
+	return &ast.TypeAssertExpr{x, typ}
+}
+
+func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "IndexOrSlice"))
+	}
+
+	lbrack := p.expect(token.LBRACK)
+	p.exprLev++
+	var low, high ast.Expr
+	isSlice := false
+	if p.tok != token.COLON {
+		low = p.parseRhs()
+	}
+	if p.tok == token.COLON {
+		isSlice = true
+		p.next()
+		if p.tok != token.RBRACK {
+			high = p.parseRhs()
+		}
+	}
+	p.exprLev--
+	rbrack := p.expect(token.RBRACK)
+
+	if isSlice {
+		return &ast.SliceExpr{x, lbrack, low, high, rbrack}
+	}
+	return &ast.IndexExpr{x, lbrack, low, rbrack}
+}
+
+func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
+	if p.trace {
+		defer un(trace(p, "CallOrConversion"))
+	}
+
+	lparen := p.expect(token.LPAREN)
+	p.exprLev++
+	var list []ast.Expr
+	var ellipsis token.Pos
+	for p.tok != token.RPAREN && p.tok != token.EOF && !ellipsis.IsValid() {
+		list = append(list, p.parseRhs())
+		if p.tok == token.ELLIPSIS {
+			ellipsis = p.pos
+			p.next()
+		}
+		if p.tok != token.COMMA {
+			break
+		}
+		p.next()
+	}
+	p.exprLev--
+	rparen := p.expect(token.RPAREN)
+
+	return &ast.CallExpr{fun, lparen, list, ellipsis, rparen}
+}
+
+func (p *parser) parseElement(keyOk bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Element"))
+	}
+
+	if p.tok == token.LBRACE {
+		return p.parseLiteralValue(nil)
+	}
+
+	x := p.parseExpr(keyOk) // don't resolve if map key
+	if keyOk {
+		if p.tok == token.COLON {
+			colon := p.pos
+			p.next()
+			return &ast.KeyValueExpr{x, colon, p.parseElement(false)}
+		}
+		p.resolve(x) // not a map key
+	}
+
+	return x
+}
+
+func (p *parser) parseElementList() (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "ElementList"))
+	}
+
+	for p.tok != token.RBRACE && p.tok != token.EOF {
+		list = append(list, p.parseElement(true))
+		if p.tok != token.COMMA {
+			break
+		}
+		p.next()
+	}
+
+	return
+}
+
+func (p *parser) parseLiteralValue(typ ast.Expr) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "LiteralValue"))
+	}
+
+	lbrace := p.expect(token.LBRACE)
+	var elts []ast.Expr
+	p.exprLev++
+	if p.tok != token.RBRACE {
+		elts = p.parseElementList()
+	}
+	p.exprLev--
+	rbrace := p.expect(token.RBRACE)
+	return &ast.CompositeLit{typ, lbrace, elts, rbrace}
+}
+
+// checkExpr checks that x is an expression (and not a type).
+func (p *parser) checkExpr(x ast.Expr) ast.Expr {
+	switch t := unparen(x).(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.BasicLit:
+	case *ast.FuncLit:
+	case *ast.CompositeLit:
+	case *ast.ParenExpr:
+		panic("unreachable")
+	case *ast.SelectorExpr:
+	case *ast.IndexExpr:
+	case *ast.SliceExpr:
+	case *ast.TypeAssertExpr:
+		if t.Type == nil {
+			// the form X.(type) is only allowed in type switch expressions
+			p.errorExpected(x.Pos(), "expression")
+			x = &ast.BadExpr{x.Pos(), x.End()}
+		}
+	case *ast.CallExpr:
+	case *ast.StarExpr:
+	case *ast.UnaryExpr:
+		if t.Op == token.RANGE {
+			// the range operator is only allowed at the top of a for statement
+			p.errorExpected(x.Pos(), "expression")
+			x = &ast.BadExpr{x.Pos(), x.End()}
+		}
+	case *ast.BinaryExpr:
+	default:
+		// all other nodes are not proper expressions
+		p.errorExpected(x.Pos(), "expression")
+		x = &ast.BadExpr{x.Pos(), x.End()}
+	}
+	return x
+}
+
+// isTypeName reports whether x is a (qualified) TypeName.
+func isTypeName(x ast.Expr) bool {
+	switch t := x.(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.SelectorExpr:
+		_, isIdent := t.X.(*ast.Ident)
+		return isIdent
+	default:
+		return false // all other nodes are not type names
+	}
+	return true
+}
+
+// isLiteralType reports whether x is a legal composite literal type.
+func isLiteralType(x ast.Expr) bool {
+	switch t := x.(type) {
+	case *ast.BadExpr:
+	case *ast.Ident:
+	case *ast.SelectorExpr:
+		_, isIdent := t.X.(*ast.Ident)
+		return isIdent
+	case *ast.ArrayType:
+	case *ast.StructType:
+	case *ast.MapType:
+	default:
+		return false // all other nodes are not legal composite literal types
+	}
+	return true
+}
+
+// If x is of the form *T, deref returns T, otherwise it returns x.
+func deref(x ast.Expr) ast.Expr {
+	if p, isPtr := x.(*ast.StarExpr); isPtr {
+		x = p.X
+	}
+	return x
+}
+
+// If x is of the form (T), unparen returns unparen(T), otherwise it returns x.
+func unparen(x ast.Expr) ast.Expr {
+	if p, isParen := x.(*ast.ParenExpr); isParen {
+		x = unparen(p.X)
+	}
+	return x
+}
+
+// checkExprOrType checks that x is an expression or a type
+// (and not a raw type such as [...]T).
+//
+func (p *parser) checkExprOrType(x ast.Expr) ast.Expr {
+	switch t := unparen(x).(type) {
+	case *ast.ParenExpr:
+		panic("unreachable")
+	case *ast.UnaryExpr:
+		if t.Op == token.RANGE {
+			// the range operator is only allowed at the top of a for statement
+			p.errorExpected(x.Pos(), "expression")
+			x = &ast.BadExpr{x.Pos(), x.End()}
+		}
+	case *ast.ArrayType:
+		if len, isEllipsis := t.Len.(*ast.Ellipsis); isEllipsis {
+			p.error(len.Pos(), "expected array length, found '...'")
+			x = &ast.BadExpr{x.Pos(), x.End()}
+		}
+	}
+
+	// all other nodes are expressions or types
+	return x
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parsePrimaryExpr(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "PrimaryExpr"))
+	}
+
+	x := p.parseOperand(lhs)
+L:
+	for {
+		switch p.tok {
+		case token.PERIOD:
+			p.next()
+			if lhs {
+				p.resolve(x)
+			}
+			switch p.tok {
+			case token.IDENT:
+				x = p.parseSelector(p.checkExpr(x))
+			case token.LPAREN:
+				x = p.parseTypeAssertion(p.checkExpr(x))
+			default:
+				pos := p.pos
+				p.next() // make progress
+				p.errorExpected(pos, "selector or type assertion")
+				x = &ast.BadExpr{pos, p.pos}
+			}
+		case token.LBRACK:
+			if lhs {
+				p.resolve(x)
+			}
+			x = p.parseIndexOrSlice(p.checkExpr(x))
+		case token.LPAREN:
+			if lhs {
+				p.resolve(x)
+			}
+			x = p.parseCallOrConversion(p.checkExprOrType(x))
+		case token.LBRACE:
+			if isLiteralType(x) && (p.exprLev >= 0 || !isTypeName(x)) {
+				if lhs {
+					p.resolve(x)
+				}
+				x = p.parseLiteralValue(x)
+			} else {
+				break L
+			}
+		default:
+			break L
+		}
+		lhs = false // no need to try to resolve again
+	}
+
+	return x
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parseUnaryExpr(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "UnaryExpr"))
+	}
+
+	switch p.tok {
+	case token.ADD, token.SUB, token.NOT, token.XOR, token.AND, token.RANGE:
+		pos, op := p.pos, p.tok
+		p.next()
+		x := p.parseUnaryExpr(false)
+		return &ast.UnaryExpr{pos, op, p.checkExpr(x)}
+
+	case token.ARROW:
+		// channel type or receive expression
+		pos := p.pos
+		p.next()
+		if p.tok == token.CHAN {
+			p.next()
+			value := p.parseType()
+			return &ast.ChanType{pos, ast.RECV, value}
+		}
+
+		x := p.parseUnaryExpr(false)
+		return &ast.UnaryExpr{pos, token.ARROW, p.checkExpr(x)}
+
+	case token.MUL:
+		// pointer type or unary "*" expression
+		pos := p.pos
+		p.next()
+		x := p.parseUnaryExpr(false)
+		return &ast.StarExpr{pos, p.checkExprOrType(x)}
+	}
+
+	return p.parsePrimaryExpr(lhs)
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+func (p *parser) parseBinaryExpr(lhs bool, prec1 int) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "BinaryExpr"))
+	}
+
+	x := p.parseUnaryExpr(lhs)
+	for prec := p.tok.Precedence(); prec >= prec1; prec-- {
+		for p.tok.Precedence() == prec {
+			pos, op := p.pos, p.tok
+			p.next()
+			if lhs {
+				p.resolve(x)
+				lhs = false
+			}
+			y := p.parseBinaryExpr(false, prec+1)
+			x = &ast.BinaryExpr{p.checkExpr(x), pos, op, p.checkExpr(y)}
+		}
+	}
+
+	return x
+}
+
+// If lhs is set and the result is an identifier, it is not resolved.
+// TODO(gri): parseExpr may return a type or even a raw type ([..]int) -
+//            should reject when a type/raw type is obviously not allowed
+func (p *parser) parseExpr(lhs bool) ast.Expr {
+	if p.trace {
+		defer un(trace(p, "Expression"))
+	}
+
+	return p.parseBinaryExpr(lhs, token.LowestPrec+1)
+}
+
+func (p *parser) parseRhs() ast.Expr {
+	return p.parseExpr(false)
+}
+
+// ----------------------------------------------------------------------------
+// Statements
+
+func (p *parser) parseSimpleStmt(labelOk bool) ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "SimpleStmt"))
+	}
+
+	x := p.parseLhsList()
+
+	switch p.tok {
+	case
+		token.DEFINE, token.ASSIGN, token.ADD_ASSIGN,
+		token.SUB_ASSIGN, token.MUL_ASSIGN, token.QUO_ASSIGN,
+		token.REM_ASSIGN, token.AND_ASSIGN, token.OR_ASSIGN,
+		token.XOR_ASSIGN, token.SHL_ASSIGN, token.SHR_ASSIGN, token.AND_NOT_ASSIGN:
+		// assignment statement
+		pos, tok := p.pos, p.tok
+		p.next()
+		y := p.parseRhsList()
+		return &ast.AssignStmt{x, pos, tok, y}
+	}
+
+	if len(x) > 1 {
+		p.errorExpected(x[0].Pos(), "1 expression")
+		// continue with first expression
+	}
+
+	switch p.tok {
+	case token.COLON:
+		// labeled statement
+		colon := p.pos
+		p.next()
+		if label, isIdent := x[0].(*ast.Ident); labelOk && isIdent {
+			// Go spec: The scope of a label is the body of the function
+			// in which it is declared and excludes the body of any nested
+			// function.
+			stmt := &ast.LabeledStmt{label, colon, p.parseStmt()}
+			p.declare(stmt, p.labelScope, ast.Lbl, label)
+			return stmt
+		}
+		p.error(x[0].Pos(), "illegal label declaration")
+		return &ast.BadStmt{x[0].Pos(), colon + 1}
+
+	case token.ARROW:
+		// send statement
+		arrow := p.pos
+		p.next() // consume "<-"
+		y := p.parseRhs()
+		return &ast.SendStmt{x[0], arrow, y}
+
+	case token.INC, token.DEC:
+		// increment or decrement
+		s := &ast.IncDecStmt{x[0], p.pos, p.tok}
+		p.next() // consume "++" or "--"
+		return s
+	}
+
+	// expression
+	return &ast.ExprStmt{x[0]}
+}
+
+func (p *parser) parseCallExpr() *ast.CallExpr {
+	x := p.parseRhs()
+	if call, isCall := x.(*ast.CallExpr); isCall {
+		return call
+	}
+	p.errorExpected(x.Pos(), "function/method call")
+	return nil
+}
+
+func (p *parser) parseGoStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "GoStmt"))
+	}
+
+	pos := p.expect(token.GO)
+	call := p.parseCallExpr()
+	p.expectSemi()
+	if call == nil {
+		return &ast.BadStmt{pos, pos + 2} // len("go")
+	}
+
+	return &ast.GoStmt{pos, call}
+}
+
+func (p *parser) parseDeferStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "DeferStmt"))
+	}
+
+	pos := p.expect(token.DEFER)
+	call := p.parseCallExpr()
+	p.expectSemi()
+	if call == nil {
+		return &ast.BadStmt{pos, pos + 5} // len("defer")
+	}
+
+	return &ast.DeferStmt{pos, call}
+}
+
+func (p *parser) parseReturnStmt() *ast.ReturnStmt {
+	if p.trace {
+		defer un(trace(p, "ReturnStmt"))
+	}
+
+	pos := p.pos
+	p.expect(token.RETURN)
+	var x []ast.Expr
+	if p.tok != token.SEMICOLON && p.tok != token.RBRACE {
+		x = p.parseRhsList()
+	}
+	p.expectSemi()
+
+	return &ast.ReturnStmt{pos, x}
+}
+
+func (p *parser) parseBranchStmt(tok token.Token) *ast.BranchStmt {
+	if p.trace {
+		defer un(trace(p, "BranchStmt"))
+	}
+
+	pos := p.expect(tok)
+	var label *ast.Ident
+	if tok != token.FALLTHROUGH && p.tok == token.IDENT {
+		label = p.parseIdent()
+		// add to list of unresolved targets
+		n := len(p.targetStack) - 1
+		p.targetStack[n] = append(p.targetStack[n], label)
+	}
+	p.expectSemi()
+
+	return &ast.BranchStmt{pos, tok, label}
+}
+
+func (p *parser) makeExpr(s ast.Stmt) ast.Expr {
+	if s == nil {
+		return nil
+	}
+	if es, isExpr := s.(*ast.ExprStmt); isExpr {
+		return p.checkExpr(es.X)
+	}
+	p.error(s.Pos(), "expected condition, found simple statement")
+	return &ast.BadExpr{s.Pos(), s.End()}
+}
+
+func (p *parser) parseIfStmt() *ast.IfStmt {
+	if p.trace {
+		defer un(trace(p, "IfStmt"))
+	}
+
+	pos := p.expect(token.IF)
+	p.openScope()
+	defer p.closeScope()
+
+	var s ast.Stmt
+	var x ast.Expr
+	{
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok == token.SEMICOLON {
+			p.next()
+			x = p.parseRhs()
+		} else {
+			s = p.parseSimpleStmt(false)
+			if p.tok == token.SEMICOLON {
+				p.next()
+				x = p.parseRhs()
+			} else {
+				x = p.makeExpr(s)
+				s = nil
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	body := p.parseBlockStmt()
+	var else_ ast.Stmt
+	if p.tok == token.ELSE {
+		p.next()
+		else_ = p.parseStmt()
+	} else {
+		p.expectSemi()
+	}
+
+	return &ast.IfStmt{pos, s, x, body, else_}
+}
+
+func (p *parser) parseTypeList() (list []ast.Expr) {
+	if p.trace {
+		defer un(trace(p, "TypeList"))
+	}
+
+	list = append(list, p.parseType())
+	for p.tok == token.COMMA {
+		p.next()
+		list = append(list, p.parseType())
+	}
+
+	return
+}
+
+func (p *parser) parseCaseClause(exprSwitch bool) *ast.CaseClause {
+	if p.trace {
+		defer un(trace(p, "CaseClause"))
+	}
+
+	pos := p.pos
+	var list []ast.Expr
+	if p.tok == token.CASE {
+		p.next()
+		if exprSwitch {
+			list = p.parseRhsList()
+		} else {
+			list = p.parseTypeList()
+		}
+	} else {
+		p.expect(token.DEFAULT)
+	}
+
+	colon := p.expect(token.COLON)
+	p.openScope()
+	body := p.parseStmtList()
+	p.closeScope()
+
+	return &ast.CaseClause{pos, list, colon, body}
+}
+
+func isExprSwitch(s ast.Stmt) bool {
+	if s == nil {
+		return true
+	}
+	if e, ok := s.(*ast.ExprStmt); ok {
+		if a, ok := e.X.(*ast.TypeAssertExpr); ok {
+			return a.Type != nil // regular type assertion
+		}
+		return true
+	}
+	return false
+}
+
+func (p *parser) parseSwitchStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "SwitchStmt"))
+	}
+
+	pos := p.expect(token.SWITCH)
+	p.openScope()
+	defer p.closeScope()
+
+	var s1, s2 ast.Stmt
+	if p.tok != token.LBRACE {
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok != token.SEMICOLON {
+			s2 = p.parseSimpleStmt(false)
+		}
+		if p.tok == token.SEMICOLON {
+			p.next()
+			s1 = s2
+			s2 = nil
+			if p.tok != token.LBRACE {
+				s2 = p.parseSimpleStmt(false)
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	exprSwitch := isExprSwitch(s2)
+	lbrace := p.expect(token.LBRACE)
+	var list []ast.Stmt
+	for p.tok == token.CASE || p.tok == token.DEFAULT {
+		list = append(list, p.parseCaseClause(exprSwitch))
+	}
+	rbrace := p.expect(token.RBRACE)
+	p.expectSemi()
+	body := &ast.BlockStmt{lbrace, list, rbrace}
+
+	if exprSwitch {
+		return &ast.SwitchStmt{pos, s1, p.makeExpr(s2), body}
+	}
+	// type switch
+	// TODO(gri): do all the checks!
+	return &ast.TypeSwitchStmt{pos, s1, s2, body}
+}
+
+func (p *parser) parseCommClause() *ast.CommClause {
+	if p.trace {
+		defer un(trace(p, "CommClause"))
+	}
+
+	p.openScope()
+	pos := p.pos
+	var comm ast.Stmt
+	if p.tok == token.CASE {
+		p.next()
+		lhs := p.parseLhsList()
+		if p.tok == token.ARROW {
+			// SendStmt
+			if len(lhs) > 1 {
+				p.errorExpected(lhs[0].Pos(), "1 expression")
+				// continue with first expression
+			}
+			arrow := p.pos
+			p.next()
+			rhs := p.parseRhs()
+			comm = &ast.SendStmt{lhs[0], arrow, rhs}
+		} else {
+			// RecvStmt
+			pos := p.pos
+			tok := p.tok
+			var rhs ast.Expr
+			if tok == token.ASSIGN || tok == token.DEFINE {
+				// RecvStmt with assignment
+				if len(lhs) > 2 {
+					p.errorExpected(lhs[0].Pos(), "1 or 2 expressions")
+					// continue with first two expressions
+					lhs = lhs[0:2]
+				}
+				p.next()
+				rhs = p.parseRhs()
+			} else {
+				// rhs must be single receive operation
+				if len(lhs) > 1 {
+					p.errorExpected(lhs[0].Pos(), "1 expression")
+					// continue with first expression
+				}
+				rhs = lhs[0]
+				lhs = nil // there is no lhs
+			}
+			if x, isUnary := rhs.(*ast.UnaryExpr); !isUnary || x.Op != token.ARROW {
+				p.errorExpected(rhs.Pos(), "send or receive operation")
+				rhs = &ast.BadExpr{rhs.Pos(), rhs.End()}
+			}
+			if lhs != nil {
+				comm = &ast.AssignStmt{lhs, pos, tok, []ast.Expr{rhs}}
+			} else {
+				comm = &ast.ExprStmt{rhs}
+			}
+		}
+	} else {
+		p.expect(token.DEFAULT)
+	}
+
+	colon := p.expect(token.COLON)
+	body := p.parseStmtList()
+	p.closeScope()
+
+	return &ast.CommClause{pos, comm, colon, body}
+}
+
+func (p *parser) parseSelectStmt() *ast.SelectStmt {
+	if p.trace {
+		defer un(trace(p, "SelectStmt"))
+	}
+
+	pos := p.expect(token.SELECT)
+	lbrace := p.expect(token.LBRACE)
+	var list []ast.Stmt
+	for p.tok == token.CASE || p.tok == token.DEFAULT {
+		list = append(list, p.parseCommClause())
+	}
+	rbrace := p.expect(token.RBRACE)
+	p.expectSemi()
+	body := &ast.BlockStmt{lbrace, list, rbrace}
+
+	return &ast.SelectStmt{pos, body}
+}
+
+func (p *parser) parseForStmt() ast.Stmt {
+	if p.trace {
+		defer un(trace(p, "ForStmt"))
+	}
+
+	pos := p.expect(token.FOR)
+	p.openScope()
+	defer p.closeScope()
+
+	var s1, s2, s3 ast.Stmt
+	if p.tok != token.LBRACE {
+		prevLev := p.exprLev
+		p.exprLev = -1
+		if p.tok != token.SEMICOLON {
+			s2 = p.parseSimpleStmt(false)
+		}
+		if p.tok == token.SEMICOLON {
+			p.next()
+			s1 = s2
+			s2 = nil
+			if p.tok != token.SEMICOLON {
+				s2 = p.parseSimpleStmt(false)
+			}
+			p.expectSemi()
+			if p.tok != token.LBRACE {
+				s3 = p.parseSimpleStmt(false)
+			}
+		}
+		p.exprLev = prevLev
+	}
+
+	body := p.parseBlockStmt()
+	p.expectSemi()
+
+	if as, isAssign := s2.(*ast.AssignStmt); isAssign {
+		// possibly a for statement with a range clause; check assignment operator
+		if as.Tok != token.ASSIGN && as.Tok != token.DEFINE {
+			p.errorExpected(as.TokPos, "'=' or ':='")
+			return &ast.BadStmt{pos, body.End()}
+		}
+		// check lhs
+		var key, value ast.Expr
+		switch len(as.Lhs) {
+		case 2:
+			key, value = as.Lhs[0], as.Lhs[1]
+		case 1:
+			key = as.Lhs[0]
+		default:
+			p.errorExpected(as.Lhs[0].Pos(), "1 or 2 expressions")
+			return &ast.BadStmt{pos, body.End()}
+		}
+		// check rhs
+		if len(as.Rhs) != 1 {
+			p.errorExpected(as.Rhs[0].Pos(), "1 expression")
+			return &ast.BadStmt{pos, body.End()}
+		}
+		if rhs, isUnary := as.Rhs[0].(*ast.UnaryExpr); isUnary && rhs.Op == token.RANGE {
+			// rhs is range expression
+			// (any short variable declaration was handled by parseSimpleStat above)
+			return &ast.RangeStmt{pos, key, value, as.TokPos, as.Tok, rhs.X, body}
+		}
+		p.errorExpected(s2.Pos(), "range clause")
+		return &ast.BadStmt{pos, body.End()}
+	}
+
+	// regular for statement
+	return &ast.ForStmt{pos, s1, p.makeExpr(s2), s3, body}
+}
+
+func (p *parser) parseStmt() (s ast.Stmt) {
+	if p.trace {
+		defer un(trace(p, "Statement"))
+	}
+
+	switch p.tok {
+	case token.CONST, token.TYPE, token.VAR:
+		s = &ast.DeclStmt{p.parseDecl()}
+	case
+		// tokens that may start a top-level expression
+		token.IDENT, token.INT, token.FLOAT, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operand
+		token.LBRACK, token.STRUCT, // composite type
+		token.MUL, token.AND, token.ARROW, token.ADD, token.SUB, token.XOR: // unary operators
+		s = p.parseSimpleStmt(true)
+		// because of the required look-ahead, labeled statements are
+		// parsed by parseSimpleStmt - don't expect a semicolon after
+		// them
+		if _, isLabeledStmt := s.(*ast.LabeledStmt); !isLabeledStmt {
+			p.expectSemi()
+		}
+	case token.GO:
+		s = p.parseGoStmt()
+	case token.DEFER:
+		s = p.parseDeferStmt()
+	case token.RETURN:
+		s = p.parseReturnStmt()
+	case token.BREAK, token.CONTINUE, token.GOTO, token.FALLTHROUGH:
+		s = p.parseBranchStmt(p.tok)
+	case token.LBRACE:
+		s = p.parseBlockStmt()
+		p.expectSemi()
+	case token.IF:
+		s = p.parseIfStmt()
+	case token.SWITCH:
+		s = p.parseSwitchStmt()
+	case token.SELECT:
+		s = p.parseSelectStmt()
+	case token.FOR:
+		s = p.parseForStmt()
+	case token.SEMICOLON:
+		s = &ast.EmptyStmt{p.pos}
+		p.next()
+	case token.RBRACE:
+		// a semicolon may be omitted before a closing "}"
+		s = &ast.EmptyStmt{p.pos}
+	default:
+		// no statement found
+		pos := p.pos
+		p.errorExpected(pos, "statement")
+		p.next() // make progress
+		s = &ast.BadStmt{pos, p.pos}
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Declarations
+
+type parseSpecFunction func(p *parser, doc *ast.CommentGroup, iota int) ast.Spec
+
+func parseImportSpec(p *parser, doc *ast.CommentGroup, _ int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "ImportSpec"))
+	}
+
+	var ident *ast.Ident
+	switch p.tok {
+	case token.PERIOD:
+		ident = &ast.Ident{p.pos, ".", nil}
+		p.next()
+	case token.IDENT:
+		ident = p.parseIdent()
+	}
+
+	var path *ast.BasicLit
+	if p.tok == token.STRING {
+		path = &ast.BasicLit{p.pos, p.tok, p.lit}
+		p.next()
+	} else {
+		p.expect(token.STRING) // use expect() error handling
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	// collect imports
+	spec := &ast.ImportSpec{doc, ident, path, p.lineComment}
+	p.imports = append(p.imports, spec)
+
+	return spec
+}
+
+func parseConstSpec(p *parser, doc *ast.CommentGroup, iota int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "ConstSpec"))
+	}
+
+	idents := p.parseIdentList()
+	typ := p.tryType()
+	var values []ast.Expr
+	if typ != nil || p.tok == token.ASSIGN || iota == 0 {
+		p.expect(token.ASSIGN)
+		values = p.parseRhsList()
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	// Go spec: The scope of a constant or variable identifier declared inside
+	// a function begins at the end of the ConstSpec or VarSpec and ends at
+	// the end of the innermost containing block.
+	// (Global identifiers are resolved in a separate phase after parsing.)
+	spec := &ast.ValueSpec{doc, idents, typ, values, p.lineComment}
+	p.declare(spec, p.topScope, ast.Con, idents...)
+
+	return spec
+}
+
+func parseTypeSpec(p *parser, doc *ast.CommentGroup, _ int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "TypeSpec"))
+	}
+
+	ident := p.parseIdent()
+
+	// Go spec: The scope of a type identifier declared inside a function begins
+	// at the identifier in the TypeSpec and ends at the end of the innermost
+	// containing block.
+	// (Global identifiers are resolved in a separate phase after parsing.)
+	spec := &ast.TypeSpec{doc, ident, nil, nil}
+	p.declare(spec, p.topScope, ast.Typ, ident)
+
+	spec.Type = p.parseType()
+	p.expectSemi() // call before accessing p.linecomment
+	spec.Comment = p.lineComment
+
+	return spec
+}
+
+func parseVarSpec(p *parser, doc *ast.CommentGroup, _ int) ast.Spec {
+	if p.trace {
+		defer un(trace(p, "VarSpec"))
+	}
+
+	idents := p.parseIdentList()
+	typ := p.tryType()
+	var values []ast.Expr
+	if typ == nil || p.tok == token.ASSIGN {
+		p.expect(token.ASSIGN)
+		values = p.parseRhsList()
+	}
+	p.expectSemi() // call before accessing p.linecomment
+
+	// Go spec: The scope of a constant or variable identifier declared inside
+	// a function begins at the end of the ConstSpec or VarSpec and ends at
+	// the end of the innermost containing block.
+	// (Global identifiers are resolved in a separate phase after parsing.)
+	spec := &ast.ValueSpec{doc, idents, typ, values, p.lineComment}
+	p.declare(spec, p.topScope, ast.Var, idents...)
+
+	return spec
+}
+
+func (p *parser) parseGenDecl(keyword token.Token, f parseSpecFunction) *ast.GenDecl {
+	if p.trace {
+		defer un(trace(p, "GenDecl("+keyword.String()+")"))
+	}
+
+	doc := p.leadComment
+	pos := p.expect(keyword)
+	var lparen, rparen token.Pos
+	var list []ast.Spec
+	if p.tok == token.LPAREN {
+		lparen = p.pos
+		p.next()
+		for iota := 0; p.tok != token.RPAREN && p.tok != token.EOF; iota++ {
+			list = append(list, f(p, p.leadComment, iota))
+		}
+		rparen = p.expect(token.RPAREN)
+		p.expectSemi()
+	} else {
+		list = append(list, f(p, nil, 0))
+	}
+
+	return &ast.GenDecl{doc, pos, keyword, lparen, list, rparen}
+}
+
+func (p *parser) parseReceiver(scope *ast.Scope) *ast.FieldList {
+	if p.trace {
+		defer un(trace(p, "Receiver"))
+	}
+
+	pos := p.pos
+	par := p.parseParameters(scope, false)
+
+	// must have exactly one receiver
+	if par.NumFields() != 1 {
+		p.errorExpected(pos, "exactly one receiver")
+		// TODO determine a better range for BadExpr below
+		par.List = []*ast.Field{{Type: &ast.BadExpr{pos, pos}}}
+		return par
+	}
+
+	// recv type must be of the form ["*"] identifier
+	recv := par.List[0]
+	base := deref(recv.Type)
+	if _, isIdent := base.(*ast.Ident); !isIdent {
+		p.errorExpected(base.Pos(), "(unqualified) identifier")
+		par.List = []*ast.Field{{Type: &ast.BadExpr{recv.Pos(), recv.End()}}}
+	}
+
+	return par
+}
+
+func (p *parser) parseFuncDecl() *ast.FuncDecl {
+	if p.trace {
+		defer un(trace(p, "FunctionDecl"))
+	}
+
+	doc := p.leadComment
+	pos := p.expect(token.FUNC)
+	scope := ast.NewScope(p.topScope) // function scope
+
+	var recv *ast.FieldList
+	if p.tok == token.LPAREN {
+		recv = p.parseReceiver(scope)
+	}
+
+	ident := p.parseIdent()
+
+	params, results := p.parseSignature(scope)
+
+	var body *ast.BlockStmt
+	if p.tok == token.LBRACE {
+		body = p.parseBody(scope)
+	}
+	p.expectSemi()
+
+	decl := &ast.FuncDecl{doc, recv, ident, &ast.FuncType{pos, params, results}, body}
+	if recv == nil {
+		// Go spec: The scope of an identifier denoting a constant, type,
+		// variable, or function (but not method) declared at top level
+		// (outside any function) is the package block.
+		//
+		// init() functions cannot be referred to and there may
+		// be more than one - don't put them in the pkgScope
+		if ident.Name != "init" {
+			p.declare(decl, p.pkgScope, ast.Fun, ident)
+		}
+	}
+
+	return decl
+}
+
+func (p *parser) parseDecl() ast.Decl {
+	if p.trace {
+		defer un(trace(p, "Declaration"))
+	}
+
+	var f parseSpecFunction
+	switch p.tok {
+	case token.CONST:
+		f = parseConstSpec
+
+	case token.TYPE:
+		f = parseTypeSpec
+
+	case token.VAR:
+		f = parseVarSpec
+
+	case token.FUNC:
+		return p.parseFuncDecl()
+
+	default:
+		pos := p.pos
+		p.errorExpected(pos, "declaration")
+		p.next() // make progress
+		decl := &ast.BadDecl{pos, p.pos}
+		return decl
+	}
+
+	return p.parseGenDecl(p.tok, f)
+}
+
+func (p *parser) parseDeclList() (list []ast.Decl) {
+	if p.trace {
+		defer un(trace(p, "DeclList"))
+	}
+
+	for p.tok != token.EOF {
+		list = append(list, p.parseDecl())
+	}
+
+	return
+}
+
+// ----------------------------------------------------------------------------
+// Source files
+
+func (p *parser) parseFile() *ast.File {
+	if p.trace {
+		defer un(trace(p, "File"))
+	}
+
+	// package clause
+	doc := p.leadComment
+	pos := p.expect(token.PACKAGE)
+	// Go spec: The package clause is not a declaration;
+	// the package name does not appear in any scope.
+	ident := p.parseIdent()
+	if ident.Name == "_" {
+		p.error(p.pos, "invalid package name _")
+	}
+	p.expectSemi()
+
+	var decls []ast.Decl
+
+	// Don't bother parsing the rest if we had errors already.
+	// Likely not a Go source file at all.
+
+	if p.ErrorCount() == 0 && p.mode&PackageClauseOnly == 0 {
+		// import decls
+		for p.tok == token.IMPORT {
+			decls = append(decls, p.parseGenDecl(token.IMPORT, parseImportSpec))
+		}
+
+		if p.mode&ImportsOnly == 0 {
+			// rest of package body
+			for p.tok != token.EOF {
+				decls = append(decls, p.parseDecl())
+			}
+		}
+	}
+
+	assert(p.topScope == p.pkgScope, "imbalanced scopes")
+
+	// resolve global identifiers within the same file
+	i := 0
+	for _, ident := range p.unresolved {
+		// i <= index for current ident
+		assert(ident.Obj == unresolved, "object already resolved")
+		ident.Obj = p.pkgScope.Lookup(ident.Name) // also removes unresolved sentinel
+		if ident.Obj == nil {
+			p.unresolved[i] = ident
+			i++
+		}
+	}
+
+	// TODO(gri): store p.imports in AST
+	return &ast.File{doc, pos, ident, decls, p.pkgScope, p.imports, p.unresolved[0:i], p.comments}
+}
diff --git a/src/go/printer/testdata/slow.golden b/src/go/printer/testdata/slow.golden
new file mode 100644
index 0000000..43a15cb
--- /dev/null
+++ b/src/go/printer/testdata/slow.golden
@@ -0,0 +1,85 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package deepequal_test
+
+import (
+	"testing"
+	"google3/spam/archer/frontend/deepequal"
+)
+
+func TestTwoNilValues(t *testing.T) {
+	if err := deepequal.Check(nil, nil); err != nil {
+		t.Errorf("expected nil, saw %v", err)
+	}
+}
+
+type Foo struct {
+	bar	*Bar
+	bang	*Bar
+}
+
+type Bar struct {
+	baz	*Baz
+	foo	[]*Foo
+}
+
+type Baz struct {
+	entries		map[int]interface{}
+	whatever	string
+}
+
+func newFoo() *Foo {
+	return &Foo{bar: &Bar{baz: &Baz{
+		entries: map[int]interface{}{
+			42:	&Foo{},
+			21:	&Bar{},
+			11:	&Baz{whatever: "it's just a test"}}}},
+		bang: &Bar{foo: []*Foo{
+			&Foo{bar: &Bar{baz: &Baz{
+				entries: map[int]interface{}{
+					43:	&Foo{},
+					22:	&Bar{},
+					13:	&Baz{whatever: "this is nuts"}}}},
+				bang: &Bar{foo: []*Foo{
+					&Foo{bar: &Bar{baz: &Baz{
+						entries: map[int]interface{}{
+							61:	&Foo{},
+							71:	&Bar{},
+							11:	&Baz{whatever: "no, it's Go"}}}},
+						bang: &Bar{foo: []*Foo{
+							&Foo{bar: &Bar{baz: &Baz{
+								entries: map[int]interface{}{
+									0:	&Foo{},
+									-2:	&Bar{},
+									-11:	&Baz{whatever: "we need to go deeper"}}}},
+								bang: &Bar{foo: []*Foo{
+									&Foo{bar: &Bar{baz: &Baz{
+										entries: map[int]interface{}{
+											-2:	&Foo{},
+											-5:	&Bar{},
+											-7:	&Baz{whatever: "are you serious?"}}}},
+										bang:	&Bar{foo: []*Foo{}}},
+									&Foo{bar: &Bar{baz: &Baz{
+										entries: map[int]interface{}{
+											-100:	&Foo{},
+											50:	&Bar{},
+											20:	&Baz{whatever: "na, not really ..."}}}},
+										bang:	&Bar{foo: []*Foo{}}}}}}}}},
+					&Foo{bar: &Bar{baz: &Baz{
+						entries: map[int]interface{}{
+							2:	&Foo{},
+							1:	&Bar{},
+							-1:	&Baz{whatever: "... it's just a test."}}}},
+						bang:	&Bar{foo: []*Foo{}}}}}}}}}
+}
+
+func TestElaborate(t *testing.T) {
+	a := newFoo()
+	b := newFoo()
+
+	if err := deepequal.Check(a, b); err != nil {
+		t.Errorf("expected nil, saw %v", err)
+	}
+}
diff --git a/src/go/printer/testdata/slow.input b/src/go/printer/testdata/slow.input
new file mode 100644
index 0000000..0e5a23d
--- /dev/null
+++ b/src/go/printer/testdata/slow.input
@@ -0,0 +1,85 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package deepequal_test
+
+import (
+        "testing"
+        "google3/spam/archer/frontend/deepequal"
+)
+
+func TestTwoNilValues(t *testing.T) {
+        if err := deepequal.Check(nil, nil); err != nil {
+                t.Errorf("expected nil, saw %v", err)
+        }
+}
+
+type Foo struct {
+        bar *Bar
+        bang *Bar
+}
+
+type Bar struct {
+        baz *Baz
+        foo []*Foo
+}
+
+type Baz struct {
+        entries  map[int]interface{}
+        whatever string
+}
+
+func newFoo() (*Foo) {
+return &Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+42: &Foo{},
+21: &Bar{},
+11: &Baz{ whatever: "it's just a test" }}}},
+        bang: &Bar{foo: []*Foo{
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+43: &Foo{},
+22: &Bar{},
+13: &Baz{ whatever: "this is nuts" }}}},
+        bang: &Bar{foo: []*Foo{
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+61: &Foo{},
+71: &Bar{},
+11: &Baz{ whatever: "no, it's Go" }}}},
+        bang: &Bar{foo: []*Foo{
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+0: &Foo{},
+-2: &Bar{},
+-11: &Baz{ whatever: "we need to go deeper" }}}},
+        bang: &Bar{foo: []*Foo{
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+-2: &Foo{},
+-5: &Bar{},
+-7: &Baz{ whatever: "are you serious?" }}}},
+        bang: &Bar{foo: []*Foo{}}},
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+-100: &Foo{},
+50: &Bar{},
+20: &Baz{ whatever: "na, not really ..." }}}},
+        bang: &Bar{foo: []*Foo{}}}}}}}}},
+&Foo{bar: &Bar{ baz: &Baz{
+entries: map[int]interface{}{
+2: &Foo{},
+1: &Bar{},
+-1: &Baz{ whatever: "... it's just a test." }}}},
+        bang: &Bar{foo: []*Foo{}}}}}}}}}
+}
+
+func TestElaborate(t *testing.T) {
+        a := newFoo()
+        b := newFoo()
+
+        if err := deepequal.Check(a, b); err != nil {
+                t.Errorf("expected nil, saw %v", err)
+        }
+}
diff --git a/src/go/printer/testdata/statements.golden b/src/go/printer/testdata/statements.golden
new file mode 100644
index 0000000..4b13460
--- /dev/null
+++ b/src/go/printer/testdata/statements.golden
@@ -0,0 +1,644 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package statements
+
+var expr bool
+
+func use(x interface{})	{}
+
+// Formatting of multi-line return statements.
+func _f() {
+	return
+	return x, y, z
+	return T{}
+	return T{1, 2, 3},
+		x, y, z
+	return T{1, 2, 3},
+		x, y,
+		z
+	return T{1,
+		2,
+		3}
+	return T{1,
+		2,
+		3,
+	}
+	return T{
+		1,
+		2,
+		3}
+	return T{
+		1,
+		2,
+		3,
+	}
+	return T{
+		1,
+		T{1, 2, 3},
+		3,
+	}
+	return T{
+		1,
+		T{1,
+			2, 3},
+		3,
+	}
+	return T{
+		1,
+		T{1,
+			2,
+			3},
+		3,
+	}
+	return T{
+		1,
+		2,
+	}, nil
+	return T{
+			1,
+			2,
+		},
+		T{
+			x:	3,
+			y:	4,
+		}, nil
+	return T{
+			1,
+			2,
+		},
+		nil
+	return T{
+			1,
+			2,
+		},
+		T{
+			x:	3,
+			y:	4,
+		},
+		nil
+	return x + y +
+		z
+	return func() {}
+	return func() {
+			_ = 0
+		}, T{
+			1, 2,
+		}
+	return func() {
+		_ = 0
+	}
+	return func() T {
+		return T{
+			1, 2,
+		}
+	}
+}
+
+// Formatting of multi-line returns: test cases from issue 1207.
+func F() (*T, os.Error) {
+	return &T{
+			X:	1,
+			Y:	2,
+		},
+		nil
+}
+
+func G() (*T, *T, os.Error) {
+	return &T{
+			X:	1,
+			Y:	2,
+		},
+		&T{
+			X:	3,
+			Y:	4,
+		},
+		nil
+}
+
+func _() interface{} {
+	return &fileStat{
+		name:		basename(file.name),
+		size:		mkSize(d.FileSizeHigh, d.FileSizeLow),
+		modTime:	mkModTime(d.LastWriteTime),
+		mode:		mkMode(d.FileAttributes),
+		sys:		mkSysFromFI(&d),
+	}, nil
+}
+
+// Formatting of if-statement headers.
+func _() {
+	if true {
+	}
+	if true {
+	}	// no semicolon printed
+	if expr {
+	}
+	if expr {
+	}	// no semicolon printed
+	if expr {
+	}	// no parens printed
+	if expr {
+	}	// no semicolon and parens printed
+	if x := expr; true {
+		use(x)
+	}
+	if x := expr; expr {
+		use(x)
+	}
+}
+
+// Formatting of switch-statement headers.
+func _() {
+	switch {
+	}
+	switch {
+	}	// no semicolon printed
+	switch expr {
+	}
+	switch expr {
+	}	// no semicolon printed
+	switch expr {
+	}	// no parens printed
+	switch expr {
+	}	// no semicolon and parens printed
+	switch x := expr; {
+	default:
+		use(
+			x)
+	}
+	switch x := expr; expr {
+	default:
+		use(x)
+	}
+}
+
+// Formatting of switch statement bodies.
+func _() {
+	switch {
+	}
+
+	switch x := 0; x {
+	case 1:
+		use(x)
+		use(x)	// followed by an empty line
+
+	case 2:	// followed by an empty line
+
+		use(x)	// followed by an empty line
+
+	case 3:	// no empty lines
+		use(x)
+		use(x)
+	}
+
+	switch x {
+	case 0:
+		use(x)
+	case 1:	// this comment should have no effect on the previous or next line
+		use(x)
+	}
+
+	switch x := 0; x {
+	case 1:
+		x = 0
+		// this comment should be indented
+	case 2:
+		x = 0
+	// this comment should not be indented, it is aligned with the next case
+	case 3:
+		x = 0
+		/* indented comment
+		   aligned
+		   aligned
+		*/
+		// bla
+		/* and more */
+	case 4:
+		x = 0
+	/* not indented comment
+	   aligned
+	   aligned
+	*/
+	// bla
+	/* and more */
+	case 5:
+	}
+}
+
+// Formatting of selected select statements.
+func _() {
+	select {}
+	select { /* this comment should not be tab-aligned because the closing } is on the same line */
+	}
+	select {	/* this comment should be tab-aligned */
+	}
+	select {	// this comment should be tab-aligned
+	}
+	select {
+	case <-c:
+	}
+}
+
+// Formatting of for-statement headers for single-line for-loops.
+func _() {
+	for {
+	}
+	for expr {
+	}
+	for expr {
+	}	// no parens printed
+	for {
+	}	// no semicolons printed
+	for x := expr; ; {
+		use(x)
+	}
+	for expr {
+	}	// no semicolons printed
+	for expr {
+	}	// no semicolons and parens printed
+	for ; ; expr = false {
+	}
+	for x := expr; expr; {
+		use(x)
+	}
+	for x := expr; ; expr = false {
+		use(x)
+	}
+	for ; expr; expr = false {
+	}
+	for x := expr; expr; expr = false {
+		use(x)
+	}
+	for x := range []int{} {
+		use(x)
+	}
+	for x := range []int{} {
+		use(x)
+	}	// no parens printed
+}
+
+// Formatting of for-statement headers for multi-line for-loops.
+func _() {
+	for {
+	}
+	for expr {
+	}
+	for expr {
+	}	// no parens printed
+	for {
+	}	// no semicolons printed
+	for x := expr; ; {
+		use(x)
+	}
+	for expr {
+	}	// no semicolons printed
+	for expr {
+	}	// no semicolons and parens printed
+	for ; ; expr = false {
+	}
+	for x := expr; expr; {
+		use(x)
+	}
+	for x := expr; ; expr = false {
+		use(x)
+	}
+	for ; expr; expr = false {
+	}
+	for x := expr; expr; expr = false {
+		use(x)
+	}
+	for range []int{} {
+		println("foo")
+	}
+	for x := range []int{} {
+		use(x)
+	}
+	for x := range []int{} {
+		use(x)
+	}	// no parens printed
+}
+
+// Formatting of selected short single- and multi-line statements.
+func _() {
+	if cond {
+	}
+	if cond {
+	}	// multiple lines
+	if cond {
+	} else {
+	}	// else clause always requires multiple lines
+
+	for {
+	}
+	for i := 0; i < len(a); 1++ {
+	}
+	for i := 0; i < len(a); 1++ {
+		a[i] = i
+	}
+	for i := 0; i < len(a); 1++ {
+		a[i] = i
+	}	// multiple lines
+
+	for range a {
+	}
+	for _ = range a {
+	}
+	for _, _ = range a {
+	}
+	for i := range a {
+	}
+	for i := range a {
+		a[i] = i
+	}
+	for i := range a {
+		a[i] = i
+	}	// multiple lines
+
+	go func() {
+		for {
+			a <- <-b
+		}
+	}()
+	defer func() {
+		if x := recover(); x != nil {
+			err = fmt.Sprintf("error: %s", x.msg)
+		}
+	}()
+}
+
+// Don't remove mandatory parentheses around composite literals in control clauses.
+func _() {
+	// strip parentheses - no composite literals or composite literals don't start with a type name
+	if x {
+	}
+	if x {
+	}
+	if []T{} {
+	}
+	if []T{} {
+	}
+	if []T{} {
+	}
+
+	for x {
+	}
+	for x {
+	}
+	for []T{} {
+	}
+	for []T{} {
+	}
+	for []T{} {
+	}
+
+	switch x {
+	}
+	switch x {
+	}
+	switch []T{} {
+	}
+	switch []T{} {
+	}
+
+	for _ = range []T{T{42}} {
+	}
+
+	// leave parentheses - composite literals start with a type name
+	if (T{}) {
+	}
+	if (T{}) {
+	}
+	if (T{}) {
+	}
+
+	for (T{}) {
+	}
+	for (T{}) {
+	}
+	for (T{}) {
+	}
+
+	switch (T{}) {
+	}
+	switch (T{}) {
+	}
+
+	for _ = range (T1{T{42}}) {
+	}
+
+	if x == (T{42}[0]) {
+	}
+	if (x == T{42}[0]) {
+	}
+	if x == (T{42}[0]) {
+	}
+	if x == (T{42}[0]) {
+	}
+	if x == (T{42}[0]) {
+	}
+	if x == a+b*(T{42}[0]) {
+	}
+	if (x == a+b*T{42}[0]) {
+	}
+	if x == a+b*(T{42}[0]) {
+	}
+	if x == a+(b*(T{42}[0])) {
+	}
+	if x == a+b*(T{42}[0]) {
+	}
+	if (a + b*(T{42}[0])) == x {
+	}
+	if (a + b*(T{42}[0])) == x {
+	}
+
+	if struct{ x bool }{false}.x {
+	}
+	if (struct{ x bool }{false}.x) == false {
+	}
+	if struct{ x bool }{false}.x == false {
+	}
+}
+
+// Extra empty lines inside functions. Do respect source code line
+// breaks between statement boundaries but print at most one empty
+// line at a time.
+func _() {
+
+	const _ = 0
+
+	const _ = 1
+	type _ int
+	type _ float
+
+	var _ = 0
+	var x = 1
+
+	// Each use(x) call below should have at most one empty line before and after.
+	// Known bug: The first use call may have more than one empty line before
+	//            (see go/printer/nodes.go, func linebreak).
+
+	use(x)
+
+	if x < x {
+
+		use(x)
+
+	} else {
+
+		use(x)
+
+	}
+}
+
+// Formatting around labels.
+func _() {
+L:
+}
+
+func _() {
+	// this comment should be indented
+L:	// no semicolon needed
+}
+
+func _() {
+	switch 0 {
+	case 0:
+	L0:
+		;	// semicolon required
+	case 1:
+	L1:
+		;	// semicolon required
+	default:
+	L2:	// no semicolon needed
+	}
+}
+
+func _() {
+	f()
+L1:
+	f()
+L2:
+	;
+L3:
+}
+
+func _() {
+	// this comment should be indented
+L:
+}
+
+func _() {
+L:
+	_ = 0
+}
+
+func _() {
+	// this comment should be indented
+L:
+	_ = 0
+}
+
+func _() {
+	for {
+	L1:
+		_ = 0
+	L2:
+		_ = 0
+	}
+}
+
+func _() {
+	// this comment should be indented
+	for {
+	L1:
+		_ = 0
+	L2:
+		_ = 0
+	}
+}
+
+func _() {
+	if true {
+		_ = 0
+	}
+	_ = 0	// the indentation here should not be affected by the long label name
+AnOverlongLabel:
+	_ = 0
+
+	if true {
+		_ = 0
+	}
+	_ = 0
+
+L:
+	_ = 0
+}
+
+func _() {
+	for {
+		goto L
+	}
+L:
+
+	MoreCode()
+}
+
+func _() {
+	for {
+		goto L
+	}
+L:	// A comment on the same line as the label, followed by a single empty line.
+	// Known bug: There may be more than one empty line before MoreCode()
+	//            (see go/printer/nodes.go, func linebreak).
+
+	MoreCode()
+}
+
+func _() {
+	for {
+		goto L
+	}
+L:
+
+	// There should be a single empty line before this comment.
+	MoreCode()
+}
+
+func _() {
+	for {
+		goto AVeryLongLabelThatShouldNotAffectFormatting
+	}
+AVeryLongLabelThatShouldNotAffectFormatting:
+	// There should be a single empty line after this comment.
+
+	// There should be a single empty line before this comment.
+	MoreCode()
+}
+
+// Formatting of empty statements.
+func _() {
+
+}
+
+func _() {
+}
+
+func _() {
+}
+
+func _() {
+	f()
+}
+
+func _() {
+L:
+	;
+}
+
+func _() {
+L:
+	;
+	f()
+}
diff --git a/src/go/printer/testdata/statements.input b/src/go/printer/testdata/statements.input
new file mode 100644
index 0000000..cade157
--- /dev/null
+++ b/src/go/printer/testdata/statements.input
@@ -0,0 +1,555 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package statements
+
+var expr bool
+
+func use(x interface{}) {}
+
+// Formatting of multi-line return statements.
+func _f() {
+	return
+	return x, y, z
+	return T{}
+	return T{1, 2, 3},
+		x, y, z
+	return T{1, 2, 3},
+		x, y,
+		z
+	return T{1,
+		2,
+		3}
+	return T{1,
+		2,
+		3,
+	}
+	return T{
+		1,
+		2,
+		3}
+	return T{
+		1,
+		2,
+		3,
+	}
+	return T{
+		1,
+		T{1, 2, 3},
+		3,
+	}
+	return T{
+		1,
+		T{1,
+			2, 3},
+		3,
+	}
+	return T{
+		1,
+		T{1,
+			2,
+			3},
+		3,
+	}
+	return T{
+			1,
+			2,
+		}, nil
+	return T{
+			1,
+			2,
+		},
+		T{
+			x: 3,
+			y: 4,
+		}, nil
+	return T{
+			1,
+			2,
+		},
+		nil
+	return T{
+			1,
+			2,
+		},
+		T{
+			x: 3,
+			y: 4,
+		},
+		nil
+	return x + y +
+		z
+	return func() {}
+	return func() {
+		_ = 0
+	}, T{
+		1, 2,
+	}
+	return func() {
+		_ = 0
+	}
+	return func() T {
+		return T {
+			1, 2,
+		}
+	}
+}
+
+// Formatting of multi-line returns: test cases from issue 1207.
+func F() (*T, os.Error) {
+       return &T{
+               X: 1,
+               Y: 2,
+       },
+               nil
+}
+
+func G() (*T, *T, os.Error) {
+       return &T{
+               X: 1,
+               Y: 2,
+       },
+               &T{
+                       X: 3,
+                       Y: 4,
+               },
+               nil
+}
+
+func _() interface{} {
+	return &fileStat{
+			name:    basename(file.name),
+			size:    mkSize(d.FileSizeHigh, d.FileSizeLow),
+			modTime: mkModTime(d.LastWriteTime),
+			mode:    mkMode(d.FileAttributes),
+			sys:     mkSysFromFI(&d),
+		}, nil
+}
+
+// Formatting of if-statement headers.
+func _() {
+	if true {}
+	if; true {}  // no semicolon printed
+	if expr{}
+	if;expr{}  // no semicolon printed
+	if (expr){}  // no parens printed
+	if;((expr)){}  // no semicolon and parens printed
+	if x:=expr;true{
+	use(x)}
+	if x:=expr; expr {use(x)}
+}
+
+
+// Formatting of switch-statement headers.
+func _() {
+	switch {}
+	switch;{}  // no semicolon printed
+	switch expr {}
+	switch;expr{}  // no semicolon printed
+	switch (expr) {}  // no parens printed
+	switch;((expr)){}  // no semicolon and parens printed
+	switch x := expr; { default:use(
+x)
+	}
+	switch x := expr; expr {default:use(x)}
+}
+
+
+// Formatting of switch statement bodies.
+func _() {
+	switch {
+	}
+
+	switch x := 0; x {
+	case 1:
+		use(x)
+		use(x)  // followed by an empty line
+
+	case 2:  // followed by an empty line
+
+		use(x)  // followed by an empty line
+
+	case 3:  // no empty lines
+		use(x)
+		use(x)
+	}
+
+	switch x {
+	case 0:
+		use(x)
+	case 1:  // this comment should have no effect on the previous or next line
+		use(x)
+	}
+
+	switch x := 0; x {
+	case 1:
+		x = 0
+		// this comment should be indented
+	case 2:
+		x = 0
+	// this comment should not be indented, it is aligned with the next case
+	case 3:
+		x = 0
+		/* indented comment
+		   aligned
+		   aligned
+		*/
+		// bla
+		/* and more */
+	case 4:
+		x = 0
+	/* not indented comment
+	   aligned
+	   aligned
+	*/
+	// bla
+	/* and more */
+	case 5:
+	}
+}
+
+
+// Formatting of selected select statements.
+func _() {
+	select {
+	}
+	select { /* this comment should not be tab-aligned because the closing } is on the same line */ }
+	select { /* this comment should be tab-aligned */
+	}
+	select { // this comment should be tab-aligned
+	}
+	select { case <-c: }
+}
+
+
+// Formatting of for-statement headers for single-line for-loops.
+func _() {
+	for{}
+	for expr {}
+	for (expr) {}  // no parens printed
+	for;;{}  // no semicolons printed
+	for x :=expr;; {use( x)}
+	for; expr;{}  // no semicolons printed
+	for; ((expr));{}  // no semicolons and parens printed
+	for; ; expr = false {}
+	for x :=expr; expr; {use(x)}
+	for x := expr;; expr=false {use(x)}
+	for;expr;expr =false {}
+	for x := expr;expr;expr = false { use(x) }
+	for x := range []int{} { use(x) }
+	for x := range (([]int{})) { use(x) }  // no parens printed
+}
+
+
+// Formatting of for-statement headers for multi-line for-loops.
+func _() {
+	for{
+	}
+	for expr {
+	}
+	for (expr) {
+	}  // no parens printed
+	for;;{
+	}  // no semicolons printed
+	for x :=expr;; {use( x)
+	}
+	for; expr;{
+	}  // no semicolons printed
+	for; ((expr));{
+	}  // no semicolons and parens printed
+	for; ; expr = false {
+	}
+	for x :=expr; expr; {use(x)
+	}
+	for x := expr;; expr=false {use(x)
+	}
+	for;expr;expr =false {
+	}
+	for x := expr;expr;expr = false {
+	use(x)
+	}
+	for range []int{} {
+	println("foo")}
+	for x := range []int{} {
+	use(x) }
+	for x := range (([]int{})) {
+	use(x) }  // no parens printed
+}
+
+
+// Formatting of selected short single- and multi-line statements.
+func _() {
+	if cond {}
+	if cond {
+	} // multiple lines
+	if cond {} else {} // else clause always requires multiple lines
+
+	for {}
+	for i := 0; i < len(a); 1++ {}
+	for i := 0; i < len(a); 1++ { a[i] = i }
+	for i := 0; i < len(a); 1++ { a[i] = i
+	} // multiple lines
+
+	for range a{}
+	for _ = range a{}
+	for _, _ = range a{}
+	for i := range a {}
+	for i := range a { a[i] = i }
+	for i := range a { a[i] = i
+	} // multiple lines
+
+	go func() { for { a <- <-b } }()
+	defer func() { if x := recover(); x != nil { err = fmt.Sprintf("error: %s", x.msg) } }()
+}
+
+
+// Don't remove mandatory parentheses around composite literals in control clauses.
+func _() {
+	// strip parentheses - no composite literals or composite literals don't start with a type name
+	if (x) {}
+	if (((x))) {}
+	if ([]T{}) {}
+	if (([]T{})) {}
+	if ; (((([]T{})))) {}
+
+	for (x) {}
+	for (((x))) {}
+	for ([]T{}) {}
+	for (([]T{})) {}
+	for ; (((([]T{})))) ; {}
+
+	switch (x) {}
+	switch (((x))) {}
+	switch ([]T{}) {}
+	switch ; (((([]T{})))) {}
+
+	for _ = range ((([]T{T{42}}))) {}
+
+	// leave parentheses - composite literals start with a type name
+	if (T{}) {}
+	if ((T{})) {}
+	if ; ((((T{})))) {}
+
+	for (T{}) {}
+	for ((T{})) {}
+	for ; ((((T{})))) ; {}
+
+	switch (T{}) {}
+	switch ; ((((T{})))) {}
+
+	for _ = range (((T1{T{42}}))) {}
+
+	if x == (T{42}[0]) {}
+	if (x == T{42}[0]) {}
+	if (x == (T{42}[0])) {}
+	if (x == (((T{42}[0])))) {}
+	if (((x == (T{42}[0])))) {}
+	if x == a + b*(T{42}[0]) {}
+	if (x == a + b*T{42}[0]) {}
+	if (x == a + b*(T{42}[0])) {}
+	if (x == a + ((b * (T{42}[0])))) {}
+	if (((x == a + b * (T{42}[0])))) {}
+	if (((a + b * (T{42}[0])) == x)) {}
+	if (((a + b * (T{42}[0])))) == x {}
+
+	if (struct{x bool}{false}.x) {}
+	if (struct{x bool}{false}.x) == false {}
+	if (struct{x bool}{false}.x == false) {}
+}
+
+
+// Extra empty lines inside functions. Do respect source code line
+// breaks between statement boundaries but print at most one empty
+// line at a time.
+func _() {
+
+	const _ = 0
+
+	const _ = 1
+	type _ int
+	type _ float
+
+	var _ = 0
+	var x = 1
+
+	// Each use(x) call below should have at most one empty line before and after.
+	// Known bug: The first use call may have more than one empty line before
+	//            (see go/printer/nodes.go, func linebreak).
+
+
+
+	use(x)
+
+	if x < x {
+
+		use(x)
+
+	} else {
+
+		use(x)
+
+	}
+}
+
+
+// Formatting around labels.
+func _() {
+	L:
+}
+
+
+func _() {
+	// this comment should be indented
+	L: ;  // no semicolon needed
+}
+
+
+func _() {
+	switch 0 {
+	case 0:
+		L0: ;  // semicolon required
+	case 1:
+		L1: ;  // semicolon required
+	default:
+		L2: ;  // no semicolon needed
+	}
+}
+
+
+func _() {
+	f()
+L1:
+	f()
+L2:
+	;
+L3:
+}
+
+
+func _() {
+	// this comment should be indented
+	L:
+}
+
+
+func _() {
+	L: _ = 0
+}
+
+
+func _() {
+	// this comment should be indented
+	L: _ = 0
+}
+
+
+func _() {
+	for {
+	L1: _ = 0
+	L2:
+		_ = 0
+	}
+}
+
+
+func _() {
+		// this comment should be indented
+	for {
+	L1: _ = 0
+	L2:
+		_ = 0
+	}
+}
+
+
+func _() {
+	if true {
+		_ = 0
+	}
+	_ = 0  // the indentation here should not be affected by the long label name
+AnOverlongLabel:
+	_ = 0
+	
+	if true {
+		_ = 0
+	}
+	_ = 0
+
+L:	_ = 0
+}
+
+
+func _() {
+	for {
+		goto L
+	}
+L:
+
+	MoreCode()
+}
+
+
+func _() {
+	for {
+		goto L
+	}
+L:	// A comment on the same line as the label, followed by a single empty line.
+	// Known bug: There may be more than one empty line before MoreCode()
+	//            (see go/printer/nodes.go, func linebreak).
+
+
+
+
+	MoreCode()
+}
+
+
+func _() {
+	for {
+		goto L
+	}
+L:
+
+
+
+
+	// There should be a single empty line before this comment.
+	MoreCode()
+}
+
+
+func _() {
+	for {
+		goto AVeryLongLabelThatShouldNotAffectFormatting
+	}
+AVeryLongLabelThatShouldNotAffectFormatting:
+	// There should be a single empty line after this comment.
+
+	// There should be a single empty line before this comment.
+	MoreCode()
+}
+
+
+// Formatting of empty statements.
+func _() {
+	;;;;;;;;;;;;;;;;;;;;;;;;;
+}
+
+func _() {;;;;;;;;;;;;;;;;;;;;;;;;;
+}
+
+func _() {;;;;;;;;;;;;;;;;;;;;;;;;;}
+
+func _() {
+f();;;;;;;;;;;;;;;;;;;;;;;;;
+}
+
+func _() {
+L:;;;;;;;;;;;;
+}
+
+func _() {
+L:;;;;;;;;;;;;
+	f()
+}
diff --git a/src/go/scanner/errors.go b/src/go/scanner/errors.go
new file mode 100644
index 0000000..bf7bfa3
--- /dev/null
+++ b/src/go/scanner/errors.go
@@ -0,0 +1,124 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package scanner
+
+import (
+	"fmt"
+	"go/token"
+	"io"
+	"sort"
+)
+
+// In an ErrorList, an error is represented by an *Error.
+// The position Pos, if valid, points to the beginning of
+// the offending token, and the error condition is described
+// by Msg.
+//
+type Error struct {
+	Pos token.Position
+	Msg string
+}
+
+// Error implements the error interface.
+func (e Error) Error() string {
+	if e.Pos.Filename != "" || e.Pos.IsValid() {
+		// don't print "<unknown position>"
+		// TODO(gri) reconsider the semantics of Position.IsValid
+		return e.Pos.String() + ": " + e.Msg
+	}
+	return e.Msg
+}
+
+// ErrorList is a list of *Errors.
+// The zero value for an ErrorList is an empty ErrorList ready to use.
+//
+type ErrorList []*Error
+
+// Add adds an Error with given position and error message to an ErrorList.
+func (p *ErrorList) Add(pos token.Position, msg string) {
+	*p = append(*p, &Error{pos, msg})
+}
+
+// Reset resets an ErrorList to no errors.
+func (p *ErrorList) Reset() { *p = (*p)[0:0] }
+
+// ErrorList implements the sort Interface.
+func (p ErrorList) Len() int      { return len(p) }
+func (p ErrorList) Swap(i, j int) { p[i], p[j] = p[j], p[i] }
+
+func (p ErrorList) Less(i, j int) bool {
+	e := &p[i].Pos
+	f := &p[j].Pos
+	// Note that it is not sufficient to simply compare file offsets because
+	// the offsets do not reflect modified line information (through //line
+	// comments).
+	if e.Filename != f.Filename {
+		return e.Filename < f.Filename
+	}
+	if e.Line != f.Line {
+		return e.Line < f.Line
+	}
+	if e.Column != f.Column {
+		return e.Column < f.Column
+	}
+	return p[i].Msg < p[j].Msg
+}
+
+// Sort sorts an ErrorList. *Error entries are sorted by position,
+// other errors are sorted by error message, and before any *Error
+// entry.
+//
+func (p ErrorList) Sort() {
+	sort.Sort(p)
+}
+
+// RemoveMultiples sorts an ErrorList and removes all but the first error per line.
+func (p *ErrorList) RemoveMultiples() {
+	sort.Sort(p)
+	var last token.Position // initial last.Line is != any legal error line
+	i := 0
+	for _, e := range *p {
+		if e.Pos.Filename != last.Filename || e.Pos.Line != last.Line {
+			last = e.Pos
+			(*p)[i] = e
+			i++
+		}
+	}
+	(*p) = (*p)[0:i]
+}
+
+// An ErrorList implements the error interface.
+func (p ErrorList) Error() string {
+	switch len(p) {
+	case 0:
+		return "no errors"
+	case 1:
+		return p[0].Error()
+	}
+	return fmt.Sprintf("%s (and %d more errors)", p[0], len(p)-1)
+}
+
+// Err returns an error equivalent to this error list.
+// If the list is empty, Err returns nil.
+func (p ErrorList) Err() error {
+	if len(p) == 0 {
+		return nil
+	}
+	return p
+}
+
+// PrintError is a utility function that prints a list of errors to w,
+// one error per line, if the err parameter is an ErrorList. Otherwise
+// it prints the err string.
+//
+func PrintError(w io.Writer, err error) {
+	if list, ok := err.(ErrorList); ok {
+		for _, e := range list {
+			fmt.Fprintf(w, "%s\n", e)
+		}
+	} else if err != nil {
+		fmt.Fprintf(w, "%s\n", err)
+	}
+}
diff --git a/src/go/scanner/example_test.go b/src/go/scanner/example_test.go
new file mode 100644
index 0000000..9004a4a
--- /dev/null
+++ b/src/go/scanner/example_test.go
@@ -0,0 +1,46 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package scanner_test
+
+import (
+	"fmt"
+	"go/scanner"
+	"go/token"
+)
+
+func ExampleScanner_Scan() {
+	// src is the input that we want to tokenize.
+	src := []byte("cos(x) + 1i*sin(x) // Euler")
+
+	// Initialize the scanner.
+	var s scanner.Scanner
+	fset := token.NewFileSet()                      // positions are relative to fset
+	file := fset.AddFile("", fset.Base(), len(src)) // register input "file"
+	s.Init(file, src, nil /* no error handler */, scanner.ScanComments)
+
+	// Repeated calls to Scan yield the token sequence found in the input.
+	for {
+		pos, tok, lit := s.Scan()
+		if tok == token.EOF {
+			break
+		}
+		fmt.Printf("%s\t%s\t%q\n", fset.Position(pos), tok, lit)
+	}
+
+	// output:
+	// 1:1	IDENT	"cos"
+	// 1:4	(	""
+	// 1:5	IDENT	"x"
+	// 1:6	)	""
+	// 1:8	+	""
+	// 1:10	IMAG	"1i"
+	// 1:12	*	""
+	// 1:13	IDENT	"sin"
+	// 1:16	(	""
+	// 1:17	IDENT	"x"
+	// 1:18	)	""
+	// 1:20	;	"\n"
+	// 1:20	COMMENT	"// Euler"
+}
diff --git a/src/go/scanner/scanner.go b/src/go/scanner/scanner.go
new file mode 100644
index 0000000..00fe2dc
--- /dev/null
+++ b/src/go/scanner/scanner.go
@@ -0,0 +1,940 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package scanner implements a scanner for Go source text.
+// It takes a []byte as source which can then be tokenized
+// through repeated calls to the Scan method.
+//
+package scanner
+
+import (
+	"bytes"
+	"fmt"
+	"go/token"
+	"path/filepath"
+	"strconv"
+	"unicode"
+	"unicode/utf8"
+)
+
+// An ErrorHandler may be provided to Scanner.Init. If a syntax error is
+// encountered and a handler was installed, the handler is called with a
+// position and an error message. The position points to the beginning of
+// the offending token.
+//
+type ErrorHandler func(pos token.Position, msg string)
+
+// A Scanner holds the scanner's internal state while processing
+// a given text. It can be allocated as part of another data
+// structure but must be initialized via Init before use.
+//
+type Scanner struct {
+	// immutable state
+	file *token.File  // source file handle
+	dir  string       // directory portion of file.Name()
+	src  []byte       // source
+	err  ErrorHandler // error reporting; or nil
+	mode Mode         // scanning mode
+
+	// scanning state
+	ch         rune // current character
+	offset     int  // character offset
+	rdOffset   int  // reading offset (position after current character)
+	lineOffset int  // current line offset
+	insertSemi bool // insert a semicolon before next newline
+
+	// public state - ok to modify
+	ErrorCount int // number of errors encountered
+}
+
+const bom = 0xFEFF // byte order mark, only permitted as very first character
+
+// Read the next Unicode char into s.ch.
+// s.ch < 0 means end-of-file.
+//
+func (s *Scanner) next() {
+	if s.rdOffset < len(s.src) {
+		s.offset = s.rdOffset
+		if s.ch == '\n' {
+			s.lineOffset = s.offset
+			s.file.AddLine(s.offset)
+		}
+		r, w := rune(s.src[s.rdOffset]), 1
+		switch {
+		case r == 0:
+			s.error(s.offset, "illegal character NUL")
+		case r >= utf8.RuneSelf:
+			// not ASCII
+			r, w = utf8.DecodeRune(s.src[s.rdOffset:])
+			if r == utf8.RuneError && w == 1 {
+				s.error(s.offset, "illegal UTF-8 encoding")
+			} else if r == bom && s.offset > 0 {
+				s.error(s.offset, "illegal byte order mark")
+			}
+		}
+		s.rdOffset += w
+		s.ch = r
+	} else {
+		s.offset = len(s.src)
+		if s.ch == '\n' {
+			s.lineOffset = s.offset
+			s.file.AddLine(s.offset)
+		}
+		s.ch = -1 // eof
+	}
+}
+
+// peek returns the byte following the most recently read character without
+// advancing the scanner. If the scanner is at EOF, peek returns 0.
+func (s *Scanner) peek() byte {
+	if s.rdOffset < len(s.src) {
+		return s.src[s.rdOffset]
+	}
+	return 0
+}
+
+// A mode value is a set of flags (or 0).
+// They control scanner behavior.
+//
+type Mode uint
+
+const (
+	ScanComments    Mode = 1 << iota // return comments as COMMENT tokens
+	dontInsertSemis                  // do not automatically insert semicolons - for testing only
+)
+
+// Init prepares the scanner s to tokenize the text src by setting the
+// scanner at the beginning of src. The scanner uses the file set file
+// for position information and it adds line information for each line.
+// It is ok to re-use the same file when re-scanning the same file as
+// line information which is already present is ignored. Init causes a
+// panic if the file size does not match the src size.
+//
+// Calls to Scan will invoke the error handler err if they encounter a
+// syntax error and err is not nil. Also, for each error encountered,
+// the Scanner field ErrorCount is incremented by one. The mode parameter
+// determines how comments are handled.
+//
+// Note that Init may call err if there is an error in the first character
+// of the file.
+//
+func (s *Scanner) Init(file *token.File, src []byte, err ErrorHandler, mode Mode) {
+	// Explicitly initialize all fields since a scanner may be reused.
+	if file.Size() != len(src) {
+		panic(fmt.Sprintf("file size (%d) does not match src len (%d)", file.Size(), len(src)))
+	}
+	s.file = file
+	s.dir, _ = filepath.Split(file.Name())
+	s.src = src
+	s.err = err
+	s.mode = mode
+
+	s.ch = ' '
+	s.offset = 0
+	s.rdOffset = 0
+	s.lineOffset = 0
+	s.insertSemi = false
+	s.ErrorCount = 0
+
+	s.next()
+	if s.ch == bom {
+		s.next() // ignore BOM at file beginning
+	}
+}
+
+func (s *Scanner) error(offs int, msg string) {
+	if s.err != nil {
+		s.err(s.file.Position(s.file.Pos(offs)), msg)
+	}
+	s.ErrorCount++
+}
+
+func (s *Scanner) errorf(offs int, format string, args ...interface{}) {
+	s.error(offs, fmt.Sprintf(format, args...))
+}
+
+func (s *Scanner) scanComment() string {
+	// initial '/' already consumed; s.ch == '/' || s.ch == '*'
+	offs := s.offset - 1 // position of initial '/'
+	next := -1           // position immediately following the comment; < 0 means invalid comment
+	numCR := 0
+
+	if s.ch == '/' {
+		//-style comment
+		// (the final '\n' is not considered part of the comment)
+		s.next()
+		for s.ch != '\n' && s.ch >= 0 {
+			if s.ch == '\r' {
+				numCR++
+			}
+			s.next()
+		}
+		// if we are at '\n', the position following the comment is afterwards
+		next = s.offset
+		if s.ch == '\n' {
+			next++
+		}
+		goto exit
+	}
+
+	/*-style comment */
+	s.next()
+	for s.ch >= 0 {
+		ch := s.ch
+		if ch == '\r' {
+			numCR++
+		}
+		s.next()
+		if ch == '*' && s.ch == '/' {
+			s.next()
+			next = s.offset
+			goto exit
+		}
+	}
+
+	s.error(offs, "comment not terminated")
+
+exit:
+	lit := s.src[offs:s.offset]
+
+	// On Windows, a (//-comment) line may end in "\r\n".
+	// Remove the final '\r' before analyzing the text for
+	// line directives (matching the compiler). Remove any
+	// other '\r' afterwards (matching the pre-existing be-
+	// havior of the scanner).
+	if numCR > 0 && len(lit) >= 2 && lit[1] == '/' && lit[len(lit)-1] == '\r' {
+		lit = lit[:len(lit)-1]
+		numCR--
+	}
+
+	// interpret line directives
+	// (//line directives must start at the beginning of the current line)
+	if next >= 0 /* implies valid comment */ && (lit[1] == '*' || offs == s.lineOffset) && bytes.HasPrefix(lit[2:], prefix) {
+		s.updateLineInfo(next, offs, lit)
+	}
+
+	if numCR > 0 {
+		lit = stripCR(lit, lit[1] == '*')
+	}
+
+	return string(lit)
+}
+
+var prefix = []byte("line ")
+
+// updateLineInfo parses the incoming comment text at offset offs
+// as a line directive. If successful, it updates the line info table
+// for the position next per the line directive.
+func (s *Scanner) updateLineInfo(next, offs int, text []byte) {
+	// extract comment text
+	if text[1] == '*' {
+		text = text[:len(text)-2] // lop off trailing "*/"
+	}
+	text = text[7:] // lop off leading "//line " or "/*line "
+	offs += 7
+
+	i, n, ok := trailingDigits(text)
+	if i == 0 {
+		return // ignore (not a line directive)
+	}
+	// i > 0
+
+	if !ok {
+		// text has a suffix :xxx but xxx is not a number
+		s.error(offs+i, "invalid line number: "+string(text[i:]))
+		return
+	}
+
+	var line, col int
+	i2, n2, ok2 := trailingDigits(text[:i-1])
+	if ok2 {
+		//line filename:line:col
+		i, i2 = i2, i
+		line, col = n2, n
+		if col == 0 {
+			s.error(offs+i2, "invalid column number: "+string(text[i2:]))
+			return
+		}
+		text = text[:i2-1] // lop off ":col"
+	} else {
+		//line filename:line
+		line = n
+	}
+
+	if line == 0 {
+		s.error(offs+i, "invalid line number: "+string(text[i:]))
+		return
+	}
+
+	// If we have a column (//line filename:line:col form),
+	// an empty filename means to use the previous filename.
+	filename := string(text[:i-1]) // lop off ":line", and trim white space
+	if filename == "" && ok2 {
+		filename = s.file.Position(s.file.Pos(offs)).Filename
+	} else if filename != "" {
+		// Put a relative filename in the current directory.
+		// This is for compatibility with earlier releases.
+		// See issue 26671.
+		filename = filepath.Clean(filename)
+		if !filepath.IsAbs(filename) {
+			filename = filepath.Join(s.dir, filename)
+		}
+	}
+
+	s.file.AddLineColumnInfo(next, filename, line, col)
+}
+
+func trailingDigits(text []byte) (int, int, bool) {
+	i := bytes.LastIndexByte(text, ':') // look from right (Windows filenames may contain ':')
+	if i < 0 {
+		return 0, 0, false // no ":"
+	}
+	// i >= 0
+	n, err := strconv.ParseUint(string(text[i+1:]), 10, 0)
+	return i + 1, int(n), err == nil
+}
+
+func (s *Scanner) findLineEnd() bool {
+	// initial '/' already consumed
+
+	defer func(offs int) {
+		// reset scanner state to where it was upon calling findLineEnd
+		s.ch = '/'
+		s.offset = offs
+		s.rdOffset = offs + 1
+		s.next() // consume initial '/' again
+	}(s.offset - 1)
+
+	// read ahead until a newline, EOF, or non-comment token is found
+	for s.ch == '/' || s.ch == '*' {
+		if s.ch == '/' {
+			//-style comment always contains a newline
+			return true
+		}
+		/*-style comment: look for newline */
+		s.next()
+		for s.ch >= 0 {
+			ch := s.ch
+			if ch == '\n' {
+				return true
+			}
+			s.next()
+			if ch == '*' && s.ch == '/' {
+				s.next()
+				break
+			}
+		}
+		s.skipWhitespace() // s.insertSemi is set
+		if s.ch < 0 || s.ch == '\n' {
+			return true
+		}
+		if s.ch != '/' {
+			// non-comment token
+			return false
+		}
+		s.next() // consume '/'
+	}
+
+	return false
+}
+
+func isLetter(ch rune) bool {
+	return 'a' <= lower(ch) && lower(ch) <= 'z' || ch == '_' || ch >= utf8.RuneSelf && unicode.IsLetter(ch)
+}
+
+func isDigit(ch rune) bool {
+	return isDecimal(ch) || ch >= utf8.RuneSelf && unicode.IsDigit(ch)
+}
+
+func (s *Scanner) scanIdentifier() string {
+	offs := s.offset
+	for isLetter(s.ch) || isDigit(s.ch) {
+		s.next()
+	}
+	return string(s.src[offs:s.offset])
+}
+
+func digitVal(ch rune) int {
+	switch {
+	case '0' <= ch && ch <= '9':
+		return int(ch - '0')
+	case 'a' <= lower(ch) && lower(ch) <= 'f':
+		return int(lower(ch) - 'a' + 10)
+	}
+	return 16 // larger than any legal digit val
+}
+
+func lower(ch rune) rune     { return ('a' - 'A') | ch } // returns lower-case ch iff ch is ASCII letter
+func isDecimal(ch rune) bool { return '0' <= ch && ch <= '9' }
+func isHex(ch rune) bool     { return '0' <= ch && ch <= '9' || 'a' <= lower(ch) && lower(ch) <= 'f' }
+
+// digits accepts the sequence { digit | '_' }.
+// If base <= 10, digits accepts any decimal digit but records
+// the offset (relative to the source start) of a digit >= base
+// in *invalid, if *invalid < 0.
+// digits returns a bitset describing whether the sequence contained
+// digits (bit 0 is set), or separators '_' (bit 1 is set).
+func (s *Scanner) digits(base int, invalid *int) (digsep int) {
+	if base <= 10 {
+		max := rune('0' + base)
+		for isDecimal(s.ch) || s.ch == '_' {
+			ds := 1
+			if s.ch == '_' {
+				ds = 2
+			} else if s.ch >= max && *invalid < 0 {
+				*invalid = int(s.offset) // record invalid rune offset
+			}
+			digsep |= ds
+			s.next()
+		}
+	} else {
+		for isHex(s.ch) || s.ch == '_' {
+			ds := 1
+			if s.ch == '_' {
+				ds = 2
+			}
+			digsep |= ds
+			s.next()
+		}
+	}
+	return
+}
+
+func (s *Scanner) scanNumber() (token.Token, string) {
+	offs := s.offset
+	tok := token.ILLEGAL
+
+	base := 10        // number base
+	prefix := rune(0) // one of 0 (decimal), '0' (0-octal), 'x', 'o', or 'b'
+	digsep := 0       // bit 0: digit present, bit 1: '_' present
+	invalid := -1     // index of invalid digit in literal, or < 0
+
+	// integer part
+	if s.ch != '.' {
+		tok = token.INT
+		if s.ch == '0' {
+			s.next()
+			switch lower(s.ch) {
+			case 'x':
+				s.next()
+				base, prefix = 16, 'x'
+			case 'o':
+				s.next()
+				base, prefix = 8, 'o'
+			case 'b':
+				s.next()
+				base, prefix = 2, 'b'
+			default:
+				base, prefix = 8, '0'
+				digsep = 1 // leading 0
+			}
+		}
+		digsep |= s.digits(base, &invalid)
+	}
+
+	// fractional part
+	if s.ch == '.' {
+		tok = token.FLOAT
+		if prefix == 'o' || prefix == 'b' {
+			s.error(s.offset, "invalid radix point in "+litname(prefix))
+		}
+		s.next()
+		digsep |= s.digits(base, &invalid)
+	}
+
+	if digsep&1 == 0 {
+		s.error(s.offset, litname(prefix)+" has no digits")
+	}
+
+	// exponent
+	if e := lower(s.ch); e == 'e' || e == 'p' {
+		switch {
+		case e == 'e' && prefix != 0 && prefix != '0':
+			s.errorf(s.offset, "%q exponent requires decimal mantissa", s.ch)
+		case e == 'p' && prefix != 'x':
+			s.errorf(s.offset, "%q exponent requires hexadecimal mantissa", s.ch)
+		}
+		s.next()
+		tok = token.FLOAT
+		if s.ch == '+' || s.ch == '-' {
+			s.next()
+		}
+		ds := s.digits(10, nil)
+		digsep |= ds
+		if ds&1 == 0 {
+			s.error(s.offset, "exponent has no digits")
+		}
+	} else if prefix == 'x' && tok == token.FLOAT {
+		s.error(s.offset, "hexadecimal mantissa requires a 'p' exponent")
+	}
+
+	// suffix 'i'
+	if s.ch == 'i' {
+		tok = token.IMAG
+		s.next()
+	}
+
+	lit := string(s.src[offs:s.offset])
+	if tok == token.INT && invalid >= 0 {
+		s.errorf(invalid, "invalid digit %q in %s", lit[invalid-offs], litname(prefix))
+	}
+	if digsep&2 != 0 {
+		if i := invalidSep(lit); i >= 0 {
+			s.error(offs+i, "'_' must separate successive digits")
+		}
+	}
+
+	return tok, lit
+}
+
+func litname(prefix rune) string {
+	switch prefix {
+	case 'x':
+		return "hexadecimal literal"
+	case 'o', '0':
+		return "octal literal"
+	case 'b':
+		return "binary literal"
+	}
+	return "decimal literal"
+}
+
+// invalidSep returns the index of the first invalid separator in x, or -1.
+func invalidSep(x string) int {
+	x1 := ' ' // prefix char, we only care if it's 'x'
+	d := '.'  // digit, one of '_', '0' (a digit), or '.' (anything else)
+	i := 0
+
+	// a prefix counts as a digit
+	if len(x) >= 2 && x[0] == '0' {
+		x1 = lower(rune(x[1]))
+		if x1 == 'x' || x1 == 'o' || x1 == 'b' {
+			d = '0'
+			i = 2
+		}
+	}
+
+	// mantissa and exponent
+	for ; i < len(x); i++ {
+		p := d // previous digit
+		d = rune(x[i])
+		switch {
+		case d == '_':
+			if p != '0' {
+				return i
+			}
+		case isDecimal(d) || x1 == 'x' && isHex(d):
+			d = '0'
+		default:
+			if p == '_' {
+				return i - 1
+			}
+			d = '.'
+		}
+	}
+	if d == '_' {
+		return len(x) - 1
+	}
+
+	return -1
+}
+
+// scanEscape parses an escape sequence where rune is the accepted
+// escaped quote. In case of a syntax error, it stops at the offending
+// character (without consuming it) and returns false. Otherwise
+// it returns true.
+func (s *Scanner) scanEscape(quote rune) bool {
+	offs := s.offset
+
+	var n int
+	var base, max uint32
+	switch s.ch {
+	case 'a', 'b', 'f', 'n', 'r', 't', 'v', '\\', quote:
+		s.next()
+		return true
+	case '0', '1', '2', '3', '4', '5', '6', '7':
+		n, base, max = 3, 8, 255
+	case 'x':
+		s.next()
+		n, base, max = 2, 16, 255
+	case 'u':
+		s.next()
+		n, base, max = 4, 16, unicode.MaxRune
+	case 'U':
+		s.next()
+		n, base, max = 8, 16, unicode.MaxRune
+	default:
+		msg := "unknown escape sequence"
+		if s.ch < 0 {
+			msg = "escape sequence not terminated"
+		}
+		s.error(offs, msg)
+		return false
+	}
+
+	var x uint32
+	for n > 0 {
+		d := uint32(digitVal(s.ch))
+		if d >= base {
+			msg := fmt.Sprintf("illegal character %#U in escape sequence", s.ch)
+			if s.ch < 0 {
+				msg = "escape sequence not terminated"
+			}
+			s.error(s.offset, msg)
+			return false
+		}
+		x = x*base + d
+		s.next()
+		n--
+	}
+
+	if x > max || 0xD800 <= x && x < 0xE000 {
+		s.error(offs, "escape sequence is invalid Unicode code point")
+		return false
+	}
+
+	return true
+}
+
+func (s *Scanner) scanRune() string {
+	// '\'' opening already consumed
+	offs := s.offset - 1
+
+	valid := true
+	n := 0
+	for {
+		ch := s.ch
+		if ch == '\n' || ch < 0 {
+			// only report error if we don't have one already
+			if valid {
+				s.error(offs, "rune literal not terminated")
+				valid = false
+			}
+			break
+		}
+		s.next()
+		if ch == '\'' {
+			break
+		}
+		n++
+		if ch == '\\' {
+			if !s.scanEscape('\'') {
+				valid = false
+			}
+			// continue to read to closing quote
+		}
+	}
+
+	if valid && n != 1 {
+		s.error(offs, "illegal rune literal")
+	}
+
+	return string(s.src[offs:s.offset])
+}
+
+func (s *Scanner) scanString() string {
+	// '"' opening already consumed
+	offs := s.offset - 1
+
+	for {
+		ch := s.ch
+		if ch == '\n' || ch < 0 {
+			s.error(offs, "string literal not terminated")
+			break
+		}
+		s.next()
+		if ch == '"' {
+			break
+		}
+		if ch == '\\' {
+			s.scanEscape('"')
+		}
+	}
+
+	return string(s.src[offs:s.offset])
+}
+
+func stripCR(b []byte, comment bool) []byte {
+	c := make([]byte, len(b))
+	i := 0
+	for j, ch := range b {
+		// In a /*-style comment, don't strip \r from *\r/ (incl.
+		// sequences of \r from *\r\r...\r/) since the resulting
+		// */ would terminate the comment too early unless the \r
+		// is immediately following the opening /* in which case
+		// it's ok because /*/ is not closed yet (issue #11151).
+		if ch != '\r' || comment && i > len("/*") && c[i-1] == '*' && j+1 < len(b) && b[j+1] == '/' {
+			c[i] = ch
+			i++
+		}
+	}
+	return c[:i]
+}
+
+func (s *Scanner) scanRawString() string {
+	// '`' opening already consumed
+	offs := s.offset - 1
+
+	hasCR := false
+	for {
+		ch := s.ch
+		if ch < 0 {
+			s.error(offs, "raw string literal not terminated")
+			break
+		}
+		s.next()
+		if ch == '`' {
+			break
+		}
+		if ch == '\r' {
+			hasCR = true
+		}
+	}
+
+	lit := s.src[offs:s.offset]
+	if hasCR {
+		lit = stripCR(lit, false)
+	}
+
+	return string(lit)
+}
+
+func (s *Scanner) skipWhitespace() {
+	for s.ch == ' ' || s.ch == '\t' || s.ch == '\n' && !s.insertSemi || s.ch == '\r' {
+		s.next()
+	}
+}
+
+// Helper functions for scanning multi-byte tokens such as >> += >>= .
+// Different routines recognize different length tok_i based on matches
+// of ch_i. If a token ends in '=', the result is tok1 or tok3
+// respectively. Otherwise, the result is tok0 if there was no other
+// matching character, or tok2 if the matching character was ch2.
+
+func (s *Scanner) switch2(tok0, tok1 token.Token) token.Token {
+	if s.ch == '=' {
+		s.next()
+		return tok1
+	}
+	return tok0
+}
+
+func (s *Scanner) switch3(tok0, tok1 token.Token, ch2 rune, tok2 token.Token) token.Token {
+	if s.ch == '=' {
+		s.next()
+		return tok1
+	}
+	if s.ch == ch2 {
+		s.next()
+		return tok2
+	}
+	return tok0
+}
+
+func (s *Scanner) switch4(tok0, tok1 token.Token, ch2 rune, tok2, tok3 token.Token) token.Token {
+	if s.ch == '=' {
+		s.next()
+		return tok1
+	}
+	if s.ch == ch2 {
+		s.next()
+		if s.ch == '=' {
+			s.next()
+			return tok3
+		}
+		return tok2
+	}
+	return tok0
+}
+
+// Scan scans the next token and returns the token position, the token,
+// and its literal string if applicable. The source end is indicated by
+// token.EOF.
+//
+// If the returned token is a literal (token.IDENT, token.INT, token.FLOAT,
+// token.IMAG, token.CHAR, token.STRING) or token.COMMENT, the literal string
+// has the corresponding value.
+//
+// If the returned token is a keyword, the literal string is the keyword.
+//
+// If the returned token is token.SEMICOLON, the corresponding
+// literal string is ";" if the semicolon was present in the source,
+// and "\n" if the semicolon was inserted because of a newline or
+// at EOF.
+//
+// If the returned token is token.ILLEGAL, the literal string is the
+// offending character.
+//
+// In all other cases, Scan returns an empty literal string.
+//
+// For more tolerant parsing, Scan will return a valid token if
+// possible even if a syntax error was encountered. Thus, even
+// if the resulting token sequence contains no illegal tokens,
+// a client may not assume that no error occurred. Instead it
+// must check the scanner's ErrorCount or the number of calls
+// of the error handler, if there was one installed.
+//
+// Scan adds line information to the file added to the file
+// set with Init. Token positions are relative to that file
+// and thus relative to the file set.
+//
+func (s *Scanner) Scan() (pos token.Pos, tok token.Token, lit string) {
+scanAgain:
+	s.skipWhitespace()
+
+	// current token start
+	pos = s.file.Pos(s.offset)
+
+	// determine token value
+	insertSemi := false
+	switch ch := s.ch; {
+	case isLetter(ch):
+		lit = s.scanIdentifier()
+		if len(lit) > 1 {
+			// keywords are longer than one letter - avoid lookup otherwise
+			tok = token.Lookup(lit)
+			switch tok {
+			case token.IDENT, token.BREAK, token.CONTINUE, token.FALLTHROUGH, token.RETURN:
+				insertSemi = true
+			}
+		} else {
+			insertSemi = true
+			tok = token.IDENT
+		}
+	case isDecimal(ch) || ch == '.' && isDecimal(rune(s.peek())):
+		insertSemi = true
+		tok, lit = s.scanNumber()
+	default:
+		s.next() // always make progress
+		switch ch {
+		case -1:
+			if s.insertSemi {
+				s.insertSemi = false // EOF consumed
+				return pos, token.SEMICOLON, "\n"
+			}
+			tok = token.EOF
+		case '\n':
+			// we only reach here if s.insertSemi was
+			// set in the first place and exited early
+			// from s.skipWhitespace()
+			s.insertSemi = false // newline consumed
+			return pos, token.SEMICOLON, "\n"
+		case '"':
+			insertSemi = true
+			tok = token.STRING
+			lit = s.scanString()
+		case '\'':
+			insertSemi = true
+			tok = token.CHAR
+			lit = s.scanRune()
+		case '`':
+			insertSemi = true
+			tok = token.STRING
+			lit = s.scanRawString()
+		case ':':
+			tok = s.switch2(token.COLON, token.DEFINE)
+		case '.':
+			// fractions starting with a '.' are handled by outer switch
+			tok = token.PERIOD
+			if s.ch == '.' && s.peek() == '.' {
+				s.next()
+				s.next() // consume last '.'
+				tok = token.ELLIPSIS
+			}
+		case ',':
+			tok = token.COMMA
+		case ';':
+			tok = token.SEMICOLON
+			lit = ";"
+		case '(':
+			tok = token.LPAREN
+		case ')':
+			insertSemi = true
+			tok = token.RPAREN
+		case '[':
+			tok = token.LBRACK
+		case ']':
+			insertSemi = true
+			tok = token.RBRACK
+		case '{':
+			tok = token.LBRACE
+		case '}':
+			insertSemi = true
+			tok = token.RBRACE
+		case '+':
+			tok = s.switch3(token.ADD, token.ADD_ASSIGN, '+', token.INC)
+			if tok == token.INC {
+				insertSemi = true
+			}
+		case '-':
+			tok = s.switch3(token.SUB, token.SUB_ASSIGN, '-', token.DEC)
+			if tok == token.DEC {
+				insertSemi = true
+			}
+		case '*':
+			tok = s.switch2(token.MUL, token.MUL_ASSIGN)
+		case '/':
+			if s.ch == '/' || s.ch == '*' {
+				// comment
+				if s.insertSemi && s.findLineEnd() {
+					// reset position to the beginning of the comment
+					s.ch = '/'
+					s.offset = s.file.Offset(pos)
+					s.rdOffset = s.offset + 1
+					s.insertSemi = false // newline consumed
+					return pos, token.SEMICOLON, "\n"
+				}
+				comment := s.scanComment()
+				if s.mode&ScanComments == 0 {
+					// skip comment
+					s.insertSemi = false // newline consumed
+					goto scanAgain
+				}
+				tok = token.COMMENT
+				lit = comment
+			} else {
+				tok = s.switch2(token.QUO, token.QUO_ASSIGN)
+			}
+		case '%':
+			tok = s.switch2(token.REM, token.REM_ASSIGN)
+		case '^':
+			tok = s.switch2(token.XOR, token.XOR_ASSIGN)
+		case '<':
+			if s.ch == '-' {
+				s.next()
+				tok = token.ARROW
+			} else {
+				tok = s.switch4(token.LSS, token.LEQ, '<', token.SHL, token.SHL_ASSIGN)
+			}
+		case '>':
+			tok = s.switch4(token.GTR, token.GEQ, '>', token.SHR, token.SHR_ASSIGN)
+		case '=':
+			tok = s.switch2(token.ASSIGN, token.EQL)
+		case '!':
+			tok = s.switch2(token.NOT, token.NEQ)
+		case '&':
+			if s.ch == '^' {
+				s.next()
+				tok = s.switch2(token.AND_NOT, token.AND_NOT_ASSIGN)
+			} else {
+				tok = s.switch3(token.AND, token.AND_ASSIGN, '&', token.LAND)
+			}
+		case '|':
+			tok = s.switch3(token.OR, token.OR_ASSIGN, '|', token.LOR)
+		default:
+			// next reports unexpected BOMs - don't repeat
+			if ch != bom {
+				s.errorf(s.file.Offset(pos), "illegal character %#U", ch)
+			}
+			insertSemi = s.insertSemi // preserve insertSemi info
+			tok = token.ILLEGAL
+			lit = string(ch)
+		}
+	}
+	if s.mode&dontInsertSemis == 0 {
+		s.insertSemi = insertSemi
+	}
+
+	return
+}
diff --git a/src/go/scanner/scanner_test.go b/src/go/scanner/scanner_test.go
new file mode 100644
index 0000000..ab4c2dd
--- /dev/null
+++ b/src/go/scanner/scanner_test.go
@@ -0,0 +1,1113 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package scanner
+
+import (
+	"go/token"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+var fset = token.NewFileSet()
+
+const /* class */ (
+	special = iota
+	literal
+	operator
+	keyword
+)
+
+func tokenclass(tok token.Token) int {
+	switch {
+	case tok.IsLiteral():
+		return literal
+	case tok.IsOperator():
+		return operator
+	case tok.IsKeyword():
+		return keyword
+	}
+	return special
+}
+
+type elt struct {
+	tok   token.Token
+	lit   string
+	class int
+}
+
+var tokens = [...]elt{
+	// Special tokens
+	{token.COMMENT, "/* a comment */", special},
+	{token.COMMENT, "// a comment \n", special},
+	{token.COMMENT, "/*\r*/", special},
+	{token.COMMENT, "/**\r/*/", special}, // issue 11151
+	{token.COMMENT, "/**\r\r/*/", special},
+	{token.COMMENT, "//\r\n", special},
+
+	// Identifiers and basic type literals
+	{token.IDENT, "foobar", literal},
+	{token.IDENT, "a۰۱۸", literal},
+	{token.IDENT, "foo६४", literal},
+	{token.IDENT, "bar９８７６", literal},
+	{token.IDENT, "ŝ", literal},    // was bug (issue 4000)
+	{token.IDENT, "ŝfoo", literal}, // was bug (issue 4000)
+	{token.INT, "0", literal},
+	{token.INT, "1", literal},
+	{token.INT, "123456789012345678890", literal},
+	{token.INT, "01234567", literal},
+	{token.INT, "0xcafebabe", literal},
+	{token.FLOAT, "0.", literal},
+	{token.FLOAT, ".0", literal},
+	{token.FLOAT, "3.14159265", literal},
+	{token.FLOAT, "1e0", literal},
+	{token.FLOAT, "1e+100", literal},
+	{token.FLOAT, "1e-100", literal},
+	{token.FLOAT, "2.71828e-1000", literal},
+	{token.IMAG, "0i", literal},
+	{token.IMAG, "1i", literal},
+	{token.IMAG, "012345678901234567889i", literal},
+	{token.IMAG, "123456789012345678890i", literal},
+	{token.IMAG, "0.i", literal},
+	{token.IMAG, ".0i", literal},
+	{token.IMAG, "3.14159265i", literal},
+	{token.IMAG, "1e0i", literal},
+	{token.IMAG, "1e+100i", literal},
+	{token.IMAG, "1e-100i", literal},
+	{token.IMAG, "2.71828e-1000i", literal},
+	{token.CHAR, "'a'", literal},
+	{token.CHAR, "'\\000'", literal},
+	{token.CHAR, "'\\xFF'", literal},
+	{token.CHAR, "'\\uff16'", literal},
+	{token.CHAR, "'\\U0000ff16'", literal},
+	{token.STRING, "`foobar`", literal},
+	{token.STRING, "`" + `foo
+	                        bar` +
+		"`",
+		literal,
+	},
+	{token.STRING, "`\r`", literal},
+	{token.STRING, "`foo\r\nbar`", literal},
+
+	// Operators and delimiters
+	{token.ADD, "+", operator},
+	{token.SUB, "-", operator},
+	{token.MUL, "*", operator},
+	{token.QUO, "/", operator},
+	{token.REM, "%", operator},
+
+	{token.AND, "&", operator},
+	{token.OR, "|", operator},
+	{token.XOR, "^", operator},
+	{token.SHL, "<<", operator},
+	{token.SHR, ">>", operator},
+	{token.AND_NOT, "&^", operator},
+
+	{token.ADD_ASSIGN, "+=", operator},
+	{token.SUB_ASSIGN, "-=", operator},
+	{token.MUL_ASSIGN, "*=", operator},
+	{token.QUO_ASSIGN, "/=", operator},
+	{token.REM_ASSIGN, "%=", operator},
+
+	{token.AND_ASSIGN, "&=", operator},
+	{token.OR_ASSIGN, "|=", operator},
+	{token.XOR_ASSIGN, "^=", operator},
+	{token.SHL_ASSIGN, "<<=", operator},
+	{token.SHR_ASSIGN, ">>=", operator},
+	{token.AND_NOT_ASSIGN, "&^=", operator},
+
+	{token.LAND, "&&", operator},
+	{token.LOR, "||", operator},
+	{token.ARROW, "<-", operator},
+	{token.INC, "++", operator},
+	{token.DEC, "--", operator},
+
+	{token.EQL, "==", operator},
+	{token.LSS, "<", operator},
+	{token.GTR, ">", operator},
+	{token.ASSIGN, "=", operator},
+	{token.NOT, "!", operator},
+
+	{token.NEQ, "!=", operator},
+	{token.LEQ, "<=", operator},
+	{token.GEQ, ">=", operator},
+	{token.DEFINE, ":=", operator},
+	{token.ELLIPSIS, "...", operator},
+
+	{token.LPAREN, "(", operator},
+	{token.LBRACK, "[", operator},
+	{token.LBRACE, "{", operator},
+	{token.COMMA, ",", operator},
+	{token.PERIOD, ".", operator},
+
+	{token.RPAREN, ")", operator},
+	{token.RBRACK, "]", operator},
+	{token.RBRACE, "}", operator},
+	{token.SEMICOLON, ";", operator},
+	{token.COLON, ":", operator},
+
+	// Keywords
+	{token.BREAK, "break", keyword},
+	{token.CASE, "case", keyword},
+	{token.CHAN, "chan", keyword},
+	{token.CONST, "const", keyword},
+	{token.CONTINUE, "continue", keyword},
+
+	{token.DEFAULT, "default", keyword},
+	{token.DEFER, "defer", keyword},
+	{token.ELSE, "else", keyword},
+	{token.FALLTHROUGH, "fallthrough", keyword},
+	{token.FOR, "for", keyword},
+
+	{token.FUNC, "func", keyword},
+	{token.GO, "go", keyword},
+	{token.GOTO, "goto", keyword},
+	{token.IF, "if", keyword},
+	{token.IMPORT, "import", keyword},
+
+	{token.INTERFACE, "interface", keyword},
+	{token.MAP, "map", keyword},
+	{token.PACKAGE, "package", keyword},
+	{token.RANGE, "range", keyword},
+	{token.RETURN, "return", keyword},
+
+	{token.SELECT, "select", keyword},
+	{token.STRUCT, "struct", keyword},
+	{token.SWITCH, "switch", keyword},
+	{token.TYPE, "type", keyword},
+	{token.VAR, "var", keyword},
+}
+
+const whitespace = "  \t  \n\n\n" // to separate tokens
+
+var source = func() []byte {
+	var src []byte
+	for _, t := range tokens {
+		src = append(src, t.lit...)
+		src = append(src, whitespace...)
+	}
+	return src
+}()
+
+func newlineCount(s string) int {
+	n := 0
+	for i := 0; i < len(s); i++ {
+		if s[i] == '\n' {
+			n++
+		}
+	}
+	return n
+}
+
+func checkPos(t *testing.T, lit string, p token.Pos, expected token.Position) {
+	pos := fset.Position(p)
+	// Check cleaned filenames so that we don't have to worry about
+	// different os.PathSeparator values.
+	if pos.Filename != expected.Filename && filepath.Clean(pos.Filename) != filepath.Clean(expected.Filename) {
+		t.Errorf("bad filename for %q: got %s, expected %s", lit, pos.Filename, expected.Filename)
+	}
+	if pos.Offset != expected.Offset {
+		t.Errorf("bad position for %q: got %d, expected %d", lit, pos.Offset, expected.Offset)
+	}
+	if pos.Line != expected.Line {
+		t.Errorf("bad line for %q: got %d, expected %d", lit, pos.Line, expected.Line)
+	}
+	if pos.Column != expected.Column {
+		t.Errorf("bad column for %q: got %d, expected %d", lit, pos.Column, expected.Column)
+	}
+}
+
+// Verify that calling Scan() provides the correct results.
+func TestScan(t *testing.T) {
+	whitespace_linecount := newlineCount(whitespace)
+
+	// error handler
+	eh := func(_ token.Position, msg string) {
+		t.Errorf("error handler called (msg = %s)", msg)
+	}
+
+	// verify scan
+	var s Scanner
+	s.Init(fset.AddFile("", fset.Base(), len(source)), source, eh, ScanComments|dontInsertSemis)
+
+	// set up expected position
+	epos := token.Position{
+		Filename: "",
+		Offset:   0,
+		Line:     1,
+		Column:   1,
+	}
+
+	index := 0
+	for {
+		pos, tok, lit := s.Scan()
+
+		// check position
+		if tok == token.EOF {
+			// correction for EOF
+			epos.Line = newlineCount(string(source))
+			epos.Column = 2
+		}
+		checkPos(t, lit, pos, epos)
+
+		// check token
+		e := elt{token.EOF, "", special}
+		if index < len(tokens) {
+			e = tokens[index]
+			index++
+		}
+		if tok != e.tok {
+			t.Errorf("bad token for %q: got %s, expected %s", lit, tok, e.tok)
+		}
+
+		// check token class
+		if tokenclass(tok) != e.class {
+			t.Errorf("bad class for %q: got %d, expected %d", lit, tokenclass(tok), e.class)
+		}
+
+		// check literal
+		elit := ""
+		switch e.tok {
+		case token.COMMENT:
+			// no CRs in comments
+			elit = string(stripCR([]byte(e.lit), e.lit[1] == '*'))
+			//-style comment literal doesn't contain newline
+			if elit[1] == '/' {
+				elit = elit[0 : len(elit)-1]
+			}
+		case token.IDENT:
+			elit = e.lit
+		case token.SEMICOLON:
+			elit = ";"
+		default:
+			if e.tok.IsLiteral() {
+				// no CRs in raw string literals
+				elit = e.lit
+				if elit[0] == '`' {
+					elit = string(stripCR([]byte(elit), false))
+				}
+			} else if e.tok.IsKeyword() {
+				elit = e.lit
+			}
+		}
+		if lit != elit {
+			t.Errorf("bad literal for %q: got %q, expected %q", lit, lit, elit)
+		}
+
+		if tok == token.EOF {
+			break
+		}
+
+		// update position
+		epos.Offset += len(e.lit) + len(whitespace)
+		epos.Line += newlineCount(e.lit) + whitespace_linecount
+
+	}
+
+	if s.ErrorCount != 0 {
+		t.Errorf("found %d errors", s.ErrorCount)
+	}
+}
+
+func TestStripCR(t *testing.T) {
+	for _, test := range []struct{ have, want string }{
+		{"//\n", "//\n"},
+		{"//\r\n", "//\n"},
+		{"//\r\r\r\n", "//\n"},
+		{"//\r*\r/\r\n", "//*/\n"},
+		{"/**/", "/**/"},
+		{"/*\r/*/", "/*/*/"},
+		{"/*\r*/", "/**/"},
+		{"/**\r/*/", "/**\r/*/"},
+		{"/*\r/\r*\r/*/", "/*/*\r/*/"},
+		{"/*\r\r\r\r*/", "/**/"},
+	} {
+		got := string(stripCR([]byte(test.have), len(test.have) >= 2 && test.have[1] == '*'))
+		if got != test.want {
+			t.Errorf("stripCR(%q) = %q; want %q", test.have, got, test.want)
+		}
+	}
+}
+
+func checkSemi(t *testing.T, line string, mode Mode) {
+	var S Scanner
+	file := fset.AddFile("TestSemis", fset.Base(), len(line))
+	S.Init(file, []byte(line), nil, mode)
+	pos, tok, lit := S.Scan()
+	for tok != token.EOF {
+		if tok == token.ILLEGAL {
+			// the illegal token literal indicates what
+			// kind of semicolon literal to expect
+			semiLit := "\n"
+			if lit[0] == '#' {
+				semiLit = ";"
+			}
+			// next token must be a semicolon
+			semiPos := file.Position(pos)
+			semiPos.Offset++
+			semiPos.Column++
+			pos, tok, lit = S.Scan()
+			if tok == token.SEMICOLON {
+				if lit != semiLit {
+					t.Errorf(`bad literal for %q: got %q, expected %q`, line, lit, semiLit)
+				}
+				checkPos(t, line, pos, semiPos)
+			} else {
+				t.Errorf("bad token for %q: got %s, expected ;", line, tok)
+			}
+		} else if tok == token.SEMICOLON {
+			t.Errorf("bad token for %q: got ;, expected no ;", line)
+		}
+		pos, tok, lit = S.Scan()
+	}
+}
+
+var lines = []string{
+	// # indicates a semicolon present in the source
+	// $ indicates an automatically inserted semicolon
+	"",
+	"\ufeff#;", // first BOM is ignored
+	"#;",
+	"foo$\n",
+	"123$\n",
+	"1.2$\n",
+	"'x'$\n",
+	`"x"` + "$\n",
+	"`x`$\n",
+
+	"+\n",
+	"-\n",
+	"*\n",
+	"/\n",
+	"%\n",
+
+	"&\n",
+	"|\n",
+	"^\n",
+	"<<\n",
+	">>\n",
+	"&^\n",
+
+	"+=\n",
+	"-=\n",
+	"*=\n",
+	"/=\n",
+	"%=\n",
+
+	"&=\n",
+	"|=\n",
+	"^=\n",
+	"<<=\n",
+	">>=\n",
+	"&^=\n",
+
+	"&&\n",
+	"||\n",
+	"<-\n",
+	"++$\n",
+	"--$\n",
+
+	"==\n",
+	"<\n",
+	">\n",
+	"=\n",
+	"!\n",
+
+	"!=\n",
+	"<=\n",
+	">=\n",
+	":=\n",
+	"...\n",
+
+	"(\n",
+	"[\n",
+	"{\n",
+	",\n",
+	".\n",
+
+	")$\n",
+	"]$\n",
+	"}$\n",
+	"#;\n",
+	":\n",
+
+	"break$\n",
+	"case\n",
+	"chan\n",
+	"const\n",
+	"continue$\n",
+
+	"default\n",
+	"defer\n",
+	"else\n",
+	"fallthrough$\n",
+	"for\n",
+
+	"func\n",
+	"go\n",
+	"goto\n",
+	"if\n",
+	"import\n",
+
+	"interface\n",
+	"map\n",
+	"package\n",
+	"range\n",
+	"return$\n",
+
+	"select\n",
+	"struct\n",
+	"switch\n",
+	"type\n",
+	"var\n",
+
+	"foo$//comment\n",
+	"foo$//comment",
+	"foo$/*comment*/\n",
+	"foo$/*\n*/",
+	"foo$/*comment*/    \n",
+	"foo$/*\n*/    ",
+
+	"foo    $// comment\n",
+	"foo    $// comment",
+	"foo    $/*comment*/\n",
+	"foo    $/*\n*/",
+	"foo    $/*  */ /* \n */ bar$/**/\n",
+	"foo    $/*0*/ /*1*/ /*2*/\n",
+
+	"foo    $/*comment*/    \n",
+	"foo    $/*0*/ /*1*/ /*2*/    \n",
+	"foo	$/**/ /*-------------*/       /*----\n*/bar       $/*  \n*/baa$\n",
+	"foo    $/* an EOF terminates a line */",
+	"foo    $/* an EOF terminates a line */ /*",
+	"foo    $/* an EOF terminates a line */ //",
+
+	"package main$\n\nfunc main() {\n\tif {\n\t\treturn /* */ }$\n}$\n",
+	"package main$",
+}
+
+func TestSemis(t *testing.T) {
+	for _, line := range lines {
+		checkSemi(t, line, 0)
+		checkSemi(t, line, ScanComments)
+
+		// if the input ended in newlines, the input must tokenize the
+		// same with or without those newlines
+		for i := len(line) - 1; i >= 0 && line[i] == '\n'; i-- {
+			checkSemi(t, line[0:i], 0)
+			checkSemi(t, line[0:i], ScanComments)
+		}
+	}
+}
+
+type segment struct {
+	srcline      string // a line of source text
+	filename     string // filename for current token; error message for invalid line directives
+	line, column int    // line and column for current token; error position for invalid line directives
+}
+
+var segments = []segment{
+	// exactly one token per line since the test consumes one token per segment
+	{"  line1", "TestLineDirectives", 1, 3},
+	{"\nline2", "TestLineDirectives", 2, 1},
+	{"\nline3  //line File1.go:100", "TestLineDirectives", 3, 1}, // bad line comment, ignored
+	{"\nline4", "TestLineDirectives", 4, 1},
+	{"\n//line File1.go:100\n  line100", "File1.go", 100, 0},
+	{"\n//line  \t :42\n  line1", " \t ", 42, 0},
+	{"\n//line File2.go:200\n  line200", "File2.go", 200, 0},
+	{"\n//line foo\t:42\n  line42", "foo\t", 42, 0},
+	{"\n //line foo:42\n  line43", "foo\t", 44, 0}, // bad line comment, ignored (use existing, prior filename)
+	{"\n//line foo 42\n  line44", "foo\t", 46, 0},  // bad line comment, ignored (use existing, prior filename)
+	{"\n//line /bar:42\n  line45", "/bar", 42, 0},
+	{"\n//line ./foo:42\n  line46", "foo", 42, 0},
+	{"\n//line a/b/c/File1.go:100\n  line100", "a/b/c/File1.go", 100, 0},
+	{"\n//line c:\\bar:42\n  line200", "c:\\bar", 42, 0},
+	{"\n//line c:\\dir\\File1.go:100\n  line201", "c:\\dir\\File1.go", 100, 0},
+
+	// tests for new line directive syntax
+	{"\n//line :100\na1", "", 100, 0}, // missing filename means empty filename
+	{"\n//line bar:100\nb1", "bar", 100, 0},
+	{"\n//line :100:10\nc1", "bar", 100, 10}, // missing filename means current filename
+	{"\n//line foo:100:10\nd1", "foo", 100, 10},
+
+	{"\n/*line :100*/a2", "", 100, 0}, // missing filename means empty filename
+	{"\n/*line bar:100*/b2", "bar", 100, 0},
+	{"\n/*line :100:10*/c2", "bar", 100, 10}, // missing filename means current filename
+	{"\n/*line foo:100:10*/d2", "foo", 100, 10},
+	{"\n/*line foo:100:10*/    e2", "foo", 100, 14}, // line-directive relative column
+	{"\n/*line foo:100:10*/\n\nf2", "foo", 102, 1},  // absolute column since on new line
+}
+
+var dirsegments = []segment{
+	// exactly one token per line since the test consumes one token per segment
+	{"  line1", "TestLineDir/TestLineDirectives", 1, 3},
+	{"\n//line File1.go:100\n  line100", "TestLineDir/File1.go", 100, 0},
+}
+
+var dirUnixSegments = []segment{
+	{"\n//line /bar:42\n  line42", "/bar", 42, 0},
+}
+
+var dirWindowsSegments = []segment{
+	{"\n//line c:\\bar:42\n  line42", "c:\\bar", 42, 0},
+}
+
+// Verify that line directives are interpreted correctly.
+func TestLineDirectives(t *testing.T) {
+	testSegments(t, segments, "TestLineDirectives")
+	testSegments(t, dirsegments, "TestLineDir/TestLineDirectives")
+	if runtime.GOOS == "windows" {
+		testSegments(t, dirWindowsSegments, "TestLineDir/TestLineDirectives")
+	} else {
+		testSegments(t, dirUnixSegments, "TestLineDir/TestLineDirectives")
+	}
+}
+
+func testSegments(t *testing.T, segments []segment, filename string) {
+	var src string
+	for _, e := range segments {
+		src += e.srcline
+	}
+
+	// verify scan
+	var S Scanner
+	file := fset.AddFile(filename, fset.Base(), len(src))
+	S.Init(file, []byte(src), func(pos token.Position, msg string) { t.Error(Error{pos, msg}) }, dontInsertSemis)
+	for _, s := range segments {
+		p, _, lit := S.Scan()
+		pos := file.Position(p)
+		checkPos(t, lit, p, token.Position{
+			Filename: s.filename,
+			Offset:   pos.Offset,
+			Line:     s.line,
+			Column:   s.column,
+		})
+	}
+
+	if S.ErrorCount != 0 {
+		t.Errorf("got %d errors", S.ErrorCount)
+	}
+}
+
+// The filename is used for the error message in these test cases.
+// The first line directive is valid and used to control the expected error line.
+var invalidSegments = []segment{
+	{"\n//line :1:1\n//line foo:42 extra text\ndummy", "invalid line number: 42 extra text", 1, 12},
+	{"\n//line :2:1\n//line foobar:\ndummy", "invalid line number: ", 2, 15},
+	{"\n//line :5:1\n//line :0\ndummy", "invalid line number: 0", 5, 9},
+	{"\n//line :10:1\n//line :1:0\ndummy", "invalid column number: 0", 10, 11},
+	{"\n//line :1:1\n//line :foo:0\ndummy", "invalid line number: 0", 1, 13}, // foo is considered part of the filename
+}
+
+// Verify that invalid line directives get the correct error message.
+func TestInvalidLineDirectives(t *testing.T) {
+	// make source
+	var src string
+	for _, e := range invalidSegments {
+		src += e.srcline
+	}
+
+	// verify scan
+	var S Scanner
+	var s segment // current segment
+	file := fset.AddFile(filepath.Join("dir", "TestInvalidLineDirectives"), fset.Base(), len(src))
+	S.Init(file, []byte(src), func(pos token.Position, msg string) {
+		if msg != s.filename {
+			t.Errorf("got error %q; want %q", msg, s.filename)
+		}
+		if pos.Line != s.line || pos.Column != s.column {
+			t.Errorf("got position %d:%d; want %d:%d", pos.Line, pos.Column, s.line, s.column)
+		}
+	}, dontInsertSemis)
+	for _, s = range invalidSegments {
+		S.Scan()
+	}
+
+	if S.ErrorCount != len(invalidSegments) {
+		t.Errorf("go %d errors; want %d", S.ErrorCount, len(invalidSegments))
+	}
+}
+
+// Verify that initializing the same scanner more than once works correctly.
+func TestInit(t *testing.T) {
+	var s Scanner
+
+	// 1st init
+	src1 := "if true { }"
+	f1 := fset.AddFile("src1", fset.Base(), len(src1))
+	s.Init(f1, []byte(src1), nil, dontInsertSemis)
+	if f1.Size() != len(src1) {
+		t.Errorf("bad file size: got %d, expected %d", f1.Size(), len(src1))
+	}
+	s.Scan()              // if
+	s.Scan()              // true
+	_, tok, _ := s.Scan() // {
+	if tok != token.LBRACE {
+		t.Errorf("bad token: got %s, expected %s", tok, token.LBRACE)
+	}
+
+	// 2nd init
+	src2 := "go true { ]"
+	f2 := fset.AddFile("src2", fset.Base(), len(src2))
+	s.Init(f2, []byte(src2), nil, dontInsertSemis)
+	if f2.Size() != len(src2) {
+		t.Errorf("bad file size: got %d, expected %d", f2.Size(), len(src2))
+	}
+	_, tok, _ = s.Scan() // go
+	if tok != token.GO {
+		t.Errorf("bad token: got %s, expected %s", tok, token.GO)
+	}
+
+	if s.ErrorCount != 0 {
+		t.Errorf("found %d errors", s.ErrorCount)
+	}
+}
+
+func TestStdErrorHander(t *testing.T) {
+	const src = "@\n" + // illegal character, cause an error
+		"@ @\n" + // two errors on the same line
+		"//line File2:20\n" +
+		"@\n" + // different file, but same line
+		"//line File2:1\n" +
+		"@ @\n" + // same file, decreasing line number
+		"//line File1:1\n" +
+		"@ @ @" // original file, line 1 again
+
+	var list ErrorList
+	eh := func(pos token.Position, msg string) { list.Add(pos, msg) }
+
+	var s Scanner
+	s.Init(fset.AddFile("File1", fset.Base(), len(src)), []byte(src), eh, dontInsertSemis)
+	for {
+		if _, tok, _ := s.Scan(); tok == token.EOF {
+			break
+		}
+	}
+
+	if len(list) != s.ErrorCount {
+		t.Errorf("found %d errors, expected %d", len(list), s.ErrorCount)
+	}
+
+	if len(list) != 9 {
+		t.Errorf("found %d raw errors, expected 9", len(list))
+		PrintError(os.Stderr, list)
+	}
+
+	list.Sort()
+	if len(list) != 9 {
+		t.Errorf("found %d sorted errors, expected 9", len(list))
+		PrintError(os.Stderr, list)
+	}
+
+	list.RemoveMultiples()
+	if len(list) != 4 {
+		t.Errorf("found %d one-per-line errors, expected 4", len(list))
+		PrintError(os.Stderr, list)
+	}
+}
+
+type errorCollector struct {
+	cnt int            // number of errors encountered
+	msg string         // last error message encountered
+	pos token.Position // last error position encountered
+}
+
+func checkError(t *testing.T, src string, tok token.Token, pos int, lit, err string) {
+	var s Scanner
+	var h errorCollector
+	eh := func(pos token.Position, msg string) {
+		h.cnt++
+		h.msg = msg
+		h.pos = pos
+	}
+	s.Init(fset.AddFile("", fset.Base(), len(src)), []byte(src), eh, ScanComments|dontInsertSemis)
+	_, tok0, lit0 := s.Scan()
+	if tok0 != tok {
+		t.Errorf("%q: got %s, expected %s", src, tok0, tok)
+	}
+	if tok0 != token.ILLEGAL && lit0 != lit {
+		t.Errorf("%q: got literal %q, expected %q", src, lit0, lit)
+	}
+	cnt := 0
+	if err != "" {
+		cnt = 1
+	}
+	if h.cnt != cnt {
+		t.Errorf("%q: got cnt %d, expected %d", src, h.cnt, cnt)
+	}
+	if h.msg != err {
+		t.Errorf("%q: got msg %q, expected %q", src, h.msg, err)
+	}
+	if h.pos.Offset != pos {
+		t.Errorf("%q: got offset %d, expected %d", src, h.pos.Offset, pos)
+	}
+}
+
+var errors = []struct {
+	src string
+	tok token.Token
+	pos int
+	lit string
+	err string
+}{
+	{"\a", token.ILLEGAL, 0, "", "illegal character U+0007"},
+	{`#`, token.ILLEGAL, 0, "", "illegal character U+0023 '#'"},
+	{`…`, token.ILLEGAL, 0, "", "illegal character U+2026 '…'"},
+	{"..", token.PERIOD, 0, "", ""}, // two periods, not invalid token (issue #28112)
+	{`' '`, token.CHAR, 0, `' '`, ""},
+	{`''`, token.CHAR, 0, `''`, "illegal rune literal"},
+	{`'12'`, token.CHAR, 0, `'12'`, "illegal rune literal"},
+	{`'123'`, token.CHAR, 0, `'123'`, "illegal rune literal"},
+	{`'\0'`, token.CHAR, 3, `'\0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\07'`, token.CHAR, 4, `'\07'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\8'`, token.CHAR, 2, `'\8'`, "unknown escape sequence"},
+	{`'\08'`, token.CHAR, 3, `'\08'`, "illegal character U+0038 '8' in escape sequence"},
+	{`'\x'`, token.CHAR, 3, `'\x'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\x0'`, token.CHAR, 4, `'\x0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\x0g'`, token.CHAR, 4, `'\x0g'`, "illegal character U+0067 'g' in escape sequence"},
+	{`'\u'`, token.CHAR, 3, `'\u'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u0'`, token.CHAR, 4, `'\u0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u00'`, token.CHAR, 5, `'\u00'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u000'`, token.CHAR, 6, `'\u000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\u000`, token.CHAR, 6, `'\u000`, "escape sequence not terminated"},
+	{`'\u0000'`, token.CHAR, 0, `'\u0000'`, ""},
+	{`'\U'`, token.CHAR, 3, `'\U'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0'`, token.CHAR, 4, `'\U0'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U00'`, token.CHAR, 5, `'\U00'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U000'`, token.CHAR, 6, `'\U000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0000'`, token.CHAR, 7, `'\U0000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U00000'`, token.CHAR, 8, `'\U00000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U000000'`, token.CHAR, 9, `'\U000000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0000000'`, token.CHAR, 10, `'\U0000000'`, "illegal character U+0027 ''' in escape sequence"},
+	{`'\U0000000`, token.CHAR, 10, `'\U0000000`, "escape sequence not terminated"},
+	{`'\U00000000'`, token.CHAR, 0, `'\U00000000'`, ""},
+	{`'\Uffffffff'`, token.CHAR, 2, `'\Uffffffff'`, "escape sequence is invalid Unicode code point"},
+	{`'`, token.CHAR, 0, `'`, "rune literal not terminated"},
+	{`'\`, token.CHAR, 2, `'\`, "escape sequence not terminated"},
+	{"'\n", token.CHAR, 0, "'", "rune literal not terminated"},
+	{"'\n   ", token.CHAR, 0, "'", "rune literal not terminated"},
+	{`""`, token.STRING, 0, `""`, ""},
+	{`"abc`, token.STRING, 0, `"abc`, "string literal not terminated"},
+	{"\"abc\n", token.STRING, 0, `"abc`, "string literal not terminated"},
+	{"\"abc\n   ", token.STRING, 0, `"abc`, "string literal not terminated"},
+	{"``", token.STRING, 0, "``", ""},
+	{"`", token.STRING, 0, "`", "raw string literal not terminated"},
+	{"/**/", token.COMMENT, 0, "/**/", ""},
+	{"/*", token.COMMENT, 0, "/*", "comment not terminated"},
+	{"077", token.INT, 0, "077", ""},
+	{"078.", token.FLOAT, 0, "078.", ""},
+	{"07801234567.", token.FLOAT, 0, "07801234567.", ""},
+	{"078e0", token.FLOAT, 0, "078e0", ""},
+	{"0E", token.FLOAT, 2, "0E", "exponent has no digits"}, // issue 17621
+	{"078", token.INT, 2, "078", "invalid digit '8' in octal literal"},
+	{"07090000008", token.INT, 3, "07090000008", "invalid digit '9' in octal literal"},
+	{"0x", token.INT, 2, "0x", "hexadecimal literal has no digits"},
+	{"\"abc\x00def\"", token.STRING, 4, "\"abc\x00def\"", "illegal character NUL"},
+	{"\"abc\x80def\"", token.STRING, 4, "\"abc\x80def\"", "illegal UTF-8 encoding"},
+	{"\ufeff\ufeff", token.ILLEGAL, 3, "\ufeff\ufeff", "illegal byte order mark"},                        // only first BOM is ignored
+	{"//\ufeff", token.COMMENT, 2, "//\ufeff", "illegal byte order mark"},                                // only first BOM is ignored
+	{"'\ufeff" + `'`, token.CHAR, 1, "'\ufeff" + `'`, "illegal byte order mark"},                         // only first BOM is ignored
+	{`"` + "abc\ufeffdef" + `"`, token.STRING, 4, `"` + "abc\ufeffdef" + `"`, "illegal byte order mark"}, // only first BOM is ignored
+}
+
+func TestScanErrors(t *testing.T) {
+	for _, e := range errors {
+		checkError(t, e.src, e.tok, e.pos, e.lit, e.err)
+	}
+}
+
+// Verify that no comments show up as literal values when skipping comments.
+func TestIssue10213(t *testing.T) {
+	const src = `
+		var (
+			A = 1 // foo
+		)
+
+		var (
+			B = 2
+			// foo
+		)
+
+		var C = 3 // foo
+
+		var D = 4
+		// foo
+
+		func anycode() {
+		// foo
+		}
+	`
+	var s Scanner
+	s.Init(fset.AddFile("", fset.Base(), len(src)), []byte(src), nil, 0)
+	for {
+		pos, tok, lit := s.Scan()
+		class := tokenclass(tok)
+		if lit != "" && class != keyword && class != literal && tok != token.SEMICOLON {
+			t.Errorf("%s: tok = %s, lit = %q", fset.Position(pos), tok, lit)
+		}
+		if tok <= token.EOF {
+			break
+		}
+	}
+}
+
+func TestIssue28112(t *testing.T) {
+	const src = "... .. 0.. .." // make sure to have stand-alone ".." immediately before EOF to test EOF behavior
+	tokens := []token.Token{token.ELLIPSIS, token.PERIOD, token.PERIOD, token.FLOAT, token.PERIOD, token.PERIOD, token.PERIOD, token.EOF}
+	var s Scanner
+	s.Init(fset.AddFile("", fset.Base(), len(src)), []byte(src), nil, 0)
+	for _, want := range tokens {
+		pos, got, lit := s.Scan()
+		if got != want {
+			t.Errorf("%s: got %s, want %s", fset.Position(pos), got, want)
+		}
+		// literals expect to have a (non-empty) literal string and we don't care about other tokens for this test
+		if tokenclass(got) == literal && lit == "" {
+			t.Errorf("%s: for %s got empty literal string", fset.Position(pos), got)
+		}
+	}
+}
+
+func BenchmarkScan(b *testing.B) {
+	b.StopTimer()
+	fset := token.NewFileSet()
+	file := fset.AddFile("", fset.Base(), len(source))
+	var s Scanner
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		s.Init(file, source, nil, ScanComments)
+		for {
+			_, tok, _ := s.Scan()
+			if tok == token.EOF {
+				break
+			}
+		}
+	}
+}
+
+func BenchmarkScanFile(b *testing.B) {
+	b.StopTimer()
+	const filename = "scanner.go"
+	src, err := os.ReadFile(filename)
+	if err != nil {
+		panic(err)
+	}
+	fset := token.NewFileSet()
+	file := fset.AddFile(filename, fset.Base(), len(src))
+	b.SetBytes(int64(len(src)))
+	var s Scanner
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		s.Init(file, src, nil, ScanComments)
+		for {
+			_, tok, _ := s.Scan()
+			if tok == token.EOF {
+				break
+			}
+		}
+	}
+}
+
+func TestNumbers(t *testing.T) {
+	for _, test := range []struct {
+		tok              token.Token
+		src, tokens, err string
+	}{
+		// binaries
+		{token.INT, "0b0", "0b0", ""},
+		{token.INT, "0b1010", "0b1010", ""},
+		{token.INT, "0B1110", "0B1110", ""},
+
+		{token.INT, "0b", "0b", "binary literal has no digits"},
+		{token.INT, "0b0190", "0b0190", "invalid digit '9' in binary literal"},
+		{token.INT, "0b01a0", "0b01 a0", ""}, // only accept 0-9
+
+		{token.FLOAT, "0b.", "0b.", "invalid radix point in binary literal"},
+		{token.FLOAT, "0b.1", "0b.1", "invalid radix point in binary literal"},
+		{token.FLOAT, "0b1.0", "0b1.0", "invalid radix point in binary literal"},
+		{token.FLOAT, "0b1e10", "0b1e10", "'e' exponent requires decimal mantissa"},
+		{token.FLOAT, "0b1P-1", "0b1P-1", "'P' exponent requires hexadecimal mantissa"},
+
+		{token.IMAG, "0b10i", "0b10i", ""},
+		{token.IMAG, "0b10.0i", "0b10.0i", "invalid radix point in binary literal"},
+
+		// octals
+		{token.INT, "0o0", "0o0", ""},
+		{token.INT, "0o1234", "0o1234", ""},
+		{token.INT, "0O1234", "0O1234", ""},
+
+		{token.INT, "0o", "0o", "octal literal has no digits"},
+		{token.INT, "0o8123", "0o8123", "invalid digit '8' in octal literal"},
+		{token.INT, "0o1293", "0o1293", "invalid digit '9' in octal literal"},
+		{token.INT, "0o12a3", "0o12 a3", ""}, // only accept 0-9
+
+		{token.FLOAT, "0o.", "0o.", "invalid radix point in octal literal"},
+		{token.FLOAT, "0o.2", "0o.2", "invalid radix point in octal literal"},
+		{token.FLOAT, "0o1.2", "0o1.2", "invalid radix point in octal literal"},
+		{token.FLOAT, "0o1E+2", "0o1E+2", "'E' exponent requires decimal mantissa"},
+		{token.FLOAT, "0o1p10", "0o1p10", "'p' exponent requires hexadecimal mantissa"},
+
+		{token.IMAG, "0o10i", "0o10i", ""},
+		{token.IMAG, "0o10e0i", "0o10e0i", "'e' exponent requires decimal mantissa"},
+
+		// 0-octals
+		{token.INT, "0", "0", ""},
+		{token.INT, "0123", "0123", ""},
+
+		{token.INT, "08123", "08123", "invalid digit '8' in octal literal"},
+		{token.INT, "01293", "01293", "invalid digit '9' in octal literal"},
+		{token.INT, "0F.", "0 F .", ""}, // only accept 0-9
+		{token.INT, "0123F.", "0123 F .", ""},
+		{token.INT, "0123456x", "0123456 x", ""},
+
+		// decimals
+		{token.INT, "1", "1", ""},
+		{token.INT, "1234", "1234", ""},
+
+		{token.INT, "1f", "1 f", ""}, // only accept 0-9
+
+		{token.IMAG, "0i", "0i", ""},
+		{token.IMAG, "0678i", "0678i", ""},
+
+		// decimal floats
+		{token.FLOAT, "0.", "0.", ""},
+		{token.FLOAT, "123.", "123.", ""},
+		{token.FLOAT, "0123.", "0123.", ""},
+
+		{token.FLOAT, ".0", ".0", ""},
+		{token.FLOAT, ".123", ".123", ""},
+		{token.FLOAT, ".0123", ".0123", ""},
+
+		{token.FLOAT, "0.0", "0.0", ""},
+		{token.FLOAT, "123.123", "123.123", ""},
+		{token.FLOAT, "0123.0123", "0123.0123", ""},
+
+		{token.FLOAT, "0e0", "0e0", ""},
+		{token.FLOAT, "123e+0", "123e+0", ""},
+		{token.FLOAT, "0123E-1", "0123E-1", ""},
+
+		{token.FLOAT, "0.e+1", "0.e+1", ""},
+		{token.FLOAT, "123.E-10", "123.E-10", ""},
+		{token.FLOAT, "0123.e123", "0123.e123", ""},
+
+		{token.FLOAT, ".0e-1", ".0e-1", ""},
+		{token.FLOAT, ".123E+10", ".123E+10", ""},
+		{token.FLOAT, ".0123E123", ".0123E123", ""},
+
+		{token.FLOAT, "0.0e1", "0.0e1", ""},
+		{token.FLOAT, "123.123E-10", "123.123E-10", ""},
+		{token.FLOAT, "0123.0123e+456", "0123.0123e+456", ""},
+
+		{token.FLOAT, "0e", "0e", "exponent has no digits"},
+		{token.FLOAT, "0E+", "0E+", "exponent has no digits"},
+		{token.FLOAT, "1e+f", "1e+ f", "exponent has no digits"},
+		{token.FLOAT, "0p0", "0p0", "'p' exponent requires hexadecimal mantissa"},
+		{token.FLOAT, "1.0P-1", "1.0P-1", "'P' exponent requires hexadecimal mantissa"},
+
+		{token.IMAG, "0.i", "0.i", ""},
+		{token.IMAG, ".123i", ".123i", ""},
+		{token.IMAG, "123.123i", "123.123i", ""},
+		{token.IMAG, "123e+0i", "123e+0i", ""},
+		{token.IMAG, "123.E-10i", "123.E-10i", ""},
+		{token.IMAG, ".123E+10i", ".123E+10i", ""},
+
+		// hexadecimals
+		{token.INT, "0x0", "0x0", ""},
+		{token.INT, "0x1234", "0x1234", ""},
+		{token.INT, "0xcafef00d", "0xcafef00d", ""},
+		{token.INT, "0XCAFEF00D", "0XCAFEF00D", ""},
+
+		{token.INT, "0x", "0x", "hexadecimal literal has no digits"},
+		{token.INT, "0x1g", "0x1 g", ""},
+
+		{token.IMAG, "0xf00i", "0xf00i", ""},
+
+		// hexadecimal floats
+		{token.FLOAT, "0x0p0", "0x0p0", ""},
+		{token.FLOAT, "0x12efp-123", "0x12efp-123", ""},
+		{token.FLOAT, "0xABCD.p+0", "0xABCD.p+0", ""},
+		{token.FLOAT, "0x.0189P-0", "0x.0189P-0", ""},
+		{token.FLOAT, "0x1.ffffp+1023", "0x1.ffffp+1023", ""},
+
+		{token.FLOAT, "0x.", "0x.", "hexadecimal literal has no digits"},
+		{token.FLOAT, "0x0.", "0x0.", "hexadecimal mantissa requires a 'p' exponent"},
+		{token.FLOAT, "0x.0", "0x.0", "hexadecimal mantissa requires a 'p' exponent"},
+		{token.FLOAT, "0x1.1", "0x1.1", "hexadecimal mantissa requires a 'p' exponent"},
+		{token.FLOAT, "0x1.1e0", "0x1.1e0", "hexadecimal mantissa requires a 'p' exponent"},
+		{token.FLOAT, "0x1.2gp1a", "0x1.2 gp1a", "hexadecimal mantissa requires a 'p' exponent"},
+		{token.FLOAT, "0x0p", "0x0p", "exponent has no digits"},
+		{token.FLOAT, "0xeP-", "0xeP-", "exponent has no digits"},
+		{token.FLOAT, "0x1234PAB", "0x1234P AB", "exponent has no digits"},
+		{token.FLOAT, "0x1.2p1a", "0x1.2p1 a", ""},
+
+		{token.IMAG, "0xf00.bap+12i", "0xf00.bap+12i", ""},
+
+		// separators
+		{token.INT, "0b_1000_0001", "0b_1000_0001", ""},
+		{token.INT, "0o_600", "0o_600", ""},
+		{token.INT, "0_466", "0_466", ""},
+		{token.INT, "1_000", "1_000", ""},
+		{token.FLOAT, "1_000.000_1", "1_000.000_1", ""},
+		{token.IMAG, "10e+1_2_3i", "10e+1_2_3i", ""},
+		{token.INT, "0x_f00d", "0x_f00d", ""},
+		{token.FLOAT, "0x_f00d.0p1_2", "0x_f00d.0p1_2", ""},
+
+		{token.INT, "0b__1000", "0b__1000", "'_' must separate successive digits"},
+		{token.INT, "0o60___0", "0o60___0", "'_' must separate successive digits"},
+		{token.INT, "0466_", "0466_", "'_' must separate successive digits"},
+		{token.FLOAT, "1_.", "1_.", "'_' must separate successive digits"},
+		{token.FLOAT, "0._1", "0._1", "'_' must separate successive digits"},
+		{token.FLOAT, "2.7_e0", "2.7_e0", "'_' must separate successive digits"},
+		{token.IMAG, "10e+12_i", "10e+12_i", "'_' must separate successive digits"},
+		{token.INT, "0x___0", "0x___0", "'_' must separate successive digits"},
+		{token.FLOAT, "0x1.0_p0", "0x1.0_p0", "'_' must separate successive digits"},
+	} {
+		var s Scanner
+		var err string
+		s.Init(fset.AddFile("", fset.Base(), len(test.src)), []byte(test.src), func(_ token.Position, msg string) {
+			if err == "" {
+				err = msg
+			}
+		}, 0)
+		for i, want := range strings.Split(test.tokens, " ") {
+			err = ""
+			_, tok, lit := s.Scan()
+
+			// compute lit where for tokens where lit is not defined
+			switch tok {
+			case token.PERIOD:
+				lit = "."
+			case token.ADD:
+				lit = "+"
+			case token.SUB:
+				lit = "-"
+			}
+
+			if i == 0 {
+				if tok != test.tok {
+					t.Errorf("%q: got token %s; want %s", test.src, tok, test.tok)
+				}
+				if err != test.err {
+					t.Errorf("%q: got error %q; want %q", test.src, err, test.err)
+				}
+			}
+
+			if lit != want {
+				t.Errorf("%q: got literal %q (%s); want %s", test.src, lit, tok, want)
+			}
+		}
+
+		// make sure we read all
+		_, tok, _ := s.Scan()
+		if tok == token.SEMICOLON {
+			_, tok, _ = s.Scan()
+		}
+		if tok != token.EOF {
+			t.Errorf("%q: got %s; want EOF", test.src, tok)
+		}
+	}
+}
diff --git a/src/go/token/example_test.go b/src/go/token/example_test.go
new file mode 100644
index 0000000..0011703
--- /dev/null
+++ b/src/go/token/example_test.go
@@ -0,0 +1,77 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token_test
+
+import (
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/token"
+)
+
+func Example_retrievePositionInfo() {
+	fset := token.NewFileSet()
+
+	const src = `package main
+
+import "fmt"
+
+import "go/token"
+
+//line :1:5
+type p = token.Pos
+
+const bad = token.NoPos
+
+//line fake.go:42:11
+func ok(pos p) bool {
+	return pos != bad
+}
+
+/*line :7:9*/func main() {
+	fmt.Println(ok(bad) == bad.IsValid())
+}
+`
+
+	f, err := parser.ParseFile(fset, "main.go", src, 0)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	// Print the location and kind of each declaration in f.
+	for _, decl := range f.Decls {
+		// Get the filename, line, and column back via the file set.
+		// We get both the relative and absolute position.
+		// The relative position is relative to the last line directive.
+		// The absolute position is the exact position in the source.
+		pos := decl.Pos()
+		relPosition := fset.Position(pos)
+		absPosition := fset.PositionFor(pos, false)
+
+		// Either a FuncDecl or GenDecl, since we exit on error.
+		kind := "func"
+		if gen, ok := decl.(*ast.GenDecl); ok {
+			kind = gen.Tok.String()
+		}
+
+		// If the relative and absolute positions differ, show both.
+		fmtPosition := relPosition.String()
+		if relPosition != absPosition {
+			fmtPosition += "[" + absPosition.String() + "]"
+		}
+
+		fmt.Printf("%s: %s\n", fmtPosition, kind)
+	}
+
+	//Output:
+	//
+	// main.go:3:1: import
+	// main.go:5:1: import
+	// main.go:1:5[main.go:8:1]: type
+	// main.go:3:1[main.go:10:1]: const
+	// fake.go:42:11[main.go:13:1]: func
+	// fake.go:7:9[main.go:17:14]: func
+}
diff --git a/src/go/token/position.go b/src/go/token/position.go
new file mode 100644
index 0000000..a21f5fd
--- /dev/null
+++ b/src/go/token/position.go
@@ -0,0 +1,552 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token
+
+import (
+	"fmt"
+	"sort"
+	"sync"
+)
+
+// -----------------------------------------------------------------------------
+// Positions
+
+// Position describes an arbitrary source position
+// including the file, line, and column location.
+// A Position is valid if the line number is > 0.
+//
+type Position struct {
+	Filename string // filename, if any
+	Offset   int    // offset, starting at 0
+	Line     int    // line number, starting at 1
+	Column   int    // column number, starting at 1 (byte count)
+}
+
+// IsValid reports whether the position is valid.
+func (pos *Position) IsValid() bool { return pos.Line > 0 }
+
+// String returns a string in one of several forms:
+//
+//	file:line:column    valid position with file name
+//	file:line           valid position with file name but no column (column == 0)
+//	line:column         valid position without file name
+//	line                valid position without file name and no column (column == 0)
+//	file                invalid position with file name
+//	-                   invalid position without file name
+//
+func (pos Position) String() string {
+	s := pos.Filename
+	if pos.IsValid() {
+		if s != "" {
+			s += ":"
+		}
+		s += fmt.Sprintf("%d", pos.Line)
+		if pos.Column != 0 {
+			s += fmt.Sprintf(":%d", pos.Column)
+		}
+	}
+	if s == "" {
+		s = "-"
+	}
+	return s
+}
+
+// Pos is a compact encoding of a source position within a file set.
+// It can be converted into a Position for a more convenient, but much
+// larger, representation.
+//
+// The Pos value for a given file is a number in the range [base, base+size],
+// where base and size are specified when a file is added to the file set.
+// The difference between a Pos value and the corresponding file base
+// corresponds to the byte offset of that position (represented by the Pos value)
+// from the beginning of the file. Thus, the file base offset is the Pos value
+// representing the first byte in the file.
+//
+// To create the Pos value for a specific source offset (measured in bytes),
+// first add the respective file to the current file set using FileSet.AddFile
+// and then call File.Pos(offset) for that file. Given a Pos value p
+// for a specific file set fset, the corresponding Position value is
+// obtained by calling fset.Position(p).
+//
+// Pos values can be compared directly with the usual comparison operators:
+// If two Pos values p and q are in the same file, comparing p and q is
+// equivalent to comparing the respective source file offsets. If p and q
+// are in different files, p < q is true if the file implied by p was added
+// to the respective file set before the file implied by q.
+//
+type Pos int
+
+// The zero value for Pos is NoPos; there is no file and line information
+// associated with it, and NoPos.IsValid() is false. NoPos is always
+// smaller than any other Pos value. The corresponding Position value
+// for NoPos is the zero value for Position.
+//
+const NoPos Pos = 0
+
+// IsValid reports whether the position is valid.
+func (p Pos) IsValid() bool {
+	return p != NoPos
+}
+
+// -----------------------------------------------------------------------------
+// File
+
+// A File is a handle for a file belonging to a FileSet.
+// A File has a name, size, and line offset table.
+//
+type File struct {
+	set  *FileSet
+	name string // file name as provided to AddFile
+	base int    // Pos value range for this file is [base...base+size]
+	size int    // file size as provided to AddFile
+
+	// lines and infos are protected by mutex
+	mutex sync.Mutex
+	lines []int // lines contains the offset of the first character for each line (the first entry is always 0)
+	infos []lineInfo
+}
+
+// Name returns the file name of file f as registered with AddFile.
+func (f *File) Name() string {
+	return f.name
+}
+
+// Base returns the base offset of file f as registered with AddFile.
+func (f *File) Base() int {
+	return f.base
+}
+
+// Size returns the size of file f as registered with AddFile.
+func (f *File) Size() int {
+	return f.size
+}
+
+// LineCount returns the number of lines in file f.
+func (f *File) LineCount() int {
+	f.mutex.Lock()
+	n := len(f.lines)
+	f.mutex.Unlock()
+	return n
+}
+
+// AddLine adds the line offset for a new line.
+// The line offset must be larger than the offset for the previous line
+// and smaller than the file size; otherwise the line offset is ignored.
+//
+func (f *File) AddLine(offset int) {
+	f.mutex.Lock()
+	if i := len(f.lines); (i == 0 || f.lines[i-1] < offset) && offset < f.size {
+		f.lines = append(f.lines, offset)
+	}
+	f.mutex.Unlock()
+}
+
+// MergeLine merges a line with the following line. It is akin to replacing
+// the newline character at the end of the line with a space (to not change the
+// remaining offsets). To obtain the line number, consult e.g. Position.Line.
+// MergeLine will panic if given an invalid line number.
+//
+func (f *File) MergeLine(line int) {
+	if line < 1 {
+		panic(fmt.Sprintf("invalid line number %d (should be >= 1)", line))
+	}
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+	if line >= len(f.lines) {
+		panic(fmt.Sprintf("invalid line number %d (should be < %d)", line, len(f.lines)))
+	}
+	// To merge the line numbered <line> with the line numbered <line+1>,
+	// we need to remove the entry in lines corresponding to the line
+	// numbered <line+1>. The entry in lines corresponding to the line
+	// numbered <line+1> is located at index <line>, since indices in lines
+	// are 0-based and line numbers are 1-based.
+	copy(f.lines[line:], f.lines[line+1:])
+	f.lines = f.lines[:len(f.lines)-1]
+}
+
+// SetLines sets the line offsets for a file and reports whether it succeeded.
+// The line offsets are the offsets of the first character of each line;
+// for instance for the content "ab\nc\n" the line offsets are {0, 3}.
+// An empty file has an empty line offset table.
+// Each line offset must be larger than the offset for the previous line
+// and smaller than the file size; otherwise SetLines fails and returns
+// false.
+// Callers must not mutate the provided slice after SetLines returns.
+//
+func (f *File) SetLines(lines []int) bool {
+	// verify validity of lines table
+	size := f.size
+	for i, offset := range lines {
+		if i > 0 && offset <= lines[i-1] || size <= offset {
+			return false
+		}
+	}
+
+	// set lines table
+	f.mutex.Lock()
+	f.lines = lines
+	f.mutex.Unlock()
+	return true
+}
+
+// SetLinesForContent sets the line offsets for the given file content.
+// It ignores position-altering //line comments.
+func (f *File) SetLinesForContent(content []byte) {
+	var lines []int
+	line := 0
+	for offset, b := range content {
+		if line >= 0 {
+			lines = append(lines, line)
+		}
+		line = -1
+		if b == '\n' {
+			line = offset + 1
+		}
+	}
+
+	// set lines table
+	f.mutex.Lock()
+	f.lines = lines
+	f.mutex.Unlock()
+}
+
+// LineStart returns the Pos value of the start of the specified line.
+// It ignores any alternative positions set using AddLineColumnInfo.
+// LineStart panics if the 1-based line number is invalid.
+func (f *File) LineStart(line int) Pos {
+	if line < 1 {
+		panic(fmt.Sprintf("invalid line number %d (should be >= 1)", line))
+	}
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+	if line > len(f.lines) {
+		panic(fmt.Sprintf("invalid line number %d (should be < %d)", line, len(f.lines)))
+	}
+	return Pos(f.base + f.lines[line-1])
+}
+
+// A lineInfo object describes alternative file, line, and column
+// number information (such as provided via a //line directive)
+// for a given file offset.
+type lineInfo struct {
+	// fields are exported to make them accessible to gob
+	Offset       int
+	Filename     string
+	Line, Column int
+}
+
+// AddLineInfo is like AddLineColumnInfo with a column = 1 argument.
+// It is here for backward-compatibility for code prior to Go 1.11.
+//
+func (f *File) AddLineInfo(offset int, filename string, line int) {
+	f.AddLineColumnInfo(offset, filename, line, 1)
+}
+
+// AddLineColumnInfo adds alternative file, line, and column number
+// information for a given file offset. The offset must be larger
+// than the offset for the previously added alternative line info
+// and smaller than the file size; otherwise the information is
+// ignored.
+//
+// AddLineColumnInfo is typically used to register alternative position
+// information for line directives such as //line filename:line:column.
+//
+func (f *File) AddLineColumnInfo(offset int, filename string, line, column int) {
+	f.mutex.Lock()
+	if i := len(f.infos); i == 0 || f.infos[i-1].Offset < offset && offset < f.size {
+		f.infos = append(f.infos, lineInfo{offset, filename, line, column})
+	}
+	f.mutex.Unlock()
+}
+
+// Pos returns the Pos value for the given file offset;
+// the offset must be <= f.Size().
+// f.Pos(f.Offset(p)) == p.
+//
+func (f *File) Pos(offset int) Pos {
+	if offset > f.size {
+		panic(fmt.Sprintf("invalid file offset %d (should be <= %d)", offset, f.size))
+	}
+	return Pos(f.base + offset)
+}
+
+// Offset returns the offset for the given file position p;
+// p must be a valid Pos value in that file.
+// f.Offset(f.Pos(offset)) == offset.
+//
+func (f *File) Offset(p Pos) int {
+	if int(p) < f.base || int(p) > f.base+f.size {
+		panic(fmt.Sprintf("invalid Pos value %d (should be in [%d, %d[)", p, f.base, f.base+f.size))
+	}
+	return int(p) - f.base
+}
+
+// Line returns the line number for the given file position p;
+// p must be a Pos value in that file or NoPos.
+//
+func (f *File) Line(p Pos) int {
+	return f.Position(p).Line
+}
+
+func searchLineInfos(a []lineInfo, x int) int {
+	return sort.Search(len(a), func(i int) bool { return a[i].Offset > x }) - 1
+}
+
+// unpack returns the filename and line and column number for a file offset.
+// If adjusted is set, unpack will return the filename and line information
+// possibly adjusted by //line comments; otherwise those comments are ignored.
+//
+func (f *File) unpack(offset int, adjusted bool) (filename string, line, column int) {
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+	filename = f.name
+	if i := searchInts(f.lines, offset); i >= 0 {
+		line, column = i+1, offset-f.lines[i]+1
+	}
+	if adjusted && len(f.infos) > 0 {
+		// few files have extra line infos
+		if i := searchLineInfos(f.infos, offset); i >= 0 {
+			alt := &f.infos[i]
+			filename = alt.Filename
+			if i := searchInts(f.lines, alt.Offset); i >= 0 {
+				// i+1 is the line at which the alternative position was recorded
+				d := line - (i + 1) // line distance from alternative position base
+				line = alt.Line + d
+				if alt.Column == 0 {
+					// alternative column is unknown => relative column is unknown
+					// (the current specification for line directives requires
+					// this to apply until the next PosBase/line directive,
+					// not just until the new newline)
+					column = 0
+				} else if d == 0 {
+					// the alternative position base is on the current line
+					// => column is relative to alternative column
+					column = alt.Column + (offset - alt.Offset)
+				}
+			}
+		}
+	}
+	return
+}
+
+func (f *File) position(p Pos, adjusted bool) (pos Position) {
+	offset := int(p) - f.base
+	pos.Offset = offset
+	pos.Filename, pos.Line, pos.Column = f.unpack(offset, adjusted)
+	return
+}
+
+// PositionFor returns the Position value for the given file position p.
+// If adjusted is set, the position may be adjusted by position-altering
+// //line comments; otherwise those comments are ignored.
+// p must be a Pos value in f or NoPos.
+//
+func (f *File) PositionFor(p Pos, adjusted bool) (pos Position) {
+	if p != NoPos {
+		if int(p) < f.base || int(p) > f.base+f.size {
+			panic(fmt.Sprintf("invalid Pos value %d (should be in [%d, %d[)", p, f.base, f.base+f.size))
+		}
+		pos = f.position(p, adjusted)
+	}
+	return
+}
+
+// Position returns the Position value for the given file position p.
+// Calling f.Position(p) is equivalent to calling f.PositionFor(p, true).
+//
+func (f *File) Position(p Pos) (pos Position) {
+	return f.PositionFor(p, true)
+}
+
+// -----------------------------------------------------------------------------
+// FileSet
+
+// A FileSet represents a set of source files.
+// Methods of file sets are synchronized; multiple goroutines
+// may invoke them concurrently.
+//
+// The byte offsets for each file in a file set are mapped into
+// distinct (integer) intervals, one interval [base, base+size]
+// per file. Base represents the first byte in the file, and size
+// is the corresponding file size. A Pos value is a value in such
+// an interval. By determining the interval a Pos value belongs
+// to, the file, its file base, and thus the byte offset (position)
+// the Pos value is representing can be computed.
+//
+// When adding a new file, a file base must be provided. That can
+// be any integer value that is past the end of any interval of any
+// file already in the file set. For convenience, FileSet.Base provides
+// such a value, which is simply the end of the Pos interval of the most
+// recently added file, plus one. Unless there is a need to extend an
+// interval later, using the FileSet.Base should be used as argument
+// for FileSet.AddFile.
+//
+type FileSet struct {
+	mutex sync.RWMutex // protects the file set
+	base  int          // base offset for the next file
+	files []*File      // list of files in the order added to the set
+	last  *File        // cache of last file looked up
+}
+
+// NewFileSet creates a new file set.
+func NewFileSet() *FileSet {
+	return &FileSet{
+		base: 1, // 0 == NoPos
+	}
+}
+
+// Base returns the minimum base offset that must be provided to
+// AddFile when adding the next file.
+//
+func (s *FileSet) Base() int {
+	s.mutex.RLock()
+	b := s.base
+	s.mutex.RUnlock()
+	return b
+
+}
+
+// AddFile adds a new file with a given filename, base offset, and file size
+// to the file set s and returns the file. Multiple files may have the same
+// name. The base offset must not be smaller than the FileSet's Base(), and
+// size must not be negative. As a special case, if a negative base is provided,
+// the current value of the FileSet's Base() is used instead.
+//
+// Adding the file will set the file set's Base() value to base + size + 1
+// as the minimum base value for the next file. The following relationship
+// exists between a Pos value p for a given file offset offs:
+//
+//	int(p) = base + offs
+//
+// with offs in the range [0, size] and thus p in the range [base, base+size].
+// For convenience, File.Pos may be used to create file-specific position
+// values from a file offset.
+//
+func (s *FileSet) AddFile(filename string, base, size int) *File {
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+	if base < 0 {
+		base = s.base
+	}
+	if base < s.base {
+		panic(fmt.Sprintf("invalid base %d (should be >= %d)", base, s.base))
+	}
+	if size < 0 {
+		panic(fmt.Sprintf("invalid size %d (should be >= 0)", size))
+	}
+	// base >= s.base && size >= 0
+	f := &File{set: s, name: filename, base: base, size: size, lines: []int{0}}
+	base += size + 1 // +1 because EOF also has a position
+	if base < 0 {
+		panic("token.Pos offset overflow (> 2G of source code in file set)")
+	}
+	// add the file to the file set
+	s.base = base
+	s.files = append(s.files, f)
+	s.last = f
+	return f
+}
+
+// Iterate calls f for the files in the file set in the order they were added
+// until f returns false.
+//
+func (s *FileSet) Iterate(f func(*File) bool) {
+	for i := 0; ; i++ {
+		var file *File
+		s.mutex.RLock()
+		if i < len(s.files) {
+			file = s.files[i]
+		}
+		s.mutex.RUnlock()
+		if file == nil || !f(file) {
+			break
+		}
+	}
+}
+
+func searchFiles(a []*File, x int) int {
+	return sort.Search(len(a), func(i int) bool { return a[i].base > x }) - 1
+}
+
+func (s *FileSet) file(p Pos) *File {
+	s.mutex.RLock()
+	// common case: p is in last file
+	if f := s.last; f != nil && f.base <= int(p) && int(p) <= f.base+f.size {
+		s.mutex.RUnlock()
+		return f
+	}
+	// p is not in last file - search all files
+	if i := searchFiles(s.files, int(p)); i >= 0 {
+		f := s.files[i]
+		// f.base <= int(p) by definition of searchFiles
+		if int(p) <= f.base+f.size {
+			s.mutex.RUnlock()
+			s.mutex.Lock()
+			s.last = f // race is ok - s.last is only a cache
+			s.mutex.Unlock()
+			return f
+		}
+	}
+	s.mutex.RUnlock()
+	return nil
+}
+
+// File returns the file that contains the position p.
+// If no such file is found (for instance for p == NoPos),
+// the result is nil.
+//
+func (s *FileSet) File(p Pos) (f *File) {
+	if p != NoPos {
+		f = s.file(p)
+	}
+	return
+}
+
+// PositionFor converts a Pos p in the fileset into a Position value.
+// If adjusted is set, the position may be adjusted by position-altering
+// //line comments; otherwise those comments are ignored.
+// p must be a Pos value in s or NoPos.
+//
+func (s *FileSet) PositionFor(p Pos, adjusted bool) (pos Position) {
+	if p != NoPos {
+		if f := s.file(p); f != nil {
+			return f.position(p, adjusted)
+		}
+	}
+	return
+}
+
+// Position converts a Pos p in the fileset into a Position value.
+// Calling s.Position(p) is equivalent to calling s.PositionFor(p, true).
+//
+func (s *FileSet) Position(p Pos) (pos Position) {
+	return s.PositionFor(p, true)
+}
+
+// -----------------------------------------------------------------------------
+// Helper functions
+
+func searchInts(a []int, x int) int {
+	// This function body is a manually inlined version of:
+	//
+	//   return sort.Search(len(a), func(i int) bool { return a[i] > x }) - 1
+	//
+	// With better compiler optimizations, this may not be needed in the
+	// future, but at the moment this change improves the go/printer
+	// benchmark performance by ~30%. This has a direct impact on the
+	// speed of gofmt and thus seems worthwhile (2011-04-29).
+	// TODO(gri): Remove this when compilers have caught up.
+	i, j := 0, len(a)
+	for i < j {
+		h := i + (j-i)/2 // avoid overflow when computing h
+		// i ≤ h < j
+		if a[h] <= x {
+			i = h + 1
+		} else {
+			j = h
+		}
+	}
+	return i - 1
+}
diff --git a/src/go/token/position_test.go b/src/go/token/position_test.go
new file mode 100644
index 0000000..7d465df
--- /dev/null
+++ b/src/go/token/position_test.go
@@ -0,0 +1,341 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token
+
+import (
+	"fmt"
+	"math/rand"
+	"sync"
+	"testing"
+)
+
+func checkPos(t *testing.T, msg string, got, want Position) {
+	if got.Filename != want.Filename {
+		t.Errorf("%s: got filename = %q; want %q", msg, got.Filename, want.Filename)
+	}
+	if got.Offset != want.Offset {
+		t.Errorf("%s: got offset = %d; want %d", msg, got.Offset, want.Offset)
+	}
+	if got.Line != want.Line {
+		t.Errorf("%s: got line = %d; want %d", msg, got.Line, want.Line)
+	}
+	if got.Column != want.Column {
+		t.Errorf("%s: got column = %d; want %d", msg, got.Column, want.Column)
+	}
+}
+
+func TestNoPos(t *testing.T) {
+	if NoPos.IsValid() {
+		t.Errorf("NoPos should not be valid")
+	}
+	var fset *FileSet
+	checkPos(t, "nil NoPos", fset.Position(NoPos), Position{})
+	fset = NewFileSet()
+	checkPos(t, "fset NoPos", fset.Position(NoPos), Position{})
+}
+
+var tests = []struct {
+	filename string
+	source   []byte // may be nil
+	size     int
+	lines    []int
+}{
+	{"a", []byte{}, 0, []int{}},
+	{"b", []byte("01234"), 5, []int{0}},
+	{"c", []byte("\n\n\n\n\n\n\n\n\n"), 9, []int{0, 1, 2, 3, 4, 5, 6, 7, 8}},
+	{"d", nil, 100, []int{0, 5, 10, 20, 30, 70, 71, 72, 80, 85, 90, 99}},
+	{"e", nil, 777, []int{0, 80, 100, 120, 130, 180, 267, 455, 500, 567, 620}},
+	{"f", []byte("package p\n\nimport \"fmt\""), 23, []int{0, 10, 11}},
+	{"g", []byte("package p\n\nimport \"fmt\"\n"), 24, []int{0, 10, 11}},
+	{"h", []byte("package p\n\nimport \"fmt\"\n "), 25, []int{0, 10, 11, 24}},
+}
+
+func linecol(lines []int, offs int) (int, int) {
+	prevLineOffs := 0
+	for line, lineOffs := range lines {
+		if offs < lineOffs {
+			return line, offs - prevLineOffs + 1
+		}
+		prevLineOffs = lineOffs
+	}
+	return len(lines), offs - prevLineOffs + 1
+}
+
+func verifyPositions(t *testing.T, fset *FileSet, f *File, lines []int) {
+	for offs := 0; offs < f.Size(); offs++ {
+		p := f.Pos(offs)
+		offs2 := f.Offset(p)
+		if offs2 != offs {
+			t.Errorf("%s, Offset: got offset %d; want %d", f.Name(), offs2, offs)
+		}
+		line, col := linecol(lines, offs)
+		msg := fmt.Sprintf("%s (offs = %d, p = %d)", f.Name(), offs, p)
+		checkPos(t, msg, f.Position(f.Pos(offs)), Position{f.Name(), offs, line, col})
+		checkPos(t, msg, fset.Position(p), Position{f.Name(), offs, line, col})
+	}
+}
+
+func makeTestSource(size int, lines []int) []byte {
+	src := make([]byte, size)
+	for _, offs := range lines {
+		if offs > 0 {
+			src[offs-1] = '\n'
+		}
+	}
+	return src
+}
+
+func TestPositions(t *testing.T) {
+	const delta = 7 // a non-zero base offset increment
+	fset := NewFileSet()
+	for _, test := range tests {
+		// verify consistency of test case
+		if test.source != nil && len(test.source) != test.size {
+			t.Errorf("%s: inconsistent test case: got file size %d; want %d", test.filename, len(test.source), test.size)
+		}
+
+		// add file and verify name and size
+		f := fset.AddFile(test.filename, fset.Base()+delta, test.size)
+		if f.Name() != test.filename {
+			t.Errorf("got filename %q; want %q", f.Name(), test.filename)
+		}
+		if f.Size() != test.size {
+			t.Errorf("%s: got file size %d; want %d", f.Name(), f.Size(), test.size)
+		}
+		if fset.File(f.Pos(0)) != f {
+			t.Errorf("%s: f.Pos(0) was not found in f", f.Name())
+		}
+
+		// add lines individually and verify all positions
+		for i, offset := range test.lines {
+			f.AddLine(offset)
+			if f.LineCount() != i+1 {
+				t.Errorf("%s, AddLine: got line count %d; want %d", f.Name(), f.LineCount(), i+1)
+			}
+			// adding the same offset again should be ignored
+			f.AddLine(offset)
+			if f.LineCount() != i+1 {
+				t.Errorf("%s, AddLine: got unchanged line count %d; want %d", f.Name(), f.LineCount(), i+1)
+			}
+			verifyPositions(t, fset, f, test.lines[0:i+1])
+		}
+
+		// add lines with SetLines and verify all positions
+		if ok := f.SetLines(test.lines); !ok {
+			t.Errorf("%s: SetLines failed", f.Name())
+		}
+		if f.LineCount() != len(test.lines) {
+			t.Errorf("%s, SetLines: got line count %d; want %d", f.Name(), f.LineCount(), len(test.lines))
+		}
+		verifyPositions(t, fset, f, test.lines)
+
+		// add lines with SetLinesForContent and verify all positions
+		src := test.source
+		if src == nil {
+			// no test source available - create one from scratch
+			src = makeTestSource(test.size, test.lines)
+		}
+		f.SetLinesForContent(src)
+		if f.LineCount() != len(test.lines) {
+			t.Errorf("%s, SetLinesForContent: got line count %d; want %d", f.Name(), f.LineCount(), len(test.lines))
+		}
+		verifyPositions(t, fset, f, test.lines)
+	}
+}
+
+func TestLineInfo(t *testing.T) {
+	fset := NewFileSet()
+	f := fset.AddFile("foo", fset.Base(), 500)
+	lines := []int{0, 42, 77, 100, 210, 220, 277, 300, 333, 401}
+	// add lines individually and provide alternative line information
+	for _, offs := range lines {
+		f.AddLine(offs)
+		f.AddLineInfo(offs, "bar", 42)
+	}
+	// verify positions for all offsets
+	for offs := 0; offs <= f.Size(); offs++ {
+		p := f.Pos(offs)
+		_, col := linecol(lines, offs)
+		msg := fmt.Sprintf("%s (offs = %d, p = %d)", f.Name(), offs, p)
+		checkPos(t, msg, f.Position(f.Pos(offs)), Position{"bar", offs, 42, col})
+		checkPos(t, msg, fset.Position(p), Position{"bar", offs, 42, col})
+	}
+}
+
+func TestFiles(t *testing.T) {
+	fset := NewFileSet()
+	for i, test := range tests {
+		base := fset.Base()
+		if i%2 == 1 {
+			// Setting a negative base is equivalent to
+			// fset.Base(), so test some of each.
+			base = -1
+		}
+		fset.AddFile(test.filename, base, test.size)
+		j := 0
+		fset.Iterate(func(f *File) bool {
+			if f.Name() != tests[j].filename {
+				t.Errorf("got filename = %s; want %s", f.Name(), tests[j].filename)
+			}
+			j++
+			return true
+		})
+		if j != i+1 {
+			t.Errorf("got %d files; want %d", j, i+1)
+		}
+	}
+}
+
+// FileSet.File should return nil if Pos is past the end of the FileSet.
+func TestFileSetPastEnd(t *testing.T) {
+	fset := NewFileSet()
+	for _, test := range tests {
+		fset.AddFile(test.filename, fset.Base(), test.size)
+	}
+	if f := fset.File(Pos(fset.Base())); f != nil {
+		t.Errorf("got %v, want nil", f)
+	}
+}
+
+func TestFileSetCacheUnlikely(t *testing.T) {
+	fset := NewFileSet()
+	offsets := make(map[string]int)
+	for _, test := range tests {
+		offsets[test.filename] = fset.Base()
+		fset.AddFile(test.filename, fset.Base(), test.size)
+	}
+	for file, pos := range offsets {
+		f := fset.File(Pos(pos))
+		if f.Name() != file {
+			t.Errorf("got %q at position %d, want %q", f.Name(), pos, file)
+		}
+	}
+}
+
+// issue 4345. Test that concurrent use of FileSet.Pos does not trigger a
+// race in the FileSet position cache.
+func TestFileSetRace(t *testing.T) {
+	fset := NewFileSet()
+	for i := 0; i < 100; i++ {
+		fset.AddFile(fmt.Sprintf("file-%d", i), fset.Base(), 1031)
+	}
+	max := int32(fset.Base())
+	var stop sync.WaitGroup
+	r := rand.New(rand.NewSource(7))
+	for i := 0; i < 2; i++ {
+		r := rand.New(rand.NewSource(r.Int63()))
+		stop.Add(1)
+		go func() {
+			for i := 0; i < 1000; i++ {
+				fset.Position(Pos(r.Int31n(max)))
+			}
+			stop.Done()
+		}()
+	}
+	stop.Wait()
+}
+
+// issue 16548. Test that concurrent use of File.AddLine and FileSet.PositionFor
+// does not trigger a race in the FileSet position cache.
+func TestFileSetRace2(t *testing.T) {
+	const N = 1e3
+	var (
+		fset = NewFileSet()
+		file = fset.AddFile("", -1, N)
+		ch   = make(chan int, 2)
+	)
+
+	go func() {
+		for i := 0; i < N; i++ {
+			file.AddLine(i)
+		}
+		ch <- 1
+	}()
+
+	go func() {
+		pos := file.Pos(0)
+		for i := 0; i < N; i++ {
+			fset.PositionFor(pos, false)
+		}
+		ch <- 1
+	}()
+
+	<-ch
+	<-ch
+}
+
+func TestPositionFor(t *testing.T) {
+	src := []byte(`
+foo
+b
+ar
+//line :100
+foobar
+//line bar:3
+done
+`)
+
+	const filename = "foo"
+	fset := NewFileSet()
+	f := fset.AddFile(filename, fset.Base(), len(src))
+	f.SetLinesForContent(src)
+
+	// verify position info
+	for i, offs := range f.lines {
+		got1 := f.PositionFor(f.Pos(offs), false)
+		got2 := f.PositionFor(f.Pos(offs), true)
+		got3 := f.Position(f.Pos(offs))
+		want := Position{filename, offs, i + 1, 1}
+		checkPos(t, "1. PositionFor unadjusted", got1, want)
+		checkPos(t, "1. PositionFor adjusted", got2, want)
+		checkPos(t, "1. Position", got3, want)
+	}
+
+	// manually add //line info on lines l1, l2
+	const l1, l2 = 5, 7
+	f.AddLineInfo(f.lines[l1-1], "", 100)
+	f.AddLineInfo(f.lines[l2-1], "bar", 3)
+
+	// unadjusted position info must remain unchanged
+	for i, offs := range f.lines {
+		got1 := f.PositionFor(f.Pos(offs), false)
+		want := Position{filename, offs, i + 1, 1}
+		checkPos(t, "2. PositionFor unadjusted", got1, want)
+	}
+
+	// adjusted position info should have changed
+	for i, offs := range f.lines {
+		got2 := f.PositionFor(f.Pos(offs), true)
+		got3 := f.Position(f.Pos(offs))
+		want := Position{filename, offs, i + 1, 1}
+		// manually compute wanted filename and line
+		line := want.Line
+		if i+1 >= l1 {
+			want.Filename = ""
+			want.Line = line - l1 + 100
+		}
+		if i+1 >= l2 {
+			want.Filename = "bar"
+			want.Line = line - l2 + 3
+		}
+		checkPos(t, "3. PositionFor adjusted", got2, want)
+		checkPos(t, "3. Position", got3, want)
+	}
+}
+
+func TestLineStart(t *testing.T) {
+	const src = "one\ntwo\nthree\n"
+	fset := NewFileSet()
+	f := fset.AddFile("input", -1, len(src))
+	f.SetLinesForContent([]byte(src))
+
+	for line := 1; line <= 3; line++ {
+		pos := f.LineStart(line)
+		position := fset.Position(pos)
+		if position.Line != line || position.Column != 1 {
+			t.Errorf("LineStart(%d) returned wrong pos %d: %s", line, pos, position)
+		}
+	}
+}
diff --git a/src/go/token/serialize.go b/src/go/token/serialize.go
new file mode 100644
index 0000000..d0ea345
--- /dev/null
+++ b/src/go/token/serialize.go
@@ -0,0 +1,71 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token
+
+type serializedFile struct {
+	// fields correspond 1:1 to fields with same (lower-case) name in File
+	Name  string
+	Base  int
+	Size  int
+	Lines []int
+	Infos []lineInfo
+}
+
+type serializedFileSet struct {
+	Base  int
+	Files []serializedFile
+}
+
+// Read calls decode to deserialize a file set into s; s must not be nil.
+func (s *FileSet) Read(decode func(interface{}) error) error {
+	var ss serializedFileSet
+	if err := decode(&ss); err != nil {
+		return err
+	}
+
+	s.mutex.Lock()
+	s.base = ss.Base
+	files := make([]*File, len(ss.Files))
+	for i := 0; i < len(ss.Files); i++ {
+		f := &ss.Files[i]
+		files[i] = &File{
+			set:   s,
+			name:  f.Name,
+			base:  f.Base,
+			size:  f.Size,
+			lines: f.Lines,
+			infos: f.Infos,
+		}
+	}
+	s.files = files
+	s.last = nil
+	s.mutex.Unlock()
+
+	return nil
+}
+
+// Write calls encode to serialize the file set s.
+func (s *FileSet) Write(encode func(interface{}) error) error {
+	var ss serializedFileSet
+
+	s.mutex.Lock()
+	ss.Base = s.base
+	files := make([]serializedFile, len(s.files))
+	for i, f := range s.files {
+		f.mutex.Lock()
+		files[i] = serializedFile{
+			Name:  f.name,
+			Base:  f.base,
+			Size:  f.size,
+			Lines: append([]int(nil), f.lines...),
+			Infos: append([]lineInfo(nil), f.infos...),
+		}
+		f.mutex.Unlock()
+	}
+	ss.Files = files
+	s.mutex.Unlock()
+
+	return encode(ss)
+}
diff --git a/src/go/token/serialize_test.go b/src/go/token/serialize_test.go
new file mode 100644
index 0000000..4e925ad
--- /dev/null
+++ b/src/go/token/serialize_test.go
@@ -0,0 +1,111 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token
+
+import (
+	"bytes"
+	"encoding/gob"
+	"fmt"
+	"testing"
+)
+
+// equal returns nil if p and q describe the same file set;
+// otherwise it returns an error describing the discrepancy.
+func equal(p, q *FileSet) error {
+	if p == q {
+		// avoid deadlock if p == q
+		return nil
+	}
+
+	// not strictly needed for the test
+	p.mutex.Lock()
+	q.mutex.Lock()
+	defer q.mutex.Unlock()
+	defer p.mutex.Unlock()
+
+	if p.base != q.base {
+		return fmt.Errorf("different bases: %d != %d", p.base, q.base)
+	}
+
+	if len(p.files) != len(q.files) {
+		return fmt.Errorf("different number of files: %d != %d", len(p.files), len(q.files))
+	}
+
+	for i, f := range p.files {
+		g := q.files[i]
+		if f.set != p {
+			return fmt.Errorf("wrong fileset for %q", f.name)
+		}
+		if g.set != q {
+			return fmt.Errorf("wrong fileset for %q", g.name)
+		}
+		if f.name != g.name {
+			return fmt.Errorf("different filenames: %q != %q", f.name, g.name)
+		}
+		if f.base != g.base {
+			return fmt.Errorf("different base for %q: %d != %d", f.name, f.base, g.base)
+		}
+		if f.size != g.size {
+			return fmt.Errorf("different size for %q: %d != %d", f.name, f.size, g.size)
+		}
+		for j, l := range f.lines {
+			m := g.lines[j]
+			if l != m {
+				return fmt.Errorf("different offsets for %q", f.name)
+			}
+		}
+		for j, l := range f.infos {
+			m := g.infos[j]
+			if l.Offset != m.Offset || l.Filename != m.Filename || l.Line != m.Line {
+				return fmt.Errorf("different infos for %q", f.name)
+			}
+		}
+	}
+
+	// we don't care about .last - it's just a cache
+	return nil
+}
+
+func checkSerialize(t *testing.T, p *FileSet) {
+	var buf bytes.Buffer
+	encode := func(x interface{}) error {
+		return gob.NewEncoder(&buf).Encode(x)
+	}
+	if err := p.Write(encode); err != nil {
+		t.Errorf("writing fileset failed: %s", err)
+		return
+	}
+	q := NewFileSet()
+	decode := func(x interface{}) error {
+		return gob.NewDecoder(&buf).Decode(x)
+	}
+	if err := q.Read(decode); err != nil {
+		t.Errorf("reading fileset failed: %s", err)
+		return
+	}
+	if err := equal(p, q); err != nil {
+		t.Errorf("filesets not identical: %s", err)
+	}
+}
+
+func TestSerialization(t *testing.T) {
+	p := NewFileSet()
+	checkSerialize(t, p)
+	// add some files
+	for i := 0; i < 10; i++ {
+		f := p.AddFile(fmt.Sprintf("file%d", i), p.Base()+i, i*100)
+		checkSerialize(t, p)
+		// add some lines and alternative file infos
+		line := 1000
+		for offs := 0; offs < f.Size(); offs += 40 + i {
+			f.AddLine(offs)
+			if offs%7 == 0 {
+				f.AddLineInfo(offs, fmt.Sprintf("file%d", offs), line)
+				line += 33
+			}
+		}
+		checkSerialize(t, p)
+	}
+}
diff --git a/src/go/token/token.go b/src/go/token/token.go
new file mode 100644
index 0000000..96a1079
--- /dev/null
+++ b/src/go/token/token.go
@@ -0,0 +1,340 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package token defines constants representing the lexical tokens of the Go
+// programming language and basic operations on tokens (printing, predicates).
+//
+package token
+
+import (
+	"strconv"
+	"unicode"
+	"unicode/utf8"
+)
+
+// Token is the set of lexical tokens of the Go programming language.
+type Token int
+
+// The list of tokens.
+const (
+	// Special tokens
+	ILLEGAL Token = iota
+	EOF
+	COMMENT
+
+	literal_beg
+	// Identifiers and basic type literals
+	// (these tokens stand for classes of literals)
+	IDENT  // main
+	INT    // 12345
+	FLOAT  // 123.45
+	IMAG   // 123.45i
+	CHAR   // 'a'
+	STRING // "abc"
+	literal_end
+
+	operator_beg
+	// Operators and delimiters
+	ADD // +
+	SUB // -
+	MUL // *
+	QUO // /
+	REM // %
+
+	AND     // &
+	OR      // |
+	XOR     // ^
+	SHL     // <<
+	SHR     // >>
+	AND_NOT // &^
+
+	ADD_ASSIGN // +=
+	SUB_ASSIGN // -=
+	MUL_ASSIGN // *=
+	QUO_ASSIGN // /=
+	REM_ASSIGN // %=
+
+	AND_ASSIGN     // &=
+	OR_ASSIGN      // |=
+	XOR_ASSIGN     // ^=
+	SHL_ASSIGN     // <<=
+	SHR_ASSIGN     // >>=
+	AND_NOT_ASSIGN // &^=
+
+	LAND  // &&
+	LOR   // ||
+	ARROW // <-
+	INC   // ++
+	DEC   // --
+
+	EQL    // ==
+	LSS    // <
+	GTR    // >
+	ASSIGN // =
+	NOT    // !
+
+	NEQ      // !=
+	LEQ      // <=
+	GEQ      // >=
+	DEFINE   // :=
+	ELLIPSIS // ...
+
+	LPAREN // (
+	LBRACK // [
+	LBRACE // {
+	COMMA  // ,
+	PERIOD // .
+
+	RPAREN    // )
+	RBRACK    // ]
+	RBRACE    // }
+	SEMICOLON // ;
+	COLON     // :
+	operator_end
+
+	keyword_beg
+	// Keywords
+	BREAK
+	CASE
+	CHAN
+	CONST
+	CONTINUE
+
+	DEFAULT
+	DEFER
+	ELSE
+	FALLTHROUGH
+	FOR
+
+	FUNC
+	GO
+	GOTO
+	IF
+	IMPORT
+
+	INTERFACE
+	MAP
+	PACKAGE
+	RANGE
+	RETURN
+
+	SELECT
+	STRUCT
+	SWITCH
+	TYPE
+	VAR
+	keyword_end
+)
+
+var tokens = [...]string{
+	ILLEGAL: "ILLEGAL",
+
+	EOF:     "EOF",
+	COMMENT: "COMMENT",
+
+	IDENT:  "IDENT",
+	INT:    "INT",
+	FLOAT:  "FLOAT",
+	IMAG:   "IMAG",
+	CHAR:   "CHAR",
+	STRING: "STRING",
+
+	ADD: "+",
+	SUB: "-",
+	MUL: "*",
+	QUO: "/",
+	REM: "%",
+
+	AND:     "&",
+	OR:      "|",
+	XOR:     "^",
+	SHL:     "<<",
+	SHR:     ">>",
+	AND_NOT: "&^",
+
+	ADD_ASSIGN: "+=",
+	SUB_ASSIGN: "-=",
+	MUL_ASSIGN: "*=",
+	QUO_ASSIGN: "/=",
+	REM_ASSIGN: "%=",
+
+	AND_ASSIGN:     "&=",
+	OR_ASSIGN:      "|=",
+	XOR_ASSIGN:     "^=",
+	SHL_ASSIGN:     "<<=",
+	SHR_ASSIGN:     ">>=",
+	AND_NOT_ASSIGN: "&^=",
+
+	LAND:  "&&",
+	LOR:   "||",
+	ARROW: "<-",
+	INC:   "++",
+	DEC:   "--",
+
+	EQL:    "==",
+	LSS:    "<",
+	GTR:    ">",
+	ASSIGN: "=",
+	NOT:    "!",
+
+	NEQ:      "!=",
+	LEQ:      "<=",
+	GEQ:      ">=",
+	DEFINE:   ":=",
+	ELLIPSIS: "...",
+
+	LPAREN: "(",
+	LBRACK: "[",
+	LBRACE: "{",
+	COMMA:  ",",
+	PERIOD: ".",
+
+	RPAREN:    ")",
+	RBRACK:    "]",
+	RBRACE:    "}",
+	SEMICOLON: ";",
+	COLON:     ":",
+
+	BREAK:    "break",
+	CASE:     "case",
+	CHAN:     "chan",
+	CONST:    "const",
+	CONTINUE: "continue",
+
+	DEFAULT:     "default",
+	DEFER:       "defer",
+	ELSE:        "else",
+	FALLTHROUGH: "fallthrough",
+	FOR:         "for",
+
+	FUNC:   "func",
+	GO:     "go",
+	GOTO:   "goto",
+	IF:     "if",
+	IMPORT: "import",
+
+	INTERFACE: "interface",
+	MAP:       "map",
+	PACKAGE:   "package",
+	RANGE:     "range",
+	RETURN:    "return",
+
+	SELECT: "select",
+	STRUCT: "struct",
+	SWITCH: "switch",
+	TYPE:   "type",
+	VAR:    "var",
+}
+
+// String returns the string corresponding to the token tok.
+// For operators, delimiters, and keywords the string is the actual
+// token character sequence (e.g., for the token ADD, the string is
+// "+"). For all other tokens the string corresponds to the token
+// constant name (e.g. for the token IDENT, the string is "IDENT").
+//
+func (tok Token) String() string {
+	s := ""
+	if 0 <= tok && tok < Token(len(tokens)) {
+		s = tokens[tok]
+	}
+	if s == "" {
+		s = "token(" + strconv.Itoa(int(tok)) + ")"
+	}
+	return s
+}
+
+// A set of constants for precedence-based expression parsing.
+// Non-operators have lowest precedence, followed by operators
+// starting with precedence 1 up to unary operators. The highest
+// precedence serves as "catch-all" precedence for selector,
+// indexing, and other operator and delimiter tokens.
+//
+const (
+	LowestPrec  = 0 // non-operators
+	UnaryPrec   = 6
+	HighestPrec = 7
+)
+
+// Precedence returns the operator precedence of the binary
+// operator op. If op is not a binary operator, the result
+// is LowestPrecedence.
+//
+func (op Token) Precedence() int {
+	switch op {
+	case LOR:
+		return 1
+	case LAND:
+		return 2
+	case EQL, NEQ, LSS, LEQ, GTR, GEQ:
+		return 3
+	case ADD, SUB, OR, XOR:
+		return 4
+	case MUL, QUO, REM, SHL, SHR, AND, AND_NOT:
+		return 5
+	}
+	return LowestPrec
+}
+
+var keywords map[string]Token
+
+func init() {
+	keywords = make(map[string]Token)
+	for i := keyword_beg + 1; i < keyword_end; i++ {
+		keywords[tokens[i]] = i
+	}
+}
+
+// Lookup maps an identifier to its keyword token or IDENT (if not a keyword).
+//
+func Lookup(ident string) Token {
+	if tok, is_keyword := keywords[ident]; is_keyword {
+		return tok
+	}
+	return IDENT
+}
+
+// Predicates
+
+// IsLiteral returns true for tokens corresponding to identifiers
+// and basic type literals; it returns false otherwise.
+//
+func (tok Token) IsLiteral() bool { return literal_beg < tok && tok < literal_end }
+
+// IsOperator returns true for tokens corresponding to operators and
+// delimiters; it returns false otherwise.
+//
+func (tok Token) IsOperator() bool { return operator_beg < tok && tok < operator_end }
+
+// IsKeyword returns true for tokens corresponding to keywords;
+// it returns false otherwise.
+//
+func (tok Token) IsKeyword() bool { return keyword_beg < tok && tok < keyword_end }
+
+// IsExported reports whether name starts with an upper-case letter.
+//
+func IsExported(name string) bool {
+	ch, _ := utf8.DecodeRuneInString(name)
+	return unicode.IsUpper(ch)
+}
+
+// IsKeyword reports whether name is a Go keyword, such as "func" or "return".
+//
+func IsKeyword(name string) bool {
+	// TODO: opt: use a perfect hash function instead of a global map.
+	_, ok := keywords[name]
+	return ok
+}
+
+// IsIdentifier reports whether name is a Go identifier, that is, a non-empty
+// string made up of letters, digits, and underscores, where the first character
+// is not a digit. Keywords are not identifiers.
+//
+func IsIdentifier(name string) bool {
+	for i, c := range name {
+		if !unicode.IsLetter(c) && c != '_' && (i == 0 || !unicode.IsDigit(c)) {
+			return false
+		}
+	}
+	return name != "" && !IsKeyword(name)
+}
diff --git a/src/go/token/token_test.go b/src/go/token/token_test.go
new file mode 100644
index 0000000..eff38cc
--- /dev/null
+++ b/src/go/token/token_test.go
@@ -0,0 +1,33 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package token
+
+import "testing"
+
+func TestIsIdentifier(t *testing.T) {
+	tests := []struct {
+		name string
+		in   string
+		want bool
+	}{
+		{"Empty", "", false},
+		{"Space", " ", false},
+		{"SpaceSuffix", "foo ", false},
+		{"Number", "123", false},
+		{"Keyword", "func", false},
+
+		{"LettersASCII", "foo", true},
+		{"MixedASCII", "_bar123", true},
+		{"UppercaseKeyword", "Func", true},
+		{"LettersUnicode", "fóö", true},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			if got := IsIdentifier(test.in); got != test.want {
+				t.Fatalf("IsIdentifier(%q) = %t, want %v", test.in, got, test.want)
+			}
+		})
+	}
+}
diff --git a/src/go/types/api.go b/src/go/types/api.go
new file mode 100644
index 0000000..d625959
--- /dev/null
+++ b/src/go/types/api.go
@@ -0,0 +1,408 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package types declares the data types and implements
+// the algorithms for type-checking of Go packages. Use
+// Config.Check to invoke the type checker for a package.
+// Alternatively, create a new type checker with NewChecker
+// and invoke it incrementally by calling Checker.Files.
+//
+// Type-checking consists of several interdependent phases:
+//
+// Name resolution maps each identifier (ast.Ident) in the program to the
+// language object (Object) it denotes.
+// Use Info.{Defs,Uses,Implicits} for the results of name resolution.
+//
+// Constant folding computes the exact constant value (constant.Value)
+// for every expression (ast.Expr) that is a compile-time constant.
+// Use Info.Types[expr].Value for the results of constant folding.
+//
+// Type inference computes the type (Type) of every expression (ast.Expr)
+// and checks for compliance with the language specification.
+// Use Info.Types[expr].Type for the results of type inference.
+//
+// For a tutorial, see https://golang.org/s/types-tutorial.
+//
+package types
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/constant"
+	"go/token"
+)
+
+// An Error describes a type-checking error; it implements the error interface.
+// A "soft" error is an error that still permits a valid interpretation of a
+// package (such as "unused variable"); "hard" errors may lead to unpredictable
+// behavior if ignored.
+type Error struct {
+	Fset *token.FileSet // file set for interpretation of Pos
+	Pos  token.Pos      // error position
+	Msg  string         // error message
+	Soft bool           // if set, error is "soft"
+
+	// go116code is a future API, unexported as the set of error codes is large
+	// and likely to change significantly during experimentation. Tools wishing
+	// to preview this feature may read go116code using reflection (see
+	// errorcodes_test.go), but beware that there is no guarantee of future
+	// compatibility.
+	go116code  errorCode
+	go116start token.Pos
+	go116end   token.Pos
+}
+
+// Error returns an error string formatted as follows:
+// filename:line:column: message
+func (err Error) Error() string {
+	return fmt.Sprintf("%s: %s", err.Fset.Position(err.Pos), err.Msg)
+}
+
+// An Importer resolves import paths to Packages.
+//
+// CAUTION: This interface does not support the import of locally
+// vendored packages. See https://golang.org/s/go15vendor.
+// If possible, external implementations should implement ImporterFrom.
+type Importer interface {
+	// Import returns the imported package for the given import path.
+	// The semantics is like for ImporterFrom.ImportFrom except that
+	// dir and mode are ignored (since they are not present).
+	Import(path string) (*Package, error)
+}
+
+// ImportMode is reserved for future use.
+type ImportMode int
+
+// An ImporterFrom resolves import paths to packages; it
+// supports vendoring per https://golang.org/s/go15vendor.
+// Use go/importer to obtain an ImporterFrom implementation.
+type ImporterFrom interface {
+	// Importer is present for backward-compatibility. Calling
+	// Import(path) is the same as calling ImportFrom(path, "", 0);
+	// i.e., locally vendored packages may not be found.
+	// The types package does not call Import if an ImporterFrom
+	// is present.
+	Importer
+
+	// ImportFrom returns the imported package for the given import
+	// path when imported by a package file located in dir.
+	// If the import failed, besides returning an error, ImportFrom
+	// is encouraged to cache and return a package anyway, if one
+	// was created. This will reduce package inconsistencies and
+	// follow-on type checker errors due to the missing package.
+	// The mode value must be 0; it is reserved for future use.
+	// Two calls to ImportFrom with the same path and dir must
+	// return the same package.
+	ImportFrom(path, dir string, mode ImportMode) (*Package, error)
+}
+
+// A Config specifies the configuration for type checking.
+// The zero value for Config is a ready-to-use default configuration.
+type Config struct {
+	// If IgnoreFuncBodies is set, function bodies are not
+	// type-checked.
+	IgnoreFuncBodies bool
+
+	// If FakeImportC is set, `import "C"` (for packages requiring Cgo)
+	// declares an empty "C" package and errors are omitted for qualified
+	// identifiers referring to package C (which won't find an object).
+	// This feature is intended for the standard library cmd/api tool.
+	//
+	// Caution: Effects may be unpredictable due to follow-on errors.
+	//          Do not use casually!
+	FakeImportC bool
+
+	// If go115UsesCgo is set, the type checker expects the
+	// _cgo_gotypes.go file generated by running cmd/cgo to be
+	// provided as a package source file. Qualified identifiers
+	// referring to package C will be resolved to cgo-provided
+	// declarations within _cgo_gotypes.go.
+	//
+	// It is an error to set both FakeImportC and go115UsesCgo.
+	go115UsesCgo bool
+
+	// If Error != nil, it is called with each error found
+	// during type checking; err has dynamic type Error.
+	// Secondary errors (for instance, to enumerate all types
+	// involved in an invalid recursive type declaration) have
+	// error strings that start with a '\t' character.
+	// If Error == nil, type-checking stops with the first
+	// error found.
+	Error func(err error)
+
+	// An importer is used to import packages referred to from
+	// import declarations.
+	// If the installed importer implements ImporterFrom, the type
+	// checker calls ImportFrom instead of Import.
+	// The type checker reports an error if an importer is needed
+	// but none was installed.
+	Importer Importer
+
+	// If Sizes != nil, it provides the sizing functions for package unsafe.
+	// Otherwise SizesFor("gc", "amd64") is used instead.
+	Sizes Sizes
+
+	// If DisableUnusedImportCheck is set, packages are not checked
+	// for unused imports.
+	DisableUnusedImportCheck bool
+}
+
+func srcimporter_setUsesCgo(conf *Config) {
+	conf.go115UsesCgo = true
+}
+
+// Info holds result type information for a type-checked package.
+// Only the information for which a map is provided is collected.
+// If the package has type errors, the collected information may
+// be incomplete.
+type Info struct {
+	// Types maps expressions to their types, and for constant
+	// expressions, also their values. Invalid expressions are
+	// omitted.
+	//
+	// For (possibly parenthesized) identifiers denoting built-in
+	// functions, the recorded signatures are call-site specific:
+	// if the call result is not a constant, the recorded type is
+	// an argument-specific signature. Otherwise, the recorded type
+	// is invalid.
+	//
+	// The Types map does not record the type of every identifier,
+	// only those that appear where an arbitrary expression is
+	// permitted. For instance, the identifier f in a selector
+	// expression x.f is found only in the Selections map, the
+	// identifier z in a variable declaration 'var z int' is found
+	// only in the Defs map, and identifiers denoting packages in
+	// qualified identifiers are collected in the Uses map.
+	Types map[ast.Expr]TypeAndValue
+
+	// Defs maps identifiers to the objects they define (including
+	// package names, dots "." of dot-imports, and blank "_" identifiers).
+	// For identifiers that do not denote objects (e.g., the package name
+	// in package clauses, or symbolic variables t in t := x.(type) of
+	// type switch headers), the corresponding objects are nil.
+	//
+	// For an embedded field, Defs returns the field *Var it defines.
+	//
+	// Invariant: Defs[id] == nil || Defs[id].Pos() == id.Pos()
+	Defs map[*ast.Ident]Object
+
+	// Uses maps identifiers to the objects they denote.
+	//
+	// For an embedded field, Uses returns the *TypeName it denotes.
+	//
+	// Invariant: Uses[id].Pos() != id.Pos()
+	Uses map[*ast.Ident]Object
+
+	// Implicits maps nodes to their implicitly declared objects, if any.
+	// The following node and object types may appear:
+	//
+	//     node               declared object
+	//
+	//     *ast.ImportSpec    *PkgName for imports without renames
+	//     *ast.CaseClause    type-specific *Var for each type switch case clause (incl. default)
+	//     *ast.Field         anonymous parameter *Var (incl. unnamed results)
+	//
+	Implicits map[ast.Node]Object
+
+	// Selections maps selector expressions (excluding qualified identifiers)
+	// to their corresponding selections.
+	Selections map[*ast.SelectorExpr]*Selection
+
+	// Scopes maps ast.Nodes to the scopes they define. Package scopes are not
+	// associated with a specific node but with all files belonging to a package.
+	// Thus, the package scope can be found in the type-checked Package object.
+	// Scopes nest, with the Universe scope being the outermost scope, enclosing
+	// the package scope, which contains (one or more) files scopes, which enclose
+	// function scopes which in turn enclose statement and function literal scopes.
+	// Note that even though package-level functions are declared in the package
+	// scope, the function scopes are embedded in the file scope of the file
+	// containing the function declaration.
+	//
+	// The following node types may appear in Scopes:
+	//
+	//     *ast.File
+	//     *ast.FuncType
+	//     *ast.BlockStmt
+	//     *ast.IfStmt
+	//     *ast.SwitchStmt
+	//     *ast.TypeSwitchStmt
+	//     *ast.CaseClause
+	//     *ast.CommClause
+	//     *ast.ForStmt
+	//     *ast.RangeStmt
+	//
+	Scopes map[ast.Node]*Scope
+
+	// InitOrder is the list of package-level initializers in the order in which
+	// they must be executed. Initializers referring to variables related by an
+	// initialization dependency appear in topological order, the others appear
+	// in source order. Variables without an initialization expression do not
+	// appear in this list.
+	InitOrder []*Initializer
+}
+
+// TypeOf returns the type of expression e, or nil if not found.
+// Precondition: the Types, Uses and Defs maps are populated.
+//
+func (info *Info) TypeOf(e ast.Expr) Type {
+	if t, ok := info.Types[e]; ok {
+		return t.Type
+	}
+	if id, _ := e.(*ast.Ident); id != nil {
+		if obj := info.ObjectOf(id); obj != nil {
+			return obj.Type()
+		}
+	}
+	return nil
+}
+
+// ObjectOf returns the object denoted by the specified id,
+// or nil if not found.
+//
+// If id is an embedded struct field, ObjectOf returns the field (*Var)
+// it defines, not the type (*TypeName) it uses.
+//
+// Precondition: the Uses and Defs maps are populated.
+//
+func (info *Info) ObjectOf(id *ast.Ident) Object {
+	if obj := info.Defs[id]; obj != nil {
+		return obj
+	}
+	return info.Uses[id]
+}
+
+// TypeAndValue reports the type and value (for constants)
+// of the corresponding expression.
+type TypeAndValue struct {
+	mode  operandMode
+	Type  Type
+	Value constant.Value
+}
+
+// IsVoid reports whether the corresponding expression
+// is a function call without results.
+func (tv TypeAndValue) IsVoid() bool {
+	return tv.mode == novalue
+}
+
+// IsType reports whether the corresponding expression specifies a type.
+func (tv TypeAndValue) IsType() bool {
+	return tv.mode == typexpr
+}
+
+// IsBuiltin reports whether the corresponding expression denotes
+// a (possibly parenthesized) built-in function.
+func (tv TypeAndValue) IsBuiltin() bool {
+	return tv.mode == builtin
+}
+
+// IsValue reports whether the corresponding expression is a value.
+// Builtins are not considered values. Constant values have a non-
+// nil Value.
+func (tv TypeAndValue) IsValue() bool {
+	switch tv.mode {
+	case constant_, variable, mapindex, value, commaok, commaerr:
+		return true
+	}
+	return false
+}
+
+// IsNil reports whether the corresponding expression denotes the
+// predeclared value nil.
+func (tv TypeAndValue) IsNil() bool {
+	return tv.mode == value && tv.Type == Typ[UntypedNil]
+}
+
+// Addressable reports whether the corresponding expression
+// is addressable (https://golang.org/ref/spec#Address_operators).
+func (tv TypeAndValue) Addressable() bool {
+	return tv.mode == variable
+}
+
+// Assignable reports whether the corresponding expression
+// is assignable to (provided a value of the right type).
+func (tv TypeAndValue) Assignable() bool {
+	return tv.mode == variable || tv.mode == mapindex
+}
+
+// HasOk reports whether the corresponding expression may be
+// used on the rhs of a comma-ok assignment.
+func (tv TypeAndValue) HasOk() bool {
+	return tv.mode == commaok || tv.mode == mapindex
+}
+
+// An Initializer describes a package-level variable, or a list of variables in case
+// of a multi-valued initialization expression, and the corresponding initialization
+// expression.
+type Initializer struct {
+	Lhs []*Var // var Lhs = Rhs
+	Rhs ast.Expr
+}
+
+func (init *Initializer) String() string {
+	var buf bytes.Buffer
+	for i, lhs := range init.Lhs {
+		if i > 0 {
+			buf.WriteString(", ")
+		}
+		buf.WriteString(lhs.Name())
+	}
+	buf.WriteString(" = ")
+	WriteExpr(&buf, init.Rhs)
+	return buf.String()
+}
+
+// Check type-checks a package and returns the resulting package object and
+// the first error if any. Additionally, if info != nil, Check populates each
+// of the non-nil maps in the Info struct.
+//
+// The package is marked as complete if no errors occurred, otherwise it is
+// incomplete. See Config.Error for controlling behavior in the presence of
+// errors.
+//
+// The package is specified by a list of *ast.Files and corresponding
+// file set, and the package path the package is identified with.
+// The clean path must not be empty or dot (".").
+func (conf *Config) Check(path string, fset *token.FileSet, files []*ast.File, info *Info) (*Package, error) {
+	pkg := NewPackage(path, "")
+	return pkg, NewChecker(conf, fset, pkg, info).Files(files)
+}
+
+// AssertableTo reports whether a value of type V can be asserted to have type T.
+func AssertableTo(V *Interface, T Type) bool {
+	m, _ := (*Checker)(nil).assertableTo(V, T)
+	return m == nil
+}
+
+// AssignableTo reports whether a value of type V is assignable to a variable of type T.
+func AssignableTo(V, T Type) bool {
+	x := operand{mode: value, typ: V}
+	ok, _ := x.assignableTo(nil, T, nil) // check not needed for non-constant x
+	return ok
+}
+
+// ConvertibleTo reports whether a value of type V is convertible to a value of type T.
+func ConvertibleTo(V, T Type) bool {
+	x := operand{mode: value, typ: V}
+	return x.convertibleTo(nil, T) // check not needed for non-constant x
+}
+
+// Implements reports whether type V implements interface T.
+func Implements(V Type, T *Interface) bool {
+	f, _ := MissingMethod(V, T, true)
+	return f == nil
+}
+
+// Identical reports whether x and y are identical types.
+// Receivers of Signature types are ignored.
+func Identical(x, y Type) bool {
+	return (*Checker)(nil).identical(x, y)
+}
+
+// IdenticalIgnoreTags reports whether x and y are identical types if tags are ignored.
+// Receivers of Signature types are ignored.
+func IdenticalIgnoreTags(x, y Type) bool {
+	return (*Checker)(nil).identicalIgnoreTags(x, y)
+}
diff --git a/src/go/types/api_test.go b/src/go/types/api_test.go
new file mode 100644
index 0000000..75cebc9
--- /dev/null
+++ b/src/go/types/api_test.go
@@ -0,0 +1,1514 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types_test
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/importer"
+	"go/parser"
+	"go/token"
+	"internal/testenv"
+	"reflect"
+	"regexp"
+	"strings"
+	"testing"
+
+	. "go/types"
+)
+
+func pkgFor(path, source string, info *Info) (*Package, error) {
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, path, source, 0)
+	if err != nil {
+		return nil, err
+	}
+	conf := Config{Importer: importer.Default()}
+	return conf.Check(f.Name.Name, fset, []*ast.File{f}, info)
+}
+
+func mustTypecheck(t *testing.T, path, source string, info *Info) string {
+	pkg, err := pkgFor(path, source, info)
+	if err != nil {
+		name := path
+		if pkg != nil {
+			name = "package " + pkg.Name()
+		}
+		t.Fatalf("%s: didn't type-check (%s)", name, err)
+	}
+	return pkg.Name()
+}
+
+func mayTypecheck(t *testing.T, path, source string, info *Info) string {
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, path, source, 0)
+	if f == nil { // ignore errors unless f is nil
+		t.Fatalf("%s: unable to parse: %s", path, err)
+	}
+	conf := Config{
+		Error:    func(err error) {},
+		Importer: importer.Default(),
+	}
+	pkg, _ := conf.Check(f.Name.Name, fset, []*ast.File{f}, info)
+	return pkg.Name()
+}
+
+func TestValuesInfo(t *testing.T) {
+	var tests = []struct {
+		src  string
+		expr string // constant expression
+		typ  string // constant type
+		val  string // constant value
+	}{
+		{`package a0; const _ = false`, `false`, `untyped bool`, `false`},
+		{`package a1; const _ = 0`, `0`, `untyped int`, `0`},
+		{`package a2; const _ = 'A'`, `'A'`, `untyped rune`, `65`},
+		{`package a3; const _ = 0.`, `0.`, `untyped float`, `0`},
+		{`package a4; const _ = 0i`, `0i`, `untyped complex`, `(0 + 0i)`},
+		{`package a5; const _ = "foo"`, `"foo"`, `untyped string`, `"foo"`},
+
+		{`package b0; var _ = false`, `false`, `bool`, `false`},
+		{`package b1; var _ = 0`, `0`, `int`, `0`},
+		{`package b2; var _ = 'A'`, `'A'`, `rune`, `65`},
+		{`package b3; var _ = 0.`, `0.`, `float64`, `0`},
+		{`package b4; var _ = 0i`, `0i`, `complex128`, `(0 + 0i)`},
+		{`package b5; var _ = "foo"`, `"foo"`, `string`, `"foo"`},
+
+		{`package c0a; var _ = bool(false)`, `false`, `bool`, `false`},
+		{`package c0b; var _ = bool(false)`, `bool(false)`, `bool`, `false`},
+		{`package c0c; type T bool; var _ = T(false)`, `T(false)`, `c0c.T`, `false`},
+
+		{`package c1a; var _ = int(0)`, `0`, `int`, `0`},
+		{`package c1b; var _ = int(0)`, `int(0)`, `int`, `0`},
+		{`package c1c; type T int; var _ = T(0)`, `T(0)`, `c1c.T`, `0`},
+
+		{`package c2a; var _ = rune('A')`, `'A'`, `rune`, `65`},
+		{`package c2b; var _ = rune('A')`, `rune('A')`, `rune`, `65`},
+		{`package c2c; type T rune; var _ = T('A')`, `T('A')`, `c2c.T`, `65`},
+
+		{`package c3a; var _ = float32(0.)`, `0.`, `float32`, `0`},
+		{`package c3b; var _ = float32(0.)`, `float32(0.)`, `float32`, `0`},
+		{`package c3c; type T float32; var _ = T(0.)`, `T(0.)`, `c3c.T`, `0`},
+
+		{`package c4a; var _ = complex64(0i)`, `0i`, `complex64`, `(0 + 0i)`},
+		{`package c4b; var _ = complex64(0i)`, `complex64(0i)`, `complex64`, `(0 + 0i)`},
+		{`package c4c; type T complex64; var _ = T(0i)`, `T(0i)`, `c4c.T`, `(0 + 0i)`},
+
+		{`package c5a; var _ = string("foo")`, `"foo"`, `string`, `"foo"`},
+		{`package c5b; var _ = string("foo")`, `string("foo")`, `string`, `"foo"`},
+		{`package c5c; type T string; var _ = T("foo")`, `T("foo")`, `c5c.T`, `"foo"`},
+		{`package c5d; var _ = string(65)`, `65`, `untyped int`, `65`},
+		{`package c5e; var _ = string('A')`, `'A'`, `untyped rune`, `65`},
+		{`package c5f; type T string; var _ = T('A')`, `'A'`, `untyped rune`, `65`},
+		{`package c5g; var s uint; var _ = string(1 << s)`, `1 << s`, `untyped int`, ``},
+
+		{`package d0; var _ = []byte("foo")`, `"foo"`, `string`, `"foo"`},
+		{`package d1; var _ = []byte(string("foo"))`, `"foo"`, `string`, `"foo"`},
+		{`package d2; var _ = []byte(string("foo"))`, `string("foo")`, `string`, `"foo"`},
+		{`package d3; type T []byte; var _ = T("foo")`, `"foo"`, `string`, `"foo"`},
+
+		{`package e0; const _ = float32( 1e-200)`, `float32(1e-200)`, `float32`, `0`},
+		{`package e1; const _ = float32(-1e-200)`, `float32(-1e-200)`, `float32`, `0`},
+		{`package e2; const _ = float64( 1e-2000)`, `float64(1e-2000)`, `float64`, `0`},
+		{`package e3; const _ = float64(-1e-2000)`, `float64(-1e-2000)`, `float64`, `0`},
+		{`package e4; const _ = complex64( 1e-200)`, `complex64(1e-200)`, `complex64`, `(0 + 0i)`},
+		{`package e5; const _ = complex64(-1e-200)`, `complex64(-1e-200)`, `complex64`, `(0 + 0i)`},
+		{`package e6; const _ = complex128( 1e-2000)`, `complex128(1e-2000)`, `complex128`, `(0 + 0i)`},
+		{`package e7; const _ = complex128(-1e-2000)`, `complex128(-1e-2000)`, `complex128`, `(0 + 0i)`},
+
+		{`package f0 ; var _ float32 =  1e-200`, `1e-200`, `float32`, `0`},
+		{`package f1 ; var _ float32 = -1e-200`, `-1e-200`, `float32`, `0`},
+		{`package f2a; var _ float64 =  1e-2000`, `1e-2000`, `float64`, `0`},
+		{`package f3a; var _ float64 = -1e-2000`, `-1e-2000`, `float64`, `0`},
+		{`package f2b; var _         =  1e-2000`, `1e-2000`, `float64`, `0`},
+		{`package f3b; var _         = -1e-2000`, `-1e-2000`, `float64`, `0`},
+		{`package f4 ; var _ complex64  =  1e-200 `, `1e-200`, `complex64`, `(0 + 0i)`},
+		{`package f5 ; var _ complex64  = -1e-200 `, `-1e-200`, `complex64`, `(0 + 0i)`},
+		{`package f6a; var _ complex128 =  1e-2000i`, `1e-2000i`, `complex128`, `(0 + 0i)`},
+		{`package f7a; var _ complex128 = -1e-2000i`, `-1e-2000i`, `complex128`, `(0 + 0i)`},
+		{`package f6b; var _            =  1e-2000i`, `1e-2000i`, `complex128`, `(0 + 0i)`},
+		{`package f7b; var _            = -1e-2000i`, `-1e-2000i`, `complex128`, `(0 + 0i)`},
+
+		{`package g0; const (a = len([iota]int{}); b; c); const _ = c`, `c`, `int`, `2`}, // issue #22341
+	}
+
+	for _, test := range tests {
+		info := Info{
+			Types: make(map[ast.Expr]TypeAndValue),
+		}
+		name := mustTypecheck(t, "ValuesInfo", test.src, &info)
+
+		// look for expression
+		var expr ast.Expr
+		for e := range info.Types {
+			if ExprString(e) == test.expr {
+				expr = e
+				break
+			}
+		}
+		if expr == nil {
+			t.Errorf("package %s: no expression found for %s", name, test.expr)
+			continue
+		}
+		tv := info.Types[expr]
+
+		// check that type is correct
+		if got := tv.Type.String(); got != test.typ {
+			t.Errorf("package %s: got type %s; want %s", name, got, test.typ)
+			continue
+		}
+
+		// if we have a constant, check that value is correct
+		if tv.Value != nil {
+			if got := tv.Value.ExactString(); got != test.val {
+				t.Errorf("package %s: got value %s; want %s", name, got, test.val)
+			}
+		} else {
+			if test.val != "" {
+				t.Errorf("package %s: no constant found; want %s", name, test.val)
+			}
+		}
+	}
+}
+
+func TestTypesInfo(t *testing.T) {
+	var tests = []struct {
+		src  string
+		expr string // expression
+		typ  string // value type
+	}{
+		// single-valued expressions of untyped constants
+		{`package b0; var x interface{} = false`, `false`, `bool`},
+		{`package b1; var x interface{} = 0`, `0`, `int`},
+		{`package b2; var x interface{} = 0.`, `0.`, `float64`},
+		{`package b3; var x interface{} = 0i`, `0i`, `complex128`},
+		{`package b4; var x interface{} = "foo"`, `"foo"`, `string`},
+
+		// comma-ok expressions
+		{`package p0; var x interface{}; var _, _ = x.(int)`,
+			`x.(int)`,
+			`(int, bool)`,
+		},
+		{`package p1; var x interface{}; func _() { _, _ = x.(int) }`,
+			`x.(int)`,
+			`(int, bool)`,
+		},
+		{`package p2a; type mybool bool; var m map[string]complex128; var b mybool; func _() { _, b = m["foo"] }`,
+			`m["foo"]`,
+			`(complex128, p2a.mybool)`,
+		},
+		{`package p2b; var m map[string]complex128; var b bool; func _() { _, b = m["foo"] }`,
+			`m["foo"]`,
+			`(complex128, bool)`,
+		},
+		{`package p3; var c chan string; var _, _ = <-c`,
+			`<-c`,
+			`(string, bool)`,
+		},
+
+		// issue 6796
+		{`package issue6796_a; var x interface{}; var _, _ = (x.(int))`,
+			`x.(int)`,
+			`(int, bool)`,
+		},
+		{`package issue6796_b; var c chan string; var _, _ = (<-c)`,
+			`(<-c)`,
+			`(string, bool)`,
+		},
+		{`package issue6796_c; var c chan string; var _, _ = (<-c)`,
+			`<-c`,
+			`(string, bool)`,
+		},
+		{`package issue6796_d; var c chan string; var _, _ = ((<-c))`,
+			`(<-c)`,
+			`(string, bool)`,
+		},
+		{`package issue6796_e; func f(c chan string) { _, _ = ((<-c)) }`,
+			`(<-c)`,
+			`(string, bool)`,
+		},
+
+		// issue 7060
+		{`package issue7060_a; var ( m map[int]string; x, ok = m[0] )`,
+			`m[0]`,
+			`(string, bool)`,
+		},
+		{`package issue7060_b; var ( m map[int]string; x, ok interface{} = m[0] )`,
+			`m[0]`,
+			`(string, bool)`,
+		},
+		{`package issue7060_c; func f(x interface{}, ok bool, m map[int]string) { x, ok = m[0] }`,
+			`m[0]`,
+			`(string, bool)`,
+		},
+		{`package issue7060_d; var ( ch chan string; x, ok = <-ch )`,
+			`<-ch`,
+			`(string, bool)`,
+		},
+		{`package issue7060_e; var ( ch chan string; x, ok interface{} = <-ch )`,
+			`<-ch`,
+			`(string, bool)`,
+		},
+		{`package issue7060_f; func f(x interface{}, ok bool, ch chan string) { x, ok = <-ch }`,
+			`<-ch`,
+			`(string, bool)`,
+		},
+
+		// issue 28277
+		{`package issue28277_a; func f(...int)`,
+			`...int`,
+			`[]int`,
+		},
+		{`package issue28277_b; func f(a, b int, c ...[]struct{})`,
+			`...[]struct{}`,
+			`[][]struct{}`,
+		},
+
+		// tests for broken code that doesn't parse or type-check
+		{`package x0; func _() { var x struct {f string}; x.f := 0 }`, `x.f`, `string`},
+		{`package x1; func _() { var z string; type x struct {f string}; y := &x{q: z}}`, `z`, `string`},
+		{`package x2; func _() { var a, b string; type x struct {f string}; z := &x{f: a; f: b;}}`, `b`, `string`},
+		{`package x3; var x = panic("");`, `panic`, `func(interface{})`},
+		{`package x4; func _() { panic("") }`, `panic`, `func(interface{})`},
+		{`package x5; func _() { var x map[string][...]int; x = map[string][...]int{"": {1,2,3}} }`, `x`, `map[string][-1]int`},
+	}
+
+	for _, test := range tests {
+		info := Info{Types: make(map[ast.Expr]TypeAndValue)}
+		name := mayTypecheck(t, "TypesInfo", test.src, &info)
+
+		// look for expression type
+		var typ Type
+		for e, tv := range info.Types {
+			if ExprString(e) == test.expr {
+				typ = tv.Type
+				break
+			}
+		}
+		if typ == nil {
+			t.Errorf("package %s: no type found for %s", name, test.expr)
+			continue
+		}
+
+		// check that type is correct
+		if got := typ.String(); got != test.typ {
+			t.Errorf("package %s: got %s; want %s", name, got, test.typ)
+		}
+	}
+}
+
+func TestImplicitsInfo(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	var tests = []struct {
+		src  string
+		want string
+	}{
+		{`package p2; import . "fmt"; var _ = Println`, ""},           // no Implicits entry
+		{`package p0; import local "fmt"; var _ = local.Println`, ""}, // no Implicits entry
+		{`package p1; import "fmt"; var _ = fmt.Println`, "importSpec: package fmt"},
+
+		{`package p3; func f(x interface{}) { switch x.(type) { case int: } }`, ""}, // no Implicits entry
+		{`package p4; func f(x interface{}) { switch t := x.(type) { case int: _ = t } }`, "caseClause: var t int"},
+		{`package p5; func f(x interface{}) { switch t := x.(type) { case int, uint: _ = t } }`, "caseClause: var t interface{}"},
+		{`package p6; func f(x interface{}) { switch t := x.(type) { default: _ = t } }`, "caseClause: var t interface{}"},
+
+		{`package p7; func f(x int) {}`, ""}, // no Implicits entry
+		{`package p8; func f(int) {}`, "field: var  int"},
+		{`package p9; func f() (complex64) { return 0 }`, "field: var  complex64"},
+		{`package p10; type T struct{}; func (*T) f() {}`, "field: var  *p10.T"},
+	}
+
+	for _, test := range tests {
+		info := Info{
+			Implicits: make(map[ast.Node]Object),
+		}
+		name := mustTypecheck(t, "ImplicitsInfo", test.src, &info)
+
+		// the test cases expect at most one Implicits entry
+		if len(info.Implicits) > 1 {
+			t.Errorf("package %s: %d Implicits entries found", name, len(info.Implicits))
+			continue
+		}
+
+		// extract Implicits entry, if any
+		var got string
+		for n, obj := range info.Implicits {
+			switch x := n.(type) {
+			case *ast.ImportSpec:
+				got = "importSpec"
+			case *ast.CaseClause:
+				got = "caseClause"
+			case *ast.Field:
+				got = "field"
+			default:
+				t.Fatalf("package %s: unexpected %T", name, x)
+			}
+			got += ": " + obj.String()
+		}
+
+		// verify entry
+		if got != test.want {
+			t.Errorf("package %s: got %q; want %q", name, got, test.want)
+		}
+	}
+}
+
+func predString(tv TypeAndValue) string {
+	var buf bytes.Buffer
+	pred := func(b bool, s string) {
+		if b {
+			if buf.Len() > 0 {
+				buf.WriteString(", ")
+			}
+			buf.WriteString(s)
+		}
+	}
+
+	pred(tv.IsVoid(), "void")
+	pred(tv.IsType(), "type")
+	pred(tv.IsBuiltin(), "builtin")
+	pred(tv.IsValue() && tv.Value != nil, "const")
+	pred(tv.IsValue() && tv.Value == nil, "value")
+	pred(tv.IsNil(), "nil")
+	pred(tv.Addressable(), "addressable")
+	pred(tv.Assignable(), "assignable")
+	pred(tv.HasOk(), "hasOk")
+
+	if buf.Len() == 0 {
+		return "invalid"
+	}
+	return buf.String()
+}
+
+func TestPredicatesInfo(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	var tests = []struct {
+		src  string
+		expr string
+		pred string
+	}{
+		// void
+		{`package n0; func f() { f() }`, `f()`, `void`},
+
+		// types
+		{`package t0; type _ int`, `int`, `type`},
+		{`package t1; type _ []int`, `[]int`, `type`},
+		{`package t2; type _ func()`, `func()`, `type`},
+		{`package t3; type _ func(int)`, `int`, `type`},
+		{`package t3; type _ func(...int)`, `...int`, `type`},
+
+		// built-ins
+		{`package b0; var _ = len("")`, `len`, `builtin`},
+		{`package b1; var _ = (len)("")`, `(len)`, `builtin`},
+
+		// constants
+		{`package c0; var _ = 42`, `42`, `const`},
+		{`package c1; var _ = "foo" + "bar"`, `"foo" + "bar"`, `const`},
+		{`package c2; const (i = 1i; _ = i)`, `i`, `const`},
+
+		// values
+		{`package v0; var (a, b int; _ = a + b)`, `a + b`, `value`},
+		{`package v1; var _ = &[]int{1}`, `([]int literal)`, `value`},
+		{`package v2; var _ = func(){}`, `(func() literal)`, `value`},
+		{`package v4; func f() { _ = f }`, `f`, `value`},
+		{`package v3; var _ *int = nil`, `nil`, `value, nil`},
+		{`package v3; var _ *int = (nil)`, `(nil)`, `value, nil`},
+
+		// addressable (and thus assignable) operands
+		{`package a0; var (x int; _ = x)`, `x`, `value, addressable, assignable`},
+		{`package a1; var (p *int; _ = *p)`, `*p`, `value, addressable, assignable`},
+		{`package a2; var (s []int; _ = s[0])`, `s[0]`, `value, addressable, assignable`},
+		{`package a3; var (s struct{f int}; _ = s.f)`, `s.f`, `value, addressable, assignable`},
+		{`package a4; var (a [10]int; _ = a[0])`, `a[0]`, `value, addressable, assignable`},
+		{`package a5; func _(x int) { _ = x }`, `x`, `value, addressable, assignable`},
+		{`package a6; func _()(x int) { _ = x; return }`, `x`, `value, addressable, assignable`},
+		{`package a7; type T int; func (x T) _() { _ = x }`, `x`, `value, addressable, assignable`},
+		// composite literals are not addressable
+
+		// assignable but not addressable values
+		{`package s0; var (m map[int]int; _ = m[0])`, `m[0]`, `value, assignable, hasOk`},
+		{`package s1; var (m map[int]int; _, _ = m[0])`, `m[0]`, `value, assignable, hasOk`},
+
+		// hasOk expressions
+		{`package k0; var (ch chan int; _ = <-ch)`, `<-ch`, `value, hasOk`},
+		{`package k1; var (ch chan int; _, _ = <-ch)`, `<-ch`, `value, hasOk`},
+
+		// missing entries
+		// - package names are collected in the Uses map
+		// - identifiers being declared are collected in the Defs map
+		{`package m0; import "os"; func _() { _ = os.Stdout }`, `os`, `<missing>`},
+		{`package m1; import p "os"; func _() { _ = p.Stdout }`, `p`, `<missing>`},
+		{`package m2; const c = 0`, `c`, `<missing>`},
+		{`package m3; type T int`, `T`, `<missing>`},
+		{`package m4; var v int`, `v`, `<missing>`},
+		{`package m5; func f() {}`, `f`, `<missing>`},
+		{`package m6; func _(x int) {}`, `x`, `<missing>`},
+		{`package m6; func _()(x int) { return }`, `x`, `<missing>`},
+		{`package m6; type T int; func (x T) _() {}`, `x`, `<missing>`},
+	}
+
+	for _, test := range tests {
+		info := Info{Types: make(map[ast.Expr]TypeAndValue)}
+		name := mustTypecheck(t, "PredicatesInfo", test.src, &info)
+
+		// look for expression predicates
+		got := "<missing>"
+		for e, tv := range info.Types {
+			//println(name, ExprString(e))
+			if ExprString(e) == test.expr {
+				got = predString(tv)
+				break
+			}
+		}
+
+		if got != test.pred {
+			t.Errorf("package %s: got %s; want %s", name, got, test.pred)
+		}
+	}
+}
+
+func TestScopesInfo(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	var tests = []struct {
+		src    string
+		scopes []string // list of scope descriptors of the form kind:varlist
+	}{
+		{`package p0`, []string{
+			"file:",
+		}},
+		{`package p1; import ( "fmt"; m "math"; _ "os" ); var ( _ = fmt.Println; _ = m.Pi )`, []string{
+			"file:fmt m",
+		}},
+		{`package p2; func _() {}`, []string{
+			"file:", "func:",
+		}},
+		{`package p3; func _(x, y int) {}`, []string{
+			"file:", "func:x y",
+		}},
+		{`package p4; func _(x, y int) { x, z := 1, 2; _ = z }`, []string{
+			"file:", "func:x y z", // redeclaration of x
+		}},
+		{`package p5; func _(x, y int) (u, _ int) { return }`, []string{
+			"file:", "func:u x y",
+		}},
+		{`package p6; func _() { { var x int; _ = x } }`, []string{
+			"file:", "func:", "block:x",
+		}},
+		{`package p7; func _() { if true {} }`, []string{
+			"file:", "func:", "if:", "block:",
+		}},
+		{`package p8; func _() { if x := 0; x < 0 { y := x; _ = y } }`, []string{
+			"file:", "func:", "if:x", "block:y",
+		}},
+		{`package p9; func _() { switch x := 0; x {} }`, []string{
+			"file:", "func:", "switch:x",
+		}},
+		{`package p10; func _() { switch x := 0; x { case 1: y := x; _ = y; default: }}`, []string{
+			"file:", "func:", "switch:x", "case:y", "case:",
+		}},
+		{`package p11; func _(t interface{}) { switch t.(type) {} }`, []string{
+			"file:", "func:t", "type switch:",
+		}},
+		{`package p12; func _(t interface{}) { switch t := t; t.(type) {} }`, []string{
+			"file:", "func:t", "type switch:t",
+		}},
+		{`package p13; func _(t interface{}) { switch x := t.(type) { case int: _ = x } }`, []string{
+			"file:", "func:t", "type switch:", "case:x", // x implicitly declared
+		}},
+		{`package p14; func _() { select{} }`, []string{
+			"file:", "func:",
+		}},
+		{`package p15; func _(c chan int) { select{ case <-c: } }`, []string{
+			"file:", "func:c", "comm:",
+		}},
+		{`package p16; func _(c chan int) { select{ case i := <-c: x := i; _ = x} }`, []string{
+			"file:", "func:c", "comm:i x",
+		}},
+		{`package p17; func _() { for{} }`, []string{
+			"file:", "func:", "for:", "block:",
+		}},
+		{`package p18; func _(n int) { for i := 0; i < n; i++ { _ = i } }`, []string{
+			"file:", "func:n", "for:i", "block:",
+		}},
+		{`package p19; func _(a []int) { for i := range a { _ = i} }`, []string{
+			"file:", "func:a", "range:i", "block:",
+		}},
+		{`package p20; var s int; func _(a []int) { for i, x := range a { s += x; _ = i } }`, []string{
+			"file:", "func:a", "range:i x", "block:",
+		}},
+	}
+
+	for _, test := range tests {
+		info := Info{Scopes: make(map[ast.Node]*Scope)}
+		name := mustTypecheck(t, "ScopesInfo", test.src, &info)
+
+		// number of scopes must match
+		if len(info.Scopes) != len(test.scopes) {
+			t.Errorf("package %s: got %d scopes; want %d", name, len(info.Scopes), len(test.scopes))
+		}
+
+		// scope descriptions must match
+		for node, scope := range info.Scopes {
+			kind := "<unknown node kind>"
+			switch node.(type) {
+			case *ast.File:
+				kind = "file"
+			case *ast.FuncType:
+				kind = "func"
+			case *ast.BlockStmt:
+				kind = "block"
+			case *ast.IfStmt:
+				kind = "if"
+			case *ast.SwitchStmt:
+				kind = "switch"
+			case *ast.TypeSwitchStmt:
+				kind = "type switch"
+			case *ast.CaseClause:
+				kind = "case"
+			case *ast.CommClause:
+				kind = "comm"
+			case *ast.ForStmt:
+				kind = "for"
+			case *ast.RangeStmt:
+				kind = "range"
+			}
+
+			// look for matching scope description
+			desc := kind + ":" + strings.Join(scope.Names(), " ")
+			found := false
+			for _, d := range test.scopes {
+				if desc == d {
+					found = true
+					break
+				}
+			}
+			if !found {
+				t.Errorf("package %s: no matching scope found for %s", name, desc)
+			}
+		}
+	}
+}
+
+func TestInitOrderInfo(t *testing.T) {
+	var tests = []struct {
+		src   string
+		inits []string
+	}{
+		{`package p0; var (x = 1; y = x)`, []string{
+			"x = 1", "y = x",
+		}},
+		{`package p1; var (a = 1; b = 2; c = 3)`, []string{
+			"a = 1", "b = 2", "c = 3",
+		}},
+		{`package p2; var (a, b, c = 1, 2, 3)`, []string{
+			"a = 1", "b = 2", "c = 3",
+		}},
+		{`package p3; var _ = f(); func f() int { return 1 }`, []string{
+			"_ = f()", // blank var
+		}},
+		{`package p4; var (a = 0; x = y; y = z; z = 0)`, []string{
+			"a = 0", "z = 0", "y = z", "x = y",
+		}},
+		{`package p5; var (a, _ = m[0]; m map[int]string)`, []string{
+			"a, _ = m[0]", // blank var
+		}},
+		{`package p6; var a, b = f(); func f() (_, _ int) { return z, z }; var z = 0`, []string{
+			"z = 0", "a, b = f()",
+		}},
+		{`package p7; var (a = func() int { return b }(); b = 1)`, []string{
+			"b = 1", "a = (func() int literal)()",
+		}},
+		{`package p8; var (a, b = func() (_, _ int) { return c, c }(); c = 1)`, []string{
+			"c = 1", "a, b = (func() (_, _ int) literal)()",
+		}},
+		{`package p9; type T struct{}; func (T) m() int { _ = y; return 0 }; var x, y = T.m, 1`, []string{
+			"y = 1", "x = T.m",
+		}},
+		{`package p10; var (d = c + b; a = 0; b = 0; c = 0)`, []string{
+			"a = 0", "b = 0", "c = 0", "d = c + b",
+		}},
+		{`package p11; var (a = e + c; b = d + c; c = 0; d = 0; e = 0)`, []string{
+			"c = 0", "d = 0", "b = d + c", "e = 0", "a = e + c",
+		}},
+		// emit an initializer for n:1 initializations only once (not for each node
+		// on the lhs which may appear in different order in the dependency graph)
+		{`package p12; var (a = x; b = 0; x, y = m[0]; m map[int]int)`, []string{
+			"b = 0", "x, y = m[0]", "a = x",
+		}},
+		// test case from spec section on package initialization
+		{`package p12
+
+		var (
+			a = c + b
+			b = f()
+			c = f()
+			d = 3
+		)
+
+		func f() int {
+			d++
+			return d
+		}`, []string{
+			"d = 3", "b = f()", "c = f()", "a = c + b",
+		}},
+		// test case for issue 7131
+		{`package main
+
+		var counter int
+		func next() int { counter++; return counter }
+
+		var _ = makeOrder()
+		func makeOrder() []int { return []int{f, b, d, e, c, a} }
+
+		var a       = next()
+		var b, c    = next(), next()
+		var d, e, f = next(), next(), next()
+		`, []string{
+			"a = next()", "b = next()", "c = next()", "d = next()", "e = next()", "f = next()", "_ = makeOrder()",
+		}},
+		// test case for issue 10709
+		{`package p13
+
+		var (
+		    v = t.m()
+		    t = makeT(0)
+		)
+
+		type T struct{}
+
+		func (T) m() int { return 0 }
+
+		func makeT(n int) T {
+		    if n > 0 {
+		        return makeT(n-1)
+		    }
+		    return T{}
+		}`, []string{
+			"t = makeT(0)", "v = t.m()",
+		}},
+		// test case for issue 10709: same as test before, but variable decls swapped
+		{`package p14
+
+		var (
+		    t = makeT(0)
+		    v = t.m()
+		)
+
+		type T struct{}
+
+		func (T) m() int { return 0 }
+
+		func makeT(n int) T {
+		    if n > 0 {
+		        return makeT(n-1)
+		    }
+		    return T{}
+		}`, []string{
+			"t = makeT(0)", "v = t.m()",
+		}},
+		// another candidate possibly causing problems with issue 10709
+		{`package p15
+
+		var y1 = f1()
+
+		func f1() int { return g1() }
+		func g1() int { f1(); return x1 }
+
+		var x1 = 0
+
+		var y2 = f2()
+
+		func f2() int { return g2() }
+		func g2() int { return x2 }
+
+		var x2 = 0`, []string{
+			"x1 = 0", "y1 = f1()", "x2 = 0", "y2 = f2()",
+		}},
+	}
+
+	for _, test := range tests {
+		info := Info{}
+		name := mustTypecheck(t, "InitOrderInfo", test.src, &info)
+
+		// number of initializers must match
+		if len(info.InitOrder) != len(test.inits) {
+			t.Errorf("package %s: got %d initializers; want %d", name, len(info.InitOrder), len(test.inits))
+			continue
+		}
+
+		// initializers must match
+		for i, want := range test.inits {
+			got := info.InitOrder[i].String()
+			if got != want {
+				t.Errorf("package %s, init %d: got %s; want %s", name, i, got, want)
+				continue
+			}
+		}
+	}
+}
+
+func TestMultiFileInitOrder(t *testing.T) {
+	fset := token.NewFileSet()
+	mustParse := func(src string) *ast.File {
+		f, err := parser.ParseFile(fset, "main", src, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return f
+	}
+
+	fileA := mustParse(`package main; var a = 1`)
+	fileB := mustParse(`package main; var b = 2`)
+
+	// The initialization order must not depend on the parse
+	// order of the files, only on the presentation order to
+	// the type-checker.
+	for _, test := range []struct {
+		files []*ast.File
+		want  string
+	}{
+		{[]*ast.File{fileA, fileB}, "[a = 1 b = 2]"},
+		{[]*ast.File{fileB, fileA}, "[b = 2 a = 1]"},
+	} {
+		var info Info
+		if _, err := new(Config).Check("main", fset, test.files, &info); err != nil {
+			t.Fatal(err)
+		}
+		if got := fmt.Sprint(info.InitOrder); got != test.want {
+			t.Fatalf("got %s; want %s", got, test.want)
+		}
+	}
+}
+
+func TestFiles(t *testing.T) {
+	var sources = []string{
+		"package p; type T struct{}; func (T) m1() {}",
+		"package p; func (T) m2() {}; var x interface{ m1(); m2() } = T{}",
+		"package p; func (T) m3() {}; var y interface{ m1(); m2(); m3() } = T{}",
+		"package p",
+	}
+
+	var conf Config
+	fset := token.NewFileSet()
+	pkg := NewPackage("p", "p")
+	var info Info
+	check := NewChecker(&conf, fset, pkg, &info)
+
+	for i, src := range sources {
+		filename := fmt.Sprintf("sources%d", i)
+		f, err := parser.ParseFile(fset, filename, src, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err := check.Files([]*ast.File{f}); err != nil {
+			t.Error(err)
+		}
+	}
+
+	// check InitOrder is [x y]
+	var vars []string
+	for _, init := range info.InitOrder {
+		for _, v := range init.Lhs {
+			vars = append(vars, v.Name())
+		}
+	}
+	if got, want := fmt.Sprint(vars), "[x y]"; got != want {
+		t.Errorf("InitOrder == %s, want %s", got, want)
+	}
+}
+
+type testImporter map[string]*Package
+
+func (m testImporter) Import(path string) (*Package, error) {
+	if pkg := m[path]; pkg != nil {
+		return pkg, nil
+	}
+	return nil, fmt.Errorf("package %q not found", path)
+}
+
+func TestSelection(t *testing.T) {
+	selections := make(map[*ast.SelectorExpr]*Selection)
+
+	fset := token.NewFileSet()
+	imports := make(testImporter)
+	conf := Config{Importer: imports}
+	makePkg := func(path, src string) {
+		f, err := parser.ParseFile(fset, path+".go", src, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+		pkg, err := conf.Check(path, fset, []*ast.File{f}, &Info{Selections: selections})
+		if err != nil {
+			t.Fatal(err)
+		}
+		imports[path] = pkg
+	}
+
+	const libSrc = `
+package lib
+type T float64
+const C T = 3
+var V T
+func F() {}
+func (T) M() {}
+`
+	const mainSrc = `
+package main
+import "lib"
+
+type A struct {
+	*B
+	C
+}
+
+type B struct {
+	b int
+}
+
+func (B) f(int)
+
+type C struct {
+	c int
+}
+
+func (C) g()
+func (*C) h()
+
+func main() {
+	// qualified identifiers
+	var _ lib.T
+        _ = lib.C
+        _ = lib.F
+        _ = lib.V
+	_ = lib.T.M
+
+	// fields
+	_ = A{}.B
+	_ = new(A).B
+
+	_ = A{}.C
+	_ = new(A).C
+
+	_ = A{}.b
+	_ = new(A).b
+
+	_ = A{}.c
+	_ = new(A).c
+
+	// methods
+        _ = A{}.f
+        _ = new(A).f
+        _ = A{}.g
+        _ = new(A).g
+        _ = new(A).h
+
+        _ = B{}.f
+        _ = new(B).f
+
+        _ = C{}.g
+        _ = new(C).g
+        _ = new(C).h
+
+	// method expressions
+        _ = A.f
+        _ = (*A).f
+        _ = B.f
+        _ = (*B).f
+}`
+
+	wantOut := map[string][2]string{
+		"lib.T.M": {"method expr (lib.T) M(lib.T)", ".[0]"},
+
+		"A{}.B":    {"field (main.A) B *main.B", ".[0]"},
+		"new(A).B": {"field (*main.A) B *main.B", "->[0]"},
+		"A{}.C":    {"field (main.A) C main.C", ".[1]"},
+		"new(A).C": {"field (*main.A) C main.C", "->[1]"},
+		"A{}.b":    {"field (main.A) b int", "->[0 0]"},
+		"new(A).b": {"field (*main.A) b int", "->[0 0]"},
+		"A{}.c":    {"field (main.A) c int", ".[1 0]"},
+		"new(A).c": {"field (*main.A) c int", "->[1 0]"},
+
+		"A{}.f":    {"method (main.A) f(int)", "->[0 0]"},
+		"new(A).f": {"method (*main.A) f(int)", "->[0 0]"},
+		"A{}.g":    {"method (main.A) g()", ".[1 0]"},
+		"new(A).g": {"method (*main.A) g()", "->[1 0]"},
+		"new(A).h": {"method (*main.A) h()", "->[1 1]"}, // TODO(gri) should this report .[1 1] ?
+		"B{}.f":    {"method (main.B) f(int)", ".[0]"},
+		"new(B).f": {"method (*main.B) f(int)", "->[0]"},
+		"C{}.g":    {"method (main.C) g()", ".[0]"},
+		"new(C).g": {"method (*main.C) g()", "->[0]"},
+		"new(C).h": {"method (*main.C) h()", "->[1]"}, // TODO(gri) should this report .[1] ?
+
+		"A.f":    {"method expr (main.A) f(main.A, int)", "->[0 0]"},
+		"(*A).f": {"method expr (*main.A) f(*main.A, int)", "->[0 0]"},
+		"B.f":    {"method expr (main.B) f(main.B, int)", ".[0]"},
+		"(*B).f": {"method expr (*main.B) f(*main.B, int)", "->[0]"},
+	}
+
+	makePkg("lib", libSrc)
+	makePkg("main", mainSrc)
+
+	for e, sel := range selections {
+		_ = sel.String() // assertion: must not panic
+
+		start := fset.Position(e.Pos()).Offset
+		end := fset.Position(e.End()).Offset
+		syntax := mainSrc[start:end] // (all SelectorExprs are in main, not lib)
+
+		direct := "."
+		if sel.Indirect() {
+			direct = "->"
+		}
+		got := [2]string{
+			sel.String(),
+			fmt.Sprintf("%s%v", direct, sel.Index()),
+		}
+		want := wantOut[syntax]
+		if want != got {
+			t.Errorf("%s: got %q; want %q", syntax, got, want)
+		}
+		delete(wantOut, syntax)
+
+		// We must explicitly assert properties of the
+		// Signature's receiver since it doesn't participate
+		// in Identical() or String().
+		sig, _ := sel.Type().(*Signature)
+		if sel.Kind() == MethodVal {
+			got := sig.Recv().Type()
+			want := sel.Recv()
+			if !Identical(got, want) {
+				t.Errorf("%s: Recv() = %s, want %s", syntax, got, want)
+			}
+		} else if sig != nil && sig.Recv() != nil {
+			t.Errorf("%s: signature has receiver %s", sig, sig.Recv().Type())
+		}
+	}
+	// Assert that all wantOut entries were used exactly once.
+	for syntax := range wantOut {
+		t.Errorf("no ast.Selection found with syntax %q", syntax)
+	}
+}
+
+func TestIssue8518(t *testing.T) {
+	fset := token.NewFileSet()
+	imports := make(testImporter)
+	conf := Config{
+		Error:    func(err error) { t.Log(err) }, // don't exit after first error
+		Importer: imports,
+	}
+	makePkg := func(path, src string) {
+		f, err := parser.ParseFile(fset, path, src, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+		pkg, _ := conf.Check(path, fset, []*ast.File{f}, nil) // errors logged via conf.Error
+		imports[path] = pkg
+	}
+
+	const libSrc = `
+package a
+import "missing"
+const C1 = foo
+const C2 = missing.C
+`
+
+	const mainSrc = `
+package main
+import "a"
+var _ = a.C1
+var _ = a.C2
+`
+
+	makePkg("a", libSrc)
+	makePkg("main", mainSrc) // don't crash when type-checking this package
+}
+
+func TestLookupFieldOrMethod(t *testing.T) {
+	// Test cases assume a lookup of the form a.f or x.f, where a stands for an
+	// addressable value, and x for a non-addressable value (even though a variable
+	// for ease of test case writing).
+	var tests = []struct {
+		src      string
+		found    bool
+		index    []int
+		indirect bool
+	}{
+		// field lookups
+		{"var x T; type T struct{}", false, nil, false},
+		{"var x T; type T struct{ f int }", true, []int{0}, false},
+		{"var x T; type T struct{ a, b, f, c int }", true, []int{2}, false},
+
+		// method lookups
+		{"var a T; type T struct{}; func (T) f() {}", true, []int{0}, false},
+		{"var a *T; type T struct{}; func (T) f() {}", true, []int{0}, true},
+		{"var a T; type T struct{}; func (*T) f() {}", true, []int{0}, false},
+		{"var a *T; type T struct{}; func (*T) f() {}", true, []int{0}, true}, // TODO(gri) should this report indirect = false?
+
+		// collisions
+		{"type ( E1 struct{ f int }; E2 struct{ f int }; x struct{ E1; *E2 })", false, []int{1, 0}, false},
+		{"type ( E1 struct{ f int }; E2 struct{}; x struct{ E1; *E2 }); func (E2) f() {}", false, []int{1, 0}, false},
+
+		// outside methodset
+		// (*T).f method exists, but value of type T is not addressable
+		{"var x T; type T struct{}; func (*T) f() {}", false, nil, true},
+	}
+
+	for _, test := range tests {
+		pkg, err := pkgFor("test", "package p;"+test.src, nil)
+		if err != nil {
+			t.Errorf("%s: incorrect test case: %s", test.src, err)
+			continue
+		}
+
+		obj := pkg.Scope().Lookup("a")
+		if obj == nil {
+			if obj = pkg.Scope().Lookup("x"); obj == nil {
+				t.Errorf("%s: incorrect test case - no object a or x", test.src)
+				continue
+			}
+		}
+
+		f, index, indirect := LookupFieldOrMethod(obj.Type(), obj.Name() == "a", pkg, "f")
+		if (f != nil) != test.found {
+			if f == nil {
+				t.Errorf("%s: got no object; want one", test.src)
+			} else {
+				t.Errorf("%s: got object = %v; want none", test.src, f)
+			}
+		}
+		if !sameSlice(index, test.index) {
+			t.Errorf("%s: got index = %v; want %v", test.src, index, test.index)
+		}
+		if indirect != test.indirect {
+			t.Errorf("%s: got indirect = %v; want %v", test.src, indirect, test.indirect)
+		}
+	}
+}
+
+func sameSlice(a, b []int) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i, x := range a {
+		if x != b[i] {
+			return false
+		}
+	}
+	return true
+}
+
+// TestScopeLookupParent ensures that (*Scope).LookupParent returns
+// the correct result at various positions with the source.
+func TestScopeLookupParent(t *testing.T) {
+	fset := token.NewFileSet()
+	imports := make(testImporter)
+	conf := Config{Importer: imports}
+	mustParse := func(src string) *ast.File {
+		f, err := parser.ParseFile(fset, "dummy.go", src, parser.ParseComments)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return f
+	}
+	var info Info
+	makePkg := func(path string, files ...*ast.File) {
+		var err error
+		imports[path], err = conf.Check(path, fset, files, &info)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	makePkg("lib", mustParse("package lib; var X int"))
+	// Each /*name=kind:line*/ comment makes the test look up the
+	// name at that point and checks that it resolves to a decl of
+	// the specified kind and line number.  "undef" means undefined.
+	mainSrc := `
+/*lib=pkgname:5*/ /*X=var:1*/ /*Pi=const:8*/ /*T=typename:9*/ /*Y=var:10*/ /*F=func:12*/
+package main
+
+import "lib"
+import . "lib"
+
+const Pi = 3.1415
+type T struct{}
+var Y, _ = lib.X, X
+
+func F(){
+	const pi, e = 3.1415, /*pi=undef*/ 2.71828 /*pi=const:13*/ /*e=const:13*/
+	type /*t=undef*/ t /*t=typename:14*/ *t
+	print(Y) /*Y=var:10*/
+	x, Y := Y, /*x=undef*/ /*Y=var:10*/ Pi /*x=var:16*/ /*Y=var:16*/ ; _ = x; _ = Y
+	var F = /*F=func:12*/ F /*F=var:17*/ ; _ = F
+
+	var a []int
+	for i, x := range /*i=undef*/ /*x=var:16*/ a /*i=var:20*/ /*x=var:20*/ { _ = i; _ = x }
+
+	var i interface{}
+	switch y := i.(type) { /*y=undef*/
+	case /*y=undef*/ int /*y=var:23*/ :
+	case float32, /*y=undef*/ float64 /*y=var:23*/ :
+	default /*y=var:23*/:
+		println(y)
+	}
+	/*y=undef*/
+
+        switch int := i.(type) {
+        case /*int=typename:0*/ int /*int=var:31*/ :
+        	println(int)
+        default /*int=var:31*/ :
+        }
+}
+/*main=undef*/
+`
+
+	info.Uses = make(map[*ast.Ident]Object)
+	f := mustParse(mainSrc)
+	makePkg("main", f)
+	mainScope := imports["main"].Scope()
+	rx := regexp.MustCompile(`^/\*(\w*)=([\w:]*)\*/$`)
+	for _, group := range f.Comments {
+		for _, comment := range group.List {
+			// Parse the assertion in the comment.
+			m := rx.FindStringSubmatch(comment.Text)
+			if m == nil {
+				t.Errorf("%s: bad comment: %s",
+					fset.Position(comment.Pos()), comment.Text)
+				continue
+			}
+			name, want := m[1], m[2]
+
+			// Look up the name in the innermost enclosing scope.
+			inner := mainScope.Innermost(comment.Pos())
+			if inner == nil {
+				t.Errorf("%s: at %s: can't find innermost scope",
+					fset.Position(comment.Pos()), comment.Text)
+				continue
+			}
+			got := "undef"
+			if _, obj := inner.LookupParent(name, comment.Pos()); obj != nil {
+				kind := strings.ToLower(strings.TrimPrefix(reflect.TypeOf(obj).String(), "*types."))
+				got = fmt.Sprintf("%s:%d", kind, fset.Position(obj.Pos()).Line)
+			}
+			if got != want {
+				t.Errorf("%s: at %s: %s resolved to %s, want %s",
+					fset.Position(comment.Pos()), comment.Text, name, got, want)
+			}
+		}
+	}
+
+	// Check that for each referring identifier,
+	// a lookup of its name on the innermost
+	// enclosing scope returns the correct object.
+
+	for id, wantObj := range info.Uses {
+		inner := mainScope.Innermost(id.Pos())
+		if inner == nil {
+			t.Errorf("%s: can't find innermost scope enclosing %q",
+				fset.Position(id.Pos()), id.Name)
+			continue
+		}
+
+		// Exclude selectors and qualified identifiers---lexical
+		// refs only.  (Ideally, we'd see if the AST parent is a
+		// SelectorExpr, but that requires PathEnclosingInterval
+		// from golang.org/x/tools/go/ast/astutil.)
+		if id.Name == "X" {
+			continue
+		}
+
+		_, gotObj := inner.LookupParent(id.Name, id.Pos())
+		if gotObj != wantObj {
+			t.Errorf("%s: got %v, want %v",
+				fset.Position(id.Pos()), gotObj, wantObj)
+			continue
+		}
+	}
+}
+
+func TestConvertibleTo(t *testing.T) {
+	for _, test := range []struct {
+		v, t Type
+		want bool
+	}{
+		{Typ[Int], Typ[Int], true},
+		{Typ[Int], Typ[Float32], true},
+		{newDefined(Typ[Int]), Typ[Int], true},
+		{newDefined(new(Struct)), new(Struct), true},
+		{newDefined(Typ[Int]), new(Struct), false},
+		{Typ[UntypedInt], Typ[Int], true},
+		// Untyped string values are not permitted by the spec, so the below
+		// behavior is undefined.
+		{Typ[UntypedString], Typ[String], true},
+	} {
+		if got := ConvertibleTo(test.v, test.t); got != test.want {
+			t.Errorf("ConvertibleTo(%v, %v) = %t, want %t", test.v, test.t, got, test.want)
+		}
+	}
+}
+
+func TestAssignableTo(t *testing.T) {
+	for _, test := range []struct {
+		v, t Type
+		want bool
+	}{
+		{Typ[Int], Typ[Int], true},
+		{Typ[Int], Typ[Float32], false},
+		{newDefined(Typ[Int]), Typ[Int], false},
+		{newDefined(new(Struct)), new(Struct), true},
+		{Typ[UntypedBool], Typ[Bool], true},
+		{Typ[UntypedString], Typ[Bool], false},
+		// Neither untyped string nor untyped numeric assignments arise during
+		// normal type checking, so the below behavior is technically undefined by
+		// the spec.
+		{Typ[UntypedString], Typ[String], true},
+		{Typ[UntypedInt], Typ[Int], true},
+	} {
+		if got := AssignableTo(test.v, test.t); got != test.want {
+			t.Errorf("AssignableTo(%v, %v) = %t, want %t", test.v, test.t, got, test.want)
+		}
+	}
+}
+
+func TestIdentical_issue15173(t *testing.T) {
+	// Identical should allow nil arguments and be symmetric.
+	for _, test := range []struct {
+		x, y Type
+		want bool
+	}{
+		{Typ[Int], Typ[Int], true},
+		{Typ[Int], nil, false},
+		{nil, Typ[Int], false},
+		{nil, nil, true},
+	} {
+		if got := Identical(test.x, test.y); got != test.want {
+			t.Errorf("Identical(%v, %v) = %t", test.x, test.y, got)
+		}
+	}
+}
+
+func TestIssue15305(t *testing.T) {
+	const src = "package p; func f() int16; var _ = f(undef)"
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "issue15305.go", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	conf := Config{
+		Error: func(err error) {}, // allow errors
+	}
+	info := &Info{
+		Types: make(map[ast.Expr]TypeAndValue),
+	}
+	conf.Check("p", fset, []*ast.File{f}, info) // ignore result
+	for e, tv := range info.Types {
+		if _, ok := e.(*ast.CallExpr); ok {
+			if tv.Type != Typ[Int16] {
+				t.Errorf("CallExpr has type %v, want int16", tv.Type)
+			}
+			return
+		}
+	}
+	t.Errorf("CallExpr has no type")
+}
+
+// TestCompositeLitTypes verifies that Info.Types registers the correct
+// types for composite literal expressions and composite literal type
+// expressions.
+func TestCompositeLitTypes(t *testing.T) {
+	for _, test := range []struct {
+		lit, typ string
+	}{
+		{`[16]byte{}`, `[16]byte`},
+		{`[...]byte{}`, `[0]byte`},                // test for issue #14092
+		{`[...]int{1, 2, 3}`, `[3]int`},           // test for issue #14092
+		{`[...]int{90: 0, 98: 1, 2}`, `[100]int`}, // test for issue #14092
+		{`[]int{}`, `[]int`},
+		{`map[string]bool{"foo": true}`, `map[string]bool`},
+		{`struct{}{}`, `struct{}`},
+		{`struct{x, y int; z complex128}{}`, `struct{x int; y int; z complex128}`},
+	} {
+		fset := token.NewFileSet()
+		f, err := parser.ParseFile(fset, test.lit, "package p; var _ = "+test.lit, 0)
+		if err != nil {
+			t.Fatalf("%s: %v", test.lit, err)
+		}
+
+		info := &Info{
+			Types: make(map[ast.Expr]TypeAndValue),
+		}
+		if _, err = new(Config).Check("p", fset, []*ast.File{f}, info); err != nil {
+			t.Fatalf("%s: %v", test.lit, err)
+		}
+
+		cmptype := func(x ast.Expr, want string) {
+			tv, ok := info.Types[x]
+			if !ok {
+				t.Errorf("%s: no Types entry found", test.lit)
+				return
+			}
+			if tv.Type == nil {
+				t.Errorf("%s: type is nil", test.lit)
+				return
+			}
+			if got := tv.Type.String(); got != want {
+				t.Errorf("%s: got %v, want %s", test.lit, got, want)
+			}
+		}
+
+		// test type of composite literal expression
+		rhs := f.Decls[0].(*ast.GenDecl).Specs[0].(*ast.ValueSpec).Values[0]
+		cmptype(rhs, test.typ)
+
+		// test type of composite literal type expression
+		cmptype(rhs.(*ast.CompositeLit).Type, test.typ)
+	}
+}
+
+// TestObjectParents verifies that objects have parent scopes or not
+// as specified by the Object interface.
+func TestObjectParents(t *testing.T) {
+	const src = `
+package p
+
+const C = 0
+
+type T1 struct {
+	a, b int
+	T2
+}
+
+type T2 interface {
+	im1()
+	im2()
+}
+
+func (T1) m1() {}
+func (*T1) m2() {}
+
+func f(x int) { y := x; print(y) }
+`
+
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "src", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	info := &Info{
+		Defs: make(map[*ast.Ident]Object),
+	}
+	if _, err = new(Config).Check("p", fset, []*ast.File{f}, info); err != nil {
+		t.Fatal(err)
+	}
+
+	for ident, obj := range info.Defs {
+		if obj == nil {
+			// only package names and implicit vars have a nil object
+			// (in this test we only need to handle the package name)
+			if ident.Name != "p" {
+				t.Errorf("%v has nil object", ident)
+			}
+			continue
+		}
+
+		// struct fields, type-associated and interface methods
+		// have no parent scope
+		wantParent := true
+		switch obj := obj.(type) {
+		case *Var:
+			if obj.IsField() {
+				wantParent = false
+			}
+		case *Func:
+			if obj.Type().(*Signature).Recv() != nil { // method
+				wantParent = false
+			}
+		}
+
+		gotParent := obj.Parent() != nil
+		switch {
+		case gotParent && !wantParent:
+			t.Errorf("%v: want no parent, got %s", ident, obj.Parent())
+		case !gotParent && wantParent:
+			t.Errorf("%v: no parent found", ident)
+		}
+	}
+}
+
+// TestFailedImport tests that we don't get follow-on errors
+// elsewhere in a package due to failing to import a package.
+func TestFailedImport(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	const src = `
+package p
+
+import foo "go/types/thisdirectorymustnotexistotherwisethistestmayfail/foo" // should only see an error here
+
+const c = foo.C
+type T = foo.T
+var v T = c
+func f(x T) T { return foo.F(x) }
+`
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "src", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	files := []*ast.File{f}
+
+	// type-check using all possible importers
+	for _, compiler := range []string{"gc", "gccgo", "source"} {
+		errcount := 0
+		conf := Config{
+			Error: func(err error) {
+				// we should only see the import error
+				if errcount > 0 || !strings.Contains(err.Error(), "could not import") {
+					t.Errorf("for %s importer, got unexpected error: %v", compiler, err)
+				}
+				errcount++
+			},
+			Importer: importer.For(compiler, nil),
+		}
+
+		info := &Info{
+			Uses: make(map[*ast.Ident]Object),
+		}
+		pkg, _ := conf.Check("p", fset, files, info)
+		if pkg == nil {
+			t.Errorf("for %s importer, type-checking failed to return a package", compiler)
+			continue
+		}
+
+		imports := pkg.Imports()
+		if len(imports) != 1 {
+			t.Errorf("for %s importer, got %d imports, want 1", compiler, len(imports))
+			continue
+		}
+		imp := imports[0]
+		if imp.Name() != "foo" {
+			t.Errorf(`for %s importer, got %q, want "foo"`, compiler, imp.Name())
+			continue
+		}
+
+		// verify that all uses of foo refer to the imported package foo (imp)
+		for ident, obj := range info.Uses {
+			if ident.Name == "foo" {
+				if obj, ok := obj.(*PkgName); ok {
+					if obj.Imported() != imp {
+						t.Errorf("%s resolved to %v; want %v", ident, obj.Imported(), imp)
+					}
+				} else {
+					t.Errorf("%s resolved to %v; want package name", ident, obj)
+				}
+			}
+		}
+	}
+}
diff --git a/src/go/types/assignments.go b/src/go/types/assignments.go
new file mode 100644
index 0000000..616564b
--- /dev/null
+++ b/src/go/types/assignments.go
@@ -0,0 +1,357 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements initialization and assignment checks.
+
+package types
+
+import (
+	"errors"
+	"go/ast"
+	"go/token"
+)
+
+// assignment reports whether x can be assigned to a variable of type T,
+// if necessary by attempting to convert untyped values to the appropriate
+// type. context describes the context in which the assignment takes place.
+// Use T == nil to indicate assignment to an untyped blank identifier.
+// x.mode is set to invalid if the assignment failed.
+func (check *Checker) assignment(x *operand, T Type, context string) {
+	check.singleValue(x)
+
+	switch x.mode {
+	case invalid:
+		return // error reported before
+	case constant_, variable, mapindex, value, commaok, commaerr:
+		// ok
+	default:
+		unreachable()
+	}
+
+	if isUntyped(x.typ) {
+		target := T
+		// spec: "If an untyped constant is assigned to a variable of interface
+		// type or the blank identifier, the constant is first converted to type
+		// bool, rune, int, float64, complex128 or string respectively, depending
+		// on whether the value is a boolean, rune, integer, floating-point,
+		// complex, or string constant."
+		if T == nil || IsInterface(T) {
+			if T == nil && x.typ == Typ[UntypedNil] {
+				check.errorf(x, _UntypedNil, "use of untyped nil in %s", context)
+				x.mode = invalid
+				return
+			}
+			target = Default(x.typ)
+		}
+		if err := check.canConvertUntyped(x, target); err != nil {
+			msg := check.sprintf("cannot use %s as %s value in %s", x, target, context)
+			code := _IncompatibleAssign
+			var ierr Error
+			if errors.As(err, &ierr) {
+				// Preserve these inner errors, as they are informative.
+				switch ierr.go116code {
+				case _TruncatedFloat:
+					msg += " (truncated)"
+					code = ierr.go116code
+				case _NumericOverflow:
+					msg += " (overflows)"
+					code = ierr.go116code
+				}
+			}
+			check.error(x, code, msg)
+			x.mode = invalid
+			return
+		}
+	}
+	// x.typ is typed
+
+	// spec: "If a left-hand side is the blank identifier, any typed or
+	// non-constant value except for the predeclared identifier nil may
+	// be assigned to it."
+	if T == nil {
+		return
+	}
+
+	reason := ""
+	if ok, code := x.assignableTo(check, T, &reason); !ok {
+		if reason != "" {
+			check.errorf(x, code, "cannot use %s as %s value in %s: %s", x, T, context, reason)
+		} else {
+			check.errorf(x, code, "cannot use %s as %s value in %s", x, T, context)
+		}
+		x.mode = invalid
+	}
+}
+
+func (check *Checker) initConst(lhs *Const, x *operand) {
+	if x.mode == invalid || x.typ == Typ[Invalid] || lhs.typ == Typ[Invalid] {
+		if lhs.typ == nil {
+			lhs.typ = Typ[Invalid]
+		}
+		return
+	}
+
+	// rhs must be a constant
+	if x.mode != constant_ {
+		check.errorf(x, _InvalidConstInit, "%s is not constant", x)
+		if lhs.typ == nil {
+			lhs.typ = Typ[Invalid]
+		}
+		return
+	}
+	assert(isConstType(x.typ))
+
+	// If the lhs doesn't have a type yet, use the type of x.
+	if lhs.typ == nil {
+		lhs.typ = x.typ
+	}
+
+	check.assignment(x, lhs.typ, "constant declaration")
+	if x.mode == invalid {
+		return
+	}
+
+	lhs.val = x.val
+}
+
+func (check *Checker) initVar(lhs *Var, x *operand, context string) Type {
+	if x.mode == invalid || x.typ == Typ[Invalid] || lhs.typ == Typ[Invalid] {
+		if lhs.typ == nil {
+			lhs.typ = Typ[Invalid]
+		}
+		return nil
+	}
+
+	// If the lhs doesn't have a type yet, use the type of x.
+	if lhs.typ == nil {
+		typ := x.typ
+		if isUntyped(typ) {
+			// convert untyped types to default types
+			if typ == Typ[UntypedNil] {
+				check.errorf(x, _UntypedNil, "use of untyped nil in %s", context)
+				lhs.typ = Typ[Invalid]
+				return nil
+			}
+			typ = Default(typ)
+		}
+		lhs.typ = typ
+	}
+
+	check.assignment(x, lhs.typ, context)
+	if x.mode == invalid {
+		return nil
+	}
+
+	return x.typ
+}
+
+func (check *Checker) assignVar(lhs ast.Expr, x *operand) Type {
+	if x.mode == invalid || x.typ == Typ[Invalid] {
+		return nil
+	}
+
+	// Determine if the lhs is a (possibly parenthesized) identifier.
+	ident, _ := unparen(lhs).(*ast.Ident)
+
+	// Don't evaluate lhs if it is the blank identifier.
+	if ident != nil && ident.Name == "_" {
+		check.recordDef(ident, nil)
+		check.assignment(x, nil, "assignment to _ identifier")
+		if x.mode == invalid {
+			return nil
+		}
+		return x.typ
+	}
+
+	// If the lhs is an identifier denoting a variable v, this assignment
+	// is not a 'use' of v. Remember current value of v.used and restore
+	// after evaluating the lhs via check.expr.
+	var v *Var
+	var v_used bool
+	if ident != nil {
+		if obj := check.lookup(ident.Name); obj != nil {
+			// It's ok to mark non-local variables, but ignore variables
+			// from other packages to avoid potential race conditions with
+			// dot-imported variables.
+			if w, _ := obj.(*Var); w != nil && w.pkg == check.pkg {
+				v = w
+				v_used = v.used
+			}
+		}
+	}
+
+	var z operand
+	check.expr(&z, lhs)
+	if v != nil {
+		v.used = v_used // restore v.used
+	}
+
+	if z.mode == invalid || z.typ == Typ[Invalid] {
+		return nil
+	}
+
+	// spec: "Each left-hand side operand must be addressable, a map index
+	// expression, or the blank identifier. Operands may be parenthesized."
+	switch z.mode {
+	case invalid:
+		return nil
+	case variable, mapindex:
+		// ok
+	default:
+		if sel, ok := z.expr.(*ast.SelectorExpr); ok {
+			var op operand
+			check.expr(&op, sel.X)
+			if op.mode == mapindex {
+				check.errorf(&z, _UnaddressableFieldAssign, "cannot assign to struct field %s in map", ExprString(z.expr))
+				return nil
+			}
+		}
+		check.errorf(&z, _UnassignableOperand, "cannot assign to %s", &z)
+		return nil
+	}
+
+	check.assignment(x, z.typ, "assignment")
+	if x.mode == invalid {
+		return nil
+	}
+
+	return x.typ
+}
+
+// If returnPos is valid, initVars is called to type-check the assignment of
+// return expressions, and returnPos is the position of the return statement.
+func (check *Checker) initVars(lhs []*Var, rhs []ast.Expr, returnPos token.Pos) {
+	l := len(lhs)
+	get, r, commaOk := unpack(func(x *operand, i int) { check.multiExpr(x, rhs[i]) }, len(rhs), l == 2 && !returnPos.IsValid())
+	if get == nil || l != r {
+		// invalidate lhs and use rhs
+		for _, obj := range lhs {
+			if obj.typ == nil {
+				obj.typ = Typ[Invalid]
+			}
+		}
+		if get == nil {
+			return // error reported by unpack
+		}
+		check.useGetter(get, r)
+		if returnPos.IsValid() {
+			check.errorf(atPos(returnPos), _WrongResultCount, "wrong number of return values (want %d, got %d)", l, r)
+			return
+		}
+		check.errorf(rhs[0], _WrongAssignCount, "cannot initialize %d variables with %d values", l, r)
+		return
+	}
+
+	context := "assignment"
+	if returnPos.IsValid() {
+		context = "return statement"
+	}
+
+	var x operand
+	if commaOk {
+		var a [2]Type
+		for i := range a {
+			get(&x, i)
+			a[i] = check.initVar(lhs[i], &x, context)
+		}
+		check.recordCommaOkTypes(rhs[0], a)
+		return
+	}
+
+	for i, lhs := range lhs {
+		get(&x, i)
+		check.initVar(lhs, &x, context)
+	}
+}
+
+func (check *Checker) assignVars(lhs, rhs []ast.Expr) {
+	l := len(lhs)
+	get, r, commaOk := unpack(func(x *operand, i int) { check.multiExpr(x, rhs[i]) }, len(rhs), l == 2)
+	if get == nil {
+		check.useLHS(lhs...)
+		return // error reported by unpack
+	}
+	if l != r {
+		check.useGetter(get, r)
+		check.errorf(rhs[0], _WrongAssignCount, "cannot assign %d values to %d variables", r, l)
+		return
+	}
+
+	var x operand
+	if commaOk {
+		var a [2]Type
+		for i := range a {
+			get(&x, i)
+			a[i] = check.assignVar(lhs[i], &x)
+		}
+		check.recordCommaOkTypes(rhs[0], a)
+		return
+	}
+
+	for i, lhs := range lhs {
+		get(&x, i)
+		check.assignVar(lhs, &x)
+	}
+}
+
+func (check *Checker) shortVarDecl(pos positioner, lhs, rhs []ast.Expr) {
+	top := len(check.delayed)
+	scope := check.scope
+
+	// collect lhs variables
+	var newVars []*Var
+	var lhsVars = make([]*Var, len(lhs))
+	for i, lhs := range lhs {
+		var obj *Var
+		if ident, _ := lhs.(*ast.Ident); ident != nil {
+			// Use the correct obj if the ident is redeclared. The
+			// variable's scope starts after the declaration; so we
+			// must use Scope.Lookup here and call Scope.Insert
+			// (via check.declare) later.
+			name := ident.Name
+			if alt := scope.Lookup(name); alt != nil {
+				// redeclared object must be a variable
+				if alt, _ := alt.(*Var); alt != nil {
+					obj = alt
+				} else {
+					check.errorf(lhs, _UnassignableOperand, "cannot assign to %s", lhs)
+				}
+				check.recordUse(ident, alt)
+			} else {
+				// declare new variable, possibly a blank (_) variable
+				obj = NewVar(ident.Pos(), check.pkg, name, nil)
+				if name != "_" {
+					newVars = append(newVars, obj)
+				}
+				check.recordDef(ident, obj)
+			}
+		} else {
+			check.useLHS(lhs)
+			check.invalidAST(lhs, "cannot declare %s", lhs)
+		}
+		if obj == nil {
+			obj = NewVar(lhs.Pos(), check.pkg, "_", nil) // dummy variable
+		}
+		lhsVars[i] = obj
+	}
+
+	check.initVars(lhsVars, rhs, token.NoPos)
+
+	// process function literals in rhs expressions before scope changes
+	check.processDelayed(top)
+
+	// declare new variables
+	if len(newVars) > 0 {
+		// spec: "The scope of a constant or variable identifier declared inside
+		// a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl
+		// for short variable declarations) and ends at the end of the innermost
+		// containing block."
+		scopePos := rhs[len(rhs)-1].End()
+		for _, obj := range newVars {
+			check.declare(scope, nil, obj, scopePos) // recordObject already called
+		}
+	} else {
+		check.softErrorf(pos, _NoNewVar, "no new variables on left side of :=")
+	}
+}
diff --git a/src/go/types/builtins.go b/src/go/types/builtins.go
new file mode 100644
index 0000000..fd35f78
--- /dev/null
+++ b/src/go/types/builtins.go
@@ -0,0 +1,697 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements typechecking of builtin function calls.
+
+package types
+
+import (
+	"go/ast"
+	"go/constant"
+	"go/token"
+)
+
+// builtin type-checks a call to the built-in specified by id and
+// reports whether the call is valid, with *x holding the result;
+// but x.expr is not set. If the call is invalid, the result is
+// false, and *x is undefined.
+//
+func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ bool) {
+	// append is the only built-in that permits the use of ... for the last argument
+	bin := predeclaredFuncs[id]
+	if call.Ellipsis.IsValid() && id != _Append {
+		check.invalidOp(atPos(call.Ellipsis),
+			_InvalidDotDotDot,
+			"invalid use of ... with built-in %s", bin.name)
+		check.use(call.Args...)
+		return
+	}
+
+	// For len(x) and cap(x) we need to know if x contains any function calls or
+	// receive operations. Save/restore current setting and set hasCallOrRecv to
+	// false for the evaluation of x so that we can check it afterwards.
+	// Note: We must do this _before_ calling unpack because unpack evaluates the
+	//       first argument before we even call arg(x, 0)!
+	if id == _Len || id == _Cap {
+		defer func(b bool) {
+			check.hasCallOrRecv = b
+		}(check.hasCallOrRecv)
+		check.hasCallOrRecv = false
+	}
+
+	// determine actual arguments
+	var arg getter
+	nargs := len(call.Args)
+	switch id {
+	default:
+		// make argument getter
+		arg, nargs, _ = unpack(func(x *operand, i int) { check.multiExpr(x, call.Args[i]) }, nargs, false)
+		if arg == nil {
+			return
+		}
+		// evaluate first argument, if present
+		if nargs > 0 {
+			arg(x, 0)
+			if x.mode == invalid {
+				return
+			}
+		}
+	case _Make, _New, _Offsetof, _Trace:
+		// arguments require special handling
+	}
+
+	// check argument count
+	{
+		msg := ""
+		if nargs < bin.nargs {
+			msg = "not enough"
+		} else if !bin.variadic && nargs > bin.nargs {
+			msg = "too many"
+		}
+		if msg != "" {
+			check.invalidOp(inNode(call, call.Rparen), _WrongArgCount, "%s arguments for %s (expected %d, found %d)", msg, call, bin.nargs, nargs)
+			return
+		}
+	}
+
+	switch id {
+	case _Append:
+		// append(s S, x ...T) S, where T is the element type of S
+		// spec: "The variadic function append appends zero or more values x to s of type
+		// S, which must be a slice type, and returns the resulting slice, also of type S.
+		// The values x are passed to a parameter of type ...T where T is the element type
+		// of S and the respective parameter passing rules apply."
+		S := x.typ
+		var T Type
+		if s, _ := S.Underlying().(*Slice); s != nil {
+			T = s.elem
+		} else {
+			check.invalidArg(x, _InvalidAppend, "%s is not a slice", x)
+			return
+		}
+
+		// remember arguments that have been evaluated already
+		alist := []operand{*x}
+
+		// spec: "As a special case, append also accepts a first argument assignable
+		// to type []byte with a second argument of string type followed by ... .
+		// This form appends the bytes of the string.
+		if nargs == 2 && call.Ellipsis.IsValid() {
+			if ok, _ := x.assignableTo(check, NewSlice(universeByte), nil); ok {
+				arg(x, 1)
+				if x.mode == invalid {
+					return
+				}
+				if isString(x.typ) {
+					if check.Types != nil {
+						sig := makeSig(S, S, x.typ)
+						sig.variadic = true
+						check.recordBuiltinType(call.Fun, sig)
+					}
+					x.mode = value
+					x.typ = S
+					break
+				}
+				alist = append(alist, *x)
+				// fallthrough
+			}
+		}
+
+		// check general case by creating custom signature
+		sig := makeSig(S, S, NewSlice(T)) // []T required for variadic signature
+		sig.variadic = true
+		check.arguments(x, call, sig, func(x *operand, i int) {
+			// only evaluate arguments that have not been evaluated before
+			if i < len(alist) {
+				*x = alist[i]
+				return
+			}
+			arg(x, i)
+		}, nargs)
+		// ok to continue even if check.arguments reported errors
+
+		x.mode = value
+		x.typ = S
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, sig)
+		}
+
+	case _Cap, _Len:
+		// cap(x)
+		// len(x)
+		mode := invalid
+		var typ Type
+		var val constant.Value
+		switch typ = implicitArrayDeref(x.typ.Underlying()); t := typ.(type) {
+		case *Basic:
+			if isString(t) && id == _Len {
+				if x.mode == constant_ {
+					mode = constant_
+					val = constant.MakeInt64(int64(len(constant.StringVal(x.val))))
+				} else {
+					mode = value
+				}
+			}
+
+		case *Array:
+			mode = value
+			// spec: "The expressions len(s) and cap(s) are constants
+			// if the type of s is an array or pointer to an array and
+			// the expression s does not contain channel receives or
+			// function calls; in this case s is not evaluated."
+			if !check.hasCallOrRecv {
+				mode = constant_
+				if t.len >= 0 {
+					val = constant.MakeInt64(t.len)
+				} else {
+					val = constant.MakeUnknown()
+				}
+			}
+
+		case *Slice, *Chan:
+			mode = value
+
+		case *Map:
+			if id == _Len {
+				mode = value
+			}
+		}
+
+		if mode == invalid && typ != Typ[Invalid] {
+			code := _InvalidCap
+			if id == _Len {
+				code = _InvalidLen
+			}
+			check.invalidArg(x, code, "%s for %s", x, bin.name)
+			return
+		}
+
+		x.mode = mode
+		x.typ = Typ[Int]
+		x.val = val
+		if check.Types != nil && mode != constant_ {
+			check.recordBuiltinType(call.Fun, makeSig(x.typ, typ))
+		}
+
+	case _Close:
+		// close(c)
+		c, _ := x.typ.Underlying().(*Chan)
+		if c == nil {
+			check.invalidArg(x, _InvalidClose, "%s is not a channel", x)
+			return
+		}
+		if c.dir == RecvOnly {
+			check.invalidArg(x, _InvalidClose, "%s must not be a receive-only channel", x)
+			return
+		}
+
+		x.mode = novalue
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(nil, c))
+		}
+
+	case _Complex:
+		// complex(x, y floatT) complexT
+		var y operand
+		arg(&y, 1)
+		if y.mode == invalid {
+			return
+		}
+
+		// convert or check untyped arguments
+		d := 0
+		if isUntyped(x.typ) {
+			d |= 1
+		}
+		if isUntyped(y.typ) {
+			d |= 2
+		}
+		switch d {
+		case 0:
+			// x and y are typed => nothing to do
+		case 1:
+			// only x is untyped => convert to type of y
+			check.convertUntyped(x, y.typ)
+		case 2:
+			// only y is untyped => convert to type of x
+			check.convertUntyped(&y, x.typ)
+		case 3:
+			// x and y are untyped =>
+			// 1) if both are constants, convert them to untyped
+			//    floating-point numbers if possible,
+			// 2) if one of them is not constant (possible because
+			//    it contains a shift that is yet untyped), convert
+			//    both of them to float64 since they must have the
+			//    same type to succeed (this will result in an error
+			//    because shifts of floats are not permitted)
+			if x.mode == constant_ && y.mode == constant_ {
+				toFloat := func(x *operand) {
+					if isNumeric(x.typ) && constant.Sign(constant.Imag(x.val)) == 0 {
+						x.typ = Typ[UntypedFloat]
+					}
+				}
+				toFloat(x)
+				toFloat(&y)
+			} else {
+				check.convertUntyped(x, Typ[Float64])
+				check.convertUntyped(&y, Typ[Float64])
+				// x and y should be invalid now, but be conservative
+				// and check below
+			}
+		}
+		if x.mode == invalid || y.mode == invalid {
+			return
+		}
+
+		// both argument types must be identical
+		if !check.identical(x.typ, y.typ) {
+			check.invalidArg(x, _InvalidComplex, "mismatched types %s and %s", x.typ, y.typ)
+			return
+		}
+
+		// the argument types must be of floating-point type
+		if !isFloat(x.typ) {
+			check.invalidArg(x, _InvalidComplex, "arguments have type %s, expected floating-point", x.typ)
+			return
+		}
+
+		// if both arguments are constants, the result is a constant
+		if x.mode == constant_ && y.mode == constant_ {
+			x.val = constant.BinaryOp(constant.ToFloat(x.val), token.ADD, constant.MakeImag(constant.ToFloat(y.val)))
+		} else {
+			x.mode = value
+		}
+
+		// determine result type
+		var res BasicKind
+		switch x.typ.Underlying().(*Basic).kind {
+		case Float32:
+			res = Complex64
+		case Float64:
+			res = Complex128
+		case UntypedFloat:
+			res = UntypedComplex
+		default:
+			unreachable()
+		}
+		resTyp := Typ[res]
+
+		if check.Types != nil && x.mode != constant_ {
+			check.recordBuiltinType(call.Fun, makeSig(resTyp, x.typ, x.typ))
+		}
+
+		x.typ = resTyp
+
+	case _Copy:
+		// copy(x, y []T) int
+		var dst Type
+		if t, _ := x.typ.Underlying().(*Slice); t != nil {
+			dst = t.elem
+		}
+
+		var y operand
+		arg(&y, 1)
+		if y.mode == invalid {
+			return
+		}
+		var src Type
+		switch t := y.typ.Underlying().(type) {
+		case *Basic:
+			if isString(y.typ) {
+				src = universeByte
+			}
+		case *Slice:
+			src = t.elem
+		}
+
+		if dst == nil || src == nil {
+			check.invalidArg(x, _InvalidCopy, "copy expects slice arguments; found %s and %s", x, &y)
+			return
+		}
+
+		if !check.identical(dst, src) {
+			check.invalidArg(x, _InvalidCopy, "arguments to copy %s and %s have different element types %s and %s", x, &y, dst, src)
+			return
+		}
+
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(Typ[Int], x.typ, y.typ))
+		}
+		x.mode = value
+		x.typ = Typ[Int]
+
+	case _Delete:
+		// delete(m, k)
+		m, _ := x.typ.Underlying().(*Map)
+		if m == nil {
+			check.invalidArg(x, _InvalidDelete, "%s is not a map", x)
+			return
+		}
+		arg(x, 1) // k
+		if x.mode == invalid {
+			return
+		}
+
+		if ok, code := x.assignableTo(check, m.key, nil); !ok {
+			check.invalidArg(x, code, "%s is not assignable to %s", x, m.key)
+			return
+		}
+
+		x.mode = novalue
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(nil, m, m.key))
+		}
+
+	case _Imag, _Real:
+		// imag(complexT) floatT
+		// real(complexT) floatT
+
+		// convert or check untyped argument
+		if isUntyped(x.typ) {
+			if x.mode == constant_ {
+				// an untyped constant number can always be considered
+				// as a complex constant
+				if isNumeric(x.typ) {
+					x.typ = Typ[UntypedComplex]
+				}
+			} else {
+				// an untyped non-constant argument may appear if
+				// it contains a (yet untyped non-constant) shift
+				// expression: convert it to complex128 which will
+				// result in an error (shift of complex value)
+				check.convertUntyped(x, Typ[Complex128])
+				// x should be invalid now, but be conservative and check
+				if x.mode == invalid {
+					return
+				}
+			}
+		}
+
+		// the argument must be of complex type
+		if !isComplex(x.typ) {
+			code := _InvalidImag
+			if id == _Real {
+				code = _InvalidReal
+			}
+			check.invalidArg(x, code, "argument has type %s, expected complex type", x.typ)
+			return
+		}
+
+		// if the argument is a constant, the result is a constant
+		if x.mode == constant_ {
+			if id == _Real {
+				x.val = constant.Real(x.val)
+			} else {
+				x.val = constant.Imag(x.val)
+			}
+		} else {
+			x.mode = value
+		}
+
+		// determine result type
+		var res BasicKind
+		switch x.typ.Underlying().(*Basic).kind {
+		case Complex64:
+			res = Float32
+		case Complex128:
+			res = Float64
+		case UntypedComplex:
+			res = UntypedFloat
+		default:
+			unreachable()
+		}
+		resTyp := Typ[res]
+
+		if check.Types != nil && x.mode != constant_ {
+			check.recordBuiltinType(call.Fun, makeSig(resTyp, x.typ))
+		}
+
+		x.typ = resTyp
+
+	case _Make:
+		// make(T, n)
+		// make(T, n, m)
+		// (no argument evaluated yet)
+		arg0 := call.Args[0]
+		T := check.typ(arg0)
+		if T == Typ[Invalid] {
+			return
+		}
+
+		var min int // minimum number of arguments
+		switch T.Underlying().(type) {
+		case *Slice:
+			min = 2
+		case *Map, *Chan:
+			min = 1
+		default:
+			check.invalidArg(arg0, _InvalidMake, "cannot make %s; type must be slice, map, or channel", arg0)
+			return
+		}
+		if nargs < min || min+1 < nargs {
+			check.errorf(call, _WrongArgCount, "%v expects %d or %d arguments; found %d", call, min, min+1, nargs)
+			return
+		}
+		types := []Type{T}
+		var sizes []int64 // constant integer arguments, if any
+		for _, arg := range call.Args[1:] {
+			typ, size := check.index(arg, -1) // ok to continue with typ == Typ[Invalid]
+			types = append(types, typ)
+			if size >= 0 {
+				sizes = append(sizes, size)
+			}
+		}
+		if len(sizes) == 2 && sizes[0] > sizes[1] {
+			check.invalidArg(call.Args[1], _SwappedMakeArgs, "length and capacity swapped")
+			// safe to continue
+		}
+		x.mode = value
+		x.typ = T
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(x.typ, types...))
+		}
+
+	case _New:
+		// new(T)
+		// (no argument evaluated yet)
+		T := check.typ(call.Args[0])
+		if T == Typ[Invalid] {
+			return
+		}
+
+		x.mode = value
+		x.typ = &Pointer{base: T}
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(x.typ, T))
+		}
+
+	case _Panic:
+		// panic(x)
+		// record panic call if inside a function with result parameters
+		// (for use in Checker.isTerminating)
+		if check.sig != nil && check.sig.results.Len() > 0 {
+			// function has result parameters
+			p := check.isPanic
+			if p == nil {
+				// allocate lazily
+				p = make(map[*ast.CallExpr]bool)
+				check.isPanic = p
+			}
+			p[call] = true
+		}
+
+		check.assignment(x, &emptyInterface, "argument to panic")
+		if x.mode == invalid {
+			return
+		}
+
+		x.mode = novalue
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(nil, &emptyInterface))
+		}
+
+	case _Print, _Println:
+		// print(x, y, ...)
+		// println(x, y, ...)
+		var params []Type
+		if nargs > 0 {
+			params = make([]Type, nargs)
+			for i := 0; i < nargs; i++ {
+				if i > 0 {
+					arg(x, i) // first argument already evaluated
+				}
+				check.assignment(x, nil, "argument to "+predeclaredFuncs[id].name)
+				if x.mode == invalid {
+					// TODO(gri) "use" all arguments?
+					return
+				}
+				params[i] = x.typ
+			}
+		}
+
+		x.mode = novalue
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(nil, params...))
+		}
+
+	case _Recover:
+		// recover() interface{}
+		x.mode = value
+		x.typ = &emptyInterface
+		if check.Types != nil {
+			check.recordBuiltinType(call.Fun, makeSig(x.typ))
+		}
+
+	case _Alignof:
+		// unsafe.Alignof(x T) uintptr
+		check.assignment(x, nil, "argument to unsafe.Alignof")
+		if x.mode == invalid {
+			return
+		}
+
+		x.mode = constant_
+		x.val = constant.MakeInt64(check.conf.alignof(x.typ))
+		x.typ = Typ[Uintptr]
+		// result is constant - no need to record signature
+
+	case _Offsetof:
+		// unsafe.Offsetof(x T) uintptr, where x must be a selector
+		// (no argument evaluated yet)
+		arg0 := call.Args[0]
+		selx, _ := unparen(arg0).(*ast.SelectorExpr)
+		if selx == nil {
+			check.invalidArg(arg0, _BadOffsetofSyntax, "%s is not a selector expression", arg0)
+			check.use(arg0)
+			return
+		}
+
+		check.expr(x, selx.X)
+		if x.mode == invalid {
+			return
+		}
+
+		base := derefStructPtr(x.typ)
+		sel := selx.Sel.Name
+		obj, index, indirect := check.lookupFieldOrMethod(base, false, check.pkg, sel)
+		switch obj.(type) {
+		case nil:
+			check.invalidArg(x, _MissingFieldOrMethod, "%s has no single field %s", base, sel)
+			return
+		case *Func:
+			// TODO(gri) Using derefStructPtr may result in methods being found
+			// that don't actually exist. An error either way, but the error
+			// message is confusing. See: https://play.golang.org/p/al75v23kUy ,
+			// but go/types reports: "invalid argument: x.m is a method value".
+			check.invalidArg(arg0, _InvalidOffsetof, "%s is a method value", arg0)
+			return
+		}
+		if indirect {
+			check.invalidArg(x, _InvalidOffsetof, "field %s is embedded via a pointer in %s", sel, base)
+			return
+		}
+
+		// TODO(gri) Should we pass x.typ instead of base (and indirect report if derefStructPtr indirected)?
+		check.recordSelection(selx, FieldVal, base, obj, index, false)
+
+		offs := check.conf.offsetof(base, index)
+		x.mode = constant_
+		x.val = constant.MakeInt64(offs)
+		x.typ = Typ[Uintptr]
+		// result is constant - no need to record signature
+
+	case _Sizeof:
+		// unsafe.Sizeof(x T) uintptr
+		check.assignment(x, nil, "argument to unsafe.Sizeof")
+		if x.mode == invalid {
+			return
+		}
+
+		x.mode = constant_
+		x.val = constant.MakeInt64(check.conf.sizeof(x.typ))
+		x.typ = Typ[Uintptr]
+		// result is constant - no need to record signature
+
+	case _Assert:
+		// assert(pred) causes a typechecker error if pred is false.
+		// The result of assert is the value of pred if there is no error.
+		// Note: assert is only available in self-test mode.
+		if x.mode != constant_ || !isBoolean(x.typ) {
+			check.invalidArg(x, _Test, "%s is not a boolean constant", x)
+			return
+		}
+		if x.val.Kind() != constant.Bool {
+			check.errorf(x, _Test, "internal error: value of %s should be a boolean constant", x)
+			return
+		}
+		if !constant.BoolVal(x.val) {
+			check.errorf(call, _Test, "%v failed", call)
+			// compile-time assertion failure - safe to continue
+		}
+		// result is constant - no need to record signature
+
+	case _Trace:
+		// trace(x, y, z, ...) dumps the positions, expressions, and
+		// values of its arguments. The result of trace is the value
+		// of the first argument.
+		// Note: trace is only available in self-test mode.
+		// (no argument evaluated yet)
+		if nargs == 0 {
+			check.dump("%v: trace() without arguments", call.Pos())
+			x.mode = novalue
+			break
+		}
+		var t operand
+		x1 := x
+		for _, arg := range call.Args {
+			check.rawExpr(x1, arg, nil) // permit trace for types, e.g.: new(trace(T))
+			check.dump("%v: %s", x1.Pos(), x1)
+			x1 = &t // use incoming x only for first argument
+		}
+		// trace is only available in test mode - no need to record signature
+
+	default:
+		unreachable()
+	}
+
+	return true
+}
+
+// makeSig makes a signature for the given argument and result types.
+// Default types are used for untyped arguments, and res may be nil.
+func makeSig(res Type, args ...Type) *Signature {
+	list := make([]*Var, len(args))
+	for i, param := range args {
+		list[i] = NewVar(token.NoPos, nil, "", Default(param))
+	}
+	params := NewTuple(list...)
+	var result *Tuple
+	if res != nil {
+		assert(!isUntyped(res))
+		result = NewTuple(NewVar(token.NoPos, nil, "", res))
+	}
+	return &Signature{params: params, results: result}
+}
+
+// implicitArrayDeref returns A if typ is of the form *A and A is an array;
+// otherwise it returns typ.
+//
+func implicitArrayDeref(typ Type) Type {
+	if p, ok := typ.(*Pointer); ok {
+		if a, ok := p.base.Underlying().(*Array); ok {
+			return a
+		}
+	}
+	return typ
+}
+
+// unparen returns e with any enclosing parentheses stripped.
+func unparen(e ast.Expr) ast.Expr {
+	for {
+		p, ok := e.(*ast.ParenExpr)
+		if !ok {
+			return e
+		}
+		e = p.X
+	}
+}
diff --git a/src/go/types/builtins_test.go b/src/go/types/builtins_test.go
new file mode 100644
index 0000000..cfd19d5
--- /dev/null
+++ b/src/go/types/builtins_test.go
@@ -0,0 +1,220 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types_test
+
+import (
+	"fmt"
+	"go/ast"
+	"go/importer"
+	"go/parser"
+	"testing"
+
+	. "go/types"
+)
+
+var builtinCalls = []struct {
+	name, src, sig string
+}{
+	{"append", `var s []int; _ = append(s)`, `func([]int, ...int) []int`},
+	{"append", `var s []int; _ = append(s, 0)`, `func([]int, ...int) []int`},
+	{"append", `var s []int; _ = (append)(s, 0)`, `func([]int, ...int) []int`},
+	{"append", `var s []byte; _ = ((append))(s, 0)`, `func([]byte, ...byte) []byte`},
+	{"append", `var s []byte; _ = append(s, "foo"...)`, `func([]byte, string...) []byte`},
+	{"append", `type T []byte; var s T; var str string; _ = append(s, str...)`, `func(p.T, string...) p.T`},
+	{"append", `type T []byte; type U string; var s T; var str U; _ = append(s, str...)`, `func(p.T, p.U...) p.T`},
+
+	{"cap", `var s [10]int; _ = cap(s)`, `invalid type`},  // constant
+	{"cap", `var s [10]int; _ = cap(&s)`, `invalid type`}, // constant
+	{"cap", `var s []int64; _ = cap(s)`, `func([]int64) int`},
+	{"cap", `var c chan<-bool; _ = cap(c)`, `func(chan<- bool) int`},
+
+	{"len", `_ = len("foo")`, `invalid type`}, // constant
+	{"len", `var s string; _ = len(s)`, `func(string) int`},
+	{"len", `var s [10]int; _ = len(s)`, `invalid type`},  // constant
+	{"len", `var s [10]int; _ = len(&s)`, `invalid type`}, // constant
+	{"len", `var s []int64; _ = len(s)`, `func([]int64) int`},
+	{"len", `var c chan<-bool; _ = len(c)`, `func(chan<- bool) int`},
+	{"len", `var m map[string]float32; _ = len(m)`, `func(map[string]float32) int`},
+
+	{"close", `var c chan int; close(c)`, `func(chan int)`},
+	{"close", `var c chan<- chan string; close(c)`, `func(chan<- chan string)`},
+
+	{"complex", `_ = complex(1, 0)`, `invalid type`}, // constant
+	{"complex", `var re float32; _ = complex(re, 1.0)`, `func(float32, float32) complex64`},
+	{"complex", `var im float64; _ = complex(1, im)`, `func(float64, float64) complex128`},
+	{"complex", `type F32 float32; var re, im F32; _ = complex(re, im)`, `func(p.F32, p.F32) complex64`},
+	{"complex", `type F64 float64; var re, im F64; _ = complex(re, im)`, `func(p.F64, p.F64) complex128`},
+
+	{"copy", `var src, dst []byte; copy(dst, src)`, `func([]byte, []byte) int`},
+	{"copy", `type T [][]int; var src, dst T; _ = copy(dst, src)`, `func(p.T, p.T) int`},
+	{"copy", `var src string; var dst []byte; copy(dst, src)`, `func([]byte, string) int`},
+	{"copy", `type T string; type U []byte; var src T; var dst U; copy(dst, src)`, `func(p.U, p.T) int`},
+	{"copy", `var dst []byte; copy(dst, "hello")`, `func([]byte, string) int`},
+
+	{"delete", `var m map[string]bool; delete(m, "foo")`, `func(map[string]bool, string)`},
+	{"delete", `type (K string; V int); var m map[K]V; delete(m, "foo")`, `func(map[p.K]p.V, p.K)`},
+
+	{"imag", `_ = imag(1i)`, `invalid type`}, // constant
+	{"imag", `var c complex64; _ = imag(c)`, `func(complex64) float32`},
+	{"imag", `var c complex128; _ = imag(c)`, `func(complex128) float64`},
+	{"imag", `type C64 complex64; var c C64; _ = imag(c)`, `func(p.C64) float32`},
+	{"imag", `type C128 complex128; var c C128; _ = imag(c)`, `func(p.C128) float64`},
+
+	{"real", `_ = real(1i)`, `invalid type`}, // constant
+	{"real", `var c complex64; _ = real(c)`, `func(complex64) float32`},
+	{"real", `var c complex128; _ = real(c)`, `func(complex128) float64`},
+	{"real", `type C64 complex64; var c C64; _ = real(c)`, `func(p.C64) float32`},
+	{"real", `type C128 complex128; var c C128; _ = real(c)`, `func(p.C128) float64`},
+
+	{"make", `_ = make([]int, 10)`, `func([]int, int) []int`},
+	{"make", `type T []byte; _ = make(T, 10, 20)`, `func(p.T, int, int) p.T`},
+
+	// issue #37349
+	{"make", `              _ = make([]int, 0   )`, `func([]int, int) []int`},
+	{"make", `var l    int; _ = make([]int, l   )`, `func([]int, int) []int`},
+	{"make", `              _ = make([]int, 0, 0)`, `func([]int, int, int) []int`},
+	{"make", `var l    int; _ = make([]int, l, 0)`, `func([]int, int, int) []int`},
+	{"make", `var    c int; _ = make([]int, 0, c)`, `func([]int, int, int) []int`},
+	{"make", `var l, c int; _ = make([]int, l, c)`, `func([]int, int, int) []int`},
+
+	// issue #37393
+	{"make", `                _ = make([]int       , 0   )`, `func([]int, int) []int`},
+	{"make", `var l    byte ; _ = make([]int8      , l   )`, `func([]int8, byte) []int8`},
+	{"make", `                _ = make([]int16     , 0, 0)`, `func([]int16, int, int) []int16`},
+	{"make", `var l    int16; _ = make([]string    , l, 0)`, `func([]string, int16, int) []string`},
+	{"make", `var    c int32; _ = make([]float64   , 0, c)`, `func([]float64, int, int32) []float64`},
+	{"make", `var l, c uint ; _ = make([]complex128, l, c)`, `func([]complex128, uint, uint) []complex128`},
+
+	{"new", `_ = new(int)`, `func(int) *int`},
+	{"new", `type T struct{}; _ = new(T)`, `func(p.T) *p.T`},
+
+	{"panic", `panic(0)`, `func(interface{})`},
+	{"panic", `panic("foo")`, `func(interface{})`},
+
+	{"print", `print()`, `func()`},
+	{"print", `print(0)`, `func(int)`},
+	{"print", `print(1, 2.0, "foo", true)`, `func(int, float64, string, bool)`},
+
+	{"println", `println()`, `func()`},
+	{"println", `println(0)`, `func(int)`},
+	{"println", `println(1, 2.0, "foo", true)`, `func(int, float64, string, bool)`},
+
+	{"recover", `recover()`, `func() interface{}`},
+	{"recover", `_ = recover()`, `func() interface{}`},
+
+	{"Alignof", `_ = unsafe.Alignof(0)`, `invalid type`},                 // constant
+	{"Alignof", `var x struct{}; _ = unsafe.Alignof(x)`, `invalid type`}, // constant
+
+	{"Offsetof", `var x struct{f bool}; _ = unsafe.Offsetof(x.f)`, `invalid type`},           // constant
+	{"Offsetof", `var x struct{_ int; f bool}; _ = unsafe.Offsetof((&x).f)`, `invalid type`}, // constant
+
+	{"Sizeof", `_ = unsafe.Sizeof(0)`, `invalid type`},                 // constant
+	{"Sizeof", `var x struct{}; _ = unsafe.Sizeof(x)`, `invalid type`}, // constant
+
+	{"assert", `assert(true)`, `invalid type`},                                    // constant
+	{"assert", `type B bool; const pred B = 1 < 2; assert(pred)`, `invalid type`}, // constant
+
+	// no tests for trace since it produces output as a side-effect
+}
+
+func TestBuiltinSignatures(t *testing.T) {
+	DefPredeclaredTestFuncs()
+
+	seen := map[string]bool{"trace": true} // no test for trace built-in; add it manually
+	for _, call := range builtinCalls {
+		testBuiltinSignature(t, call.name, call.src, call.sig)
+		seen[call.name] = true
+	}
+
+	// make sure we didn't miss one
+	for _, name := range Universe.Names() {
+		if _, ok := Universe.Lookup(name).(*Builtin); ok && !seen[name] {
+			t.Errorf("missing test for %s", name)
+		}
+	}
+	for _, name := range Unsafe.Scope().Names() {
+		if _, ok := Unsafe.Scope().Lookup(name).(*Builtin); ok && !seen[name] {
+			t.Errorf("missing test for unsafe.%s", name)
+		}
+	}
+}
+
+func testBuiltinSignature(t *testing.T, name, src0, want string) {
+	src := fmt.Sprintf(`package p; import "unsafe"; type _ unsafe.Pointer /* use unsafe */; func _() { %s }`, src0)
+	f, err := parser.ParseFile(fset, "", src, 0)
+	if err != nil {
+		t.Errorf("%s: %s", src0, err)
+		return
+	}
+
+	conf := Config{Importer: importer.Default()}
+	uses := make(map[*ast.Ident]Object)
+	types := make(map[ast.Expr]TypeAndValue)
+	_, err = conf.Check(f.Name.Name, fset, []*ast.File{f}, &Info{Uses: uses, Types: types})
+	if err != nil {
+		t.Errorf("%s: %s", src0, err)
+		return
+	}
+
+	// find called function
+	n := 0
+	var fun ast.Expr
+	for x := range types {
+		if call, _ := x.(*ast.CallExpr); call != nil {
+			fun = call.Fun
+			n++
+		}
+	}
+	if n != 1 {
+		t.Errorf("%s: got %d CallExprs; want 1", src0, n)
+		return
+	}
+
+	// check recorded types for fun and descendents (may be parenthesized)
+	for {
+		// the recorded type for the built-in must match the wanted signature
+		typ := types[fun].Type
+		if typ == nil {
+			t.Errorf("%s: no type recorded for %s", src0, ExprString(fun))
+			return
+		}
+		if got := typ.String(); got != want {
+			t.Errorf("%s: got type %s; want %s", src0, got, want)
+			return
+		}
+
+		// called function must be a (possibly parenthesized, qualified)
+		// identifier denoting the expected built-in
+		switch p := fun.(type) {
+		case *ast.Ident:
+			obj := uses[p]
+			if obj == nil {
+				t.Errorf("%s: no object found for %s", src0, p)
+				return
+			}
+			bin, _ := obj.(*Builtin)
+			if bin == nil {
+				t.Errorf("%s: %s does not denote a built-in", src0, p)
+				return
+			}
+			if bin.Name() != name {
+				t.Errorf("%s: got built-in %s; want %s", src0, bin.Name(), name)
+				return
+			}
+			return // we're done
+
+		case *ast.ParenExpr:
+			fun = p.X // unpack
+
+		case *ast.SelectorExpr:
+			// built-in from package unsafe - ignore details
+			return // we're done
+
+		default:
+			t.Errorf("%s: invalid function call", src0)
+			return
+		}
+	}
+}
diff --git a/src/go/types/call.go b/src/go/types/call.go
new file mode 100644
index 0000000..6765b17
--- /dev/null
+++ b/src/go/types/call.go
@@ -0,0 +1,569 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements typechecking of call and selector expressions.
+
+package types
+
+import (
+	"go/ast"
+	"go/token"
+	"strings"
+	"unicode"
+)
+
+func (check *Checker) call(x *operand, e *ast.CallExpr) exprKind {
+	check.exprOrType(x, e.Fun)
+
+	switch x.mode {
+	case invalid:
+		check.use(e.Args...)
+		x.mode = invalid
+		x.expr = e
+		return statement
+
+	case typexpr:
+		// conversion
+		T := x.typ
+		x.mode = invalid
+		switch n := len(e.Args); n {
+		case 0:
+			check.errorf(inNode(e, e.Rparen), _WrongArgCount, "missing argument in conversion to %s", T)
+		case 1:
+			check.expr(x, e.Args[0])
+			if x.mode != invalid {
+				if e.Ellipsis.IsValid() {
+					check.errorf(e.Args[0], _BadDotDotDotSyntax, "invalid use of ... in conversion to %s", T)
+					break
+				}
+				check.conversion(x, T)
+			}
+		default:
+			check.use(e.Args...)
+			check.errorf(e.Args[n-1], _WrongArgCount, "too many arguments in conversion to %s", T)
+		}
+		x.expr = e
+		return conversion
+
+	case builtin:
+		id := x.id
+		if !check.builtin(x, e, id) {
+			x.mode = invalid
+		}
+		x.expr = e
+		// a non-constant result implies a function call
+		if x.mode != invalid && x.mode != constant_ {
+			check.hasCallOrRecv = true
+		}
+		return predeclaredFuncs[id].kind
+
+	default:
+		// function/method call
+		cgocall := x.mode == cgofunc
+
+		sig, _ := x.typ.Underlying().(*Signature)
+		if sig == nil {
+			check.invalidOp(x, _InvalidCall, "cannot call non-function %s", x)
+			x.mode = invalid
+			x.expr = e
+			return statement
+		}
+
+		arg, n, _ := unpack(func(x *operand, i int) { check.multiExpr(x, e.Args[i]) }, len(e.Args), false)
+		if arg != nil {
+			check.arguments(x, e, sig, arg, n)
+		} else {
+			x.mode = invalid
+		}
+
+		// determine result
+		switch sig.results.Len() {
+		case 0:
+			x.mode = novalue
+		case 1:
+			if cgocall {
+				x.mode = commaerr
+			} else {
+				x.mode = value
+			}
+			x.typ = sig.results.vars[0].typ // unpack tuple
+		default:
+			x.mode = value
+			x.typ = sig.results
+		}
+
+		x.expr = e
+		check.hasCallOrRecv = true
+
+		return statement
+	}
+}
+
+// use type-checks each argument.
+// Useful to make sure expressions are evaluated
+// (and variables are "used") in the presence of other errors.
+// The arguments may be nil.
+func (check *Checker) use(arg ...ast.Expr) {
+	var x operand
+	for _, e := range arg {
+		// The nil check below is necessary since certain AST fields
+		// may legally be nil (e.g., the ast.SliceExpr.High field).
+		if e != nil {
+			check.rawExpr(&x, e, nil)
+		}
+	}
+}
+
+// useLHS is like use, but doesn't "use" top-level identifiers.
+// It should be called instead of use if the arguments are
+// expressions on the lhs of an assignment.
+// The arguments must not be nil.
+func (check *Checker) useLHS(arg ...ast.Expr) {
+	var x operand
+	for _, e := range arg {
+		// If the lhs is an identifier denoting a variable v, this assignment
+		// is not a 'use' of v. Remember current value of v.used and restore
+		// after evaluating the lhs via check.rawExpr.
+		var v *Var
+		var v_used bool
+		if ident, _ := unparen(e).(*ast.Ident); ident != nil {
+			// never type-check the blank name on the lhs
+			if ident.Name == "_" {
+				continue
+			}
+			if _, obj := check.scope.LookupParent(ident.Name, token.NoPos); obj != nil {
+				// It's ok to mark non-local variables, but ignore variables
+				// from other packages to avoid potential race conditions with
+				// dot-imported variables.
+				if w, _ := obj.(*Var); w != nil && w.pkg == check.pkg {
+					v = w
+					v_used = v.used
+				}
+			}
+		}
+		check.rawExpr(&x, e, nil)
+		if v != nil {
+			v.used = v_used // restore v.used
+		}
+	}
+}
+
+// useGetter is like use, but takes a getter instead of a list of expressions.
+// It should be called instead of use if a getter is present to avoid repeated
+// evaluation of the first argument (since the getter was likely obtained via
+// unpack, which may have evaluated the first argument already).
+func (check *Checker) useGetter(get getter, n int) {
+	var x operand
+	for i := 0; i < n; i++ {
+		get(&x, i)
+	}
+}
+
+// A getter sets x as the i'th operand, where 0 <= i < n and n is the total
+// number of operands (context-specific, and maintained elsewhere). A getter
+// type-checks the i'th operand; the details of the actual check are getter-
+// specific.
+type getter func(x *operand, i int)
+
+// unpack takes a getter get and a number of operands n. If n == 1, unpack
+// calls the incoming getter for the first operand. If that operand is
+// invalid, unpack returns (nil, 0, false). Otherwise, if that operand is a
+// function call, or a comma-ok expression and allowCommaOk is set, the result
+// is a new getter and operand count providing access to the function results,
+// or comma-ok values, respectively. The third result value reports if it
+// is indeed the comma-ok case. In all other cases, the incoming getter and
+// operand count are returned unchanged, and the third result value is false.
+//
+// In other words, if there's exactly one operand that - after type-checking
+// by calling get - stands for multiple operands, the resulting getter provides
+// access to those operands instead.
+//
+// If the returned getter is called at most once for a given operand index i
+// (including i == 0), that operand is guaranteed to cause only one call of
+// the incoming getter with that i.
+//
+func unpack(get getter, n int, allowCommaOk bool) (getter, int, bool) {
+	if n != 1 {
+		// zero or multiple values
+		return get, n, false
+	}
+	// possibly result of an n-valued function call or comma,ok value
+	var x0 operand
+	get(&x0, 0)
+	if x0.mode == invalid {
+		return nil, 0, false
+	}
+
+	if t, ok := x0.typ.(*Tuple); ok {
+		// result of an n-valued function call
+		return func(x *operand, i int) {
+			x.mode = value
+			x.expr = x0.expr
+			x.typ = t.At(i).typ
+		}, t.Len(), false
+	}
+
+	if x0.mode == mapindex || x0.mode == commaok || x0.mode == commaerr {
+		// comma-ok value
+		if allowCommaOk {
+			a := [2]Type{x0.typ, Typ[UntypedBool]}
+			if x0.mode == commaerr {
+				a[1] = universeError
+			}
+			return func(x *operand, i int) {
+				x.mode = value
+				x.expr = x0.expr
+				x.typ = a[i]
+			}, 2, true
+		}
+		x0.mode = value
+	}
+
+	// single value
+	return func(x *operand, i int) {
+		if i != 0 {
+			unreachable()
+		}
+		*x = x0
+	}, 1, false
+}
+
+// arguments checks argument passing for the call with the given signature.
+// The arg function provides the operand for the i'th argument.
+func (check *Checker) arguments(x *operand, call *ast.CallExpr, sig *Signature, arg getter, n int) {
+	if call.Ellipsis.IsValid() {
+		// last argument is of the form x...
+		if !sig.variadic {
+			check.errorf(atPos(call.Ellipsis), _NonVariadicDotDotDot, "cannot use ... in call to non-variadic %s", call.Fun)
+			check.useGetter(arg, n)
+			return
+		}
+		if len(call.Args) == 1 && n > 1 {
+			// f()... is not permitted if f() is multi-valued
+			check.errorf(atPos(call.Ellipsis), _InvalidDotDotDotOperand, "cannot use ... with %d-valued %s", n, call.Args[0])
+			check.useGetter(arg, n)
+			return
+		}
+	}
+
+	// evaluate arguments
+	context := check.sprintf("argument to %s", call.Fun)
+	for i := 0; i < n; i++ {
+		arg(x, i)
+		if x.mode != invalid {
+			var ellipsis token.Pos
+			if i == n-1 && call.Ellipsis.IsValid() {
+				ellipsis = call.Ellipsis
+			}
+			check.argument(sig, i, x, ellipsis, context)
+		}
+	}
+
+	// check argument count
+	if sig.variadic {
+		// a variadic function accepts an "empty"
+		// last argument: count one extra
+		n++
+	}
+	if n < sig.params.Len() {
+		check.errorf(inNode(call, call.Rparen), _WrongArgCount, "too few arguments in call to %s", call.Fun)
+		// ok to continue
+	}
+}
+
+// argument checks passing of argument x to the i'th parameter of the given signature.
+// If ellipsis is valid, the argument is followed by ... at that position in the call.
+func (check *Checker) argument(sig *Signature, i int, x *operand, ellipsis token.Pos, context string) {
+	check.singleValue(x)
+	if x.mode == invalid {
+		return
+	}
+
+	n := sig.params.Len()
+
+	// determine parameter type
+	var typ Type
+	switch {
+	case i < n:
+		typ = sig.params.vars[i].typ
+	case sig.variadic:
+		typ = sig.params.vars[n-1].typ
+		if debug {
+			if _, ok := typ.(*Slice); !ok {
+				check.dump("%v: expected unnamed slice type, got %s", sig.params.vars[n-1].Pos(), typ)
+			}
+		}
+	default:
+		check.errorf(x, _WrongArgCount, "too many arguments")
+		return
+	}
+
+	if ellipsis.IsValid() {
+		if i != n-1 {
+			check.errorf(atPos(ellipsis), _MisplacedDotDotDot, "can only use ... with matching parameter")
+			return
+		}
+		// argument is of the form x... and x is single-valued
+		if _, ok := x.typ.Underlying().(*Slice); !ok && x.typ != Typ[UntypedNil] { // see issue #18268
+			check.errorf(x, _InvalidDotDotDotOperand, "cannot use %s as parameter of type %s", x, typ)
+			return
+		}
+	} else if sig.variadic && i >= n-1 {
+		// use the variadic parameter slice's element type
+		typ = typ.(*Slice).elem
+	}
+
+	check.assignment(x, typ, context)
+}
+
+var cgoPrefixes = [...]string{
+	"_Ciconst_",
+	"_Cfconst_",
+	"_Csconst_",
+	"_Ctype_",
+	"_Cvar_", // actually a pointer to the var
+	"_Cfpvar_fp_",
+	"_Cfunc_",
+	"_Cmacro_", // function to evaluate the expanded expression
+}
+
+func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
+	// these must be declared before the "goto Error" statements
+	var (
+		obj      Object
+		index    []int
+		indirect bool
+	)
+
+	sel := e.Sel.Name
+	// If the identifier refers to a package, handle everything here
+	// so we don't need a "package" mode for operands: package names
+	// can only appear in qualified identifiers which are mapped to
+	// selector expressions.
+	if ident, ok := e.X.(*ast.Ident); ok {
+		obj := check.lookup(ident.Name)
+		if pname, _ := obj.(*PkgName); pname != nil {
+			assert(pname.pkg == check.pkg)
+			check.recordUse(ident, pname)
+			pname.used = true
+			pkg := pname.imported
+
+			var exp Object
+			funcMode := value
+			if pkg.cgo {
+				// cgo special cases C.malloc: it's
+				// rewritten to _CMalloc and does not
+				// support two-result calls.
+				if sel == "malloc" {
+					sel = "_CMalloc"
+				} else {
+					funcMode = cgofunc
+				}
+				for _, prefix := range cgoPrefixes {
+					// cgo objects are part of the current package (in file
+					// _cgo_gotypes.go). Use regular lookup.
+					_, exp = check.scope.LookupParent(prefix+sel, check.pos)
+					if exp != nil {
+						break
+					}
+				}
+				if exp == nil {
+					check.errorf(e.Sel, _UndeclaredImportedName, "%s not declared by package C", sel)
+					goto Error
+				}
+				check.objDecl(exp, nil)
+			} else {
+				exp = pkg.scope.Lookup(sel)
+				if exp == nil {
+					if !pkg.fake {
+						check.errorf(e.Sel, _UndeclaredImportedName, "%s not declared by package %s", sel, pkg.name)
+					}
+					goto Error
+				}
+				if !exp.Exported() {
+					check.errorf(e.Sel, _UnexportedName, "%s not exported by package %s", sel, pkg.name)
+					// ok to continue
+				}
+			}
+			check.recordUse(e.Sel, exp)
+
+			// Simplified version of the code for *ast.Idents:
+			// - imported objects are always fully initialized
+			switch exp := exp.(type) {
+			case *Const:
+				assert(exp.Val() != nil)
+				x.mode = constant_
+				x.typ = exp.typ
+				x.val = exp.val
+			case *TypeName:
+				x.mode = typexpr
+				x.typ = exp.typ
+			case *Var:
+				x.mode = variable
+				x.typ = exp.typ
+				if pkg.cgo && strings.HasPrefix(exp.name, "_Cvar_") {
+					x.typ = x.typ.(*Pointer).base
+				}
+			case *Func:
+				x.mode = funcMode
+				x.typ = exp.typ
+				if pkg.cgo && strings.HasPrefix(exp.name, "_Cmacro_") {
+					x.mode = value
+					x.typ = x.typ.(*Signature).results.vars[0].typ
+				}
+			case *Builtin:
+				x.mode = builtin
+				x.typ = exp.typ
+				x.id = exp.id
+			default:
+				check.dump("unexpected object %v", exp)
+				unreachable()
+			}
+			x.expr = e
+			return
+		}
+	}
+
+	check.exprOrType(x, e.X)
+	if x.mode == invalid {
+		goto Error
+	}
+
+	obj, index, indirect = check.lookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, sel)
+	if obj == nil {
+		switch {
+		case index != nil:
+			// TODO(gri) should provide actual type where the conflict happens
+			check.errorf(e.Sel, _AmbiguousSelector, "ambiguous selector %s.%s", x.expr, sel)
+		case indirect:
+			check.errorf(e.Sel, _InvalidMethodExpr, "cannot call pointer method %s on %s", sel, x.typ)
+		default:
+			// Check if capitalization of sel matters and provide better error
+			// message in that case.
+			if len(sel) > 0 {
+				var changeCase string
+				if r := rune(sel[0]); unicode.IsUpper(r) {
+					changeCase = string(unicode.ToLower(r)) + sel[1:]
+				} else {
+					changeCase = string(unicode.ToUpper(r)) + sel[1:]
+				}
+				if obj, _, _ = check.lookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, changeCase); obj != nil {
+					check.errorf(e.Sel, _MissingFieldOrMethod, "%s.%s undefined (type %s has no field or method %s, but does have %s)", x.expr, sel, x.typ, sel, changeCase)
+					break
+				}
+			}
+			check.errorf(e.Sel, _MissingFieldOrMethod, "%s.%s undefined (type %s has no field or method %s)", x.expr, sel, x.typ, sel)
+		}
+		goto Error
+	}
+
+	// methods may not have a fully set up signature yet
+	if m, _ := obj.(*Func); m != nil {
+		check.objDecl(m, nil)
+	}
+
+	if x.mode == typexpr {
+		// method expression
+		m, _ := obj.(*Func)
+		if m == nil {
+			// TODO(gri) should check if capitalization of sel matters and provide better error message in that case
+			check.errorf(e.Sel, _MissingFieldOrMethod, "%s.%s undefined (type %s has no method %s)", x.expr, sel, x.typ, sel)
+			goto Error
+		}
+
+		check.recordSelection(e, MethodExpr, x.typ, m, index, indirect)
+
+		// the receiver type becomes the type of the first function
+		// argument of the method expression's function type
+		var params []*Var
+		sig := m.typ.(*Signature)
+		if sig.params != nil {
+			params = sig.params.vars
+		}
+		x.mode = value
+		x.typ = &Signature{
+			params:   NewTuple(append([]*Var{NewVar(token.NoPos, check.pkg, "", x.typ)}, params...)...),
+			results:  sig.results,
+			variadic: sig.variadic,
+		}
+
+		check.addDeclDep(m)
+
+	} else {
+		// regular selector
+		switch obj := obj.(type) {
+		case *Var:
+			check.recordSelection(e, FieldVal, x.typ, obj, index, indirect)
+			if x.mode == variable || indirect {
+				x.mode = variable
+			} else {
+				x.mode = value
+			}
+			x.typ = obj.typ
+
+		case *Func:
+			// TODO(gri) If we needed to take into account the receiver's
+			// addressability, should we report the type &(x.typ) instead?
+			check.recordSelection(e, MethodVal, x.typ, obj, index, indirect)
+
+			if debug {
+				// Verify that LookupFieldOrMethod and MethodSet.Lookup agree.
+				// TODO(gri) This only works because we call LookupFieldOrMethod
+				// _before_ calling NewMethodSet: LookupFieldOrMethod completes
+				// any incomplete interfaces so they are available to NewMethodSet
+				// (which assumes that interfaces have been completed already).
+				typ := x.typ
+				if x.mode == variable {
+					// If typ is not an (unnamed) pointer or an interface,
+					// use *typ instead, because the method set of *typ
+					// includes the methods of typ.
+					// Variables are addressable, so we can always take their
+					// address.
+					if _, ok := typ.(*Pointer); !ok && !IsInterface(typ) {
+						typ = &Pointer{base: typ}
+					}
+				}
+				// If we created a synthetic pointer type above, we will throw
+				// away the method set computed here after use.
+				// TODO(gri) Method set computation should probably always compute
+				// both, the value and the pointer receiver method set and represent
+				// them in a single structure.
+				// TODO(gri) Consider also using a method set cache for the lifetime
+				// of checker once we rely on MethodSet lookup instead of individual
+				// lookup.
+				mset := NewMethodSet(typ)
+				if m := mset.Lookup(check.pkg, sel); m == nil || m.obj != obj {
+					check.dump("%v: (%s).%v -> %s", e.Pos(), typ, obj.name, m)
+					check.dump("%s\n", mset)
+					// Caution: MethodSets are supposed to be used externally
+					// only (after all interface types were completed). It's
+					// now possible that we get here incorrectly. Not urgent
+					// to fix since we only run this code in debug mode.
+					// TODO(gri) fix this eventually.
+					panic("method sets and lookup don't agree")
+				}
+			}
+
+			x.mode = value
+
+			// remove receiver
+			sig := *obj.typ.(*Signature)
+			sig.recv = nil
+			x.typ = &sig
+
+			check.addDeclDep(obj)
+
+		default:
+			unreachable()
+		}
+	}
+
+	// everything went well
+	x.expr = e
+	return
+
+Error:
+	x.mode = invalid
+	x.expr = e
+}
diff --git a/src/go/types/check.go b/src/go/types/check.go
new file mode 100644
index 0000000..280792e
--- /dev/null
+++ b/src/go/types/check.go
@@ -0,0 +1,417 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements the Check function, which drives type-checking.
+
+package types
+
+import (
+	"errors"
+	"go/ast"
+	"go/constant"
+	"go/token"
+)
+
+// debugging/development support
+const (
+	debug = false // leave on during development
+	trace = false // turn on for detailed type resolution traces
+)
+
+// If Strict is set, the type-checker enforces additional
+// rules not specified by the Go 1 spec, but which will
+// catch guaranteed run-time errors if the respective
+// code is executed. In other words, programs passing in
+// Strict mode are Go 1 compliant, but not all Go 1 programs
+// will pass in Strict mode. The additional rules are:
+//
+// - A type assertion x.(T) where T is an interface type
+//   is invalid if any (statically known) method that exists
+//   for both x and T have different signatures.
+//
+const strict = false
+
+// exprInfo stores information about an untyped expression.
+type exprInfo struct {
+	isLhs bool // expression is lhs operand of a shift with delayed type-check
+	mode  operandMode
+	typ   *Basic
+	val   constant.Value // constant value; or nil (if not a constant)
+}
+
+// A context represents the context within which an object is type-checked.
+type context struct {
+	decl          *declInfo              // package-level declaration whose init expression/function body is checked
+	scope         *Scope                 // top-most scope for lookups
+	pos           token.Pos              // if valid, identifiers are looked up as if at position pos (used by Eval)
+	iota          constant.Value         // value of iota in a constant declaration; nil otherwise
+	errpos        positioner             // if set, identifier position of a constant with inherited initializer
+	sig           *Signature             // function signature if inside a function; nil otherwise
+	isPanic       map[*ast.CallExpr]bool // set of panic call expressions (used for termination check)
+	hasLabel      bool                   // set if a function makes use of labels (only ~1% of functions); unused outside functions
+	hasCallOrRecv bool                   // set if an expression contains a function call or channel receive operation
+}
+
+// lookup looks up name in the current context and returns the matching object, or nil.
+func (ctxt *context) lookup(name string) Object {
+	_, obj := ctxt.scope.LookupParent(name, ctxt.pos)
+	return obj
+}
+
+// An importKey identifies an imported package by import path and source directory
+// (directory containing the file containing the import). In practice, the directory
+// may always be the same, or may not matter. Given an (import path, directory), an
+// importer must always return the same package (but given two different import paths,
+// an importer may still return the same package by mapping them to the same package
+// paths).
+type importKey struct {
+	path, dir string
+}
+
+// A Checker maintains the state of the type checker.
+// It must be created with NewChecker.
+type Checker struct {
+	// package information
+	// (initialized by NewChecker, valid for the life-time of checker)
+	conf *Config
+	fset *token.FileSet
+	pkg  *Package
+	*Info
+	objMap map[Object]*declInfo       // maps package-level objects and (non-interface) methods to declaration info
+	impMap map[importKey]*Package     // maps (import path, source directory) to (complete or fake) package
+	posMap map[*Interface][]token.Pos // maps interface types to lists of embedded interface positions
+	pkgCnt map[string]int             // counts number of imported packages with a given name (for better error messages)
+
+	// information collected during type-checking of a set of package files
+	// (initialized by Files, valid only for the duration of check.Files;
+	// maps and lists are allocated on demand)
+	files            []*ast.File                             // package files
+	unusedDotImports map[*Scope]map[*Package]*ast.ImportSpec // unused dot-imported packages
+
+	firstErr error                 // first error encountered
+	methods  map[*TypeName][]*Func // maps package scope type names to associated non-blank (non-interface) methods
+	untyped  map[ast.Expr]exprInfo // map of expressions without final type
+	delayed  []func()              // stack of delayed action segments; segments are processed in FIFO order
+	finals   []func()              // list of final actions; processed at the end of type-checking the current set of files
+	objPath  []Object              // path of object dependencies during type inference (for cycle reporting)
+
+	// context within which the current object is type-checked
+	// (valid only for the duration of type-checking a specific object)
+	context
+
+	// debugging
+	indent int // indentation for tracing
+}
+
+// addUnusedImport adds the position of a dot-imported package
+// pkg to the map of dot imports for the given file scope.
+func (check *Checker) addUnusedDotImport(scope *Scope, pkg *Package, spec *ast.ImportSpec) {
+	mm := check.unusedDotImports
+	if mm == nil {
+		mm = make(map[*Scope]map[*Package]*ast.ImportSpec)
+		check.unusedDotImports = mm
+	}
+	m := mm[scope]
+	if m == nil {
+		m = make(map[*Package]*ast.ImportSpec)
+		mm[scope] = m
+	}
+	m[pkg] = spec
+}
+
+// addDeclDep adds the dependency edge (check.decl -> to) if check.decl exists
+func (check *Checker) addDeclDep(to Object) {
+	from := check.decl
+	if from == nil {
+		return // not in a package-level init expression
+	}
+	if _, found := check.objMap[to]; !found {
+		return // to is not a package-level object
+	}
+	from.addDep(to)
+}
+
+func (check *Checker) rememberUntyped(e ast.Expr, lhs bool, mode operandMode, typ *Basic, val constant.Value) {
+	m := check.untyped
+	if m == nil {
+		m = make(map[ast.Expr]exprInfo)
+		check.untyped = m
+	}
+	m[e] = exprInfo{lhs, mode, typ, val}
+}
+
+// later pushes f on to the stack of actions that will be processed later;
+// either at the end of the current statement, or in case of a local constant
+// or variable declaration, before the constant or variable is in scope
+// (so that f still sees the scope before any new declarations).
+func (check *Checker) later(f func()) {
+	check.delayed = append(check.delayed, f)
+}
+
+// atEnd adds f to the list of actions processed at the end
+// of type-checking, before initialization order computation.
+// Actions added by atEnd are processed after any actions
+// added by later.
+func (check *Checker) atEnd(f func()) {
+	check.finals = append(check.finals, f)
+}
+
+// push pushes obj onto the object path and returns its index in the path.
+func (check *Checker) push(obj Object) int {
+	check.objPath = append(check.objPath, obj)
+	return len(check.objPath) - 1
+}
+
+// pop pops and returns the topmost object from the object path.
+func (check *Checker) pop() Object {
+	i := len(check.objPath) - 1
+	obj := check.objPath[i]
+	check.objPath[i] = nil
+	check.objPath = check.objPath[:i]
+	return obj
+}
+
+// NewChecker returns a new Checker instance for a given package.
+// Package files may be added incrementally via checker.Files.
+func NewChecker(conf *Config, fset *token.FileSet, pkg *Package, info *Info) *Checker {
+	// make sure we have a configuration
+	if conf == nil {
+		conf = new(Config)
+	}
+
+	// make sure we have an info struct
+	if info == nil {
+		info = new(Info)
+	}
+
+	return &Checker{
+		conf:   conf,
+		fset:   fset,
+		pkg:    pkg,
+		Info:   info,
+		objMap: make(map[Object]*declInfo),
+		impMap: make(map[importKey]*Package),
+		posMap: make(map[*Interface][]token.Pos),
+		pkgCnt: make(map[string]int),
+	}
+}
+
+// initFiles initializes the files-specific portion of checker.
+// The provided files must all belong to the same package.
+func (check *Checker) initFiles(files []*ast.File) {
+	// start with a clean slate (check.Files may be called multiple times)
+	check.files = nil
+	check.unusedDotImports = nil
+
+	check.firstErr = nil
+	check.methods = nil
+	check.untyped = nil
+	check.delayed = nil
+	check.finals = nil
+
+	// determine package name and collect valid files
+	pkg := check.pkg
+	for _, file := range files {
+		switch name := file.Name.Name; pkg.name {
+		case "":
+			if name != "_" {
+				pkg.name = name
+			} else {
+				check.errorf(file.Name, _BlankPkgName, "invalid package name _")
+			}
+			fallthrough
+
+		case name:
+			check.files = append(check.files, file)
+
+		default:
+			check.errorf(atPos(file.Package), _MismatchedPkgName, "package %s; expected %s", name, pkg.name)
+			// ignore this file
+		}
+	}
+}
+
+// A bailout panic is used for early termination.
+type bailout struct{}
+
+func (check *Checker) handleBailout(err *error) {
+	switch p := recover().(type) {
+	case nil, bailout:
+		// normal return or early exit
+		*err = check.firstErr
+	default:
+		// re-panic
+		panic(p)
+	}
+}
+
+// Files checks the provided files as part of the checker's package.
+func (check *Checker) Files(files []*ast.File) error { return check.checkFiles(files) }
+
+var errBadCgo = errors.New("cannot use FakeImportC and go115UsesCgo together")
+
+func (check *Checker) checkFiles(files []*ast.File) (err error) {
+	if check.conf.FakeImportC && check.conf.go115UsesCgo {
+		return errBadCgo
+	}
+
+	defer check.handleBailout(&err)
+
+	check.initFiles(files)
+
+	check.collectObjects()
+
+	check.packageObjects()
+
+	check.processDelayed(0) // incl. all functions
+	check.processFinals()
+
+	check.initOrder()
+
+	if !check.conf.DisableUnusedImportCheck {
+		check.unusedImports()
+	}
+
+	check.recordUntyped()
+
+	check.pkg.complete = true
+	return
+}
+
+// processDelayed processes all delayed actions pushed after top.
+func (check *Checker) processDelayed(top int) {
+	// If each delayed action pushes a new action, the
+	// stack will continue to grow during this loop.
+	// However, it is only processing functions (which
+	// are processed in a delayed fashion) that may
+	// add more actions (such as nested functions), so
+	// this is a sufficiently bounded process.
+	for i := top; i < len(check.delayed); i++ {
+		check.delayed[i]() // may append to check.delayed
+	}
+	assert(top <= len(check.delayed)) // stack must not have shrunk
+	check.delayed = check.delayed[:top]
+}
+
+func (check *Checker) processFinals() {
+	n := len(check.finals)
+	for _, f := range check.finals {
+		f() // must not append to check.finals
+	}
+	if len(check.finals) != n {
+		panic("internal error: final action list grew")
+	}
+}
+
+func (check *Checker) recordUntyped() {
+	if !debug && check.Types == nil {
+		return // nothing to do
+	}
+
+	for x, info := range check.untyped {
+		if debug && isTyped(info.typ) {
+			check.dump("%v: %s (type %s) is typed", x.Pos(), x, info.typ)
+			unreachable()
+		}
+		check.recordTypeAndValue(x, info.mode, info.typ, info.val)
+	}
+}
+
+func (check *Checker) recordTypeAndValue(x ast.Expr, mode operandMode, typ Type, val constant.Value) {
+	assert(x != nil)
+	assert(typ != nil)
+	if mode == invalid {
+		return // omit
+	}
+	if mode == constant_ {
+		assert(val != nil)
+		assert(typ == Typ[Invalid] || isConstType(typ))
+	}
+	if m := check.Types; m != nil {
+		m[x] = TypeAndValue{mode, typ, val}
+	}
+}
+
+func (check *Checker) recordBuiltinType(f ast.Expr, sig *Signature) {
+	// f must be a (possibly parenthesized) identifier denoting a built-in
+	// (built-ins in package unsafe always produce a constant result and
+	// we don't record their signatures, so we don't see qualified idents
+	// here): record the signature for f and possible children.
+	for {
+		check.recordTypeAndValue(f, builtin, sig, nil)
+		switch p := f.(type) {
+		case *ast.Ident:
+			return // we're done
+		case *ast.ParenExpr:
+			f = p.X
+		default:
+			unreachable()
+		}
+	}
+}
+
+func (check *Checker) recordCommaOkTypes(x ast.Expr, a [2]Type) {
+	assert(x != nil)
+	if a[0] == nil || a[1] == nil {
+		return
+	}
+	assert(isTyped(a[0]) && isTyped(a[1]) && (isBoolean(a[1]) || a[1] == universeError))
+	if m := check.Types; m != nil {
+		for {
+			tv := m[x]
+			assert(tv.Type != nil) // should have been recorded already
+			pos := x.Pos()
+			tv.Type = NewTuple(
+				NewVar(pos, check.pkg, "", a[0]),
+				NewVar(pos, check.pkg, "", a[1]),
+			)
+			m[x] = tv
+			// if x is a parenthesized expression (p.X), update p.X
+			p, _ := x.(*ast.ParenExpr)
+			if p == nil {
+				break
+			}
+			x = p.X
+		}
+	}
+}
+
+func (check *Checker) recordDef(id *ast.Ident, obj Object) {
+	assert(id != nil)
+	if m := check.Defs; m != nil {
+		m[id] = obj
+	}
+}
+
+func (check *Checker) recordUse(id *ast.Ident, obj Object) {
+	assert(id != nil)
+	assert(obj != nil)
+	if m := check.Uses; m != nil {
+		m[id] = obj
+	}
+}
+
+func (check *Checker) recordImplicit(node ast.Node, obj Object) {
+	assert(node != nil)
+	assert(obj != nil)
+	if m := check.Implicits; m != nil {
+		m[node] = obj
+	}
+}
+
+func (check *Checker) recordSelection(x *ast.SelectorExpr, kind SelectionKind, recv Type, obj Object, index []int, indirect bool) {
+	assert(obj != nil && (recv == nil || len(index) > 0))
+	check.recordUse(x.Sel, obj)
+	if m := check.Selections; m != nil {
+		m[x] = &Selection{kind, recv, obj, index, indirect}
+	}
+}
+
+func (check *Checker) recordScope(node ast.Node, scope *Scope) {
+	assert(node != nil)
+	assert(scope != nil)
+	if m := check.Scopes; m != nil {
+		m[node] = scope
+	}
+}
diff --git a/src/go/types/check_test.go b/src/go/types/check_test.go
new file mode 100644
index 0000000..ce31dab
--- /dev/null
+++ b/src/go/types/check_test.go
@@ -0,0 +1,349 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements a typechecker test harness. The packages specified
+// in tests are typechecked. Error messages reported by the typechecker are
+// compared against the error messages expected in the test files.
+//
+// Expected errors are indicated in the test files by putting a comment
+// of the form /* ERROR "rx" */ immediately following an offending token.
+// The harness will verify that an error matching the regular expression
+// rx is reported at that source position. Consecutive comments may be
+// used to indicate multiple errors for the same token position.
+//
+// For instance, the following test file indicates that a "not declared"
+// error should be reported for the undeclared variable x:
+//
+//	package p
+//	func f() {
+//		_ = x /* ERROR "not declared" */ + 1
+//	}
+
+// TODO(gri) Also collect strict mode errors of the form /* STRICT ... */
+//           and test against strict mode.
+
+package types_test
+
+import (
+	"flag"
+	"go/ast"
+	"go/importer"
+	"go/parser"
+	"go/scanner"
+	"go/token"
+	"internal/testenv"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+
+	. "go/types"
+)
+
+var (
+	haltOnError = flag.Bool("halt", false, "halt on error")
+	listErrors  = flag.Bool("errlist", false, "list errors")
+	testFiles   = flag.String("files", "", "space-separated list of test files")
+)
+
+// The test filenames do not end in .go so that they are invisible
+// to gofmt since they contain comments that must not change their
+// positions relative to surrounding tokens.
+
+// Each tests entry is list of files belonging to the same package.
+var tests = [][]string{
+	{"testdata/errors.src"},
+	{"testdata/importdecl0a.src", "testdata/importdecl0b.src"},
+	{"testdata/importdecl1a.src", "testdata/importdecl1b.src"},
+	{"testdata/importC.src"}, // special handling in checkFiles
+	{"testdata/cycles.src"},
+	{"testdata/cycles1.src"},
+	{"testdata/cycles2.src"},
+	{"testdata/cycles3.src"},
+	{"testdata/cycles4.src"},
+	{"testdata/cycles5.src"},
+	{"testdata/init0.src"},
+	{"testdata/init1.src"},
+	{"testdata/init2.src"},
+	{"testdata/decls0.src"},
+	{"testdata/decls1.src"},
+	{"testdata/decls2a.src", "testdata/decls2b.src"},
+	{"testdata/decls3.src"},
+	{"testdata/decls4.src"},
+	{"testdata/decls5.src"},
+	{"testdata/const0.src"},
+	{"testdata/const1.src"},
+	{"testdata/constdecl.src"},
+	{"testdata/vardecl.src"},
+	{"testdata/expr0.src"},
+	{"testdata/expr1.src"},
+	{"testdata/expr2.src"},
+	{"testdata/expr3.src"},
+	{"testdata/methodsets.src"},
+	{"testdata/shifts.src"},
+	{"testdata/builtins.src"},
+	{"testdata/conversions.src"},
+	{"testdata/conversions2.src"},
+	{"testdata/stmt0.src"},
+	{"testdata/stmt1.src"},
+	{"testdata/gotos.src"},
+	{"testdata/labels.src"},
+	{"testdata/literals.src"},
+	{"testdata/issues.src"},
+	{"testdata/blank.src"},
+	{"testdata/issue25008b.src", "testdata/issue25008a.src"}, // order (b before a) is crucial!
+}
+
+var fset = token.NewFileSet()
+
+// Positioned errors are of the form filename:line:column: message .
+var posMsgRx = regexp.MustCompile(`^(.*:[0-9]+:[0-9]+): *(.*)`)
+
+// splitError splits an error's error message into a position string
+// and the actual error message. If there's no position information,
+// pos is the empty string, and msg is the entire error message.
+//
+func splitError(err error) (pos, msg string) {
+	msg = err.Error()
+	if m := posMsgRx.FindStringSubmatch(msg); len(m) == 3 {
+		pos = m[1]
+		msg = m[2]
+	}
+	return
+}
+
+func parseFiles(t *testing.T, filenames []string) ([]*ast.File, []error) {
+	var files []*ast.File
+	var errlist []error
+	for _, filename := range filenames {
+		file, err := parser.ParseFile(fset, filename, nil, parser.AllErrors)
+		if file == nil {
+			t.Fatalf("%s: %s", filename, err)
+		}
+		files = append(files, file)
+		if err != nil {
+			if list, _ := err.(scanner.ErrorList); len(list) > 0 {
+				for _, err := range list {
+					errlist = append(errlist, err)
+				}
+			} else {
+				errlist = append(errlist, err)
+			}
+		}
+	}
+	return files, errlist
+}
+
+// ERROR comments must start with text `ERROR "rx"` or `ERROR rx` where
+// rx is a regular expression that matches the expected error message.
+// Space around "rx" or rx is ignored. Use the form `ERROR HERE "rx"`
+// for error messages that are located immediately after rather than
+// at a token's position.
+//
+var errRx = regexp.MustCompile(`^ *ERROR *(HERE)? *"?([^"]*)"?`)
+
+// errMap collects the regular expressions of ERROR comments found
+// in files and returns them as a map of error positions to error messages.
+//
+func errMap(t *testing.T, testname string, files []*ast.File) map[string][]string {
+	// map of position strings to lists of error message patterns
+	errmap := make(map[string][]string)
+
+	for _, file := range files {
+		filename := fset.Position(file.Package).Filename
+		src, err := os.ReadFile(filename)
+		if err != nil {
+			t.Fatalf("%s: could not read %s", testname, filename)
+		}
+
+		var s scanner.Scanner
+		s.Init(fset.AddFile(filename, -1, len(src)), src, nil, scanner.ScanComments)
+		var prev token.Pos // position of last non-comment, non-semicolon token
+		var here token.Pos // position immediately after the token at position prev
+
+	scanFile:
+		for {
+			pos, tok, lit := s.Scan()
+			switch tok {
+			case token.EOF:
+				break scanFile
+			case token.COMMENT:
+				if lit[1] == '*' {
+					lit = lit[:len(lit)-2] // strip trailing */
+				}
+				if s := errRx.FindStringSubmatch(lit[2:]); len(s) == 3 {
+					pos := prev
+					if s[1] == "HERE" {
+						pos = here
+					}
+					p := fset.Position(pos).String()
+					errmap[p] = append(errmap[p], strings.TrimSpace(s[2]))
+				}
+			case token.SEMICOLON:
+				// ignore automatically inserted semicolon
+				if lit == "\n" {
+					continue scanFile
+				}
+				fallthrough
+			default:
+				prev = pos
+				var l int // token length
+				if tok.IsLiteral() {
+					l = len(lit)
+				} else {
+					l = len(tok.String())
+				}
+				here = prev + token.Pos(l)
+			}
+		}
+	}
+
+	return errmap
+}
+
+func eliminate(t *testing.T, errmap map[string][]string, errlist []error) {
+	for _, err := range errlist {
+		pos, gotMsg := splitError(err)
+		list := errmap[pos]
+		index := -1 // list index of matching message, if any
+		// we expect one of the messages in list to match the error at pos
+		for i, wantRx := range list {
+			rx, err := regexp.Compile(wantRx)
+			if err != nil {
+				t.Errorf("%s: %v", pos, err)
+				continue
+			}
+			if rx.MatchString(gotMsg) {
+				index = i
+				break
+			}
+		}
+		if index >= 0 {
+			// eliminate from list
+			if n := len(list) - 1; n > 0 {
+				// not the last entry - swap in last element and shorten list by 1
+				list[index] = list[n]
+				errmap[pos] = list[:n]
+			} else {
+				// last entry - remove list from map
+				delete(errmap, pos)
+			}
+		} else {
+			t.Errorf("%s: no error expected: %q", pos, gotMsg)
+		}
+	}
+}
+
+func checkFiles(t *testing.T, testfiles []string) {
+	// parse files and collect parser errors
+	files, errlist := parseFiles(t, testfiles)
+
+	pkgName := "<no package>"
+	if len(files) > 0 {
+		pkgName = files[0].Name.Name
+	}
+
+	if *listErrors && len(errlist) > 0 {
+		t.Errorf("--- %s:", pkgName)
+		for _, err := range errlist {
+			t.Error(err)
+		}
+	}
+
+	// typecheck and collect typechecker errors
+	var conf Config
+	// special case for importC.src
+	if len(testfiles) == 1 && strings.HasSuffix(testfiles[0], "importC.src") {
+		conf.FakeImportC = true
+	}
+	conf.Importer = importer.Default()
+	conf.Error = func(err error) {
+		if *haltOnError {
+			defer panic(err)
+		}
+		if *listErrors {
+			t.Error(err)
+			return
+		}
+		// Ignore secondary error messages starting with "\t";
+		// they are clarifying messages for a primary error.
+		if !strings.Contains(err.Error(), ": \t") {
+			errlist = append(errlist, err)
+		}
+	}
+	conf.Check(pkgName, fset, files, nil)
+
+	if *listErrors {
+		return
+	}
+
+	for _, err := range errlist {
+		err, ok := err.(Error)
+		if !ok {
+			continue
+		}
+		code := readCode(err)
+		if code == 0 {
+			t.Errorf("missing error code: %v", err)
+		}
+	}
+
+	// match and eliminate errors;
+	// we are expecting the following errors
+	errmap := errMap(t, pkgName, files)
+	eliminate(t, errmap, errlist)
+
+	// there should be no expected errors left
+	if len(errmap) > 0 {
+		t.Errorf("--- %s: %d source positions with expected (but not reported) errors:", pkgName, len(errmap))
+		for pos, list := range errmap {
+			for _, rx := range list {
+				t.Errorf("%s: %q", pos, rx)
+			}
+		}
+	}
+}
+
+func TestCheck(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	// Declare builtins for testing.
+	DefPredeclaredTestFuncs()
+
+	// If explicit test files are specified, only check those.
+	if files := *testFiles; files != "" {
+		checkFiles(t, strings.Split(files, " "))
+		return
+	}
+
+	// Otherwise, run all the tests.
+	for _, files := range tests {
+		checkFiles(t, files)
+	}
+}
+
+func TestFixedBugs(t *testing.T) { testDir(t, "fixedbugs") }
+
+func testDir(t *testing.T, dir string) {
+	testenv.MustHaveGoBuild(t)
+
+	dirs, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, d := range dirs {
+		testname := filepath.Base(d.Name())
+		testname = strings.TrimSuffix(testname, filepath.Ext(testname))
+		t.Run(testname, func(t *testing.T) {
+			filename := filepath.Join(dir, d.Name())
+			if d.IsDir() {
+				t.Errorf("skipped directory %q", filename)
+				return
+			}
+			checkFiles(t, []string{filename})
+		})
+	}
+}
diff --git a/src/go/types/conversions.go b/src/go/types/conversions.go
new file mode 100644
index 0000000..1cab1cc
--- /dev/null
+++ b/src/go/types/conversions.go
@@ -0,0 +1,163 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements typechecking of conversions.
+
+package types
+
+import (
+	"go/constant"
+	"unicode"
+)
+
+// Conversion type-checks the conversion T(x).
+// The result is in x.
+func (check *Checker) conversion(x *operand, T Type) {
+	constArg := x.mode == constant_
+
+	var ok bool
+	switch {
+	case constArg && isConstType(T):
+		// constant conversion
+		switch t := T.Underlying().(*Basic); {
+		case representableConst(x.val, check, t, &x.val):
+			ok = true
+		case isInteger(x.typ) && isString(t):
+			codepoint := unicode.ReplacementChar
+			if i, ok := constant.Uint64Val(x.val); ok && i <= unicode.MaxRune {
+				codepoint = rune(i)
+			}
+			x.val = constant.MakeString(string(codepoint))
+			ok = true
+		}
+	case x.convertibleTo(check, T):
+		// non-constant conversion
+		x.mode = value
+		ok = true
+	}
+
+	if !ok {
+		check.errorf(x, _InvalidConversion, "cannot convert %s to %s", x, T)
+		x.mode = invalid
+		return
+	}
+
+	// The conversion argument types are final. For untyped values the
+	// conversion provides the type, per the spec: "A constant may be
+	// given a type explicitly by a constant declaration or conversion,...".
+	if isUntyped(x.typ) {
+		final := T
+		// - For conversions to interfaces, use the argument's default type.
+		// - For conversions of untyped constants to non-constant types, also
+		//   use the default type (e.g., []byte("foo") should report string
+		//   not []byte as type for the constant "foo").
+		// - Keep untyped nil for untyped nil arguments.
+		// - For integer to string conversions, keep the argument type.
+		//   (See also the TODO below.)
+		if IsInterface(T) || constArg && !isConstType(T) {
+			final = Default(x.typ)
+		} else if isInteger(x.typ) && isString(T) {
+			final = x.typ
+		}
+		check.updateExprType(x.expr, final, true)
+	}
+
+	x.typ = T
+}
+
+// TODO(gri) convertibleTo checks if T(x) is valid. It assumes that the type
+// of x is fully known, but that's not the case for say string(1<<s + 1.0):
+// Here, the type of 1<<s + 1.0 will be UntypedFloat which will lead to the
+// (correct!) refusal of the conversion. But the reported error is essentially
+// "cannot convert untyped float value to string", yet the correct error (per
+// the spec) is that we cannot shift a floating-point value: 1 in 1<<s should
+// be converted to UntypedFloat because of the addition of 1.0. Fixing this
+// is tricky because we'd have to run updateExprType on the argument first.
+// (Issue #21982.)
+
+// convertibleTo reports whether T(x) is valid.
+// The check parameter may be nil if convertibleTo is invoked through an
+// exported API call, i.e., when all methods have been type-checked.
+func (x *operand) convertibleTo(check *Checker, T Type) bool {
+	// "x is assignable to T"
+	if ok, _ := x.assignableTo(check, T, nil); ok {
+		return true
+	}
+
+	// "x's type and T have identical underlying types if tags are ignored"
+	V := x.typ
+	Vu := V.Underlying()
+	Tu := T.Underlying()
+	if check.identicalIgnoreTags(Vu, Tu) {
+		return true
+	}
+
+	// "x's type and T are unnamed pointer types and their pointer base types
+	// have identical underlying types if tags are ignored"
+	if V, ok := V.(*Pointer); ok {
+		if T, ok := T.(*Pointer); ok {
+			if check.identicalIgnoreTags(V.base.Underlying(), T.base.Underlying()) {
+				return true
+			}
+		}
+	}
+
+	// "x's type and T are both integer or floating point types"
+	if (isInteger(V) || isFloat(V)) && (isInteger(T) || isFloat(T)) {
+		return true
+	}
+
+	// "x's type and T are both complex types"
+	if isComplex(V) && isComplex(T) {
+		return true
+	}
+
+	// "x is an integer or a slice of bytes or runes and T is a string type"
+	if (isInteger(V) || isBytesOrRunes(Vu)) && isString(T) {
+		return true
+	}
+
+	// "x is a string and T is a slice of bytes or runes"
+	if isString(V) && isBytesOrRunes(Tu) {
+		return true
+	}
+
+	// package unsafe:
+	// "any pointer or value of underlying type uintptr can be converted into a unsafe.Pointer"
+	if (isPointer(Vu) || isUintptr(Vu)) && isUnsafePointer(T) {
+		return true
+	}
+	// "and vice versa"
+	if isUnsafePointer(V) && (isPointer(Tu) || isUintptr(Tu)) {
+		return true
+	}
+
+	return false
+}
+
+func isUintptr(typ Type) bool {
+	t, ok := typ.Underlying().(*Basic)
+	return ok && t.kind == Uintptr
+}
+
+func isUnsafePointer(typ Type) bool {
+	// TODO(gri): Is this (typ.Underlying() instead of just typ) correct?
+	//            The spec does not say so, but gc claims it is. See also
+	//            issue 6326.
+	t, ok := typ.Underlying().(*Basic)
+	return ok && t.kind == UnsafePointer
+}
+
+func isPointer(typ Type) bool {
+	_, ok := typ.Underlying().(*Pointer)
+	return ok
+}
+
+func isBytesOrRunes(typ Type) bool {
+	if s, ok := typ.(*Slice); ok {
+		t, ok := s.elem.Underlying().(*Basic)
+		return ok && (t.kind == Byte || t.kind == Rune)
+	}
+	return false
+}
diff --git a/src/go/types/decl.go b/src/go/types/decl.go
new file mode 100644
index 0000000..1f0bc35
--- /dev/null
+++ b/src/go/types/decl.go
@@ -0,0 +1,855 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types
+
+import (
+	"go/ast"
+	"go/constant"
+	"go/token"
+)
+
+func (check *Checker) reportAltDecl(obj Object) {
+	if pos := obj.Pos(); pos.IsValid() {
+		// We use "other" rather than "previous" here because
+		// the first declaration seen may not be textually
+		// earlier in the source.
+		check.errorf(obj, _DuplicateDecl, "\tother declaration of %s", obj.Name()) // secondary error, \t indented
+	}
+}
+
+func (check *Checker) declare(scope *Scope, id *ast.Ident, obj Object, pos token.Pos) {
+	// spec: "The blank identifier, represented by the underscore
+	// character _, may be used in a declaration like any other
+	// identifier but the declaration does not introduce a new
+	// binding."
+	if obj.Name() != "_" {
+		if alt := scope.Insert(obj); alt != nil {
+			check.errorf(obj, _DuplicateDecl, "%s redeclared in this block", obj.Name())
+			check.reportAltDecl(alt)
+			return
+		}
+		obj.setScopePos(pos)
+	}
+	if id != nil {
+		check.recordDef(id, obj)
+	}
+}
+
+// pathString returns a string of the form a->b-> ... ->g for a path [a, b, ... g].
+func pathString(path []Object) string {
+	var s string
+	for i, p := range path {
+		if i > 0 {
+			s += "->"
+		}
+		s += p.Name()
+	}
+	return s
+}
+
+// objDecl type-checks the declaration of obj in its respective (file) context.
+// For the meaning of def, see Checker.definedType, in typexpr.go.
+func (check *Checker) objDecl(obj Object, def *Named) {
+	if trace {
+		check.trace(obj.Pos(), "-- checking %s (%s, objPath = %s)", obj, obj.color(), pathString(check.objPath))
+		check.indent++
+		defer func() {
+			check.indent--
+			check.trace(obj.Pos(), "=> %s (%s)", obj, obj.color())
+		}()
+	}
+
+	// Checking the declaration of obj means inferring its type
+	// (and possibly its value, for constants).
+	// An object's type (and thus the object) may be in one of
+	// three states which are expressed by colors:
+	//
+	// - an object whose type is not yet known is painted white (initial color)
+	// - an object whose type is in the process of being inferred is painted grey
+	// - an object whose type is fully inferred is painted black
+	//
+	// During type inference, an object's color changes from white to grey
+	// to black (pre-declared objects are painted black from the start).
+	// A black object (i.e., its type) can only depend on (refer to) other black
+	// ones. White and grey objects may depend on white and black objects.
+	// A dependency on a grey object indicates a cycle which may or may not be
+	// valid.
+	//
+	// When objects turn grey, they are pushed on the object path (a stack);
+	// they are popped again when they turn black. Thus, if a grey object (a
+	// cycle) is encountered, it is on the object path, and all the objects
+	// it depends on are the remaining objects on that path. Color encoding
+	// is such that the color value of a grey object indicates the index of
+	// that object in the object path.
+
+	// During type-checking, white objects may be assigned a type without
+	// traversing through objDecl; e.g., when initializing constants and
+	// variables. Update the colors of those objects here (rather than
+	// everywhere where we set the type) to satisfy the color invariants.
+	if obj.color() == white && obj.Type() != nil {
+		obj.setColor(black)
+		return
+	}
+
+	switch obj.color() {
+	case white:
+		assert(obj.Type() == nil)
+		// All color values other than white and black are considered grey.
+		// Because black and white are < grey, all values >= grey are grey.
+		// Use those values to encode the object's index into the object path.
+		obj.setColor(grey + color(check.push(obj)))
+		defer func() {
+			check.pop().setColor(black)
+		}()
+
+	case black:
+		assert(obj.Type() != nil)
+		return
+
+	default:
+		// Color values other than white or black are considered grey.
+		fallthrough
+
+	case grey:
+		// We have a cycle.
+		// In the existing code, this is marked by a non-nil type
+		// for the object except for constants and variables whose
+		// type may be non-nil (known), or nil if it depends on the
+		// not-yet known initialization value.
+		// In the former case, set the type to Typ[Invalid] because
+		// we have an initialization cycle. The cycle error will be
+		// reported later, when determining initialization order.
+		// TODO(gri) Report cycle here and simplify initialization
+		// order code.
+		switch obj := obj.(type) {
+		case *Const:
+			if check.cycle(obj) || obj.typ == nil {
+				obj.typ = Typ[Invalid]
+			}
+
+		case *Var:
+			if check.cycle(obj) || obj.typ == nil {
+				obj.typ = Typ[Invalid]
+			}
+
+		case *TypeName:
+			if check.cycle(obj) {
+				// break cycle
+				// (without this, calling underlying()
+				// below may lead to an endless loop
+				// if we have a cycle for a defined
+				// (*Named) type)
+				obj.typ = Typ[Invalid]
+			}
+
+		case *Func:
+			if check.cycle(obj) {
+				// Don't set obj.typ to Typ[Invalid] here
+				// because plenty of code type-asserts that
+				// functions have a *Signature type. Grey
+				// functions have their type set to an empty
+				// signature which makes it impossible to
+				// initialize a variable with the function.
+			}
+
+		default:
+			unreachable()
+		}
+		assert(obj.Type() != nil)
+		return
+	}
+
+	d := check.objMap[obj]
+	if d == nil {
+		check.dump("%v: %s should have been declared", obj.Pos(), obj)
+		unreachable()
+	}
+
+	// save/restore current context and setup object context
+	defer func(ctxt context) {
+		check.context = ctxt
+	}(check.context)
+	check.context = context{
+		scope: d.file,
+	}
+
+	// Const and var declarations must not have initialization
+	// cycles. We track them by remembering the current declaration
+	// in check.decl. Initialization expressions depending on other
+	// consts, vars, or functions, add dependencies to the current
+	// check.decl.
+	switch obj := obj.(type) {
+	case *Const:
+		check.decl = d // new package-level const decl
+		check.constDecl(obj, d.typ, d.init, d.inherited)
+	case *Var:
+		check.decl = d // new package-level var decl
+		check.varDecl(obj, d.lhs, d.typ, d.init)
+	case *TypeName:
+		// invalid recursive types are detected via path
+		check.typeDecl(obj, d.typ, def, d.alias)
+	case *Func:
+		// functions may be recursive - no need to track dependencies
+		check.funcDecl(obj, d)
+	default:
+		unreachable()
+	}
+}
+
+// cycle checks if the cycle starting with obj is valid and
+// reports an error if it is not.
+func (check *Checker) cycle(obj Object) (isCycle bool) {
+	// The object map contains the package scope objects and the non-interface methods.
+	if debug {
+		info := check.objMap[obj]
+		inObjMap := info != nil && (info.fdecl == nil || info.fdecl.Recv == nil) // exclude methods
+		isPkgObj := obj.Parent() == check.pkg.scope
+		if isPkgObj != inObjMap {
+			check.dump("%v: inconsistent object map for %s (isPkgObj = %v, inObjMap = %v)", obj.Pos(), obj, isPkgObj, inObjMap)
+			unreachable()
+		}
+	}
+
+	// Count cycle objects.
+	assert(obj.color() >= grey)
+	start := obj.color() - grey // index of obj in objPath
+	cycle := check.objPath[start:]
+	nval := 0 // number of (constant or variable) values in the cycle
+	ndef := 0 // number of type definitions in the cycle
+	for _, obj := range cycle {
+		switch obj := obj.(type) {
+		case *Const, *Var:
+			nval++
+		case *TypeName:
+			// Determine if the type name is an alias or not. For
+			// package-level objects, use the object map which
+			// provides syntactic information (which doesn't rely
+			// on the order in which the objects are set up). For
+			// local objects, we can rely on the order, so use
+			// the object's predicate.
+			// TODO(gri) It would be less fragile to always access
+			// the syntactic information. We should consider storing
+			// this information explicitly in the object.
+			var alias bool
+			if d := check.objMap[obj]; d != nil {
+				alias = d.alias // package-level object
+			} else {
+				alias = obj.IsAlias() // function local object
+			}
+			if !alias {
+				ndef++
+			}
+		case *Func:
+			// ignored for now
+		default:
+			unreachable()
+		}
+	}
+
+	if trace {
+		check.trace(obj.Pos(), "## cycle detected: objPath = %s->%s (len = %d)", pathString(cycle), obj.Name(), len(cycle))
+		check.trace(obj.Pos(), "## cycle contains: %d values, %d type definitions", nval, ndef)
+		defer func() {
+			if isCycle {
+				check.trace(obj.Pos(), "=> error: cycle is invalid")
+			}
+		}()
+	}
+
+	// A cycle involving only constants and variables is invalid but we
+	// ignore them here because they are reported via the initialization
+	// cycle check.
+	if nval == len(cycle) {
+		return false
+	}
+
+	// A cycle involving only types (and possibly functions) must have at least
+	// one type definition to be permitted: If there is no type definition, we
+	// have a sequence of alias type names which will expand ad infinitum.
+	if nval == 0 && ndef > 0 {
+		return false // cycle is permitted
+	}
+
+	check.cycleError(cycle)
+
+	return true
+}
+
+type typeInfo uint
+
+// validType verifies that the given type does not "expand" infinitely
+// producing a cycle in the type graph. Cycles are detected by marking
+// defined types.
+// (Cycles involving alias types, as in "type A = [10]A" are detected
+// earlier, via the objDecl cycle detection mechanism.)
+func (check *Checker) validType(typ Type, path []Object) typeInfo {
+	const (
+		unknown typeInfo = iota
+		marked
+		valid
+		invalid
+	)
+
+	switch t := typ.(type) {
+	case *Array:
+		return check.validType(t.elem, path)
+
+	case *Struct:
+		for _, f := range t.fields {
+			if check.validType(f.typ, path) == invalid {
+				return invalid
+			}
+		}
+
+	case *Interface:
+		for _, etyp := range t.embeddeds {
+			if check.validType(etyp, path) == invalid {
+				return invalid
+			}
+		}
+
+	case *Named:
+		// don't touch the type if it is from a different package or the Universe scope
+		// (doing so would lead to a race condition - was issue #35049)
+		if t.obj.pkg != check.pkg {
+			return valid
+		}
+
+		// don't report a 2nd error if we already know the type is invalid
+		// (e.g., if a cycle was detected earlier, via Checker.underlying).
+		if t.underlying == Typ[Invalid] {
+			t.info = invalid
+			return invalid
+		}
+
+		switch t.info {
+		case unknown:
+			t.info = marked
+			t.info = check.validType(t.orig, append(path, t.obj)) // only types of current package added to path
+		case marked:
+			// cycle detected
+			for i, tn := range path {
+				if t.obj.pkg != check.pkg {
+					panic("internal error: type cycle via package-external type")
+				}
+				if tn == t.obj {
+					check.cycleError(path[i:])
+					t.info = invalid
+					t.underlying = Typ[Invalid]
+					return t.info
+				}
+			}
+			panic("internal error: cycle start not found")
+		}
+		return t.info
+	}
+
+	return valid
+}
+
+// cycleError reports a declaration cycle starting with
+// the object in cycle that is "first" in the source.
+func (check *Checker) cycleError(cycle []Object) {
+	// TODO(gri) Should we start with the last (rather than the first) object in the cycle
+	//           since that is the earliest point in the source where we start seeing the
+	//           cycle? That would be more consistent with other error messages.
+	i := firstInSrc(cycle)
+	obj := cycle[i]
+	check.errorf(obj, _InvalidDeclCycle, "illegal cycle in declaration of %s", obj.Name())
+	for range cycle {
+		check.errorf(obj, _InvalidDeclCycle, "\t%s refers to", obj.Name()) // secondary error, \t indented
+		i++
+		if i >= len(cycle) {
+			i = 0
+		}
+		obj = cycle[i]
+	}
+	check.errorf(obj, _InvalidDeclCycle, "\t%s", obj.Name())
+}
+
+// firstInSrc reports the index of the object with the "smallest"
+// source position in path. path must not be empty.
+func firstInSrc(path []Object) int {
+	fst, pos := 0, path[0].Pos()
+	for i, t := range path[1:] {
+		if t.Pos() < pos {
+			fst, pos = i+1, t.Pos()
+		}
+	}
+	return fst
+}
+
+type (
+	decl interface {
+		node() ast.Node
+	}
+
+	importDecl struct{ spec *ast.ImportSpec }
+	constDecl  struct {
+		spec      *ast.ValueSpec
+		iota      int
+		typ       ast.Expr
+		init      []ast.Expr
+		inherited bool
+	}
+	varDecl  struct{ spec *ast.ValueSpec }
+	typeDecl struct{ spec *ast.TypeSpec }
+	funcDecl struct{ decl *ast.FuncDecl }
+)
+
+func (d importDecl) node() ast.Node { return d.spec }
+func (d constDecl) node() ast.Node  { return d.spec }
+func (d varDecl) node() ast.Node    { return d.spec }
+func (d typeDecl) node() ast.Node   { return d.spec }
+func (d funcDecl) node() ast.Node   { return d.decl }
+
+func (check *Checker) walkDecls(decls []ast.Decl, f func(decl)) {
+	for _, d := range decls {
+		check.walkDecl(d, f)
+	}
+}
+
+func (check *Checker) walkDecl(d ast.Decl, f func(decl)) {
+	switch d := d.(type) {
+	case *ast.BadDecl:
+		// ignore
+	case *ast.GenDecl:
+		var last *ast.ValueSpec // last ValueSpec with type or init exprs seen
+		for iota, s := range d.Specs {
+			switch s := s.(type) {
+			case *ast.ImportSpec:
+				f(importDecl{s})
+			case *ast.ValueSpec:
+				switch d.Tok {
+				case token.CONST:
+					// determine which initialization expressions to use
+					inherited := true
+					switch {
+					case s.Type != nil || len(s.Values) > 0:
+						last = s
+						inherited = false
+					case last == nil:
+						last = new(ast.ValueSpec) // make sure last exists
+						inherited = false
+					}
+					check.arityMatch(s, last)
+					f(constDecl{spec: s, iota: iota, typ: last.Type, init: last.Values, inherited: inherited})
+				case token.VAR:
+					check.arityMatch(s, nil)
+					f(varDecl{s})
+				default:
+					check.invalidAST(s, "invalid token %s", d.Tok)
+				}
+			case *ast.TypeSpec:
+				f(typeDecl{s})
+			default:
+				check.invalidAST(s, "unknown ast.Spec node %T", s)
+			}
+		}
+	case *ast.FuncDecl:
+		f(funcDecl{d})
+	default:
+		check.invalidAST(d, "unknown ast.Decl node %T", d)
+	}
+}
+
+func (check *Checker) constDecl(obj *Const, typ, init ast.Expr, inherited bool) {
+	assert(obj.typ == nil)
+
+	// use the correct value of iota
+	defer func(iota constant.Value, errpos positioner) {
+		check.iota = iota
+		check.errpos = errpos
+	}(check.iota, check.errpos)
+	check.iota = obj.val
+	check.errpos = nil
+
+	// provide valid constant value under all circumstances
+	obj.val = constant.MakeUnknown()
+
+	// determine type, if any
+	if typ != nil {
+		t := check.typ(typ)
+		if !isConstType(t) {
+			// don't report an error if the type is an invalid C (defined) type
+			// (issue #22090)
+			if t.Underlying() != Typ[Invalid] {
+				check.errorf(typ, _InvalidConstType, "invalid constant type %s", t)
+			}
+			obj.typ = Typ[Invalid]
+			return
+		}
+		obj.typ = t
+	}
+
+	// check initialization
+	var x operand
+	if init != nil {
+		if inherited {
+			// The initialization expression is inherited from a previous
+			// constant declaration, and (error) positions refer to that
+			// expression and not the current constant declaration. Use
+			// the constant identifier position for any errors during
+			// init expression evaluation since that is all we have
+			// (see issues #42991, #42992).
+			check.errpos = atPos(obj.pos)
+		}
+		check.expr(&x, init)
+	}
+	check.initConst(obj, &x)
+}
+
+func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init ast.Expr) {
+	assert(obj.typ == nil)
+
+	// determine type, if any
+	if typ != nil {
+		obj.typ = check.typ(typ)
+		// We cannot spread the type to all lhs variables if there
+		// are more than one since that would mark them as checked
+		// (see Checker.objDecl) and the assignment of init exprs,
+		// if any, would not be checked.
+		//
+		// TODO(gri) If we have no init expr, we should distribute
+		// a given type otherwise we need to re-evalate the type
+		// expr for each lhs variable, leading to duplicate work.
+	}
+
+	// check initialization
+	if init == nil {
+		if typ == nil {
+			// error reported before by arityMatch
+			obj.typ = Typ[Invalid]
+		}
+		return
+	}
+
+	if lhs == nil || len(lhs) == 1 {
+		assert(lhs == nil || lhs[0] == obj)
+		var x operand
+		check.expr(&x, init)
+		check.initVar(obj, &x, "variable declaration")
+		return
+	}
+
+	if debug {
+		// obj must be one of lhs
+		found := false
+		for _, lhs := range lhs {
+			if obj == lhs {
+				found = true
+				break
+			}
+		}
+		if !found {
+			panic("inconsistent lhs")
+		}
+	}
+
+	// We have multiple variables on the lhs and one init expr.
+	// Make sure all variables have been given the same type if
+	// one was specified, otherwise they assume the type of the
+	// init expression values (was issue #15755).
+	if typ != nil {
+		for _, lhs := range lhs {
+			lhs.typ = obj.typ
+		}
+	}
+
+	check.initVars(lhs, []ast.Expr{init}, token.NoPos)
+}
+
+// underlying returns the underlying type of typ; possibly by following
+// forward chains of named types. Such chains only exist while named types
+// are incomplete. If an underlying type is found, resolve the chain by
+// setting the underlying type for each defined type in the chain before
+// returning it.
+//
+// If no underlying type is found, a cycle error is reported and Typ[Invalid]
+// is used as underlying type for each defined type in the chain and returned
+// as result.
+func (check *Checker) underlying(typ Type) Type {
+	// If typ is not a defined type, its underlying type is itself.
+	n0, _ := typ.(*Named)
+	if n0 == nil {
+		return typ // nothing to do
+	}
+
+	// If the underlying type of a defined type is not a defined
+	// type, then that is the desired underlying type.
+	typ = n0.underlying
+	n, _ := typ.(*Named)
+	if n == nil {
+		return typ // common case
+	}
+
+	// Otherwise, follow the forward chain.
+	seen := map[*Named]int{n0: 0}
+	path := []Object{n0.obj}
+	for {
+		typ = n.underlying
+		n1, _ := typ.(*Named)
+		if n1 == nil {
+			break // end of chain
+		}
+
+		seen[n] = len(seen)
+		path = append(path, n.obj)
+		n = n1
+
+		if i, ok := seen[n]; ok {
+			// cycle
+			check.cycleError(path[i:])
+			typ = Typ[Invalid]
+			break
+		}
+	}
+
+	for n := range seen {
+		// We should never have to update the underlying type of an imported type;
+		// those underlying types should have been resolved during the import.
+		// Also, doing so would lead to a race condition (was issue #31749).
+		if n.obj.pkg != check.pkg {
+			panic("internal error: imported type with unresolved underlying type")
+		}
+		n.underlying = typ
+	}
+
+	return typ
+}
+
+func (n *Named) setUnderlying(typ Type) {
+	if n != nil {
+		n.underlying = typ
+	}
+}
+
+func (check *Checker) typeDecl(obj *TypeName, typ ast.Expr, def *Named, alias bool) {
+	assert(obj.typ == nil)
+
+	check.later(func() {
+		check.validType(obj.typ, nil)
+	})
+
+	if alias {
+
+		obj.typ = Typ[Invalid]
+		obj.typ = check.typ(typ)
+
+	} else {
+
+		named := &Named{obj: obj}
+		def.setUnderlying(named)
+		obj.typ = named // make sure recursive type declarations terminate
+
+		// determine underlying type of named
+		named.orig = check.definedType(typ, named)
+
+		// The underlying type of named may be itself a named type that is
+		// incomplete:
+		//
+		//	type (
+		//		A B
+		//		B *C
+		//		C A
+		//	)
+		//
+		// The type of C is the (named) type of A which is incomplete,
+		// and which has as its underlying type the named type B.
+		// Determine the (final, unnamed) underlying type by resolving
+		// any forward chain.
+		named.underlying = check.underlying(named)
+
+	}
+
+	check.addMethodDecls(obj)
+}
+
+func (check *Checker) addMethodDecls(obj *TypeName) {
+	// get associated methods
+	// (Checker.collectObjects only collects methods with non-blank names;
+	// Checker.resolveBaseTypeName ensures that obj is not an alias name
+	// if it has attached methods.)
+	methods := check.methods[obj]
+	if methods == nil {
+		return
+	}
+	delete(check.methods, obj)
+	assert(!check.objMap[obj].alias) // don't use TypeName.IsAlias (requires fully set up object)
+
+	// use an objset to check for name conflicts
+	var mset objset
+
+	// spec: "If the base type is a struct type, the non-blank method
+	// and field names must be distinct."
+	base, _ := obj.typ.(*Named) // shouldn't fail but be conservative
+	if base != nil {
+		if t, _ := base.underlying.(*Struct); t != nil {
+			for _, fld := range t.fields {
+				if fld.name != "_" {
+					assert(mset.insert(fld) == nil)
+				}
+			}
+		}
+
+		// Checker.Files may be called multiple times; additional package files
+		// may add methods to already type-checked types. Add pre-existing methods
+		// so that we can detect redeclarations.
+		for _, m := range base.methods {
+			assert(m.name != "_")
+			assert(mset.insert(m) == nil)
+		}
+	}
+
+	// add valid methods
+	for _, m := range methods {
+		// spec: "For a base type, the non-blank names of methods bound
+		// to it must be unique."
+		assert(m.name != "_")
+		if alt := mset.insert(m); alt != nil {
+			switch alt.(type) {
+			case *Var:
+				check.errorf(m, _DuplicateFieldAndMethod, "field and method with the same name %s", m.name)
+			case *Func:
+				check.errorf(m, _DuplicateMethod, "method %s already declared for %s", m.name, obj)
+			default:
+				unreachable()
+			}
+			check.reportAltDecl(alt)
+			continue
+		}
+
+		if base != nil {
+			base.methods = append(base.methods, m)
+		}
+	}
+}
+
+func (check *Checker) funcDecl(obj *Func, decl *declInfo) {
+	assert(obj.typ == nil)
+
+	// func declarations cannot use iota
+	assert(check.iota == nil)
+
+	sig := new(Signature)
+	obj.typ = sig // guard against cycles
+	fdecl := decl.fdecl
+	check.funcType(sig, fdecl.Recv, fdecl.Type)
+	if sig.recv == nil && obj.name == "init" && (sig.params.Len() > 0 || sig.results.Len() > 0) {
+		check.errorf(fdecl, _InvalidInitSig, "func init must have no arguments and no return values")
+		// ok to continue
+	}
+
+	// function body must be type-checked after global declarations
+	// (functions implemented elsewhere have no body)
+	if !check.conf.IgnoreFuncBodies && fdecl.Body != nil {
+		check.later(func() {
+			check.funcBody(decl, obj.name, sig, fdecl.Body, nil)
+		})
+	}
+}
+
+func (check *Checker) declStmt(d ast.Decl) {
+	pkg := check.pkg
+
+	check.walkDecl(d, func(d decl) {
+		switch d := d.(type) {
+		case constDecl:
+			top := len(check.delayed)
+
+			// declare all constants
+			lhs := make([]*Const, len(d.spec.Names))
+			for i, name := range d.spec.Names {
+				obj := NewConst(name.Pos(), pkg, name.Name, nil, constant.MakeInt64(int64(d.iota)))
+				lhs[i] = obj
+
+				var init ast.Expr
+				if i < len(d.init) {
+					init = d.init[i]
+				}
+
+				check.constDecl(obj, d.typ, init, d.inherited)
+			}
+
+			// process function literals in init expressions before scope changes
+			check.processDelayed(top)
+
+			// spec: "The scope of a constant or variable identifier declared
+			// inside a function begins at the end of the ConstSpec or VarSpec
+			// (ShortVarDecl for short variable declarations) and ends at the
+			// end of the innermost containing block."
+			scopePos := d.spec.End()
+			for i, name := range d.spec.Names {
+				check.declare(check.scope, name, lhs[i], scopePos)
+			}
+
+		case varDecl:
+			top := len(check.delayed)
+
+			lhs0 := make([]*Var, len(d.spec.Names))
+			for i, name := range d.spec.Names {
+				lhs0[i] = NewVar(name.Pos(), pkg, name.Name, nil)
+			}
+
+			// initialize all variables
+			for i, obj := range lhs0 {
+				var lhs []*Var
+				var init ast.Expr
+				switch len(d.spec.Values) {
+				case len(d.spec.Names):
+					// lhs and rhs match
+					init = d.spec.Values[i]
+				case 1:
+					// rhs is expected to be a multi-valued expression
+					lhs = lhs0
+					init = d.spec.Values[0]
+				default:
+					if i < len(d.spec.Values) {
+						init = d.spec.Values[i]
+					}
+				}
+				check.varDecl(obj, lhs, d.spec.Type, init)
+				if len(d.spec.Values) == 1 {
+					// If we have a single lhs variable we are done either way.
+					// If we have a single rhs expression, it must be a multi-
+					// valued expression, in which case handling the first lhs
+					// variable will cause all lhs variables to have a type
+					// assigned, and we are done as well.
+					if debug {
+						for _, obj := range lhs0 {
+							assert(obj.typ != nil)
+						}
+					}
+					break
+				}
+			}
+
+			// process function literals in init expressions before scope changes
+			check.processDelayed(top)
+
+			// declare all variables
+			// (only at this point are the variable scopes (parents) set)
+			scopePos := d.spec.End() // see constant declarations
+			for i, name := range d.spec.Names {
+				// see constant declarations
+				check.declare(check.scope, name, lhs0[i], scopePos)
+			}
+
+		case typeDecl:
+			obj := NewTypeName(d.spec.Name.Pos(), pkg, d.spec.Name.Name, nil)
+			// spec: "The scope of a type identifier declared inside a function
+			// begins at the identifier in the TypeSpec and ends at the end of
+			// the innermost containing block."
+			scopePos := d.spec.Name.Pos()
+			check.declare(check.scope, d.spec.Name, obj, scopePos)
+			// mark and unmark type before calling typeDecl; its type is still nil (see Checker.objDecl)
+			obj.setColor(grey + color(check.push(obj)))
+			check.typeDecl(obj, d.spec.Type, nil, d.spec.Assign.IsValid())
+			check.pop().setColor(black)
+		default:
+			check.invalidAST(d.node(), "unknown ast.Decl node %T", d.node())
+		}
+	})
+}
diff --git a/src/go/types/errorcodes.go b/src/go/types/errorcodes.go
new file mode 100644
index 0000000..c01a12c
--- /dev/null
+++ b/src/go/types/errorcodes.go
@@ -0,0 +1,1366 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types
+
+type errorCode int
+
+// This file defines the error codes that can be produced during type-checking.
+// Collectively, these codes provide an identifier that may be used to
+// implement special handling for certain types of errors.
+//
+// Error codes should be fine-grained enough that the exact nature of the error
+// can be easily determined, but coarse enough that they are not an
+// implementation detail of the type checking algorithm. As a rule-of-thumb,
+// errors should be considered equivalent if there is a theoretical refactoring
+// of the type checker in which they are emitted in exactly one place. For
+// example, the type checker emits different error messages for "too many
+// arguments" and "too few arguments", but one can imagine an alternative type
+// checker where this check instead just emits a single "wrong number of
+// arguments", so these errors should have the same code.
+//
+// Error code names should be as brief as possible while retaining accuracy and
+// distinctiveness. In most cases names should start with an adjective
+// describing the nature of the error (e.g. "invalid", "unused", "misplaced"),
+// and end with a noun identifying the relevant language object. For example,
+// "_DuplicateDecl" or "_InvalidSliceExpr". For brevity, naming follows the
+// convention that "bad" implies a problem with syntax, and "invalid" implies a
+// problem with types.
+
+const (
+	_ errorCode = iota
+
+	// _Test is reserved for errors that only apply while in self-test mode.
+	_Test
+
+	/* package names */
+
+	// _BlankPkgName occurs when a package name is the blank identifier "_".
+	//
+	// Per the spec:
+	//  "The PackageName must not be the blank identifier."
+	_BlankPkgName
+
+	// _MismatchedPkgName occurs when a file's package name doesn't match the
+	// package name already established by other files.
+	_MismatchedPkgName
+
+	// _InvalidPkgUse occurs when a package identifier is used outside of a
+	// selector expression.
+	//
+	// Example:
+	//  import "fmt"
+	//
+	//  var _ = fmt
+	_InvalidPkgUse
+
+	/* imports */
+
+	// _BadImportPath occurs when an import path is not valid.
+	_BadImportPath
+
+	// _BrokenImport occurs when importing a package fails.
+	//
+	// Example:
+	//  import "amissingpackage"
+	_BrokenImport
+
+	// _ImportCRenamed occurs when the special import "C" is renamed. "C" is a
+	// pseudo-package, and must not be renamed.
+	//
+	// Example:
+	//  import _ "C"
+	_ImportCRenamed
+
+	// _UnusedImport occurs when an import is unused.
+	//
+	// Example:
+	//  import "fmt"
+	//
+	//  func main() {}
+	_UnusedImport
+
+	/* initialization */
+
+	// _InvalidInitCycle occurs when an invalid cycle is detected within the
+	// initialization graph.
+	//
+	// Example:
+	//  var x int = f()
+	//
+	//  func f() int { return x }
+	_InvalidInitCycle
+
+	/* decls */
+
+	// _DuplicateDecl occurs when an identifier is declared multiple times.
+	//
+	// Example:
+	//  var x = 1
+	//  var x = 2
+	_DuplicateDecl
+
+	// _InvalidDeclCycle occurs when a declaration cycle is not valid.
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  type T struct {
+	//  	a [n]int
+	//  }
+	//
+	//  var n = unsafe.Sizeof(T{})
+	_InvalidDeclCycle
+
+	// _InvalidTypeCycle occurs when a cycle in type definitions results in a
+	// type that is not well-defined.
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  type T [unsafe.Sizeof(T{})]int
+	_InvalidTypeCycle
+
+	/* decls > const */
+
+	// _InvalidConstInit occurs when a const declaration has a non-constant
+	// initializer.
+	//
+	// Example:
+	//  var x int
+	//  const _ = x
+	_InvalidConstInit
+
+	// _InvalidConstVal occurs when a const value cannot be converted to its
+	// target type.
+	//
+	// TODO(findleyr): this error code and example are not very clear. Consider
+	// removing it.
+	//
+	// Example:
+	//  const _ = 1 << "hello"
+	_InvalidConstVal
+
+	// _InvalidConstType occurs when the underlying type in a const declaration
+	// is not a valid constant type.
+	//
+	// Example:
+	//  const c *int = 4
+	_InvalidConstType
+
+	/* decls > var (+ other variable assignment codes) */
+
+	// _UntypedNil occurs when the predeclared (untyped) value nil is used to
+	// initialize a variable declared without an explicit type.
+	//
+	// Example:
+	//  var x = nil
+	_UntypedNil
+
+	// _WrongAssignCount occurs when the number of values on the right-hand side
+	// of an assignment or or initialization expression does not match the number
+	// of variables on the left-hand side.
+	//
+	// Example:
+	//  var x = 1, 2
+	_WrongAssignCount
+
+	// _UnassignableOperand occurs when the left-hand side of an assignment is
+	// not assignable.
+	//
+	// Example:
+	//  func f() {
+	//  	const c = 1
+	//  	c = 2
+	//  }
+	_UnassignableOperand
+
+	// _NoNewVar occurs when a short variable declaration (':=') does not declare
+	// new variables.
+	//
+	// Example:
+	//  func f() {
+	//  	x := 1
+	//  	x := 2
+	//  }
+	_NoNewVar
+
+	// _MultiValAssignOp occurs when an assignment operation (+=, *=, etc) does
+	// not have single-valued left-hand or right-hand side.
+	//
+	// Per the spec:
+	//  "In assignment operations, both the left- and right-hand expression lists
+	//  must contain exactly one single-valued expression"
+	//
+	// Example:
+	//  func f() int {
+	//  	x, y := 1, 2
+	//  	x, y += 1
+	//  	return x + y
+	//  }
+	_MultiValAssignOp
+
+	// _InvalidIfaceAssign occurs when a value of type T is used as an
+	// interface, but T does not implement a method of the expected interface.
+	//
+	// Example:
+	//  type I interface {
+	//  	f()
+	//  }
+	//
+	//  type T int
+	//
+	//  var x I = T(1)
+	_InvalidIfaceAssign
+
+	// _InvalidChanAssign occurs when a chan assignment is invalid.
+	//
+	// Per the spec, a value x is assignable to a channel type T if:
+	//  "x is a bidirectional channel value, T is a channel type, x's type V and
+	//  T have identical element types, and at least one of V or T is not a
+	//  defined type."
+	//
+	// Example:
+	//  type T1 chan int
+	//  type T2 chan int
+	//
+	//  var x T1
+	//  // Invalid assignment because both types are named
+	//  var _ T2 = x
+	_InvalidChanAssign
+
+	// _IncompatibleAssign occurs when the type of the right-hand side expression
+	// in an assignment cannot be assigned to the type of the variable being
+	// assigned.
+	//
+	// Example:
+	//  var x []int
+	//  var _ int = x
+	_IncompatibleAssign
+
+	// _UnaddressableFieldAssign occurs when trying to assign to a struct field
+	// in a map value.
+	//
+	// Example:
+	//  func f() {
+	//  	m := make(map[string]struct{i int})
+	//  	m["foo"].i = 42
+	//  }
+	_UnaddressableFieldAssign
+
+	/* decls > type (+ other type expression codes) */
+
+	// _NotAType occurs when the identifier used as the underlying type in a type
+	// declaration or the right-hand side of a type alias does not denote a type.
+	//
+	// Example:
+	//  var S = 2
+	//
+	//  type T S
+	_NotAType
+
+	// _InvalidArrayLen occurs when an array length is not a constant value.
+	//
+	// Example:
+	//  var n = 3
+	//  var _ = [n]int{}
+	_InvalidArrayLen
+
+	// _BlankIfaceMethod occurs when a method name is '_'.
+	//
+	// Per the spec:
+	//  "The name of each explicitly specified method must be unique and not
+	//  blank."
+	//
+	// Example:
+	//  type T interface {
+	//  	_(int)
+	//  }
+	_BlankIfaceMethod
+
+	// _IncomparableMapKey occurs when a map key type does not support the == and
+	// != operators.
+	//
+	// Per the spec:
+	//  "The comparison operators == and != must be fully defined for operands of
+	//  the key type; thus the key type must not be a function, map, or slice."
+	//
+	// Example:
+	//  var x map[T]int
+	//
+	//  type T []int
+	_IncomparableMapKey
+
+	// _InvalidIfaceEmbed occurs when a non-interface type is embedded in an
+	// interface.
+	//
+	// Example:
+	//  type T struct {}
+	//
+	//  func (T) m()
+	//
+	//  type I interface {
+	//  	T
+	//  }
+	_InvalidIfaceEmbed
+
+	// _InvalidPtrEmbed occurs when an embedded field is of the pointer form *T,
+	// and T itself is itself a pointer, an unsafe.Pointer, or an interface.
+	//
+	// Per the spec:
+	//  "An embedded field must be specified as a type name T or as a pointer to
+	//  a non-interface type name *T, and T itself may not be a pointer type."
+	//
+	// Example:
+	//  type T *int
+	//
+	//  type S struct {
+	//  	*T
+	//  }
+	_InvalidPtrEmbed
+
+	/* decls > func and method */
+
+	// _BadRecv occurs when a method declaration does not have exactly one
+	// receiver parameter.
+	//
+	// Example:
+	//  func () _() {}
+	_BadRecv
+
+	// _InvalidRecv occurs when a receiver type expression is not of the form T
+	// or *T, or T is a pointer type.
+	//
+	// Example:
+	//  type T struct {}
+	//
+	//  func (**T) m() {}
+	_InvalidRecv
+
+	// _DuplicateFieldAndMethod occurs when an identifier appears as both a field
+	// and method name.
+	//
+	// Example:
+	//  type T struct {
+	//  	m int
+	//  }
+	//
+	//  func (T) m() {}
+	_DuplicateFieldAndMethod
+
+	// _DuplicateMethod occurs when two methods on the same receiver type have
+	// the same name.
+	//
+	// Example:
+	//  type T struct {}
+	//  func (T) m() {}
+	//  func (T) m(i int) int { return i }
+	_DuplicateMethod
+
+	/* decls > special */
+
+	// _InvalidBlank occurs when a blank identifier is used as a value or type.
+	//
+	// Per the spec:
+	//  "The blank identifier may appear as an operand only on the left-hand side
+	//  of an assignment."
+	//
+	// Example:
+	//  var x = _
+	_InvalidBlank
+
+	// _InvalidIota occurs when the predeclared identifier iota is used outside
+	// of a constant declaration.
+	//
+	// Example:
+	//  var x = iota
+	_InvalidIota
+
+	// _MissingInitBody occurs when an init function is missing its body.
+	//
+	// Example:
+	//  func init()
+	_MissingInitBody
+
+	// _InvalidInitSig occurs when an init function declares parameters or
+	// results.
+	//
+	// Example:
+	//  func init() int { return 1 }
+	_InvalidInitSig
+
+	// _InvalidInitDecl occurs when init is declared as anything other than a
+	// function.
+	//
+	// Example:
+	//  var init = 1
+	_InvalidInitDecl
+
+	// _InvalidMainDecl occurs when main is declared as anything other than a
+	// function, in a main package.
+	_InvalidMainDecl
+
+	/* exprs */
+
+	// _TooManyValues occurs when a function returns too many values for the
+	// expression context in which it is used.
+	//
+	// Example:
+	//  func ReturnTwo() (int, int) {
+	//  	return 1, 2
+	//  }
+	//
+	//  var x = ReturnTwo()
+	_TooManyValues
+
+	// _NotAnExpr occurs when a type expression is used where a value expression
+	// is expected.
+	//
+	// Example:
+	//  type T struct {}
+	//
+	//  func f() {
+	//  	T
+	//  }
+	_NotAnExpr
+
+	/* exprs > const */
+
+	// _TruncatedFloat occurs when a float constant is truncated to an integer
+	// value.
+	//
+	// Example:
+	//  var _ int = 98.6
+	_TruncatedFloat
+
+	// _NumericOverflow occurs when a numeric constant overflows its target type.
+	//
+	// Example:
+	//  var x int8 = 1000
+	_NumericOverflow
+
+	/* exprs > operation */
+
+	// _UndefinedOp occurs when an operator is not defined for the type(s) used
+	// in an operation.
+	//
+	// Example:
+	//  var c = "a" - "b"
+	_UndefinedOp
+
+	// _MismatchedTypes occurs when operand types are incompatible in a binary
+	// operation.
+	//
+	// Example:
+	//  var a = "hello"
+	//  var b = 1
+	//  var c = a - b
+	_MismatchedTypes
+
+	// _DivByZero occurs when a division operation is provable at compile
+	// time to be a division by zero.
+	//
+	// Example:
+	//  const divisor = 0
+	//  var x int = 1/divisor
+	_DivByZero
+
+	// _NonNumericIncDec occurs when an increment or decrement operator is
+	// applied to a non-numeric value.
+	//
+	// Example:
+	//  func f() {
+	//  	var c = "c"
+	//  	c++
+	//  }
+	_NonNumericIncDec
+
+	/* exprs > ptr */
+
+	// _UnaddressableOperand occurs when the & operator is applied to an
+	// unaddressable expression.
+	//
+	// Example:
+	//  var x = &1
+	_UnaddressableOperand
+
+	// _InvalidIndirection occurs when a non-pointer value is indirected via the
+	// '*' operator.
+	//
+	// Example:
+	//  var x int
+	//  var y = *x
+	_InvalidIndirection
+
+	/* exprs > [] */
+
+	// _NonIndexableOperand occurs when an index operation is applied to a value
+	// that cannot be indexed.
+	//
+	// Example:
+	//  var x = 1
+	//  var y = x[1]
+	_NonIndexableOperand
+
+	// _InvalidIndex occurs when an index argument is not of integer type,
+	// negative, or out-of-bounds.
+	//
+	// Example:
+	//  var s = [...]int{1,2,3}
+	//  var x = s[5]
+	//
+	// Example:
+	//  var s = []int{1,2,3}
+	//  var _ = s[-1]
+	//
+	// Example:
+	//  var s = []int{1,2,3}
+	//  var i string
+	//  var _ = s[i]
+	_InvalidIndex
+
+	// _SwappedSliceIndices occurs when constant indices in a slice expression
+	// are decreasing in value.
+	//
+	// Example:
+	//  var _ = []int{1,2,3}[2:1]
+	_SwappedSliceIndices
+
+	/* operators > slice */
+
+	// _NonSliceableOperand occurs when a slice operation is applied to a value
+	// whose type is not sliceable, or is unaddressable.
+	//
+	// Example:
+	//  var x = [...]int{1, 2, 3}[:1]
+	//
+	// Example:
+	//  var x = 1
+	//  var y = 1[:1]
+	_NonSliceableOperand
+
+	// _InvalidSliceExpr occurs when a three-index slice expression (a[x:y:z]) is
+	// applied to a string.
+	//
+	// Example:
+	//  var s = "hello"
+	//  var x = s[1:2:3]
+	_InvalidSliceExpr
+
+	/* exprs > shift */
+
+	// _InvalidShiftCount occurs when the right-hand side of a shift operation is
+	// either non-integer, negative, or too large.
+	//
+	// Example:
+	//  var (
+	//  	x string
+	//  	y int = 1 << x
+	//  )
+	_InvalidShiftCount
+
+	// _InvalidShiftOperand occurs when the shifted operand is not an integer.
+	//
+	// Example:
+	//  var s = "hello"
+	//  var x = s << 2
+	_InvalidShiftOperand
+
+	/* exprs > chan */
+
+	// _InvalidReceive occurs when there is a channel receive from a value that
+	// is either not a channel, or is a send-only channel.
+	//
+	// Example:
+	//  func f() {
+	//  	var x = 1
+	//  	<-x
+	//  }
+	_InvalidReceive
+
+	// _InvalidSend occurs when there is a channel send to a value that is not a
+	// channel, or is a receive-only channel.
+	//
+	// Example:
+	//  func f() {
+	//  	var x = 1
+	//  	x <- "hello!"
+	//  }
+	_InvalidSend
+
+	/* exprs > literal */
+
+	// _DuplicateLitKey occurs when an index is duplicated in a slice, array, or
+	// map literal.
+	//
+	// Example:
+	//  var _ = []int{0:1, 0:2}
+	//
+	// Example:
+	//  var _ = map[string]int{"a": 1, "a": 2}
+	_DuplicateLitKey
+
+	// _MissingLitKey occurs when a map literal is missing a key expression.
+	//
+	// Example:
+	//  var _ = map[string]int{1}
+	_MissingLitKey
+
+	// _InvalidLitIndex occurs when the key in a key-value element of a slice or
+	// array literal is not an integer constant.
+	//
+	// Example:
+	//  var i = 0
+	//  var x = []string{i: "world"}
+	_InvalidLitIndex
+
+	// _OversizeArrayLit occurs when an array literal exceeds its length.
+	//
+	// Example:
+	//  var _ = [2]int{1,2,3}
+	_OversizeArrayLit
+
+	// _MixedStructLit occurs when a struct literal contains a mix of positional
+	// and named elements.
+	//
+	// Example:
+	//  var _ = struct{i, j int}{i: 1, 2}
+	_MixedStructLit
+
+	// _InvalidStructLit occurs when a positional struct literal has an incorrect
+	// number of values.
+	//
+	// Example:
+	//  var _ = struct{i, j int}{1,2,3}
+	_InvalidStructLit
+
+	// _MissingLitField occurs when a struct literal refers to a field that does
+	// not exist on the struct type.
+	//
+	// Example:
+	//  var _ = struct{i int}{j: 2}
+	_MissingLitField
+
+	// _DuplicateLitField occurs when a struct literal contains duplicated
+	// fields.
+	//
+	// Example:
+	//  var _ = struct{i int}{i: 1, i: 2}
+	_DuplicateLitField
+
+	// _UnexportedLitField occurs when a positional struct literal implicitly
+	// assigns an unexported field of an imported type.
+	_UnexportedLitField
+
+	// _InvalidLitField occurs when a field name is not a valid identifier.
+	//
+	// Example:
+	//  var _ = struct{i int}{1: 1}
+	_InvalidLitField
+
+	// _UntypedLit occurs when a composite literal omits a required type
+	// identifier.
+	//
+	// Example:
+	//  type outer struct{
+	//  	inner struct { i int }
+	//  }
+	//
+	//  var _ = outer{inner: {1}}
+	_UntypedLit
+
+	// _InvalidLit occurs when a composite literal expression does not match its
+	// type.
+	//
+	// Example:
+	//  type P *struct{
+	//  	x int
+	//  }
+	//  var _ = P {}
+	_InvalidLit
+
+	/* exprs > selector */
+
+	// _AmbiguousSelector occurs when a selector is ambiguous.
+	//
+	// Example:
+	//  type E1 struct { i int }
+	//  type E2 struct { i int }
+	//  type T struct { E1; E2 }
+	//
+	//  var x T
+	//  var _ = x.i
+	_AmbiguousSelector
+
+	// _UndeclaredImportedName occurs when a package-qualified identifier is
+	// undeclared by the imported package.
+	//
+	// Example:
+	//  import "go/types"
+	//
+	//  var _ = types.NotAnActualIdentifier
+	_UndeclaredImportedName
+
+	// _UnexportedName occurs when a selector refers to an unexported identifier
+	// of an imported package.
+	//
+	// Example:
+	//  import "reflect"
+	//
+	//  type _ reflect.flag
+	_UnexportedName
+
+	// _UndeclaredName occurs when an identifier is not declared in the current
+	// scope.
+	//
+	// Example:
+	//  var x T
+	_UndeclaredName
+
+	// _MissingFieldOrMethod occurs when a selector references a field or method
+	// that does not exist.
+	//
+	// Example:
+	//  type T struct {}
+	//
+	//  var x = T{}.f
+	_MissingFieldOrMethod
+
+	/* exprs > ... */
+
+	// _BadDotDotDotSyntax occurs when a "..." occurs in a context where it is
+	// not valid.
+	//
+	// Example:
+	//  var _ = map[int][...]int{0: {}}
+	_BadDotDotDotSyntax
+
+	// _NonVariadicDotDotDot occurs when a "..." is used on the final argument to
+	// a non-variadic function.
+	//
+	// Example:
+	//  func printArgs(s []string) {
+	//  	for _, a := range s {
+	//  		println(a)
+	//  	}
+	//  }
+	//
+	//  func f() {
+	//  	s := []string{"a", "b", "c"}
+	//  	printArgs(s...)
+	//  }
+	_NonVariadicDotDotDot
+
+	// _MisplacedDotDotDot occurs when a "..." is used somewhere other than the
+	// final argument to a function call.
+	//
+	// Example:
+	//  func printArgs(args ...int) {
+	//  	for _, a := range args {
+	//  		println(a)
+	//  	}
+	//  }
+	//
+	//  func f() {
+	//  	a := []int{1,2,3}
+	//  	printArgs(0, a...)
+	//  }
+	_MisplacedDotDotDot
+
+	// _InvalidDotDotDotOperand occurs when a "..." operator is applied to a
+	// single-valued operand.
+	//
+	// Example:
+	//  func printArgs(args ...int) {
+	//  	for _, a := range args {
+	//  		println(a)
+	//  	}
+	//  }
+	//
+	//  func f() {
+	//  	a := 1
+	//  	printArgs(a...)
+	//  }
+	//
+	// Example:
+	//  func args() (int, int) {
+	//  	return 1, 2
+	//  }
+	//
+	//  func printArgs(args ...int) {
+	//  	for _, a := range args {
+	//  		println(a)
+	//  	}
+	//  }
+	//
+	//  func g() {
+	//  	printArgs(args()...)
+	//  }
+	_InvalidDotDotDotOperand
+
+	// _InvalidDotDotDot occurs when a "..." is used in a non-variadic built-in
+	// function.
+	//
+	// Example:
+	//  var s = []int{1, 2, 3}
+	//  var l = len(s...)
+	_InvalidDotDotDot
+
+	/* exprs > built-in */
+
+	// _UncalledBuiltin occurs when a built-in function is used as a
+	// function-valued expression, instead of being called.
+	//
+	// Per the spec:
+	//  "The built-in functions do not have standard Go types, so they can only
+	//  appear in call expressions; they cannot be used as function values."
+	//
+	// Example:
+	//  var _ = copy
+	_UncalledBuiltin
+
+	// _InvalidAppend occurs when append is called with a first argument that is
+	// not a slice.
+	//
+	// Example:
+	//  var _ = append(1, 2)
+	_InvalidAppend
+
+	// _InvalidCap occurs when an argument to the cap built-in function is not of
+	// supported type.
+	//
+	// See https://golang.org/ref/spec#Length_and_capacity for information on
+	// which underlying types are supported as arguments to cap and len.
+	//
+	// Example:
+	//  var s = 2
+	//  var x = cap(s)
+	_InvalidCap
+
+	// _InvalidClose occurs when close(...) is called with an argument that is
+	// not of channel type, or that is a receive-only channel.
+	//
+	// Example:
+	//  func f() {
+	//  	var x int
+	//  	close(x)
+	//  }
+	_InvalidClose
+
+	// _InvalidCopy occurs when the arguments are not of slice type or do not
+	// have compatible type.
+	//
+	// See https://golang.org/ref/spec#Appending_and_copying_slices for more
+	// information on the type requirements for the copy built-in.
+	//
+	// Example:
+	//  func f() {
+	//  	var x []int
+	//  	y := []int64{1,2,3}
+	//  	copy(x, y)
+	//  }
+	_InvalidCopy
+
+	// _InvalidComplex occurs when the complex built-in function is called with
+	// arguments with incompatible types.
+	//
+	// Example:
+	//  var _ = complex(float32(1), float64(2))
+	_InvalidComplex
+
+	// _InvalidDelete occurs when the delete built-in function is called with a
+	// first argument that is not a map.
+	//
+	// Example:
+	//  func f() {
+	//  	m := "hello"
+	//  	delete(m, "e")
+	//  }
+	_InvalidDelete
+
+	// _InvalidImag occurs when the imag built-in function is called with an
+	// argument that does not have complex type.
+	//
+	// Example:
+	//  var _ = imag(int(1))
+	_InvalidImag
+
+	// _InvalidLen occurs when an argument to the len built-in function is not of
+	// supported type.
+	//
+	// See https://golang.org/ref/spec#Length_and_capacity for information on
+	// which underlying types are supported as arguments to cap and len.
+	//
+	// Example:
+	//  var s = 2
+	//  var x = len(s)
+	_InvalidLen
+
+	// _SwappedMakeArgs occurs when make is called with three arguments, and its
+	// length argument is larger than its capacity argument.
+	//
+	// Example:
+	//  var x = make([]int, 3, 2)
+	_SwappedMakeArgs
+
+	// _InvalidMake occurs when make is called with an unsupported type argument.
+	//
+	// See https://golang.org/ref/spec#Making_slices_maps_and_channels for
+	// information on the types that may be created using make.
+	//
+	// Example:
+	//  var x = make(int)
+	_InvalidMake
+
+	// _InvalidReal occurs when the real built-in function is called with an
+	// argument that does not have complex type.
+	//
+	// Example:
+	//  var _ = real(int(1))
+	_InvalidReal
+
+	/* exprs > assertion */
+
+	// _InvalidAssert occurs when a type assertion is applied to a
+	// value that is not of interface type.
+	//
+	// Example:
+	//  var x = 1
+	//  var _ = x.(float64)
+	_InvalidAssert
+
+	// _ImpossibleAssert occurs for a type assertion x.(T) when the value x of
+	// interface cannot have dynamic type T, due to a missing or mismatching
+	// method on T.
+	//
+	// Example:
+	//  type T int
+	//
+	//  func (t *T) m() int { return int(*t) }
+	//
+	//  type I interface { m() int }
+	//
+	//  var x I
+	//  var _ = x.(T)
+	_ImpossibleAssert
+
+	/* exprs > conversion */
+
+	// _InvalidConversion occurs when the argument type cannot be converted to the
+	// target.
+	//
+	// See https://golang.org/ref/spec#Conversions for the rules of
+	// convertibility.
+	//
+	// Example:
+	//  var x float64
+	//  var _ = string(x)
+	_InvalidConversion
+
+	// _InvalidUntypedConversion occurs when an there is no valid implicit
+	// conversion from an untyped value satisfying the type constraints of the
+	// context in which it is used.
+	//
+	// Example:
+	//  var _ = 1 + ""
+	_InvalidUntypedConversion
+
+	/* offsetof */
+
+	// _BadOffsetofSyntax occurs when unsafe.Offsetof is called with an argument
+	// that is not a selector expression.
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  var x int
+	//  var _ = unsafe.Offsetof(x)
+	_BadOffsetofSyntax
+
+	// _InvalidOffsetof occurs when unsafe.Offsetof is called with a method
+	// selector, rather than a field selector, or when the field is embedded via
+	// a pointer.
+	//
+	// Per the spec:
+	//
+	//  "If f is an embedded field, it must be reachable without pointer
+	//  indirections through fields of the struct. "
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  type T struct { f int }
+	//  type S struct { *T }
+	//  var s S
+	//  var _ = unsafe.Offsetof(s.f)
+	//
+	// Example:
+	//  import "unsafe"
+	//
+	//  type S struct{}
+	//
+	//  func (S) m() {}
+	//
+	//  var s S
+	//  var _ = unsafe.Offsetof(s.m)
+	_InvalidOffsetof
+
+	/* control flow > scope */
+
+	// _UnusedExpr occurs when a side-effect free expression is used as a
+	// statement. Such a statement has no effect.
+	//
+	// Example:
+	//  func f(i int) {
+	//  	i*i
+	//  }
+	_UnusedExpr
+
+	// _UnusedVar occurs when a variable is declared but unused.
+	//
+	// Example:
+	//  func f() {
+	//  	x := 1
+	//  }
+	_UnusedVar
+
+	// _MissingReturn occurs when a function with results is missing a return
+	// statement.
+	//
+	// Example:
+	//  func f() int {}
+	_MissingReturn
+
+	// _WrongResultCount occurs when a return statement returns an incorrect
+	// number of values.
+	//
+	// Example:
+	//  func ReturnOne() int {
+	//  	return 1, 2
+	//  }
+	_WrongResultCount
+
+	// _OutOfScopeResult occurs when the name of a value implicitly returned by
+	// an empty return statement is shadowed in a nested scope.
+	//
+	// Example:
+	//  func factor(n int) (i int) {
+	//  	for i := 2; i < n; i++ {
+	//  		if n%i == 0 {
+	//  			return
+	//  		}
+	//  	}
+	//  	return 0
+	//  }
+	_OutOfScopeResult
+
+	/* control flow > if */
+
+	// _InvalidCond occurs when an if condition is not a boolean expression.
+	//
+	// Example:
+	//  func checkReturn(i int) {
+	//  	if i {
+	//  		panic("non-zero return")
+	//  	}
+	//  }
+	_InvalidCond
+
+	/* control flow > for */
+
+	// _InvalidPostDecl occurs when there is a declaration in a for-loop post
+	// statement.
+	//
+	// Example:
+	//  func f() {
+	//  	for i := 0; i < 10; j := 0 {}
+	//  }
+	_InvalidPostDecl
+
+	// _InvalidChanRange occurs when a send-only channel used in a range
+	// expression.
+	//
+	// Example:
+	//  func sum(c chan<- int) {
+	//  	s := 0
+	//  	for i := range c {
+	//  		s += i
+	//  	}
+	//  }
+	_InvalidChanRange
+
+	// _InvalidIterVar occurs when two iteration variables are used while ranging
+	// over a channel.
+	//
+	// Example:
+	//  func f(c chan int) {
+	//  	for k, v := range c {
+	//  		println(k, v)
+	//  	}
+	//  }
+	_InvalidIterVar
+
+	// _InvalidRangeExpr occurs when the type of a range expression is not array,
+	// slice, string, map, or channel.
+	//
+	// Example:
+	//  func f(i int) {
+	//  	for j := range i {
+	//  		println(j)
+	//  	}
+	//  }
+	_InvalidRangeExpr
+
+	/* control flow > switch */
+
+	// _MisplacedBreak occurs when a break statement is not within a for, switch,
+	// or select statement of the innermost function definition.
+	//
+	// Example:
+	//  func f() {
+	//  	break
+	//  }
+	_MisplacedBreak
+
+	// _MisplacedContinue occurs when a continue statement is not within a for
+	// loop of the innermost function definition.
+	//
+	// Example:
+	//  func sumeven(n int) int {
+	//  	proceed := func() {
+	//  		continue
+	//  	}
+	//  	sum := 0
+	//  	for i := 1; i <= n; i++ {
+	//  		if i % 2 != 0 {
+	//  			proceed()
+	//  		}
+	//  		sum += i
+	//  	}
+	//  	return sum
+	//  }
+	_MisplacedContinue
+
+	// _MisplacedFallthrough occurs when a fallthrough statement is not within an
+	// expression switch.
+	//
+	// Example:
+	//  func typename(i interface{}) string {
+	//  	switch i.(type) {
+	//  	case int64:
+	//  		fallthrough
+	//  	case int:
+	//  		return "int"
+	//  	}
+	//  	return "unsupported"
+	//  }
+	_MisplacedFallthrough
+
+	// _DuplicateCase occurs when a type or expression switch has duplicate
+	// cases.
+	//
+	// Example:
+	//  func printInt(i int) {
+	//  	switch i {
+	//  	case 1:
+	//  		println("one")
+	//  	case 1:
+	//  		println("One")
+	//  	}
+	//  }
+	_DuplicateCase
+
+	// _DuplicateDefault occurs when a type or expression switch has multiple
+	// default clauses.
+	//
+	// Example:
+	//  func printInt(i int) {
+	//  	switch i {
+	//  	case 1:
+	//  		println("one")
+	//  	default:
+	//  		println("One")
+	//  	default:
+	//  		println("1")
+	//  	}
+	//  }
+	_DuplicateDefault
+
+	// _BadTypeKeyword occurs when a .(type) expression is used anywhere other
+	// than a type switch.
+	//
+	// Example:
+	//  type I interface {
+	//  	m()
+	//  }
+	//  var t I
+	//  var _ = t.(type)
+	_BadTypeKeyword
+
+	// _InvalidTypeSwitch occurs when .(type) is used on an expression that is
+	// not of interface type.
+	//
+	// Example:
+	//  func f(i int) {
+	//  	switch x := i.(type) {}
+	//  }
+	_InvalidTypeSwitch
+
+	// _InvalidExprSwitch occurs when a switch expression is not comparable.
+	//
+	// Example:
+	//  func _() {
+	//  	var a struct{ _ func() }
+	//  	switch a /* ERROR cannot switch on a */ {
+	//  	}
+	//  }
+	_InvalidExprSwitch
+
+	/* control flow > select */
+
+	// _InvalidSelectCase occurs when a select case is not a channel send or
+	// receive.
+	//
+	// Example:
+	//  func checkChan(c <-chan int) bool {
+	//  	select {
+	//  	case c:
+	//  		return true
+	//  	default:
+	//  		return false
+	//  	}
+	//  }
+	_InvalidSelectCase
+
+	/* control flow > labels and jumps */
+
+	// _UndeclaredLabel occurs when an undeclared label is jumped to.
+	//
+	// Example:
+	//  func f() {
+	//  	goto L
+	//  }
+	_UndeclaredLabel
+
+	// _DuplicateLabel occurs when a label is declared more than once.
+	//
+	// Example:
+	//  func f() int {
+	//  L:
+	//  L:
+	//  	return 1
+	//  }
+	_DuplicateLabel
+
+	// _MisplacedLabel occurs when a break or continue label is not on a for,
+	// switch, or select statement.
+	//
+	// Example:
+	//  func f() {
+	//  L:
+	//  	a := []int{1,2,3}
+	//  	for _, e := range a {
+	//  		if e > 10 {
+	//  			break L
+	//  		}
+	//  		println(a)
+	//  	}
+	//  }
+	_MisplacedLabel
+
+	// _UnusedLabel occurs when a label is declared but not used.
+	//
+	// Example:
+	//  func f() {
+	//  L:
+	//  }
+	_UnusedLabel
+
+	// _JumpOverDecl occurs when a label jumps over a variable declaration.
+	//
+	// Example:
+	//  func f() int {
+	//  	goto L
+	//  	x := 2
+	//  L:
+	//  	x++
+	//  	return x
+	//  }
+	_JumpOverDecl
+
+	// _JumpIntoBlock occurs when a forward jump goes to a label inside a nested
+	// block.
+	//
+	// Example:
+	//  func f(x int) {
+	//  	goto L
+	//  	if x > 0 {
+	//  	L:
+	//  		print("inside block")
+	//  	}
+	// }
+	_JumpIntoBlock
+
+	/* control flow > calls */
+
+	// _InvalidMethodExpr occurs when a pointer method is called but the argument
+	// is not addressable.
+	//
+	// Example:
+	//  type T struct {}
+	//
+	//  func (*T) m() int { return 1 }
+	//
+	//  var _ = T.m(T{})
+	_InvalidMethodExpr
+
+	// _WrongArgCount occurs when too few or too many arguments are passed by a
+	// function call.
+	//
+	// Example:
+	//  func f(i int) {}
+	//  var x = f()
+	_WrongArgCount
+
+	// _InvalidCall occurs when an expression is called that is not of function
+	// type.
+	//
+	// Example:
+	//  var x = "x"
+	//  var y = x()
+	_InvalidCall
+
+	/* control flow > suspended */
+
+	// _UnusedResults occurs when a restricted expression-only built-in function
+	// is suspended via go or defer. Such a suspension discards the results of
+	// these side-effect free built-in functions, and therefore is ineffectual.
+	//
+	// Example:
+	//  func f(a []int) int {
+	//  	defer len(a)
+	//  	return i
+	//  }
+	_UnusedResults
+
+	// _InvalidDefer occurs when a deferred expression is not a function call,
+	// for example if the expression is a type conversion.
+	//
+	// Example:
+	//  func f(i int) int {
+	//  	defer int32(i)
+	//  	return i
+	//  }
+	_InvalidDefer
+
+	// _InvalidGo occurs when a go expression is not a function call, for example
+	// if the expression is a type conversion.
+	//
+	// Example:
+	//  func f(i int) int {
+	//  	go int32(i)
+	//  	return i
+	//  }
+	_InvalidGo
+)
diff --git a/src/go/types/errorcodes_test.go b/src/go/types/errorcodes_test.go
new file mode 100644
index 0000000..5da1cda
--- /dev/null
+++ b/src/go/types/errorcodes_test.go
@@ -0,0 +1,197 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types_test
+
+import (
+	"fmt"
+	"go/ast"
+	"go/constant"
+	"go/importer"
+	"go/parser"
+	"go/token"
+	"reflect"
+	"strings"
+	"testing"
+
+	. "go/types"
+)
+
+func TestErrorCodeExamples(t *testing.T) {
+	walkCodes(t, func(name string, value int, spec *ast.ValueSpec) {
+		t.Run(name, func(t *testing.T) {
+			doc := spec.Doc.Text()
+			examples := strings.Split(doc, "Example:")
+			for i := 1; i < len(examples); i++ {
+				example := examples[i]
+				err := checkExample(t, example)
+				if err == nil {
+					t.Fatalf("no error in example #%d", i)
+				}
+				typerr, ok := err.(Error)
+				if !ok {
+					t.Fatalf("not a types.Error: %v", err)
+				}
+				if got := readCode(typerr); got != value {
+					t.Errorf("%s: example #%d returned code %d (%s), want %d", name, i, got, err, value)
+				}
+			}
+		})
+	})
+}
+
+func walkCodes(t *testing.T, f func(string, int, *ast.ValueSpec)) {
+	t.Helper()
+	fset := token.NewFileSet()
+	files, err := pkgFiles(fset, ".", parser.ParseComments) // from self_test.go
+	if err != nil {
+		t.Fatal(err)
+	}
+	conf := Config{Importer: importer.Default()}
+	info := &Info{
+		Types: make(map[ast.Expr]TypeAndValue),
+		Defs:  make(map[*ast.Ident]Object),
+		Uses:  make(map[*ast.Ident]Object),
+	}
+	_, err = conf.Check("types", fset, files, info)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, file := range files {
+		for _, decl := range file.Decls {
+			decl, ok := decl.(*ast.GenDecl)
+			if !ok || decl.Tok != token.CONST {
+				continue
+			}
+			for _, spec := range decl.Specs {
+				spec, ok := spec.(*ast.ValueSpec)
+				if !ok || len(spec.Names) == 0 {
+					continue
+				}
+				obj := info.ObjectOf(spec.Names[0])
+				if named, ok := obj.Type().(*Named); ok && named.Obj().Name() == "errorCode" {
+					if len(spec.Names) != 1 {
+						t.Fatalf("bad Code declaration for %q: got %d names, want exactly 1", spec.Names[0].Name, len(spec.Names))
+					}
+					codename := spec.Names[0].Name
+					value := int(constant.Val(obj.(*Const).Val()).(int64))
+					f(codename, value, spec)
+				}
+			}
+		}
+	}
+}
+
+func readCode(err Error) int {
+	v := reflect.ValueOf(err)
+	return int(v.FieldByName("go116code").Int())
+}
+
+func checkExample(t *testing.T, example string) error {
+	t.Helper()
+	fset := token.NewFileSet()
+	src := fmt.Sprintf("package p\n\n%s", example)
+	file, err := parser.ParseFile(fset, "example.go", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	conf := Config{
+		FakeImportC: true,
+		Importer:    importer.Default(),
+	}
+	_, err = conf.Check("example", fset, []*ast.File{file}, nil)
+	return err
+}
+
+func TestErrorCodeStyle(t *testing.T) {
+	// The set of error codes is large and intended to be self-documenting, so
+	// this test enforces some style conventions.
+	forbiddenInIdent := []string{
+		// use invalid instead
+		"illegal",
+		// words with a common short-form
+		"argument",
+		"assertion",
+		"assignment",
+		"boolean",
+		"channel",
+		"condition",
+		"declaration",
+		"expression",
+		"function",
+		"initial", // use init for initializer, initialization, etc.
+		"integer",
+		"interface",
+		"iterat", // use iter for iterator, iteration, etc.
+		"literal",
+		"operation",
+		"package",
+		"pointer",
+		"receiver",
+		"signature",
+		"statement",
+		"variable",
+	}
+	forbiddenInComment := []string{
+		// lhs and rhs should be spelled-out.
+		"lhs", "rhs",
+		// builtin should be hyphenated.
+		"builtin",
+		// Use dot-dot-dot.
+		"ellipsis",
+	}
+	nameHist := make(map[int]int)
+	longestName := ""
+	maxValue := 0
+
+	walkCodes(t, func(name string, value int, spec *ast.ValueSpec) {
+		if name == "_" {
+			return
+		}
+		nameHist[len(name)]++
+		if value > maxValue {
+			maxValue = value
+		}
+		if len(name) > len(longestName) {
+			longestName = name
+		}
+		if token.IsExported(name) {
+			// This is an experimental API, and errorCode values should not be
+			// exported.
+			t.Errorf("%q is exported", name)
+		}
+		if name[0] != '_' || !token.IsExported(name[1:]) {
+			t.Errorf("%q should start with _, followed by an exported identifier", name)
+		}
+		lower := strings.ToLower(name)
+		for _, bad := range forbiddenInIdent {
+			if strings.Contains(lower, bad) {
+				t.Errorf("%q contains forbidden word %q", name, bad)
+			}
+		}
+		doc := spec.Doc.Text()
+		if !strings.HasPrefix(doc, name) {
+			t.Errorf("doc for %q does not start with identifier", name)
+		}
+		lowerComment := strings.ToLower(strings.TrimPrefix(doc, name))
+		for _, bad := range forbiddenInComment {
+			if strings.Contains(lowerComment, bad) {
+				t.Errorf("doc for %q contains forbidden word %q", name, bad)
+			}
+		}
+	})
+
+	if testing.Verbose() {
+		var totChars, totCount int
+		for chars, count := range nameHist {
+			totChars += chars * count
+			totCount += count
+		}
+		avg := float64(totChars) / float64(totCount)
+		fmt.Println()
+		fmt.Printf("%d error codes\n", totCount)
+		fmt.Printf("average length: %.2f chars\n", avg)
+		fmt.Printf("max length: %d (%s)\n", len(longestName), longestName)
+	}
+}
diff --git a/src/go/types/errors.go b/src/go/types/errors.go
new file mode 100644
index 0000000..a219501
--- /dev/null
+++ b/src/go/types/errors.go
@@ -0,0 +1,227 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements various error reporters.
+
+package types
+
+import (
+	"errors"
+	"fmt"
+	"go/ast"
+	"go/token"
+	"strconv"
+	"strings"
+)
+
+func assert(p bool) {
+	if !p {
+		panic("assertion failed")
+	}
+}
+
+func unreachable() {
+	panic("unreachable")
+}
+
+func (check *Checker) qualifier(pkg *Package) string {
+	// Qualify the package unless it's the package being type-checked.
+	if pkg != check.pkg {
+		// If the same package name was used by multiple packages, display the full path.
+		if check.pkgCnt[pkg.name] > 1 {
+			return strconv.Quote(pkg.path)
+		}
+		return pkg.name
+	}
+	return ""
+}
+
+func (check *Checker) sprintf(format string, args ...interface{}) string {
+	for i, arg := range args {
+		switch a := arg.(type) {
+		case nil:
+			arg = "<nil>"
+		case operand:
+			panic("internal error: should always pass *operand")
+		case *operand:
+			arg = operandString(a, check.qualifier)
+		case token.Pos:
+			arg = check.fset.Position(a).String()
+		case ast.Expr:
+			arg = ExprString(a)
+		case Object:
+			arg = ObjectString(a, check.qualifier)
+		case Type:
+			arg = TypeString(a, check.qualifier)
+		}
+		args[i] = arg
+	}
+	return fmt.Sprintf(format, args...)
+}
+
+func (check *Checker) trace(pos token.Pos, format string, args ...interface{}) {
+	fmt.Printf("%s:\t%s%s\n",
+		check.fset.Position(pos),
+		strings.Repeat(".  ", check.indent),
+		check.sprintf(format, args...),
+	)
+}
+
+// dump is only needed for debugging
+func (check *Checker) dump(format string, args ...interface{}) {
+	fmt.Println(check.sprintf(format, args...))
+}
+
+func (check *Checker) err(err error) {
+	if err == nil {
+		return
+	}
+	var e Error
+	isInternal := errors.As(err, &e)
+	// Cheap trick: Don't report errors with messages containing
+	// "invalid operand" or "invalid type" as those tend to be
+	// follow-on errors which don't add useful information. Only
+	// exclude them if these strings are not at the beginning,
+	// and only if we have at least one error already reported.
+	isInvalidErr := isInternal && (strings.Index(e.Msg, "invalid operand") > 0 || strings.Index(e.Msg, "invalid type") > 0)
+	if check.firstErr != nil && isInvalidErr {
+		return
+	}
+
+	if check.errpos != nil && isInternal {
+		// If we have an internal error and the errpos override is set, use it to
+		// augment our error positioning.
+		// TODO(rFindley) we may also want to augment the error message and refer
+		// to the position (pos) in the original expression.
+		span := spanOf(check.errpos)
+		e.Pos = span.pos
+		e.go116start = span.start
+		e.go116end = span.end
+		err = e
+	}
+
+	if check.firstErr == nil {
+		check.firstErr = err
+	}
+
+	if trace {
+		pos := e.Pos
+		msg := e.Msg
+		if !isInternal {
+			msg = err.Error()
+			pos = token.NoPos
+		}
+		check.trace(pos, "ERROR: %s", msg)
+	}
+
+	f := check.conf.Error
+	if f == nil {
+		panic(bailout{}) // report only first error
+	}
+	f(err)
+}
+
+func (check *Checker) newError(at positioner, code errorCode, soft bool, msg string) error {
+	span := spanOf(at)
+	return Error{
+		Fset:       check.fset,
+		Pos:        span.pos,
+		Msg:        msg,
+		Soft:       soft,
+		go116code:  code,
+		go116start: span.start,
+		go116end:   span.end,
+	}
+}
+
+// newErrorf creates a new Error, but does not handle it.
+func (check *Checker) newErrorf(at positioner, code errorCode, soft bool, format string, args ...interface{}) error {
+	msg := check.sprintf(format, args...)
+	return check.newError(at, code, soft, msg)
+}
+
+func (check *Checker) error(at positioner, code errorCode, msg string) {
+	check.err(check.newError(at, code, false, msg))
+}
+
+func (check *Checker) errorf(at positioner, code errorCode, format string, args ...interface{}) {
+	check.error(at, code, check.sprintf(format, args...))
+}
+
+func (check *Checker) softErrorf(at positioner, code errorCode, format string, args ...interface{}) {
+	check.err(check.newErrorf(at, code, true, format, args...))
+}
+
+func (check *Checker) invalidAST(at positioner, format string, args ...interface{}) {
+	check.errorf(at, 0, "invalid AST: "+format, args...)
+}
+
+func (check *Checker) invalidArg(at positioner, code errorCode, format string, args ...interface{}) {
+	check.errorf(at, code, "invalid argument: "+format, args...)
+}
+
+func (check *Checker) invalidOp(at positioner, code errorCode, format string, args ...interface{}) {
+	check.errorf(at, code, "invalid operation: "+format, args...)
+}
+
+// The positioner interface is used to extract the position of type-checker
+// errors.
+type positioner interface {
+	Pos() token.Pos
+}
+
+// posSpan holds a position range along with a highlighted position within that
+// range. This is used for positioning errors, with pos by convention being the
+// first position in the source where the error is known to exist, and start
+// and end defining the full span of syntax being considered when the error was
+// detected. Invariant: start <= pos < end || start == pos == end.
+type posSpan struct {
+	start, pos, end token.Pos
+}
+
+func (e posSpan) Pos() token.Pos {
+	return e.pos
+}
+
+// inNode creates a posSpan for the given node.
+// Invariant: node.Pos() <= pos < node.End() (node.End() is the position of the
+// first byte after node within the source).
+func inNode(node ast.Node, pos token.Pos) posSpan {
+	start, end := node.Pos(), node.End()
+	if debug {
+		assert(start <= pos && pos < end)
+	}
+	return posSpan{start, pos, end}
+}
+
+// atPos wraps a token.Pos to implement the positioner interface.
+type atPos token.Pos
+
+func (s atPos) Pos() token.Pos {
+	return token.Pos(s)
+}
+
+// spanOf extracts an error span from the given positioner. By default this is
+// the trivial span starting and ending at pos, but this span is expanded when
+// the argument naturally corresponds to a span of source code.
+func spanOf(at positioner) posSpan {
+	switch x := at.(type) {
+	case nil:
+		panic("internal error: nil")
+	case posSpan:
+		return x
+	case ast.Node:
+		pos := x.Pos()
+		return posSpan{pos, pos, x.End()}
+	case *operand:
+		if x.expr != nil {
+			pos := x.Pos()
+			return posSpan{pos, pos, x.expr.End()}
+		}
+		return posSpan{token.NoPos, token.NoPos, token.NoPos}
+	default:
+		pos := at.Pos()
+		return posSpan{pos, pos, pos}
+	}
+}
diff --git a/src/go/types/eval.go b/src/go/types/eval.go
new file mode 100644
index 0000000..5125960
--- /dev/null
+++ b/src/go/types/eval.go
@@ -0,0 +1,99 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types
+
+import (
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/token"
+)
+
+// Eval returns the type and, if constant, the value for the
+// expression expr, evaluated at position pos of package pkg,
+// which must have been derived from type-checking an AST with
+// complete position information relative to the provided file
+// set.
+//
+// The meaning of the parameters fset, pkg, and pos is the
+// same as in CheckExpr. An error is returned if expr cannot
+// be parsed successfully, or the resulting expr AST cannot be
+// type-checked.
+func Eval(fset *token.FileSet, pkg *Package, pos token.Pos, expr string) (_ TypeAndValue, err error) {
+	// parse expressions
+	node, err := parser.ParseExprFrom(fset, "eval", expr, 0)
+	if err != nil {
+		return TypeAndValue{}, err
+	}
+
+	info := &Info{
+		Types: make(map[ast.Expr]TypeAndValue),
+	}
+	err = CheckExpr(fset, pkg, pos, node, info)
+	return info.Types[node], err
+}
+
+// CheckExpr type checks the expression expr as if it had appeared at
+// position pos of package pkg. Type information about the expression
+// is recorded in info.
+//
+// If pkg == nil, the Universe scope is used and the provided
+// position pos is ignored. If pkg != nil, and pos is invalid,
+// the package scope is used. Otherwise, pos must belong to the
+// package.
+//
+// An error is returned if pos is not within the package or
+// if the node cannot be type-checked.
+//
+// Note: Eval and CheckExpr should not be used instead of running Check
+// to compute types and values, but in addition to Check, as these
+// functions ignore the context in which an expression is used (e.g., an
+// assignment). Thus, top-level untyped constants will return an
+// untyped type rather then the respective context-specific type.
+//
+func CheckExpr(fset *token.FileSet, pkg *Package, pos token.Pos, expr ast.Expr, info *Info) (err error) {
+	// determine scope
+	var scope *Scope
+	if pkg == nil {
+		scope = Universe
+		pos = token.NoPos
+	} else if !pos.IsValid() {
+		scope = pkg.scope
+	} else {
+		// The package scope extent (position information) may be
+		// incorrect (files spread across a wide range of fset
+		// positions) - ignore it and just consider its children
+		// (file scopes).
+		for _, fscope := range pkg.scope.children {
+			if scope = fscope.Innermost(pos); scope != nil {
+				break
+			}
+		}
+		if scope == nil || debug {
+			s := scope
+			for s != nil && s != pkg.scope {
+				s = s.parent
+			}
+			// s == nil || s == pkg.scope
+			if s == nil {
+				return fmt.Errorf("no position %s found in package %s", fset.Position(pos), pkg.name)
+			}
+		}
+	}
+
+	// initialize checker
+	check := NewChecker(nil, fset, pkg, info)
+	check.scope = scope
+	check.pos = pos
+	defer check.handleBailout(&err)
+
+	// evaluate node
+	var x operand
+	check.rawExpr(&x, expr, nil)
+	check.processDelayed(0) // incl. all functions
+	check.recordUntyped()
+
+	return nil
+}
diff --git a/src/go/types/eval_test.go b/src/go/types/eval_test.go
new file mode 100644
index 0000000..d940bf0
--- /dev/null
+++ b/src/go/types/eval_test.go
@@ -0,0 +1,297 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains tests for Eval.
+
+package types_test
+
+import (
+	"fmt"
+	"go/ast"
+	"go/importer"
+	"go/parser"
+	"go/token"
+	"internal/testenv"
+	"strings"
+	"testing"
+
+	. "go/types"
+)
+
+func testEval(t *testing.T, fset *token.FileSet, pkg *Package, pos token.Pos, expr string, typ Type, typStr, valStr string) {
+	gotTv, err := Eval(fset, pkg, pos, expr)
+	if err != nil {
+		t.Errorf("Eval(%q) failed: %s", expr, err)
+		return
+	}
+	if gotTv.Type == nil {
+		t.Errorf("Eval(%q) got nil type but no error", expr)
+		return
+	}
+
+	// compare types
+	if typ != nil {
+		// we have a type, check identity
+		if !Identical(gotTv.Type, typ) {
+			t.Errorf("Eval(%q) got type %s, want %s", expr, gotTv.Type, typ)
+			return
+		}
+	} else {
+		// we have a string, compare type string
+		gotStr := gotTv.Type.String()
+		if gotStr != typStr {
+			t.Errorf("Eval(%q) got type %s, want %s", expr, gotStr, typStr)
+			return
+		}
+	}
+
+	// compare values
+	gotStr := ""
+	if gotTv.Value != nil {
+		gotStr = gotTv.Value.ExactString()
+	}
+	if gotStr != valStr {
+		t.Errorf("Eval(%q) got value %s, want %s", expr, gotStr, valStr)
+	}
+}
+
+func TestEvalBasic(t *testing.T) {
+	fset := token.NewFileSet()
+	for _, typ := range Typ[Bool : String+1] {
+		testEval(t, fset, nil, token.NoPos, typ.Name(), typ, "", "")
+	}
+}
+
+func TestEvalComposite(t *testing.T) {
+	fset := token.NewFileSet()
+	for _, test := range independentTestTypes {
+		testEval(t, fset, nil, token.NoPos, test.src, nil, test.str, "")
+	}
+}
+
+func TestEvalArith(t *testing.T) {
+	var tests = []string{
+		`true`,
+		`false == false`,
+		`12345678 + 87654321 == 99999999`,
+		`10 * 20 == 200`,
+		`(1<<1000)*2 >> 100 == 2<<900`,
+		`"foo" + "bar" == "foobar"`,
+		`"abc" <= "bcd"`,
+		`len([10]struct{}{}) == 2*5`,
+	}
+	fset := token.NewFileSet()
+	for _, test := range tests {
+		testEval(t, fset, nil, token.NoPos, test, Typ[UntypedBool], "", "true")
+	}
+}
+
+func TestEvalPos(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	// The contents of /*-style comments are of the form
+	//	expr => value, type
+	// where value may be the empty string.
+	// Each expr is evaluated at the position of the comment
+	// and the result is compared with the expected value
+	// and type.
+	var sources = []string{
+		`
+		package p
+		import "fmt"
+		import m "math"
+		const c = 3.0
+		type T []int
+		func f(a int, s string) float64 {
+			fmt.Println("calling f")
+			_ = m.Pi // use package math
+			const d int = c + 1
+			var x int
+			x = a + len(s)
+			return float64(x)
+			/* true => true, untyped bool */
+			/* fmt.Println => , func(a ...interface{}) (n int, err error) */
+			/* c => 3, untyped float */
+			/* T => , p.T */
+			/* a => , int */
+			/* s => , string */
+			/* d => 4, int */
+			/* x => , int */
+			/* d/c => 1, int */
+			/* c/2 => 3/2, untyped float */
+			/* m.Pi < m.E => false, untyped bool */
+		}
+		`,
+		`
+		package p
+		/* c => 3, untyped float */
+		type T1 /* T1 => , p.T1 */ struct {}
+		var v1 /* v1 => , int */ = 42
+		func /* f1 => , func(v1 float64) */ f1(v1 float64) {
+			/* f1 => , func(v1 float64) */
+			/* v1 => , float64 */
+			var c /* c => 3, untyped float */ = "foo" /* c => , string */
+			{
+				var c struct {
+					c /* c => , string */ int
+				}
+				/* c => , struct{c int} */
+				_ = c
+			}
+			_ = func(a, b, c int) /* c => , string */ {
+				/* c => , int */
+			}
+			_ = c
+			type FT /* FT => , p.FT */ interface{}
+		}
+		`,
+		`
+		package p
+		/* T => , p.T */
+		`,
+		`
+		package p
+		import "io"
+		type R = io.Reader
+		func _() {
+			/* interface{R}.Read => , func(interface{io.Reader}, p []byte) (n int, err error) */
+			_ = func() {
+				/* interface{io.Writer}.Write => , func(interface{io.Writer}, p []byte) (n int, err error) */
+				type io interface {} // must not shadow io in line above
+			}
+			type R interface {} // must not shadow R in first line of this function body
+		}
+		`,
+	}
+
+	fset := token.NewFileSet()
+	var files []*ast.File
+	for i, src := range sources {
+		file, err := parser.ParseFile(fset, "p", src, parser.ParseComments)
+		if err != nil {
+			t.Fatalf("could not parse file %d: %s", i, err)
+		}
+		files = append(files, file)
+	}
+
+	conf := Config{Importer: importer.Default()}
+	pkg, err := conf.Check("p", fset, files, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, file := range files {
+		for _, group := range file.Comments {
+			for _, comment := range group.List {
+				s := comment.Text
+				if len(s) >= 4 && s[:2] == "/*" && s[len(s)-2:] == "*/" {
+					str, typ := split(s[2:len(s)-2], ", ")
+					str, val := split(str, "=>")
+					testEval(t, fset, pkg, comment.Pos(), str, nil, typ, val)
+				}
+			}
+		}
+	}
+}
+
+// split splits string s at the first occurrence of s.
+func split(s, sep string) (string, string) {
+	i := strings.Index(s, sep)
+	return strings.TrimSpace(s[:i]), strings.TrimSpace(s[i+len(sep):])
+}
+
+func TestCheckExpr(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	// Each comment has the form /* expr => object */:
+	// expr is an identifier or selector expression that is passed
+	// to CheckExpr at the position of the comment, and object is
+	// the string form of the object it denotes.
+	const src = `
+package p
+
+import "fmt"
+
+const c = 3.0
+type T []int
+type S struct{ X int }
+
+func f(a int, s string) S {
+	/* fmt.Println => func fmt.Println(a ...interface{}) (n int, err error) */
+	/* fmt.Stringer.String => func (fmt.Stringer).String() string */
+	fmt.Println("calling f")
+
+	var fmt struct{ Println int }
+	/* fmt => var fmt struct{Println int} */
+	/* fmt.Println => field Println int */
+	/* f(1, "").X => field X int */
+	fmt.Println = 1
+
+	/* append => builtin append */
+
+	/* new(S).X => field X int */
+
+	return S{}
+}`
+
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "p", src, parser.ParseComments)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	conf := Config{Importer: importer.Default()}
+	pkg, err := conf.Check("p", fset, []*ast.File{f}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	checkExpr := func(pos token.Pos, str string) (Object, error) {
+		expr, err := parser.ParseExprFrom(fset, "eval", str, 0)
+		if err != nil {
+			return nil, err
+		}
+
+		info := &Info{
+			Uses:       make(map[*ast.Ident]Object),
+			Selections: make(map[*ast.SelectorExpr]*Selection),
+		}
+		if err := CheckExpr(fset, pkg, pos, expr, info); err != nil {
+			return nil, fmt.Errorf("CheckExpr(%q) failed: %s", str, err)
+		}
+		switch expr := expr.(type) {
+		case *ast.Ident:
+			if obj, ok := info.Uses[expr]; ok {
+				return obj, nil
+			}
+		case *ast.SelectorExpr:
+			if sel, ok := info.Selections[expr]; ok {
+				return sel.Obj(), nil
+			}
+			if obj, ok := info.Uses[expr.Sel]; ok {
+				return obj, nil // qualified identifier
+			}
+		}
+		return nil, fmt.Errorf("no object for %s", str)
+	}
+
+	for _, group := range f.Comments {
+		for _, comment := range group.List {
+			s := comment.Text
+			if len(s) >= 4 && strings.HasPrefix(s, "/*") && strings.HasSuffix(s, "*/") {
+				pos := comment.Pos()
+				expr, wantObj := split(s[2:len(s)-2], "=>")
+				obj, err := checkExpr(pos, expr)
+				if err != nil {
+					t.Errorf("%s: %s", fset.Position(pos), err)
+					continue
+				}
+				if obj.String() != wantObj {
+					t.Errorf("%s: checkExpr(%s) = %s, want %v",
+						fset.Position(pos), expr, obj, wantObj)
+				}
+			}
+		}
+	}
+}
diff --git a/src/go/types/example_test.go b/src/go/types/example_test.go
new file mode 100644
index 0000000..3747f3b
--- /dev/null
+++ b/src/go/types/example_test.go
@@ -0,0 +1,334 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Only run where builders (build.golang.org) have
+// access to compiled packages for import.
+//
+// +build !arm,!arm64
+
+package types_test
+
+// This file shows examples of basic usage of the go/types API.
+//
+// To locate a Go package, use (*go/build.Context).Import.
+// To load, parse, and type-check a complete Go program
+// from source, use golang.org/x/tools/go/loader.
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/format"
+	"go/importer"
+	"go/parser"
+	"go/token"
+	"go/types"
+	"log"
+	"regexp"
+	"sort"
+	"strings"
+)
+
+// ExampleScope prints the tree of Scopes of a package created from a
+// set of parsed files.
+func ExampleScope() {
+	// Parse the source files for a package.
+	fset := token.NewFileSet()
+	var files []*ast.File
+	for _, file := range []struct{ name, input string }{
+		{"main.go", `
+package main
+import "fmt"
+func main() {
+	freezing := FToC(-18)
+	fmt.Println(freezing, Boiling) }
+`},
+		{"celsius.go", `
+package main
+import "fmt"
+type Celsius float64
+func (c Celsius) String() string { return fmt.Sprintf("%g°C", c) }
+func FToC(f float64) Celsius { return Celsius(f - 32 / 9 * 5) }
+const Boiling Celsius = 100
+func Unused() { {}; {{ var x int; _ = x }} } // make sure empty block scopes get printed
+`},
+	} {
+		f, err := parser.ParseFile(fset, file.name, file.input, 0)
+		if err != nil {
+			log.Fatal(err)
+		}
+		files = append(files, f)
+	}
+
+	// Type-check a package consisting of these files.
+	// Type information for the imported "fmt" package
+	// comes from $GOROOT/pkg/$GOOS_$GOOARCH/fmt.a.
+	conf := types.Config{Importer: importer.Default()}
+	pkg, err := conf.Check("temperature", fset, files, nil)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Print the tree of scopes.
+	// For determinism, we redact addresses.
+	var buf bytes.Buffer
+	pkg.Scope().WriteTo(&buf, 0, true)
+	rx := regexp.MustCompile(` 0x[a-fA-F0-9]*`)
+	fmt.Println(rx.ReplaceAllString(buf.String(), ""))
+
+	// Output:
+	// package "temperature" scope {
+	// .  const temperature.Boiling temperature.Celsius
+	// .  type temperature.Celsius float64
+	// .  func temperature.FToC(f float64) temperature.Celsius
+	// .  func temperature.Unused()
+	// .  func temperature.main()
+	// .  main.go scope {
+	// .  .  package fmt
+	// .  .  function scope {
+	// .  .  .  var freezing temperature.Celsius
+	// .  .  }
+	// .  }
+	// .  celsius.go scope {
+	// .  .  package fmt
+	// .  .  function scope {
+	// .  .  .  var c temperature.Celsius
+	// .  .  }
+	// .  .  function scope {
+	// .  .  .  var f float64
+	// .  .  }
+	// .  .  function scope {
+	// .  .  .  block scope {
+	// .  .  .  }
+	// .  .  .  block scope {
+	// .  .  .  .  block scope {
+	// .  .  .  .  .  var x int
+	// .  .  .  .  }
+	// .  .  .  }
+	// .  .  }
+	// .  }
+	// }
+}
+
+// ExampleMethodSet prints the method sets of various types.
+func ExampleMethodSet() {
+	// Parse a single source file.
+	const input = `
+package temperature
+import "fmt"
+type Celsius float64
+func (c Celsius) String() string  { return fmt.Sprintf("%g°C", c) }
+func (c *Celsius) SetF(f float64) { *c = Celsius(f - 32 / 9 * 5) }
+
+type S struct { I; m int }
+type I interface { m() byte }
+`
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "celsius.go", input, 0)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Type-check a package consisting of this file.
+	// Type information for the imported packages
+	// comes from $GOROOT/pkg/$GOOS_$GOOARCH/fmt.a.
+	conf := types.Config{Importer: importer.Default()}
+	pkg, err := conf.Check("temperature", fset, []*ast.File{f}, nil)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Print the method sets of Celsius and *Celsius.
+	celsius := pkg.Scope().Lookup("Celsius").Type()
+	for _, t := range []types.Type{celsius, types.NewPointer(celsius)} {
+		fmt.Printf("Method set of %s:\n", t)
+		mset := types.NewMethodSet(t)
+		for i := 0; i < mset.Len(); i++ {
+			fmt.Println(mset.At(i))
+		}
+		fmt.Println()
+	}
+
+	// Print the method set of S.
+	styp := pkg.Scope().Lookup("S").Type()
+	fmt.Printf("Method set of %s:\n", styp)
+	fmt.Println(types.NewMethodSet(styp))
+
+	// Output:
+	// Method set of temperature.Celsius:
+	// method (temperature.Celsius) String() string
+	//
+	// Method set of *temperature.Celsius:
+	// method (*temperature.Celsius) SetF(f float64)
+	// method (*temperature.Celsius) String() string
+	//
+	// Method set of temperature.S:
+	// MethodSet {}
+}
+
+// ExampleInfo prints various facts recorded by the type checker in a
+// types.Info struct: definitions of and references to each named object,
+// and the type, value, and mode of every expression in the package.
+func ExampleInfo() {
+	// Parse a single source file.
+	const input = `
+package fib
+
+type S string
+
+var a, b, c = len(b), S(c), "hello"
+
+func fib(x int) int {
+	if x < 2 {
+		return x
+	}
+	return fib(x-1) - fib(x-2)
+}`
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "fib.go", input, 0)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Type-check the package.
+	// We create an empty map for each kind of input
+	// we're interested in, and Check populates them.
+	info := types.Info{
+		Types: make(map[ast.Expr]types.TypeAndValue),
+		Defs:  make(map[*ast.Ident]types.Object),
+		Uses:  make(map[*ast.Ident]types.Object),
+	}
+	var conf types.Config
+	pkg, err := conf.Check("fib", fset, []*ast.File{f}, &info)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Print package-level variables in initialization order.
+	fmt.Printf("InitOrder: %v\n\n", info.InitOrder)
+
+	// For each named object, print the line and
+	// column of its definition and each of its uses.
+	fmt.Println("Defs and Uses of each named object:")
+	usesByObj := make(map[types.Object][]string)
+	for id, obj := range info.Uses {
+		posn := fset.Position(id.Pos())
+		lineCol := fmt.Sprintf("%d:%d", posn.Line, posn.Column)
+		usesByObj[obj] = append(usesByObj[obj], lineCol)
+	}
+	var items []string
+	for obj, uses := range usesByObj {
+		sort.Strings(uses)
+		item := fmt.Sprintf("%s:\n  defined at %s\n  used at %s",
+			types.ObjectString(obj, types.RelativeTo(pkg)),
+			fset.Position(obj.Pos()),
+			strings.Join(uses, ", "))
+		items = append(items, item)
+	}
+	sort.Strings(items) // sort by line:col, in effect
+	fmt.Println(strings.Join(items, "\n"))
+	fmt.Println()
+
+	fmt.Println("Types and Values of each expression:")
+	items = nil
+	for expr, tv := range info.Types {
+		var buf bytes.Buffer
+		posn := fset.Position(expr.Pos())
+		tvstr := tv.Type.String()
+		if tv.Value != nil {
+			tvstr += " = " + tv.Value.String()
+		}
+		// line:col | expr | mode : type = value
+		fmt.Fprintf(&buf, "%2d:%2d | %-19s | %-7s : %s",
+			posn.Line, posn.Column, exprString(fset, expr),
+			mode(tv), tvstr)
+		items = append(items, buf.String())
+	}
+	sort.Strings(items)
+	fmt.Println(strings.Join(items, "\n"))
+
+	// Output:
+	// InitOrder: [c = "hello" b = S(c) a = len(b)]
+	//
+	// Defs and Uses of each named object:
+	// builtin len:
+	//   defined at -
+	//   used at 6:15
+	// func fib(x int) int:
+	//   defined at fib.go:8:6
+	//   used at 12:20, 12:9
+	// type S string:
+	//   defined at fib.go:4:6
+	//   used at 6:23
+	// type int:
+	//   defined at -
+	//   used at 8:12, 8:17
+	// type string:
+	//   defined at -
+	//   used at 4:8
+	// var b S:
+	//   defined at fib.go:6:8
+	//   used at 6:19
+	// var c string:
+	//   defined at fib.go:6:11
+	//   used at 6:25
+	// var x int:
+	//   defined at fib.go:8:10
+	//   used at 10:10, 12:13, 12:24, 9:5
+	//
+	// Types and Values of each expression:
+	//  4: 8 | string              | type    : string
+	//  6:15 | len                 | builtin : func(string) int
+	//  6:15 | len(b)              | value   : int
+	//  6:19 | b                   | var     : fib.S
+	//  6:23 | S                   | type    : fib.S
+	//  6:23 | S(c)                | value   : fib.S
+	//  6:25 | c                   | var     : string
+	//  6:29 | "hello"             | value   : string = "hello"
+	//  8:12 | int                 | type    : int
+	//  8:17 | int                 | type    : int
+	//  9: 5 | x                   | var     : int
+	//  9: 5 | x < 2               | value   : untyped bool
+	//  9: 9 | 2                   | value   : int = 2
+	// 10:10 | x                   | var     : int
+	// 12: 9 | fib                 | value   : func(x int) int
+	// 12: 9 | fib(x - 1)          | value   : int
+	// 12: 9 | fib(x-1) - fib(x-2) | value   : int
+	// 12:13 | x                   | var     : int
+	// 12:13 | x - 1               | value   : int
+	// 12:15 | 1                   | value   : int = 1
+	// 12:20 | fib                 | value   : func(x int) int
+	// 12:20 | fib(x - 2)          | value   : int
+	// 12:24 | x                   | var     : int
+	// 12:24 | x - 2               | value   : int
+	// 12:26 | 2                   | value   : int = 2
+}
+
+func mode(tv types.TypeAndValue) string {
+	switch {
+	case tv.IsVoid():
+		return "void"
+	case tv.IsType():
+		return "type"
+	case tv.IsBuiltin():
+		return "builtin"
+	case tv.IsNil():
+		return "nil"
+	case tv.Assignable():
+		if tv.Addressable() {
+			return "var"
+		}
+		return "mapindex"
+	case tv.IsValue():
+		return "value"
+	default:
+		return "unknown"
+	}
+}
+
+func exprString(fset *token.FileSet, expr ast.Expr) string {
+	var buf bytes.Buffer
+	format.Node(&buf, fset, expr)
+	return buf.String()
+}
diff --git a/src/go/types/expr.go b/src/go/types/expr.go
new file mode 100644
index 0000000..eb20561
--- /dev/null
+++ b/src/go/types/expr.go
@@ -0,0 +1,1709 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements typechecking of expressions.
+
+package types
+
+import (
+	"fmt"
+	"go/ast"
+	"go/constant"
+	"go/token"
+	"math"
+)
+
+/*
+Basic algorithm:
+
+Expressions are checked recursively, top down. Expression checker functions
+are generally of the form:
+
+  func f(x *operand, e *ast.Expr, ...)
+
+where e is the expression to be checked, and x is the result of the check.
+The check performed by f may fail in which case x.mode == invalid, and
+related error messages will have been issued by f.
+
+If a hint argument is present, it is the composite literal element type
+of an outer composite literal; it is used to type-check composite literal
+elements that have no explicit type specification in the source
+(e.g.: []T{{...}, {...}}, the hint is the type T in this case).
+
+All expressions are checked via rawExpr, which dispatches according
+to expression kind. Upon returning, rawExpr is recording the types and
+constant values for all expressions that have an untyped type (those types
+may change on the way up in the expression tree). Usually these are constants,
+but the results of comparisons or non-constant shifts of untyped constants
+may also be untyped, but not constant.
+
+Untyped expressions may eventually become fully typed (i.e., not untyped),
+typically when the value is assigned to a variable, or is used otherwise.
+The updateExprType method is used to record this final type and update
+the recorded types: the type-checked expression tree is again traversed down,
+and the new type is propagated as needed. Untyped constant expression values
+that become fully typed must now be representable by the full type (constant
+sub-expression trees are left alone except for their roots). This mechanism
+ensures that a client sees the actual (run-time) type an untyped value would
+have. It also permits type-checking of lhs shift operands "as if the shift
+were not present": when updateExprType visits an untyped lhs shift operand
+and assigns it it's final type, that type must be an integer type, and a
+constant lhs must be representable as an integer.
+
+When an expression gets its final type, either on the way out from rawExpr,
+on the way down in updateExprType, or at the end of the type checker run,
+the type (and constant value, if any) is recorded via Info.Types, if present.
+*/
+
+type opPredicates map[token.Token]func(Type) bool
+
+var unaryOpPredicates = opPredicates{
+	token.ADD: isNumeric,
+	token.SUB: isNumeric,
+	token.XOR: isInteger,
+	token.NOT: isBoolean,
+}
+
+func (check *Checker) op(m opPredicates, x *operand, op token.Token) bool {
+	if pred := m[op]; pred != nil {
+		if !pred(x.typ) {
+			check.invalidOp(x, _UndefinedOp, "operator %s not defined for %s", op, x)
+			return false
+		}
+	} else {
+		check.invalidAST(x, "unknown operator %s", op)
+		return false
+	}
+	return true
+}
+
+// The unary expression e may be nil. It's passed in for better error messages only.
+func (check *Checker) unary(x *operand, e *ast.UnaryExpr, op token.Token) {
+	switch op {
+	case token.AND:
+		// spec: "As an exception to the addressability
+		// requirement x may also be a composite literal."
+		if _, ok := unparen(x.expr).(*ast.CompositeLit); !ok && x.mode != variable {
+			check.invalidOp(x, _UnaddressableOperand, "cannot take address of %s", x)
+			x.mode = invalid
+			return
+		}
+		x.mode = value
+		x.typ = &Pointer{base: x.typ}
+		return
+
+	case token.ARROW:
+		typ, ok := x.typ.Underlying().(*Chan)
+		if !ok {
+			check.invalidOp(x, _InvalidReceive, "cannot receive from non-channel %s", x)
+			x.mode = invalid
+			return
+		}
+		if typ.dir == SendOnly {
+			check.invalidOp(x, _InvalidReceive, "cannot receive from send-only channel %s", x)
+			x.mode = invalid
+			return
+		}
+		x.mode = commaok
+		x.typ = typ.elem
+		check.hasCallOrRecv = true
+		return
+	}
+
+	if !check.op(unaryOpPredicates, x, op) {
+		x.mode = invalid
+		return
+	}
+
+	if x.mode == constant_ {
+		typ := x.typ.Underlying().(*Basic)
+		var prec uint
+		if isUnsigned(typ) {
+			prec = uint(check.conf.sizeof(typ) * 8)
+		}
+		x.val = constant.UnaryOp(op, x.val, prec)
+		// Typed constants must be representable in
+		// their type after each constant operation.
+		if isTyped(typ) {
+			if e != nil {
+				x.expr = e // for better error message
+			}
+			check.representable(x, typ)
+		}
+		return
+	}
+
+	x.mode = value
+	// x.typ remains unchanged
+}
+
+func isShift(op token.Token) bool {
+	return op == token.SHL || op == token.SHR
+}
+
+func isComparison(op token.Token) bool {
+	// Note: tokens are not ordered well to make this much easier
+	switch op {
+	case token.EQL, token.NEQ, token.LSS, token.LEQ, token.GTR, token.GEQ:
+		return true
+	}
+	return false
+}
+
+func fitsFloat32(x constant.Value) bool {
+	f32, _ := constant.Float32Val(x)
+	f := float64(f32)
+	return !math.IsInf(f, 0)
+}
+
+func roundFloat32(x constant.Value) constant.Value {
+	f32, _ := constant.Float32Val(x)
+	f := float64(f32)
+	if !math.IsInf(f, 0) {
+		return constant.MakeFloat64(f)
+	}
+	return nil
+}
+
+func fitsFloat64(x constant.Value) bool {
+	f, _ := constant.Float64Val(x)
+	return !math.IsInf(f, 0)
+}
+
+func roundFloat64(x constant.Value) constant.Value {
+	f, _ := constant.Float64Val(x)
+	if !math.IsInf(f, 0) {
+		return constant.MakeFloat64(f)
+	}
+	return nil
+}
+
+// representableConst reports whether x can be represented as
+// value of the given basic type and for the configuration
+// provided (only needed for int/uint sizes).
+//
+// If rounded != nil, *rounded is set to the rounded value of x for
+// representable floating-point and complex values, and to an Int
+// value for integer values; it is left alone otherwise.
+// It is ok to provide the addressof the first argument for rounded.
+//
+// The check parameter may be nil if representableConst is invoked
+// (indirectly) through an exported API call (AssignableTo, ConvertibleTo)
+// because we don't need the Checker's config for those calls.
+func representableConst(x constant.Value, check *Checker, typ *Basic, rounded *constant.Value) bool {
+	if x.Kind() == constant.Unknown {
+		return true // avoid follow-up errors
+	}
+
+	var conf *Config
+	if check != nil {
+		conf = check.conf
+	}
+
+	switch {
+	case isInteger(typ):
+		x := constant.ToInt(x)
+		if x.Kind() != constant.Int {
+			return false
+		}
+		if rounded != nil {
+			*rounded = x
+		}
+		if x, ok := constant.Int64Val(x); ok {
+			switch typ.kind {
+			case Int:
+				var s = uint(conf.sizeof(typ)) * 8
+				return int64(-1)<<(s-1) <= x && x <= int64(1)<<(s-1)-1
+			case Int8:
+				const s = 8
+				return -1<<(s-1) <= x && x <= 1<<(s-1)-1
+			case Int16:
+				const s = 16
+				return -1<<(s-1) <= x && x <= 1<<(s-1)-1
+			case Int32:
+				const s = 32
+				return -1<<(s-1) <= x && x <= 1<<(s-1)-1
+			case Int64, UntypedInt:
+				return true
+			case Uint, Uintptr:
+				if s := uint(conf.sizeof(typ)) * 8; s < 64 {
+					return 0 <= x && x <= int64(1)<<s-1
+				}
+				return 0 <= x
+			case Uint8:
+				const s = 8
+				return 0 <= x && x <= 1<<s-1
+			case Uint16:
+				const s = 16
+				return 0 <= x && x <= 1<<s-1
+			case Uint32:
+				const s = 32
+				return 0 <= x && x <= 1<<s-1
+			case Uint64:
+				return 0 <= x
+			default:
+				unreachable()
+			}
+		}
+		// x does not fit into int64
+		switch n := constant.BitLen(x); typ.kind {
+		case Uint, Uintptr:
+			var s = uint(conf.sizeof(typ)) * 8
+			return constant.Sign(x) >= 0 && n <= int(s)
+		case Uint64:
+			return constant.Sign(x) >= 0 && n <= 64
+		case UntypedInt:
+			return true
+		}
+
+	case isFloat(typ):
+		x := constant.ToFloat(x)
+		if x.Kind() != constant.Float {
+			return false
+		}
+		switch typ.kind {
+		case Float32:
+			if rounded == nil {
+				return fitsFloat32(x)
+			}
+			r := roundFloat32(x)
+			if r != nil {
+				*rounded = r
+				return true
+			}
+		case Float64:
+			if rounded == nil {
+				return fitsFloat64(x)
+			}
+			r := roundFloat64(x)
+			if r != nil {
+				*rounded = r
+				return true
+			}
+		case UntypedFloat:
+			return true
+		default:
+			unreachable()
+		}
+
+	case isComplex(typ):
+		x := constant.ToComplex(x)
+		if x.Kind() != constant.Complex {
+			return false
+		}
+		switch typ.kind {
+		case Complex64:
+			if rounded == nil {
+				return fitsFloat32(constant.Real(x)) && fitsFloat32(constant.Imag(x))
+			}
+			re := roundFloat32(constant.Real(x))
+			im := roundFloat32(constant.Imag(x))
+			if re != nil && im != nil {
+				*rounded = constant.BinaryOp(re, token.ADD, constant.MakeImag(im))
+				return true
+			}
+		case Complex128:
+			if rounded == nil {
+				return fitsFloat64(constant.Real(x)) && fitsFloat64(constant.Imag(x))
+			}
+			re := roundFloat64(constant.Real(x))
+			im := roundFloat64(constant.Imag(x))
+			if re != nil && im != nil {
+				*rounded = constant.BinaryOp(re, token.ADD, constant.MakeImag(im))
+				return true
+			}
+		case UntypedComplex:
+			return true
+		default:
+			unreachable()
+		}
+
+	case isString(typ):
+		return x.Kind() == constant.String
+
+	case isBoolean(typ):
+		return x.Kind() == constant.Bool
+	}
+
+	return false
+}
+
+// representable checks that a constant operand is representable in the given
+// basic type.
+func (check *Checker) representable(x *operand, typ *Basic) {
+	if err := check.isRepresentable(x, typ); err != nil {
+		x.mode = invalid
+		check.err(err)
+	}
+}
+
+func (check *Checker) isRepresentable(x *operand, typ *Basic) error {
+	assert(x.mode == constant_)
+	if !representableConst(x.val, check, typ, &x.val) {
+		var msg string
+		var code errorCode
+		if isNumeric(x.typ) && isNumeric(typ) {
+			// numeric conversion : error msg
+			//
+			// integer -> integer : overflows
+			// integer -> float   : overflows (actually not possible)
+			// float   -> integer : truncated
+			// float   -> float   : overflows
+			//
+			if !isInteger(x.typ) && isInteger(typ) {
+				msg = "%s truncated to %s"
+				code = _TruncatedFloat
+			} else {
+				msg = "%s overflows %s"
+				code = _NumericOverflow
+			}
+		} else {
+			msg = "cannot convert %s to %s"
+			code = _InvalidConstVal
+		}
+		return check.newErrorf(x, code, false, msg, x, typ)
+	}
+	return nil
+}
+
+// updateExprType updates the type of x to typ and invokes itself
+// recursively for the operands of x, depending on expression kind.
+// If typ is still an untyped and not the final type, updateExprType
+// only updates the recorded untyped type for x and possibly its
+// operands. Otherwise (i.e., typ is not an untyped type anymore,
+// or it is the final type for x), the type and value are recorded.
+// Also, if x is a constant, it must be representable as a value of typ,
+// and if x is the (formerly untyped) lhs operand of a non-constant
+// shift, it must be an integer value.
+//
+func (check *Checker) updateExprType(x ast.Expr, typ Type, final bool) {
+	old, found := check.untyped[x]
+	if !found {
+		return // nothing to do
+	}
+
+	// update operands of x if necessary
+	switch x := x.(type) {
+	case *ast.BadExpr,
+		*ast.FuncLit,
+		*ast.CompositeLit,
+		*ast.IndexExpr,
+		*ast.SliceExpr,
+		*ast.TypeAssertExpr,
+		*ast.StarExpr,
+		*ast.KeyValueExpr,
+		*ast.ArrayType,
+		*ast.StructType,
+		*ast.FuncType,
+		*ast.InterfaceType,
+		*ast.MapType,
+		*ast.ChanType:
+		// These expression are never untyped - nothing to do.
+		// The respective sub-expressions got their final types
+		// upon assignment or use.
+		if debug {
+			check.dump("%v: found old type(%s): %s (new: %s)", x.Pos(), x, old.typ, typ)
+			unreachable()
+		}
+		return
+
+	case *ast.CallExpr:
+		// Resulting in an untyped constant (e.g., built-in complex).
+		// The respective calls take care of calling updateExprType
+		// for the arguments if necessary.
+
+	case *ast.Ident, *ast.BasicLit, *ast.SelectorExpr:
+		// An identifier denoting a constant, a constant literal,
+		// or a qualified identifier (imported untyped constant).
+		// No operands to take care of.
+
+	case *ast.ParenExpr:
+		check.updateExprType(x.X, typ, final)
+
+	case *ast.UnaryExpr:
+		// If x is a constant, the operands were constants.
+		// The operands don't need to be updated since they
+		// never get "materialized" into a typed value. If
+		// left in the untyped map, they will be processed
+		// at the end of the type check.
+		if old.val != nil {
+			break
+		}
+		check.updateExprType(x.X, typ, final)
+
+	case *ast.BinaryExpr:
+		if old.val != nil {
+			break // see comment for unary expressions
+		}
+		if isComparison(x.Op) {
+			// The result type is independent of operand types
+			// and the operand types must have final types.
+		} else if isShift(x.Op) {
+			// The result type depends only on lhs operand.
+			// The rhs type was updated when checking the shift.
+			check.updateExprType(x.X, typ, final)
+		} else {
+			// The operand types match the result type.
+			check.updateExprType(x.X, typ, final)
+			check.updateExprType(x.Y, typ, final)
+		}
+
+	default:
+		unreachable()
+	}
+
+	// If the new type is not final and still untyped, just
+	// update the recorded type.
+	if !final && isUntyped(typ) {
+		old.typ = typ.Underlying().(*Basic)
+		check.untyped[x] = old
+		return
+	}
+
+	// Otherwise we have the final (typed or untyped type).
+	// Remove it from the map of yet untyped expressions.
+	delete(check.untyped, x)
+
+	if old.isLhs {
+		// If x is the lhs of a shift, its final type must be integer.
+		// We already know from the shift check that it is representable
+		// as an integer if it is a constant.
+		if !isInteger(typ) {
+			check.invalidOp(x, _InvalidShiftOperand, "shifted operand %s (type %s) must be integer", x, typ)
+			return
+		}
+		// Even if we have an integer, if the value is a constant we
+		// still must check that it is representable as the specific
+		// int type requested (was issue #22969). Fall through here.
+	}
+	if old.val != nil {
+		// If x is a constant, it must be representable as a value of typ.
+		c := operand{old.mode, x, old.typ, old.val, 0}
+		check.convertUntyped(&c, typ)
+		if c.mode == invalid {
+			return
+		}
+	}
+
+	// Everything's fine, record final type and value for x.
+	check.recordTypeAndValue(x, old.mode, typ, old.val)
+}
+
+// updateExprVal updates the value of x to val.
+func (check *Checker) updateExprVal(x ast.Expr, val constant.Value) {
+	if info, ok := check.untyped[x]; ok {
+		info.val = val
+		check.untyped[x] = info
+	}
+}
+
+// convertUntyped attempts to set the type of an untyped value to the target type.
+func (check *Checker) convertUntyped(x *operand, target Type) {
+	if err := check.canConvertUntyped(x, target); err != nil {
+		x.mode = invalid
+		check.err(err)
+	}
+}
+
+func (check *Checker) canConvertUntyped(x *operand, target Type) error {
+	if x.mode == invalid || isTyped(x.typ) || target == Typ[Invalid] {
+		return nil
+	}
+
+	if isUntyped(target) {
+		// both x and target are untyped
+		xkind := x.typ.(*Basic).kind
+		tkind := target.(*Basic).kind
+		if isNumeric(x.typ) && isNumeric(target) {
+			if xkind < tkind {
+				x.typ = target
+				check.updateExprType(x.expr, target, false)
+			}
+		} else if xkind != tkind {
+			return check.newErrorf(x, _InvalidUntypedConversion, false, "cannot convert %s to %s", x, target)
+		}
+		return nil
+	}
+
+	if t, ok := target.Underlying().(*Basic); ok && x.mode == constant_ {
+		if err := check.isRepresentable(x, t); err != nil {
+			return err
+		}
+		// Expression value may have been rounded - update if needed.
+		check.updateExprVal(x.expr, x.val)
+	} else {
+		newTarget := check.implicitType(x, target)
+		if newTarget == nil {
+			return check.newErrorf(x, _InvalidUntypedConversion, false, "cannot convert %s to %s", x, target)
+		}
+		target = newTarget
+	}
+	x.typ = target
+	// Even though implicitType can return UntypedNil, this value is final: the
+	// predeclared identifier nil has no type.
+	check.updateExprType(x.expr, target, true)
+	return nil
+}
+
+// implicitType returns the implicit type of x when used in a context where the
+// target type is expected. If no such implicit conversion is possible, it
+// returns nil.
+func (check *Checker) implicitType(x *operand, target Type) Type {
+	assert(isUntyped(x.typ))
+	switch t := target.Underlying().(type) {
+	case *Basic:
+		assert(x.mode != constant_)
+		// Non-constant untyped values may appear as the
+		// result of comparisons (untyped bool), intermediate
+		// (delayed-checked) rhs operands of shifts, and as
+		// the value nil.
+		switch x.typ.(*Basic).kind {
+		case UntypedBool:
+			if !isBoolean(target) {
+				return nil
+			}
+		case UntypedInt, UntypedRune, UntypedFloat, UntypedComplex:
+			if !isNumeric(target) {
+				return nil
+			}
+		case UntypedString:
+			// Non-constant untyped string values are not permitted by the spec and
+			// should not occur during normal typechecking passes, but this path is
+			// reachable via the AssignableTo API.
+			if !isString(target) {
+				return nil
+			}
+		case UntypedNil:
+			// Unsafe.Pointer is a basic type that includes nil.
+			if !hasNil(target) {
+				return nil
+			}
+		default:
+			return nil
+		}
+	case *Interface:
+		// Values must have concrete dynamic types. If the value is nil,
+		// keep it untyped (this is important for tools such as go vet which
+		// need the dynamic type for argument checking of say, print
+		// functions)
+		if x.isNil() {
+			return Typ[UntypedNil]
+		}
+		// cannot assign untyped values to non-empty interfaces
+		check.completeInterface(t)
+		if !t.Empty() {
+			return nil
+		}
+		return Default(x.typ)
+	case *Pointer, *Signature, *Slice, *Map, *Chan:
+		if !x.isNil() {
+			return nil
+		}
+		// Keep nil untyped - see comment for interfaces, above.
+		return Typ[UntypedNil]
+	default:
+		return nil
+	}
+	return target
+}
+
+func (check *Checker) comparison(x, y *operand, op token.Token) {
+	// spec: "In any comparison, the first operand must be assignable
+	// to the type of the second operand, or vice versa."
+	err := ""
+	var code errorCode
+	xok, _ := x.assignableTo(check, y.typ, nil)
+	yok, _ := y.assignableTo(check, x.typ, nil)
+	if xok || yok {
+		defined := false
+		switch op {
+		case token.EQL, token.NEQ:
+			// spec: "The equality operators == and != apply to operands that are comparable."
+			defined = Comparable(x.typ) && Comparable(y.typ) || x.isNil() && hasNil(y.typ) || y.isNil() && hasNil(x.typ)
+		case token.LSS, token.LEQ, token.GTR, token.GEQ:
+			// spec: The ordering operators <, <=, >, and >= apply to operands that are ordered."
+			defined = isOrdered(x.typ) && isOrdered(y.typ)
+		default:
+			unreachable()
+		}
+		if !defined {
+			typ := x.typ
+			if x.isNil() {
+				typ = y.typ
+			}
+			err = check.sprintf("operator %s not defined for %s", op, typ)
+			code = _UndefinedOp
+		}
+	} else {
+		err = check.sprintf("mismatched types %s and %s", x.typ, y.typ)
+		code = _MismatchedTypes
+	}
+
+	if err != "" {
+		check.errorf(x, code, "cannot compare %s %s %s (%s)", x.expr, op, y.expr, err)
+		x.mode = invalid
+		return
+	}
+
+	if x.mode == constant_ && y.mode == constant_ {
+		x.val = constant.MakeBool(constant.Compare(x.val, op, y.val))
+		// The operands are never materialized; no need to update
+		// their types.
+	} else {
+		x.mode = value
+		// The operands have now their final types, which at run-
+		// time will be materialized. Update the expression trees.
+		// If the current types are untyped, the materialized type
+		// is the respective default type.
+		check.updateExprType(x.expr, Default(x.typ), true)
+		check.updateExprType(y.expr, Default(y.typ), true)
+	}
+
+	// spec: "Comparison operators compare two operands and yield
+	//        an untyped boolean value."
+	x.typ = Typ[UntypedBool]
+}
+
+func (check *Checker) shift(x, y *operand, e *ast.BinaryExpr, op token.Token) {
+	untypedx := isUntyped(x.typ)
+
+	var xval constant.Value
+	if x.mode == constant_ {
+		xval = constant.ToInt(x.val)
+	}
+
+	if isInteger(x.typ) || untypedx && xval != nil && xval.Kind() == constant.Int {
+		// The lhs is of integer type or an untyped constant representable
+		// as an integer. Nothing to do.
+	} else {
+		// shift has no chance
+		check.invalidOp(x, _InvalidShiftOperand, "shifted operand %s must be integer", x)
+		x.mode = invalid
+		return
+	}
+
+	// spec: "The right operand in a shift expression must have integer type
+	// or be an untyped constant representable by a value of type uint."
+	switch {
+	case isInteger(y.typ):
+		// nothing to do
+	case isUntyped(y.typ):
+		check.convertUntyped(y, Typ[Uint])
+		if y.mode == invalid {
+			x.mode = invalid
+			return
+		}
+	default:
+		check.invalidOp(y, _InvalidShiftCount, "shift count %s must be integer", y)
+		x.mode = invalid
+		return
+	}
+
+	var yval constant.Value
+	if y.mode == constant_ {
+		// rhs must be an integer value
+		// (Either it was of an integer type already, or it was
+		// untyped and successfully converted to a uint above.)
+		yval = constant.ToInt(y.val)
+		assert(yval.Kind() == constant.Int)
+		if constant.Sign(yval) < 0 {
+			check.invalidOp(y, _InvalidShiftCount, "negative shift count %s", y)
+			x.mode = invalid
+			return
+		}
+	}
+
+	if x.mode == constant_ {
+		if y.mode == constant_ {
+			// rhs must be within reasonable bounds in constant shifts
+			const shiftBound = 1023 - 1 + 52 // so we can express smallestFloat64
+			s, ok := constant.Uint64Val(yval)
+			if !ok || s > shiftBound {
+				check.invalidOp(y, _InvalidShiftCount, "invalid shift count %s", y)
+				x.mode = invalid
+				return
+			}
+			// The lhs is representable as an integer but may not be an integer
+			// (e.g., 2.0, an untyped float) - this can only happen for untyped
+			// non-integer numeric constants. Correct the type so that the shift
+			// result is of integer type.
+			if !isInteger(x.typ) {
+				x.typ = Typ[UntypedInt]
+			}
+			// x is a constant so xval != nil and it must be of Int kind.
+			x.val = constant.Shift(xval, op, uint(s))
+			// Typed constants must be representable in
+			// their type after each constant operation.
+			if isTyped(x.typ) {
+				if e != nil {
+					x.expr = e // for better error message
+				}
+				check.representable(x, x.typ.Underlying().(*Basic))
+			}
+			return
+		}
+
+		// non-constant shift with constant lhs
+		if untypedx {
+			// spec: "If the left operand of a non-constant shift
+			// expression is an untyped constant, the type of the
+			// constant is what it would be if the shift expression
+			// were replaced by its left operand alone.".
+			//
+			// Delay operand checking until we know the final type
+			// by marking the lhs expression as lhs shift operand.
+			//
+			// Usually (in correct programs), the lhs expression
+			// is in the untyped map. However, it is possible to
+			// create incorrect programs where the same expression
+			// is evaluated twice (via a declaration cycle) such
+			// that the lhs expression type is determined in the
+			// first round and thus deleted from the map, and then
+			// not found in the second round (double insertion of
+			// the same expr node still just leads to one entry for
+			// that node, and it can only be deleted once).
+			// Be cautious and check for presence of entry.
+			// Example: var e, f = int(1<<""[f]) // issue 11347
+			if info, found := check.untyped[x.expr]; found {
+				info.isLhs = true
+				check.untyped[x.expr] = info
+			}
+			// keep x's type
+			x.mode = value
+			return
+		}
+	}
+
+	// non-constant shift - lhs must be an integer
+	if !isInteger(x.typ) {
+		check.invalidOp(x, _InvalidShiftOperand, "shifted operand %s must be integer", x)
+		x.mode = invalid
+		return
+	}
+
+	x.mode = value
+}
+
+var binaryOpPredicates = opPredicates{
+	token.ADD: func(typ Type) bool { return isNumeric(typ) || isString(typ) },
+	token.SUB: isNumeric,
+	token.MUL: isNumeric,
+	token.QUO: isNumeric,
+	token.REM: isInteger,
+
+	token.AND:     isInteger,
+	token.OR:      isInteger,
+	token.XOR:     isInteger,
+	token.AND_NOT: isInteger,
+
+	token.LAND: isBoolean,
+	token.LOR:  isBoolean,
+}
+
+// The binary expression e may be nil. It's passed in for better error messages only.
+func (check *Checker) binary(x *operand, e *ast.BinaryExpr, lhs, rhs ast.Expr, op token.Token, opPos token.Pos) {
+	var y operand
+
+	check.expr(x, lhs)
+	check.expr(&y, rhs)
+
+	if x.mode == invalid {
+		return
+	}
+	if y.mode == invalid {
+		x.mode = invalid
+		x.expr = y.expr
+		return
+	}
+
+	if isShift(op) {
+		check.shift(x, &y, e, op)
+		return
+	}
+
+	check.convertUntyped(x, y.typ)
+	if x.mode == invalid {
+		return
+	}
+	check.convertUntyped(&y, x.typ)
+	if y.mode == invalid {
+		x.mode = invalid
+		return
+	}
+
+	if isComparison(op) {
+		check.comparison(x, &y, op)
+		return
+	}
+
+	if !check.identical(x.typ, y.typ) {
+		// only report an error if we have valid types
+		// (otherwise we had an error reported elsewhere already)
+		if x.typ != Typ[Invalid] && y.typ != Typ[Invalid] {
+			var posn positioner = x
+			if e != nil {
+				posn = e
+			}
+			check.invalidOp(posn, _MismatchedTypes, "mismatched types %s and %s", x.typ, y.typ)
+		}
+		x.mode = invalid
+		return
+	}
+
+	if !check.op(binaryOpPredicates, x, op) {
+		x.mode = invalid
+		return
+	}
+
+	if op == token.QUO || op == token.REM {
+		// check for zero divisor
+		if (x.mode == constant_ || isInteger(x.typ)) && y.mode == constant_ && constant.Sign(y.val) == 0 {
+			check.invalidOp(&y, _DivByZero, "division by zero")
+			x.mode = invalid
+			return
+		}
+
+		// check for divisor underflow in complex division (see issue 20227)
+		if x.mode == constant_ && y.mode == constant_ && isComplex(x.typ) {
+			re, im := constant.Real(y.val), constant.Imag(y.val)
+			re2, im2 := constant.BinaryOp(re, token.MUL, re), constant.BinaryOp(im, token.MUL, im)
+			if constant.Sign(re2) == 0 && constant.Sign(im2) == 0 {
+				check.invalidOp(&y, _DivByZero, "division by zero")
+				x.mode = invalid
+				return
+			}
+		}
+	}
+
+	if x.mode == constant_ && y.mode == constant_ {
+		xval := x.val
+		yval := y.val
+		typ := x.typ.Underlying().(*Basic)
+		// force integer division of integer operands
+		if op == token.QUO && isInteger(typ) {
+			op = token.QUO_ASSIGN
+		}
+		x.val = constant.BinaryOp(xval, op, yval)
+		// report error if valid operands lead to an invalid result
+		if xval.Kind() != constant.Unknown && yval.Kind() != constant.Unknown && x.val.Kind() == constant.Unknown {
+			// TODO(gri) We should report exactly what went wrong. At the
+			//           moment we don't have the (go/constant) API for that.
+			//           See also TODO in go/constant/value.go.
+			check.errorf(atPos(opPos), _InvalidConstVal, "constant result is not representable")
+			// TODO(gri) Should we mark operands with unknown values as invalid?
+		}
+		// Typed constants must be representable in
+		// their type after each constant operation.
+		if isTyped(typ) {
+			if e != nil {
+				x.expr = e // for better error message
+			}
+			check.representable(x, typ)
+		}
+		return
+	}
+
+	x.mode = value
+	// x.typ is unchanged
+}
+
+// index checks an index expression for validity.
+// If max >= 0, it is the upper bound for index.
+// If the result typ is != Typ[Invalid], index is valid and typ is its (possibly named) integer type.
+// If the result val >= 0, index is valid and val is its constant int value.
+func (check *Checker) index(index ast.Expr, max int64) (typ Type, val int64) {
+	typ = Typ[Invalid]
+	val = -1
+
+	var x operand
+	check.expr(&x, index)
+	if x.mode == invalid {
+		return
+	}
+
+	// an untyped constant must be representable as Int
+	check.convertUntyped(&x, Typ[Int])
+	if x.mode == invalid {
+		return
+	}
+
+	// the index must be of integer type
+	if !isInteger(x.typ) {
+		check.invalidArg(&x, _InvalidIndex, "index %s must be integer", &x)
+		return
+	}
+
+	if x.mode != constant_ {
+		return x.typ, -1
+	}
+
+	// a constant index i must be in bounds
+	if constant.Sign(x.val) < 0 {
+		check.invalidArg(&x, _InvalidIndex, "index %s must not be negative", &x)
+		return
+	}
+
+	v, valid := constant.Int64Val(constant.ToInt(x.val))
+	if !valid || max >= 0 && v >= max {
+		check.errorf(&x, _InvalidIndex, "index %s is out of bounds", &x)
+		return
+	}
+
+	// 0 <= v [ && v < max ]
+	return Typ[Int], v
+}
+
+// indexElts checks the elements (elts) of an array or slice composite literal
+// against the literal's element type (typ), and the element indices against
+// the literal length if known (length >= 0). It returns the length of the
+// literal (maximum index value + 1).
+//
+func (check *Checker) indexedElts(elts []ast.Expr, typ Type, length int64) int64 {
+	visited := make(map[int64]bool, len(elts))
+	var index, max int64
+	for _, e := range elts {
+		// determine and check index
+		validIndex := false
+		eval := e
+		if kv, _ := e.(*ast.KeyValueExpr); kv != nil {
+			if typ, i := check.index(kv.Key, length); typ != Typ[Invalid] {
+				if i >= 0 {
+					index = i
+					validIndex = true
+				} else {
+					check.errorf(e, _InvalidLitIndex, "index %s must be integer constant", kv.Key)
+				}
+			}
+			eval = kv.Value
+		} else if length >= 0 && index >= length {
+			check.errorf(e, _OversizeArrayLit, "index %d is out of bounds (>= %d)", index, length)
+		} else {
+			validIndex = true
+		}
+
+		// if we have a valid index, check for duplicate entries
+		if validIndex {
+			if visited[index] {
+				check.errorf(e, _DuplicateLitKey, "duplicate index %d in array or slice literal", index)
+			}
+			visited[index] = true
+		}
+		index++
+		if index > max {
+			max = index
+		}
+
+		// check element against composite literal element type
+		var x operand
+		check.exprWithHint(&x, eval, typ)
+		check.assignment(&x, typ, "array or slice literal")
+	}
+	return max
+}
+
+// exprKind describes the kind of an expression; the kind
+// determines if an expression is valid in 'statement context'.
+type exprKind int
+
+const (
+	conversion exprKind = iota
+	expression
+	statement
+)
+
+// rawExpr typechecks expression e and initializes x with the expression
+// value or type. If an error occurred, x.mode is set to invalid.
+// If hint != nil, it is the type of a composite literal element.
+//
+func (check *Checker) rawExpr(x *operand, e ast.Expr, hint Type) exprKind {
+	if trace {
+		check.trace(e.Pos(), "%s", e)
+		check.indent++
+		defer func() {
+			check.indent--
+			check.trace(e.Pos(), "=> %s", x)
+		}()
+	}
+
+	kind := check.exprInternal(x, e, hint)
+
+	// convert x into a user-friendly set of values
+	// TODO(gri) this code can be simplified
+	var typ Type
+	var val constant.Value
+	switch x.mode {
+	case invalid:
+		typ = Typ[Invalid]
+	case novalue:
+		typ = (*Tuple)(nil)
+	case constant_:
+		typ = x.typ
+		val = x.val
+	default:
+		typ = x.typ
+	}
+	assert(x.expr != nil && typ != nil)
+
+	if isUntyped(typ) {
+		// delay type and value recording until we know the type
+		// or until the end of type checking
+		check.rememberUntyped(x.expr, false, x.mode, typ.(*Basic), val)
+	} else {
+		check.recordTypeAndValue(e, x.mode, typ, val)
+	}
+
+	return kind
+}
+
+// exprInternal contains the core of type checking of expressions.
+// Must only be called by rawExpr.
+//
+func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
+	// make sure x has a valid state in case of bailout
+	// (was issue 5770)
+	x.mode = invalid
+	x.typ = Typ[Invalid]
+
+	switch e := e.(type) {
+	case *ast.BadExpr:
+		goto Error // error was reported before
+
+	case *ast.Ident:
+		check.ident(x, e, nil, false)
+
+	case *ast.Ellipsis:
+		// ellipses are handled explicitly where they are legal
+		// (array composite literals and parameter lists)
+		check.error(e, _BadDotDotDotSyntax, "invalid use of '...'")
+		goto Error
+
+	case *ast.BasicLit:
+		x.setConst(e.Kind, e.Value)
+		if x.mode == invalid {
+			// The parser already establishes syntactic correctness.
+			// If we reach here it's because of number under-/overflow.
+			// TODO(gri) setConst (and in turn the go/constant package)
+			// should return an error describing the issue.
+			check.errorf(e, _InvalidConstVal, "malformed constant: %s", e.Value)
+			goto Error
+		}
+
+	case *ast.FuncLit:
+		if sig, ok := check.typ(e.Type).(*Signature); ok {
+			// Anonymous functions are considered part of the
+			// init expression/func declaration which contains
+			// them: use existing package-level declaration info.
+			decl := check.decl // capture for use in closure below
+			iota := check.iota // capture for use in closure below (#22345)
+			// Don't type-check right away because the function may
+			// be part of a type definition to which the function
+			// body refers. Instead, type-check as soon as possible,
+			// but before the enclosing scope contents changes (#22992).
+			check.later(func() {
+				check.funcBody(decl, "<function literal>", sig, e.Body, iota)
+			})
+			x.mode = value
+			x.typ = sig
+		} else {
+			check.invalidAST(e, "invalid function literal %s", e)
+			goto Error
+		}
+
+	case *ast.CompositeLit:
+		var typ, base Type
+
+		switch {
+		case e.Type != nil:
+			// composite literal type present - use it
+			// [...]T array types may only appear with composite literals.
+			// Check for them here so we don't have to handle ... in general.
+			if atyp, _ := e.Type.(*ast.ArrayType); atyp != nil && atyp.Len != nil {
+				if ellip, _ := atyp.Len.(*ast.Ellipsis); ellip != nil && ellip.Elt == nil {
+					// We have an "open" [...]T array type.
+					// Create a new ArrayType with unknown length (-1)
+					// and finish setting it up after analyzing the literal.
+					typ = &Array{len: -1, elem: check.typ(atyp.Elt)}
+					base = typ
+					break
+				}
+			}
+			typ = check.typ(e.Type)
+			base = typ
+
+		case hint != nil:
+			// no composite literal type present - use hint (element type of enclosing type)
+			typ = hint
+			base, _ = deref(typ.Underlying()) // *T implies &T{}
+
+		default:
+			// TODO(gri) provide better error messages depending on context
+			check.error(e, _UntypedLit, "missing type in composite literal")
+			goto Error
+		}
+
+		switch utyp := base.Underlying().(type) {
+		case *Struct:
+			if len(e.Elts) == 0 {
+				break
+			}
+			fields := utyp.fields
+			if _, ok := e.Elts[0].(*ast.KeyValueExpr); ok {
+				// all elements must have keys
+				visited := make([]bool, len(fields))
+				for _, e := range e.Elts {
+					kv, _ := e.(*ast.KeyValueExpr)
+					if kv == nil {
+						check.error(e, _MixedStructLit, "mixture of field:value and value elements in struct literal")
+						continue
+					}
+					key, _ := kv.Key.(*ast.Ident)
+					// do all possible checks early (before exiting due to errors)
+					// so we don't drop information on the floor
+					check.expr(x, kv.Value)
+					if key == nil {
+						check.errorf(kv, _InvalidLitField, "invalid field name %s in struct literal", kv.Key)
+						continue
+					}
+					i := fieldIndex(utyp.fields, check.pkg, key.Name)
+					if i < 0 {
+						check.errorf(kv, _MissingLitField, "unknown field %s in struct literal", key.Name)
+						continue
+					}
+					fld := fields[i]
+					check.recordUse(key, fld)
+					etyp := fld.typ
+					check.assignment(x, etyp, "struct literal")
+					// 0 <= i < len(fields)
+					if visited[i] {
+						check.errorf(kv, _DuplicateLitField, "duplicate field name %s in struct literal", key.Name)
+						continue
+					}
+					visited[i] = true
+				}
+			} else {
+				// no element must have a key
+				for i, e := range e.Elts {
+					if kv, _ := e.(*ast.KeyValueExpr); kv != nil {
+						check.error(kv, _MixedStructLit, "mixture of field:value and value elements in struct literal")
+						continue
+					}
+					check.expr(x, e)
+					if i >= len(fields) {
+						check.error(x, _InvalidStructLit, "too many values in struct literal")
+						break // cannot continue
+					}
+					// i < len(fields)
+					fld := fields[i]
+					if !fld.Exported() && fld.pkg != check.pkg {
+						check.errorf(x,
+							_UnexportedLitField,
+							"implicit assignment to unexported field %s in %s literal", fld.name, typ)
+						continue
+					}
+					etyp := fld.typ
+					check.assignment(x, etyp, "struct literal")
+				}
+				if len(e.Elts) < len(fields) {
+					check.error(inNode(e, e.Rbrace), _InvalidStructLit, "too few values in struct literal")
+					// ok to continue
+				}
+			}
+
+		case *Array:
+			// Prevent crash if the array referred to is not yet set up. Was issue #18643.
+			// This is a stop-gap solution. Should use Checker.objPath to report entire
+			// path starting with earliest declaration in the source. TODO(gri) fix this.
+			if utyp.elem == nil {
+				check.error(e, _InvalidTypeCycle, "illegal cycle in type declaration")
+				goto Error
+			}
+			n := check.indexedElts(e.Elts, utyp.elem, utyp.len)
+			// If we have an array of unknown length (usually [...]T arrays, but also
+			// arrays [n]T where n is invalid) set the length now that we know it and
+			// record the type for the array (usually done by check.typ which is not
+			// called for [...]T). We handle [...]T arrays and arrays with invalid
+			// length the same here because it makes sense to "guess" the length for
+			// the latter if we have a composite literal; e.g. for [n]int{1, 2, 3}
+			// where n is invalid for some reason, it seems fair to assume it should
+			// be 3 (see also Checked.arrayLength and issue #27346).
+			if utyp.len < 0 {
+				utyp.len = n
+				// e.Type is missing if we have a composite literal element
+				// that is itself a composite literal with omitted type. In
+				// that case there is nothing to record (there is no type in
+				// the source at that point).
+				if e.Type != nil {
+					check.recordTypeAndValue(e.Type, typexpr, utyp, nil)
+				}
+			}
+
+		case *Slice:
+			// Prevent crash if the slice referred to is not yet set up.
+			// See analogous comment for *Array.
+			if utyp.elem == nil {
+				check.error(e, _InvalidTypeCycle, "illegal cycle in type declaration")
+				goto Error
+			}
+			check.indexedElts(e.Elts, utyp.elem, -1)
+
+		case *Map:
+			// Prevent crash if the map referred to is not yet set up.
+			// See analogous comment for *Array.
+			if utyp.key == nil || utyp.elem == nil {
+				check.error(e, _InvalidTypeCycle, "illegal cycle in type declaration")
+				goto Error
+			}
+			visited := make(map[interface{}][]Type, len(e.Elts))
+			for _, e := range e.Elts {
+				kv, _ := e.(*ast.KeyValueExpr)
+				if kv == nil {
+					check.error(e, _MissingLitKey, "missing key in map literal")
+					continue
+				}
+				check.exprWithHint(x, kv.Key, utyp.key)
+				check.assignment(x, utyp.key, "map literal")
+				if x.mode == invalid {
+					continue
+				}
+				if x.mode == constant_ {
+					duplicate := false
+					// if the key is of interface type, the type is also significant when checking for duplicates
+					xkey := keyVal(x.val)
+					if _, ok := utyp.key.Underlying().(*Interface); ok {
+						for _, vtyp := range visited[xkey] {
+							if check.identical(vtyp, x.typ) {
+								duplicate = true
+								break
+							}
+						}
+						visited[xkey] = append(visited[xkey], x.typ)
+					} else {
+						_, duplicate = visited[xkey]
+						visited[xkey] = nil
+					}
+					if duplicate {
+						check.errorf(x, _DuplicateLitKey, "duplicate key %s in map literal", x.val)
+						continue
+					}
+				}
+				check.exprWithHint(x, kv.Value, utyp.elem)
+				check.assignment(x, utyp.elem, "map literal")
+			}
+
+		default:
+			// when "using" all elements unpack KeyValueExpr
+			// explicitly because check.use doesn't accept them
+			for _, e := range e.Elts {
+				if kv, _ := e.(*ast.KeyValueExpr); kv != nil {
+					// Ideally, we should also "use" kv.Key but we can't know
+					// if it's an externally defined struct key or not. Going
+					// forward anyway can lead to other errors. Give up instead.
+					e = kv.Value
+				}
+				check.use(e)
+			}
+			// if utyp is invalid, an error was reported before
+			if utyp != Typ[Invalid] {
+				check.errorf(e, _InvalidLit, "invalid composite literal type %s", typ)
+				goto Error
+			}
+		}
+
+		x.mode = value
+		x.typ = typ
+
+	case *ast.ParenExpr:
+		kind := check.rawExpr(x, e.X, nil)
+		x.expr = e
+		return kind
+
+	case *ast.SelectorExpr:
+		check.selector(x, e)
+
+	case *ast.IndexExpr:
+		check.expr(x, e.X)
+		if x.mode == invalid {
+			check.use(e.Index)
+			goto Error
+		}
+
+		valid := false
+		length := int64(-1) // valid if >= 0
+		switch typ := x.typ.Underlying().(type) {
+		case *Basic:
+			if isString(typ) {
+				valid = true
+				if x.mode == constant_ {
+					length = int64(len(constant.StringVal(x.val)))
+				}
+				// an indexed string always yields a byte value
+				// (not a constant) even if the string and the
+				// index are constant
+				x.mode = value
+				x.typ = universeByte // use 'byte' name
+			}
+
+		case *Array:
+			valid = true
+			length = typ.len
+			if x.mode != variable {
+				x.mode = value
+			}
+			x.typ = typ.elem
+
+		case *Pointer:
+			if typ, _ := typ.base.Underlying().(*Array); typ != nil {
+				valid = true
+				length = typ.len
+				x.mode = variable
+				x.typ = typ.elem
+			}
+
+		case *Slice:
+			valid = true
+			x.mode = variable
+			x.typ = typ.elem
+
+		case *Map:
+			var key operand
+			check.expr(&key, e.Index)
+			check.assignment(&key, typ.key, "map index")
+			// ok to continue even if indexing failed - map element type is known
+			x.mode = mapindex
+			x.typ = typ.elem
+			x.expr = e
+			return expression
+		}
+
+		if !valid {
+			check.invalidOp(x, _NonIndexableOperand, "cannot index %s", x)
+			goto Error
+		}
+
+		if e.Index == nil {
+			check.invalidAST(e, "missing index for %s", x)
+			goto Error
+		}
+
+		check.index(e.Index, length)
+		// ok to continue
+
+	case *ast.SliceExpr:
+		check.expr(x, e.X)
+		if x.mode == invalid {
+			check.use(e.Low, e.High, e.Max)
+			goto Error
+		}
+
+		valid := false
+		length := int64(-1) // valid if >= 0
+		switch typ := x.typ.Underlying().(type) {
+		case *Basic:
+			if isString(typ) {
+				if e.Slice3 {
+					check.invalidOp(x, _InvalidSliceExpr, "3-index slice of string")
+					goto Error
+				}
+				valid = true
+				if x.mode == constant_ {
+					length = int64(len(constant.StringVal(x.val)))
+				}
+				// spec: "For untyped string operands the result
+				// is a non-constant value of type string."
+				if typ.kind == UntypedString {
+					x.typ = Typ[String]
+				}
+			}
+
+		case *Array:
+			valid = true
+			length = typ.len
+			if x.mode != variable {
+				check.invalidOp(x, _NonSliceableOperand, "cannot slice %s (value not addressable)", x)
+				goto Error
+			}
+			x.typ = &Slice{elem: typ.elem}
+
+		case *Pointer:
+			if typ, _ := typ.base.Underlying().(*Array); typ != nil {
+				valid = true
+				length = typ.len
+				x.typ = &Slice{elem: typ.elem}
+			}
+
+		case *Slice:
+			valid = true
+			// x.typ doesn't change
+		}
+
+		if !valid {
+			check.invalidOp(x, _NonSliceableOperand, "cannot slice %s", x)
+			goto Error
+		}
+
+		x.mode = value
+
+		// spec: "Only the first index may be omitted; it defaults to 0."
+		if e.Slice3 && (e.High == nil || e.Max == nil) {
+			check.invalidAST(inNode(e, e.Rbrack), "2nd and 3rd index required in 3-index slice")
+			goto Error
+		}
+
+		// check indices
+		var ind [3]int64
+		for i, expr := range []ast.Expr{e.Low, e.High, e.Max} {
+			x := int64(-1)
+			switch {
+			case expr != nil:
+				// The "capacity" is only known statically for strings, arrays,
+				// and pointers to arrays, and it is the same as the length for
+				// those types.
+				max := int64(-1)
+				if length >= 0 {
+					max = length + 1
+				}
+				if _, v := check.index(expr, max); v >= 0 {
+					x = v
+				}
+			case i == 0:
+				// default is 0 for the first index
+				x = 0
+			case length >= 0:
+				// default is length (== capacity) otherwise
+				x = length
+			}
+			ind[i] = x
+		}
+
+		// constant indices must be in range
+		// (check.index already checks that existing indices >= 0)
+	L:
+		for i, x := range ind[:len(ind)-1] {
+			if x > 0 {
+				for _, y := range ind[i+1:] {
+					if y >= 0 && x > y {
+						check.errorf(inNode(e, e.Rbrack), _SwappedSliceIndices, "swapped slice indices: %d > %d", x, y)
+						break L // only report one error, ok to continue
+					}
+				}
+			}
+		}
+
+	case *ast.TypeAssertExpr:
+		check.expr(x, e.X)
+		if x.mode == invalid {
+			goto Error
+		}
+		xtyp, _ := x.typ.Underlying().(*Interface)
+		if xtyp == nil {
+			check.invalidOp(x, _InvalidAssert, "%s is not an interface", x)
+			goto Error
+		}
+		// x.(type) expressions are handled explicitly in type switches
+		if e.Type == nil {
+			// Don't use invalidAST because this can occur in the AST produced by
+			// go/parser.
+			check.error(e, _BadTypeKeyword, "use of .(type) outside type switch")
+			goto Error
+		}
+		T := check.typ(e.Type)
+		if T == Typ[Invalid] {
+			goto Error
+		}
+		check.typeAssertion(x, x, xtyp, T)
+		x.mode = commaok
+		x.typ = T
+
+	case *ast.CallExpr:
+		return check.call(x, e)
+
+	case *ast.StarExpr:
+		check.exprOrType(x, e.X)
+		switch x.mode {
+		case invalid:
+			goto Error
+		case typexpr:
+			x.typ = &Pointer{base: x.typ}
+		default:
+			if typ, ok := x.typ.Underlying().(*Pointer); ok {
+				x.mode = variable
+				x.typ = typ.base
+			} else {
+				check.invalidOp(x, _InvalidIndirection, "cannot indirect %s", x)
+				goto Error
+			}
+		}
+
+	case *ast.UnaryExpr:
+		check.expr(x, e.X)
+		if x.mode == invalid {
+			goto Error
+		}
+		check.unary(x, e, e.Op)
+		if x.mode == invalid {
+			goto Error
+		}
+		if e.Op == token.ARROW {
+			x.expr = e
+			return statement // receive operations may appear in statement context
+		}
+
+	case *ast.BinaryExpr:
+		check.binary(x, e, e.X, e.Y, e.Op, e.OpPos)
+		if x.mode == invalid {
+			goto Error
+		}
+
+	case *ast.KeyValueExpr:
+		// key:value expressions are handled in composite literals
+		check.invalidAST(e, "no key:value expected")
+		goto Error
+
+	case *ast.ArrayType, *ast.StructType, *ast.FuncType,
+		*ast.InterfaceType, *ast.MapType, *ast.ChanType:
+		x.mode = typexpr
+		x.typ = check.typ(e)
+		// Note: rawExpr (caller of exprInternal) will call check.recordTypeAndValue
+		// even though check.typ has already called it. This is fine as both
+		// times the same expression and type are recorded. It is also not a
+		// performance issue because we only reach here for composite literal
+		// types, which are comparatively rare.
+
+	default:
+		panic(fmt.Sprintf("%s: unknown expression type %T", check.fset.Position(e.Pos()), e))
+	}
+
+	// everything went well
+	x.expr = e
+	return expression
+
+Error:
+	x.mode = invalid
+	x.expr = e
+	return statement // avoid follow-up errors
+}
+
+func keyVal(x constant.Value) interface{} {
+	switch x.Kind() {
+	case constant.Bool:
+		return constant.BoolVal(x)
+	case constant.String:
+		return constant.StringVal(x)
+	case constant.Int:
+		if v, ok := constant.Int64Val(x); ok {
+			return v
+		}
+		if v, ok := constant.Uint64Val(x); ok {
+			return v
+		}
+	case constant.Float:
+		v, _ := constant.Float64Val(x)
+		return v
+	case constant.Complex:
+		r, _ := constant.Float64Val(constant.Real(x))
+		i, _ := constant.Float64Val(constant.Imag(x))
+		return complex(r, i)
+	}
+	return x
+}
+
+// typeAssertion checks that x.(T) is legal; xtyp must be the type of x.
+func (check *Checker) typeAssertion(at positioner, x *operand, xtyp *Interface, T Type) {
+	method, wrongType := check.assertableTo(xtyp, T)
+	if method == nil {
+		return
+	}
+	var msg string
+	if wrongType != nil {
+		if check.identical(method.typ, wrongType.typ) {
+			msg = fmt.Sprintf("missing method %s (%s has pointer receiver)", method.name, method.name)
+		} else {
+			msg = fmt.Sprintf("wrong type for method %s (have %s, want %s)", method.name, wrongType.typ, method.typ)
+		}
+	} else {
+		msg = "missing method " + method.name
+	}
+	check.errorf(at, _ImpossibleAssert, "%s cannot have dynamic type %s (%s)", x, T, msg)
+}
+
+func (check *Checker) singleValue(x *operand) {
+	if x.mode == value {
+		// tuple types are never named - no need for underlying type below
+		if t, ok := x.typ.(*Tuple); ok {
+			assert(t.Len() != 1)
+			check.errorf(x, _TooManyValues, "%d-valued %s where single value is expected", t.Len(), x)
+			x.mode = invalid
+		}
+	}
+}
+
+// expr typechecks expression e and initializes x with the expression value.
+// The result must be a single value.
+// If an error occurred, x.mode is set to invalid.
+//
+func (check *Checker) expr(x *operand, e ast.Expr) {
+	check.multiExpr(x, e)
+	check.singleValue(x)
+}
+
+// multiExpr is like expr but the result may be a multi-value.
+func (check *Checker) multiExpr(x *operand, e ast.Expr) {
+	check.rawExpr(x, e, nil)
+	var msg string
+	var code errorCode
+	switch x.mode {
+	default:
+		return
+	case novalue:
+		msg = "%s used as value"
+		code = _TooManyValues
+	case builtin:
+		msg = "%s must be called"
+		code = _UncalledBuiltin
+	case typexpr:
+		msg = "%s is not an expression"
+		code = _NotAnExpr
+	}
+	check.errorf(x, code, msg, x)
+	x.mode = invalid
+}
+
+// exprWithHint typechecks expression e and initializes x with the expression value;
+// hint is the type of a composite literal element.
+// If an error occurred, x.mode is set to invalid.
+//
+func (check *Checker) exprWithHint(x *operand, e ast.Expr, hint Type) {
+	assert(hint != nil)
+	check.rawExpr(x, e, hint)
+	check.singleValue(x)
+	var msg string
+	var code errorCode
+	switch x.mode {
+	default:
+		return
+	case novalue:
+		msg = "%s used as value"
+		code = _TooManyValues
+	case builtin:
+		msg = "%s must be called"
+		code = _UncalledBuiltin
+	case typexpr:
+		msg = "%s is not an expression"
+		code = _NotAnExpr
+	}
+	check.errorf(x, code, msg, x)
+	x.mode = invalid
+}
+
+// exprOrType typechecks expression or type e and initializes x with the expression value or type.
+// If an error occurred, x.mode is set to invalid.
+//
+func (check *Checker) exprOrType(x *operand, e ast.Expr) {
+	check.rawExpr(x, e, nil)
+	check.singleValue(x)
+	if x.mode == novalue {
+		check.errorf(x, _NotAnExpr, "%s used as value or type", x)
+		x.mode = invalid
+	}
+}
diff --git a/src/go/types/exprstring.go b/src/go/types/exprstring.go
new file mode 100644
index 0000000..28d605f
--- /dev/null
+++ b/src/go/types/exprstring.go
@@ -0,0 +1,224 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements printing of expressions.
+
+package types
+
+import (
+	"bytes"
+	"go/ast"
+)
+
+// ExprString returns the (possibly shortened) string representation for x.
+// Shortened representations are suitable for user interfaces but may not
+// necessarily follow Go syntax.
+func ExprString(x ast.Expr) string {
+	var buf bytes.Buffer
+	WriteExpr(&buf, x)
+	return buf.String()
+}
+
+// WriteExpr writes the (possibly shortened) string representation for x to buf.
+// Shortened representations are suitable for user interfaces but may not
+// necessarily follow Go syntax.
+func WriteExpr(buf *bytes.Buffer, x ast.Expr) {
+	// The AST preserves source-level parentheses so there is
+	// no need to introduce them here to correct for different
+	// operator precedences. (This assumes that the AST was
+	// generated by a Go parser.)
+
+	switch x := x.(type) {
+	default:
+		buf.WriteString("(bad expr)") // nil, ast.BadExpr, ast.KeyValueExpr
+
+	case *ast.Ident:
+		buf.WriteString(x.Name)
+
+	case *ast.Ellipsis:
+		buf.WriteString("...")
+		if x.Elt != nil {
+			WriteExpr(buf, x.Elt)
+		}
+
+	case *ast.BasicLit:
+		buf.WriteString(x.Value)
+
+	case *ast.FuncLit:
+		buf.WriteByte('(')
+		WriteExpr(buf, x.Type)
+		buf.WriteString(" literal)") // shortened
+
+	case *ast.CompositeLit:
+		buf.WriteByte('(')
+		WriteExpr(buf, x.Type)
+		buf.WriteString(" literal)") // shortened
+
+	case *ast.ParenExpr:
+		buf.WriteByte('(')
+		WriteExpr(buf, x.X)
+		buf.WriteByte(')')
+
+	case *ast.SelectorExpr:
+		WriteExpr(buf, x.X)
+		buf.WriteByte('.')
+		buf.WriteString(x.Sel.Name)
+
+	case *ast.IndexExpr:
+		WriteExpr(buf, x.X)
+		buf.WriteByte('[')
+		WriteExpr(buf, x.Index)
+		buf.WriteByte(']')
+
+	case *ast.SliceExpr:
+		WriteExpr(buf, x.X)
+		buf.WriteByte('[')
+		if x.Low != nil {
+			WriteExpr(buf, x.Low)
+		}
+		buf.WriteByte(':')
+		if x.High != nil {
+			WriteExpr(buf, x.High)
+		}
+		if x.Slice3 {
+			buf.WriteByte(':')
+			if x.Max != nil {
+				WriteExpr(buf, x.Max)
+			}
+		}
+		buf.WriteByte(']')
+
+	case *ast.TypeAssertExpr:
+		WriteExpr(buf, x.X)
+		buf.WriteString(".(")
+		WriteExpr(buf, x.Type)
+		buf.WriteByte(')')
+
+	case *ast.CallExpr:
+		WriteExpr(buf, x.Fun)
+		buf.WriteByte('(')
+		for i, arg := range x.Args {
+			if i > 0 {
+				buf.WriteString(", ")
+			}
+			WriteExpr(buf, arg)
+		}
+		if x.Ellipsis.IsValid() {
+			buf.WriteString("...")
+		}
+		buf.WriteByte(')')
+
+	case *ast.StarExpr:
+		buf.WriteByte('*')
+		WriteExpr(buf, x.X)
+
+	case *ast.UnaryExpr:
+		buf.WriteString(x.Op.String())
+		WriteExpr(buf, x.X)
+
+	case *ast.BinaryExpr:
+		WriteExpr(buf, x.X)
+		buf.WriteByte(' ')
+		buf.WriteString(x.Op.String())
+		buf.WriteByte(' ')
+		WriteExpr(buf, x.Y)
+
+	case *ast.ArrayType:
+		buf.WriteByte('[')
+		if x.Len != nil {
+			WriteExpr(buf, x.Len)
+		}
+		buf.WriteByte(']')
+		WriteExpr(buf, x.Elt)
+
+	case *ast.StructType:
+		buf.WriteString("struct{")
+		writeFieldList(buf, x.Fields, "; ", false)
+		buf.WriteByte('}')
+
+	case *ast.FuncType:
+		buf.WriteString("func")
+		writeSigExpr(buf, x)
+
+	case *ast.InterfaceType:
+		buf.WriteString("interface{")
+		writeFieldList(buf, x.Methods, "; ", true)
+		buf.WriteByte('}')
+
+	case *ast.MapType:
+		buf.WriteString("map[")
+		WriteExpr(buf, x.Key)
+		buf.WriteByte(']')
+		WriteExpr(buf, x.Value)
+
+	case *ast.ChanType:
+		var s string
+		switch x.Dir {
+		case ast.SEND:
+			s = "chan<- "
+		case ast.RECV:
+			s = "<-chan "
+		default:
+			s = "chan "
+		}
+		buf.WriteString(s)
+		WriteExpr(buf, x.Value)
+	}
+}
+
+func writeSigExpr(buf *bytes.Buffer, sig *ast.FuncType) {
+	buf.WriteByte('(')
+	writeFieldList(buf, sig.Params, ", ", false)
+	buf.WriteByte(')')
+
+	res := sig.Results
+	n := res.NumFields()
+	if n == 0 {
+		// no result
+		return
+	}
+
+	buf.WriteByte(' ')
+	if n == 1 && len(res.List[0].Names) == 0 {
+		// single unnamed result
+		WriteExpr(buf, res.List[0].Type)
+		return
+	}
+
+	// multiple or named result(s)
+	buf.WriteByte('(')
+	writeFieldList(buf, res, ", ", false)
+	buf.WriteByte(')')
+}
+
+func writeFieldList(buf *bytes.Buffer, fields *ast.FieldList, sep string, iface bool) {
+	for i, f := range fields.List {
+		if i > 0 {
+			buf.WriteString(sep)
+		}
+
+		// field list names
+		for i, name := range f.Names {
+			if i > 0 {
+				buf.WriteString(", ")
+			}
+			buf.WriteString(name.Name)
+		}
+
+		// types of interface methods consist of signatures only
+		if sig, _ := f.Type.(*ast.FuncType); sig != nil && iface {
+			writeSigExpr(buf, sig)
+			continue
+		}
+
+		// named fields are separated with a blank from the field type
+		if len(f.Names) > 0 {
+			buf.WriteByte(' ')
+		}
+
+		WriteExpr(buf, f.Type)
+
+		// ignore tag
+	}
+}
diff --git a/src/go/types/exprstring_test.go b/src/go/types/exprstring_test.go
new file mode 100644
index 0000000..5110288
--- /dev/null
+++ b/src/go/types/exprstring_test.go
@@ -0,0 +1,94 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types_test
+
+import (
+	"go/parser"
+	"testing"
+
+	. "go/types"
+)
+
+var testExprs = []testEntry{
+	// basic type literals
+	dup("x"),
+	dup("true"),
+	dup("42"),
+	dup("3.1415"),
+	dup("2.71828i"),
+	dup(`'a'`),
+	dup(`"foo"`),
+	dup("`bar`"),
+
+	// func and composite literals
+	{"func(){}", "(func() literal)"},
+	{"func(x int) complex128 {}", "(func(x int) complex128 literal)"},
+	{"[]int{1, 2, 3}", "([]int literal)"},
+
+	// non-type expressions
+	dup("(x)"),
+	dup("x.f"),
+	dup("a[i]"),
+
+	dup("s[:]"),
+	dup("s[i:]"),
+	dup("s[:j]"),
+	dup("s[i:j]"),
+	dup("s[:j:k]"),
+	dup("s[i:j:k]"),
+
+	dup("x.(T)"),
+
+	dup("x.([10]int)"),
+	dup("x.([...]int)"),
+
+	dup("x.(struct{})"),
+	dup("x.(struct{x int; y, z float32; E})"),
+
+	dup("x.(func())"),
+	dup("x.(func(x int))"),
+	dup("x.(func() int)"),
+	dup("x.(func(x, y int, z float32) (r int))"),
+	dup("x.(func(a, b, c int))"),
+	dup("x.(func(x ...T))"),
+
+	dup("x.(interface{})"),
+	dup("x.(interface{m(); n(x int); E})"),
+	dup("x.(interface{m(); n(x int) T; E; F})"),
+
+	dup("x.(map[K]V)"),
+
+	dup("x.(chan E)"),
+	dup("x.(<-chan E)"),
+	dup("x.(chan<- chan int)"),
+	dup("x.(chan<- <-chan int)"),
+	dup("x.(<-chan chan int)"),
+	dup("x.(chan (<-chan int))"),
+
+	dup("f()"),
+	dup("f(x)"),
+	dup("int(x)"),
+	dup("f(x, x + y)"),
+	dup("f(s...)"),
+	dup("f(a, s...)"),
+
+	dup("*x"),
+	dup("&x"),
+	dup("x + y"),
+	dup("x + y << (2 * s)"),
+}
+
+func TestExprString(t *testing.T) {
+	for _, test := range testExprs {
+		x, err := parser.ParseExpr(test.src)
+		if err != nil {
+			t.Errorf("%s: %s", test.src, err)
+			continue
+		}
+		if got := ExprString(x); got != test.str {
+			t.Errorf("%s: got %s, want %s", test.src, got, test.str)
+		}
+	}
+}
diff --git a/src/go/types/fixedbugs/issue20583.src b/src/go/types/fixedbugs/issue20583.src
new file mode 100644
index 0000000..d26dbad
--- /dev/null
+++ b/src/go/types/fixedbugs/issue20583.src
@@ -0,0 +1,14 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue20583
+
+const (
+	_ = 6e886451608 /* ERROR malformed constant */ /2
+	_ = 6e886451608i /* ERROR malformed constant */ /2
+	_ = 0 * 1e+1000000000 // ERROR malformed constant
+
+	x = 1e100000000
+	_ = x*x*x*x*x*x* /* ERROR not representable */ x
+)
diff --git a/src/go/types/fixedbugs/issue23203a.src b/src/go/types/fixedbugs/issue23203a.src
new file mode 100644
index 0000000..48cb588
--- /dev/null
+++ b/src/go/types/fixedbugs/issue23203a.src
@@ -0,0 +1,14 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "unsafe"
+
+type T struct{}
+
+func (T) m1()                         {}
+func (T) m2([unsafe.Sizeof(T.m1)]int) {}
+
+func main() {}
diff --git a/src/go/types/fixedbugs/issue23203b.src b/src/go/types/fixedbugs/issue23203b.src
new file mode 100644
index 0000000..638ec6c
--- /dev/null
+++ b/src/go/types/fixedbugs/issue23203b.src
@@ -0,0 +1,14 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "unsafe"
+
+type T struct{}
+
+func (T) m2([unsafe.Sizeof(T.m1)]int) {}
+func (T) m1()                         {}
+
+func main() {}
diff --git a/src/go/types/fixedbugs/issue26390.src b/src/go/types/fixedbugs/issue26390.src
new file mode 100644
index 0000000..9e0101f
--- /dev/null
+++ b/src/go/types/fixedbugs/issue26390.src
@@ -0,0 +1,13 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// stand-alone test to ensure case is triggered
+
+package issue26390
+
+type A = T
+
+func (t *T) m() *A { return t }
+
+type T struct{}
diff --git a/src/go/types/fixedbugs/issue28251.src b/src/go/types/fixedbugs/issue28251.src
new file mode 100644
index 0000000..cd79e0e
--- /dev/null
+++ b/src/go/types/fixedbugs/issue28251.src
@@ -0,0 +1,65 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains test cases for various forms of
+// method receiver declarations, per the spec clarification
+// https://golang.org/cl/142757.
+
+package issue28251
+
+// test case from issue28251
+type T struct{}
+
+type T0 = *T
+
+func (T0) m() {}
+
+func _() { (&T{}).m() }
+
+// various alternative forms
+type (
+        T1 = (((T)))
+)
+
+func ((*(T1))) m1() {}
+func _() { (T{}).m2() }
+func _() { (&T{}).m2() }
+
+type (
+        T2 = (((T3)))
+        T3 = T
+)
+
+func (T2) m2() {}
+func _() { (T{}).m2() }
+func _() { (&T{}).m2() }
+
+type (
+        T4 = ((*(T5)))
+        T5 = T
+)
+
+func (T4) m4() {}
+func _() { (T{}).m4 /* ERROR "cannot call pointer method m4 on T" */ () }
+func _() { (&T{}).m4() }
+
+type (
+        T6 = (((T7)))
+        T7 = (*(T8))
+        T8 = T
+)
+
+func (T6) m6() {}
+func _() { (T{}).m6 /* ERROR "cannot call pointer method m6 on T" */ () }
+func _() { (&T{}).m6() }
+
+type (
+        T9 = *T10
+        T10 = *T11
+        T11 = T
+)
+
+func (T9 /* ERROR invalid receiver \*\*T */ ) m9() {}
+func _() { (T{}).m9 /* ERROR has no field or method m9 */ () }
+func _() { (&T{}).m9 /* ERROR has no field or method m9 */ () }
diff --git a/src/go/types/fixedbugs/issue42695.src b/src/go/types/fixedbugs/issue42695.src
new file mode 100644
index 0000000..d0d6200
--- /dev/null
+++ b/src/go/types/fixedbugs/issue42695.src
@@ -0,0 +1,17 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue42695
+
+const _ = 6e5518446744 // ERROR malformed constant
+const _ uint8 = 6e5518446744 // ERROR malformed constant
+
+var _ = 6e5518446744 // ERROR malformed constant
+var _ uint8 = 6e5518446744 // ERROR malformed constant
+
+func f(x int) int {
+        return x + 6e5518446744 // ERROR malformed constant
+}
+
+var _ = f(6e5518446744 /* ERROR malformed constant */ )
diff --git a/src/go/types/fixedbugs/issue43110.src b/src/go/types/fixedbugs/issue43110.src
new file mode 100644
index 0000000..4a46945
--- /dev/null
+++ b/src/go/types/fixedbugs/issue43110.src
@@ -0,0 +1,43 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type P *struct{}
+
+func _() {
+	// want an error even if the switch is empty
+	var a struct{ _ func() }
+	switch a /* ERROR cannot switch on a */ {
+	}
+
+	switch a /* ERROR cannot switch on a */ {
+	case a: // no follow-on error here
+	}
+
+	// this is ok because f can be compared to nil
+	var f func()
+	switch f {
+	}
+
+	switch f {
+	case nil:
+	}
+
+	switch (func())(nil) {
+	case nil:
+	}
+
+	switch (func())(nil) {
+	case f /* ERROR cannot compare */ :
+	}
+
+	switch nil /* ERROR use of untyped nil in switch expression */ {
+	}
+
+	// this is ok
+	switch P(nil) {
+	case P(nil):
+	}
+}
diff --git a/src/go/types/fixedbugs/issue43124.src b/src/go/types/fixedbugs/issue43124.src
new file mode 100644
index 0000000..f429f74
--- /dev/null
+++ b/src/go/types/fixedbugs/issue43124.src
@@ -0,0 +1,16 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+var _ = int(0 /* ERROR invalid use of \.\.\. in conversion to int */ ...)
+
+// test case from issue
+
+type M []string
+
+var (
+	x = []string{"a", "b"}
+	_ = M(x /* ERROR invalid use of \.\.\. in conversion to M */ ...)
+)
diff --git a/src/go/types/fixedbugs/issue6977.src b/src/go/types/fixedbugs/issue6977.src
new file mode 100644
index 0000000..8f4e9ba
--- /dev/null
+++ b/src/go/types/fixedbugs/issue6977.src
@@ -0,0 +1,82 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "io"
+
+// Alan's initial report.
+
+type I interface { f(); String() string }
+type J interface { g(); String() string }
+
+type IJ1 = interface { I; J }
+type IJ2 = interface { f(); g(); String() string }
+
+var _ = (*IJ1)(nil) == (*IJ2)(nil) // static assert that IJ1 and IJ2 are identical types
+
+// The canonical example.
+
+type ReadWriteCloser interface { io.ReadCloser; io.WriteCloser }
+
+// Some more cases.
+
+type M interface { m() }
+type M32 interface { m() int32 }
+type M64 interface { m() int64 }
+
+type U1 interface { m() }
+type U2 interface { m(); M }
+type U3 interface { M; m() }
+type U4 interface { M; M; M }
+type U5 interface { U1; U2; U3; U4 }
+
+type U6 interface { m(); m /* ERROR duplicate method */ () }
+type U7 interface { M32 /* ERROR duplicate method */ ; m() }
+type U8 interface { m(); M32 /* ERROR duplicate method */ }
+type U9 interface { M32; M64 /* ERROR duplicate method */ }
+
+// Verify that repeated embedding of the same interface(s)
+// eliminates duplicate methods early (rather than at the
+// end) to prevent exponential memory and time use.
+// Without early elimination, computing T29 may take dozens
+// of minutes.
+type (
+        T0 interface { m() }
+        T1 interface { T0; T0 }
+        T2 interface { T1; T1 }
+        T3 interface { T2; T2 }
+        T4 interface { T3; T3 }
+        T5 interface { T4; T4 }
+        T6 interface { T5; T5 }
+        T7 interface { T6; T6 }
+        T8 interface { T7; T7 }
+        T9 interface { T8; T8 }
+
+        T10 interface { T9; T9 }
+        T11 interface { T10; T10 }
+        T12 interface { T11; T11 }
+        T13 interface { T12; T12 }
+        T14 interface { T13; T13 }
+        T15 interface { T14; T14 }
+        T16 interface { T15; T15 }
+        T17 interface { T16; T16 }
+        T18 interface { T17; T17 }
+        T19 interface { T18; T18 }
+
+        T20 interface { T19; T19 }
+        T21 interface { T20; T20 }
+        T22 interface { T21; T21 }
+        T23 interface { T22; T22 }
+        T24 interface { T23; T23 }
+        T25 interface { T24; T24 }
+        T26 interface { T25; T25 }
+        T27 interface { T26; T26 }
+        T28 interface { T27; T27 }
+        T29 interface { T28; T28 }
+)
+
+// Verify that m is present.
+var x T29
+var _ = x.m
diff --git a/src/go/types/gccgosizes.go b/src/go/types/gccgosizes.go
new file mode 100644
index 0000000..d5c92c6
--- /dev/null
+++ b/src/go/types/gccgosizes.go
@@ -0,0 +1,40 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This is a copy of the file generated during the gccgo build process.
+// Last update 2019-01-22.
+
+package types
+
+var gccgoArchSizes = map[string]*StdSizes{
+	"386":         {4, 4},
+	"alpha":       {8, 8},
+	"amd64":       {8, 8},
+	"amd64p32":    {4, 8},
+	"arm":         {4, 8},
+	"armbe":       {4, 8},
+	"arm64":       {8, 8},
+	"arm64be":     {8, 8},
+	"ia64":        {8, 8},
+	"m68k":        {4, 2},
+	"mips":        {4, 8},
+	"mipsle":      {4, 8},
+	"mips64":      {8, 8},
+	"mips64le":    {8, 8},
+	"mips64p32":   {4, 8},
+	"mips64p32le": {4, 8},
+	"nios2":       {4, 8},
+	"ppc":         {4, 8},
+	"ppc64":       {8, 8},
+	"ppc64le":     {8, 8},
+	"riscv":       {4, 8},
+	"riscv64":     {8, 8},
+	"s390":        {4, 8},
+	"s390x":       {8, 8},
+	"sh":          {4, 8},
+	"shbe":        {4, 8},
+	"sparc":       {4, 8},
+	"sparc64":     {8, 8},
+	"wasm":        {8, 8},
+}
diff --git a/src/go/types/gotype.go b/src/go/types/gotype.go
new file mode 100644
index 0000000..52709df
--- /dev/null
+++ b/src/go/types/gotype.go
@@ -0,0 +1,354 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build ignore
+
+// Build this command explicitly: go build gotype.go
+
+/*
+The gotype command, like the front-end of a Go compiler, parses and
+type-checks a single Go package. Errors are reported if the analysis
+fails; otherwise gotype is quiet (unless -v is set).
+
+Without a list of paths, gotype reads from standard input, which
+must provide a single Go source file defining a complete package.
+
+With a single directory argument, gotype checks the Go files in
+that directory, comprising a single package. Use -t to include the
+(in-package) _test.go files. Use -x to type check only external
+test files.
+
+Otherwise, each path must be the filename of a Go file belonging
+to the same package.
+
+Imports are processed by importing directly from the source of
+imported packages (default), or by importing from compiled and
+installed packages (by setting -c to the respective compiler).
+
+The -c flag must be set to a compiler ("gc", "gccgo") when type-
+checking packages containing imports with relative import paths
+(import "./mypkg") because the source importer cannot know which
+files to include for such packages.
+
+Usage:
+	gotype [flags] [path...]
+
+The flags are:
+	-t
+		include local test files in a directory (ignored if -x is provided)
+	-x
+		consider only external test files in a directory
+	-e
+		report all errors (not just the first 10)
+	-v
+		verbose mode
+	-c
+		compiler used for installed packages (gc, gccgo, or source); default: source
+
+Flags controlling additional output:
+	-ast
+		print AST
+	-trace
+		print parse trace
+	-comments
+		parse comments (ignored unless -ast or -trace is provided)
+	-panic
+		panic on first error
+
+Examples:
+
+To check the files a.go, b.go, and c.go:
+
+	gotype a.go b.go c.go
+
+To check an entire package including (in-package) tests in the directory dir and print the processed files:
+
+	gotype -t -v dir
+
+To check the external test package (if any) in the current directory, based on installed packages compiled with
+cmd/compile:
+
+	gotype -c=gc -x .
+
+To verify the output of a pipe:
+
+	echo "package foo" | gotype
+
+*/
+package main
+
+import (
+	"flag"
+	"fmt"
+	"go/ast"
+	"go/build"
+	"go/importer"
+	"go/parser"
+	"go/scanner"
+	"go/token"
+	"go/types"
+	"io"
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+)
+
+var (
+	// main operation modes
+	testFiles  = flag.Bool("t", false, "include in-package test files in a directory")
+	xtestFiles = flag.Bool("x", false, "consider only external test files in a directory")
+	allErrors  = flag.Bool("e", false, "report all errors, not just the first 10")
+	verbose    = flag.Bool("v", false, "verbose mode")
+	compiler   = flag.String("c", "source", "compiler used for installed packages (gc, gccgo, or source)")
+
+	// additional output control
+	printAST      = flag.Bool("ast", false, "print AST")
+	printTrace    = flag.Bool("trace", false, "print parse trace")
+	parseComments = flag.Bool("comments", false, "parse comments (ignored unless -ast or -trace is provided)")
+	panicOnError  = flag.Bool("panic", false, "panic on first error")
+)
+
+var (
+	fset       = token.NewFileSet()
+	errorCount = 0
+	sequential = false
+	parserMode parser.Mode
+)
+
+func initParserMode() {
+	if *allErrors {
+		parserMode |= parser.AllErrors
+	}
+	if *printAST {
+		sequential = true
+	}
+	if *printTrace {
+		parserMode |= parser.Trace
+		sequential = true
+	}
+	if *parseComments && (*printAST || *printTrace) {
+		parserMode |= parser.ParseComments
+	}
+}
+
+const usageString = `usage: gotype [flags] [path ...]
+
+The gotype command, like the front-end of a Go compiler, parses and
+type-checks a single Go package. Errors are reported if the analysis
+fails; otherwise gotype is quiet (unless -v is set).
+
+Without a list of paths, gotype reads from standard input, which
+must provide a single Go source file defining a complete package.
+
+With a single directory argument, gotype checks the Go files in
+that directory, comprising a single package. Use -t to include the
+(in-package) _test.go files. Use -x to type check only external
+test files.
+
+Otherwise, each path must be the filename of a Go file belonging
+to the same package.
+
+Imports are processed by importing directly from the source of
+imported packages (default), or by importing from compiled and
+installed packages (by setting -c to the respective compiler).
+
+The -c flag must be set to a compiler ("gc", "gccgo") when type-
+checking packages containing imports with relative import paths
+(import "./mypkg") because the source importer cannot know which
+files to include for such packages.
+`
+
+func usage() {
+	fmt.Fprintln(os.Stderr, usageString)
+	flag.PrintDefaults()
+	os.Exit(2)
+}
+
+func report(err error) {
+	if *panicOnError {
+		panic(err)
+	}
+	scanner.PrintError(os.Stderr, err)
+	if list, ok := err.(scanner.ErrorList); ok {
+		errorCount += len(list)
+		return
+	}
+	errorCount++
+}
+
+// parse may be called concurrently
+func parse(filename string, src interface{}) (*ast.File, error) {
+	if *verbose {
+		fmt.Println(filename)
+	}
+	file, err := parser.ParseFile(fset, filename, src, parserMode) // ok to access fset concurrently
+	if *printAST {
+		ast.Print(fset, file)
+	}
+	return file, err
+}
+
+func parseStdin() (*ast.File, error) {
+	src, err := io.ReadAll(os.Stdin)
+	if err != nil {
+		return nil, err
+	}
+	return parse("<standard input>", src)
+}
+
+func parseFiles(dir string, filenames []string) ([]*ast.File, error) {
+	files := make([]*ast.File, len(filenames))
+	errors := make([]error, len(filenames))
+
+	var wg sync.WaitGroup
+	for i, filename := range filenames {
+		wg.Add(1)
+		go func(i int, filepath string) {
+			defer wg.Done()
+			files[i], errors[i] = parse(filepath, nil)
+		}(i, filepath.Join(dir, filename))
+		if sequential {
+			wg.Wait()
+		}
+	}
+	wg.Wait()
+
+	// If there are errors, return the first one for deterministic results.
+	var first error
+	for _, err := range errors {
+		if err != nil {
+			first = err
+			// If we have an error, some files may be nil.
+			// Remove them. (The go/parser always returns
+			// a possibly partial AST even in the presence
+			// of errors, except if the file doesn't exist
+			// in the first place, in which case it cannot
+			// matter.)
+			i := 0
+			for _, f := range files {
+				if f != nil {
+					files[i] = f
+					i++
+				}
+			}
+			files = files[:i]
+			break
+		}
+	}
+
+	return files, first
+}
+
+func parseDir(dir string) ([]*ast.File, error) {
+	ctxt := build.Default
+	pkginfo, err := ctxt.ImportDir(dir, 0)
+	if _, nogo := err.(*build.NoGoError); err != nil && !nogo {
+		return nil, err
+	}
+
+	if *xtestFiles {
+		return parseFiles(dir, pkginfo.XTestGoFiles)
+	}
+
+	filenames := append(pkginfo.GoFiles, pkginfo.CgoFiles...)
+	if *testFiles {
+		filenames = append(filenames, pkginfo.TestGoFiles...)
+	}
+	return parseFiles(dir, filenames)
+}
+
+func getPkgFiles(args []string) ([]*ast.File, error) {
+	if len(args) == 0 {
+		// stdin
+		file, err := parseStdin()
+		if err != nil {
+			return nil, err
+		}
+		return []*ast.File{file}, nil
+	}
+
+	if len(args) == 1 {
+		// possibly a directory
+		path := args[0]
+		info, err := os.Stat(path)
+		if err != nil {
+			return nil, err
+		}
+		if info.IsDir() {
+			return parseDir(path)
+		}
+	}
+
+	// list of files
+	return parseFiles("", args)
+}
+
+func checkPkgFiles(files []*ast.File) {
+	type bailout struct{}
+
+	// if checkPkgFiles is called multiple times, set up conf only once
+	conf := types.Config{
+		FakeImportC: true,
+		Error: func(err error) {
+			if !*allErrors && errorCount >= 10 {
+				panic(bailout{})
+			}
+			report(err)
+		},
+		Importer: importer.ForCompiler(fset, *compiler, nil),
+		Sizes:    types.SizesFor(build.Default.Compiler, build.Default.GOARCH),
+	}
+
+	defer func() {
+		switch p := recover().(type) {
+		case nil, bailout:
+			// normal return or early exit
+		default:
+			// re-panic
+			panic(p)
+		}
+	}()
+
+	const path = "pkg" // any non-empty string will do for now
+	conf.Check(path, fset, files, nil)
+}
+
+func printStats(d time.Duration) {
+	fileCount := 0
+	lineCount := 0
+	fset.Iterate(func(f *token.File) bool {
+		fileCount++
+		lineCount += f.LineCount()
+		return true
+	})
+
+	fmt.Printf(
+		"%s (%d files, %d lines, %d lines/s)\n",
+		d, fileCount, lineCount, int64(float64(lineCount)/d.Seconds()),
+	)
+}
+
+func main() {
+	flag.Usage = usage
+	flag.Parse()
+	initParserMode()
+
+	start := time.Now()
+
+	files, err := getPkgFiles(flag.Args())
+	if err != nil {
+		report(err)
+		// ok to continue (files may be empty, but not nil)
+	}
+
+	checkPkgFiles(files)
+	if errorCount > 0 {
+		os.Exit(2)
+	}
+
+	if *verbose {
+		printStats(time.Since(start))
+	}
+}
diff --git a/src/go/types/hilbert_test.go b/src/go/types/hilbert_test.go
new file mode 100644
index 0000000..77954d2
--- /dev/null
+++ b/src/go/types/hilbert_test.go
@@ -0,0 +1,222 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types_test
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"go/ast"
+	"go/importer"
+	"go/parser"
+	"go/token"
+	"os"
+	"testing"
+
+	. "go/types"
+)
+
+var (
+	H   = flag.Int("H", 5, "Hilbert matrix size")
+	out = flag.String("out", "", "write generated program to out")
+)
+
+func TestHilbert(t *testing.T) {
+	// generate source
+	src := program(*H, *out)
+	if *out != "" {
+		os.WriteFile(*out, src, 0666)
+		return
+	}
+
+	// parse source
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "hilbert.go", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// type-check file
+	DefPredeclaredTestFuncs() // define assert built-in
+	conf := Config{Importer: importer.Default()}
+	_, err = conf.Check(f.Name.Name, fset, []*ast.File{f}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func program(n int, out string) []byte {
+	var g gen
+
+	g.p(`// Code generated by: go test -run=Hilbert -H=%d -out=%q. DO NOT EDIT.
+
+// +`+`build ignore
+
+// This program tests arbitrary precision constant arithmetic
+// by generating the constant elements of a Hilbert matrix H,
+// its inverse I, and the product P = H*I. The product should
+// be the identity matrix.
+package main
+
+func main() {
+	if !ok {
+		printProduct()
+		return
+	}
+	println("PASS")
+}
+
+`, n, out)
+	g.hilbert(n)
+	g.inverse(n)
+	g.product(n)
+	g.verify(n)
+	g.printProduct(n)
+	g.binomials(2*n - 1)
+	g.factorials(2*n - 1)
+
+	return g.Bytes()
+}
+
+type gen struct {
+	bytes.Buffer
+}
+
+func (g *gen) p(format string, args ...interface{}) {
+	fmt.Fprintf(&g.Buffer, format, args...)
+}
+
+func (g *gen) hilbert(n int) {
+	g.p(`// Hilbert matrix, n = %d
+const (
+`, n)
+	for i := 0; i < n; i++ {
+		g.p("\t")
+		for j := 0; j < n; j++ {
+			if j > 0 {
+				g.p(", ")
+			}
+			g.p("h%d_%d", i, j)
+		}
+		if i == 0 {
+			g.p(" = ")
+			for j := 0; j < n; j++ {
+				if j > 0 {
+					g.p(", ")
+				}
+				g.p("1.0/(iota + %d)", j+1)
+			}
+		}
+		g.p("\n")
+	}
+	g.p(")\n\n")
+}
+
+func (g *gen) inverse(n int) {
+	g.p(`// Inverse Hilbert matrix
+const (
+`)
+	for i := 0; i < n; i++ {
+		for j := 0; j < n; j++ {
+			s := "+"
+			if (i+j)&1 != 0 {
+				s = "-"
+			}
+			g.p("\ti%d_%d = %s%d * b%d_%d * b%d_%d * b%d_%d * b%d_%d\n",
+				i, j, s, i+j+1, n+i, n-j-1, n+j, n-i-1, i+j, i, i+j, i)
+		}
+		g.p("\n")
+	}
+	g.p(")\n\n")
+}
+
+func (g *gen) product(n int) {
+	g.p(`// Product matrix
+const (
+`)
+	for i := 0; i < n; i++ {
+		for j := 0; j < n; j++ {
+			g.p("\tp%d_%d = ", i, j)
+			for k := 0; k < n; k++ {
+				if k > 0 {
+					g.p(" + ")
+				}
+				g.p("h%d_%d*i%d_%d", i, k, k, j)
+			}
+			g.p("\n")
+		}
+		g.p("\n")
+	}
+	g.p(")\n\n")
+}
+
+func (g *gen) verify(n int) {
+	g.p(`// Verify that product is the identity matrix
+const ok =
+`)
+	for i := 0; i < n; i++ {
+		for j := 0; j < n; j++ {
+			if j == 0 {
+				g.p("\t")
+			} else {
+				g.p(" && ")
+			}
+			v := 0
+			if i == j {
+				v = 1
+			}
+			g.p("p%d_%d == %d", i, j, v)
+		}
+		g.p(" &&\n")
+	}
+	g.p("\ttrue\n\n")
+
+	// verify ok at type-check time
+	if *out == "" {
+		g.p("const _ = assert(ok)\n\n")
+	}
+}
+
+func (g *gen) printProduct(n int) {
+	g.p("func printProduct() {\n")
+	for i := 0; i < n; i++ {
+		g.p("\tprintln(")
+		for j := 0; j < n; j++ {
+			if j > 0 {
+				g.p(", ")
+			}
+			g.p("p%d_%d", i, j)
+		}
+		g.p(")\n")
+	}
+	g.p("}\n\n")
+}
+
+func (g *gen) binomials(n int) {
+	g.p(`// Binomials
+const (
+`)
+	for j := 0; j <= n; j++ {
+		if j > 0 {
+			g.p("\n")
+		}
+		for k := 0; k <= j; k++ {
+			g.p("\tb%d_%d = f%d / (f%d*f%d)\n", j, k, j, k, j-k)
+		}
+	}
+	g.p(")\n\n")
+}
+
+func (g *gen) factorials(n int) {
+	g.p(`// Factorials
+const (
+	f0 = 1
+	f1 = 1
+`)
+	for i := 2; i <= n; i++ {
+		g.p("\tf%d = f%d * %d\n", i, i-1, i)
+	}
+	g.p(")\n\n")
+}
diff --git a/src/go/types/initorder.go b/src/go/types/initorder.go
new file mode 100644
index 0000000..77a739c
--- /dev/null
+++ b/src/go/types/initorder.go
@@ -0,0 +1,297 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types
+
+import (
+	"container/heap"
+	"fmt"
+)
+
+// initOrder computes the Info.InitOrder for package variables.
+func (check *Checker) initOrder() {
+	// An InitOrder may already have been computed if a package is
+	// built from several calls to (*Checker).Files. Clear it.
+	check.Info.InitOrder = check.Info.InitOrder[:0]
+
+	// Compute the object dependency graph and initialize
+	// a priority queue with the list of graph nodes.
+	pq := nodeQueue(dependencyGraph(check.objMap))
+	heap.Init(&pq)
+
+	const debug = false
+	if debug {
+		fmt.Printf("Computing initialization order for %s\n\n", check.pkg)
+		fmt.Println("Object dependency graph:")
+		for obj, d := range check.objMap {
+			// only print objects that may appear in the dependency graph
+			if obj, _ := obj.(dependency); obj != nil {
+				if len(d.deps) > 0 {
+					fmt.Printf("\t%s depends on\n", obj.Name())
+					for dep := range d.deps {
+						fmt.Printf("\t\t%s\n", dep.Name())
+					}
+				} else {
+					fmt.Printf("\t%s has no dependencies\n", obj.Name())
+				}
+			}
+		}
+		fmt.Println()
+
+		fmt.Println("Transposed object dependency graph (functions eliminated):")
+		for _, n := range pq {
+			fmt.Printf("\t%s depends on %d nodes\n", n.obj.Name(), n.ndeps)
+			for p := range n.pred {
+				fmt.Printf("\t\t%s is dependent\n", p.obj.Name())
+			}
+		}
+		fmt.Println()
+
+		fmt.Println("Processing nodes:")
+	}
+
+	// Determine initialization order by removing the highest priority node
+	// (the one with the fewest dependencies) and its edges from the graph,
+	// repeatedly, until there are no nodes left.
+	// In a valid Go program, those nodes always have zero dependencies (after
+	// removing all incoming dependencies), otherwise there are initialization
+	// cycles.
+	emitted := make(map[*declInfo]bool)
+	for len(pq) > 0 {
+		// get the next node
+		n := heap.Pop(&pq).(*graphNode)
+
+		if debug {
+			fmt.Printf("\t%s (src pos %d) depends on %d nodes now\n",
+				n.obj.Name(), n.obj.order(), n.ndeps)
+		}
+
+		// if n still depends on other nodes, we have a cycle
+		if n.ndeps > 0 {
+			cycle := findPath(check.objMap, n.obj, n.obj, make(map[Object]bool))
+			// If n.obj is not part of the cycle (e.g., n.obj->b->c->d->c),
+			// cycle will be nil. Don't report anything in that case since
+			// the cycle is reported when the algorithm gets to an object
+			// in the cycle.
+			// Furthermore, once an object in the cycle is encountered,
+			// the cycle will be broken (dependency count will be reduced
+			// below), and so the remaining nodes in the cycle don't trigger
+			// another error (unless they are part of multiple cycles).
+			if cycle != nil {
+				check.reportCycle(cycle)
+			}
+			// Ok to continue, but the variable initialization order
+			// will be incorrect at this point since it assumes no
+			// cycle errors.
+		}
+
+		// reduce dependency count of all dependent nodes
+		// and update priority queue
+		for p := range n.pred {
+			p.ndeps--
+			heap.Fix(&pq, p.index)
+		}
+
+		// record the init order for variables with initializers only
+		v, _ := n.obj.(*Var)
+		info := check.objMap[v]
+		if v == nil || !info.hasInitializer() {
+			continue
+		}
+
+		// n:1 variable declarations such as: a, b = f()
+		// introduce a node for each lhs variable (here: a, b);
+		// but they all have the same initializer - emit only
+		// one, for the first variable seen
+		if emitted[info] {
+			continue // initializer already emitted, if any
+		}
+		emitted[info] = true
+
+		infoLhs := info.lhs // possibly nil (see declInfo.lhs field comment)
+		if infoLhs == nil {
+			infoLhs = []*Var{v}
+		}
+		init := &Initializer{infoLhs, info.init}
+		check.Info.InitOrder = append(check.Info.InitOrder, init)
+	}
+
+	if debug {
+		fmt.Println()
+		fmt.Println("Initialization order:")
+		for _, init := range check.Info.InitOrder {
+			fmt.Printf("\t%s\n", init)
+		}
+		fmt.Println()
+	}
+}
+
+// findPath returns the (reversed) list of objects []Object{to, ... from}
+// such that there is a path of object dependencies from 'from' to 'to'.
+// If there is no such path, the result is nil.
+func findPath(objMap map[Object]*declInfo, from, to Object, seen map[Object]bool) []Object {
+	if seen[from] {
+		return nil
+	}
+	seen[from] = true
+
+	for d := range objMap[from].deps {
+		if d == to {
+			return []Object{d}
+		}
+		if P := findPath(objMap, d, to, seen); P != nil {
+			return append(P, d)
+		}
+	}
+
+	return nil
+}
+
+// reportCycle reports an error for the given cycle.
+func (check *Checker) reportCycle(cycle []Object) {
+	obj := cycle[0]
+	check.errorf(obj, _InvalidInitCycle, "initialization cycle for %s", obj.Name())
+	// subtle loop: print cycle[i] for i = 0, n-1, n-2, ... 1 for len(cycle) = n
+	for i := len(cycle) - 1; i >= 0; i-- {
+		check.errorf(obj, _InvalidInitCycle, "\t%s refers to", obj.Name()) // secondary error, \t indented
+		obj = cycle[i]
+	}
+	// print cycle[0] again to close the cycle
+	check.errorf(obj, _InvalidInitCycle, "\t%s", obj.Name())
+}
+
+// ----------------------------------------------------------------------------
+// Object dependency graph
+
+// A dependency is an object that may be a dependency in an initialization
+// expression. Only constants, variables, and functions can be dependencies.
+// Constants are here because constant expression cycles are reported during
+// initialization order computation.
+type dependency interface {
+	Object
+	isDependency()
+}
+
+// A graphNode represents a node in the object dependency graph.
+// Each node p in n.pred represents an edge p->n, and each node
+// s in n.succ represents an edge n->s; with a->b indicating that
+// a depends on b.
+type graphNode struct {
+	obj        dependency // object represented by this node
+	pred, succ nodeSet    // consumers and dependencies of this node (lazily initialized)
+	index      int        // node index in graph slice/priority queue
+	ndeps      int        // number of outstanding dependencies before this object can be initialized
+}
+
+type nodeSet map[*graphNode]bool
+
+func (s *nodeSet) add(p *graphNode) {
+	if *s == nil {
+		*s = make(nodeSet)
+	}
+	(*s)[p] = true
+}
+
+// dependencyGraph computes the object dependency graph from the given objMap,
+// with any function nodes removed. The resulting graph contains only constants
+// and variables.
+func dependencyGraph(objMap map[Object]*declInfo) []*graphNode {
+	// M is the dependency (Object) -> graphNode mapping
+	M := make(map[dependency]*graphNode)
+	for obj := range objMap {
+		// only consider nodes that may be an initialization dependency
+		if obj, _ := obj.(dependency); obj != nil {
+			M[obj] = &graphNode{obj: obj}
+		}
+	}
+
+	// compute edges for graph M
+	// (We need to include all nodes, even isolated ones, because they still need
+	// to be scheduled for initialization in correct order relative to other nodes.)
+	for obj, n := range M {
+		// for each dependency obj -> d (= deps[i]), create graph edges n->s and s->n
+		for d := range objMap[obj].deps {
+			// only consider nodes that may be an initialization dependency
+			if d, _ := d.(dependency); d != nil {
+				d := M[d]
+				n.succ.add(d)
+				d.pred.add(n)
+			}
+		}
+	}
+
+	// remove function nodes and collect remaining graph nodes in G
+	// (Mutually recursive functions may introduce cycles among themselves
+	// which are permitted. Yet such cycles may incorrectly inflate the dependency
+	// count for variables which in turn may not get scheduled for initialization
+	// in correct order.)
+	var G []*graphNode
+	for obj, n := range M {
+		if _, ok := obj.(*Func); ok {
+			// connect each predecessor p of n with each successor s
+			// and drop the function node (don't collect it in G)
+			for p := range n.pred {
+				// ignore self-cycles
+				if p != n {
+					// Each successor s of n becomes a successor of p, and
+					// each predecessor p of n becomes a predecessor of s.
+					for s := range n.succ {
+						// ignore self-cycles
+						if s != n {
+							p.succ.add(s)
+							s.pred.add(p)
+							delete(s.pred, n) // remove edge to n
+						}
+					}
+					delete(p.succ, n) // remove edge to n
+				}
+			}
+		} else {
+			// collect non-function nodes
+			G = append(G, n)
+		}
+	}
+
+	// fill in index and ndeps fields
+	for i, n := range G {
+		n.index = i
+		n.ndeps = len(n.succ)
+	}
+
+	return G
+}
+
+// ----------------------------------------------------------------------------
+// Priority queue
+
+// nodeQueue implements the container/heap interface;
+// a nodeQueue may be used as a priority queue.
+type nodeQueue []*graphNode
+
+func (a nodeQueue) Len() int { return len(a) }
+
+func (a nodeQueue) Swap(i, j int) {
+	x, y := a[i], a[j]
+	a[i], a[j] = y, x
+	x.index, y.index = j, i
+}
+
+func (a nodeQueue) Less(i, j int) bool {
+	x, y := a[i], a[j]
+	// nodes are prioritized by number of incoming dependencies (1st key)
+	// and source order (2nd key)
+	return x.ndeps < y.ndeps || x.ndeps == y.ndeps && x.obj.order() < y.obj.order()
+}
+
+func (a *nodeQueue) Push(x interface{}) {
+	panic("unreachable")
+}
+
+func (a *nodeQueue) Pop() interface{} {
+	n := len(*a)
+	x := (*a)[n-1]
+	x.index = -1 // for safety
+	*a = (*a)[:n-1]
+	return x
+}
diff --git a/src/go/types/issues_test.go b/src/go/types/issues_test.go
new file mode 100644
index 0000000..34850eb
--- /dev/null
+++ b/src/go/types/issues_test.go
@@ -0,0 +1,551 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements tests for various issues.
+
+package types_test
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/importer"
+	"go/parser"
+	"go/token"
+	"internal/testenv"
+	"sort"
+	"strings"
+	"testing"
+
+	. "go/types"
+)
+
+func mustParse(t *testing.T, src string) *ast.File {
+	f, err := parser.ParseFile(fset, "", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return f
+}
+func TestIssue5770(t *testing.T) {
+	f := mustParse(t, `package p; type S struct{T}`)
+	conf := Config{Importer: importer.Default()}
+	_, err := conf.Check(f.Name.Name, fset, []*ast.File{f}, nil) // do not crash
+	want := "undeclared name: T"
+	if err == nil || !strings.Contains(err.Error(), want) {
+		t.Errorf("got: %v; want: %s", err, want)
+	}
+}
+
+func TestIssue5849(t *testing.T) {
+	src := `
+package p
+var (
+	s uint
+	_ = uint8(8)
+	_ = uint16(16) << s
+	_ = uint32(32 << s)
+	_ = uint64(64 << s + s)
+	_ = (interface{})("foo")
+	_ = (interface{})(nil)
+)`
+	f := mustParse(t, src)
+
+	var conf Config
+	types := make(map[ast.Expr]TypeAndValue)
+	_, err := conf.Check(f.Name.Name, fset, []*ast.File{f}, &Info{Types: types})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for x, tv := range types {
+		var want Type
+		switch x := x.(type) {
+		case *ast.BasicLit:
+			switch x.Value {
+			case `8`:
+				want = Typ[Uint8]
+			case `16`:
+				want = Typ[Uint16]
+			case `32`:
+				want = Typ[Uint32]
+			case `64`:
+				want = Typ[Uint] // because of "+ s", s is of type uint
+			case `"foo"`:
+				want = Typ[String]
+			}
+		case *ast.Ident:
+			if x.Name == "nil" {
+				want = Typ[UntypedNil]
+			}
+		}
+		if want != nil && !Identical(tv.Type, want) {
+			t.Errorf("got %s; want %s", tv.Type, want)
+		}
+	}
+}
+
+func TestIssue6413(t *testing.T) {
+	src := `
+package p
+func f() int {
+	defer f()
+	go f()
+	return 0
+}
+`
+	f := mustParse(t, src)
+
+	var conf Config
+	types := make(map[ast.Expr]TypeAndValue)
+	_, err := conf.Check(f.Name.Name, fset, []*ast.File{f}, &Info{Types: types})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	want := Typ[Int]
+	n := 0
+	for x, tv := range types {
+		if _, ok := x.(*ast.CallExpr); ok {
+			if tv.Type != want {
+				t.Errorf("%s: got %s; want %s", fset.Position(x.Pos()), tv.Type, want)
+			}
+			n++
+		}
+	}
+
+	if n != 2 {
+		t.Errorf("got %d CallExprs; want 2", n)
+	}
+}
+
+func TestIssue7245(t *testing.T) {
+	src := `
+package p
+func (T) m() (res bool) { return }
+type T struct{} // receiver type after method declaration
+`
+	f := mustParse(t, src)
+
+	var conf Config
+	defs := make(map[*ast.Ident]Object)
+	_, err := conf.Check(f.Name.Name, fset, []*ast.File{f}, &Info{Defs: defs})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	m := f.Decls[0].(*ast.FuncDecl)
+	res1 := defs[m.Name].(*Func).Type().(*Signature).Results().At(0)
+	res2 := defs[m.Type.Results.List[0].Names[0]].(*Var)
+
+	if res1 != res2 {
+		t.Errorf("got %s (%p) != %s (%p)", res1, res2, res1, res2)
+	}
+}
+
+// This tests that uses of existing vars on the LHS of an assignment
+// are Uses, not Defs; and also that the (illegal) use of a non-var on
+// the LHS of an assignment is a Use nonetheless.
+func TestIssue7827(t *testing.T) {
+	const src = `
+package p
+func _() {
+	const w = 1        // defs w
+        x, y := 2, 3       // defs x, y
+        w, x, z := 4, 5, 6 // uses w, x, defs z; error: cannot assign to w
+        _, _, _ = x, y, z  // uses x, y, z
+}
+`
+	f := mustParse(t, src)
+
+	const want = `L3 defs func p._()
+L4 defs const w untyped int
+L5 defs var x int
+L5 defs var y int
+L6 defs var z int
+L6 uses const w untyped int
+L6 uses var x int
+L7 uses var x int
+L7 uses var y int
+L7 uses var z int`
+
+	// don't abort at the first error
+	conf := Config{Error: func(err error) { t.Log(err) }}
+	defs := make(map[*ast.Ident]Object)
+	uses := make(map[*ast.Ident]Object)
+	_, err := conf.Check(f.Name.Name, fset, []*ast.File{f}, &Info{Defs: defs, Uses: uses})
+	if s := fmt.Sprint(err); !strings.HasSuffix(s, "cannot assign to w") {
+		t.Errorf("Check: unexpected error: %s", s)
+	}
+
+	var facts []string
+	for id, obj := range defs {
+		if obj != nil {
+			fact := fmt.Sprintf("L%d defs %s", fset.Position(id.Pos()).Line, obj)
+			facts = append(facts, fact)
+		}
+	}
+	for id, obj := range uses {
+		fact := fmt.Sprintf("L%d uses %s", fset.Position(id.Pos()).Line, obj)
+		facts = append(facts, fact)
+	}
+	sort.Strings(facts)
+
+	got := strings.Join(facts, "\n")
+	if got != want {
+		t.Errorf("Unexpected defs/uses\ngot:\n%s\nwant:\n%s", got, want)
+	}
+}
+
+// This tests that the package associated with the types.Object.Pkg method
+// is the type's package independent of the order in which the imports are
+// listed in the sources src1, src2 below.
+// The actual issue is in go/internal/gcimporter which has a corresponding
+// test; we leave this test here to verify correct behavior at the go/types
+// level.
+func TestIssue13898(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	const src0 = `
+package main
+
+import "go/types"
+
+func main() {
+	var info types.Info
+	for _, obj := range info.Uses {
+		_ = obj.Pkg()
+	}
+}
+`
+	// like src0, but also imports go/importer
+	const src1 = `
+package main
+
+import (
+	"go/types"
+	_ "go/importer"
+)
+
+func main() {
+	var info types.Info
+	for _, obj := range info.Uses {
+		_ = obj.Pkg()
+	}
+}
+`
+	// like src1 but with different import order
+	// (used to fail with this issue)
+	const src2 = `
+package main
+
+import (
+	_ "go/importer"
+	"go/types"
+)
+
+func main() {
+	var info types.Info
+	for _, obj := range info.Uses {
+		_ = obj.Pkg()
+	}
+}
+`
+	f := func(test, src string) {
+		f := mustParse(t, src)
+		cfg := Config{Importer: importer.Default()}
+		info := Info{Uses: make(map[*ast.Ident]Object)}
+		_, err := cfg.Check("main", fset, []*ast.File{f}, &info)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		var pkg *Package
+		count := 0
+		for id, obj := range info.Uses {
+			if id.Name == "Pkg" {
+				pkg = obj.Pkg()
+				count++
+			}
+		}
+		if count != 1 {
+			t.Fatalf("%s: got %d entries named Pkg; want 1", test, count)
+		}
+		if pkg.Name() != "types" {
+			t.Fatalf("%s: got %v; want package types", test, pkg)
+		}
+	}
+
+	f("src0", src0)
+	f("src1", src1)
+	f("src2", src2)
+}
+
+func TestIssue22525(t *testing.T) {
+	f := mustParse(t, `package p; func f() { var a, b, c, d, e int }`)
+
+	got := "\n"
+	conf := Config{Error: func(err error) { got += err.Error() + "\n" }}
+	conf.Check(f.Name.Name, fset, []*ast.File{f}, nil) // do not crash
+	want := `
+1:27: a declared but not used
+1:30: b declared but not used
+1:33: c declared but not used
+1:36: d declared but not used
+1:39: e declared but not used
+`
+	if got != want {
+		t.Errorf("got: %swant: %s", got, want)
+	}
+}
+
+func TestIssue25627(t *testing.T) {
+	const prefix = `package p; import "unsafe"; type P *struct{}; type I interface{}; type T `
+	// The src strings (without prefix) are constructed such that the number of semicolons
+	// plus one corresponds to the number of fields expected in the respective struct.
+	for _, src := range []string{
+		`struct { x Missing }`,
+		`struct { Missing }`,
+		`struct { *Missing }`,
+		`struct { unsafe.Pointer }`,
+		`struct { P }`,
+		`struct { *I }`,
+		`struct { a int; b Missing; *Missing }`,
+	} {
+		f := mustParse(t, prefix+src)
+
+		cfg := Config{Importer: importer.Default(), Error: func(err error) {}}
+		info := &Info{Types: make(map[ast.Expr]TypeAndValue)}
+		_, err := cfg.Check(f.Name.Name, fset, []*ast.File{f}, info)
+		if err != nil {
+			if _, ok := err.(Error); !ok {
+				t.Fatal(err)
+			}
+		}
+
+		ast.Inspect(f, func(n ast.Node) bool {
+			if spec, _ := n.(*ast.TypeSpec); spec != nil {
+				if tv, ok := info.Types[spec.Type]; ok && spec.Name.Name == "T" {
+					want := strings.Count(src, ";") + 1
+					if got := tv.Type.(*Struct).NumFields(); got != want {
+						t.Errorf("%s: got %d fields; want %d", src, got, want)
+					}
+				}
+			}
+			return true
+		})
+	}
+}
+
+func TestIssue28005(t *testing.T) {
+	// method names must match defining interface name for this test
+	// (see last comment in this function)
+	sources := [...]string{
+		"package p; type A interface{ A() }",
+		"package p; type B interface{ B() }",
+		"package p; type X interface{ A; B }",
+	}
+
+	// compute original file ASTs
+	var orig [len(sources)]*ast.File
+	for i, src := range sources {
+		orig[i] = mustParse(t, src)
+	}
+
+	// run the test for all order permutations of the incoming files
+	for _, perm := range [][len(sources)]int{
+		{0, 1, 2},
+		{0, 2, 1},
+		{1, 0, 2},
+		{1, 2, 0},
+		{2, 0, 1},
+		{2, 1, 0},
+	} {
+		// create file order permutation
+		files := make([]*ast.File, len(sources))
+		for i := range perm {
+			files[i] = orig[perm[i]]
+		}
+
+		// type-check package with given file order permutation
+		var conf Config
+		info := &Info{Defs: make(map[*ast.Ident]Object)}
+		_, err := conf.Check("", fset, files, info)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// look for interface object X
+		var obj Object
+		for name, def := range info.Defs {
+			if name.Name == "X" {
+				obj = def
+				break
+			}
+		}
+		if obj == nil {
+			t.Fatal("interface not found")
+		}
+		iface := obj.Type().Underlying().(*Interface) // I must be an interface
+
+		// Each iface method m is embedded; and m's receiver base type name
+		// must match the method's name per the choice in the source file.
+		for i := 0; i < iface.NumMethods(); i++ {
+			m := iface.Method(i)
+			recvName := m.Type().(*Signature).Recv().Type().(*Named).Obj().Name()
+			if recvName != m.Name() {
+				t.Errorf("perm %v: got recv %s; want %s", perm, recvName, m.Name())
+			}
+		}
+	}
+}
+
+func TestIssue28282(t *testing.T) {
+	// create type interface { error }
+	et := Universe.Lookup("error").Type()
+	it := NewInterfaceType(nil, []Type{et})
+	it.Complete()
+	// verify that after completing the interface, the embedded method remains unchanged
+	want := et.Underlying().(*Interface).Method(0)
+	got := it.Method(0)
+	if got != want {
+		t.Fatalf("%s.Method(0): got %q (%p); want %q (%p)", it, got, got, want, want)
+	}
+	// verify that lookup finds the same method in both interfaces (redundant check)
+	obj, _, _ := LookupFieldOrMethod(et, false, nil, "Error")
+	if obj != want {
+		t.Fatalf("%s.Lookup: got %q (%p); want %q (%p)", et, obj, obj, want, want)
+	}
+	obj, _, _ = LookupFieldOrMethod(it, false, nil, "Error")
+	if obj != want {
+		t.Fatalf("%s.Lookup: got %q (%p); want %q (%p)", it, obj, obj, want, want)
+	}
+}
+
+func TestIssue29029(t *testing.T) {
+	f1 := mustParse(t, `package p; type A interface { M() }`)
+	f2 := mustParse(t, `package p; var B interface { A }`)
+
+	// printInfo prints the *Func definitions recorded in info, one *Func per line.
+	printInfo := func(info *Info) string {
+		var buf bytes.Buffer
+		for _, obj := range info.Defs {
+			if fn, ok := obj.(*Func); ok {
+				fmt.Fprintln(&buf, fn)
+			}
+		}
+		return buf.String()
+	}
+
+	// The *Func (method) definitions for package p must be the same
+	// independent on whether f1 and f2 are type-checked together, or
+	// incrementally.
+
+	// type-check together
+	var conf Config
+	info := &Info{Defs: make(map[*ast.Ident]Object)}
+	check := NewChecker(&conf, fset, NewPackage("", "p"), info)
+	if err := check.Files([]*ast.File{f1, f2}); err != nil {
+		t.Fatal(err)
+	}
+	want := printInfo(info)
+
+	// type-check incrementally
+	info = &Info{Defs: make(map[*ast.Ident]Object)}
+	check = NewChecker(&conf, fset, NewPackage("", "p"), info)
+	if err := check.Files([]*ast.File{f1}); err != nil {
+		t.Fatal(err)
+	}
+	if err := check.Files([]*ast.File{f2}); err != nil {
+		t.Fatal(err)
+	}
+	got := printInfo(info)
+
+	if got != want {
+		t.Errorf("\ngot : %swant: %s", got, want)
+	}
+}
+
+func TestIssue34151(t *testing.T) {
+	const asrc = `package a; type I interface{ M() }; type T struct { F interface { I } }`
+	const bsrc = `package b; import "a"; type T struct { F interface { a.I } }; var _ = a.T(T{})`
+
+	a, err := pkgFor("a", asrc, nil)
+	if err != nil {
+		t.Fatalf("package %s failed to typecheck: %v", a.Name(), err)
+	}
+
+	bast := mustParse(t, bsrc)
+	conf := Config{Importer: importHelper{a}}
+	b, err := conf.Check(bast.Name.Name, fset, []*ast.File{bast}, nil)
+	if err != nil {
+		t.Errorf("package %s failed to typecheck: %v", b.Name(), err)
+	}
+}
+
+type importHelper struct {
+	pkg *Package
+}
+
+func (h importHelper) Import(path string) (*Package, error) {
+	if path != h.pkg.Path() {
+		return nil, fmt.Errorf("got package path %q; want %q", path, h.pkg.Path())
+	}
+	return h.pkg, nil
+}
+
+// TestIssue34921 verifies that we don't update an imported type's underlying
+// type when resolving an underlying type. Specifically, when determining the
+// underlying type of b.T (which is the underlying type of a.T, which is int)
+// we must not set the underlying type of a.T again since that would lead to
+// a race condition if package b is imported elsewhere, in a package that is
+// concurrently type-checked.
+func TestIssue34921(t *testing.T) {
+	defer func() {
+		if r := recover(); r != nil {
+			t.Error(r)
+		}
+	}()
+
+	var sources = []string{
+		`package a; type T int`,
+		`package b; import "a"; type T a.T`,
+	}
+
+	var pkg *Package
+	for _, src := range sources {
+		f := mustParse(t, src)
+		conf := Config{Importer: importHelper{pkg}}
+		res, err := conf.Check(f.Name.Name, fset, []*ast.File{f}, nil)
+		if err != nil {
+			t.Errorf("%q failed to typecheck: %v", src, err)
+		}
+		pkg = res // res is imported by the next package in this test
+	}
+}
+
+func TestIssue43088(t *testing.T) {
+	// type T1 struct {
+	//         _ T2
+	// }
+	//
+	// type T2 struct {
+	//         _ struct {
+	//                 _ T2
+	//         }
+	// }
+	n1 := NewTypeName(token.NoPos, nil, "T1", nil)
+	T1 := NewNamed(n1, nil, nil)
+	n2 := NewTypeName(token.NoPos, nil, "T2", nil)
+	T2 := NewNamed(n2, nil, nil)
+	s1 := NewStruct([]*Var{NewField(token.NoPos, nil, "_", T2, false)}, nil)
+	T1.SetUnderlying(s1)
+	s2 := NewStruct([]*Var{NewField(token.NoPos, nil, "_", T2, false)}, nil)
+	s3 := NewStruct([]*Var{NewField(token.NoPos, nil, "_", s2, false)}, nil)
+	T2.SetUnderlying(s3)
+
+	// These calls must terminate (no endless recursion).
+	Comparable(T1)
+	Comparable(T2)
+}
diff --git a/src/go/types/labels.go b/src/go/types/labels.go
new file mode 100644
index 0000000..8cf6e63
--- /dev/null
+++ b/src/go/types/labels.go
@@ -0,0 +1,272 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types
+
+import (
+	"go/ast"
+	"go/token"
+)
+
+// labels checks correct label use in body.
+func (check *Checker) labels(body *ast.BlockStmt) {
+	// set of all labels in this body
+	all := NewScope(nil, body.Pos(), body.End(), "label")
+
+	fwdJumps := check.blockBranches(all, nil, nil, body.List)
+
+	// If there are any forward jumps left, no label was found for
+	// the corresponding goto statements. Either those labels were
+	// never defined, or they are inside blocks and not reachable
+	// for the respective gotos.
+	for _, jmp := range fwdJumps {
+		var msg string
+		var code errorCode
+		name := jmp.Label.Name
+		if alt := all.Lookup(name); alt != nil {
+			msg = "goto %s jumps into block"
+			alt.(*Label).used = true // avoid another error
+			code = _JumpIntoBlock
+		} else {
+			msg = "label %s not declared"
+			code = _UndeclaredLabel
+		}
+		check.errorf(jmp.Label, code, msg, name)
+	}
+
+	// spec: "It is illegal to define a label that is never used."
+	for _, obj := range all.elems {
+		if lbl := obj.(*Label); !lbl.used {
+			check.softErrorf(lbl, _UnusedLabel, "label %s declared but not used", lbl.name)
+		}
+	}
+}
+
+// A block tracks label declarations in a block and its enclosing blocks.
+type block struct {
+	parent *block                      // enclosing block
+	lstmt  *ast.LabeledStmt            // labeled statement to which this block belongs, or nil
+	labels map[string]*ast.LabeledStmt // allocated lazily
+}
+
+// insert records a new label declaration for the current block.
+// The label must not have been declared before in any block.
+func (b *block) insert(s *ast.LabeledStmt) {
+	name := s.Label.Name
+	if debug {
+		assert(b.gotoTarget(name) == nil)
+	}
+	labels := b.labels
+	if labels == nil {
+		labels = make(map[string]*ast.LabeledStmt)
+		b.labels = labels
+	}
+	labels[name] = s
+}
+
+// gotoTarget returns the labeled statement in the current
+// or an enclosing block with the given label name, or nil.
+func (b *block) gotoTarget(name string) *ast.LabeledStmt {
+	for s := b; s != nil; s = s.parent {
+		if t := s.labels[name]; t != nil {
+			return t
+		}
+	}
+	return nil
+}
+
+// enclosingTarget returns the innermost enclosing labeled
+// statement with the given label name, or nil.
+func (b *block) enclosingTarget(name string) *ast.LabeledStmt {
+	for s := b; s != nil; s = s.parent {
+		if t := s.lstmt; t != nil && t.Label.Name == name {
+			return t
+		}
+	}
+	return nil
+}
+
+// blockBranches processes a block's statement list and returns the set of outgoing forward jumps.
+// all is the scope of all declared labels, parent the set of labels declared in the immediately
+// enclosing block, and lstmt is the labeled statement this block is associated with (or nil).
+func (check *Checker) blockBranches(all *Scope, parent *block, lstmt *ast.LabeledStmt, list []ast.Stmt) []*ast.BranchStmt {
+	b := &block{parent: parent, lstmt: lstmt}
+
+	var (
+		varDeclPos         token.Pos
+		fwdJumps, badJumps []*ast.BranchStmt
+	)
+
+	// All forward jumps jumping over a variable declaration are possibly
+	// invalid (they may still jump out of the block and be ok).
+	// recordVarDecl records them for the given position.
+	recordVarDecl := func(pos token.Pos) {
+		varDeclPos = pos
+		badJumps = append(badJumps[:0], fwdJumps...) // copy fwdJumps to badJumps
+	}
+
+	jumpsOverVarDecl := func(jmp *ast.BranchStmt) bool {
+		if varDeclPos.IsValid() {
+			for _, bad := range badJumps {
+				if jmp == bad {
+					return true
+				}
+			}
+		}
+		return false
+	}
+
+	blockBranches := func(lstmt *ast.LabeledStmt, list []ast.Stmt) {
+		// Unresolved forward jumps inside the nested block
+		// become forward jumps in the current block.
+		fwdJumps = append(fwdJumps, check.blockBranches(all, b, lstmt, list)...)
+	}
+
+	var stmtBranches func(ast.Stmt)
+	stmtBranches = func(s ast.Stmt) {
+		switch s := s.(type) {
+		case *ast.DeclStmt:
+			if d, _ := s.Decl.(*ast.GenDecl); d != nil && d.Tok == token.VAR {
+				recordVarDecl(d.Pos())
+			}
+
+		case *ast.LabeledStmt:
+			// declare non-blank label
+			if name := s.Label.Name; name != "_" {
+				lbl := NewLabel(s.Label.Pos(), check.pkg, name)
+				if alt := all.Insert(lbl); alt != nil {
+					check.softErrorf(lbl, _DuplicateLabel, "label %s already declared", name)
+					check.reportAltDecl(alt)
+					// ok to continue
+				} else {
+					b.insert(s)
+					check.recordDef(s.Label, lbl)
+				}
+				// resolve matching forward jumps and remove them from fwdJumps
+				i := 0
+				for _, jmp := range fwdJumps {
+					if jmp.Label.Name == name {
+						// match
+						lbl.used = true
+						check.recordUse(jmp.Label, lbl)
+						if jumpsOverVarDecl(jmp) {
+							check.softErrorf(
+								jmp.Label,
+								_JumpOverDecl,
+								"goto %s jumps over variable declaration at line %d",
+								name,
+								check.fset.Position(varDeclPos).Line,
+							)
+							// ok to continue
+						}
+					} else {
+						// no match - record new forward jump
+						fwdJumps[i] = jmp
+						i++
+					}
+				}
+				fwdJumps = fwdJumps[:i]
+				lstmt = s
+			}
+			stmtBranches(s.Stmt)
+
+		case *ast.BranchStmt:
+			if s.Label == nil {
+				return // checked in 1st pass (check.stmt)
+			}
+
+			// determine and validate target
+			name := s.Label.Name
+			switch s.Tok {
+			case token.BREAK:
+				// spec: "If there is a label, it must be that of an enclosing
+				// "for", "switch", or "select" statement, and that is the one
+				// whose execution terminates."
+				valid := false
+				if t := b.enclosingTarget(name); t != nil {
+					switch t.Stmt.(type) {
+					case *ast.SwitchStmt, *ast.TypeSwitchStmt, *ast.SelectStmt, *ast.ForStmt, *ast.RangeStmt:
+						valid = true
+					}
+				}
+				if !valid {
+					check.errorf(s.Label, _MisplacedLabel, "invalid break label %s", name)
+					return
+				}
+
+			case token.CONTINUE:
+				// spec: "If there is a label, it must be that of an enclosing
+				// "for" statement, and that is the one whose execution advances."
+				valid := false
+				if t := b.enclosingTarget(name); t != nil {
+					switch t.Stmt.(type) {
+					case *ast.ForStmt, *ast.RangeStmt:
+						valid = true
+					}
+				}
+				if !valid {
+					check.errorf(s.Label, _MisplacedLabel, "invalid continue label %s", name)
+					return
+				}
+
+			case token.GOTO:
+				if b.gotoTarget(name) == nil {
+					// label may be declared later - add branch to forward jumps
+					fwdJumps = append(fwdJumps, s)
+					return
+				}
+
+			default:
+				check.invalidAST(s, "branch statement: %s %s", s.Tok, name)
+				return
+			}
+
+			// record label use
+			obj := all.Lookup(name)
+			obj.(*Label).used = true
+			check.recordUse(s.Label, obj)
+
+		case *ast.AssignStmt:
+			if s.Tok == token.DEFINE {
+				recordVarDecl(s.Pos())
+			}
+
+		case *ast.BlockStmt:
+			blockBranches(lstmt, s.List)
+
+		case *ast.IfStmt:
+			stmtBranches(s.Body)
+			if s.Else != nil {
+				stmtBranches(s.Else)
+			}
+
+		case *ast.CaseClause:
+			blockBranches(nil, s.Body)
+
+		case *ast.SwitchStmt:
+			stmtBranches(s.Body)
+
+		case *ast.TypeSwitchStmt:
+			stmtBranches(s.Body)
+
+		case *ast.CommClause:
+			blockBranches(nil, s.Body)
+
+		case *ast.SelectStmt:
+			stmtBranches(s.Body)
+
+		case *ast.ForStmt:
+			stmtBranches(s.Body)
+
+		case *ast.RangeStmt:
+			stmtBranches(s.Body)
+		}
+	}
+
+	for _, s := range list {
+		stmtBranches(s)
+	}
+
+	return fwdJumps
+}
diff --git a/src/go/types/lookup.go b/src/go/types/lookup.go
new file mode 100644
index 0000000..3c9ff18
--- /dev/null
+++ b/src/go/types/lookup.go
@@ -0,0 +1,400 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements various field and method lookup functions.
+
+package types
+
+// LookupFieldOrMethod looks up a field or method with given package and name
+// in T and returns the corresponding *Var or *Func, an index sequence, and a
+// bool indicating if there were any pointer indirections on the path to the
+// field or method. If addressable is set, T is the type of an addressable
+// variable (only matters for method lookups).
+//
+// The last index entry is the field or method index in the (possibly embedded)
+// type where the entry was found, either:
+//
+//	1) the list of declared methods of a named type; or
+//	2) the list of all methods (method set) of an interface type; or
+//	3) the list of fields of a struct type.
+//
+// The earlier index entries are the indices of the embedded struct fields
+// traversed to get to the found entry, starting at depth 0.
+//
+// If no entry is found, a nil object is returned. In this case, the returned
+// index and indirect values have the following meaning:
+//
+//	- If index != nil, the index sequence points to an ambiguous entry
+//	(the same name appeared more than once at the same embedding level).
+//
+//	- If indirect is set, a method with a pointer receiver type was found
+//      but there was no pointer on the path from the actual receiver type to
+//	the method's formal receiver base type, nor was the receiver addressable.
+//
+func LookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
+	return (*Checker)(nil).lookupFieldOrMethod(T, addressable, pkg, name)
+}
+
+// Internal use of Checker.lookupFieldOrMethod: If the obj result is a method
+// associated with a concrete (non-interface) type, the method's signature
+// may not be fully set up. Call Checker.objDecl(obj, nil) before accessing
+// the method's type.
+// TODO(gri) Now that we provide the *Checker, we can probably remove this
+// caveat by calling Checker.objDecl from lookupFieldOrMethod. Investigate.
+
+// lookupFieldOrMethod is like the external version but completes interfaces
+// as necessary.
+func (check *Checker) lookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
+	// Methods cannot be associated to a named pointer type
+	// (spec: "The type denoted by T is called the receiver base type;
+	// it must not be a pointer or interface type and it must be declared
+	// in the same package as the method.").
+	// Thus, if we have a named pointer type, proceed with the underlying
+	// pointer type but discard the result if it is a method since we would
+	// not have found it for T (see also issue 8590).
+	if t, _ := T.(*Named); t != nil {
+		if p, _ := t.underlying.(*Pointer); p != nil {
+			obj, index, indirect = check.rawLookupFieldOrMethod(p, false, pkg, name)
+			if _, ok := obj.(*Func); ok {
+				return nil, nil, false
+			}
+			return
+		}
+	}
+
+	return check.rawLookupFieldOrMethod(T, addressable, pkg, name)
+}
+
+// TODO(gri) The named type consolidation and seen maps below must be
+//           indexed by unique keys for a given type. Verify that named
+//           types always have only one representation (even when imported
+//           indirectly via different packages.)
+
+// rawLookupFieldOrMethod should only be called by lookupFieldOrMethod and missingMethod.
+func (check *Checker) rawLookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (obj Object, index []int, indirect bool) {
+	// WARNING: The code in this function is extremely subtle - do not modify casually!
+	//          This function and NewMethodSet should be kept in sync.
+
+	if name == "_" {
+		return // blank fields/methods are never found
+	}
+
+	typ, isPtr := deref(T)
+
+	// *typ where typ is an interface has no methods.
+	if isPtr && IsInterface(typ) {
+		return
+	}
+
+	// Start with typ as single entry at shallowest depth.
+	current := []embeddedType{{typ, nil, isPtr, false}}
+
+	// Named types that we have seen already, allocated lazily.
+	// Used to avoid endless searches in case of recursive types.
+	// Since only Named types can be used for recursive types, we
+	// only need to track those.
+	// (If we ever allow type aliases to construct recursive types,
+	// we must use type identity rather than pointer equality for
+	// the map key comparison, as we do in consolidateMultiples.)
+	var seen map[*Named]bool
+
+	// search current depth
+	for len(current) > 0 {
+		var next []embeddedType // embedded types found at current depth
+
+		// look for (pkg, name) in all types at current depth
+		for _, e := range current {
+			typ := e.typ
+
+			// If we have a named type, we may have associated methods.
+			// Look for those first.
+			if named, _ := typ.(*Named); named != nil {
+				if seen[named] {
+					// We have seen this type before, at a more shallow depth
+					// (note that multiples of this type at the current depth
+					// were consolidated before). The type at that depth shadows
+					// this same type at the current depth, so we can ignore
+					// this one.
+					continue
+				}
+				if seen == nil {
+					seen = make(map[*Named]bool)
+				}
+				seen[named] = true
+
+				// look for a matching attached method
+				if i, m := lookupMethod(named.methods, pkg, name); m != nil {
+					// potential match
+					// caution: method may not have a proper signature yet
+					index = concat(e.index, i)
+					if obj != nil || e.multiples {
+						return nil, index, false // collision
+					}
+					obj = m
+					indirect = e.indirect
+					continue // we can't have a matching field or interface method
+				}
+
+				// continue with underlying type
+				typ = named.underlying
+			}
+
+			switch t := typ.(type) {
+			case *Struct:
+				// look for a matching field and collect embedded types
+				for i, f := range t.fields {
+					if f.sameId(pkg, name) {
+						assert(f.typ != nil)
+						index = concat(e.index, i)
+						if obj != nil || e.multiples {
+							return nil, index, false // collision
+						}
+						obj = f
+						indirect = e.indirect
+						continue // we can't have a matching interface method
+					}
+					// Collect embedded struct fields for searching the next
+					// lower depth, but only if we have not seen a match yet
+					// (if we have a match it is either the desired field or
+					// we have a name collision on the same depth; in either
+					// case we don't need to look further).
+					// Embedded fields are always of the form T or *T where
+					// T is a type name. If e.typ appeared multiple times at
+					// this depth, f.typ appears multiple times at the next
+					// depth.
+					if obj == nil && f.embedded {
+						typ, isPtr := deref(f.typ)
+						// TODO(gri) optimization: ignore types that can't
+						// have fields or methods (only Named, Struct, and
+						// Interface types need to be considered).
+						next = append(next, embeddedType{typ, concat(e.index, i), e.indirect || isPtr, e.multiples})
+					}
+				}
+
+			case *Interface:
+				// look for a matching method
+				// TODO(gri) t.allMethods is sorted - use binary search
+				check.completeInterface(t)
+				if i, m := lookupMethod(t.allMethods, pkg, name); m != nil {
+					assert(m.typ != nil)
+					index = concat(e.index, i)
+					if obj != nil || e.multiples {
+						return nil, index, false // collision
+					}
+					obj = m
+					indirect = e.indirect
+				}
+			}
+		}
+
+		if obj != nil {
+			// found a potential match
+			// spec: "A method call x.m() is valid if the method set of (the type of) x
+			//        contains m and the argument list can be assigned to the parameter
+			//        list of m. If x is addressable and &x's method set contains m, x.m()
+			//        is shorthand for (&x).m()".
+			if f, _ := obj.(*Func); f != nil && ptrRecv(f) && !indirect && !addressable {
+				return nil, nil, true // pointer/addressable receiver required
+			}
+			return
+		}
+
+		current = check.consolidateMultiples(next)
+	}
+
+	return nil, nil, false // not found
+}
+
+// embeddedType represents an embedded type
+type embeddedType struct {
+	typ       Type
+	index     []int // embedded field indices, starting with index at depth 0
+	indirect  bool  // if set, there was a pointer indirection on the path to this field
+	multiples bool  // if set, typ appears multiple times at this depth
+}
+
+// consolidateMultiples collects multiple list entries with the same type
+// into a single entry marked as containing multiples. The result is the
+// consolidated list.
+func (check *Checker) consolidateMultiples(list []embeddedType) []embeddedType {
+	if len(list) <= 1 {
+		return list // at most one entry - nothing to do
+	}
+
+	n := 0                     // number of entries w/ unique type
+	prev := make(map[Type]int) // index at which type was previously seen
+	for _, e := range list {
+		if i, found := check.lookupType(prev, e.typ); found {
+			list[i].multiples = true
+			// ignore this entry
+		} else {
+			prev[e.typ] = n
+			list[n] = e
+			n++
+		}
+	}
+	return list[:n]
+}
+
+func (check *Checker) lookupType(m map[Type]int, typ Type) (int, bool) {
+	// fast path: maybe the types are equal
+	if i, found := m[typ]; found {
+		return i, true
+	}
+
+	for t, i := range m {
+		if check.identical(t, typ) {
+			return i, true
+		}
+	}
+
+	return 0, false
+}
+
+// MissingMethod returns (nil, false) if V implements T, otherwise it
+// returns a missing method required by T and whether it is missing or
+// just has the wrong type.
+//
+// For non-interface types V, or if static is set, V implements T if all
+// methods of T are present in V. Otherwise (V is an interface and static
+// is not set), MissingMethod only checks that methods of T which are also
+// present in V have matching types (e.g., for a type assertion x.(T) where
+// x is of interface type V).
+//
+func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType bool) {
+	m, typ := (*Checker)(nil).missingMethod(V, T, static)
+	return m, typ != nil
+}
+
+// missingMethod is like MissingMethod but accepts a receiver.
+// The receiver may be nil if missingMethod is invoked through
+// an exported API call (such as MissingMethod), i.e., when all
+// methods have been type-checked.
+// If the type has the correctly named method, but with the wrong
+// signature, the existing method is returned as well.
+// To improve error messages, also report the wrong signature
+// when the method exists on *V instead of V.
+func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method, wrongType *Func) {
+	check.completeInterface(T)
+
+	// fast path for common case
+	if T.Empty() {
+		return
+	}
+
+	if ityp, _ := V.Underlying().(*Interface); ityp != nil {
+		check.completeInterface(ityp)
+		// TODO(gri) allMethods is sorted - can do this more efficiently
+		for _, m := range T.allMethods {
+			_, obj := lookupMethod(ityp.allMethods, m.pkg, m.name)
+			switch {
+			case obj == nil:
+				if static {
+					return m, nil
+				}
+			case !check.identical(obj.Type(), m.typ):
+				return m, obj
+			}
+		}
+		return
+	}
+
+	// A concrete type implements T if it implements all methods of T.
+	for _, m := range T.allMethods {
+		obj, _, _ := check.rawLookupFieldOrMethod(V, false, m.pkg, m.name)
+
+		// Check if *V implements this method of T.
+		if obj == nil {
+			ptr := NewPointer(V)
+			obj, _, _ = check.rawLookupFieldOrMethod(ptr, false, m.pkg, m.name)
+			if obj != nil {
+				return m, obj.(*Func)
+			}
+		}
+
+		// we must have a method (not a field of matching function type)
+		f, _ := obj.(*Func)
+		if f == nil {
+			return m, nil
+		}
+
+		// methods may not have a fully set up signature yet
+		if check != nil {
+			check.objDecl(f, nil)
+		}
+
+		if !check.identical(f.typ, m.typ) {
+			return m, f
+		}
+	}
+
+	return
+}
+
+// assertableTo reports whether a value of type V can be asserted to have type T.
+// It returns (nil, false) as affirmative answer. Otherwise it returns a missing
+// method required by V and whether it is missing or just has the wrong type.
+// The receiver may be nil if assertableTo is invoked through an exported API call
+// (such as AssertableTo), i.e., when all methods have been type-checked.
+func (check *Checker) assertableTo(V *Interface, T Type) (method, wrongType *Func) {
+	// no static check is required if T is an interface
+	// spec: "If T is an interface type, x.(T) asserts that the
+	//        dynamic type of x implements the interface T."
+	if _, ok := T.Underlying().(*Interface); ok && !strict {
+		return
+	}
+	return check.missingMethod(T, V, false)
+}
+
+// deref dereferences typ if it is a *Pointer and returns its base and true.
+// Otherwise it returns (typ, false).
+func deref(typ Type) (Type, bool) {
+	if p, _ := typ.(*Pointer); p != nil {
+		return p.base, true
+	}
+	return typ, false
+}
+
+// derefStructPtr dereferences typ if it is a (named or unnamed) pointer to a
+// (named or unnamed) struct and returns its base. Otherwise it returns typ.
+func derefStructPtr(typ Type) Type {
+	if p, _ := typ.Underlying().(*Pointer); p != nil {
+		if _, ok := p.base.Underlying().(*Struct); ok {
+			return p.base
+		}
+	}
+	return typ
+}
+
+// concat returns the result of concatenating list and i.
+// The result does not share its underlying array with list.
+func concat(list []int, i int) []int {
+	var t []int
+	t = append(t, list...)
+	return append(t, i)
+}
+
+// fieldIndex returns the index for the field with matching package and name, or a value < 0.
+func fieldIndex(fields []*Var, pkg *Package, name string) int {
+	if name != "_" {
+		for i, f := range fields {
+			if f.sameId(pkg, name) {
+				return i
+			}
+		}
+	}
+	return -1
+}
+
+// lookupMethod returns the index of and method with matching package and name, or (-1, nil).
+func lookupMethod(methods []*Func, pkg *Package, name string) (int, *Func) {
+	if name != "_" {
+		for i, m := range methods {
+			if m.sameId(pkg, name) {
+				return i, m
+			}
+		}
+	}
+	return -1, nil
+}
diff --git a/src/go/types/methodset.go b/src/go/types/methodset.go
new file mode 100644
index 0000000..c34d732
--- /dev/null
+++ b/src/go/types/methodset.go
@@ -0,0 +1,260 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements method sets.
+
+package types
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// A MethodSet is an ordered set of concrete or abstract (interface) methods;
+// a method is a MethodVal selection, and they are ordered by ascending m.Obj().Id().
+// The zero value for a MethodSet is a ready-to-use empty method set.
+type MethodSet struct {
+	list []*Selection
+}
+
+func (s *MethodSet) String() string {
+	if s.Len() == 0 {
+		return "MethodSet {}"
+	}
+
+	var buf strings.Builder
+	fmt.Fprintln(&buf, "MethodSet {")
+	for _, f := range s.list {
+		fmt.Fprintf(&buf, "\t%s\n", f)
+	}
+	fmt.Fprintln(&buf, "}")
+	return buf.String()
+}
+
+// Len returns the number of methods in s.
+func (s *MethodSet) Len() int { return len(s.list) }
+
+// At returns the i'th method in s for 0 <= i < s.Len().
+func (s *MethodSet) At(i int) *Selection { return s.list[i] }
+
+// Lookup returns the method with matching package and name, or nil if not found.
+func (s *MethodSet) Lookup(pkg *Package, name string) *Selection {
+	if s.Len() == 0 {
+		return nil
+	}
+
+	key := Id(pkg, name)
+	i := sort.Search(len(s.list), func(i int) bool {
+		m := s.list[i]
+		return m.obj.Id() >= key
+	})
+	if i < len(s.list) {
+		m := s.list[i]
+		if m.obj.Id() == key {
+			return m
+		}
+	}
+	return nil
+}
+
+// Shared empty method set.
+var emptyMethodSet MethodSet
+
+// Note: NewMethodSet is intended for external use only as it
+//       requires interfaces to be complete. If may be used
+//       internally if LookupFieldOrMethod completed the same
+//       interfaces beforehand.
+
+// NewMethodSet returns the method set for the given type T.
+// It always returns a non-nil method set, even if it is empty.
+func NewMethodSet(T Type) *MethodSet {
+	// WARNING: The code in this function is extremely subtle - do not modify casually!
+	//          This function and lookupFieldOrMethod should be kept in sync.
+
+	// method set up to the current depth, allocated lazily
+	var base methodSet
+
+	typ, isPtr := deref(T)
+
+	// *typ where typ is an interface has no methods.
+	if isPtr && IsInterface(typ) {
+		return &emptyMethodSet
+	}
+
+	// Start with typ as single entry at shallowest depth.
+	current := []embeddedType{{typ, nil, isPtr, false}}
+
+	// Named types that we have seen already, allocated lazily.
+	// Used to avoid endless searches in case of recursive types.
+	// Since only Named types can be used for recursive types, we
+	// only need to track those.
+	// (If we ever allow type aliases to construct recursive types,
+	// we must use type identity rather than pointer equality for
+	// the map key comparison, as we do in consolidateMultiples.)
+	var seen map[*Named]bool
+
+	// collect methods at current depth
+	for len(current) > 0 {
+		var next []embeddedType // embedded types found at current depth
+
+		// field and method sets at current depth, indexed by names (Id's), and allocated lazily
+		var fset map[string]bool // we only care about the field names
+		var mset methodSet
+
+		for _, e := range current {
+			typ := e.typ
+
+			// If we have a named type, we may have associated methods.
+			// Look for those first.
+			if named, _ := typ.(*Named); named != nil {
+				if seen[named] {
+					// We have seen this type before, at a more shallow depth
+					// (note that multiples of this type at the current depth
+					// were consolidated before). The type at that depth shadows
+					// this same type at the current depth, so we can ignore
+					// this one.
+					continue
+				}
+				if seen == nil {
+					seen = make(map[*Named]bool)
+				}
+				seen[named] = true
+
+				mset = mset.add(named.methods, e.index, e.indirect, e.multiples)
+
+				// continue with underlying type
+				typ = named.underlying
+			}
+
+			switch t := typ.(type) {
+			case *Struct:
+				for i, f := range t.fields {
+					if fset == nil {
+						fset = make(map[string]bool)
+					}
+					fset[f.Id()] = true
+
+					// Embedded fields are always of the form T or *T where
+					// T is a type name. If typ appeared multiple times at
+					// this depth, f.Type appears multiple times at the next
+					// depth.
+					if f.embedded {
+						typ, isPtr := deref(f.typ)
+						// TODO(gri) optimization: ignore types that can't
+						// have fields or methods (only Named, Struct, and
+						// Interface types need to be considered).
+						next = append(next, embeddedType{typ, concat(e.index, i), e.indirect || isPtr, e.multiples})
+					}
+				}
+
+			case *Interface:
+				mset = mset.add(t.allMethods, e.index, true, e.multiples)
+			}
+		}
+
+		// Add methods and collisions at this depth to base if no entries with matching
+		// names exist already.
+		for k, m := range mset {
+			if _, found := base[k]; !found {
+				// Fields collide with methods of the same name at this depth.
+				if fset[k] {
+					m = nil // collision
+				}
+				if base == nil {
+					base = make(methodSet)
+				}
+				base[k] = m
+			}
+		}
+
+		// Add all (remaining) fields at this depth as collisions (since they will
+		// hide any method further down) if no entries with matching names exist already.
+		for k := range fset {
+			if _, found := base[k]; !found {
+				if base == nil {
+					base = make(methodSet)
+				}
+				base[k] = nil // collision
+			}
+		}
+
+		// It's ok to call consolidateMultiples with a nil *Checker because
+		// MethodSets are not used internally (outside debug mode). When used
+		// externally, interfaces are expected to be completed and then we do
+		// not need a *Checker to complete them when (indirectly) calling
+		// Checker.identical via consolidateMultiples.
+		current = (*Checker)(nil).consolidateMultiples(next)
+	}
+
+	if len(base) == 0 {
+		return &emptyMethodSet
+	}
+
+	// collect methods
+	var list []*Selection
+	for _, m := range base {
+		if m != nil {
+			m.recv = T
+			list = append(list, m)
+		}
+	}
+	// sort by unique name
+	sort.Slice(list, func(i, j int) bool {
+		return list[i].obj.Id() < list[j].obj.Id()
+	})
+	return &MethodSet{list}
+}
+
+// A methodSet is a set of methods and name collisions.
+// A collision indicates that multiple methods with the
+// same unique id, or a field with that id appeared.
+type methodSet map[string]*Selection // a nil entry indicates a name collision
+
+// Add adds all functions in list to the method set s.
+// If multiples is set, every function in list appears multiple times
+// and is treated as a collision.
+func (s methodSet) add(list []*Func, index []int, indirect bool, multiples bool) methodSet {
+	if len(list) == 0 {
+		return s
+	}
+	if s == nil {
+		s = make(methodSet)
+	}
+	for i, f := range list {
+		key := f.Id()
+		// if f is not in the set, add it
+		if !multiples {
+			// TODO(gri) A found method may not be added because it's not in the method set
+			// (!indirect && ptrRecv(f)). A 2nd method on the same level may be in the method
+			// set and may not collide with the first one, thus leading to a false positive.
+			// Is that possible? Investigate.
+			if _, found := s[key]; !found && (indirect || !ptrRecv(f)) {
+				s[key] = &Selection{MethodVal, nil, f, concat(index, i), indirect}
+				continue
+			}
+		}
+		s[key] = nil // collision
+	}
+	return s
+}
+
+// ptrRecv reports whether the receiver is of the form *T.
+func ptrRecv(f *Func) bool {
+	// If a method's receiver type is set, use that as the source of truth for the receiver.
+	// Caution: Checker.funcDecl (decl.go) marks a function by setting its type to an empty
+	// signature. We may reach here before the signature is fully set up: we must explicitly
+	// check if the receiver is set (we cannot just look for non-nil f.typ).
+	if sig, _ := f.typ.(*Signature); sig != nil && sig.recv != nil {
+		_, isPtr := deref(sig.recv.typ)
+		return isPtr
+	}
+
+	// If a method's type is not set it may be a method/function that is:
+	// 1) client-supplied (via NewFunc with no signature), or
+	// 2) internally created but not yet type-checked.
+	// For case 1) we can't do anything; the client must know what they are doing.
+	// For case 2) we can use the information gathered by the resolver.
+	return f.hasPtrRecv
+}
diff --git a/src/go/types/object.go b/src/go/types/object.go
new file mode 100644
index 0000000..374b24d
--- /dev/null
+++ b/src/go/types/object.go
@@ -0,0 +1,486 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types
+
+import (
+	"bytes"
+	"fmt"
+	"go/constant"
+	"go/token"
+)
+
+// An Object describes a named language entity such as a package,
+// constant, type, variable, function (incl. methods), or label.
+// All objects implement the Object interface.
+//
+type Object interface {
+	Parent() *Scope // scope in which this object is declared; nil for methods and struct fields
+	Pos() token.Pos // position of object identifier in declaration
+	Pkg() *Package  // package to which this object belongs; nil for labels and objects in the Universe scope
+	Name() string   // package local object name
+	Type() Type     // object type
+	Exported() bool // reports whether the name starts with a capital letter
+	Id() string     // object name if exported, qualified name if not exported (see func Id)
+
+	// String returns a human-readable string of the object.
+	String() string
+
+	// order reflects a package-level object's source order: if object
+	// a is before object b in the source, then a.order() < b.order().
+	// order returns a value > 0 for package-level objects; it returns
+	// 0 for all other objects (including objects in file scopes).
+	order() uint32
+
+	// color returns the object's color.
+	color() color
+
+	// setOrder sets the order number of the object. It must be > 0.
+	setOrder(uint32)
+
+	// setColor sets the object's color. It must not be white.
+	setColor(color color)
+
+	// setParent sets the parent scope of the object.
+	setParent(*Scope)
+
+	// sameId reports whether obj.Id() and Id(pkg, name) are the same.
+	sameId(pkg *Package, name string) bool
+
+	// scopePos returns the start position of the scope of this Object
+	scopePos() token.Pos
+
+	// setScopePos sets the start position of the scope for this Object.
+	setScopePos(pos token.Pos)
+}
+
+// Id returns name if it is exported, otherwise it
+// returns the name qualified with the package path.
+func Id(pkg *Package, name string) string {
+	if token.IsExported(name) {
+		return name
+	}
+	// unexported names need the package path for differentiation
+	// (if there's no package, make sure we don't start with '.'
+	// as that may change the order of methods between a setup
+	// inside a package and outside a package - which breaks some
+	// tests)
+	path := "_"
+	// pkg is nil for objects in Universe scope and possibly types
+	// introduced via Eval (see also comment in object.sameId)
+	if pkg != nil && pkg.path != "" {
+		path = pkg.path
+	}
+	return path + "." + name
+}
+
+// An object implements the common parts of an Object.
+type object struct {
+	parent    *Scope
+	pos       token.Pos
+	pkg       *Package
+	name      string
+	typ       Type
+	order_    uint32
+	color_    color
+	scopePos_ token.Pos
+}
+
+// color encodes the color of an object (see Checker.objDecl for details).
+type color uint32
+
+// An object may be painted in one of three colors.
+// Color values other than white or black are considered grey.
+const (
+	white color = iota
+	black
+	grey // must be > white and black
+)
+
+func (c color) String() string {
+	switch c {
+	case white:
+		return "white"
+	case black:
+		return "black"
+	default:
+		return "grey"
+	}
+}
+
+// colorFor returns the (initial) color for an object depending on
+// whether its type t is known or not.
+func colorFor(t Type) color {
+	if t != nil {
+		return black
+	}
+	return white
+}
+
+// Parent returns the scope in which the object is declared.
+// The result is nil for methods and struct fields.
+func (obj *object) Parent() *Scope { return obj.parent }
+
+// Pos returns the declaration position of the object's identifier.
+func (obj *object) Pos() token.Pos { return obj.pos }
+
+// Pkg returns the package to which the object belongs.
+// The result is nil for labels and objects in the Universe scope.
+func (obj *object) Pkg() *Package { return obj.pkg }
+
+// Name returns the object's (package-local, unqualified) name.
+func (obj *object) Name() string { return obj.name }
+
+// Type returns the object's type.
+func (obj *object) Type() Type { return obj.typ }
+
+// Exported reports whether the object is exported (starts with a capital letter).
+// It doesn't take into account whether the object is in a local (function) scope
+// or not.
+func (obj *object) Exported() bool { return token.IsExported(obj.name) }
+
+// Id is a wrapper for Id(obj.Pkg(), obj.Name()).
+func (obj *object) Id() string { return Id(obj.pkg, obj.name) }
+
+func (obj *object) String() string      { panic("abstract") }
+func (obj *object) order() uint32       { return obj.order_ }
+func (obj *object) color() color        { return obj.color_ }
+func (obj *object) scopePos() token.Pos { return obj.scopePos_ }
+
+func (obj *object) setParent(parent *Scope)   { obj.parent = parent }
+func (obj *object) setOrder(order uint32)     { assert(order > 0); obj.order_ = order }
+func (obj *object) setColor(color color)      { assert(color != white); obj.color_ = color }
+func (obj *object) setScopePos(pos token.Pos) { obj.scopePos_ = pos }
+
+func (obj *object) sameId(pkg *Package, name string) bool {
+	// spec:
+	// "Two identifiers are different if they are spelled differently,
+	// or if they appear in different packages and are not exported.
+	// Otherwise, they are the same."
+	if name != obj.name {
+		return false
+	}
+	// obj.Name == name
+	if obj.Exported() {
+		return true
+	}
+	// not exported, so packages must be the same (pkg == nil for
+	// fields in Universe scope; this can only happen for types
+	// introduced via Eval)
+	if pkg == nil || obj.pkg == nil {
+		return pkg == obj.pkg
+	}
+	// pkg != nil && obj.pkg != nil
+	return pkg.path == obj.pkg.path
+}
+
+// A PkgName represents an imported Go package.
+// PkgNames don't have a type.
+type PkgName struct {
+	object
+	imported *Package
+	used     bool // set if the package was used
+}
+
+// NewPkgName returns a new PkgName object representing an imported package.
+// The remaining arguments set the attributes found with all Objects.
+func NewPkgName(pos token.Pos, pkg *Package, name string, imported *Package) *PkgName {
+	return &PkgName{object{nil, pos, pkg, name, Typ[Invalid], 0, black, token.NoPos}, imported, false}
+}
+
+// Imported returns the package that was imported.
+// It is distinct from Pkg(), which is the package containing the import statement.
+func (obj *PkgName) Imported() *Package { return obj.imported }
+
+// A Const represents a declared constant.
+type Const struct {
+	object
+	val constant.Value
+}
+
+// NewConst returns a new constant with value val.
+// The remaining arguments set the attributes found with all Objects.
+func NewConst(pos token.Pos, pkg *Package, name string, typ Type, val constant.Value) *Const {
+	return &Const{object{nil, pos, pkg, name, typ, 0, colorFor(typ), token.NoPos}, val}
+}
+
+// Val returns the constant's value.
+func (obj *Const) Val() constant.Value { return obj.val }
+
+func (*Const) isDependency() {} // a constant may be a dependency of an initialization expression
+
+// A TypeName represents a name for a (defined or alias) type.
+type TypeName struct {
+	object
+}
+
+// NewTypeName returns a new type name denoting the given typ.
+// The remaining arguments set the attributes found with all Objects.
+//
+// The typ argument may be a defined (Named) type or an alias type.
+// It may also be nil such that the returned TypeName can be used as
+// argument for NewNamed, which will set the TypeName's type as a side-
+// effect.
+func NewTypeName(pos token.Pos, pkg *Package, name string, typ Type) *TypeName {
+	return &TypeName{object{nil, pos, pkg, name, typ, 0, colorFor(typ), token.NoPos}}
+}
+
+// IsAlias reports whether obj is an alias name for a type.
+func (obj *TypeName) IsAlias() bool {
+	switch t := obj.typ.(type) {
+	case nil:
+		return false
+	case *Basic:
+		// unsafe.Pointer is not an alias.
+		if obj.pkg == Unsafe {
+			return false
+		}
+		// Any user-defined type name for a basic type is an alias for a
+		// basic type (because basic types are pre-declared in the Universe
+		// scope, outside any package scope), and so is any type name with
+		// a different name than the name of the basic type it refers to.
+		// Additionally, we need to look for "byte" and "rune" because they
+		// are aliases but have the same names (for better error messages).
+		return obj.pkg != nil || t.name != obj.name || t == universeByte || t == universeRune
+	case *Named:
+		return obj != t.obj
+	default:
+		return true
+	}
+}
+
+// A Variable represents a declared variable (including function parameters and results, and struct fields).
+type Var struct {
+	object
+	embedded bool // if set, the variable is an embedded struct field, and name is the type name
+	isField  bool // var is struct field
+	used     bool // set if the variable was used
+}
+
+// NewVar returns a new variable.
+// The arguments set the attributes found with all Objects.
+func NewVar(pos token.Pos, pkg *Package, name string, typ Type) *Var {
+	return &Var{object: object{nil, pos, pkg, name, typ, 0, colorFor(typ), token.NoPos}}
+}
+
+// NewParam returns a new variable representing a function parameter.
+func NewParam(pos token.Pos, pkg *Package, name string, typ Type) *Var {
+	return &Var{object: object{nil, pos, pkg, name, typ, 0, colorFor(typ), token.NoPos}, used: true} // parameters are always 'used'
+}
+
+// NewField returns a new variable representing a struct field.
+// For embedded fields, the name is the unqualified type name
+/// under which the field is accessible.
+func NewField(pos token.Pos, pkg *Package, name string, typ Type, embedded bool) *Var {
+	return &Var{object: object{nil, pos, pkg, name, typ, 0, colorFor(typ), token.NoPos}, embedded: embedded, isField: true}
+}
+
+// Anonymous reports whether the variable is an embedded field.
+// Same as Embedded; only present for backward-compatibility.
+func (obj *Var) Anonymous() bool { return obj.embedded }
+
+// Embedded reports whether the variable is an embedded field.
+func (obj *Var) Embedded() bool { return obj.embedded }
+
+// IsField reports whether the variable is a struct field.
+func (obj *Var) IsField() bool { return obj.isField }
+
+func (*Var) isDependency() {} // a variable may be a dependency of an initialization expression
+
+// A Func represents a declared function, concrete method, or abstract
+// (interface) method. Its Type() is always a *Signature.
+// An abstract method may belong to many interfaces due to embedding.
+type Func struct {
+	object
+	hasPtrRecv bool // only valid for methods that don't have a type yet
+}
+
+// NewFunc returns a new function with the given signature, representing
+// the function's type.
+func NewFunc(pos token.Pos, pkg *Package, name string, sig *Signature) *Func {
+	// don't store a nil signature
+	var typ Type
+	if sig != nil {
+		typ = sig
+	}
+	return &Func{object{nil, pos, pkg, name, typ, 0, colorFor(typ), token.NoPos}, false}
+}
+
+// FullName returns the package- or receiver-type-qualified name of
+// function or method obj.
+func (obj *Func) FullName() string {
+	var buf bytes.Buffer
+	writeFuncName(&buf, obj, nil)
+	return buf.String()
+}
+
+// Scope returns the scope of the function's body block.
+func (obj *Func) Scope() *Scope { return obj.typ.(*Signature).scope }
+
+func (*Func) isDependency() {} // a function may be a dependency of an initialization expression
+
+// A Label represents a declared label.
+// Labels don't have a type.
+type Label struct {
+	object
+	used bool // set if the label was used
+}
+
+// NewLabel returns a new label.
+func NewLabel(pos token.Pos, pkg *Package, name string) *Label {
+	return &Label{object{pos: pos, pkg: pkg, name: name, typ: Typ[Invalid], color_: black}, false}
+}
+
+// A Builtin represents a built-in function.
+// Builtins don't have a valid type.
+type Builtin struct {
+	object
+	id builtinId
+}
+
+func newBuiltin(id builtinId) *Builtin {
+	return &Builtin{object{name: predeclaredFuncs[id].name, typ: Typ[Invalid], color_: black}, id}
+}
+
+// Nil represents the predeclared value nil.
+type Nil struct {
+	object
+}
+
+func writeObject(buf *bytes.Buffer, obj Object, qf Qualifier) {
+	var tname *TypeName
+	typ := obj.Type()
+
+	switch obj := obj.(type) {
+	case *PkgName:
+		fmt.Fprintf(buf, "package %s", obj.Name())
+		if path := obj.imported.path; path != "" && path != obj.name {
+			fmt.Fprintf(buf, " (%q)", path)
+		}
+		return
+
+	case *Const:
+		buf.WriteString("const")
+
+	case *TypeName:
+		tname = obj
+		buf.WriteString("type")
+
+	case *Var:
+		if obj.isField {
+			buf.WriteString("field")
+		} else {
+			buf.WriteString("var")
+		}
+
+	case *Func:
+		buf.WriteString("func ")
+		writeFuncName(buf, obj, qf)
+		if typ != nil {
+			WriteSignature(buf, typ.(*Signature), qf)
+		}
+		return
+
+	case *Label:
+		buf.WriteString("label")
+		typ = nil
+
+	case *Builtin:
+		buf.WriteString("builtin")
+		typ = nil
+
+	case *Nil:
+		buf.WriteString("nil")
+		return
+
+	default:
+		panic(fmt.Sprintf("writeObject(%T)", obj))
+	}
+
+	buf.WriteByte(' ')
+
+	// For package-level objects, qualify the name.
+	if obj.Pkg() != nil && obj.Pkg().scope.Lookup(obj.Name()) == obj {
+		writePackage(buf, obj.Pkg(), qf)
+	}
+	buf.WriteString(obj.Name())
+
+	if typ == nil {
+		return
+	}
+
+	if tname != nil {
+		// We have a type object: Don't print anything more for
+		// basic types since there's no more information (names
+		// are the same; see also comment in TypeName.IsAlias).
+		if _, ok := typ.(*Basic); ok {
+			return
+		}
+		if tname.IsAlias() {
+			buf.WriteString(" =")
+		} else {
+			typ = typ.Underlying()
+		}
+	}
+
+	buf.WriteByte(' ')
+	WriteType(buf, typ, qf)
+}
+
+func writePackage(buf *bytes.Buffer, pkg *Package, qf Qualifier) {
+	if pkg == nil {
+		return
+	}
+	var s string
+	if qf != nil {
+		s = qf(pkg)
+	} else {
+		s = pkg.Path()
+	}
+	if s != "" {
+		buf.WriteString(s)
+		buf.WriteByte('.')
+	}
+}
+
+// ObjectString returns the string form of obj.
+// The Qualifier controls the printing of
+// package-level objects, and may be nil.
+func ObjectString(obj Object, qf Qualifier) string {
+	var buf bytes.Buffer
+	writeObject(&buf, obj, qf)
+	return buf.String()
+}
+
+func (obj *PkgName) String() string  { return ObjectString(obj, nil) }
+func (obj *Const) String() string    { return ObjectString(obj, nil) }
+func (obj *TypeName) String() string { return ObjectString(obj, nil) }
+func (obj *Var) String() string      { return ObjectString(obj, nil) }
+func (obj *Func) String() string     { return ObjectString(obj, nil) }
+func (obj *Label) String() string    { return ObjectString(obj, nil) }
+func (obj *Builtin) String() string  { return ObjectString(obj, nil) }
+func (obj *Nil) String() string      { return ObjectString(obj, nil) }
+
+func writeFuncName(buf *bytes.Buffer, f *Func, qf Qualifier) {
+	if f.typ != nil {
+		sig := f.typ.(*Signature)
+		if recv := sig.Recv(); recv != nil {
+			buf.WriteByte('(')
+			if _, ok := recv.Type().(*Interface); ok {
+				// gcimporter creates abstract methods of
+				// named interfaces using the interface type
+				// (not the named type) as the receiver.
+				// Don't print it in full.
+				buf.WriteString("interface")
+			} else {
+				WriteType(buf, recv.Type(), qf)
+			}
+			buf.WriteByte(')')
+			buf.WriteByte('.')
+		} else if f.pkg != nil {
+			writePackage(buf, f.pkg, qf)
+		}
+	}
+	buf.WriteString(f.name)
+}
diff --git a/src/go/types/object_test.go b/src/go/types/object_test.go
new file mode 100644
index 0000000..2b6057b
--- /dev/null
+++ b/src/go/types/object_test.go
@@ -0,0 +1,86 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types
+
+import (
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"testing"
+)
+
+func TestIsAlias(t *testing.T) {
+	check := func(obj *TypeName, want bool) {
+		if got := obj.IsAlias(); got != want {
+			t.Errorf("%v: got IsAlias = %v; want %v", obj, got, want)
+		}
+	}
+
+	// predeclared types
+	check(Unsafe.Scope().Lookup("Pointer").(*TypeName), false)
+	for _, name := range Universe.Names() {
+		if obj, _ := Universe.Lookup(name).(*TypeName); obj != nil {
+			check(obj, name == "byte" || name == "rune")
+		}
+	}
+
+	// various other types
+	pkg := NewPackage("p", "p")
+	t1 := NewTypeName(0, pkg, "t1", nil)
+	n1 := NewNamed(t1, new(Struct), nil)
+	for _, test := range []struct {
+		name  *TypeName
+		alias bool
+	}{
+		{NewTypeName(0, nil, "t0", nil), false},            // no type yet
+		{NewTypeName(0, pkg, "t0", nil), false},            // no type yet
+		{t1, false},                                        // type name refers to named type and vice versa
+		{NewTypeName(0, nil, "t2", &emptyInterface), true}, // type name refers to unnamed type
+		{NewTypeName(0, pkg, "t3", n1), true},              // type name refers to named type with different type name
+		{NewTypeName(0, nil, "t4", Typ[Int32]), true},      // type name refers to basic type with different name
+		{NewTypeName(0, nil, "int32", Typ[Int32]), false},  // type name refers to basic type with same name
+		{NewTypeName(0, pkg, "int32", Typ[Int32]), true},   // type name is declared in user-defined package (outside Universe)
+		{NewTypeName(0, nil, "rune", Typ[Rune]), true},     // type name refers to basic type rune which is an alias already
+	} {
+		check(test.name, test.alias)
+	}
+}
+
+// TestEmbeddedMethod checks that an embedded method is represented by
+// the same Func Object as the original method. See also issue #34421.
+func TestEmbeddedMethod(t *testing.T) {
+	const src = `package p; type I interface { error }`
+
+	// type-check src
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, 0)
+	if err != nil {
+		t.Fatalf("parse failed: %s", err)
+	}
+	var conf Config
+	pkg, err := conf.Check(f.Name.Name, fset, []*ast.File{f}, nil)
+	if err != nil {
+		t.Fatalf("typecheck failed: %s", err)
+	}
+
+	// get original error.Error method
+	eface := Universe.Lookup("error")
+	orig, _, _ := LookupFieldOrMethod(eface.Type(), false, nil, "Error")
+	if orig == nil {
+		t.Fatalf("original error.Error not found")
+	}
+
+	// get embedded error.Error method
+	iface := pkg.Scope().Lookup("I")
+	embed, _, _ := LookupFieldOrMethod(iface.Type(), false, nil, "Error")
+	if embed == nil {
+		t.Fatalf("embedded error.Error not found")
+	}
+
+	// original and embedded Error object should be identical
+	if orig != embed {
+		t.Fatalf("%s (%p) != %s (%p)", orig, orig, embed, embed)
+	}
+}
diff --git a/src/go/types/objset.go b/src/go/types/objset.go
new file mode 100644
index 0000000..55eb74a
--- /dev/null
+++ b/src/go/types/objset.go
@@ -0,0 +1,31 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements objsets.
+//
+// An objset is similar to a Scope but objset elements
+// are identified by their unique id, instead of their
+// object name.
+
+package types
+
+// An objset is a set of objects identified by their unique id.
+// The zero value for objset is a ready-to-use empty objset.
+type objset map[string]Object // initialized lazily
+
+// insert attempts to insert an object obj into objset s.
+// If s already contains an alternative object alt with
+// the same name, insert leaves s unchanged and returns alt.
+// Otherwise it inserts obj and returns nil.
+func (s *objset) insert(obj Object) Object {
+	id := obj.Id()
+	if alt := (*s)[id]; alt != nil {
+		return alt
+	}
+	if *s == nil {
+		*s = make(map[string]Object)
+	}
+	(*s)[id] = obj
+	return nil
+}
diff --git a/src/go/types/operand.go b/src/go/types/operand.go
new file mode 100644
index 0000000..3e1ac31
--- /dev/null
+++ b/src/go/types/operand.go
@@ -0,0 +1,283 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file defines operands and associated operations.
+
+package types
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/constant"
+	"go/token"
+)
+
+// An operandMode specifies the (addressing) mode of an operand.
+type operandMode byte
+
+const (
+	invalid   operandMode = iota // operand is invalid
+	novalue                      // operand represents no value (result of a function call w/o result)
+	builtin                      // operand is a built-in function
+	typexpr                      // operand is a type
+	constant_                    // operand is a constant; the operand's typ is a Basic type
+	variable                     // operand is an addressable variable
+	mapindex                     // operand is a map index expression (acts like a variable on lhs, commaok on rhs of an assignment)
+	value                        // operand is a computed value
+	commaok                      // like value, but operand may be used in a comma,ok expression
+	commaerr                     // like commaok, but second value is error, not boolean
+	cgofunc                      // operand is a cgo function
+)
+
+var operandModeString = [...]string{
+	invalid:   "invalid operand",
+	novalue:   "no value",
+	builtin:   "built-in",
+	typexpr:   "type",
+	constant_: "constant",
+	variable:  "variable",
+	mapindex:  "map index expression",
+	value:     "value",
+	commaok:   "comma, ok expression",
+	commaerr:  "comma, error expression",
+	cgofunc:   "cgo function",
+}
+
+// An operand represents an intermediate value during type checking.
+// Operands have an (addressing) mode, the expression evaluating to
+// the operand, the operand's type, a value for constants, and an id
+// for built-in functions.
+// The zero value of operand is a ready to use invalid operand.
+//
+type operand struct {
+	mode operandMode
+	expr ast.Expr
+	typ  Type
+	val  constant.Value
+	id   builtinId
+}
+
+// Pos returns the position of the expression corresponding to x.
+// If x is invalid the position is token.NoPos.
+//
+func (x *operand) Pos() token.Pos {
+	// x.expr may not be set if x is invalid
+	if x.expr == nil {
+		return token.NoPos
+	}
+	return x.expr.Pos()
+}
+
+// Operand string formats
+// (not all "untyped" cases can appear due to the type system,
+// but they fall out naturally here)
+//
+// mode       format
+//
+// invalid    <expr> (               <mode>                    )
+// novalue    <expr> (               <mode>                    )
+// builtin    <expr> (               <mode>                    )
+// typexpr    <expr> (               <mode>                    )
+//
+// constant   <expr> (<untyped kind> <mode>                    )
+// constant   <expr> (               <mode>       of type <typ>)
+// constant   <expr> (<untyped kind> <mode> <val>              )
+// constant   <expr> (               <mode> <val> of type <typ>)
+//
+// variable   <expr> (<untyped kind> <mode>                    )
+// variable   <expr> (               <mode>       of type <typ>)
+//
+// mapindex   <expr> (<untyped kind> <mode>                    )
+// mapindex   <expr> (               <mode>       of type <typ>)
+//
+// value      <expr> (<untyped kind> <mode>                    )
+// value      <expr> (               <mode>       of type <typ>)
+//
+// commaok    <expr> (<untyped kind> <mode>                    )
+// commaok    <expr> (               <mode>       of type <typ>)
+//
+// commaerr   <expr> (<untyped kind> <mode>                    )
+// commaerr   <expr> (               <mode>       of type <typ>)
+//
+// cgofunc    <expr> (<untyped kind> <mode>                    )
+// cgofunc    <expr> (               <mode>       of type <typ>)
+//
+func operandString(x *operand, qf Qualifier) string {
+	var buf bytes.Buffer
+
+	var expr string
+	if x.expr != nil {
+		expr = ExprString(x.expr)
+	} else {
+		switch x.mode {
+		case builtin:
+			expr = predeclaredFuncs[x.id].name
+		case typexpr:
+			expr = TypeString(x.typ, qf)
+		case constant_:
+			expr = x.val.String()
+		}
+	}
+
+	// <expr> (
+	if expr != "" {
+		buf.WriteString(expr)
+		buf.WriteString(" (")
+	}
+
+	// <untyped kind>
+	hasType := false
+	switch x.mode {
+	case invalid, novalue, builtin, typexpr:
+		// no type
+	default:
+		// should have a type, but be cautious (don't crash during printing)
+		if x.typ != nil {
+			if isUntyped(x.typ) {
+				buf.WriteString(x.typ.(*Basic).name)
+				buf.WriteByte(' ')
+				break
+			}
+			hasType = true
+		}
+	}
+
+	// <mode>
+	buf.WriteString(operandModeString[x.mode])
+
+	// <val>
+	if x.mode == constant_ {
+		if s := x.val.String(); s != expr {
+			buf.WriteByte(' ')
+			buf.WriteString(s)
+		}
+	}
+
+	// <typ>
+	if hasType {
+		if x.typ != Typ[Invalid] {
+			buf.WriteString(" of type ")
+			WriteType(&buf, x.typ, qf)
+		} else {
+			buf.WriteString(" with invalid type")
+		}
+	}
+
+	// )
+	if expr != "" {
+		buf.WriteByte(')')
+	}
+
+	return buf.String()
+}
+
+func (x *operand) String() string {
+	return operandString(x, nil)
+}
+
+// setConst sets x to the untyped constant for literal lit.
+func (x *operand) setConst(tok token.Token, lit string) {
+	var kind BasicKind
+	switch tok {
+	case token.INT:
+		kind = UntypedInt
+	case token.FLOAT:
+		kind = UntypedFloat
+	case token.IMAG:
+		kind = UntypedComplex
+	case token.CHAR:
+		kind = UntypedRune
+	case token.STRING:
+		kind = UntypedString
+	default:
+		unreachable()
+	}
+
+	val := constant.MakeFromLiteral(lit, tok, 0)
+	if val.Kind() == constant.Unknown {
+		x.mode = invalid
+		x.typ = Typ[Invalid]
+		return
+	}
+	x.mode = constant_
+	x.typ = Typ[kind]
+	x.val = val
+}
+
+// isNil reports whether x is the nil value.
+func (x *operand) isNil() bool {
+	return x.mode == value && x.typ == Typ[UntypedNil]
+}
+
+// assignableTo reports whether x is assignable to a variable of type T. If the
+// result is false and a non-nil reason is provided, it may be set to a more
+// detailed explanation of the failure (result != ""). The check parameter may
+// be nil if assignableTo is invoked through an exported API call, i.e., when
+// all methods have been type-checked.
+func (x *operand) assignableTo(check *Checker, T Type, reason *string) (bool, errorCode) {
+	if x.mode == invalid || T == Typ[Invalid] {
+		return true, 0 // avoid spurious errors
+	}
+
+	V := x.typ
+
+	// x's type is identical to T
+	if check.identical(V, T) {
+		return true, 0
+	}
+
+	Vu := V.Underlying()
+	Tu := T.Underlying()
+
+	// x is an untyped value representable by a value of type T.
+	if isUntyped(Vu) {
+		if t, ok := Tu.(*Basic); ok && x.mode == constant_ {
+			return representableConst(x.val, check, t, nil), _IncompatibleAssign
+		}
+		return check.implicitType(x, Tu) != nil, _IncompatibleAssign
+	}
+	// Vu is typed
+
+	// x's type V and T have identical underlying types and at least one of V or
+	// T is not a named type.
+	if check.identical(Vu, Tu) && (!isNamed(V) || !isNamed(T)) {
+		return true, 0
+	}
+
+	// T is an interface type and x implements T
+	if Ti, ok := Tu.(*Interface); ok {
+		if m, wrongType := check.missingMethod(V, Ti, true); m != nil /* Implements(V, Ti) */ {
+			if reason != nil {
+				if wrongType != nil {
+					if check.identical(m.typ, wrongType.typ) {
+						*reason = fmt.Sprintf("missing method %s (%s has pointer receiver)", m.name, m.name)
+					} else {
+						*reason = fmt.Sprintf("wrong type for method %s (have %s, want %s)", m.Name(), wrongType.typ, m.typ)
+					}
+
+				} else {
+					*reason = "missing method " + m.Name()
+				}
+			}
+			return false, _InvalidIfaceAssign
+		}
+		return true, 0
+	}
+
+	// x is a bidirectional channel value, T is a channel
+	// type, x's type V and T have identical element types,
+	// and at least one of V or T is not a named type
+	if Vc, ok := Vu.(*Chan); ok && Vc.dir == SendRecv {
+		if Tc, ok := Tu.(*Chan); ok && check.identical(Vc.elem, Tc.elem) {
+			if !isNamed(V) || !isNamed(T) {
+				return true, 0
+			} else {
+				return false, _InvalidChanAssign
+			}
+		}
+	}
+
+	return false, _IncompatibleAssign
+}
diff --git a/src/go/types/package.go b/src/go/types/package.go
new file mode 100644
index 0000000..7b89def
--- /dev/null
+++ b/src/go/types/package.go
@@ -0,0 +1,65 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types
+
+import (
+	"fmt"
+	"go/token"
+)
+
+// A Package describes a Go package.
+type Package struct {
+	path     string
+	name     string
+	scope    *Scope
+	complete bool
+	imports  []*Package
+	fake     bool // scope lookup errors are silently dropped if package is fake (internal use only)
+	cgo      bool // uses of this package will be rewritten into uses of declarations from _cgo_gotypes.go
+}
+
+// NewPackage returns a new Package for the given package path and name.
+// The package is not complete and contains no explicit imports.
+func NewPackage(path, name string) *Package {
+	scope := NewScope(Universe, token.NoPos, token.NoPos, fmt.Sprintf("package %q", path))
+	return &Package{path: path, name: name, scope: scope}
+}
+
+// Path returns the package path.
+func (pkg *Package) Path() string { return pkg.path }
+
+// Name returns the package name.
+func (pkg *Package) Name() string { return pkg.name }
+
+// SetName sets the package name.
+func (pkg *Package) SetName(name string) { pkg.name = name }
+
+// Scope returns the (complete or incomplete) package scope
+// holding the objects declared at package level (TypeNames,
+// Consts, Vars, and Funcs).
+func (pkg *Package) Scope() *Scope { return pkg.scope }
+
+// A package is complete if its scope contains (at least) all
+// exported objects; otherwise it is incomplete.
+func (pkg *Package) Complete() bool { return pkg.complete }
+
+// MarkComplete marks a package as complete.
+func (pkg *Package) MarkComplete() { pkg.complete = true }
+
+// Imports returns the list of packages directly imported by
+// pkg; the list is in source order.
+//
+// If pkg was loaded from export data, Imports includes packages that
+// provide package-level objects referenced by pkg. This may be more or
+// less than the set of packages directly imported by pkg's source code.
+func (pkg *Package) Imports() []*Package { return pkg.imports }
+
+// SetImports sets the list of explicitly imported packages to list.
+// It is the caller's responsibility to make sure list elements are unique.
+func (pkg *Package) SetImports(list []*Package) { pkg.imports = list }
+
+func (pkg *Package) String() string {
+	return fmt.Sprintf("package %s (%q)", pkg.name, pkg.path)
+}
diff --git a/src/go/types/predicates.go b/src/go/types/predicates.go
new file mode 100644
index 0000000..148edbf
--- /dev/null
+++ b/src/go/types/predicates.go
@@ -0,0 +1,340 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements commonly used type predicates.
+
+package types
+
+import "sort"
+
+func isNamed(typ Type) bool {
+	if _, ok := typ.(*Basic); ok {
+		return ok
+	}
+	_, ok := typ.(*Named)
+	return ok
+}
+
+func isBoolean(typ Type) bool {
+	t, ok := typ.Underlying().(*Basic)
+	return ok && t.info&IsBoolean != 0
+}
+
+func isInteger(typ Type) bool {
+	t, ok := typ.Underlying().(*Basic)
+	return ok && t.info&IsInteger != 0
+}
+
+func isUnsigned(typ Type) bool {
+	t, ok := typ.Underlying().(*Basic)
+	return ok && t.info&IsUnsigned != 0
+}
+
+func isFloat(typ Type) bool {
+	t, ok := typ.Underlying().(*Basic)
+	return ok && t.info&IsFloat != 0
+}
+
+func isComplex(typ Type) bool {
+	t, ok := typ.Underlying().(*Basic)
+	return ok && t.info&IsComplex != 0
+}
+
+func isNumeric(typ Type) bool {
+	t, ok := typ.Underlying().(*Basic)
+	return ok && t.info&IsNumeric != 0
+}
+
+func isString(typ Type) bool {
+	t, ok := typ.Underlying().(*Basic)
+	return ok && t.info&IsString != 0
+}
+
+func isTyped(typ Type) bool {
+	t, ok := typ.Underlying().(*Basic)
+	return !ok || t.info&IsUntyped == 0
+}
+
+func isUntyped(typ Type) bool {
+	t, ok := typ.Underlying().(*Basic)
+	return ok && t.info&IsUntyped != 0
+}
+
+func isOrdered(typ Type) bool {
+	t, ok := typ.Underlying().(*Basic)
+	return ok && t.info&IsOrdered != 0
+}
+
+func isConstType(typ Type) bool {
+	t, ok := typ.Underlying().(*Basic)
+	return ok && t.info&IsConstType != 0
+}
+
+// IsInterface reports whether typ is an interface type.
+func IsInterface(typ Type) bool {
+	_, ok := typ.Underlying().(*Interface)
+	return ok
+}
+
+// Comparable reports whether values of type T are comparable.
+func Comparable(T Type) bool {
+	return comparable(T, nil)
+}
+
+func comparable(T Type, seen map[Type]bool) bool {
+	if seen[T] {
+		return true
+	}
+	if seen == nil {
+		seen = make(map[Type]bool)
+	}
+	seen[T] = true
+
+	switch t := T.Underlying().(type) {
+	case *Basic:
+		// assume invalid types to be comparable
+		// to avoid follow-up errors
+		return t.kind != UntypedNil
+	case *Pointer, *Interface, *Chan:
+		return true
+	case *Struct:
+		for _, f := range t.fields {
+			if !comparable(f.typ, seen) {
+				return false
+			}
+		}
+		return true
+	case *Array:
+		return comparable(t.elem, seen)
+	}
+	return false
+}
+
+// hasNil reports whether a type includes the nil value.
+func hasNil(typ Type) bool {
+	switch t := typ.Underlying().(type) {
+	case *Basic:
+		return t.kind == UnsafePointer
+	case *Slice, *Pointer, *Signature, *Interface, *Map, *Chan:
+		return true
+	}
+	return false
+}
+
+// identical reports whether x and y are identical types.
+// Receivers of Signature types are ignored.
+func (check *Checker) identical(x, y Type) bool {
+	return check.identical0(x, y, true, nil)
+}
+
+// identicalIgnoreTags reports whether x and y are identical types if tags are ignored.
+// Receivers of Signature types are ignored.
+func (check *Checker) identicalIgnoreTags(x, y Type) bool {
+	return check.identical0(x, y, false, nil)
+}
+
+// An ifacePair is a node in a stack of interface type pairs compared for identity.
+type ifacePair struct {
+	x, y *Interface
+	prev *ifacePair
+}
+
+func (p *ifacePair) identical(q *ifacePair) bool {
+	return p.x == q.x && p.y == q.y || p.x == q.y && p.y == q.x
+}
+
+func (check *Checker) identical0(x, y Type, cmpTags bool, p *ifacePair) bool {
+	if x == y {
+		return true
+	}
+
+	switch x := x.(type) {
+	case *Basic:
+		// Basic types are singletons except for the rune and byte
+		// aliases, thus we cannot solely rely on the x == y check
+		// above. See also comment in TypeName.IsAlias.
+		if y, ok := y.(*Basic); ok {
+			return x.kind == y.kind
+		}
+
+	case *Array:
+		// Two array types are identical if they have identical element types
+		// and the same array length.
+		if y, ok := y.(*Array); ok {
+			// If one or both array lengths are unknown (< 0) due to some error,
+			// assume they are the same to avoid spurious follow-on errors.
+			return (x.len < 0 || y.len < 0 || x.len == y.len) && check.identical0(x.elem, y.elem, cmpTags, p)
+		}
+
+	case *Slice:
+		// Two slice types are identical if they have identical element types.
+		if y, ok := y.(*Slice); ok {
+			return check.identical0(x.elem, y.elem, cmpTags, p)
+		}
+
+	case *Struct:
+		// Two struct types are identical if they have the same sequence of fields,
+		// and if corresponding fields have the same names, and identical types,
+		// and identical tags. Two embedded fields are considered to have the same
+		// name. Lower-case field names from different packages are always different.
+		if y, ok := y.(*Struct); ok {
+			if x.NumFields() == y.NumFields() {
+				for i, f := range x.fields {
+					g := y.fields[i]
+					if f.embedded != g.embedded ||
+						cmpTags && x.Tag(i) != y.Tag(i) ||
+						!f.sameId(g.pkg, g.name) ||
+						!check.identical0(f.typ, g.typ, cmpTags, p) {
+						return false
+					}
+				}
+				return true
+			}
+		}
+
+	case *Pointer:
+		// Two pointer types are identical if they have identical base types.
+		if y, ok := y.(*Pointer); ok {
+			return check.identical0(x.base, y.base, cmpTags, p)
+		}
+
+	case *Tuple:
+		// Two tuples types are identical if they have the same number of elements
+		// and corresponding elements have identical types.
+		if y, ok := y.(*Tuple); ok {
+			if x.Len() == y.Len() {
+				if x != nil {
+					for i, v := range x.vars {
+						w := y.vars[i]
+						if !check.identical0(v.typ, w.typ, cmpTags, p) {
+							return false
+						}
+					}
+				}
+				return true
+			}
+		}
+
+	case *Signature:
+		// Two function types are identical if they have the same number of parameters
+		// and result values, corresponding parameter and result types are identical,
+		// and either both functions are variadic or neither is. Parameter and result
+		// names are not required to match.
+		if y, ok := y.(*Signature); ok {
+			return x.variadic == y.variadic &&
+				check.identical0(x.params, y.params, cmpTags, p) &&
+				check.identical0(x.results, y.results, cmpTags, p)
+		}
+
+	case *Interface:
+		// Two interface types are identical if they have the same set of methods with
+		// the same names and identical function types. Lower-case method names from
+		// different packages are always different. The order of the methods is irrelevant.
+		if y, ok := y.(*Interface); ok {
+			// If identical0 is called (indirectly) via an external API entry point
+			// (such as Identical, IdenticalIgnoreTags, etc.), check is nil. But in
+			// that case, interfaces are expected to be complete and lazy completion
+			// here is not needed.
+			if check != nil {
+				check.completeInterface(x)
+				check.completeInterface(y)
+			}
+			a := x.allMethods
+			b := y.allMethods
+			if len(a) == len(b) {
+				// Interface types are the only types where cycles can occur
+				// that are not "terminated" via named types; and such cycles
+				// can only be created via method parameter types that are
+				// anonymous interfaces (directly or indirectly) embedding
+				// the current interface. Example:
+				//
+				//    type T interface {
+				//        m() interface{T}
+				//    }
+				//
+				// If two such (differently named) interfaces are compared,
+				// endless recursion occurs if the cycle is not detected.
+				//
+				// If x and y were compared before, they must be equal
+				// (if they were not, the recursion would have stopped);
+				// search the ifacePair stack for the same pair.
+				//
+				// This is a quadratic algorithm, but in practice these stacks
+				// are extremely short (bounded by the nesting depth of interface
+				// type declarations that recur via parameter types, an extremely
+				// rare occurrence). An alternative implementation might use a
+				// "visited" map, but that is probably less efficient overall.
+				q := &ifacePair{x, y, p}
+				for p != nil {
+					if p.identical(q) {
+						return true // same pair was compared before
+					}
+					p = p.prev
+				}
+				if debug {
+					assert(sort.IsSorted(byUniqueMethodName(a)))
+					assert(sort.IsSorted(byUniqueMethodName(b)))
+				}
+				for i, f := range a {
+					g := b[i]
+					if f.Id() != g.Id() || !check.identical0(f.typ, g.typ, cmpTags, q) {
+						return false
+					}
+				}
+				return true
+			}
+		}
+
+	case *Map:
+		// Two map types are identical if they have identical key and value types.
+		if y, ok := y.(*Map); ok {
+			return check.identical0(x.key, y.key, cmpTags, p) && check.identical0(x.elem, y.elem, cmpTags, p)
+		}
+
+	case *Chan:
+		// Two channel types are identical if they have identical value types
+		// and the same direction.
+		if y, ok := y.(*Chan); ok {
+			return x.dir == y.dir && check.identical0(x.elem, y.elem, cmpTags, p)
+		}
+
+	case *Named:
+		// Two named types are identical if their type names originate
+		// in the same type declaration.
+		if y, ok := y.(*Named); ok {
+			return x.obj == y.obj
+		}
+
+	case nil:
+
+	default:
+		unreachable()
+	}
+
+	return false
+}
+
+// Default returns the default "typed" type for an "untyped" type;
+// it returns the incoming type for all other types. The default type
+// for untyped nil is untyped nil.
+//
+func Default(typ Type) Type {
+	if t, ok := typ.(*Basic); ok {
+		switch t.kind {
+		case UntypedBool:
+			return Typ[Bool]
+		case UntypedInt:
+			return Typ[Int]
+		case UntypedRune:
+			return universeRune // use 'rune' name
+		case UntypedFloat:
+			return Typ[Float64]
+		case UntypedComplex:
+			return Typ[Complex128]
+		case UntypedString:
+			return Typ[String]
+		}
+	}
+	return typ
+}
diff --git a/src/go/types/resolver.go b/src/go/types/resolver.go
new file mode 100644
index 0000000..b637f8b
--- /dev/null
+++ b/src/go/types/resolver.go
@@ -0,0 +1,614 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types
+
+import (
+	"fmt"
+	"go/ast"
+	"go/constant"
+	"go/token"
+	"sort"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+// A declInfo describes a package-level const, type, var, or func declaration.
+type declInfo struct {
+	file      *Scope        // scope of file containing this declaration
+	lhs       []*Var        // lhs of n:1 variable declarations, or nil
+	typ       ast.Expr      // type, or nil
+	init      ast.Expr      // init/orig expression, or nil
+	inherited bool          // if set, the init expression is inherited from a previous constant declaration
+	fdecl     *ast.FuncDecl // func declaration, or nil
+	alias     bool          // type alias declaration
+
+	// The deps field tracks initialization expression dependencies.
+	deps map[Object]bool // lazily initialized
+}
+
+// hasInitializer reports whether the declared object has an initialization
+// expression or function body.
+func (d *declInfo) hasInitializer() bool {
+	return d.init != nil || d.fdecl != nil && d.fdecl.Body != nil
+}
+
+// addDep adds obj to the set of objects d's init expression depends on.
+func (d *declInfo) addDep(obj Object) {
+	m := d.deps
+	if m == nil {
+		m = make(map[Object]bool)
+		d.deps = m
+	}
+	m[obj] = true
+}
+
+// arityMatch checks that the lhs and rhs of a const or var decl
+// have the appropriate number of names and init exprs. For const
+// decls, init is the value spec providing the init exprs; for
+// var decls, init is nil (the init exprs are in s in this case).
+func (check *Checker) arityMatch(s, init *ast.ValueSpec) {
+	l := len(s.Names)
+	r := len(s.Values)
+	if init != nil {
+		r = len(init.Values)
+	}
+
+	const code = _WrongAssignCount
+	switch {
+	case init == nil && r == 0:
+		// var decl w/o init expr
+		if s.Type == nil {
+			check.errorf(s, code, "missing type or init expr")
+		}
+	case l < r:
+		if l < len(s.Values) {
+			// init exprs from s
+			n := s.Values[l]
+			check.errorf(n, code, "extra init expr %s", n)
+			// TODO(gri) avoid declared but not used error here
+		} else {
+			// init exprs "inherited"
+			check.errorf(s, code, "extra init expr at %s", check.fset.Position(init.Pos()))
+			// TODO(gri) avoid declared but not used error here
+		}
+	case l > r && (init != nil || r != 1):
+		n := s.Names[r]
+		check.errorf(n, code, "missing init expr for %s", n)
+	}
+}
+
+func validatedImportPath(path string) (string, error) {
+	s, err := strconv.Unquote(path)
+	if err != nil {
+		return "", err
+	}
+	if s == "" {
+		return "", fmt.Errorf("empty string")
+	}
+	const illegalChars = `!"#$%&'()*,:;<=>?[\]^{|}` + "`\uFFFD"
+	for _, r := range s {
+		if !unicode.IsGraphic(r) || unicode.IsSpace(r) || strings.ContainsRune(illegalChars, r) {
+			return s, fmt.Errorf("invalid character %#U", r)
+		}
+	}
+	return s, nil
+}
+
+// declarePkgObj declares obj in the package scope, records its ident -> obj mapping,
+// and updates check.objMap. The object must not be a function or method.
+func (check *Checker) declarePkgObj(ident *ast.Ident, obj Object, d *declInfo) {
+	assert(ident.Name == obj.Name())
+
+	// spec: "A package-scope or file-scope identifier with name init
+	// may only be declared to be a function with this (func()) signature."
+	if ident.Name == "init" {
+		check.errorf(ident, _InvalidInitDecl, "cannot declare init - must be func")
+		return
+	}
+
+	// spec: "The main package must have package name main and declare
+	// a function main that takes no arguments and returns no value."
+	if ident.Name == "main" && check.pkg.name == "main" {
+		check.errorf(ident, _InvalidMainDecl, "cannot declare main - must be func")
+		return
+	}
+
+	check.declare(check.pkg.scope, ident, obj, token.NoPos)
+	check.objMap[obj] = d
+	obj.setOrder(uint32(len(check.objMap)))
+}
+
+// filename returns a filename suitable for debugging output.
+func (check *Checker) filename(fileNo int) string {
+	file := check.files[fileNo]
+	if pos := file.Pos(); pos.IsValid() {
+		return check.fset.File(pos).Name()
+	}
+	return fmt.Sprintf("file[%d]", fileNo)
+}
+
+func (check *Checker) importPackage(pos token.Pos, path, dir string) *Package {
+	// If we already have a package for the given (path, dir)
+	// pair, use it instead of doing a full import.
+	// Checker.impMap only caches packages that are marked Complete
+	// or fake (dummy packages for failed imports). Incomplete but
+	// non-fake packages do require an import to complete them.
+	key := importKey{path, dir}
+	imp := check.impMap[key]
+	if imp != nil {
+		return imp
+	}
+
+	// no package yet => import it
+	if path == "C" && (check.conf.FakeImportC || check.conf.go115UsesCgo) {
+		imp = NewPackage("C", "C")
+		imp.fake = true // package scope is not populated
+		imp.cgo = check.conf.go115UsesCgo
+	} else {
+		// ordinary import
+		var err error
+		if importer := check.conf.Importer; importer == nil {
+			err = fmt.Errorf("Config.Importer not installed")
+		} else if importerFrom, ok := importer.(ImporterFrom); ok {
+			imp, err = importerFrom.ImportFrom(path, dir, 0)
+			if imp == nil && err == nil {
+				err = fmt.Errorf("Config.Importer.ImportFrom(%s, %s, 0) returned nil but no error", path, dir)
+			}
+		} else {
+			imp, err = importer.Import(path)
+			if imp == nil && err == nil {
+				err = fmt.Errorf("Config.Importer.Import(%s) returned nil but no error", path)
+			}
+		}
+		// make sure we have a valid package name
+		// (errors here can only happen through manipulation of packages after creation)
+		if err == nil && imp != nil && (imp.name == "_" || imp.name == "") {
+			err = fmt.Errorf("invalid package name: %q", imp.name)
+			imp = nil // create fake package below
+		}
+		if err != nil {
+			check.errorf(atPos(pos), _BrokenImport, "could not import %s (%s)", path, err)
+			if imp == nil {
+				// create a new fake package
+				// come up with a sensible package name (heuristic)
+				name := path
+				if i := len(name); i > 0 && name[i-1] == '/' {
+					name = name[:i-1]
+				}
+				if i := strings.LastIndex(name, "/"); i >= 0 {
+					name = name[i+1:]
+				}
+				imp = NewPackage(path, name)
+			}
+			// continue to use the package as best as we can
+			imp.fake = true // avoid follow-up lookup failures
+		}
+	}
+
+	// package should be complete or marked fake, but be cautious
+	if imp.complete || imp.fake {
+		check.impMap[key] = imp
+		check.pkgCnt[imp.name]++
+		return imp
+	}
+
+	// something went wrong (importer may have returned incomplete package without error)
+	return nil
+}
+
+// collectObjects collects all file and package objects and inserts them
+// into their respective scopes. It also performs imports and associates
+// methods with receiver base type names.
+func (check *Checker) collectObjects() {
+	pkg := check.pkg
+
+	// pkgImports is the set of packages already imported by any package file seen
+	// so far. Used to avoid duplicate entries in pkg.imports. Allocate and populate
+	// it (pkg.imports may not be empty if we are checking test files incrementally).
+	// Note that pkgImports is keyed by package (and thus package path), not by an
+	// importKey value. Two different importKey values may map to the same package
+	// which is why we cannot use the check.impMap here.
+	var pkgImports = make(map[*Package]bool)
+	for _, imp := range pkg.imports {
+		pkgImports[imp] = true
+	}
+
+	var methods []*Func // list of methods with non-blank _ names
+	for fileNo, file := range check.files {
+		// The package identifier denotes the current package,
+		// but there is no corresponding package object.
+		check.recordDef(file.Name, nil)
+
+		// Use the actual source file extent rather than *ast.File extent since the
+		// latter doesn't include comments which appear at the start or end of the file.
+		// Be conservative and use the *ast.File extent if we don't have a *token.File.
+		pos, end := file.Pos(), file.End()
+		if f := check.fset.File(file.Pos()); f != nil {
+			pos, end = token.Pos(f.Base()), token.Pos(f.Base()+f.Size())
+		}
+		fileScope := NewScope(check.pkg.scope, pos, end, check.filename(fileNo))
+		check.recordScope(file, fileScope)
+
+		// determine file directory, necessary to resolve imports
+		// FileName may be "" (typically for tests) in which case
+		// we get "." as the directory which is what we would want.
+		fileDir := dir(check.fset.Position(file.Name.Pos()).Filename)
+
+		check.walkDecls(file.Decls, func(d decl) {
+			switch d := d.(type) {
+			case importDecl:
+				// import package
+				path, err := validatedImportPath(d.spec.Path.Value)
+				if err != nil {
+					check.errorf(d.spec.Path, _BadImportPath, "invalid import path (%s)", err)
+					return
+				}
+
+				imp := check.importPackage(d.spec.Path.Pos(), path, fileDir)
+				if imp == nil {
+					return
+				}
+
+				// add package to list of explicit imports
+				// (this functionality is provided as a convenience
+				// for clients; it is not needed for type-checking)
+				if !pkgImports[imp] {
+					pkgImports[imp] = true
+					pkg.imports = append(pkg.imports, imp)
+				}
+
+				// local name overrides imported package name
+				name := imp.name
+				if d.spec.Name != nil {
+					name = d.spec.Name.Name
+					if path == "C" {
+						// match cmd/compile (not prescribed by spec)
+						check.errorf(d.spec.Name, _ImportCRenamed, `cannot rename import "C"`)
+						return
+					}
+					if name == "init" {
+						check.errorf(d.spec.Name, _InvalidInitDecl, "cannot declare init - must be func")
+						return
+					}
+				}
+
+				obj := NewPkgName(d.spec.Pos(), pkg, name, imp)
+				if d.spec.Name != nil {
+					// in a dot-import, the dot represents the package
+					check.recordDef(d.spec.Name, obj)
+				} else {
+					check.recordImplicit(d.spec, obj)
+				}
+
+				if path == "C" {
+					// match cmd/compile (not prescribed by spec)
+					obj.used = true
+				}
+
+				// add import to file scope
+				if name == "." {
+					// merge imported scope with file scope
+					for _, obj := range imp.scope.elems {
+						// A package scope may contain non-exported objects,
+						// do not import them!
+						if obj.Exported() {
+							// declare dot-imported object
+							// (Do not use check.declare because it modifies the object
+							// via Object.setScopePos, which leads to a race condition;
+							// the object may be imported into more than one file scope
+							// concurrently. See issue #32154.)
+							if alt := fileScope.Insert(obj); alt != nil {
+								check.errorf(d.spec.Name, _DuplicateDecl, "%s redeclared in this block", obj.Name())
+								check.reportAltDecl(alt)
+							}
+						}
+					}
+					// add position to set of dot-import positions for this file
+					// (this is only needed for "imported but not used" errors)
+					check.addUnusedDotImport(fileScope, imp, d.spec)
+				} else {
+					// declare imported package object in file scope
+					// (no need to provide s.Name since we called check.recordDef earlier)
+					check.declare(fileScope, nil, obj, token.NoPos)
+				}
+			case constDecl:
+				// declare all constants
+				for i, name := range d.spec.Names {
+					obj := NewConst(name.Pos(), pkg, name.Name, nil, constant.MakeInt64(int64(d.iota)))
+
+					var init ast.Expr
+					if i < len(d.init) {
+						init = d.init[i]
+					}
+
+					d := &declInfo{file: fileScope, typ: d.typ, init: init, inherited: d.inherited}
+					check.declarePkgObj(name, obj, d)
+				}
+
+			case varDecl:
+				lhs := make([]*Var, len(d.spec.Names))
+				// If there's exactly one rhs initializer, use
+				// the same declInfo d1 for all lhs variables
+				// so that each lhs variable depends on the same
+				// rhs initializer (n:1 var declaration).
+				var d1 *declInfo
+				if len(d.spec.Values) == 1 {
+					// The lhs elements are only set up after the for loop below,
+					// but that's ok because declareVar only collects the declInfo
+					// for a later phase.
+					d1 = &declInfo{file: fileScope, lhs: lhs, typ: d.spec.Type, init: d.spec.Values[0]}
+				}
+
+				// declare all variables
+				for i, name := range d.spec.Names {
+					obj := NewVar(name.Pos(), pkg, name.Name, nil)
+					lhs[i] = obj
+
+					di := d1
+					if di == nil {
+						// individual assignments
+						var init ast.Expr
+						if i < len(d.spec.Values) {
+							init = d.spec.Values[i]
+						}
+						di = &declInfo{file: fileScope, typ: d.spec.Type, init: init}
+					}
+
+					check.declarePkgObj(name, obj, di)
+				}
+			case typeDecl:
+				obj := NewTypeName(d.spec.Name.Pos(), pkg, d.spec.Name.Name, nil)
+				check.declarePkgObj(d.spec.Name, obj, &declInfo{file: fileScope, typ: d.spec.Type, alias: d.spec.Assign.IsValid()})
+			case funcDecl:
+				info := &declInfo{file: fileScope, fdecl: d.decl}
+				name := d.decl.Name.Name
+				obj := NewFunc(d.decl.Name.Pos(), pkg, name, nil)
+				if d.decl.Recv == nil {
+					// regular function
+					if name == "init" {
+						// don't declare init functions in the package scope - they are invisible
+						obj.parent = pkg.scope
+						check.recordDef(d.decl.Name, obj)
+						// init functions must have a body
+						if d.decl.Body == nil {
+							check.softErrorf(obj, _MissingInitBody, "missing function body")
+						}
+					} else {
+						check.declare(pkg.scope, d.decl.Name, obj, token.NoPos)
+					}
+				} else {
+					// method
+					// (Methods with blank _ names are never found; no need to collect
+					// them for later type association. They will still be type-checked
+					// with all the other functions.)
+					if name != "_" {
+						methods = append(methods, obj)
+					}
+					check.recordDef(d.decl.Name, obj)
+				}
+				// Methods are not package-level objects but we still track them in the
+				// object map so that we can handle them like regular functions (if the
+				// receiver is invalid); also we need their fdecl info when associating
+				// them with their receiver base type, below.
+				check.objMap[obj] = info
+				obj.setOrder(uint32(len(check.objMap)))
+			}
+		})
+	}
+
+	// verify that objects in package and file scopes have different names
+	for _, scope := range check.pkg.scope.children /* file scopes */ {
+		for _, obj := range scope.elems {
+			if alt := pkg.scope.Lookup(obj.Name()); alt != nil {
+				if pkg, ok := obj.(*PkgName); ok {
+					check.errorf(alt, _DuplicateDecl, "%s already declared through import of %s", alt.Name(), pkg.Imported())
+					check.reportAltDecl(pkg)
+				} else {
+					check.errorf(alt, _DuplicateDecl, "%s already declared through dot-import of %s", alt.Name(), obj.Pkg())
+					// TODO(gri) dot-imported objects don't have a position; reportAltDecl won't print anything
+					check.reportAltDecl(obj)
+				}
+			}
+		}
+	}
+
+	// Now that we have all package scope objects and all methods,
+	// associate methods with receiver base type name where possible.
+	// Ignore methods that have an invalid receiver. They will be
+	// type-checked later, with regular functions.
+	if methods == nil {
+		return // nothing to do
+	}
+	check.methods = make(map[*TypeName][]*Func)
+	for _, f := range methods {
+		fdecl := check.objMap[f].fdecl
+		if list := fdecl.Recv.List; len(list) > 0 {
+			// f is a method.
+			// Determine the receiver base type and associate f with it.
+			ptr, base := check.resolveBaseTypeName(list[0].Type)
+			if base != nil {
+				f.hasPtrRecv = ptr
+				check.methods[base] = append(check.methods[base], f)
+			}
+		}
+	}
+}
+
+// resolveBaseTypeName returns the non-alias base type name for typ, and whether
+// there was a pointer indirection to get to it. The base type name must be declared
+// in package scope, and there can be at most one pointer indirection. If no such type
+// name exists, the returned base is nil.
+func (check *Checker) resolveBaseTypeName(typ ast.Expr) (ptr bool, base *TypeName) {
+	// Algorithm: Starting from a type expression, which may be a name,
+	// we follow that type through alias declarations until we reach a
+	// non-alias type name. If we encounter anything but pointer types or
+	// parentheses we're done. If we encounter more than one pointer type
+	// we're done.
+	var seen map[*TypeName]bool
+	for {
+		typ = unparen(typ)
+
+		// check if we have a pointer type
+		if pexpr, _ := typ.(*ast.StarExpr); pexpr != nil {
+			// if we've already seen a pointer, we're done
+			if ptr {
+				return false, nil
+			}
+			ptr = true
+			typ = unparen(pexpr.X) // continue with pointer base type
+		}
+
+		// typ must be a name
+		name, _ := typ.(*ast.Ident)
+		if name == nil {
+			return false, nil
+		}
+
+		// name must denote an object found in the current package scope
+		// (note that dot-imported objects are not in the package scope!)
+		obj := check.pkg.scope.Lookup(name.Name)
+		if obj == nil {
+			return false, nil
+		}
+
+		// the object must be a type name...
+		tname, _ := obj.(*TypeName)
+		if tname == nil {
+			return false, nil
+		}
+
+		// ... which we have not seen before
+		if seen[tname] {
+			return false, nil
+		}
+
+		// we're done if tdecl defined tname as a new type
+		// (rather than an alias)
+		tdecl := check.objMap[tname] // must exist for objects in package scope
+		if !tdecl.alias {
+			return ptr, tname
+		}
+
+		// otherwise, continue resolving
+		typ = tdecl.typ
+		if seen == nil {
+			seen = make(map[*TypeName]bool)
+		}
+		seen[tname] = true
+	}
+}
+
+// packageObjects typechecks all package objects, but not function bodies.
+func (check *Checker) packageObjects() {
+	// process package objects in source order for reproducible results
+	objList := make([]Object, len(check.objMap))
+	i := 0
+	for obj := range check.objMap {
+		objList[i] = obj
+		i++
+	}
+	sort.Sort(inSourceOrder(objList))
+
+	// add new methods to already type-checked types (from a prior Checker.Files call)
+	for _, obj := range objList {
+		if obj, _ := obj.(*TypeName); obj != nil && obj.typ != nil {
+			check.addMethodDecls(obj)
+		}
+	}
+
+	// We process non-alias declarations first, in order to avoid situations where
+	// the type of an alias declaration is needed before it is available. In general
+	// this is still not enough, as it is possible to create sufficiently convoluted
+	// recursive type definitions that will cause a type alias to be needed before it
+	// is available (see issue #25838 for examples).
+	// As an aside, the cmd/compiler suffers from the same problem (#25838).
+	var aliasList []*TypeName
+	// phase 1
+	for _, obj := range objList {
+		// If we have a type alias, collect it for the 2nd phase.
+		if tname, _ := obj.(*TypeName); tname != nil && check.objMap[tname].alias {
+			aliasList = append(aliasList, tname)
+			continue
+		}
+
+		check.objDecl(obj, nil)
+	}
+	// phase 2
+	for _, obj := range aliasList {
+		check.objDecl(obj, nil)
+	}
+
+	// At this point we may have a non-empty check.methods map; this means that not all
+	// entries were deleted at the end of typeDecl because the respective receiver base
+	// types were not found. In that case, an error was reported when declaring those
+	// methods. We can now safely discard this map.
+	check.methods = nil
+}
+
+// inSourceOrder implements the sort.Sort interface.
+type inSourceOrder []Object
+
+func (a inSourceOrder) Len() int           { return len(a) }
+func (a inSourceOrder) Less(i, j int) bool { return a[i].order() < a[j].order() }
+func (a inSourceOrder) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+
+// unusedImports checks for unused imports.
+func (check *Checker) unusedImports() {
+	// if function bodies are not checked, packages' uses are likely missing - don't check
+	if check.conf.IgnoreFuncBodies {
+		return
+	}
+
+	// spec: "It is illegal (...) to directly import a package without referring to
+	// any of its exported identifiers. To import a package solely for its side-effects
+	// (initialization), use the blank identifier as explicit package name."
+
+	// check use of regular imported packages
+	for _, scope := range check.pkg.scope.children /* file scopes */ {
+		for _, obj := range scope.elems {
+			if obj, ok := obj.(*PkgName); ok {
+				// Unused "blank imports" are automatically ignored
+				// since _ identifiers are not entered into scopes.
+				if !obj.used {
+					path := obj.imported.path
+					base := pkgName(path)
+					if obj.name == base {
+						check.softErrorf(obj, _UnusedImport, "%q imported but not used", path)
+					} else {
+						check.softErrorf(obj, _UnusedImport, "%q imported but not used as %s", path, obj.name)
+					}
+				}
+			}
+		}
+	}
+
+	// check use of dot-imported packages
+	for _, unusedDotImports := range check.unusedDotImports {
+		for pkg, pos := range unusedDotImports {
+			check.softErrorf(pos, _UnusedImport, "%q imported but not used", pkg.path)
+		}
+	}
+}
+
+// pkgName returns the package name (last element) of an import path.
+func pkgName(path string) string {
+	if i := strings.LastIndex(path, "/"); i >= 0 {
+		path = path[i+1:]
+	}
+	return path
+}
+
+// dir makes a good-faith attempt to return the directory
+// portion of path. If path is empty, the result is ".".
+// (Per the go/build package dependency tests, we cannot import
+// path/filepath and simply use filepath.Dir.)
+func dir(path string) string {
+	if i := strings.LastIndexAny(path, `/\`); i > 0 {
+		return path[:i]
+	}
+	// i <= 0
+	return "."
+}
diff --git a/src/go/types/resolver_test.go b/src/go/types/resolver_test.go
new file mode 100644
index 0000000..4bb63b6
--- /dev/null
+++ b/src/go/types/resolver_test.go
@@ -0,0 +1,216 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types_test
+
+import (
+	"fmt"
+	"go/ast"
+	"go/importer"
+	"go/parser"
+	"go/token"
+	"internal/testenv"
+	"sort"
+	"testing"
+
+	. "go/types"
+)
+
+type resolveTestImporter struct {
+	importer ImporterFrom
+	imported map[string]bool
+}
+
+func (imp *resolveTestImporter) Import(string) (*Package, error) {
+	panic("should not be called")
+}
+
+func (imp *resolveTestImporter) ImportFrom(path, srcDir string, mode ImportMode) (*Package, error) {
+	if mode != 0 {
+		panic("mode must be 0")
+	}
+	if imp.importer == nil {
+		imp.importer = importer.Default().(ImporterFrom)
+		imp.imported = make(map[string]bool)
+	}
+	pkg, err := imp.importer.ImportFrom(path, srcDir, mode)
+	if err != nil {
+		return nil, err
+	}
+	imp.imported[path] = true
+	return pkg, nil
+}
+
+func TestResolveIdents(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	sources := []string{
+		`
+		package p
+		import "fmt"
+		import "math"
+		const pi = math.Pi
+		func sin(x float64) float64 {
+			return math.Sin(x)
+		}
+		var Println = fmt.Println
+		`,
+		`
+		package p
+		import "fmt"
+		type errorStringer struct { fmt.Stringer; error }
+		func f() string {
+			_ = "foo"
+			return fmt.Sprintf("%d", g())
+		}
+		func g() (x int) { return }
+		`,
+		`
+		package p
+		import . "go/parser"
+		import "sync"
+		func h() Mode { return ImportsOnly }
+		var _, x int = 1, 2
+		func init() {}
+		type T struct{ *sync.Mutex; a, b, c int}
+		type I interface{ m() }
+		var _ = T{a: 1, b: 2, c: 3}
+		func (_ T) m() {}
+		func (T) _() {}
+		var i I
+		var _ = i.m
+		func _(s []int) { for i, x := range s { _, _ = i, x } }
+		func _(x interface{}) {
+			switch x := x.(type) {
+			case int:
+				_ = x
+			}
+			switch {} // implicit 'true' tag
+		}
+		`,
+		`
+		package p
+		type S struct{}
+		func (T) _() {}
+		func (T) _() {}
+		`,
+		`
+		package p
+		func _() {
+		L0:
+		L1:
+			goto L0
+			for {
+				goto L1
+			}
+			if true {
+				goto L2
+			}
+		L2:
+		}
+		`,
+	}
+
+	pkgnames := []string{
+		"fmt",
+		"math",
+	}
+
+	// parse package files
+	fset := token.NewFileSet()
+	var files []*ast.File
+	for i, src := range sources {
+		f, err := parser.ParseFile(fset, fmt.Sprintf("sources[%d]", i), src, parser.DeclarationErrors)
+		if err != nil {
+			t.Fatal(err)
+		}
+		files = append(files, f)
+	}
+
+	// resolve and type-check package AST
+	importer := new(resolveTestImporter)
+	conf := Config{Importer: importer}
+	uses := make(map[*ast.Ident]Object)
+	defs := make(map[*ast.Ident]Object)
+	_, err := conf.Check("testResolveIdents", fset, files, &Info{Defs: defs, Uses: uses})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// check that all packages were imported
+	for _, name := range pkgnames {
+		if !importer.imported[name] {
+			t.Errorf("package %s not imported", name)
+		}
+	}
+
+	// check that qualified identifiers are resolved
+	for _, f := range files {
+		ast.Inspect(f, func(n ast.Node) bool {
+			if s, ok := n.(*ast.SelectorExpr); ok {
+				if x, ok := s.X.(*ast.Ident); ok {
+					obj := uses[x]
+					if obj == nil {
+						t.Errorf("%s: unresolved qualified identifier %s", fset.Position(x.Pos()), x.Name)
+						return false
+					}
+					if _, ok := obj.(*PkgName); ok && uses[s.Sel] == nil {
+						t.Errorf("%s: unresolved selector %s", fset.Position(s.Sel.Pos()), s.Sel.Name)
+						return false
+					}
+					return false
+				}
+				return false
+			}
+			return true
+		})
+	}
+
+	for id, obj := range uses {
+		if obj == nil {
+			t.Errorf("%s: Uses[%s] == nil", fset.Position(id.Pos()), id.Name)
+		}
+	}
+
+	// check that each identifier in the source is found in uses or defs or both
+	var both []string
+	for _, f := range files {
+		ast.Inspect(f, func(n ast.Node) bool {
+			if x, ok := n.(*ast.Ident); ok {
+				var objects int
+				if _, found := uses[x]; found {
+					objects |= 1
+					delete(uses, x)
+				}
+				if _, found := defs[x]; found {
+					objects |= 2
+					delete(defs, x)
+				}
+				if objects == 0 {
+					t.Errorf("%s: unresolved identifier %s", fset.Position(x.Pos()), x.Name)
+				} else if objects == 3 {
+					both = append(both, x.Name)
+				}
+				return false
+			}
+			return true
+		})
+	}
+
+	// check the expected set of idents that are simultaneously uses and defs
+	sort.Strings(both)
+	if got, want := fmt.Sprint(both), "[Mutex Stringer error]"; got != want {
+		t.Errorf("simultaneous uses/defs = %s, want %s", got, want)
+	}
+
+	// any left-over identifiers didn't exist in the source
+	for x := range uses {
+		t.Errorf("%s: identifier %s not present in source", fset.Position(x.Pos()), x.Name)
+	}
+	for x := range defs {
+		t.Errorf("%s: identifier %s not present in source", fset.Position(x.Pos()), x.Name)
+	}
+
+	// TODO(gri) add tests to check ImplicitObj callbacks
+}
diff --git a/src/go/types/return.go b/src/go/types/return.go
new file mode 100644
index 0000000..2d34a70
--- /dev/null
+++ b/src/go/types/return.go
@@ -0,0 +1,184 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements isTerminating.
+
+package types
+
+import (
+	"go/ast"
+	"go/token"
+)
+
+// isTerminating reports if s is a terminating statement.
+// If s is labeled, label is the label name; otherwise s
+// is "".
+func (check *Checker) isTerminating(s ast.Stmt, label string) bool {
+	switch s := s.(type) {
+	default:
+		unreachable()
+
+	case *ast.BadStmt, *ast.DeclStmt, *ast.EmptyStmt, *ast.SendStmt,
+		*ast.IncDecStmt, *ast.AssignStmt, *ast.GoStmt, *ast.DeferStmt,
+		*ast.RangeStmt:
+		// no chance
+
+	case *ast.LabeledStmt:
+		return check.isTerminating(s.Stmt, s.Label.Name)
+
+	case *ast.ExprStmt:
+		// calling the predeclared (possibly parenthesized) panic() function is terminating
+		if call, ok := unparen(s.X).(*ast.CallExpr); ok && check.isPanic[call] {
+			return true
+		}
+
+	case *ast.ReturnStmt:
+		return true
+
+	case *ast.BranchStmt:
+		if s.Tok == token.GOTO || s.Tok == token.FALLTHROUGH {
+			return true
+		}
+
+	case *ast.BlockStmt:
+		return check.isTerminatingList(s.List, "")
+
+	case *ast.IfStmt:
+		if s.Else != nil &&
+			check.isTerminating(s.Body, "") &&
+			check.isTerminating(s.Else, "") {
+			return true
+		}
+
+	case *ast.SwitchStmt:
+		return check.isTerminatingSwitch(s.Body, label)
+
+	case *ast.TypeSwitchStmt:
+		return check.isTerminatingSwitch(s.Body, label)
+
+	case *ast.SelectStmt:
+		for _, s := range s.Body.List {
+			cc := s.(*ast.CommClause)
+			if !check.isTerminatingList(cc.Body, "") || hasBreakList(cc.Body, label, true) {
+				return false
+			}
+
+		}
+		return true
+
+	case *ast.ForStmt:
+		if s.Cond == nil && !hasBreak(s.Body, label, true) {
+			return true
+		}
+	}
+
+	return false
+}
+
+func (check *Checker) isTerminatingList(list []ast.Stmt, label string) bool {
+	// trailing empty statements are permitted - skip them
+	for i := len(list) - 1; i >= 0; i-- {
+		if _, ok := list[i].(*ast.EmptyStmt); !ok {
+			return check.isTerminating(list[i], label)
+		}
+	}
+	return false // all statements are empty
+}
+
+func (check *Checker) isTerminatingSwitch(body *ast.BlockStmt, label string) bool {
+	hasDefault := false
+	for _, s := range body.List {
+		cc := s.(*ast.CaseClause)
+		if cc.List == nil {
+			hasDefault = true
+		}
+		if !check.isTerminatingList(cc.Body, "") || hasBreakList(cc.Body, label, true) {
+			return false
+		}
+	}
+	return hasDefault
+}
+
+// TODO(gri) For nested breakable statements, the current implementation of hasBreak
+//	     will traverse the same subtree repeatedly, once for each label. Replace
+//           with a single-pass label/break matching phase.
+
+// hasBreak reports if s is or contains a break statement
+// referring to the label-ed statement or implicit-ly the
+// closest outer breakable statement.
+func hasBreak(s ast.Stmt, label string, implicit bool) bool {
+	switch s := s.(type) {
+	default:
+		unreachable()
+
+	case *ast.BadStmt, *ast.DeclStmt, *ast.EmptyStmt, *ast.ExprStmt,
+		*ast.SendStmt, *ast.IncDecStmt, *ast.AssignStmt, *ast.GoStmt,
+		*ast.DeferStmt, *ast.ReturnStmt:
+		// no chance
+
+	case *ast.LabeledStmt:
+		return hasBreak(s.Stmt, label, implicit)
+
+	case *ast.BranchStmt:
+		if s.Tok == token.BREAK {
+			if s.Label == nil {
+				return implicit
+			}
+			if s.Label.Name == label {
+				return true
+			}
+		}
+
+	case *ast.BlockStmt:
+		return hasBreakList(s.List, label, implicit)
+
+	case *ast.IfStmt:
+		if hasBreak(s.Body, label, implicit) ||
+			s.Else != nil && hasBreak(s.Else, label, implicit) {
+			return true
+		}
+
+	case *ast.CaseClause:
+		return hasBreakList(s.Body, label, implicit)
+
+	case *ast.SwitchStmt:
+		if label != "" && hasBreak(s.Body, label, false) {
+			return true
+		}
+
+	case *ast.TypeSwitchStmt:
+		if label != "" && hasBreak(s.Body, label, false) {
+			return true
+		}
+
+	case *ast.CommClause:
+		return hasBreakList(s.Body, label, implicit)
+
+	case *ast.SelectStmt:
+		if label != "" && hasBreak(s.Body, label, false) {
+			return true
+		}
+
+	case *ast.ForStmt:
+		if label != "" && hasBreak(s.Body, label, false) {
+			return true
+		}
+
+	case *ast.RangeStmt:
+		if label != "" && hasBreak(s.Body, label, false) {
+			return true
+		}
+	}
+
+	return false
+}
+
+func hasBreakList(list []ast.Stmt, label string, implicit bool) bool {
+	for _, s := range list {
+		if hasBreak(s, label, implicit) {
+			return true
+		}
+	}
+	return false
+}
diff --git a/src/go/types/scope.go b/src/go/types/scope.go
new file mode 100644
index 0000000..8c9d9ab
--- /dev/null
+++ b/src/go/types/scope.go
@@ -0,0 +1,182 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements Scopes.
+
+package types
+
+import (
+	"bytes"
+	"fmt"
+	"go/token"
+	"io"
+	"sort"
+	"strings"
+)
+
+// A Scope maintains a set of objects and links to its containing
+// (parent) and contained (children) scopes. Objects may be inserted
+// and looked up by name. The zero value for Scope is a ready-to-use
+// empty scope.
+type Scope struct {
+	parent   *Scope
+	children []*Scope
+	elems    map[string]Object // lazily allocated
+	pos, end token.Pos         // scope extent; may be invalid
+	comment  string            // for debugging only
+	isFunc   bool              // set if this is a function scope (internal use only)
+}
+
+// NewScope returns a new, empty scope contained in the given parent
+// scope, if any. The comment is for debugging only.
+func NewScope(parent *Scope, pos, end token.Pos, comment string) *Scope {
+	s := &Scope{parent, nil, nil, pos, end, comment, false}
+	// don't add children to Universe scope!
+	if parent != nil && parent != Universe {
+		parent.children = append(parent.children, s)
+	}
+	return s
+}
+
+// Parent returns the scope's containing (parent) scope.
+func (s *Scope) Parent() *Scope { return s.parent }
+
+// Len returns the number of scope elements.
+func (s *Scope) Len() int { return len(s.elems) }
+
+// Names returns the scope's element names in sorted order.
+func (s *Scope) Names() []string {
+	names := make([]string, len(s.elems))
+	i := 0
+	for name := range s.elems {
+		names[i] = name
+		i++
+	}
+	sort.Strings(names)
+	return names
+}
+
+// NumChildren returns the number of scopes nested in s.
+func (s *Scope) NumChildren() int { return len(s.children) }
+
+// Child returns the i'th child scope for 0 <= i < NumChildren().
+func (s *Scope) Child(i int) *Scope { return s.children[i] }
+
+// Lookup returns the object in scope s with the given name if such an
+// object exists; otherwise the result is nil.
+func (s *Scope) Lookup(name string) Object {
+	return s.elems[name]
+}
+
+// LookupParent follows the parent chain of scopes starting with s until
+// it finds a scope where Lookup(name) returns a non-nil object, and then
+// returns that scope and object. If a valid position pos is provided,
+// only objects that were declared at or before pos are considered.
+// If no such scope and object exists, the result is (nil, nil).
+//
+// Note that obj.Parent() may be different from the returned scope if the
+// object was inserted into the scope and already had a parent at that
+// time (see Insert). This can only happen for dot-imported objects
+// whose scope is the scope of the package that exported them.
+func (s *Scope) LookupParent(name string, pos token.Pos) (*Scope, Object) {
+	for ; s != nil; s = s.parent {
+		if obj := s.elems[name]; obj != nil && (!pos.IsValid() || obj.scopePos() <= pos) {
+			return s, obj
+		}
+	}
+	return nil, nil
+}
+
+// Insert attempts to insert an object obj into scope s.
+// If s already contains an alternative object alt with
+// the same name, Insert leaves s unchanged and returns alt.
+// Otherwise it inserts obj, sets the object's parent scope
+// if not already set, and returns nil.
+func (s *Scope) Insert(obj Object) Object {
+	name := obj.Name()
+	if alt := s.elems[name]; alt != nil {
+		return alt
+	}
+	if s.elems == nil {
+		s.elems = make(map[string]Object)
+	}
+	s.elems[name] = obj
+	if obj.Parent() == nil {
+		obj.setParent(s)
+	}
+	return nil
+}
+
+// Pos and End describe the scope's source code extent [pos, end).
+// The results are guaranteed to be valid only if the type-checked
+// AST has complete position information. The extent is undefined
+// for Universe and package scopes.
+func (s *Scope) Pos() token.Pos { return s.pos }
+func (s *Scope) End() token.Pos { return s.end }
+
+// Contains reports whether pos is within the scope's extent.
+// The result is guaranteed to be valid only if the type-checked
+// AST has complete position information.
+func (s *Scope) Contains(pos token.Pos) bool {
+	return s.pos <= pos && pos < s.end
+}
+
+// Innermost returns the innermost (child) scope containing
+// pos. If pos is not within any scope, the result is nil.
+// The result is also nil for the Universe scope.
+// The result is guaranteed to be valid only if the type-checked
+// AST has complete position information.
+func (s *Scope) Innermost(pos token.Pos) *Scope {
+	// Package scopes do not have extents since they may be
+	// discontiguous, so iterate over the package's files.
+	if s.parent == Universe {
+		for _, s := range s.children {
+			if inner := s.Innermost(pos); inner != nil {
+				return inner
+			}
+		}
+	}
+
+	if s.Contains(pos) {
+		for _, s := range s.children {
+			if s.Contains(pos) {
+				return s.Innermost(pos)
+			}
+		}
+		return s
+	}
+	return nil
+}
+
+// WriteTo writes a string representation of the scope to w,
+// with the scope elements sorted by name.
+// The level of indentation is controlled by n >= 0, with
+// n == 0 for no indentation.
+// If recurse is set, it also writes nested (children) scopes.
+func (s *Scope) WriteTo(w io.Writer, n int, recurse bool) {
+	const ind = ".  "
+	indn := strings.Repeat(ind, n)
+
+	fmt.Fprintf(w, "%s%s scope %p {\n", indn, s.comment, s)
+
+	indn1 := indn + ind
+	for _, name := range s.Names() {
+		fmt.Fprintf(w, "%s%s\n", indn1, s.elems[name])
+	}
+
+	if recurse {
+		for _, s := range s.children {
+			s.WriteTo(w, n+1, recurse)
+		}
+	}
+
+	fmt.Fprintf(w, "%s}\n", indn)
+}
+
+// String returns a string representation of the scope, for debugging.
+func (s *Scope) String() string {
+	var buf bytes.Buffer
+	s.WriteTo(&buf, 0, false)
+	return buf.String()
+}
diff --git a/src/go/types/selection.go b/src/go/types/selection.go
new file mode 100644
index 0000000..6ec69d2
--- /dev/null
+++ b/src/go/types/selection.go
@@ -0,0 +1,143 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements Selections.
+
+package types
+
+import (
+	"bytes"
+	"fmt"
+)
+
+// SelectionKind describes the kind of a selector expression x.f
+// (excluding qualified identifiers).
+type SelectionKind int
+
+const (
+	FieldVal   SelectionKind = iota // x.f is a struct field selector
+	MethodVal                       // x.f is a method selector
+	MethodExpr                      // x.f is a method expression
+)
+
+// A Selection describes a selector expression x.f.
+// For the declarations:
+//
+//	type T struct{ x int; E }
+//	type E struct{}
+//	func (e E) m() {}
+//	var p *T
+//
+// the following relations exist:
+//
+//	Selector    Kind          Recv    Obj    Type       Index     Indirect
+//
+//	p.x         FieldVal      T       x      int        {0}       true
+//	p.m         MethodVal     *T      m      func()     {1, 0}    true
+//	T.m         MethodExpr    T       m      func(T)    {1, 0}    false
+//
+type Selection struct {
+	kind     SelectionKind
+	recv     Type   // type of x
+	obj      Object // object denoted by x.f
+	index    []int  // path from x to x.f
+	indirect bool   // set if there was any pointer indirection on the path
+}
+
+// Kind returns the selection kind.
+func (s *Selection) Kind() SelectionKind { return s.kind }
+
+// Recv returns the type of x in x.f.
+func (s *Selection) Recv() Type { return s.recv }
+
+// Obj returns the object denoted by x.f; a *Var for
+// a field selection, and a *Func in all other cases.
+func (s *Selection) Obj() Object { return s.obj }
+
+// Type returns the type of x.f, which may be different from the type of f.
+// See Selection for more information.
+func (s *Selection) Type() Type {
+	switch s.kind {
+	case MethodVal:
+		// The type of x.f is a method with its receiver type set
+		// to the type of x.
+		sig := *s.obj.(*Func).typ.(*Signature)
+		recv := *sig.recv
+		recv.typ = s.recv
+		sig.recv = &recv
+		return &sig
+
+	case MethodExpr:
+		// The type of x.f is a function (without receiver)
+		// and an additional first argument with the same type as x.
+		// TODO(gri) Similar code is already in call.go - factor!
+		// TODO(gri) Compute this eagerly to avoid allocations.
+		sig := *s.obj.(*Func).typ.(*Signature)
+		arg0 := *sig.recv
+		sig.recv = nil
+		arg0.typ = s.recv
+		var params []*Var
+		if sig.params != nil {
+			params = sig.params.vars
+		}
+		sig.params = NewTuple(append([]*Var{&arg0}, params...)...)
+		return &sig
+	}
+
+	// In all other cases, the type of x.f is the type of x.
+	return s.obj.Type()
+}
+
+// Index describes the path from x to f in x.f.
+// The last index entry is the field or method index of the type declaring f;
+// either:
+//
+//	1) the list of declared methods of a named type; or
+//	2) the list of methods of an interface type; or
+//	3) the list of fields of a struct type.
+//
+// The earlier index entries are the indices of the embedded fields implicitly
+// traversed to get from (the type of) x to f, starting at embedding depth 0.
+func (s *Selection) Index() []int { return s.index }
+
+// Indirect reports whether any pointer indirection was required to get from
+// x to f in x.f.
+func (s *Selection) Indirect() bool { return s.indirect }
+
+func (s *Selection) String() string { return SelectionString(s, nil) }
+
+// SelectionString returns the string form of s.
+// The Qualifier controls the printing of
+// package-level objects, and may be nil.
+//
+// Examples:
+//	"field (T) f int"
+//	"method (T) f(X) Y"
+//	"method expr (T) f(X) Y"
+//
+func SelectionString(s *Selection, qf Qualifier) string {
+	var k string
+	switch s.kind {
+	case FieldVal:
+		k = "field "
+	case MethodVal:
+		k = "method "
+	case MethodExpr:
+		k = "method expr "
+	default:
+		unreachable()
+	}
+	var buf bytes.Buffer
+	buf.WriteString(k)
+	buf.WriteByte('(')
+	WriteType(&buf, s.Recv(), qf)
+	fmt.Fprintf(&buf, ") %s", s.obj.Name())
+	if T := s.Type(); s.kind == FieldVal {
+		buf.WriteByte(' ')
+		WriteType(&buf, T, qf)
+	} else {
+		WriteSignature(&buf, T.(*Signature), qf)
+	}
+	return buf.String()
+}
diff --git a/src/go/types/self_test.go b/src/go/types/self_test.go
new file mode 100644
index 0000000..262dc7b
--- /dev/null
+++ b/src/go/types/self_test.go
@@ -0,0 +1,121 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types_test
+
+import (
+	"go/ast"
+	"go/importer"
+	"go/parser"
+	"go/token"
+	"path"
+	"path/filepath"
+	"testing"
+	"time"
+
+	. "go/types"
+)
+
+func TestSelf(t *testing.T) {
+	fset := token.NewFileSet()
+	files, err := pkgFiles(fset, ".", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	conf := Config{Importer: importer.Default()}
+	_, err = conf.Check("go/types", fset, files, nil)
+	if err != nil {
+		// Importing go/constant doesn't work in the
+		// build dashboard environment. Don't report an error
+		// for now so that the build remains green.
+		// TODO(gri) fix this
+		t.Log(err) // replace w/ t.Fatal eventually
+		return
+	}
+}
+
+func BenchmarkCheck(b *testing.B) {
+	for _, p := range []string{
+		"net/http",
+		"go/parser",
+		"go/constant",
+		filepath.Join("go", "internal", "gcimporter"),
+	} {
+		b.Run(path.Base(p), func(b *testing.B) {
+			path := filepath.Join("..", "..", p)
+			for _, ignoreFuncBodies := range []bool{false, true} {
+				name := "funcbodies"
+				if ignoreFuncBodies {
+					name = "nofuncbodies"
+				}
+				b.Run(name, func(b *testing.B) {
+					b.Run("info", func(b *testing.B) {
+						runbench(b, path, ignoreFuncBodies, true)
+					})
+					b.Run("noinfo", func(b *testing.B) {
+						runbench(b, path, ignoreFuncBodies, false)
+					})
+				})
+			}
+		})
+	}
+}
+
+func runbench(b *testing.B, path string, ignoreFuncBodies, writeInfo bool) {
+	fset := token.NewFileSet()
+	files, err := pkgFiles(fset, path, 0)
+	if err != nil {
+		b.Fatal(err)
+	}
+	// determine line count
+	lines := 0
+	fset.Iterate(func(f *token.File) bool {
+		lines += f.LineCount()
+		return true
+	})
+
+	b.ResetTimer()
+	start := time.Now()
+	for i := 0; i < b.N; i++ {
+		conf := Config{
+			IgnoreFuncBodies: ignoreFuncBodies,
+			Importer:         importer.Default(),
+		}
+		var info *Info
+		if writeInfo {
+			info = &Info{
+				Types:      make(map[ast.Expr]TypeAndValue),
+				Defs:       make(map[*ast.Ident]Object),
+				Uses:       make(map[*ast.Ident]Object),
+				Implicits:  make(map[ast.Node]Object),
+				Selections: make(map[*ast.SelectorExpr]*Selection),
+				Scopes:     make(map[ast.Node]*Scope),
+			}
+		}
+		if _, err := conf.Check(path, fset, files, info); err != nil {
+			b.Fatal(err)
+		}
+	}
+	b.StopTimer()
+	b.ReportMetric(float64(lines)*float64(b.N)/time.Since(start).Seconds(), "lines/s")
+}
+
+func pkgFiles(fset *token.FileSet, path string, mode parser.Mode) ([]*ast.File, error) {
+	filenames, err := pkgFilenames(path) // from stdlib_test.go
+	if err != nil {
+		return nil, err
+	}
+
+	var files []*ast.File
+	for _, filename := range filenames {
+		file, err := parser.ParseFile(fset, filename, nil, mode)
+		if err != nil {
+			return nil, err
+		}
+		files = append(files, file)
+	}
+
+	return files, nil
+}
diff --git a/src/go/types/sizes.go b/src/go/types/sizes.go
new file mode 100644
index 0000000..6ab6157
--- /dev/null
+++ b/src/go/types/sizes.go
@@ -0,0 +1,263 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements Sizes.
+
+package types
+
+// Sizes defines the sizing functions for package unsafe.
+type Sizes interface {
+	// Alignof returns the alignment of a variable of type T.
+	// Alignof must implement the alignment guarantees required by the spec.
+	Alignof(T Type) int64
+
+	// Offsetsof returns the offsets of the given struct fields, in bytes.
+	// Offsetsof must implement the offset guarantees required by the spec.
+	Offsetsof(fields []*Var) []int64
+
+	// Sizeof returns the size of a variable of type T.
+	// Sizeof must implement the size guarantees required by the spec.
+	Sizeof(T Type) int64
+}
+
+// StdSizes is a convenience type for creating commonly used Sizes.
+// It makes the following simplifying assumptions:
+//
+//	- The size of explicitly sized basic types (int16, etc.) is the
+//	  specified size.
+//	- The size of strings and interfaces is 2*WordSize.
+//	- The size of slices is 3*WordSize.
+//	- The size of an array of n elements corresponds to the size of
+//	  a struct of n consecutive fields of the array's element type.
+//      - The size of a struct is the offset of the last field plus that
+//	  field's size. As with all element types, if the struct is used
+//	  in an array its size must first be aligned to a multiple of the
+//	  struct's alignment.
+//	- All other types have size WordSize.
+//	- Arrays and structs are aligned per spec definition; all other
+//	  types are naturally aligned with a maximum alignment MaxAlign.
+//
+// *StdSizes implements Sizes.
+//
+type StdSizes struct {
+	WordSize int64 // word size in bytes - must be >= 4 (32bits)
+	MaxAlign int64 // maximum alignment in bytes - must be >= 1
+}
+
+func (s *StdSizes) Alignof(T Type) int64 {
+	// For arrays and structs, alignment is defined in terms
+	// of alignment of the elements and fields, respectively.
+	switch t := T.Underlying().(type) {
+	case *Array:
+		// spec: "For a variable x of array type: unsafe.Alignof(x)
+		// is the same as unsafe.Alignof(x[0]), but at least 1."
+		return s.Alignof(t.elem)
+	case *Struct:
+		// spec: "For a variable x of struct type: unsafe.Alignof(x)
+		// is the largest of the values unsafe.Alignof(x.f) for each
+		// field f of x, but at least 1."
+		max := int64(1)
+		for _, f := range t.fields {
+			if a := s.Alignof(f.typ); a > max {
+				max = a
+			}
+		}
+		return max
+	case *Slice, *Interface:
+		// Multiword data structures are effectively structs
+		// in which each element has size WordSize.
+		return s.WordSize
+	case *Basic:
+		// Strings are like slices and interfaces.
+		if t.Info()&IsString != 0 {
+			return s.WordSize
+		}
+	}
+	a := s.Sizeof(T) // may be 0
+	// spec: "For a variable x of any type: unsafe.Alignof(x) is at least 1."
+	if a < 1 {
+		return 1
+	}
+	// complex{64,128} are aligned like [2]float{32,64}.
+	if isComplex(T) {
+		a /= 2
+	}
+	if a > s.MaxAlign {
+		return s.MaxAlign
+	}
+	return a
+}
+
+func (s *StdSizes) Offsetsof(fields []*Var) []int64 {
+	offsets := make([]int64, len(fields))
+	var o int64
+	for i, f := range fields {
+		a := s.Alignof(f.typ)
+		o = align(o, a)
+		offsets[i] = o
+		o += s.Sizeof(f.typ)
+	}
+	return offsets
+}
+
+var basicSizes = [...]byte{
+	Bool:       1,
+	Int8:       1,
+	Int16:      2,
+	Int32:      4,
+	Int64:      8,
+	Uint8:      1,
+	Uint16:     2,
+	Uint32:     4,
+	Uint64:     8,
+	Float32:    4,
+	Float64:    8,
+	Complex64:  8,
+	Complex128: 16,
+}
+
+func (s *StdSizes) Sizeof(T Type) int64 {
+	switch t := T.Underlying().(type) {
+	case *Basic:
+		assert(isTyped(T))
+		k := t.kind
+		if int(k) < len(basicSizes) {
+			if s := basicSizes[k]; s > 0 {
+				return int64(s)
+			}
+		}
+		if k == String {
+			return s.WordSize * 2
+		}
+	case *Array:
+		n := t.len
+		if n <= 0 {
+			return 0
+		}
+		// n > 0
+		a := s.Alignof(t.elem)
+		z := s.Sizeof(t.elem)
+		return align(z, a)*(n-1) + z
+	case *Slice:
+		return s.WordSize * 3
+	case *Struct:
+		n := t.NumFields()
+		if n == 0 {
+			return 0
+		}
+		offsets := s.Offsetsof(t.fields)
+		return offsets[n-1] + s.Sizeof(t.fields[n-1].typ)
+	case *Interface:
+		return s.WordSize * 2
+	}
+	return s.WordSize // catch-all
+}
+
+// common architecture word sizes and alignments
+var gcArchSizes = map[string]*StdSizes{
+	"386":      {4, 4},
+	"arm":      {4, 4},
+	"arm64":    {8, 8},
+	"amd64":    {8, 8},
+	"amd64p32": {4, 8},
+	"mips":     {4, 4},
+	"mipsle":   {4, 4},
+	"mips64":   {8, 8},
+	"mips64le": {8, 8},
+	"ppc64":    {8, 8},
+	"ppc64le":  {8, 8},
+	"riscv64":  {8, 8},
+	"s390x":    {8, 8},
+	"sparc64":  {8, 8},
+	"wasm":     {8, 8},
+	// When adding more architectures here,
+	// update the doc string of SizesFor below.
+}
+
+// SizesFor returns the Sizes used by a compiler for an architecture.
+// The result is nil if a compiler/architecture pair is not known.
+//
+// Supported architectures for compiler "gc":
+// "386", "arm", "arm64", "amd64", "amd64p32", "mips", "mipsle",
+// "mips64", "mips64le", "ppc64", "ppc64le", "riscv64", "s390x", "sparc64", "wasm".
+func SizesFor(compiler, arch string) Sizes {
+	var m map[string]*StdSizes
+	switch compiler {
+	case "gc":
+		m = gcArchSizes
+	case "gccgo":
+		m = gccgoArchSizes
+	default:
+		return nil
+	}
+	s, ok := m[arch]
+	if !ok {
+		return nil
+	}
+	return s
+}
+
+// stdSizes is used if Config.Sizes == nil.
+var stdSizes = SizesFor("gc", "amd64")
+
+func (conf *Config) alignof(T Type) int64 {
+	if s := conf.Sizes; s != nil {
+		if a := s.Alignof(T); a >= 1 {
+			return a
+		}
+		panic("Config.Sizes.Alignof returned an alignment < 1")
+	}
+	return stdSizes.Alignof(T)
+}
+
+func (conf *Config) offsetsof(T *Struct) []int64 {
+	var offsets []int64
+	if T.NumFields() > 0 {
+		// compute offsets on demand
+		if s := conf.Sizes; s != nil {
+			offsets = s.Offsetsof(T.fields)
+			// sanity checks
+			if len(offsets) != T.NumFields() {
+				panic("Config.Sizes.Offsetsof returned the wrong number of offsets")
+			}
+			for _, o := range offsets {
+				if o < 0 {
+					panic("Config.Sizes.Offsetsof returned an offset < 0")
+				}
+			}
+		} else {
+			offsets = stdSizes.Offsetsof(T.fields)
+		}
+	}
+	return offsets
+}
+
+// offsetof returns the offset of the field specified via
+// the index sequence relative to typ. All embedded fields
+// must be structs (rather than pointer to structs).
+func (conf *Config) offsetof(typ Type, index []int) int64 {
+	var o int64
+	for _, i := range index {
+		s := typ.Underlying().(*Struct)
+		o += conf.offsetsof(s)[i]
+		typ = s.fields[i].typ
+	}
+	return o
+}
+
+func (conf *Config) sizeof(T Type) int64 {
+	if s := conf.Sizes; s != nil {
+		if z := s.Sizeof(T); z >= 0 {
+			return z
+		}
+		panic("Config.Sizes.Sizeof returned a size < 0")
+	}
+	return stdSizes.Sizeof(T)
+}
+
+// align returns the smallest y >= x such that y % a == 0.
+func align(x, a int64) int64 {
+	y := x + a - 1
+	return y - y%a
+}
diff --git a/src/go/types/sizes_test.go b/src/go/types/sizes_test.go
new file mode 100644
index 0000000..539b4e3
--- /dev/null
+++ b/src/go/types/sizes_test.go
@@ -0,0 +1,112 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains tests for sizes.
+
+package types_test
+
+import (
+	"go/ast"
+	"go/importer"
+	"go/parser"
+	"go/token"
+	"go/types"
+	"testing"
+)
+
+// findStructType typechecks src and returns the first struct type encountered.
+func findStructType(t *testing.T, src string) *types.Struct {
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "x.go", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	info := types.Info{Types: make(map[ast.Expr]types.TypeAndValue)}
+	var conf types.Config
+	_, err = conf.Check("x", fset, []*ast.File{f}, &info)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, tv := range info.Types {
+		if ts, ok := tv.Type.(*types.Struct); ok {
+			return ts
+		}
+	}
+	t.Fatalf("failed to find a struct type in src:\n%s\n", src)
+	return nil
+}
+
+// Issue 16316
+func TestMultipleSizeUse(t *testing.T) {
+	const src = `
+package main
+
+type S struct {
+    i int
+    b bool
+    s string
+    n int
+}
+`
+	ts := findStructType(t, src)
+	sizes := types.StdSizes{WordSize: 4, MaxAlign: 4}
+	if got := sizes.Sizeof(ts); got != 20 {
+		t.Errorf("Sizeof(%v) with WordSize 4 = %d want 20", ts, got)
+	}
+	sizes = types.StdSizes{WordSize: 8, MaxAlign: 8}
+	if got := sizes.Sizeof(ts); got != 40 {
+		t.Errorf("Sizeof(%v) with WordSize 8 = %d want 40", ts, got)
+	}
+}
+
+// Issue 16464
+func TestAlignofNaclSlice(t *testing.T) {
+	const src = `
+package main
+
+var s struct {
+	x *int
+	y []byte
+}
+`
+	ts := findStructType(t, src)
+	sizes := &types.StdSizes{WordSize: 4, MaxAlign: 8}
+	var fields []*types.Var
+	// Make a copy manually :(
+	for i := 0; i < ts.NumFields(); i++ {
+		fields = append(fields, ts.Field(i))
+	}
+	offsets := sizes.Offsetsof(fields)
+	if offsets[0] != 0 || offsets[1] != 4 {
+		t.Errorf("OffsetsOf(%v) = %v want %v", ts, offsets, []int{0, 4})
+	}
+}
+
+func TestIssue16902(t *testing.T) {
+	const src = `
+package a
+
+import "unsafe"
+
+const _ = unsafe.Offsetof(struct{ x int64 }{}.x)
+`
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "x.go", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	info := types.Info{Types: make(map[ast.Expr]types.TypeAndValue)}
+	conf := types.Config{
+		Importer: importer.Default(),
+		Sizes:    &types.StdSizes{WordSize: 8, MaxAlign: 8},
+	}
+	_, err = conf.Check("x", fset, []*ast.File{f}, &info)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, tv := range info.Types {
+		_ = conf.Sizes.Sizeof(tv.Type)
+		_ = conf.Sizes.Alignof(tv.Type)
+	}
+}
diff --git a/src/go/types/stdlib_test.go b/src/go/types/stdlib_test.go
new file mode 100644
index 0000000..5ca4493
--- /dev/null
+++ b/src/go/types/stdlib_test.go
@@ -0,0 +1,326 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file tests types.Check by using it to
+// typecheck the standard library and tests.
+
+package types_test
+
+import (
+	"fmt"
+	"go/ast"
+	"go/build"
+	"go/importer"
+	"go/parser"
+	"go/scanner"
+	"go/token"
+	"internal/testenv"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+
+	. "go/types"
+)
+
+// Use the same importer for all std lib tests to
+// avoid repeated importing of the same packages.
+var stdLibImporter = importer.Default()
+
+func TestStdlib(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	pkgCount := 0
+	duration := walkPkgDirs(filepath.Join(runtime.GOROOT(), "src"), func(dir string, filenames []string) {
+		typecheck(t, dir, filenames)
+		pkgCount++
+	}, t.Error)
+
+	if testing.Verbose() {
+		fmt.Println(pkgCount, "packages typechecked in", duration)
+	}
+}
+
+// firstComment returns the contents of the first non-empty comment in
+// the given file, "skip", or the empty string. No matter the present
+// comments, if any of them contains a build tag, the result is always
+// "skip". Only comments before the "package" token and within the first
+// 4K of the file are considered.
+func firstComment(filename string) string {
+	f, err := os.Open(filename)
+	if err != nil {
+		return ""
+	}
+	defer f.Close()
+
+	var src [4 << 10]byte // read at most 4KB
+	n, _ := f.Read(src[:])
+
+	var first string
+	var s scanner.Scanner
+	s.Init(fset.AddFile("", fset.Base(), n), src[:n], nil /* ignore errors */, scanner.ScanComments)
+	for {
+		_, tok, lit := s.Scan()
+		switch tok {
+		case token.COMMENT:
+			// remove trailing */ of multi-line comment
+			if lit[1] == '*' {
+				lit = lit[:len(lit)-2]
+			}
+			contents := strings.TrimSpace(lit[2:])
+			if strings.HasPrefix(contents, "+build ") {
+				return "skip"
+			}
+			if first == "" {
+				first = contents // contents may be "" but that's ok
+			}
+			// continue as we may still see build tags
+
+		case token.PACKAGE, token.EOF:
+			return first
+		}
+	}
+}
+
+func testTestDir(t *testing.T, path string, ignore ...string) {
+	files, err := os.ReadDir(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	excluded := make(map[string]bool)
+	for _, filename := range ignore {
+		excluded[filename] = true
+	}
+
+	fset := token.NewFileSet()
+	for _, f := range files {
+		// filter directory contents
+		if f.IsDir() || !strings.HasSuffix(f.Name(), ".go") || excluded[f.Name()] {
+			continue
+		}
+
+		// get per-file instructions
+		expectErrors := false
+		filename := filepath.Join(path, f.Name())
+		if comment := firstComment(filename); comment != "" {
+			fields := strings.Fields(comment)
+			switch fields[0] {
+			case "skip", "compiledir":
+				continue // ignore this file
+			case "errorcheck":
+				expectErrors = true
+				for _, arg := range fields[1:] {
+					if arg == "-0" || arg == "-+" || arg == "-std" {
+						// Marked explicitly as not expected errors (-0),
+						// or marked as compiling runtime/stdlib, which is only done
+						// to trigger runtime/stdlib-only error output.
+						// In both cases, the code should typecheck.
+						expectErrors = false
+						break
+					}
+				}
+			}
+		}
+
+		// parse and type-check file
+		file, err := parser.ParseFile(fset, filename, nil, 0)
+		if err == nil {
+			conf := Config{Importer: stdLibImporter}
+			_, err = conf.Check(filename, fset, []*ast.File{file}, nil)
+		}
+
+		if expectErrors {
+			if err == nil {
+				t.Errorf("expected errors but found none in %s", filename)
+			}
+		} else {
+			if err != nil {
+				t.Error(err)
+			}
+		}
+	}
+}
+
+func TestStdTest(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	if testing.Short() && testenv.Builder() == "" {
+		t.Skip("skipping in short mode")
+	}
+
+	testTestDir(t, filepath.Join(runtime.GOROOT(), "test"),
+		"cmplxdivide.go", // also needs file cmplxdivide1.go - ignore
+		"directive.go",   // tests compiler rejection of bad directive placement - ignore
+		"embedfunc.go",   // tests //go:embed
+		"embedvers.go",   // tests //go:embed
+	)
+}
+
+func TestStdFixed(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	if testing.Short() && testenv.Builder() == "" {
+		t.Skip("skipping in short mode")
+	}
+
+	testTestDir(t, filepath.Join(runtime.GOROOT(), "test", "fixedbugs"),
+		"bug248.go", "bug302.go", "bug369.go", // complex test instructions - ignore
+		"issue6889.go",   // gc-specific test
+		"issue7746.go",   // large constants - consumes too much memory
+		"issue11362.go",  // canonical import path check
+		"issue16369.go",  // go/types handles this correctly - not an issue
+		"issue18459.go",  // go/types doesn't check validity of //go:xxx directives
+		"issue18882.go",  // go/types doesn't check validity of //go:xxx directives
+		"issue20232.go",  // go/types handles larger constants than gc
+		"issue20529.go",  // go/types does not have constraints on stack size
+		"issue22200.go",  // go/types does not have constraints on stack size
+		"issue22200b.go", // go/types does not have constraints on stack size
+		"issue25507.go",  // go/types does not have constraints on stack size
+		"issue20780.go",  // go/types does not have constraints on stack size
+		"issue31747.go",  // go/types does not have constraints on language level (-lang=go1.12) (see #31793)
+		"issue34329.go",  // go/types does not have constraints on language level (-lang=go1.13) (see #31793)
+		"bug251.go",      // issue #34333 which was exposed with fix for #34151
+		"issue42058a.go", // go/types does not have constraints on channel element size
+		"issue42058b.go", // go/types does not have constraints on channel element size
+	)
+}
+
+func TestStdKen(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	testTestDir(t, filepath.Join(runtime.GOROOT(), "test", "ken"))
+}
+
+// Package paths of excluded packages.
+var excluded = map[string]bool{
+	"builtin": true,
+}
+
+// typecheck typechecks the given package files.
+func typecheck(t *testing.T, path string, filenames []string) {
+	fset := token.NewFileSet()
+
+	// parse package files
+	var files []*ast.File
+	for _, filename := range filenames {
+		file, err := parser.ParseFile(fset, filename, nil, parser.AllErrors)
+		if err != nil {
+			// the parser error may be a list of individual errors; report them all
+			if list, ok := err.(scanner.ErrorList); ok {
+				for _, err := range list {
+					t.Error(err)
+				}
+				return
+			}
+			t.Error(err)
+			return
+		}
+
+		if testing.Verbose() {
+			if len(files) == 0 {
+				fmt.Println("package", file.Name.Name)
+			}
+			fmt.Println("\t", filename)
+		}
+
+		files = append(files, file)
+	}
+
+	// typecheck package files
+	conf := Config{
+		Error:    func(err error) { t.Error(err) },
+		Importer: stdLibImporter,
+	}
+	info := Info{Uses: make(map[*ast.Ident]Object)}
+	conf.Check(path, fset, files, &info)
+
+	// Perform checks of API invariants.
+
+	// All Objects have a package, except predeclared ones.
+	errorError := Universe.Lookup("error").Type().Underlying().(*Interface).ExplicitMethod(0) // (error).Error
+	for id, obj := range info.Uses {
+		predeclared := obj == Universe.Lookup(obj.Name()) || obj == errorError
+		if predeclared == (obj.Pkg() != nil) {
+			posn := fset.Position(id.Pos())
+			if predeclared {
+				t.Errorf("%s: predeclared object with package: %s", posn, obj)
+			} else {
+				t.Errorf("%s: user-defined object without package: %s", posn, obj)
+			}
+		}
+	}
+}
+
+// pkgFilenames returns the list of package filenames for the given directory.
+func pkgFilenames(dir string) ([]string, error) {
+	ctxt := build.Default
+	ctxt.CgoEnabled = false
+	pkg, err := ctxt.ImportDir(dir, 0)
+	if err != nil {
+		if _, nogo := err.(*build.NoGoError); nogo {
+			return nil, nil // no *.go files, not an error
+		}
+		return nil, err
+	}
+	if excluded[pkg.ImportPath] {
+		return nil, nil
+	}
+	var filenames []string
+	for _, name := range pkg.GoFiles {
+		filenames = append(filenames, filepath.Join(pkg.Dir, name))
+	}
+	for _, name := range pkg.TestGoFiles {
+		filenames = append(filenames, filepath.Join(pkg.Dir, name))
+	}
+	return filenames, nil
+}
+
+func walkPkgDirs(dir string, pkgh func(dir string, filenames []string), errh func(args ...interface{})) time.Duration {
+	w := walker{time.Now(), 10 * time.Millisecond, pkgh, errh}
+	w.walk(dir)
+	return time.Since(w.start)
+}
+
+type walker struct {
+	start time.Time
+	dmax  time.Duration
+	pkgh  func(dir string, filenames []string)
+	errh  func(args ...interface{})
+}
+
+func (w *walker) walk(dir string) {
+	// limit run time for short tests
+	if testing.Short() && time.Since(w.start) >= w.dmax {
+		return
+	}
+
+	files, err := os.ReadDir(dir)
+	if err != nil {
+		w.errh(err)
+		return
+	}
+
+	// apply pkgh to the files in directory dir
+	// but ignore files directly under $GOROOT/src (might be temporary test files).
+	if dir != filepath.Join(runtime.GOROOT(), "src") {
+		files, err := pkgFilenames(dir)
+		if err != nil {
+			w.errh(err)
+			return
+		}
+		if files != nil {
+			w.pkgh(dir, files)
+		}
+	}
+
+	// traverse subdirectories, but don't walk into testdata
+	for _, f := range files {
+		if f.IsDir() && f.Name() != "testdata" {
+			w.walk(filepath.Join(dir, f.Name()))
+		}
+	}
+}
diff --git a/src/go/types/stmt.go b/src/go/types/stmt.go
new file mode 100644
index 0000000..d88e471
--- /dev/null
+++ b/src/go/types/stmt.go
@@ -0,0 +1,881 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements typechecking of statements.
+
+package types
+
+import (
+	"go/ast"
+	"go/constant"
+	"go/token"
+	"sort"
+)
+
+func (check *Checker) funcBody(decl *declInfo, name string, sig *Signature, body *ast.BlockStmt, iota constant.Value) {
+	if trace {
+		check.trace(body.Pos(), "--- %s: %s", name, sig)
+		defer func() {
+			check.trace(body.End(), "--- <end>")
+		}()
+	}
+
+	// set function scope extent
+	sig.scope.pos = body.Pos()
+	sig.scope.end = body.End()
+
+	// save/restore current context and setup function context
+	// (and use 0 indentation at function start)
+	defer func(ctxt context, indent int) {
+		check.context = ctxt
+		check.indent = indent
+	}(check.context, check.indent)
+	check.context = context{
+		decl:  decl,
+		scope: sig.scope,
+		iota:  iota,
+		sig:   sig,
+	}
+	check.indent = 0
+
+	check.stmtList(0, body.List)
+
+	if check.hasLabel {
+		check.labels(body)
+	}
+
+	if sig.results.Len() > 0 && !check.isTerminating(body, "") {
+		check.error(atPos(body.Rbrace), _MissingReturn, "missing return")
+	}
+
+	// spec: "Implementation restriction: A compiler may make it illegal to
+	// declare a variable inside a function body if the variable is never used."
+	check.usage(sig.scope)
+}
+
+func (check *Checker) usage(scope *Scope) {
+	var unused []*Var
+	for _, elem := range scope.elems {
+		if v, _ := elem.(*Var); v != nil && !v.used {
+			unused = append(unused, v)
+		}
+	}
+	sort.Slice(unused, func(i, j int) bool {
+		return unused[i].pos < unused[j].pos
+	})
+	for _, v := range unused {
+		check.softErrorf(v, _UnusedVar, "%s declared but not used", v.name)
+	}
+
+	for _, scope := range scope.children {
+		// Don't go inside function literal scopes a second time;
+		// they are handled explicitly by funcBody.
+		if !scope.isFunc {
+			check.usage(scope)
+		}
+	}
+}
+
+// stmtContext is a bitset describing which
+// control-flow statements are permissible,
+// and provides additional context information
+// for better error messages.
+type stmtContext uint
+
+const (
+	// permissible control-flow statements
+	breakOk stmtContext = 1 << iota
+	continueOk
+	fallthroughOk
+
+	// additional context information
+	finalSwitchCase
+)
+
+func (check *Checker) simpleStmt(s ast.Stmt) {
+	if s != nil {
+		check.stmt(0, s)
+	}
+}
+
+func trimTrailingEmptyStmts(list []ast.Stmt) []ast.Stmt {
+	for i := len(list); i > 0; i-- {
+		if _, ok := list[i-1].(*ast.EmptyStmt); !ok {
+			return list[:i]
+		}
+	}
+	return nil
+}
+
+func (check *Checker) stmtList(ctxt stmtContext, list []ast.Stmt) {
+	ok := ctxt&fallthroughOk != 0
+	inner := ctxt &^ fallthroughOk
+	list = trimTrailingEmptyStmts(list) // trailing empty statements are "invisible" to fallthrough analysis
+	for i, s := range list {
+		inner := inner
+		if ok && i+1 == len(list) {
+			inner |= fallthroughOk
+		}
+		check.stmt(inner, s)
+	}
+}
+
+func (check *Checker) multipleDefaults(list []ast.Stmt) {
+	var first ast.Stmt
+	for _, s := range list {
+		var d ast.Stmt
+		switch c := s.(type) {
+		case *ast.CaseClause:
+			if len(c.List) == 0 {
+				d = s
+			}
+		case *ast.CommClause:
+			if c.Comm == nil {
+				d = s
+			}
+		default:
+			check.invalidAST(s, "case/communication clause expected")
+		}
+		if d != nil {
+			if first != nil {
+				check.errorf(d, _DuplicateDefault, "multiple defaults (first at %s)", check.fset.Position(first.Pos()))
+			} else {
+				first = d
+			}
+		}
+	}
+}
+
+func (check *Checker) openScope(s ast.Stmt, comment string) {
+	scope := NewScope(check.scope, s.Pos(), s.End(), comment)
+	check.recordScope(s, scope)
+	check.scope = scope
+}
+
+func (check *Checker) closeScope() {
+	check.scope = check.scope.Parent()
+}
+
+func assignOp(op token.Token) token.Token {
+	// token_test.go verifies the token ordering this function relies on
+	if token.ADD_ASSIGN <= op && op <= token.AND_NOT_ASSIGN {
+		return op + (token.ADD - token.ADD_ASSIGN)
+	}
+	return token.ILLEGAL
+}
+
+func (check *Checker) suspendedCall(keyword string, call *ast.CallExpr) {
+	var x operand
+	var msg string
+	var code errorCode
+	switch check.rawExpr(&x, call, nil) {
+	case conversion:
+		msg = "requires function call, not conversion"
+		code = _InvalidDefer
+		if keyword == "go" {
+			code = _InvalidGo
+		}
+	case expression:
+		msg = "discards result of"
+		code = _UnusedResults
+	case statement:
+		return
+	default:
+		unreachable()
+	}
+	check.errorf(&x, code, "%s %s %s", keyword, msg, &x)
+}
+
+// goVal returns the Go value for val, or nil.
+func goVal(val constant.Value) interface{} {
+	// val should exist, but be conservative and check
+	if val == nil {
+		return nil
+	}
+	// Match implementation restriction of other compilers.
+	// gc only checks duplicates for integer, floating-point
+	// and string values, so only create Go values for these
+	// types.
+	switch val.Kind() {
+	case constant.Int:
+		if x, ok := constant.Int64Val(val); ok {
+			return x
+		}
+		if x, ok := constant.Uint64Val(val); ok {
+			return x
+		}
+	case constant.Float:
+		if x, ok := constant.Float64Val(val); ok {
+			return x
+		}
+	case constant.String:
+		return constant.StringVal(val)
+	}
+	return nil
+}
+
+// A valueMap maps a case value (of a basic Go type) to a list of positions
+// where the same case value appeared, together with the corresponding case
+// types.
+// Since two case values may have the same "underlying" value but different
+// types we need to also check the value's types (e.g., byte(1) vs myByte(1))
+// when the switch expression is of interface type.
+type (
+	valueMap  map[interface{}][]valueType // underlying Go value -> valueType
+	valueType struct {
+		pos token.Pos
+		typ Type
+	}
+)
+
+func (check *Checker) caseValues(x *operand, values []ast.Expr, seen valueMap) {
+L:
+	for _, e := range values {
+		var v operand
+		check.expr(&v, e)
+		if x.mode == invalid || v.mode == invalid {
+			continue L
+		}
+		check.convertUntyped(&v, x.typ)
+		if v.mode == invalid {
+			continue L
+		}
+		// Order matters: By comparing v against x, error positions are at the case values.
+		res := v // keep original v unchanged
+		check.comparison(&res, x, token.EQL)
+		if res.mode == invalid {
+			continue L
+		}
+		if v.mode != constant_ {
+			continue L // we're done
+		}
+		// look for duplicate values
+		if val := goVal(v.val); val != nil {
+			// look for duplicate types for a given value
+			// (quadratic algorithm, but these lists tend to be very short)
+			for _, vt := range seen[val] {
+				if check.identical(v.typ, vt.typ) {
+					check.errorf(&v, _DuplicateCase, "duplicate case %s in expression switch", &v)
+					check.error(atPos(vt.pos), _DuplicateCase, "\tprevious case") // secondary error, \t indented
+					continue L
+				}
+			}
+			seen[val] = append(seen[val], valueType{v.Pos(), v.typ})
+		}
+	}
+}
+
+func (check *Checker) caseTypes(x *operand, xtyp *Interface, types []ast.Expr, seen map[Type]ast.Expr) (T Type) {
+L:
+	for _, e := range types {
+		T = check.typOrNil(e)
+		if T == Typ[Invalid] {
+			continue L
+		}
+		// look for duplicate types
+		// (quadratic algorithm, but type switches tend to be reasonably small)
+		for t, other := range seen {
+			if T == nil && t == nil || T != nil && t != nil && check.identical(T, t) {
+				// talk about "case" rather than "type" because of nil case
+				Ts := "nil"
+				if T != nil {
+					Ts = T.String()
+				}
+				check.errorf(e, _DuplicateCase, "duplicate case %s in type switch", Ts)
+				check.error(other, _DuplicateCase, "\tprevious case") // secondary error, \t indented
+				continue L
+			}
+		}
+		seen[T] = e
+		if T != nil {
+			check.typeAssertion(e, x, xtyp, T)
+		}
+	}
+	return
+}
+
+// stmt typechecks statement s.
+func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
+	// statements must end with the same top scope as they started with
+	if debug {
+		defer func(scope *Scope) {
+			// don't check if code is panicking
+			if p := recover(); p != nil {
+				panic(p)
+			}
+			assert(scope == check.scope)
+		}(check.scope)
+	}
+
+	// process collected function literals before scope changes
+	defer check.processDelayed(len(check.delayed))
+
+	inner := ctxt &^ (fallthroughOk | finalSwitchCase)
+	switch s := s.(type) {
+	case *ast.BadStmt, *ast.EmptyStmt:
+		// ignore
+
+	case *ast.DeclStmt:
+		check.declStmt(s.Decl)
+
+	case *ast.LabeledStmt:
+		check.hasLabel = true
+		check.stmt(ctxt, s.Stmt)
+
+	case *ast.ExprStmt:
+		// spec: "With the exception of specific built-in functions,
+		// function and method calls and receive operations can appear
+		// in statement context. Such statements may be parenthesized."
+		var x operand
+		kind := check.rawExpr(&x, s.X, nil)
+		var msg string
+		var code errorCode
+		switch x.mode {
+		default:
+			if kind == statement {
+				return
+			}
+			msg = "is not used"
+			code = _UnusedExpr
+		case builtin:
+			msg = "must be called"
+			code = _UncalledBuiltin
+		case typexpr:
+			msg = "is not an expression"
+			code = _NotAnExpr
+		}
+		check.errorf(&x, code, "%s %s", &x, msg)
+
+	case *ast.SendStmt:
+		var ch, x operand
+		check.expr(&ch, s.Chan)
+		check.expr(&x, s.Value)
+		if ch.mode == invalid || x.mode == invalid {
+			return
+		}
+
+		tch, ok := ch.typ.Underlying().(*Chan)
+		if !ok {
+			check.invalidOp(inNode(s, s.Arrow), _InvalidSend, "cannot send to non-chan type %s", ch.typ)
+			return
+		}
+
+		if tch.dir == RecvOnly {
+			check.invalidOp(inNode(s, s.Arrow), _InvalidSend, "cannot send to receive-only type %s", tch)
+			return
+		}
+
+		check.assignment(&x, tch.elem, "send")
+
+	case *ast.IncDecStmt:
+		var op token.Token
+		switch s.Tok {
+		case token.INC:
+			op = token.ADD
+		case token.DEC:
+			op = token.SUB
+		default:
+			check.invalidAST(inNode(s, s.TokPos), "unknown inc/dec operation %s", s.Tok)
+			return
+		}
+
+		var x operand
+		check.expr(&x, s.X)
+		if x.mode == invalid {
+			return
+		}
+		if !isNumeric(x.typ) {
+			check.invalidOp(s.X, _NonNumericIncDec, "%s%s (non-numeric type %s)", s.X, s.Tok, x.typ)
+			return
+		}
+
+		Y := &ast.BasicLit{ValuePos: s.X.Pos(), Kind: token.INT, Value: "1"} // use x's position
+		check.binary(&x, nil, s.X, Y, op, s.TokPos)
+		if x.mode == invalid {
+			return
+		}
+		check.assignVar(s.X, &x)
+
+	case *ast.AssignStmt:
+		switch s.Tok {
+		case token.ASSIGN, token.DEFINE:
+			if len(s.Lhs) == 0 {
+				check.invalidAST(s, "missing lhs in assignment")
+				return
+			}
+			if s.Tok == token.DEFINE {
+				check.shortVarDecl(inNode(s, s.TokPos), s.Lhs, s.Rhs)
+			} else {
+				// regular assignment
+				check.assignVars(s.Lhs, s.Rhs)
+			}
+
+		default:
+			// assignment operations
+			if len(s.Lhs) != 1 || len(s.Rhs) != 1 {
+				check.errorf(inNode(s, s.TokPos), _MultiValAssignOp, "assignment operation %s requires single-valued expressions", s.Tok)
+				return
+			}
+			op := assignOp(s.Tok)
+			if op == token.ILLEGAL {
+				check.invalidAST(atPos(s.TokPos), "unknown assignment operation %s", s.Tok)
+				return
+			}
+			var x operand
+			check.binary(&x, nil, s.Lhs[0], s.Rhs[0], op, s.TokPos)
+			if x.mode == invalid {
+				return
+			}
+			check.assignVar(s.Lhs[0], &x)
+		}
+
+	case *ast.GoStmt:
+		check.suspendedCall("go", s.Call)
+
+	case *ast.DeferStmt:
+		check.suspendedCall("defer", s.Call)
+
+	case *ast.ReturnStmt:
+		res := check.sig.results
+		if res.Len() > 0 {
+			// function returns results
+			// (if one, say the first, result parameter is named, all of them are named)
+			if len(s.Results) == 0 && res.vars[0].name != "" {
+				// spec: "Implementation restriction: A compiler may disallow an empty expression
+				// list in a "return" statement if a different entity (constant, type, or variable)
+				// with the same name as a result parameter is in scope at the place of the return."
+				for _, obj := range res.vars {
+					if alt := check.lookup(obj.name); alt != nil && alt != obj {
+						check.errorf(s, _OutOfScopeResult, "result parameter %s not in scope at return", obj.name)
+						check.errorf(alt, _OutOfScopeResult, "\tinner declaration of %s", obj)
+						// ok to continue
+					}
+				}
+			} else {
+				// return has results or result parameters are unnamed
+				check.initVars(res.vars, s.Results, s.Return)
+			}
+		} else if len(s.Results) > 0 {
+			check.error(s.Results[0], _WrongResultCount, "no result values expected")
+			check.use(s.Results...)
+		}
+
+	case *ast.BranchStmt:
+		if s.Label != nil {
+			check.hasLabel = true
+			return // checked in 2nd pass (check.labels)
+		}
+		switch s.Tok {
+		case token.BREAK:
+			if ctxt&breakOk == 0 {
+				check.error(s, _MisplacedBreak, "break not in for, switch, or select statement")
+			}
+		case token.CONTINUE:
+			if ctxt&continueOk == 0 {
+				check.error(s, _MisplacedContinue, "continue not in for statement")
+			}
+		case token.FALLTHROUGH:
+			if ctxt&fallthroughOk == 0 {
+				msg := "fallthrough statement out of place"
+				code := _MisplacedFallthrough
+				if ctxt&finalSwitchCase != 0 {
+					msg = "cannot fallthrough final case in switch"
+				}
+				check.error(s, code, msg)
+			}
+		default:
+			check.invalidAST(s, "branch statement: %s", s.Tok)
+		}
+
+	case *ast.BlockStmt:
+		check.openScope(s, "block")
+		defer check.closeScope()
+
+		check.stmtList(inner, s.List)
+
+	case *ast.IfStmt:
+		check.openScope(s, "if")
+		defer check.closeScope()
+
+		check.simpleStmt(s.Init)
+		var x operand
+		check.expr(&x, s.Cond)
+		if x.mode != invalid && !isBoolean(x.typ) {
+			check.error(s.Cond, _InvalidCond, "non-boolean condition in if statement")
+		}
+		check.stmt(inner, s.Body)
+		// The parser produces a correct AST but if it was modified
+		// elsewhere the else branch may be invalid. Check again.
+		switch s.Else.(type) {
+		case nil, *ast.BadStmt:
+			// valid or error already reported
+		case *ast.IfStmt, *ast.BlockStmt:
+			check.stmt(inner, s.Else)
+		default:
+			check.invalidAST(s.Else, "invalid else branch in if statement")
+		}
+
+	case *ast.SwitchStmt:
+		inner |= breakOk
+		check.openScope(s, "switch")
+		defer check.closeScope()
+
+		check.simpleStmt(s.Init)
+		var x operand
+		if s.Tag != nil {
+			check.expr(&x, s.Tag)
+			// By checking assignment of x to an invisible temporary
+			// (as a compiler would), we get all the relevant checks.
+			check.assignment(&x, nil, "switch expression")
+			if x.mode != invalid && !Comparable(x.typ) && !hasNil(x.typ) {
+				check.errorf(&x, _InvalidExprSwitch, "cannot switch on %s (%s is not comparable)", &x, x.typ)
+				x.mode = invalid
+			}
+		} else {
+			// spec: "A missing switch expression is
+			// equivalent to the boolean value true."
+			x.mode = constant_
+			x.typ = Typ[Bool]
+			x.val = constant.MakeBool(true)
+			x.expr = &ast.Ident{NamePos: s.Body.Lbrace, Name: "true"}
+		}
+
+		check.multipleDefaults(s.Body.List)
+
+		seen := make(valueMap) // map of seen case values to positions and types
+		for i, c := range s.Body.List {
+			clause, _ := c.(*ast.CaseClause)
+			if clause == nil {
+				check.invalidAST(c, "incorrect expression switch case")
+				continue
+			}
+			check.caseValues(&x, clause.List, seen)
+			check.openScope(clause, "case")
+			inner := inner
+			if i+1 < len(s.Body.List) {
+				inner |= fallthroughOk
+			} else {
+				inner |= finalSwitchCase
+			}
+			check.stmtList(inner, clause.Body)
+			check.closeScope()
+		}
+
+	case *ast.TypeSwitchStmt:
+		inner |= breakOk
+		check.openScope(s, "type switch")
+		defer check.closeScope()
+
+		check.simpleStmt(s.Init)
+
+		// A type switch guard must be of the form:
+		//
+		//     TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
+		//
+		// The parser is checking syntactic correctness;
+		// remaining syntactic errors are considered AST errors here.
+		// TODO(gri) better factoring of error handling (invalid ASTs)
+		//
+		var lhs *ast.Ident // lhs identifier or nil
+		var rhs ast.Expr
+		switch guard := s.Assign.(type) {
+		case *ast.ExprStmt:
+			rhs = guard.X
+		case *ast.AssignStmt:
+			if len(guard.Lhs) != 1 || guard.Tok != token.DEFINE || len(guard.Rhs) != 1 {
+				check.invalidAST(s, "incorrect form of type switch guard")
+				return
+			}
+
+			lhs, _ = guard.Lhs[0].(*ast.Ident)
+			if lhs == nil {
+				check.invalidAST(s, "incorrect form of type switch guard")
+				return
+			}
+
+			if lhs.Name == "_" {
+				// _ := x.(type) is an invalid short variable declaration
+				check.softErrorf(lhs, _NoNewVar, "no new variable on left side of :=")
+				lhs = nil // avoid declared but not used error below
+			} else {
+				check.recordDef(lhs, nil) // lhs variable is implicitly declared in each cause clause
+			}
+
+			rhs = guard.Rhs[0]
+
+		default:
+			check.invalidAST(s, "incorrect form of type switch guard")
+			return
+		}
+
+		// rhs must be of the form: expr.(type) and expr must be an interface
+		expr, _ := rhs.(*ast.TypeAssertExpr)
+		if expr == nil || expr.Type != nil {
+			check.invalidAST(s, "incorrect form of type switch guard")
+			return
+		}
+		var x operand
+		check.expr(&x, expr.X)
+		if x.mode == invalid {
+			return
+		}
+		xtyp, _ := x.typ.Underlying().(*Interface)
+		if xtyp == nil {
+			check.errorf(&x, _InvalidTypeSwitch, "%s is not an interface", &x)
+			return
+		}
+
+		check.multipleDefaults(s.Body.List)
+
+		var lhsVars []*Var              // list of implicitly declared lhs variables
+		seen := make(map[Type]ast.Expr) // map of seen types to positions
+		for _, s := range s.Body.List {
+			clause, _ := s.(*ast.CaseClause)
+			if clause == nil {
+				check.invalidAST(s, "incorrect type switch case")
+				continue
+			}
+			// Check each type in this type switch case.
+			T := check.caseTypes(&x, xtyp, clause.List, seen)
+			check.openScope(clause, "case")
+			// If lhs exists, declare a corresponding variable in the case-local scope.
+			if lhs != nil {
+				// spec: "The TypeSwitchGuard may include a short variable declaration.
+				// When that form is used, the variable is declared at the beginning of
+				// the implicit block in each clause. In clauses with a case listing
+				// exactly one type, the variable has that type; otherwise, the variable
+				// has the type of the expression in the TypeSwitchGuard."
+				if len(clause.List) != 1 || T == nil {
+					T = x.typ
+				}
+				obj := NewVar(lhs.Pos(), check.pkg, lhs.Name, T)
+				scopePos := clause.Pos() + token.Pos(len("default")) // for default clause (len(List) == 0)
+				if n := len(clause.List); n > 0 {
+					scopePos = clause.List[n-1].End()
+				}
+				check.declare(check.scope, nil, obj, scopePos)
+				check.recordImplicit(clause, obj)
+				// For the "declared but not used" error, all lhs variables act as
+				// one; i.e., if any one of them is 'used', all of them are 'used'.
+				// Collect them for later analysis.
+				lhsVars = append(lhsVars, obj)
+			}
+			check.stmtList(inner, clause.Body)
+			check.closeScope()
+		}
+
+		// If lhs exists, we must have at least one lhs variable that was used.
+		if lhs != nil {
+			var used bool
+			for _, v := range lhsVars {
+				if v.used {
+					used = true
+				}
+				v.used = true // avoid usage error when checking entire function
+			}
+			if !used {
+				check.softErrorf(lhs, _UnusedVar, "%s declared but not used", lhs.Name)
+			}
+		}
+
+	case *ast.SelectStmt:
+		inner |= breakOk
+
+		check.multipleDefaults(s.Body.List)
+
+		for _, s := range s.Body.List {
+			clause, _ := s.(*ast.CommClause)
+			if clause == nil {
+				continue // error reported before
+			}
+
+			// clause.Comm must be a SendStmt, RecvStmt, or default case
+			valid := false
+			var rhs ast.Expr // rhs of RecvStmt, or nil
+			switch s := clause.Comm.(type) {
+			case nil, *ast.SendStmt:
+				valid = true
+			case *ast.AssignStmt:
+				if len(s.Rhs) == 1 {
+					rhs = s.Rhs[0]
+				}
+			case *ast.ExprStmt:
+				rhs = s.X
+			}
+
+			// if present, rhs must be a receive operation
+			if rhs != nil {
+				if x, _ := unparen(rhs).(*ast.UnaryExpr); x != nil && x.Op == token.ARROW {
+					valid = true
+				}
+			}
+
+			if !valid {
+				check.error(clause.Comm, _InvalidSelectCase, "select case must be send or receive (possibly with assignment)")
+				continue
+			}
+
+			check.openScope(s, "case")
+			if clause.Comm != nil {
+				check.stmt(inner, clause.Comm)
+			}
+			check.stmtList(inner, clause.Body)
+			check.closeScope()
+		}
+
+	case *ast.ForStmt:
+		inner |= breakOk | continueOk
+		check.openScope(s, "for")
+		defer check.closeScope()
+
+		check.simpleStmt(s.Init)
+		if s.Cond != nil {
+			var x operand
+			check.expr(&x, s.Cond)
+			if x.mode != invalid && !isBoolean(x.typ) {
+				check.error(s.Cond, _InvalidCond, "non-boolean condition in for statement")
+			}
+		}
+		check.simpleStmt(s.Post)
+		// spec: "The init statement may be a short variable
+		// declaration, but the post statement must not."
+		if s, _ := s.Post.(*ast.AssignStmt); s != nil && s.Tok == token.DEFINE {
+			check.softErrorf(s, _InvalidPostDecl, "cannot declare in post statement")
+			// Don't call useLHS here because we want to use the lhs in
+			// this erroneous statement so that we don't get errors about
+			// these lhs variables being declared but not used.
+			check.use(s.Lhs...) // avoid follow-up errors
+		}
+		check.stmt(inner, s.Body)
+
+	case *ast.RangeStmt:
+		inner |= breakOk | continueOk
+		check.openScope(s, "for")
+		defer check.closeScope()
+
+		// check expression to iterate over
+		var x operand
+		check.expr(&x, s.X)
+
+		// determine key/value types
+		var key, val Type
+		if x.mode != invalid {
+			switch typ := x.typ.Underlying().(type) {
+			case *Basic:
+				if isString(typ) {
+					key = Typ[Int]
+					val = universeRune // use 'rune' name
+				}
+			case *Array:
+				key = Typ[Int]
+				val = typ.elem
+			case *Slice:
+				key = Typ[Int]
+				val = typ.elem
+			case *Pointer:
+				if typ, _ := typ.base.Underlying().(*Array); typ != nil {
+					key = Typ[Int]
+					val = typ.elem
+				}
+			case *Map:
+				key = typ.key
+				val = typ.elem
+			case *Chan:
+				key = typ.elem
+				val = Typ[Invalid]
+				if typ.dir == SendOnly {
+					check.errorf(&x, _InvalidChanRange, "cannot range over send-only channel %s", &x)
+					// ok to continue
+				}
+				if s.Value != nil {
+					check.errorf(atPos(s.Value.Pos()), _InvalidIterVar, "iteration over %s permits only one iteration variable", &x)
+					// ok to continue
+				}
+			}
+		}
+
+		if key == nil {
+			check.errorf(&x, _InvalidRangeExpr, "cannot range over %s", &x)
+			// ok to continue
+		}
+
+		// check assignment to/declaration of iteration variables
+		// (irregular assignment, cannot easily map to existing assignment checks)
+
+		// lhs expressions and initialization value (rhs) types
+		lhs := [2]ast.Expr{s.Key, s.Value}
+		rhs := [2]Type{key, val} // key, val may be nil
+
+		if s.Tok == token.DEFINE {
+			// short variable declaration; variable scope starts after the range clause
+			// (the for loop opens a new scope, so variables on the lhs never redeclare
+			// previously declared variables)
+			var vars []*Var
+			for i, lhs := range lhs {
+				if lhs == nil {
+					continue
+				}
+
+				// determine lhs variable
+				var obj *Var
+				if ident, _ := lhs.(*ast.Ident); ident != nil {
+					// declare new variable
+					name := ident.Name
+					obj = NewVar(ident.Pos(), check.pkg, name, nil)
+					check.recordDef(ident, obj)
+					// _ variables don't count as new variables
+					if name != "_" {
+						vars = append(vars, obj)
+					}
+				} else {
+					check.invalidAST(lhs, "cannot declare %s", lhs)
+					obj = NewVar(lhs.Pos(), check.pkg, "_", nil) // dummy variable
+				}
+
+				// initialize lhs variable
+				if typ := rhs[i]; typ != nil {
+					x.mode = value
+					x.expr = lhs // we don't have a better rhs expression to use here
+					x.typ = typ
+					check.initVar(obj, &x, "range clause")
+				} else {
+					obj.typ = Typ[Invalid]
+					obj.used = true // don't complain about unused variable
+				}
+			}
+
+			// declare variables
+			if len(vars) > 0 {
+				scopePos := s.X.End()
+				for _, obj := range vars {
+					// spec: "The scope of a constant or variable identifier declared inside
+					// a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl
+					// for short variable declarations) and ends at the end of the innermost
+					// containing block."
+					check.declare(check.scope, nil /* recordDef already called */, obj, scopePos)
+				}
+			} else {
+				check.error(inNode(s, s.TokPos), _NoNewVar, "no new variables on left side of :=")
+			}
+		} else {
+			// ordinary assignment
+			for i, lhs := range lhs {
+				if lhs == nil {
+					continue
+				}
+				if typ := rhs[i]; typ != nil {
+					x.mode = value
+					x.expr = lhs // we don't have a better rhs expression to use here
+					x.typ = typ
+					check.assignVar(lhs, &x)
+				}
+			}
+		}
+
+		check.stmt(inner, s.Body)
+
+	default:
+		check.invalidAST(s, "invalid statement")
+	}
+}
diff --git a/src/go/types/testdata/blank.src b/src/go/types/testdata/blank.src
new file mode 100644
index 0000000..6a2507f
--- /dev/null
+++ b/src/go/types/testdata/blank.src
@@ -0,0 +1,5 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package _ /* ERROR invalid package name */
diff --git a/src/go/types/testdata/builtins.src b/src/go/types/testdata/builtins.src
new file mode 100644
index 0000000..98830eb
--- /dev/null
+++ b/src/go/types/testdata/builtins.src
@@ -0,0 +1,902 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// builtin calls
+
+package builtins
+
+import "unsafe"
+
+func f0() {}
+
+func append1() {
+	var b byte
+	var x int
+	var s []byte
+	_ = append() // ERROR not enough arguments
+	_ = append("foo" /* ERROR not a slice */ )
+	_ = append(nil /* ERROR not a slice */ , s)
+	_ = append(x /* ERROR not a slice */ , s)
+	_ = append(s)
+	_ = append(s, nil...)
+	append /* ERROR not used */ (s)
+
+	_ = append(s, b)
+	_ = append(s, x /* ERROR cannot use x */ )
+	_ = append(s, s /* ERROR cannot use s */ )
+	_ = append(s... /* ERROR can only use ... with matching parameter */ )
+	_ = append(s, b, s... /* ERROR can only use ... with matching parameter */ )
+	_ = append(s, 1, 2, 3)
+	_ = append(s, 1, 2, 3, x /* ERROR cannot use x */ , 5, 6, 6)
+	_ = append(s, 1, 2, s... /* ERROR can only use ... with matching parameter */ )
+	_ = append([]interface{}(nil), 1, 2, "foo", x, 3.1425, false)
+
+	type S []byte
+	type T string
+	var t T
+	_ = append(s, "foo" /* ERROR cannot use .* in argument to append */ )
+	_ = append(s, "foo"...)
+	_ = append(S(s), "foo" /* ERROR cannot use .* in argument to append */ )
+	_ = append(S(s), "foo"...)
+	_ = append(s, t /* ERROR cannot use t */ )
+	_ = append(s, t...)
+	_ = append(s, T("foo")...)
+	_ = append(S(s), t /* ERROR cannot use t */ )
+	_ = append(S(s), t...)
+	_ = append(S(s), T("foo")...)
+	_ = append([]string{}, t /* ERROR cannot use t */ , "foo")
+	_ = append([]T{}, t, "foo")
+}
+
+// from the spec
+func append2() {
+	s0 := []int{0, 0}
+	s1 := append(s0, 2)                // append a single element     s1 == []int{0, 0, 2}
+	s2 := append(s1, 3, 5, 7)          // append multiple elements    s2 == []int{0, 0, 2, 3, 5, 7}
+	s3 := append(s2, s0...)            // append a slice              s3 == []int{0, 0, 2, 3, 5, 7, 0, 0}
+	s4 := append(s3[3:6], s3[2:]...)   // append overlapping slice    s4 == []int{3, 5, 7, 2, 3, 5, 7, 0, 0}
+
+	var t []interface{}
+	t = append(t, 42, 3.1415, "foo")   //                             t == []interface{}{42, 3.1415, "foo"}
+
+	var b []byte
+	b = append(b, "bar"...)            // append string contents      b == []byte{'b', 'a', 'r' }
+
+	_ = s4
+}
+
+func append3() {
+	f1 := func() (s []int) { return }
+	f2 := func() (s []int, x int) { return }
+	f3 := func() (s []int, x, y int) { return }
+	f5 := func() (s []interface{}, x int, y float32, z string, b bool) { return }
+	ff := func() (int, float32) { return 0, 0 }
+	_ = append(f0 /* ERROR used as value */ ())
+	_ = append(f1())
+	_ = append(f2())
+	_ = append(f3())
+	_ = append(f5())
+	_ = append(ff /* ERROR not a slice */ ()) // TODO(gri) better error message
+}
+
+func cap1() {
+	var a [10]bool
+	var p *[20]int
+	var c chan string
+	_ = cap() // ERROR not enough arguments
+	_ = cap(1, 2) // ERROR too many arguments
+	_ = cap(42 /* ERROR invalid */)
+	const _3 = cap(a)
+	assert(_3 == 10)
+	const _4 = cap(p)
+	assert(_4 == 20)
+	_ = cap(c)
+	cap /* ERROR not used */ (c)
+
+	// issue 4744
+	type T struct{ a [10]int }
+	const _ = cap(((*T)(nil)).a)
+
+	var s [][]byte
+	_ = cap(s)
+	_ = cap(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func cap2() {
+	f1a := func() (a [10]int) { return }
+	f1s := func() (s []int) { return }
+	f2 := func() (s []int, x int) { return }
+	_ = cap(f0 /* ERROR used as value */ ())
+	_ = cap(f1a())
+	_ = cap(f1s())
+	_ = cap(f2()) // ERROR too many arguments
+}
+
+// test cases for issue 7387
+func cap3() {
+	var f = func() int { return 0 }
+	var x = f()
+	const (
+		_ = cap([4]int{})
+		_ = cap([4]int{x})
+		_ = cap /* ERROR not constant */ ([4]int{f()})
+		_ = cap /* ERROR not constant */ ([4]int{cap([]int{})})
+		_ = cap([4]int{cap([4]int{})})
+	)
+	var y float64
+	var z complex128
+	const (
+		_ = cap([4]float64{})
+		_ = cap([4]float64{y})
+		_ = cap([4]float64{real(2i)})
+		_ = cap /* ERROR not constant */ ([4]float64{real(z)})
+	)
+	var ch chan [10]int
+	const (
+		_ = cap /* ERROR not constant */ (<-ch)
+		_ = cap /* ERROR not constant */ ([4]int{(<-ch)[0]})
+	)
+}
+
+func close1() {
+	var c chan int
+	var r <-chan int
+	close() // ERROR not enough arguments
+	close(1, 2) // ERROR too many arguments
+	close(42 /* ERROR not a channel */)
+	close(r /* ERROR receive-only channel */)
+	close(c)
+	_ = close /* ERROR used as value */ (c)
+
+	var s []chan int
+	close(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func close2() {
+	f1 := func() (ch chan int) { return }
+	f2 := func() (ch chan int, x int) { return }
+	close(f0 /* ERROR used as value */ ())
+	close(f1())
+	close(f2()) // ERROR too many arguments
+}
+
+func complex1() {
+	var i32 int32
+	var f32 float32
+	var f64 float64
+	var c64 complex64
+	var c128 complex128
+	_ = complex() // ERROR not enough arguments
+	_ = complex(1) // ERROR not enough arguments
+	_ = complex(true /* ERROR mismatched types */ , 0)
+	_ = complex(i32 /* ERROR expected floating-point */ , 0)
+	_ = complex("foo" /* ERROR mismatched types */ , 0)
+	_ = complex(c64 /* ERROR expected floating-point */ , 0)
+	_ = complex(0 /* ERROR mismatched types */ , true)
+	_ = complex(0 /* ERROR expected floating-point */ , i32)
+	_ = complex(0 /* ERROR mismatched types */ , "foo")
+	_ = complex(0 /* ERROR expected floating-point */ , c64)
+	_ = complex(f32, f32)
+	_ = complex(f32, 1)
+	_ = complex(f32, 1.0)
+	_ = complex(f32, 'a')
+	_ = complex(f64, f64)
+	_ = complex(f64, 1)
+	_ = complex(f64, 1.0)
+	_ = complex(f64, 'a')
+	_ = complex(f32 /* ERROR mismatched types */ , f64)
+	_ = complex(f64 /* ERROR mismatched types */ , f32)
+	_ = complex(1, 1)
+	_ = complex(1, 1.1)
+	_ = complex(1, 'a')
+	complex /* ERROR not used */ (1, 2)
+
+	var _ complex64 = complex(f32, f32)
+	var _ complex64 = complex /* ERROR cannot use .* in variable declaration */ (f64, f64)
+
+	var _ complex128 = complex /* ERROR cannot use .* in variable declaration */ (f32, f32)
+	var _ complex128 = complex(f64, f64)
+
+	// untyped constants
+	const _ int = complex(1, 0)
+	const _ float32 = complex(1, 0)
+	const _ complex64 = complex(1, 0)
+	const _ complex128 = complex(1, 0)
+	const _ = complex(0i, 0i)
+	const _ = complex(0i, 0)
+	const _ int = 1.0 + complex(1, 0i)
+
+	const _ int = complex /* ERROR int */ (1.1, 0)
+	const _ float32 = complex /* ERROR float32 */ (1, 2)
+
+	// untyped values
+	var s uint
+	_ = complex(1 /* ERROR integer */ <<s, 0)
+	const _ = complex /* ERROR not constant */ (1 /* ERROR integer */ <<s, 0)
+	var _ int = complex /* ERROR cannot use .* in variable declaration */ (1 /* ERROR integer */ <<s, 0)
+
+	// floating-point argument types must be identical
+	type F32 float32
+	type F64 float64
+	var x32 F32
+	var x64 F64
+	c64 = complex(x32, x32)
+	_ = complex(x32 /* ERROR mismatched types */ , f32)
+	_ = complex(f32 /* ERROR mismatched types */ , x32)
+	c128 = complex(x64, x64)
+	_ = c128
+	_ = complex(x64 /* ERROR mismatched types */ , f64)
+	_ = complex(f64 /* ERROR mismatched types */ , x64)
+
+	var t []float32
+	_ = complex(t... /* ERROR invalid use of \.\.\. */ )
+}
+
+func complex2() {
+	f1 := func() (x float32) { return }
+	f2 := func() (x, y float32) { return }
+	f3 := func() (x, y, z float32) { return }
+	_ = complex(f0 /* ERROR used as value */ ())
+	_ = complex(f1()) // ERROR not enough arguments
+	_ = complex(f2())
+	_ = complex(f3()) // ERROR too many arguments
+}
+
+func copy1() {
+	copy() // ERROR not enough arguments
+	copy("foo") // ERROR not enough arguments
+	copy([ /* ERROR copy expects slice arguments */ ...]int{}, []int{})
+	copy([ /* ERROR copy expects slice arguments */ ]int{}, [...]int{})
+	copy([ /* ERROR different element types */ ]int8{}, "foo")
+
+	// spec examples
+	var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7}
+	var s = make([]int, 6)
+	var b = make([]byte, 5)
+	n1 := copy(s, a[0:])            // n1 == 6, s == []int{0, 1, 2, 3, 4, 5}
+	n2 := copy(s, s[2:])            // n2 == 4, s == []int{2, 3, 4, 5, 4, 5}
+	n3 := copy(b, "Hello, World!")  // n3 == 5, b == []byte("Hello")
+	_, _, _ = n1, n2, n3
+
+	var t [][]int
+	copy(t, t)
+	copy(t /* ERROR copy expects slice arguments */ , nil)
+	copy(nil /* ERROR copy expects slice arguments */ , t)
+	copy(nil /* ERROR copy expects slice arguments */ , nil)
+	copy(t... /* ERROR invalid use of \.\.\. */ )
+}
+
+func copy2() {
+	f1 := func() (a []int) { return }
+	f2 := func() (a, b []int) { return }
+	f3 := func() (a, b, c []int) { return }
+	copy(f0 /* ERROR used as value */ ())
+	copy(f1()) // ERROR not enough arguments
+	copy(f2())
+	copy(f3()) // ERROR too many arguments
+}
+
+func delete1() {
+	var m map[string]int
+	var s string
+	delete() // ERROR not enough arguments
+	delete(1) // ERROR not enough arguments
+	delete(1, 2, 3) // ERROR too many arguments
+	delete(m, 0 /* ERROR not assignable */)
+	delete(m, s)
+	_ = delete /* ERROR used as value */ (m, s)
+
+	var t []map[string]string
+	delete(t... /* ERROR invalid use of \.\.\. */ )
+}
+
+func delete2() {
+	f1 := func() (m map[string]int) { return }
+	f2 := func() (m map[string]int, k string) { return }
+	f3 := func() (m map[string]int, k string, x float32) { return }
+	delete(f0 /* ERROR used as value */ ())
+	delete(f1()) // ERROR not enough arguments
+	delete(f2())
+	delete(f3()) // ERROR too many arguments
+}
+
+func imag1() {
+	var f32 float32
+	var f64 float64
+	var c64 complex64
+	var c128 complex128
+	_ = imag() // ERROR not enough arguments
+	_ = imag(1, 2) // ERROR too many arguments
+	_ = imag(10)
+	_ = imag(2.7182818)
+	_ = imag("foo" /* ERROR expected complex */)
+	_ = imag('a')
+	const _5 = imag(1 + 2i)
+	assert(_5 == 2)
+	f32 = _5
+	f64 = _5
+	const _6 = imag(0i)
+	assert(_6 == 0)
+	f32 = imag(c64)
+	f64 = imag(c128)
+	f32 = imag /* ERROR cannot use .* in assignment */ (c128)
+	f64 = imag /* ERROR cannot use .* in assignment */ (c64)
+	imag /* ERROR not used */ (c64)
+	_, _ = f32, f64
+
+	// complex type may not be predeclared
+	type C64 complex64
+	type C128 complex128
+	var x64 C64
+	var x128 C128
+	f32 = imag(x64)
+	f64 = imag(x128)
+
+	var a []complex64
+	_ = imag(a... /* ERROR invalid use of \.\.\. */ )
+
+	// if argument is untyped, result is untyped
+	const _ byte = imag(1.2 + 3i)
+	const _ complex128 = imag(1.2 + 3i)
+
+	// lhs constant shift operands are typed as complex128
+	var s uint
+	_ = imag(1 /* ERROR must be integer */ << s)
+}
+
+func imag2() {
+	f1 := func() (x complex128) { return }
+	f2 := func() (x, y complex128) { return }
+	_ = imag(f0 /* ERROR used as value */ ())
+	_ = imag(f1())
+	_ = imag(f2()) // ERROR too many arguments
+}
+
+func len1() {
+	const c = "foobar"
+	var a [10]bool
+	var p *[20]int
+	var m map[string]complex128
+	_ = len() // ERROR not enough arguments
+	_ = len(1, 2) // ERROR too many arguments
+	_ = len(42 /* ERROR invalid */)
+	const _3 = len(c)
+	assert(_3 == 6)
+	const _4 = len(a)
+	assert(_4 == 10)
+	const _5 = len(p)
+	assert(_5 == 20)
+	_ = len(m)
+	len /* ERROR not used */ (c)
+
+	// esoteric case
+	var t string
+	var hash map[interface{}][]*[10]int
+	const n = len /* ERROR not constant */ (hash[recover()][len(t)])
+	assert(n == 10) // ok because n has unknown value and no error is reported
+	var ch <-chan int
+	const nn = len /* ERROR not constant */ (hash[<-ch][len(t)])
+
+	// issue 4744
+	type T struct{ a [10]int }
+	const _ = len(((*T)(nil)).a)
+
+	var s [][]byte
+	_ = len(s)
+	_ = len(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func len2() {
+	f1 := func() (x []int) { return }
+	f2 := func() (x, y []int) { return }
+	_ = len(f0 /* ERROR used as value */ ())
+	_ = len(f1())
+	_ = len(f2()) // ERROR too many arguments
+}
+
+// test cases for issue 7387
+func len3() {
+	var f = func() int { return 0 }
+	var x = f()
+	const (
+		_ = len([4]int{})
+		_ = len([4]int{x})
+		_ = len /* ERROR not constant */ ([4]int{f()})
+		_ = len /* ERROR not constant */ ([4]int{len([]int{})})
+		_ = len([4]int{len([4]int{})})
+	)
+	var y float64
+	var z complex128
+	const (
+		_ = len([4]float64{})
+		_ = len([4]float64{y})
+		_ = len([4]float64{real(2i)})
+		_ = len /* ERROR not constant */ ([4]float64{real(z)})
+	)
+	var ch chan [10]int
+	const (
+		_ = len /* ERROR not constant */ (<-ch)
+		_ = len /* ERROR not constant */ ([4]int{(<-ch)[0]})
+	)
+}
+
+func make1() {
+	var n int
+	var m float32
+	var s uint
+
+	_ = make() // ERROR not enough arguments
+	_ = make(1 /* ERROR not a type */)
+	_ = make(int /* ERROR cannot make */)
+
+	// slices
+	_ = make/* ERROR arguments */ ([]int)
+	_ = make/* ERROR arguments */ ([]int, 2, 3, 4)
+	_ = make([]int, int /* ERROR not an expression */)
+	_ = make([]int, 10, float32 /* ERROR not an expression */)
+	_ = make([]int, "foo" /* ERROR cannot convert */)
+	_ = make([]int, 10, 2.3 /* ERROR truncated */)
+	_ = make([]int, 5, 10.0)
+	_ = make([]int, 0i)
+	_ = make([]int, 1.0)
+	_ = make([]int, 1.0<<s)
+	_ = make([]int, 1.1 /* ERROR int */ <<s)
+	_ = make([]int, - /* ERROR must not be negative */ 1, 10)
+	_ = make([]int, 0, - /* ERROR must not be negative */ 1)
+	_ = make([]int, - /* ERROR must not be negative */ 1, - /* ERROR must not be negative */ 1)
+	_ = make([]int, 1 /* ERROR overflows */ <<100, 1 /* ERROR overflows */ <<100)
+	_ = make([]int, 10 /* ERROR length and capacity swapped */ , 9)
+	_ = make([]int, 1 /* ERROR overflows */ <<100, 12345)
+	_ = make([]int, m /* ERROR must be integer */ )
+        _ = &make /* ERROR cannot take address */ ([]int, 0)
+
+	// maps
+	_ = make /* ERROR arguments */ (map[int]string, 10, 20)
+	_ = make(map[int]float32, int /* ERROR not an expression */)
+	_ = make(map[int]float32, "foo" /* ERROR cannot convert */)
+	_ = make(map[int]float32, 10)
+	_ = make(map[int]float32, n)
+	_ = make(map[int]float32, int64(n))
+	_ = make(map[string]bool, 10.0)
+	_ = make(map[string]bool, 10.0<<s)
+        _ = &make /* ERROR cannot take address */ (map[string]bool)
+
+	// channels
+	_ = make /* ERROR arguments */ (chan int, 10, 20)
+	_ = make(chan int, int /* ERROR not an expression */)
+	_ = make(chan<- int, "foo" /* ERROR cannot convert */)
+	_ = make(chan int, - /* ERROR must not be negative */ 10)
+	_ = make(<-chan float64, 10)
+	_ = make(chan chan int, n)
+	_ = make(chan string, int64(n))
+	_ = make(chan bool, 10.0)
+	_ = make(chan bool, 10.0<<s)
+        _ = &make /* ERROR cannot take address */ (chan bool)
+
+	make /* ERROR not used */ ([]int, 10)
+
+	var t []int
+	_ = make([]int, t[0], t[1])
+	_ = make([]int, t... /* ERROR invalid use of \.\.\. */ )
+}
+
+func make2() {
+	f1 /* ERROR not used */ := func() (x []int) { return }
+	_ = make(f0 /* ERROR not a type */ ())
+	_ = make(f1 /* ERROR not a type */ ())
+}
+
+func new1() {
+	_ = new() // ERROR not enough arguments
+	_ = new(1, 2) // ERROR too many arguments
+	_ = new("foo" /* ERROR not a type */)
+	p := new(float64)
+	_ = new(struct{ x, y int })
+	q := new(*float64)
+	_ = *p == **q
+	new /* ERROR not used */ (int)
+        _ = &new /* ERROR cannot take address */ (int)
+
+	_ = new(int... /* ERROR invalid use of \.\.\. */ )
+}
+
+func new2() {
+	f1 /* ERROR not used */ := func() (x []int) { return }
+	_ = new(f0 /* ERROR not a type */ ())
+	_ = new(f1 /* ERROR not a type */ ())
+}
+
+func panic1() {
+	panic() // ERROR not enough arguments
+	panic(1, 2) // ERROR too many arguments
+	panic(0)
+	panic("foo")
+	panic(false)
+	panic(1<<10)
+	panic(1 /* ERROR overflows */ <<1000)
+	_ = panic /* ERROR used as value */ (0)
+
+	var s []byte
+	panic(s)
+	panic(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func panic2() {
+	f1 := func() (x int) { return }
+	f2 := func() (x, y int) { return }
+	panic(f0 /* ERROR used as value */ ())
+	panic(f1())
+	panic(f2()) // ERROR too many arguments
+}
+
+func print1() {
+	print()
+	print(1)
+	print(1, 2)
+	print("foo")
+	print(2.718281828)
+	print(false)
+	print(1<<10)
+	print(1 /* ERROR overflows */ <<1000)
+	println(nil /* ERROR untyped nil */ )
+
+	var s []int
+	print(s... /* ERROR invalid use of \.\.\. */ )
+	_ = print /* ERROR used as value */ ()
+}
+
+func print2() {
+	f1 := func() (x int) { return }
+	f2 := func() (x, y int) { return }
+	f3 := func() (x int, y float32, z string) { return }
+	print(f0 /* ERROR used as value */ ())
+	print(f1())
+	print(f2())
+	print(f3())
+}
+
+func println1() {
+	println()
+	println(1)
+	println(1, 2)
+	println("foo")
+	println(2.718281828)
+	println(false)
+	println(1<<10)
+	println(1 /* ERROR overflows */ <<1000)
+	println(nil /* ERROR untyped nil */ )
+
+	var s []int
+	println(s... /* ERROR invalid use of \.\.\. */ )
+	_ = println /* ERROR used as value */ ()
+}
+
+func println2() {
+	f1 := func() (x int) { return }
+	f2 := func() (x, y int) { return }
+	f3 := func() (x int, y float32, z string) { return }
+	println(f0 /* ERROR used as value */ ())
+	println(f1())
+	println(f2())
+	println(f3())
+}
+
+func real1() {
+	var f32 float32
+	var f64 float64
+	var c64 complex64
+	var c128 complex128
+	_ = real() // ERROR not enough arguments
+	_ = real(1, 2) // ERROR too many arguments
+	_ = real(10)
+	_ = real(2.7182818)
+	_ = real("foo" /* ERROR expected complex */)
+	const _5 = real(1 + 2i)
+	assert(_5 == 1)
+	f32 = _5
+	f64 = _5
+	const _6 = real(0i)
+	assert(_6 == 0)
+	f32 = real(c64)
+	f64 = real(c128)
+	f32 = real /* ERROR cannot use .* in assignment */ (c128)
+	f64 = real /* ERROR cannot use .* in assignment */ (c64)
+	real /* ERROR not used */ (c64)
+
+	// complex type may not be predeclared
+	type C64 complex64
+	type C128 complex128
+	var x64 C64
+	var x128 C128
+	f32 = imag(x64)
+	f64 = imag(x128)
+	_, _ = f32, f64
+
+	var a []complex64
+	_ = real(a... /* ERROR invalid use of \.\.\. */ )
+
+	// if argument is untyped, result is untyped
+	const _ byte = real(1 + 2.3i)
+	const _ complex128 = real(1 + 2.3i)
+
+	// lhs constant shift operands are typed as complex128
+	var s uint
+	_ = real(1 /* ERROR must be integer */ << s)
+}
+
+func real2() {
+	f1 := func() (x complex128) { return }
+	f2 := func() (x, y complex128) { return }
+	_ = real(f0 /* ERROR used as value */ ())
+	_ = real(f1())
+	_ = real(f2()) // ERROR too many arguments
+}
+
+func recover1() {
+	_ = recover()
+	_ = recover(10) // ERROR too many arguments
+	recover()
+
+	var s []int
+	recover(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func recover2() {
+	f1 := func() (x int) { return }
+	f2 := func() (x, y int) { return }
+	_ = recover(f0 /* ERROR used as value */ ())
+	_ = recover(f1()) // ERROR too many arguments
+	_ = recover(f2()) // ERROR too many arguments
+}
+
+// assuming types.DefaultPtrSize == 8
+type S0 struct{      // offset
+	a bool       //  0
+	b rune       //  4
+	c *int       //  8
+	d bool       // 16
+	e complex128 // 24
+}                    // 40
+
+type S1 struct{   // offset
+	x float32 //  0
+	y string  //  8
+	z *S1     // 24
+	S0        // 32
+}                 // 72
+
+type S2 struct{ // offset
+	*S1     //  0
+}               //  8
+
+type S3 struct { // offset
+	a int64  //  0
+	b int32  //  8
+}                // 12
+
+type S4 struct { // offset
+	S3       //  0
+	int32    // 12
+}                // 16
+
+type S5 struct {   // offset
+	a [3]int32 //  0
+	b int32    // 12
+}                  // 16
+
+func (S2) m() {}
+
+func Alignof1() {
+	var x int
+	_ = unsafe.Alignof() // ERROR not enough arguments
+	_ = unsafe.Alignof(1, 2) // ERROR too many arguments
+	_ = unsafe.Alignof(int /* ERROR not an expression */)
+	_ = unsafe.Alignof(42)
+	_ = unsafe.Alignof(new(struct{}))
+	_ = unsafe.Alignof(1<<10)
+	_ = unsafe.Alignof(1 /* ERROR overflows */ <<1000)
+	_ = unsafe.Alignof(nil /* ERROR "untyped nil */ )
+	unsafe /* ERROR not used */ .Alignof(x)
+
+	var y S0
+	assert(unsafe.Alignof(y.a) == 1)
+	assert(unsafe.Alignof(y.b) == 4)
+	assert(unsafe.Alignof(y.c) == 8)
+	assert(unsafe.Alignof(y.d) == 1)
+	assert(unsafe.Alignof(y.e) == 8)
+
+	var s []byte
+	_ = unsafe.Alignof(s)
+	_ = unsafe.Alignof(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func Alignof2() {
+	f1 := func() (x int32) { return }
+	f2 := func() (x, y int32) { return }
+	_ = unsafe.Alignof(f0 /* ERROR used as value */ ())
+	assert(unsafe.Alignof(f1()) == 4)
+	_ = unsafe.Alignof(f2()) // ERROR too many arguments
+}
+
+func Offsetof1() {
+	var x struct{ f int }
+	_ = unsafe.Offsetof() // ERROR not enough arguments
+	_ = unsafe.Offsetof(1, 2) // ERROR too many arguments
+	_ = unsafe.Offsetof(int /* ERROR not a selector expression */ )
+	_ = unsafe.Offsetof(x /* ERROR not a selector expression */ )
+	_ = unsafe.Offsetof(nil /* ERROR not a selector expression */ )
+	_ = unsafe.Offsetof(x.f)
+	_ = unsafe.Offsetof((x.f))
+	_ = unsafe.Offsetof((((((((x))).f)))))
+	unsafe /* ERROR not used */ .Offsetof(x.f)
+
+	var y0 S0
+	assert(unsafe.Offsetof(y0.a) == 0)
+	assert(unsafe.Offsetof(y0.b) == 4)
+	assert(unsafe.Offsetof(y0.c) == 8)
+	assert(unsafe.Offsetof(y0.d) == 16)
+	assert(unsafe.Offsetof(y0.e) == 24)
+
+	var y1 S1
+	assert(unsafe.Offsetof(y1.x) == 0)
+	assert(unsafe.Offsetof(y1.y) == 8)
+	assert(unsafe.Offsetof(y1.z) == 24)
+	assert(unsafe.Offsetof(y1.S0) == 32)
+
+	assert(unsafe.Offsetof(y1.S0.a) == 0) // relative to S0
+	assert(unsafe.Offsetof(y1.a) == 32)   // relative to S1
+	assert(unsafe.Offsetof(y1.b) == 36)   // relative to S1
+	assert(unsafe.Offsetof(y1.c) == 40)   // relative to S1
+	assert(unsafe.Offsetof(y1.d) == 48)   // relative to S1
+	assert(unsafe.Offsetof(y1.e) == 56)   // relative to S1
+
+	var y1p *S1
+	assert(unsafe.Offsetof(y1p.S0) == 32)
+
+	type P *S1
+	var p P = y1p
+	assert(unsafe.Offsetof(p.S0) == 32)
+
+	var y2 S2
+	assert(unsafe.Offsetof(y2.S1) == 0)
+	_ = unsafe.Offsetof(y2 /* ERROR embedded via a pointer */ .x)
+	_ = unsafe.Offsetof(y2 /* ERROR method value */ .m)
+
+	var s []byte
+	_ = unsafe.Offsetof(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func Offsetof2() {
+	f1 := func() (x int32) { return }
+	f2 := func() (x, y int32) { return }
+	_ = unsafe.Offsetof(f0 /* ERROR not a selector expression */ ())
+	_ = unsafe.Offsetof(f1 /* ERROR not a selector expression */ ())
+	_ = unsafe.Offsetof(f2 /* ERROR not a selector expression */ ())
+}
+
+func Sizeof1() {
+	var x int
+	_ = unsafe.Sizeof() // ERROR not enough arguments
+	_ = unsafe.Sizeof(1, 2) // ERROR too many arguments
+	_ = unsafe.Sizeof(int /* ERROR not an expression */)
+	_ = unsafe.Sizeof(42)
+	_ = unsafe.Sizeof(new(complex128))
+	_ = unsafe.Sizeof(1<<10)
+	_ = unsafe.Sizeof(1 /* ERROR overflows */ <<1000)
+	_ = unsafe.Sizeof(nil /* ERROR untyped nil */ )
+	unsafe /* ERROR not used */ .Sizeof(x)
+
+	// basic types have size guarantees
+	assert(unsafe.Sizeof(byte(0)) == 1)
+	assert(unsafe.Sizeof(uint8(0)) == 1)
+	assert(unsafe.Sizeof(int8(0)) == 1)
+	assert(unsafe.Sizeof(uint16(0)) == 2)
+	assert(unsafe.Sizeof(int16(0)) == 2)
+	assert(unsafe.Sizeof(uint32(0)) == 4)
+	assert(unsafe.Sizeof(int32(0)) == 4)
+	assert(unsafe.Sizeof(float32(0)) == 4)
+	assert(unsafe.Sizeof(uint64(0)) == 8)
+	assert(unsafe.Sizeof(int64(0)) == 8)
+	assert(unsafe.Sizeof(float64(0)) == 8)
+	assert(unsafe.Sizeof(complex64(0)) == 8)
+	assert(unsafe.Sizeof(complex128(0)) == 16)
+
+	var y0 S0
+	assert(unsafe.Sizeof(y0.a) == 1)
+	assert(unsafe.Sizeof(y0.b) == 4)
+	assert(unsafe.Sizeof(y0.c) == 8)
+	assert(unsafe.Sizeof(y0.d) == 1)
+	assert(unsafe.Sizeof(y0.e) == 16)
+	assert(unsafe.Sizeof(y0) == 40)
+
+	var y1 S1
+	assert(unsafe.Sizeof(y1) == 72)
+
+	var y2 S2
+	assert(unsafe.Sizeof(y2) == 8)
+
+	var y3 S3
+	assert(unsafe.Sizeof(y3) == 12)
+
+	var y4 S4
+	assert(unsafe.Sizeof(y4) == 16)
+
+	var y5 S5
+	assert(unsafe.Sizeof(y5) == 16)
+
+	var a3 [10]S3
+	assert(unsafe.Sizeof(a3) == 156)
+
+	// test case for issue 5670
+	type T struct {
+		a int32
+		_ int32
+		c int32
+	}
+	assert(unsafe.Sizeof(T{}) == 12)
+
+	var s []byte
+	_ = unsafe.Sizeof(s)
+	_ = unsafe.Sizeof(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func Sizeof2() {
+	f1 := func() (x int64) { return }
+	f2 := func() (x, y int64) { return }
+	_ = unsafe.Sizeof(f0 /* ERROR used as value */ ())
+	assert(unsafe.Sizeof(f1()) == 8)
+	_ = unsafe.Sizeof(f2()) // ERROR too many arguments
+}
+
+// self-testing only
+func assert1() {
+	var x int
+	assert() /* ERROR not enough arguments */
+	assert(1, 2) /* ERROR too many arguments */
+	assert("foo" /* ERROR boolean constant */ )
+	assert(x /* ERROR boolean constant */)
+	assert(true)
+	assert /* ERROR failed */ (false)
+	_ = assert(true)
+
+	var s []byte
+	assert(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func assert2() {
+	f1 := func() (x bool) { return }
+	f2 := func() (x bool) { return }
+	assert(f0 /* ERROR used as value */ ())
+	assert(f1 /* ERROR boolean constant */ ())
+	assert(f2 /* ERROR boolean constant */ ())
+}
+
+// self-testing only
+func trace1() {
+	// Uncomment the code below to test trace - will produce console output
+	// _ = trace /* ERROR no value */ ()
+	// _ = trace(1)
+	// _ = trace(true, 1.2, '\'', "foo", 42i, "foo" <= "bar")
+
+	var s []byte
+	trace(s... /* ERROR invalid use of \.\.\. */ )
+}
+
+func trace2() {
+	f1 := func() (x int) { return }
+	f2 := func() (x int, y string) { return }
+	f3 := func() (x int, y string, z []int) { return }
+	_ = f1
+	_ = f2
+	_ = f3
+	// Uncomment the code below to test trace - will produce console output
+	// trace(f0())
+	// trace(f1())
+	// trace(f2())
+	// trace(f3())
+	// trace(f0(), 1)
+	// trace(f1(), 1, 2)
+	// trace(f2(), 1, 2, 3)
+	// trace(f3(), 1, 2, 3, 4)
+}
diff --git a/src/go/types/testdata/const0.src b/src/go/types/testdata/const0.src
new file mode 100644
index 0000000..adbbf28
--- /dev/null
+++ b/src/go/types/testdata/const0.src
@@ -0,0 +1,350 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// constant declarations
+
+package const0
+
+import "unsafe"
+
+// constants declarations must be initialized by constants
+var x = 0
+const c0 = x /* ERROR "not constant" */
+
+// typed constants must have constant types
+const _ interface /* ERROR invalid constant type */ {} = 0
+
+func _ () {
+	const _ interface /* ERROR invalid constant type */ {} = 0
+	for i := 0; i < 10; i++ {} // don't crash with non-nil iota here
+}
+
+// untyped constants
+const (
+	// boolean values
+	ub0 = false
+	ub1 = true
+	ub2 = 2 < 1
+	ub3 = ui1 == uf1
+	ub4 = true /* ERROR "cannot convert" */ == 0
+
+	// integer values
+	ui0 = 0
+	ui1 = 1
+	ui2 = 42
+	ui3 = 3141592653589793238462643383279502884197169399375105820974944592307816406286
+	ui4 = -10
+
+	ui5 = ui0 + ui1
+	ui6 = ui1 - ui1
+	ui7 = ui2 * ui1
+	ui8 = ui3 / ui3
+	ui9 = ui3 % ui3
+
+	ui10 = 1 / 0 /* ERROR "division by zero" */
+	ui11 = ui1 / 0 /* ERROR "division by zero" */
+	ui12 = ui3 / ui0 /* ERROR "division by zero" */
+	ui13 = 1 % 0 /* ERROR "division by zero" */
+	ui14 = ui1 % 0 /* ERROR "division by zero" */
+	ui15 = ui3 % ui0 /* ERROR "division by zero" */
+
+	ui16 = ui2 & ui3
+	ui17 = ui2 | ui3
+	ui18 = ui2 ^ ui3
+	ui19 = 1 /* ERROR "invalid operation" */ % 1.0
+
+	// floating point values
+	uf0 = 0.
+	uf1 = 1.
+	uf2 = 4.2e1
+	uf3 = 3.141592653589793238462643383279502884197169399375105820974944592307816406286
+	uf4 = 1e-1
+
+	uf5 = uf0 + uf1
+	uf6 = uf1 - uf1
+	uf7 = uf2 * uf1
+	uf8 = uf3 / uf3
+	uf9 = uf3 /* ERROR "not defined" */ % uf3
+
+	uf10 = 1 / 0 /* ERROR "division by zero" */
+	uf11 = uf1 / 0 /* ERROR "division by zero" */
+	uf12 = uf3 / uf0 /* ERROR "division by zero" */
+
+	uf16 = uf2 /* ERROR "not defined" */ & uf3
+	uf17 = uf2 /* ERROR "not defined" */ | uf3
+	uf18 = uf2 /* ERROR "not defined" */ ^ uf3
+
+	// complex values
+	uc0 = 0.i
+	uc1 = 1.i
+	uc2 = 4.2e1i
+	uc3 = 3.141592653589793238462643383279502884197169399375105820974944592307816406286i
+	uc4 = 1e-1i
+
+	uc5 = uc0 + uc1
+	uc6 = uc1 - uc1
+	uc7 = uc2 * uc1
+	uc8 = uc3 / uc3
+	uc9 = uc3 /* ERROR "not defined" */ % uc3
+
+	uc10 = 1 / 0 /* ERROR "division by zero" */
+	uc11 = uc1 / 0 /* ERROR "division by zero" */
+	uc12 = uc3 / uc0 /* ERROR "division by zero" */
+
+	uc16 = uc2 /* ERROR "not defined" */ & uc3
+	uc17 = uc2 /* ERROR "not defined" */ | uc3
+	uc18 = uc2 /* ERROR "not defined" */ ^ uc3
+)
+
+type (
+	mybool bool
+	myint int
+	myfloat float64
+	mycomplex complex128
+)
+
+// typed constants
+const (
+	// boolean values
+	tb0 bool = false
+	tb1 bool = true
+	tb2 mybool = 2 < 1
+	tb3 mybool = ti1 /* ERROR "mismatched types" */ == tf1
+
+	// integer values
+	ti0 int8 = ui0
+	ti1 int32 = ui1
+	ti2 int64 = ui2
+	ti3 myint = ui3 /* ERROR "overflows" */
+	ti4 myint = ui4
+
+	ti5 = ti0 /* ERROR "mismatched types" */ + ti1
+	ti6 = ti1 - ti1
+	ti7 = ti2 /* ERROR "mismatched types" */ * ti1
+	ti8 = ti3 / ti3
+	ti9 = ti3 % ti3
+
+	ti10 = 1 / 0 /* ERROR "division by zero" */
+	ti11 = ti1 / 0 /* ERROR "division by zero" */
+	ti12 = ti3 /* ERROR "mismatched types" */ / ti0
+	ti13 = 1 % 0 /* ERROR "division by zero" */
+	ti14 = ti1 % 0 /* ERROR "division by zero" */
+	ti15 = ti3 /* ERROR "mismatched types" */ % ti0
+
+	ti16 = ti2 /* ERROR "mismatched types" */ & ti3
+	ti17 = ti2 /* ERROR "mismatched types" */ | ti4
+	ti18 = ti2 ^ ti5 // no mismatched types error because the type of ti5 is unknown
+
+	// floating point values
+	tf0 float32 = 0.
+	tf1 float32 = 1.
+	tf2 float64 = 4.2e1
+	tf3 myfloat = 3.141592653589793238462643383279502884197169399375105820974944592307816406286
+	tf4 myfloat = 1e-1
+
+	tf5 = tf0 + tf1
+	tf6 = tf1 - tf1
+	tf7 = tf2 /* ERROR "mismatched types" */ * tf1
+	tf8 = tf3 / tf3
+	tf9 = tf3 /* ERROR "not defined" */ % tf3
+
+	tf10 = 1 / 0 /* ERROR "division by zero" */
+	tf11 = tf1 / 0 /* ERROR "division by zero" */
+	tf12 = tf3 /* ERROR "mismatched types" */ / tf0
+
+	tf16 = tf2 /* ERROR "mismatched types" */ & tf3
+	tf17 = tf2 /* ERROR "mismatched types" */ | tf3
+	tf18 = tf2 /* ERROR "mismatched types" */ ^ tf3
+
+	// complex values
+	tc0 = 0.i
+	tc1 = 1.i
+	tc2 = 4.2e1i
+	tc3 = 3.141592653589793238462643383279502884197169399375105820974944592307816406286i
+	tc4 = 1e-1i
+
+	tc5 = tc0 + tc1
+	tc6 = tc1 - tc1
+	tc7 = tc2 * tc1
+	tc8 = tc3 / tc3
+	tc9 = tc3 /* ERROR "not defined" */ % tc3
+
+	tc10 = 1 / 0 /* ERROR "division by zero" */
+	tc11 = tc1 / 0 /* ERROR "division by zero" */
+	tc12 = tc3 / tc0 /* ERROR "division by zero" */
+
+	tc16 = tc2 /* ERROR "not defined" */ & tc3
+	tc17 = tc2 /* ERROR "not defined" */ | tc3
+	tc18 = tc2 /* ERROR "not defined" */ ^ tc3
+)
+
+// initialization cycles
+const (
+	a /* ERROR "initialization cycle" */ = a
+	b /* ERROR "initialization cycle" */ , c /* ERROR "initialization cycle" */, d, e = e, d, c, b // TODO(gri) should only have one cycle error
+	f float64 = d
+)
+
+// multiple initialization
+const (
+	a1, a2, a3 = 7, 3.1415926, "foo"
+	b1, b2, b3 = b3, b1, 42
+	c1, c2, c3  /* ERROR "missing init expr for c3" */ = 1, 2
+	d1, d2, d3 = 1, 2, 3, 4 /* ERROR "extra init expr 4" */
+	_p0 = assert(a1 == 7)
+	_p1 = assert(a2 == 3.1415926)
+	_p2 = assert(a3 == "foo")
+	_p3 = assert(b1 == 42)
+	_p4 = assert(b2 == 42)
+	_p5 = assert(b3 == 42)
+)
+
+func _() {
+	const (
+		a1, a2, a3 = 7, 3.1415926, "foo"
+		b1, b2, b3 = b3, b1, 42
+		c1, c2, c3  /* ERROR "missing init expr for c3" */ = 1, 2
+		d1, d2, d3 = 1, 2, 3, 4 /* ERROR "extra init expr 4" */
+		_p0 = assert(a1 == 7)
+		_p1 = assert(a2 == 3.1415926)
+		_p2 = assert(a3 == "foo")
+		_p3 = assert(b1 == 42)
+		_p4 = assert(b2 == 42)
+		_p5 = assert(b3 == 42)
+	)
+}
+
+// iota
+const (
+	iota0 = iota
+	iota1 = iota
+	iota2 = iota*2
+	_a0 = assert(iota0 == 0)
+	_a1 = assert(iota1 == 1)
+	_a2 = assert(iota2 == 4)
+	iota6 = iota*3
+
+	iota7
+	iota8
+	_a3 = assert(iota7 == 21)
+	_a4 = assert(iota8 == 24)
+)
+
+const (
+	_b0 = iota
+	_b1 = assert(iota + iota2 == 5)
+	_b2 = len([iota]int{}) // iota may appear in a type!
+	_b3 = assert(_b2 == 2)
+	_b4 = len(A{})
+)
+
+type A [iota /* ERROR "cannot use iota" */ ]int
+
+// constant expressions with operands across different
+// constant declarations must use the right iota values
+const (
+	_c0 = iota
+	_c1
+	_c2
+	_x = _c2 + _d1 + _e0 // 3
+)
+
+const (
+	_d0 = iota
+	_d1
+)
+
+const (
+	_e0 = iota
+)
+
+var _ = assert(_x == 3)
+
+// special cases
+const (
+	_n0 = nil /* ERROR "not constant" */
+	_n1 = [ /* ERROR "not constant" */ ]int{}
+)
+
+// iotas must not be usable in expressions outside constant declarations
+type _ [iota /* ERROR "iota outside constant decl" */ ]byte
+var _ = iota /* ERROR "iota outside constant decl" */
+func _() {
+	_ = iota /* ERROR "iota outside constant decl" */
+	const _ = iota
+	_ = iota /* ERROR "iota outside constant decl" */
+}
+
+func _() {
+	iota := 123
+	const x = iota /* ERROR "is not constant" */
+	var y = iota
+	_ = y
+}
+
+// iotas are usable inside closures in constant declarations (#22345)
+const (
+	_ = iota
+	_ = len([iota]byte{})
+	_ = unsafe.Sizeof(iota)
+	_ = unsafe.Sizeof(func() { _ = iota })
+	_ = unsafe.Sizeof(func() { var _ = iota })
+	_ = unsafe.Sizeof(func() { const _ = iota })
+	_ = unsafe.Sizeof(func() { type _ [iota]byte })
+	_ = unsafe.Sizeof(func() { func() int { return iota }() })
+)
+
+// verify inner and outer const declarations have distinct iotas
+const (
+	zero = iota
+	one  = iota
+	_    = unsafe.Sizeof(func() {
+		var x [iota]int // [2]int
+		const (
+			Zero = iota
+			One
+			Two
+			_ = unsafe.Sizeof([iota-1]int{} == x) // assert types are equal
+			_ = unsafe.Sizeof([Two]int{} == x)    // assert types are equal
+		)
+		var z [iota]int                           // [2]int
+		_ = unsafe.Sizeof([2]int{} == z)          // assert types are equal
+	})
+	three = iota // the sequence continues
+)
+var _ [three]int = [3]int{} // assert 'three' has correct value
+
+var (
+	_ = iota /* ERROR "iota outside constant decl" */
+	_ = unsafe.Sizeof(iota  /* ERROR "iota outside constant decl" */ )
+	_ = unsafe.Sizeof(func() { _ = iota /* ERROR "iota outside constant decl" */ })
+	_ = unsafe.Sizeof(func() { var _ = iota /* ERROR "iota outside constant decl" */ })
+	_ = unsafe.Sizeof(func() { type _ [iota /* ERROR "iota outside constant decl" */ ]byte })
+	_ = unsafe.Sizeof(func() { func() int { return iota /* ERROR "iota outside constant decl" */ }() })
+)
+
+// constant arithmetic precision and rounding must lead to expected (integer) results
+var _ = []int64{
+	0.0005 * 1e9,
+	0.001 * 1e9,
+	0.005 * 1e9,
+	0.01 * 1e9,
+	0.05 * 1e9,
+	0.1 * 1e9,
+	0.5 * 1e9,
+	1 * 1e9,
+	5 * 1e9,
+}
+
+const _ = unsafe.Sizeof(func() {
+	const _ = 0
+	_ = iota
+
+	const (
+	   zero = iota
+	   one
+	)
+	assert(one == 1)
+	assert(iota == 0)
+})
diff --git a/src/go/types/testdata/const1.src b/src/go/types/testdata/const1.src
new file mode 100644
index 0000000..d827704
--- /dev/null
+++ b/src/go/types/testdata/const1.src
@@ -0,0 +1,322 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// constant conversions
+
+package const1
+
+const(
+	mi = ^int(0)
+	mu = ^uint(0)
+	mp = ^uintptr(0)
+
+	logSizeofInt     = uint(mi>>8&1 + mi>>16&1 + mi>>32&1)
+	logSizeofUint    = uint(mu>>8&1 + mu>>16&1 + mu>>32&1)
+	logSizeofUintptr = uint(mp>>8&1 + mp>>16&1 + mp>>32&1)
+)
+
+const (
+	minInt8 = -1<<(8<<iota - 1)
+	minInt16
+	minInt32
+	minInt64
+	minInt = -1<<(8<<logSizeofInt - 1)
+)
+
+const (
+	maxInt8 = 1<<(8<<iota - 1) - 1
+	maxInt16
+	maxInt32
+	maxInt64
+	maxInt = 1<<(8<<logSizeofInt - 1) - 1
+)
+
+const (
+	maxUint8 = 1<<(8<<iota) - 1
+	maxUint16
+	maxUint32
+	maxUint64
+	maxUint    = 1<<(8<<logSizeofUint) - 1
+	maxUintptr = 1<<(8<<logSizeofUintptr) - 1
+)
+
+const (
+	smallestFloat32 = 1.0 / (1<<(127 - 1 + 23))
+	smallestFloat64 = 1.0 / (1<<(1023 - 1 + 52))
+)
+
+const (
+	_ = assert(smallestFloat32 > 0)
+	_ = assert(smallestFloat64 > 0)
+)
+
+const (
+	maxFloat32 = 1<<127 * (1<<24 - 1) / (1.0<<23)
+	maxFloat64 = 1<<1023 * (1<<53 - 1) / (1.0<<52)
+)
+
+const (
+	_ int8 = minInt8 /* ERROR "overflows" */ - 1
+	_ int8 = minInt8
+	_ int8 = maxInt8
+	_ int8 = maxInt8 /* ERROR "overflows" */ + 1
+	_ int8 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = int8(minInt8 /* ERROR "cannot convert" */ - 1)
+	_ = int8(minInt8)
+	_ = int8(maxInt8)
+	_ = int8(maxInt8 /* ERROR "cannot convert" */ + 1)
+	_ = int8(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ int16 = minInt16 /* ERROR "overflows" */ - 1
+	_ int16 = minInt16
+	_ int16 = maxInt16
+	_ int16 = maxInt16 /* ERROR "overflows" */ + 1
+	_ int16 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = int16(minInt16 /* ERROR "cannot convert" */ - 1)
+	_ = int16(minInt16)
+	_ = int16(maxInt16)
+	_ = int16(maxInt16 /* ERROR "cannot convert" */ + 1)
+	_ = int16(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ int32 = minInt32 /* ERROR "overflows" */ - 1
+	_ int32 = minInt32
+	_ int32 = maxInt32
+	_ int32 = maxInt32 /* ERROR "overflows" */ + 1
+	_ int32 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = int32(minInt32 /* ERROR "cannot convert" */ - 1)
+	_ = int32(minInt32)
+	_ = int32(maxInt32)
+	_ = int32(maxInt32 /* ERROR "cannot convert" */ + 1)
+	_ = int32(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ int64 = minInt64 /* ERROR "overflows" */ - 1
+	_ int64 = minInt64
+	_ int64 = maxInt64
+	_ int64 = maxInt64 /* ERROR "overflows" */ + 1
+	_ int64 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = int64(minInt64 /* ERROR "cannot convert" */ - 1)
+	_ = int64(minInt64)
+	_ = int64(maxInt64)
+	_ = int64(maxInt64 /* ERROR "cannot convert" */ + 1)
+	_ = int64(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ int = minInt /* ERROR "overflows" */ - 1
+	_ int = minInt
+	_ int = maxInt
+	_ int = maxInt /* ERROR "overflows" */ + 1
+	_ int = smallestFloat64 /* ERROR "truncated" */
+
+	_ = int(minInt /* ERROR "cannot convert" */ - 1)
+	_ = int(minInt)
+	_ = int(maxInt)
+	_ = int(maxInt /* ERROR "cannot convert" */ + 1)
+	_ = int(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ uint8 = 0 /* ERROR "overflows" */ - 1
+	_ uint8 = 0
+	_ uint8 = maxUint8
+	_ uint8 = maxUint8 /* ERROR "overflows" */ + 1
+	_ uint8 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = uint8(0 /* ERROR "cannot convert" */ - 1)
+	_ = uint8(0)
+	_ = uint8(maxUint8)
+	_ = uint8(maxUint8 /* ERROR "cannot convert" */ + 1)
+	_ = uint8(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ uint16 = 0 /* ERROR "overflows" */ - 1
+	_ uint16 = 0
+	_ uint16 = maxUint16
+	_ uint16 = maxUint16 /* ERROR "overflows" */ + 1
+	_ uint16 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = uint16(0 /* ERROR "cannot convert" */ - 1)
+	_ = uint16(0)
+	_ = uint16(maxUint16)
+	_ = uint16(maxUint16 /* ERROR "cannot convert" */ + 1)
+	_ = uint16(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ uint32 = 0 /* ERROR "overflows" */ - 1
+	_ uint32 = 0
+	_ uint32 = maxUint32
+	_ uint32 = maxUint32 /* ERROR "overflows" */ + 1
+	_ uint32 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = uint32(0 /* ERROR "cannot convert" */ - 1)
+	_ = uint32(0)
+	_ = uint32(maxUint32)
+	_ = uint32(maxUint32 /* ERROR "cannot convert" */ + 1)
+	_ = uint32(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ uint64 = 0 /* ERROR "overflows" */ - 1
+	_ uint64 = 0
+	_ uint64 = maxUint64
+	_ uint64 = maxUint64 /* ERROR "overflows" */ + 1
+	_ uint64 = smallestFloat64 /* ERROR "truncated" */
+
+	_ = uint64(0 /* ERROR "cannot convert" */ - 1)
+	_ = uint64(0)
+	_ = uint64(maxUint64)
+	_ = uint64(maxUint64 /* ERROR "cannot convert" */ + 1)
+	_ = uint64(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ uint = 0 /* ERROR "overflows" */ - 1
+	_ uint = 0
+	_ uint = maxUint
+	_ uint = maxUint /* ERROR "overflows" */ + 1
+	_ uint = smallestFloat64 /* ERROR "truncated" */
+
+	_ = uint(0 /* ERROR "cannot convert" */ - 1)
+	_ = uint(0)
+	_ = uint(maxUint)
+	_ = uint(maxUint /* ERROR "cannot convert" */ + 1)
+	_ = uint(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ uintptr = 0 /* ERROR "overflows" */ - 1
+	_ uintptr = 0
+	_ uintptr = maxUintptr
+	_ uintptr = maxUintptr /* ERROR "overflows" */ + 1
+	_ uintptr = smallestFloat64 /* ERROR "truncated" */
+
+	_ = uintptr(0 /* ERROR "cannot convert" */ - 1)
+	_ = uintptr(0)
+	_ = uintptr(maxUintptr)
+	_ = uintptr(maxUintptr /* ERROR "cannot convert" */ + 1)
+	_ = uintptr(smallestFloat64 /* ERROR "cannot convert" */)
+)
+
+const (
+	_ float32 = minInt64
+	_ float64 = minInt64
+	_ complex64 = minInt64
+	_ complex128 = minInt64
+
+	_ = float32(minInt64)
+	_ = float64(minInt64)
+	_ = complex64(minInt64)
+	_ = complex128(minInt64)
+)
+
+const (
+	_ float32 = maxUint64
+	_ float64 = maxUint64
+	_ complex64 = maxUint64
+	_ complex128 = maxUint64
+
+	_ = float32(maxUint64)
+	_ = float64(maxUint64)
+	_ = complex64(maxUint64)
+	_ = complex128(maxUint64)
+)
+
+// TODO(gri) find smaller deltas below
+
+const delta32 = maxFloat32/(1 << 23)
+
+const (
+	_ float32 = - /* ERROR "overflow" */ (maxFloat32 + delta32)
+	_ float32 = -maxFloat32
+	_ float32 = maxFloat32
+	_ float32 = maxFloat32 /* ERROR "overflow" */ + delta32
+
+	_ = float32(- /* ERROR "cannot convert" */ (maxFloat32 + delta32))
+	_ = float32(-maxFloat32)
+	_ = float32(maxFloat32)
+	_ = float32(maxFloat32 /* ERROR "cannot convert" */ + delta32)
+
+	_ = assert(float32(smallestFloat32) == smallestFloat32)
+	_ = assert(float32(smallestFloat32/2) == 0)
+	_ = assert(float32(smallestFloat64) == 0)
+	_ = assert(float32(smallestFloat64/2) == 0)
+)
+
+const delta64 = maxFloat64/(1 << 52)
+
+const (
+	_ float64 = - /* ERROR "overflow" */ (maxFloat64 + delta64)
+	_ float64 = -maxFloat64
+	_ float64 = maxFloat64
+	_ float64 = maxFloat64 /* ERROR "overflow" */ + delta64
+
+	_ = float64(- /* ERROR "cannot convert" */ (maxFloat64 + delta64))
+	_ = float64(-maxFloat64)
+	_ = float64(maxFloat64)
+	_ = float64(maxFloat64 /* ERROR "cannot convert" */ + delta64)
+
+	_ = assert(float64(smallestFloat32) == smallestFloat32)
+	_ = assert(float64(smallestFloat32/2) == smallestFloat32/2)
+	_ = assert(float64(smallestFloat64) == smallestFloat64)
+	_ = assert(float64(smallestFloat64/2) == 0)
+)
+
+const (
+	_ complex64 = - /* ERROR "overflow" */ (maxFloat32 + delta32)
+	_ complex64 = -maxFloat32
+	_ complex64 = maxFloat32
+	_ complex64 = maxFloat32 /* ERROR "overflow" */ + delta32
+
+	_ = complex64(- /* ERROR "cannot convert" */ (maxFloat32 + delta32))
+	_ = complex64(-maxFloat32)
+	_ = complex64(maxFloat32)
+	_ = complex64(maxFloat32 /* ERROR "cannot convert" */ + delta32)
+)
+
+const (
+	_ complex128 = - /* ERROR "overflow" */ (maxFloat64 + delta64)
+	_ complex128 = -maxFloat64
+	_ complex128 = maxFloat64
+	_ complex128 = maxFloat64 /* ERROR "overflow" */ + delta64
+
+	_ = complex128(- /* ERROR "cannot convert" */ (maxFloat64 + delta64))
+	_ = complex128(-maxFloat64)
+	_ = complex128(maxFloat64)
+	_ = complex128(maxFloat64 /* ERROR "cannot convert" */ + delta64)
+)
+
+// Initialization of typed constant and conversion are the same:
+const (
+	f32 = 1 + smallestFloat32
+	x32 float32 = f32
+	y32 = float32(f32)
+	_ = assert(x32 - y32 == 0)
+)
+
+const (
+	f64 = 1 + smallestFloat64
+	x64 float64 = f64
+	y64 = float64(f64)
+	_ = assert(x64 - y64 == 0)
+)
+
+const (
+	_ = int8(-1) << 7
+	_ = int8 /* ERROR "overflows" */ (-1) << 8
+
+	_ = uint32(1) << 31
+	_ = uint32 /* ERROR "overflows" */ (1) << 32
+)
diff --git a/src/go/types/testdata/constdecl.src b/src/go/types/testdata/constdecl.src
new file mode 100644
index 0000000..680c85a
--- /dev/null
+++ b/src/go/types/testdata/constdecl.src
@@ -0,0 +1,141 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package constdecl
+
+import "math"
+import "unsafe"
+
+var v int
+
+// Const decls must be initialized by constants.
+const _ = v /* ERROR "not constant" */
+const _ = math /* ERROR "not constant" */ .Sin(0)
+const _ = int /* ERROR "not an expression" */
+
+func _() {
+	const _ = v /* ERROR "not constant" */
+	const _ = math /* ERROR "not constant" */ .Sin(0)
+	const _ = int /* ERROR "not an expression" */
+}
+
+// Identifier and expression arity must match.
+// The first error message is produced by the parser.
+// In a real-world scenario, the type-checker would not be run
+// in this case and the 2nd error message would not appear.
+const _ /* ERROR "missing constant value" */ /* ERROR "missing init expr for _" */
+const _ = 1, 2 /* ERROR "extra init expr 2" */
+
+const _ /* ERROR "missing constant value" */ /* ERROR "missing init expr for _" */ int
+const _ int = 1, 2 /* ERROR "extra init expr 2" */
+
+const (
+	_ /* ERROR "missing constant value" */ /* ERROR "missing init expr for _" */
+	_ = 1, 2 /* ERROR "extra init expr 2" */
+
+	_ /* ERROR "missing constant value" */ /* ERROR "missing init expr for _" */ int
+	_ int = 1, 2 /* ERROR "extra init expr 2" */
+)
+
+const (
+	_ = 1
+	_
+	_, _ /* ERROR "missing init expr for _" */
+	_
+)
+
+const (
+	_, _ = 1, 2
+	_, _
+	_ /* ERROR "extra init expr at" */
+	_, _
+	_, _, _ /* ERROR "missing init expr for _" */
+	_, _
+)
+
+func _() {
+	const _ /* ERROR "missing constant value" */ /* ERROR "missing init expr for _" */
+	const _ = 1, 2 /* ERROR "extra init expr 2" */
+
+	const _ /* ERROR "missing constant value" */ /* ERROR "missing init expr for _" */ int
+	const _ int = 1, 2 /* ERROR "extra init expr 2" */
+
+	const (
+		_ /* ERROR "missing constant value" */ /* ERROR "missing init expr for _" */
+		_ = 1, 2 /* ERROR "extra init expr 2" */
+
+		_ /* ERROR "missing constant value" */ /* ERROR "missing init expr for _" */ int
+		_ int = 1, 2 /* ERROR "extra init expr 2" */
+	)
+
+	const (
+		_ = 1
+		_
+		_, _ /* ERROR "missing init expr for _" */
+		_
+	)
+
+	const (
+		_, _ = 1, 2
+		_, _
+		_ /* ERROR "extra init expr at" */
+		_, _
+		_, _, _ /* ERROR "missing init expr for _" */
+		_, _
+	)
+}
+
+// Test case for constant with invalid initialization.
+// Caused panic because the constant value was not set up (gri - 7/8/2014).
+func _() {
+	const (
+	    x string = missing /* ERROR "undeclared name" */
+	    y = x + ""
+	)
+}
+
+// Test case for constants depending on function literals (see also #22992).
+const A /* ERROR initialization cycle */ = unsafe.Sizeof(func() { _ = A })
+
+func _() {
+	// The function literal below must not see a.
+	const a = unsafe.Sizeof(func() { _ = a /* ERROR "undeclared name" */ })
+	const b = unsafe.Sizeof(func() { _ = a })
+
+	// The function literal below must not see x, y, or z.
+	const x, y, z = 0, 1, unsafe.Sizeof(func() { _ = x /* ERROR "undeclared name" */ + y /* ERROR "undeclared name" */ + z /* ERROR "undeclared name" */ })
+}
+
+// Test cases for errors in inherited constant initialization expressions.
+// Errors related to inherited initialization expressions must appear at
+// the constant identifier being declared, not at the original expression
+// (issues #42991, #42992).
+const (
+	_ byte = 255 + iota
+	/* some gap */
+	_ // ERROR overflows
+	/* some gap */
+	/* some gap */ _ /* ERROR overflows */; _ /* ERROR overflows */
+	/* some gap */
+	_ = 255 + iota
+	_ = byte /* ERROR overflows */ (255) + iota
+	_ /* ERROR overflows */
+)
+
+// Test cases from issue.
+const (
+	ok = byte(iota + 253)
+	bad
+	barn
+	bard // ERROR cannot convert
+)
+
+const (
+	c = len([1 - iota]int{})
+	d
+	e // ERROR invalid array length
+	f // ERROR invalid array length
+)
+
+// TODO(gri) move extra tests from testdata/const0.src into here
diff --git a/src/go/types/testdata/conversions.src b/src/go/types/testdata/conversions.src
new file mode 100644
index 0000000..e1336c0
--- /dev/null
+++ b/src/go/types/testdata/conversions.src
@@ -0,0 +1,93 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// conversions
+
+package conversions
+
+import "unsafe"
+
+// argument count
+var (
+	_ = int() /* ERROR "missing argument" */
+	_ = int(1, 2 /* ERROR "too many arguments" */ )
+)
+
+// numeric constant conversions are in const1.src.
+
+func string_conversions() {
+	const A = string(65)
+	assert(A == "A")
+	const E = string(-1)
+	assert(E == "\uFFFD")
+	assert(E == string(1234567890))
+
+	type myint int
+	assert(A == string(myint(65)))
+
+	type mystring string
+	const _ mystring = mystring("foo")
+
+	const _ = string(true /* ERROR "cannot convert" */ )
+	const _ = string(1.2 /* ERROR "cannot convert" */ )
+	const _ = string(nil /* ERROR "cannot convert" */ )
+
+	// issues 11357, 11353: argument must be of integer type
+	_ = string(0.0 /* ERROR "cannot convert" */ )
+	_ = string(0i /* ERROR "cannot convert" */ )
+	_ = string(1 /* ERROR "cannot convert" */ + 2i)
+}
+
+func interface_conversions() {
+	type E interface{}
+
+	type I1 interface{
+		m1()
+	}
+
+	type I2 interface{
+		m1()
+		m2(x int)
+	}
+
+	type I3 interface{
+		m1()
+		m2() int
+	}
+
+	var e E
+	var i1 I1
+	var i2 I2
+	var i3 I3
+
+	_ = E(0)
+	_ = E(nil)
+	_ = E(e)
+	_ = E(i1)
+	_ = E(i2)
+
+	_ = I1(0 /* ERROR "cannot convert" */ )
+	_ = I1(nil)
+	_ = I1(i1)
+	_ = I1(e /* ERROR "cannot convert" */ )
+	_ = I1(i2)
+
+	_ = I2(nil)
+	_ = I2(i1 /* ERROR "cannot convert" */ )
+	_ = I2(i2)
+	_ = I2(i3 /* ERROR "cannot convert" */ )
+
+	_ = I3(nil)
+	_ = I3(i1 /* ERROR "cannot convert" */ )
+	_ = I3(i2 /* ERROR "cannot convert" */ )
+	_ = I3(i3)
+
+	// TODO(gri) add more tests, improve error message
+}
+
+func issue6326() {
+	type T unsafe.Pointer
+	var x T
+	_ = uintptr(x) // see issue 6326
+}
diff --git a/src/go/types/testdata/conversions2.src b/src/go/types/testdata/conversions2.src
new file mode 100644
index 0000000..93a5f18
--- /dev/null
+++ b/src/go/types/testdata/conversions2.src
@@ -0,0 +1,313 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test various valid and invalid struct assignments and conversions.
+// Does not compile.
+
+package conversions2
+
+type I interface {
+	m()
+}
+
+// conversions between structs
+
+func _() {
+	type S struct{}
+	type T struct{}
+	var s S
+	var t T
+	var u struct{}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u
+	s = S(s)
+	s = S(t)
+	s = S(u)
+	t = u
+	t = T(u)
+}
+
+func _() {
+	type S struct{ x int }
+	type T struct {
+		x int "foo"
+	}
+	var s S
+	var t T
+	var u struct {
+		x int "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = S(s)
+	s = S(t)
+	s = S(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = T(u)
+}
+
+func _() {
+	type E struct{ x int }
+	type S struct{ x E }
+	type T struct {
+		x E "foo"
+	}
+	var s S
+	var t T
+	var u struct {
+		x E "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = S(s)
+	s = S(t)
+	s = S(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = T(u)
+}
+
+func _() {
+	type S struct {
+		x struct {
+			x int "foo"
+		}
+	}
+	type T struct {
+		x struct {
+			x int "bar"
+		} "foo"
+	}
+	var s S
+	var t T
+	var u struct {
+		x struct {
+			x int "bar"
+		} "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = S(s)
+	s = S(t)
+	s = S(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = T(u)
+}
+
+func _() {
+	type E1 struct {
+		x int "foo"
+	}
+	type E2 struct {
+		x int "bar"
+	}
+	type S struct{ x E1 }
+	type T struct {
+		x E2 "foo"
+	}
+	var s S
+	var t T
+	var u struct {
+		x E2 "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = S(s)
+	s = S(t /* ERROR "cannot convert" */ )
+	s = S(u /* ERROR "cannot convert" */ )
+	t = u   // ERROR "cannot use .* in assignment"
+	t = T(u)
+}
+
+func _() {
+	type E struct{ x int }
+	type S struct {
+		f func(struct {
+			x int "foo"
+		})
+	}
+	type T struct {
+		f func(struct {
+			x int "bar"
+		})
+	}
+	var s S
+	var t T
+	var u struct{ f func(E) }
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = S(s)
+	s = S(t)
+	s = S(u /* ERROR "cannot convert" */ )
+	t = u   // ERROR "cannot use .* in assignment"
+	t = T(u /* ERROR "cannot convert" */ )
+}
+
+// conversions between pointers to structs
+
+func _() {
+	type S struct{}
+	type T struct{}
+	var s *S
+	var t *T
+	var u *struct{}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = (*T)(u)
+}
+
+func _() {
+	type S struct{ x int }
+	type T struct {
+		x int "foo"
+	}
+	var s *S
+	var t *T
+	var u *struct {
+		x int "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = (*T)(u)
+}
+
+func _() {
+	type E struct{ x int }
+	type S struct{ x E }
+	type T struct {
+		x E "foo"
+	}
+	var s *S
+	var t *T
+	var u *struct {
+		x E "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = (*T)(u)
+}
+
+func _() {
+	type S struct {
+		x struct {
+			x int "foo"
+		}
+	}
+	type T struct {
+		x struct {
+			x int "bar"
+		} "foo"
+	}
+	var s *S
+	var t *T
+	var u *struct {
+		x struct {
+			x int "bar"
+		} "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u)
+	t = u // ERROR "cannot use .* in assignment"
+	t = (*T)(u)
+}
+
+func _() {
+	type E1 struct {
+		x int "foo"
+	}
+	type E2 struct {
+		x int "bar"
+	}
+	type S struct{ x E1 }
+	type T struct {
+		x E2 "foo"
+	}
+	var s *S
+	var t *T
+	var u *struct {
+		x E2 "bar"
+	}
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t /* ERROR "cannot convert" */ )
+	s = (*S)(u /* ERROR "cannot convert" */ )
+	t = u      // ERROR "cannot use .* in assignment"
+	t = (*T)(u)
+}
+
+func _() {
+	type E struct{ x int }
+	type S struct {
+		f func(struct {
+			x int "foo"
+		})
+	}
+	type T struct {
+		f func(struct {
+			x int "bar"
+		})
+	}
+	var s *S
+	var t *T
+	var u *struct{ f func(E) }
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u /* ERROR "cannot convert" */ )
+	t = u      // ERROR "cannot use .* in assignment"
+	t = (*T)(u /* ERROR "cannot convert" */ )
+}
+
+func _() {
+	type E struct{ x int }
+	type S struct {
+		f func(*struct {
+			x int "foo"
+		})
+	}
+	type T struct {
+		f func(*struct {
+			x int "bar"
+		})
+	}
+	var s *S
+	var t *T
+	var u *struct{ f func(E) }
+	s = s
+	s = t // ERROR "cannot use .* in assignment"
+	s = u // ERROR "cannot use .* in assignment"
+	s = (*S)(s)
+	s = (*S)(t)
+	s = (*S)(u /* ERROR "cannot convert" */ )
+	t = u      // ERROR "cannot use .* in assignment"
+	t = (*T)(u /* ERROR "cannot convert" */ )
+}
diff --git a/src/go/types/testdata/cycles.src b/src/go/types/testdata/cycles.src
new file mode 100644
index 0000000..218b4ca
--- /dev/null
+++ b/src/go/types/testdata/cycles.src
@@ -0,0 +1,174 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cycles
+
+import "unsafe"
+
+type (
+	T0 int
+	T1 /* ERROR cycle */ T1
+	T2 *T2
+
+	T3 /* ERROR cycle */ T4
+	T4 T5
+	T5 T3
+
+	T6 T7
+	T7 *T8
+	T8 T6
+
+	// arrays
+	A0 /* ERROR cycle */ [10]A0
+	A1 [10]*A1
+
+	A2 /* ERROR cycle */ [10]A3
+	A3 [10]A4
+	A4 A2
+
+	A5 [10]A6
+	A6 *A5
+
+	// slices
+	L0 []L0
+
+	// structs
+	S0 /* ERROR cycle */ struct{ _ S0 }
+	S1 /* ERROR cycle */ struct{ S1 }
+	S2 struct{ _ *S2 }
+	S3 struct{ *S3 }
+
+	S4 /* ERROR cycle */ struct{ S5 }
+	S5 struct{ S6 }
+	S6 S4
+
+	// pointers
+	P0 *P0
+
+	// functions
+	F0 func(F0)
+	F1 func() F1
+	F2 func(F2) F2
+
+	// interfaces
+	I0 /* ERROR cycle */ interface{ I0 }
+
+	I1 /* ERROR cycle */ interface{ I2 }
+	I2 interface{ I3 }
+	I3 interface{ I1 }
+
+	I4 interface{ f(I4) }
+
+	// testcase for issue 5090
+	I5 interface{ f(I6) }
+	I6 interface{ I5 }
+
+	// maps
+	M0 map[M0 /* ERROR incomparable map key */ ]M0
+
+	// channels
+	C0 chan C0
+)
+
+// test case for issue #34771
+type (
+	AA /* ERROR cycle */ B
+	B C
+	C [10]D
+	D E
+	E AA
+)
+
+func _() {
+	type (
+		t1 /* ERROR cycle */ t1
+		t2 *t2
+
+		t3 t4 /* ERROR undeclared */
+		t4 t5 /* ERROR undeclared */
+		t5 t3
+
+		// arrays
+		a0 /* ERROR cycle */ [10]a0
+		a1 [10]*a1
+
+		// slices
+		l0 []l0
+
+		// structs
+		s0 /* ERROR cycle */ struct{ _ s0 }
+		s1 /* ERROR cycle */ struct{ s1 }
+		s2 struct{ _ *s2 }
+		s3 struct{ *s3 }
+
+		// pointers
+		p0 *p0
+
+		// functions
+		f0 func(f0)
+		f1 func() f1
+		f2 func(f2) f2
+
+		// interfaces
+		i0 /* ERROR cycle */ interface{ i0 }
+
+		// maps
+		m0 map[m0 /* ERROR incomparable map key */ ]m0
+
+		// channels
+		c0 chan c0
+	)
+}
+
+// test cases for issue 6667
+
+type A [10]map[A /* ERROR incomparable map key */ ]bool
+
+type S struct {
+	m map[S /* ERROR incomparable map key */ ]bool
+}
+
+// test cases for issue 7236
+// (cycle detection must not be dependent on starting point of resolution)
+
+type (
+	P1 *T9
+	T9 /* ERROR cycle */ T9
+
+	T10 /* ERROR cycle */ T10
+	P2 *T10
+)
+
+func (T11) m() {}
+
+type T11 /* ERROR cycle */ struct{ T11 }
+
+type T12 /* ERROR cycle */ struct{ T12 }
+
+func (*T12) m() {}
+
+type (
+	P3 *T13
+	T13 /* ERROR cycle */ T13
+)
+
+// test cases for issue 18643
+// (type cycle detection when non-type expressions are involved)
+type (
+	T14 [len(T14 /* ERROR cycle */ {})]int
+	T15 [][len(T15 /* ERROR cycle */ {})]int
+	T16 map[[len(T16 /* ERROR cycle */ {1:2})]int]int
+	T17 map[int][len(T17 /* ERROR cycle */ {1:2})]int
+)
+
+// Test case for types depending on function literals (see also #22992).
+type T20 chan [unsafe.Sizeof(func(ch T20){ _ = <-ch })]byte
+type T22 = chan [unsafe.Sizeof(func(ch T20){ _ = <-ch })]byte
+
+func _() {
+	type T0 func(T0)
+	type T1 /* ERROR cycle */ = func(T1)
+	type T2 chan [unsafe.Sizeof(func(ch T2){ _ = <-ch })]byte
+	type T3 /* ERROR cycle */ = chan [unsafe.Sizeof(func(ch T3){ _ = <-ch })]byte
+}
diff --git a/src/go/types/testdata/cycles1.src b/src/go/types/testdata/cycles1.src
new file mode 100644
index 0000000..ae2b38e
--- /dev/null
+++ b/src/go/types/testdata/cycles1.src
@@ -0,0 +1,77 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type (
+	A interface {
+		a() interface {
+			ABC1
+		}
+	}
+	B interface {
+		b() interface {
+			ABC2
+		}
+	}
+	C interface {
+		c() interface {
+			ABC3
+		}
+	}
+
+	AB interface {
+		A
+		B
+	}
+	BC interface {
+		B
+		C
+	}
+
+	ABC1 interface {
+		A
+		B
+		C
+	}
+	ABC2 interface {
+		AB
+		C
+	}
+	ABC3 interface {
+		A
+		BC
+	}
+)
+
+var (
+	x1 ABC1
+	x2 ABC2
+	x3 ABC3
+)
+
+func _() {
+	// all types have the same method set
+	x1 = x2
+	x2 = x1
+
+	x1 = x3
+	x3 = x1
+
+	x2 = x3
+	x3 = x2
+
+	// all methods return the same type again
+	x1 = x1.a()
+	x1 = x1.b()
+	x1 = x1.c()
+
+	x2 = x2.a()
+	x2 = x2.b()
+	x2 = x2.c()
+
+	x3 = x3.a()
+	x3 = x3.b()
+	x3 = x3.c()
+}
diff --git a/src/go/types/testdata/cycles2.src b/src/go/types/testdata/cycles2.src
new file mode 100644
index 0000000..1a7f40a
--- /dev/null
+++ b/src/go/types/testdata/cycles2.src
@@ -0,0 +1,98 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+// Test case for issue 5090
+
+type t interface {
+	f(u)
+}
+
+type u interface {
+	t
+}
+
+func _() {
+	var t t
+	var u u
+
+	t.f(t)
+	t.f(u)
+
+	u.f(t)
+	u.f(u)
+}
+
+
+// Test case for issues #6589, #33656.
+
+type A interface {
+	a() interface {
+		AB
+	}
+}
+
+type B interface {
+	b() interface {
+		AB
+	}
+}
+
+type AB interface {
+	a() interface {
+		A
+		B
+	}
+	b() interface {
+		A
+		B
+	}
+}
+
+var x AB
+var y interface {
+	A
+	B
+}
+
+var _ = x == y
+
+
+// Test case for issue 6638.
+
+type T interface {
+	m() [T(nil).m /* ERROR undefined */ ()[0]]int
+}
+
+// Variations of this test case.
+
+type T1 /* ERROR cycle */ interface {
+	m() [x1.m()[0]]int
+}
+
+var x1 T1
+
+type T2 /* ERROR cycle */ interface {
+	m() [len(x2.m())]int
+}
+
+var x2 T2
+
+type T3 /* ERROR cycle */ interface {
+	m() [unsafe.Sizeof(x3.m)]int
+}
+
+var x3 T3
+
+type T4 /* ERROR cycle */ interface {
+	m() [unsafe.Sizeof(cast4(x4.m))]int // cast is invalid but we have a cycle, so all bets are off
+}
+
+var x4 T4
+var _ = cast4(x4.m)
+
+type cast4 func()
diff --git a/src/go/types/testdata/cycles3.src b/src/go/types/testdata/cycles3.src
new file mode 100644
index 0000000..5e89b62
--- /dev/null
+++ b/src/go/types/testdata/cycles3.src
@@ -0,0 +1,60 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+var (
+	_ A = A(nil).a().b().c().d().e().f()
+	_ A = A(nil).b().c().d().e().f()
+	_ A = A(nil).c().d().e().f()
+	_ A = A(nil).d().e().f()
+	_ A = A(nil).e().f()
+	_ A = A(nil).f()
+	_ A = A(nil)
+)
+
+type (
+	A interface {
+		a() B
+		B
+	}
+
+	B interface {
+		b() C
+		C
+	}
+
+	C interface {
+		c() D
+		D
+	}
+
+	D interface {
+		d() E
+		E
+	}
+
+	E interface {
+		e() F
+		F
+	}
+
+	F interface {
+		f() A
+	}
+)
+
+type (
+	U /* ERROR cycle */ interface {
+		V
+	}
+
+	V interface {
+		v() [unsafe.Sizeof(u)]int
+	}
+)
+
+var u U
diff --git a/src/go/types/testdata/cycles4.src b/src/go/types/testdata/cycles4.src
new file mode 100644
index 0000000..445babc
--- /dev/null
+++ b/src/go/types/testdata/cycles4.src
@@ -0,0 +1,110 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+// Check that all methods of T are collected before
+// determining the result type of m (which embeds
+// all methods of T).
+
+type T interface {
+	m() interface {T}
+	E
+}
+
+var _ = T.m(nil).m().e()
+
+type E interface {
+	e() int
+}
+
+// Check that unresolved forward chains are followed
+// (see also comment in resolver.go, checker.typeDecl).
+
+var _ = C.m(nil).m().e()
+
+type A B
+
+type B interface {
+	m() interface{C}
+	E
+}
+
+type C A
+
+// Check that interface type comparison for identity
+// does not recur endlessly.
+
+type T1 interface {
+	m() interface{T1}
+}
+
+type T2 interface {
+	m() interface{T2}
+}
+
+func _(x T1, y T2) {
+	// Checking for assignability of interfaces must check
+	// if all methods of x are present in y, and that they
+	// have identical signatures. The signatures recur via
+	// the result type, which is an interface that embeds
+	// a single method m that refers to the very interface
+	// that contains it. This requires cycle detection in
+	// identity checks for interface types.
+	x = y
+}
+
+type T3 interface {
+	m() interface{T4}
+}
+
+type T4 interface {
+	m() interface{T3}
+}
+
+func _(x T1, y T3) {
+	x = y
+}
+
+// Check that interfaces are type-checked in order of
+// (embedded interface) dependencies (was issue 7158).
+
+var x1 T5 = T7(nil)
+
+type T5 interface {
+	T6
+}
+
+type T6 interface {
+	m() T7
+}
+type T7 interface {
+	T5
+}
+
+// Actual test case from issue 7158.
+
+func wrapNode() Node {
+	return wrapElement()
+}
+
+func wrapElement() Element {
+	return nil
+}
+
+type EventTarget interface {
+	AddEventListener(Event)
+}
+
+type Node interface {
+	EventTarget
+}
+
+type Element interface {
+	Node
+}
+
+type Event interface {
+	Target() Element
+}
diff --git a/src/go/types/testdata/cycles5.src b/src/go/types/testdata/cycles5.src
new file mode 100644
index 0000000..397adcc
--- /dev/null
+++ b/src/go/types/testdata/cycles5.src
@@ -0,0 +1,200 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "unsafe"
+
+// test case from issue #18395
+
+type (
+	A interface { B }
+	B interface { C }
+	C interface { D; F() A }
+	D interface { G() B }
+)
+
+var _ = A(nil).G // G must be found
+
+
+// test case from issue #21804
+
+type sourceBridge interface {
+	listVersions() ([]Version, error)
+}
+
+type Constraint interface {
+	copyTo(*ConstraintMsg)
+}
+
+type ConstraintMsg struct{}
+
+func (m *ConstraintMsg) asUnpairedVersion() UnpairedVersion {
+	return nil
+}
+
+type Version interface {
+	Constraint
+}
+
+type UnpairedVersion interface {
+	Version
+}
+
+var _ Constraint = UnpairedVersion(nil)
+
+
+// derived test case from issue #21804
+
+type (
+	_ interface{ m(B1) }
+	A1 interface{ a(D1) }
+	B1 interface{ A1 }
+	C1 interface{ B1 }
+	D1 interface{ C1 }
+)
+
+var _ A1 = C1(nil)
+
+
+// derived test case from issue #22701
+
+func F(x I4) interface{} {
+	return x.Method()
+}
+
+type Unused interface {
+	RefersToI1(a I1)
+}
+
+type I1 interface {
+	I2
+	I3
+}
+
+type I2 interface {
+	RefersToI4() I4
+}
+
+type I3 interface {
+	Method() interface{}
+}
+
+type I4 interface {
+	I1
+}
+
+
+// check embedding of error interface
+
+type Error interface{ error }
+
+var err Error
+var _ = err.Error()
+
+
+// more esoteric cases
+
+type (
+	T1 interface { T2 }
+	T2 /* ERROR cycle */ T2
+)
+
+type (
+	T3 interface { T4 }
+	T4 /* ERROR cycle */ T5
+	T5 = T6
+	T6 = T7
+	T7 = T4
+)
+
+
+// arbitrary code may appear inside an interface
+
+const n = unsafe.Sizeof(func(){})
+
+type I interface {
+	m([unsafe.Sizeof(func() { I.m(nil, [n]byte{}) })]byte)
+}
+
+
+// test cases for varias alias cycles
+
+type T10 /* ERROR cycle */ = *T10                 // issue #25141
+type T11 /* ERROR cycle */ = interface{ f(T11) }  // issue #23139
+
+// issue #18640
+type (
+	aa = bb
+	bb struct {
+		*aa
+	}
+)
+
+type (
+	a struct{ *b }
+	b = c
+	c struct{ *b }
+)
+
+// issue #24939
+type (
+	_ interface {
+		M(P)
+	}
+
+	M interface {
+		F() P
+	}
+
+	P = interface {
+		I() M
+	}
+)
+
+// issue #8699
+type T12 /* ERROR cycle */ [len(a12)]int
+var a12 = makeArray()
+func makeArray() (res T12) { return }
+
+// issue #20770
+var r /* ERROR cycle */ = newReader()
+func newReader() r
+
+// variations of the theme of #8699 and #20770
+var arr /* ERROR cycle */ = f()
+func f() [len(arr)]int
+
+// issue #25790
+func ff(ff /* ERROR not a type */ )
+func gg((gg /* ERROR not a type */ ))
+
+type T13 /* ERROR cycle */ [len(b13)]int
+var b13 T13
+
+func g1() [unsafe.Sizeof(g1)]int
+func g2() [unsafe.Sizeof(x2)]int
+var x2 = g2
+
+// verify that we get the correct sizes for the functions above
+// (note: assert is statically evaluated in go/types test mode)
+func init() {
+	assert(unsafe.Sizeof(g1) == 8)
+	assert(unsafe.Sizeof(x2) == 8)
+}
+
+func h() [h /* ERROR no value */ ()[0]]int { panic(0) }
+
+var c14 /* ERROR cycle */ T14
+type T14 [uintptr(unsafe.Sizeof(&c14))]byte
+
+// issue #34333
+type T15 /* ERROR cycle */ struct {
+	f func() T16
+	b T16
+}
+
+type T16 struct {
+	T15
+}
\ No newline at end of file
diff --git a/src/go/types/testdata/decls0.src b/src/go/types/testdata/decls0.src
new file mode 100644
index 0000000..5501b65
--- /dev/null
+++ b/src/go/types/testdata/decls0.src
@@ -0,0 +1,206 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// type declarations
+
+package decls0
+
+import "unsafe"
+
+const pi = 3.1415
+
+type (
+	N undeclared /* ERROR "undeclared" */
+	B bool
+	I int32
+	A [10]P
+	T struct {
+		x, y P
+	}
+	P *T
+	R (*R)
+	F func(A) I
+	Y interface {
+		f(A) I
+	}
+	S [](((P)))
+	M map[I]F
+	C chan<- I
+
+	// blank types must be typechecked
+	_ pi /* ERROR "not a type" */
+	_ struct{}
+	_ struct{ pi /* ERROR "not a type" */ }
+)
+
+
+// declarations of init
+const _, init /* ERROR "cannot declare init" */ , _ = 0, 1, 2
+type init /* ERROR "cannot declare init" */ struct{}
+var _, init /* ERROR "cannot declare init" */ int
+
+func init() {}
+func init /* ERROR "missing function body" */ ()
+
+func _() { const init = 0 }
+func _() { type init int }
+func _() { var init int; _ = init }
+
+// invalid array types
+type (
+	iA0 [... /* ERROR "invalid use of '...'" */ ]byte
+	// The error message below could be better. At the moment
+	// we believe an integer that is too large is not an integer.
+	// But at least we get an error.
+	iA1 [1 /* ERROR "must be integer" */ <<100]int
+	iA2 [- /* ERROR "invalid array length" */ 1]complex128
+	iA3 ["foo" /* ERROR "must be integer" */ ]string
+	iA4 [float64 /* ERROR "must be integer" */ (0)]int
+)
+
+
+type (
+	p1 pi.foo /* ERROR "no field or method foo" */
+	p2 unsafe.Pointer
+)
+
+
+type (
+	Pi pi /* ERROR "not a type" */
+
+	a /* ERROR "illegal cycle" */ a
+	a /* ERROR "redeclared" */ int
+
+	b /* ERROR "illegal cycle" */ c
+	c d
+	d e
+	e b
+
+	t *t
+
+	U V
+	V *W
+	W U
+
+	P1 *S2
+	P2 P1
+
+	S0 struct {
+	}
+	S1 struct {
+		a, b, c int
+		u, v, a /* ERROR "redeclared" */ float32
+	}
+	S2 struct {
+		S0 // embedded field
+		S0 /* ERROR "redeclared" */ int
+	}
+	S3 struct {
+		x S2
+	}
+	S4/* ERROR "illegal cycle" */ struct {
+		S4
+	}
+	S5 /* ERROR "illegal cycle" */ struct {
+		S6
+	}
+	S6 struct {
+		field S7
+	}
+	S7 struct {
+		S5
+	}
+
+	L1 []L1
+	L2 []int
+
+	A1 [10.0]int
+	A2 /* ERROR "illegal cycle" */ [10]A2
+	A3 /* ERROR "illegal cycle" */ [10]struct {
+		x A4
+	}
+	A4 [10]A3
+
+	F1 func()
+	F2 func(x, y, z float32)
+	F3 func(x, y, x /* ERROR "redeclared" */ float32)
+	F4 func() (x, y, x /* ERROR "redeclared" */ float32)
+	F5 func(x int) (x /* ERROR "redeclared" */ float32)
+	F6 func(x ...int)
+
+	I1 interface{}
+	I2 interface {
+		m1()
+	}
+	I3 interface {
+		m1()
+		m1 /* ERROR "duplicate method" */ ()
+	}
+	I4 interface {
+		m1(x, y, x /* ERROR "redeclared" */ float32)
+		m2() (x, y, x /* ERROR "redeclared" */ float32)
+		m3(x int) (x /* ERROR "redeclared" */ float32)
+	}
+	I5 interface {
+		m1(I5)
+	}
+	I6 interface {
+		S0 /* ERROR "not an interface" */
+	}
+	I7 interface {
+		I1
+		I1
+	}
+	I8 /* ERROR "illegal cycle" */ interface {
+		I8
+	}
+	I9 /* ERROR "illegal cycle" */ interface {
+		I10
+	}
+	I10 interface {
+		I11
+	}
+	I11 interface {
+		I9
+	}
+
+	C1 chan int
+	C2 <-chan int
+	C3 chan<- C3
+	C4 chan C5
+	C5 chan C6
+	C6 chan C4
+
+	M1 map[Last]string
+	M2 map[string]M2
+
+	Last int
+)
+
+// cycles in function/method declarations
+// (test cases for issues #5217, #25790 and variants)
+func f1(x f1 /* ERROR "not a type" */ ) {}
+func f2(x *f2 /* ERROR "not a type" */ ) {}
+func f3() (x f3 /* ERROR "not a type" */ ) { return }
+func f4() (x *f4 /* ERROR "not a type" */ ) { return }
+
+func (S0) m1 /* ERROR illegal cycle */ (x S0 /* ERROR value .* is not a type */ .m1) {}
+func (S0) m2 /* ERROR illegal cycle */ (x *S0 /* ERROR value .* is not a type */ .m2) {}
+func (S0) m3 /* ERROR illegal cycle */ () (x S0 /* ERROR value .* is not a type */ .m3) { return }
+func (S0) m4 /* ERROR illegal cycle */ () (x *S0 /* ERROR value .* is not a type */ .m4) { return }
+
+// interfaces may not have any blank methods
+type BlankI interface {
+	_ /* ERROR "invalid method name" */ ()
+	_ /* ERROR "invalid method name" */ (float32) int
+	m()
+}
+
+// non-interface types may have multiple blank methods
+type BlankT struct{}
+
+func (BlankT) _() {}
+func (BlankT) _(int) {}
+func (BlankT) _() int { return 0 }
+func (BlankT) _(int) int { return 0}
diff --git a/src/go/types/testdata/decls1.src b/src/go/types/testdata/decls1.src
new file mode 100644
index 0000000..f4d2eab
--- /dev/null
+++ b/src/go/types/testdata/decls1.src
@@ -0,0 +1,146 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// variable declarations
+
+package decls1
+
+import (
+	"math"
+)
+
+// Global variables without initialization
+var (
+	a, b bool
+	c byte
+	d uint8
+	r rune
+	i int
+	j, k, l int
+	x, y float32
+	xx, yy float64
+	u, v complex64
+	uu, vv complex128
+	s, t string
+	array []byte
+	iface interface{}
+
+	blank _ /* ERROR "cannot use _" */
+)
+
+// Global variables with initialization
+var (
+	s1 = i + j
+	s2 = i /* ERROR "mismatched types" */ + x
+	s3 = c + d
+	s4 = s + t
+	s5 = s /* ERROR "invalid operation" */ / t
+	s6 = array[t1]
+	s7 = array[x /* ERROR "integer" */]
+	s8 = &a
+	s10 = &42 /* ERROR "cannot take address" */
+	s11 = &v
+	s12 = -(u + *t11) / *&v
+	s13 = a /* ERROR "shifted operand" */ << d
+	s14 = i << j
+	s18 = math.Pi * 10.0
+	s19 = s1 /* ERROR "cannot call" */ ()
+ 	s20 = f0 /* ERROR "no value" */ ()
+	s21 = f6(1, s1, i)
+	s22 = f6(1, s1, uu /* ERROR "cannot use .* in argument" */ )
+
+	t1 int = i + j
+	t2 int = i /* ERROR "mismatched types" */ + x
+	t3 int = c /* ERROR "cannot use .* variable declaration" */ + d
+	t4 string = s + t
+	t5 string = s /* ERROR "invalid operation" */ / t
+	t6 byte = array[t1]
+	t7 byte = array[x /* ERROR "must be integer" */]
+	t8 *int = & /* ERROR "cannot use .* variable declaration" */ a
+	t10 *int = &42 /* ERROR "cannot take address" */
+	t11 *complex64 = &v
+	t12 complex64 = -(u + *t11) / *&v
+	t13 int = a /* ERROR "shifted operand" */ << d
+	t14 int = i << j
+	t15 math /* ERROR "not in selector" */
+	t16 math.xxx /* ERROR "not declared" */
+	t17 math /* ERROR "not a type" */ .Pi
+	t18 float64 = math.Pi * 10.0
+	t19 int = t1 /* ERROR "cannot call" */ ()
+	t20 int = f0 /* ERROR "no value" */ ()
+	t21 int = a /* ERROR "cannot use .* variable declaration" */
+)
+
+// Various more complex expressions
+var (
+	u1 = x /* ERROR "not an interface" */ .(int)
+	u2 = iface.([]int)
+	u3 = iface.(a /* ERROR "not a type" */ )
+	u4, ok = iface.(int)
+	u5, ok2, ok3 = iface /* ERROR "cannot initialize" */ .(int)
+)
+
+// Constant expression initializations
+var (
+	v1 = 1 /* ERROR "cannot convert" */ + "foo"
+	v2 = c + 255
+	v3 = c + 256 /* ERROR "overflows" */
+	v4 = r + 2147483647
+	v5 = r + 2147483648 /* ERROR "overflows" */
+	v6 = 42
+	v7 = v6 + 9223372036854775807
+	v8 = v6 + 9223372036854775808 /* ERROR "overflows" */
+	v9 = i + 1 << 10
+	v10 byte = 1024 /* ERROR "overflows" */
+	v11 = xx/yy*yy - xx
+	v12 = true && false
+	v13 = nil /* ERROR "use of untyped nil" */
+	v14 string = 257 // ERROR cannot use 257 .* as string value in variable declaration$
+	v15 int8 = 257 // ERROR cannot use 257 .* as int8 value in variable declaration .*overflows
+)
+
+// Multiple assignment expressions
+var (
+	m1a, m1b = 1, 2
+	m2a, m2b, m2c /* ERROR "missing init expr for m2c" */ = 1, 2
+	m3a, m3b = 1, 2, 3 /* ERROR "extra init expr 3" */
+)
+
+func _() {
+	var (
+		m1a, m1b = 1, 2
+		m2a, m2b, m2c /* ERROR "missing init expr for m2c" */ = 1, 2
+		m3a, m3b = 1, 2, 3 /* ERROR "extra init expr 3" */
+	)
+
+	_, _ = m1a, m1b
+	_, _, _ = m2a, m2b, m2c
+	_, _ = m3a, m3b
+}
+
+// Declaration of parameters and results
+func f0() {}
+func f1(a /* ERROR "not a type" */) {}
+func f2(a, b, c d /* ERROR "not a type" */) {}
+
+func f3() int { return 0 }
+func f4() a /* ERROR "not a type" */ { return 0 }
+func f5() (a, b, c d /* ERROR "not a type" */) { return }
+
+func f6(a, b, c int) complex128 { return 0 }
+
+// Declaration of receivers
+type T struct{}
+
+func (T) m0() {}
+func (*T) m1() {}
+func (x T) m2() {}
+func (x *T) m3() {}
+
+// Initialization functions
+func init() {}
+func /* ERROR "no arguments and no return values" */ init(int) {}
+func /* ERROR "no arguments and no return values" */ init() int { return 0 }
+func /* ERROR "no arguments and no return values" */ init(int) int { return 0 }
+func (T) init(int) int { return 0 }
diff --git a/src/go/types/testdata/decls2a.src b/src/go/types/testdata/decls2a.src
new file mode 100644
index 0000000..bdbecd9
--- /dev/null
+++ b/src/go/types/testdata/decls2a.src
@@ -0,0 +1,111 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// method declarations
+
+package decls2
+
+import "time"
+import "unsafe"
+
+// T1 declared before its methods.
+type T1 struct{
+	f int
+}
+
+func (T1) m() {}
+func (T1) m /* ERROR "already declared" */ () {}
+func (x *T1) f /* ERROR "field and method" */ () {}
+
+// Conflict between embedded field and method name,
+// with the embedded field being a basic type.
+type T1b struct {
+	int
+}
+
+func (T1b) int /* ERROR "field and method" */ () {}
+
+type T1c struct {
+	time.Time
+}
+
+func (T1c) Time /* ERROR "field and method" */ () int { return 0 }
+
+// Disabled for now: LookupFieldOrMethod will find Pointer even though
+// it's double-declared (it would cost extra in the common case to verify
+// this). But the MethodSet computation will not find it due to the name
+// collision caused by the double-declaration, leading to an internal
+// inconsistency while we are verifying one computation against the other.
+// var _ = T1c{}.Pointer
+
+// T2's method declared before the type.
+func (*T2) f /* ERROR "field and method" */ () {}
+
+type T2 struct {
+	f int
+}
+
+// Methods declared without a declared type.
+func (undeclared /* ERROR "undeclared" */) m() {}
+func (x *undeclared /* ERROR "undeclared" */) m() {}
+
+func (pi /* ERROR "not a type" */) m1() {}
+func (x pi /* ERROR "not a type" */) m2() {}
+func (x *pi /* ERROR "not a type" */ ) m3() {}
+
+// Blank types.
+type _ struct { m int }
+type _ struct { m int }
+
+func (_ /* ERROR "cannot use _" */) m() {}
+func m(_ /* ERROR "cannot use _" */) {}
+
+// Methods with receiver base type declared in another file.
+func (T3) m1() {}
+func (*T3) m2() {}
+func (x T3) m3() {}
+func (x *T3) f /* ERROR "field and method" */ () {}
+
+// Methods of non-struct type.
+type T4 func()
+
+func (self T4) m() func() { return self }
+
+// Methods associated with an interface.
+type T5 interface {
+	m() int
+}
+
+func (T5 /* ERROR "invalid receiver" */ ) m1() {}
+func (T5 /* ERROR "invalid receiver" */ ) m2() {}
+
+// Methods associated with a named pointer type.
+type ptr *int
+func (ptr /* ERROR "invalid receiver" */ ) _() {}
+func (* /* ERROR "invalid receiver" */ ptr) _() {}
+
+// Methods with zero or multiple receivers.
+func ( /* ERROR "missing receiver" */ ) _() {}
+func (T3, * /* ERROR "exactly one receiver" */ T3) _() {}
+func (T3, T3, T3 /* ERROR "exactly one receiver" */ ) _() {}
+func (a, b /* ERROR "exactly one receiver" */ T3) _() {}
+func (a, b, c /* ERROR "exactly one receiver" */ T3) _() {}
+
+// Methods associated with non-local or unnamed types.
+func (int /* ERROR "invalid receiver" */ ) m() {}
+func ([ /* ERROR "invalid receiver" */ ]int) m() {}
+func (time /* ERROR "invalid receiver" */ .Time) m() {}
+func (* /* ERROR "invalid receiver" */ time.Time) m() {}
+func (x /* ERROR "invalid receiver" */ interface{}) m() {}
+
+// Unsafe.Pointer is treated like a pointer when used as receiver type.
+type UP unsafe.Pointer
+func (UP /* ERROR "invalid" */ ) m1() {}
+func (* /* ERROR "invalid" */ UP) m2() {}
+
+// Double declarations across package files
+const c_double = 0
+type t_double int
+var v_double int
+func f_double() {}
diff --git a/src/go/types/testdata/decls2b.src b/src/go/types/testdata/decls2b.src
new file mode 100644
index 0000000..5c55750
--- /dev/null
+++ b/src/go/types/testdata/decls2b.src
@@ -0,0 +1,75 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// method declarations
+
+package decls2
+
+import "io"
+
+const pi = 3.1415
+
+func (T1) m /* ERROR "already declared" */ () {}
+func (T2) m(io.Writer) {}
+
+type T3 struct {
+	f *T3
+}
+
+type T6 struct {
+	x int
+}
+
+func (t *T6) m1() int {
+	return t.x
+}
+
+func f() {
+	var t *T6
+	t.m1()
+}
+
+// Double declarations across package files
+const c_double /* ERROR "redeclared" */ = 0
+type t_double  /* ERROR "redeclared" */ int
+var v_double /* ERROR "redeclared" */ int
+func f_double /* ERROR "redeclared" */ () {}
+
+// Blank methods need to be type-checked.
+// Verify by checking that errors are reported.
+func (T /* ERROR "undeclared" */ ) _() {}
+func (T1) _(undeclared /* ERROR "undeclared" */ ) {}
+func (T1) _() int { return "foo" /* ERROR "cannot use .* in return statement" */ }
+
+// Methods with undeclared receiver type can still be checked.
+// Verify by checking that errors are reported.
+func (Foo /* ERROR "undeclared" */ ) m() {}
+func (Foo /* ERROR "undeclared" */ ) m(undeclared /* ERROR "undeclared" */ ) {}
+func (Foo /* ERROR "undeclared" */ ) m() int { return "foo" /* ERROR "cannot use .* in return statement" */ }
+
+func (Foo /* ERROR "undeclared" */ ) _() {}
+func (Foo /* ERROR "undeclared" */ ) _(undeclared /* ERROR "undeclared" */ ) {}
+func (Foo /* ERROR "undeclared" */ ) _() int { return "foo" /* ERROR "cannot use .* in return statement" */ }
+
+// Receiver declarations are regular parameter lists;
+// receiver types may use parentheses, and the list
+// may have a trailing comma.
+type T7 struct {}
+
+func (T7) m1() {}
+func ((T7)) m2() {}
+func ((*T7)) m3() {}
+func (x *(T7),) m4() {}
+func (x (*(T7)),) m5() {}
+func (x ((*((T7)))),) m6() {}
+
+// Check that methods with parenthesized receiver are actually present (issue #23130).
+var (
+	_ = T7.m1
+	_ = T7.m2
+	_ = (*T7).m3
+	_ = (*T7).m4
+	_ = (*T7).m5
+	_ = (*T7).m6
+)
diff --git a/src/go/types/testdata/decls3.src b/src/go/types/testdata/decls3.src
new file mode 100644
index 0000000..745175c
--- /dev/null
+++ b/src/go/types/testdata/decls3.src
@@ -0,0 +1,309 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// embedded types
+
+package decls3
+
+import "unsafe"
+import "fmt"
+
+// fields with the same name at the same level cancel each other out
+
+func _() {
+	type (
+		T1 struct { X int }
+		T2 struct { X int }
+		T3 struct { T1; T2 } // X is embedded twice at the same level via T1->X, T2->X
+	)
+
+	var t T3
+	_ = t.X /* ERROR "ambiguous selector t.X" */
+}
+
+func _() {
+	type (
+		T1 struct { X int }
+		T2 struct { T1 }
+		T3 struct { T1 }
+		T4 struct { T2; T3 } // X is embedded twice at the same level via T2->T1->X, T3->T1->X
+	)
+
+	var t T4
+	_ = t.X /* ERROR "ambiguous selector t.X" */
+}
+
+func issue4355() {
+	type (
+	    T1 struct {X int}
+	    T2 struct {T1}
+	    T3 struct {T2}
+	    T4 struct {T2}
+	    T5 struct {T3; T4} // X is embedded twice at the same level via T3->T2->T1->X, T4->T2->T1->X
+	)
+
+	var t T5
+	_ = t.X /* ERROR "ambiguous selector t.X" */
+}
+
+func _() {
+	type State int
+	type A struct{ State }
+	type B struct{ fmt.State }
+	type T struct{ A; B }
+
+	var t T
+	_ = t.State /* ERROR "ambiguous selector t.State" */
+}
+
+// Embedded fields can be predeclared types.
+
+func _() {
+	type T0 struct{
+		int
+		float32
+		f int
+	}
+	var x T0
+	_ = x.int
+	_ = x.float32
+	_ = x.f
+
+	type T1 struct{
+		T0
+	}
+	var y T1
+	_ = y.int
+	_ = y.float32
+	_ = y.f
+}
+
+// Restrictions on embedded field types.
+
+func _() {
+	type I1 interface{}
+	type I2 interface{}
+	type P1 *int
+	type P2 *int
+	type UP unsafe.Pointer
+
+	type T1 struct {
+		I1
+		* /* ERROR "cannot be a pointer to an interface" */ I2
+		* /* ERROR "cannot be a pointer to an interface" */ error
+		P1 /* ERROR "cannot be a pointer" */
+		* /* ERROR "cannot be a pointer" */ P2
+	}
+
+	// unsafe.Pointers are treated like regular pointers when embedded
+	type T2 struct {
+		unsafe /* ERROR "cannot be unsafe.Pointer" */ .Pointer
+		*/* ERROR "cannot be unsafe.Pointer" */ /* ERROR "Pointer redeclared" */ unsafe.Pointer
+		UP /* ERROR "cannot be unsafe.Pointer" */
+		* /* ERROR "cannot be unsafe.Pointer" */  /* ERROR "UP redeclared" */ UP
+	}
+}
+
+// Named types that are pointers.
+
+type S struct{ x int }
+func (*S) m() {}
+type P *S
+
+func _() {
+	var s *S
+	_ = s.x
+	_ = s.m
+
+	var p P
+	_ = p.x
+	_ = p.m /* ERROR "no field or method" */
+	_ = P.m /* ERROR "no field or method" */
+}
+
+// Borrowed from the FieldByName test cases in reflect/all_test.go.
+
+type D1 struct {
+	d int
+}
+type D2 struct {
+	d int
+}
+
+type S0 struct {
+	A, B, C int
+	D1
+	D2
+}
+
+type S1 struct {
+	B int
+	S0
+}
+
+type S2 struct {
+	A int
+	*S1
+}
+
+type S1x struct {
+	S1
+}
+
+type S1y struct {
+	S1
+}
+
+type S3 struct {
+	S1x
+	S2
+	D, E int
+	*S1y
+}
+
+type S4 struct {
+	*S4
+	A int
+}
+
+// The X in S6 and S7 annihilate, but they also block the X in S8.S9.
+type S5 struct {
+	S6
+	S7
+	S8
+}
+
+type S6 struct {
+	X int
+}
+
+type S7 S6
+
+type S8 struct {
+	S9
+}
+
+type S9 struct {
+	X int
+	Y int
+}
+
+// The X in S11.S6 and S12.S6 annihilate, but they also block the X in S13.S8.S9.
+type S10 struct {
+	S11
+	S12
+	S13
+}
+
+type S11 struct {
+	S6
+}
+
+type S12 struct {
+	S6
+}
+
+type S13 struct {
+	S8
+}
+
+func _() {
+	_ = struct{}{}.Foo /* ERROR "no field or method" */
+	_ = S0{}.A
+	_ = S0{}.D /* ERROR "no field or method" */
+	_ = S1{}.A
+	_ = S1{}.B
+	_ = S1{}.S0
+	_ = S1{}.C
+	_ = S2{}.A
+	_ = S2{}.S1
+	_ = S2{}.B
+	_ = S2{}.C
+	_ = S2{}.D /* ERROR "no field or method" */
+	_ = S3{}.S1 /* ERROR "ambiguous selector \(S3 literal\).S1" */
+	_ = S3{}.A
+	_ = S3{}.B /* ERROR "ambiguous selector" \(S3 literal\).B */
+	_ = S3{}.D
+	_ = S3{}.E
+	_ = S4{}.A
+	_ = S4{}.B /* ERROR "no field or method" */
+	_ = S5{}.X /* ERROR "ambiguous selector \(S5 literal\).X" */
+	_ = S5{}.Y
+	_ = S10{}.X /* ERROR "ambiguous selector \(S10 literal\).X" */
+	_ = S10{}.Y
+}
+
+// Borrowed from the FieldByName benchmark in reflect/all_test.go.
+
+type R0 struct {
+	*R1
+	*R2
+	*R3
+	*R4
+}
+
+type R1 struct {
+	*R5
+	*R6
+	*R7
+	*R8
+}
+
+type R2 R1
+type R3 R1
+type R4 R1
+
+type R5 struct {
+	*R9
+	*R10
+	*R11
+	*R12
+}
+
+type R6 R5
+type R7 R5
+type R8 R5
+
+type R9 struct {
+	*R13
+	*R14
+	*R15
+	*R16
+}
+
+type R10 R9
+type R11 R9
+type R12 R9
+
+type R13 struct {
+	*R17
+	*R18
+	*R19
+	*R20
+}
+
+type R14 R13
+type R15 R13
+type R16 R13
+
+type R17 struct {
+	*R21
+	*R22
+	*R23
+	*R24
+}
+
+type R18 R17
+type R19 R17
+type R20 R17
+
+type R21 struct {
+	X int
+}
+
+type R22 R21
+type R23 R21
+type R24 R21
+
+var _ = R0{}.X /* ERROR "ambiguous selector \(R0 literal\).X" */
\ No newline at end of file
diff --git a/src/go/types/testdata/decls4.src b/src/go/types/testdata/decls4.src
new file mode 100644
index 0000000..140bbfd
--- /dev/null
+++ b/src/go/types/testdata/decls4.src
@@ -0,0 +1,199 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// type aliases
+
+package decls4
+
+type (
+	T0 [10]int
+	T1 []byte
+	T2 struct {
+		x int
+	}
+	T3 interface{
+		m() T2
+	}
+	T4 func(int, T0) chan T2
+)
+
+type (
+	Ai = int
+	A0 = T0
+	A1 = T1
+	A2 = T2
+	A3 = T3
+	A4 = T4
+
+	A10 = [10]int
+	A11 = []byte
+	A12 = struct {
+		x int
+	}
+	A13 = interface{
+		m() A2
+	}
+	A14 = func(int, A0) chan A2
+)
+
+// check assignment compatibility due to equality of types
+var (
+	xi_ int
+	ai Ai = xi_
+
+	x0 T0
+	a0 A0 = x0
+
+	x1 T1
+	a1 A1 = x1
+
+	x2 T2
+	a2 A2 = x2
+
+	x3 T3
+	a3 A3 = x3
+
+	x4 T4
+	a4 A4 = x4
+)
+
+// alias receiver types
+func (Ai /* ERROR "invalid receiver" */) m1() {}
+func (T0) m1() {}
+func (A0) m1 /* ERROR already declared */ () {}
+func (A0) m2 () {}
+func (A3 /* ERROR invalid receiver */ ) m1 () {}
+func (A10 /* ERROR invalid receiver */ ) m1() {}
+
+// x0 has methods m1, m2 declared via receiver type names T0 and A0
+var _ interface{ m1(); m2() } = x0
+
+// alias receiver types (test case for issue #23042)
+type T struct{}
+
+var (
+	_ = T.m
+	_ = T{}.m
+	_ interface{m()} = T{}
+)
+
+var (
+	_ = T.n
+	_ = T{}.n
+	_ interface{m(); n()} = T{}
+)
+
+type U = T
+func (U) m() {}
+
+// alias receiver types (long type declaration chains)
+type (
+	V0 = V1
+	V1 = (V2)
+	V2 = ((V3))
+	V3 = T
+)
+
+func (V0) m /* ERROR already declared */ () {}
+func (V1) n() {}
+
+// alias receiver types (invalid due to cycles)
+type (
+	W0 /* ERROR illegal cycle */ = W1
+	W1 = (W2)
+	W2 = ((W0))
+)
+
+func (W0) m() {} // no error expected (due to above cycle error)
+func (W1) n() {}
+
+// alias receiver types (invalid due to builtin underlying type)
+type (
+	B0 = B1
+	B1 = B2
+	B2 = int
+)
+
+func (B0 /* ERROR invalid receiver */ ) m() {}
+func (B1 /* ERROR invalid receiver */ ) n() {}
+
+// cycles
+type (
+	C2 /* ERROR illegal cycle */ = C2
+	C3 /* ERROR illegal cycle */ = C4
+	C4 = C3
+	C5 struct {
+		f *C6
+	}
+	C6 = C5
+	C7 /* ERROR illegal cycle */  struct {
+		f C8
+	}
+	C8 = C7
+)
+
+// embedded fields
+var (
+	s0 struct { T0 }
+	s1 struct { A0 } = s0 /* ERROR cannot use */ // embedded field names are different
+)
+
+// embedding and lookup of fields and methods
+func _(s struct{A0}) { s.A0 = x0 }
+
+type eX struct{xf int}
+
+func (eX) xm()
+
+type eY = struct{eX} // field/method set of eY includes xf, xm
+
+type eZ = *struct{eX} // field/method set of eZ includes xf, xm
+
+type eA struct {
+	eX // eX contributes xf, xm to eA
+}
+
+type eA2 struct {
+	*eX // *eX contributes xf, xm to eA
+}
+
+type eB struct {
+	eY // eY contributes xf, xm to eB
+}
+
+type eB2 struct {
+	*eY // *eY contributes xf, xm to eB
+}
+
+type eC struct {
+	eZ // eZ contributes xf, xm to eC
+}
+
+var (
+	_ = eA{}.xf
+	_ = eA{}.xm
+	_ = eA2{}.xf
+	_ = eA2{}.xm
+	_ = eB{}.xf
+	_ = eB{}.xm
+	_ = eB2{}.xf
+	_ = eB2{}.xm
+	_ = eC{}.xf
+	_ = eC{}.xm
+)
+
+// ambiguous selectors due to embedding via type aliases
+type eD struct {
+	eY
+	eZ
+}
+
+var (
+	_ = eD{}.xf /* ERROR ambiguous selector \(eD literal\).xf */
+	_ = eD{}.xm /* ERROR ambiguous selector \(eD literal\).xm */
+)
+
+var (
+	_ interface{ xm() } = eD /* ERROR missing method xm */ {}
+)
\ No newline at end of file
diff --git a/src/go/types/testdata/decls5.src b/src/go/types/testdata/decls5.src
new file mode 100644
index 0000000..88d3194
--- /dev/null
+++ b/src/go/types/testdata/decls5.src
@@ -0,0 +1,10 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+// declarations of main
+const _, main /* ERROR "cannot declare main" */ , _ = 0, 1, 2
+type main /* ERROR "cannot declare main" */ struct{}
+var _, main /* ERROR "cannot declare main" */ int
diff --git a/src/go/types/testdata/errors.src b/src/go/types/testdata/errors.src
new file mode 100644
index 0000000..ff92921
--- /dev/null
+++ b/src/go/types/testdata/errors.src
@@ -0,0 +1,60 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors
+
+// Testing precise operand formatting in error messages
+// (matching messages are regular expressions, hence the \'s).
+func f(x int, m map[string]int) {
+	// no values
+	_ = f /* ERROR "f\(0, m\) \(no value\) used as value" */ (0, m)
+
+	// built-ins
+	_ = println /* ERROR "println \(built-in\) must be called" */
+
+	// types
+	_ = complex128 /* ERROR "complex128 \(type\) is not an expression" */
+
+	// constants
+	const c1 = 991
+	const c2 float32 = 0.5
+	0 /* ERROR "0 \(untyped int constant\) is not used" */
+	c1 /* ERROR "c1 \(untyped int constant 991\) is not used" */
+	c2 /* ERROR "c2 \(constant 0.5 of type float32\) is not used" */
+	c1 /* ERROR "c1 \+ c2 \(constant 991.5 of type float32\) is not used" */ + c2
+
+	// variables
+	x /* ERROR "x \(variable of type int\) is not used" */
+
+	// values
+	x /* ERROR "x != x \(untyped bool value\) is not used" */ != x
+	x /* ERROR "x \+ x \(value of type int\) is not used" */ + x
+
+	// value, ok's
+	const s = "foo"
+	m /* ERROR "m\[s\] \(map index expression of type int\) is not used" */ [s]
+}
+
+// Valid ERROR comments can have a variety of forms.
+func _() {
+	0 /* ERROR "0 .* is not used" */
+	0 /* ERROR 0 .* is not used */
+	0 // ERROR "0 .* is not used"
+	0 // ERROR 0 .* is not used
+}
+
+// Don't report spurious errors as a consequence of earlier errors.
+// Add more tests as needed.
+func _() {
+	if err := foo /* ERROR undeclared */ (); err != nil /* no error here */ {}
+}
+
+// Use unqualified names for package-local objects.
+type T struct{}
+var _ int = T /* ERROR value of type T */ {} // use T in error message rather then errors.T
+
+// Don't report errors containing "invalid type" (issue #24182).
+func _(x *missing /* ERROR undeclared name: missing */ ) {
+	x.m() // there shouldn't be an error here referring to *invalid type
+}
diff --git a/src/go/types/testdata/expr0.src b/src/go/types/testdata/expr0.src
new file mode 100644
index 0000000..1aac726
--- /dev/null
+++ b/src/go/types/testdata/expr0.src
@@ -0,0 +1,180 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// unary expressions
+
+package expr0 
+
+type mybool bool
+
+var (
+	// bool
+	b0 = true
+	b1 bool = b0
+	b2 = !true
+	b3 = !b1
+	b4 bool = !true
+	b5 bool = !b4
+	b6 = +b0 /* ERROR "not defined" */
+	b7 = -b0 /* ERROR "not defined" */
+	b8 = ^b0 /* ERROR "not defined" */
+	b9 = *b0 /* ERROR "cannot indirect" */
+	b10 = &true /* ERROR "cannot take address" */
+	b11 = &b0
+	b12 = <-b0 /* ERROR "cannot receive" */
+	b13 = & & /* ERROR "cannot take address" */ b0
+
+	// byte
+	_ = byte(0)
+	_ = byte(- /* ERROR "cannot convert" */ 1)
+	_ = - /* ERROR "-byte\(1\) \(constant -1 of type byte\) overflows byte" */ byte(1) // test for issue 11367
+	_ = byte /* ERROR "overflows byte" */ (0) - byte(1)
+
+	// int
+	i0 = 1
+	i1 int = i0
+	i2 = +1
+	i3 = +i0
+	i4 int = +1
+	i5 int = +i4
+	i6 = -1
+	i7 = -i0
+	i8 int = -1
+	i9 int = -i4
+	i10 = !i0 /* ERROR "not defined" */
+	i11 = ^1
+	i12 = ^i0
+	i13 int = ^1
+	i14 int = ^i4
+	i15 = *i0 /* ERROR "cannot indirect" */
+	i16 = &i0
+	i17 = *i16
+	i18 = <-i16 /* ERROR "cannot receive" */
+
+	// uint
+	u0 = uint(1)
+	u1 uint = u0
+	u2 = +1
+	u3 = +u0
+	u4 uint = +1
+	u5 uint = +u4
+	u6 = -1
+	u7 = -u0
+	u8 uint = - /* ERROR "overflows" */ 1
+	u9 uint = -u4
+	u10 = !u0 /* ERROR "not defined" */
+	u11 = ^1
+	u12 = ^i0
+	u13 uint = ^ /* ERROR "overflows" */ 1
+	u14 uint = ^u4
+	u15 = *u0 /* ERROR "cannot indirect" */
+	u16 = &u0
+	u17 = *u16
+	u18 = <-u16 /* ERROR "cannot receive" */
+	u19 = ^uint(0)
+
+	// float64
+	f0 = float64(1)
+	f1 float64 = f0
+	f2 = +1
+	f3 = +f0
+	f4 float64 = +1
+	f5 float64 = +f4
+	f6 = -1
+	f7 = -f0
+	f8 float64 = -1
+	f9 float64 = -f4
+	f10 = !f0 /* ERROR "not defined" */
+	f11 = ^1
+	f12 = ^i0
+	f13 float64 = ^1
+	f14 float64 = ^f4 /* ERROR "not defined" */
+	f15 = *f0 /* ERROR "cannot indirect" */
+	f16 = &f0
+	f17 = *u16
+	f18 = <-u16 /* ERROR "cannot receive" */
+
+	// complex128
+	c0 = complex128(1)
+	c1 complex128 = c0
+	c2 = +1
+	c3 = +c0
+	c4 complex128 = +1
+	c5 complex128 = +c4
+	c6 = -1
+	c7 = -c0
+	c8 complex128 = -1
+	c9 complex128 = -c4
+	c10 = !c0 /* ERROR "not defined" */
+	c11 = ^1
+	c12 = ^i0
+	c13 complex128 = ^1
+	c14 complex128 = ^c4 /* ERROR "not defined" */
+	c15 = *c0 /* ERROR "cannot indirect" */
+	c16 = &c0
+	c17 = *u16
+	c18 = <-u16 /* ERROR "cannot receive" */
+
+	// string
+	s0 = "foo"
+	s1 = +"foo" /* ERROR "not defined" */
+	s2 = -s0 /* ERROR "not defined" */
+	s3 = !s0 /* ERROR "not defined" */
+	s4 = ^s0 /* ERROR "not defined" */
+	s5 = *s4
+	s6 = &s4
+	s7 = *s6
+	s8 = <-s7
+
+	// channel
+	ch chan int
+	rc <-chan float64
+	sc chan <- string
+	ch0 = +ch /* ERROR "not defined" */
+	ch1 = -ch /* ERROR "not defined" */
+	ch2 = !ch /* ERROR "not defined" */
+	ch3 = ^ch /* ERROR "not defined" */
+	ch4 = *ch /* ERROR "cannot indirect" */
+	ch5 = &ch
+	ch6 = *ch5
+	ch7 = <-ch
+	ch8 = <-rc
+	ch9 = <-sc /* ERROR "cannot receive" */
+	ch10, ok = <-ch
+	// ok is of type bool
+	ch11, myok = <-ch
+	_ mybool = myok /* ERROR "cannot use .* in variable declaration" */
+)
+
+// address of composite literals
+type T struct{x, y int}
+
+func f() T { return T{} }
+
+var (
+	_ = &T{1, 2}
+	_ = &[...]int{}
+	_ = &[]int{}
+	_ = &[]int{}
+	_ = &map[string]T{}
+	_ = &(T{1, 2})
+	_ = &((((T{1, 2}))))
+	_ = &f /* ERROR "cannot take address" */ ()
+)
+
+// recursive pointer types
+type P *P
+
+var (
+	p1 P = new(P)
+	p2 P = *p1
+	p3 P = &p2
+)
+
+func g() (a, b int) { return }
+
+func _() {
+	_ = -g /* ERROR 2-valued g */ ()
+	_ = <-g /* ERROR 2-valued g */ ()
+}
diff --git a/src/go/types/testdata/expr1.src b/src/go/types/testdata/expr1.src
new file mode 100644
index 0000000..4ead815
--- /dev/null
+++ b/src/go/types/testdata/expr1.src
@@ -0,0 +1,127 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// binary expressions
+
+package expr1
+
+type mybool bool
+
+func _(x, y bool, z mybool) {
+	x = x || y
+	x = x || true
+	x = x || false
+	x = x && y
+	x = x && true
+	x = x && false
+
+	z = z /* ERROR mismatched types */ || y
+	z = z || true
+	z = z || false
+	z = z /* ERROR mismatched types */ && y
+	z = z && true
+	z = z && false
+}
+
+type myint int
+
+func _(x, y int, z myint) {
+	x = x + 1
+	x = x + 1.0
+	x = x + 1.1 // ERROR truncated to int
+	x = x + y
+	x = x - y
+	x = x * y
+	x = x / y
+	x = x % y
+	x = x << y
+	x = x >> y
+
+	z = z + 1
+	z = z + 1.0
+	z = z + 1.1 // ERROR truncated to int
+	z = z /* ERROR mismatched types */ + y
+	z = z /* ERROR mismatched types */ - y
+	z = z /* ERROR mismatched types */ * y
+	z = z /* ERROR mismatched types */ / y
+	z = z /* ERROR mismatched types */ % y
+	z = z << y
+	z = z >> y
+}
+
+type myuint uint
+
+func _(x, y uint, z myuint) {
+	x = x + 1
+	x = x + - /* ERROR overflows uint */ 1
+	x = x + 1.0
+	x = x + 1.1 // ERROR truncated to uint
+	x = x + y
+	x = x - y
+	x = x * y
+	x = x / y
+	x = x % y
+	x = x << y
+	x = x >> y
+
+	z = z + 1
+	z = x + - /* ERROR overflows uint */ 1
+	z = z + 1.0
+	z = z + 1.1 // ERROR truncated to uint
+	z = z /* ERROR mismatched types */ + y
+	z = z /* ERROR mismatched types */ - y
+	z = z /* ERROR mismatched types */ * y
+	z = z /* ERROR mismatched types */ / y
+	z = z /* ERROR mismatched types */ % y
+	z = z << y
+	z = z >> y
+}
+
+type myfloat64 float64
+
+func _(x, y float64, z myfloat64) {
+	x = x + 1
+	x = x + -1
+	x = x + 1.0
+	x = x + 1.1
+	x = x + y
+	x = x - y
+	x = x * y
+	x = x / y
+	x = x /* ERROR not defined */ % y
+	x = x /* ERROR operand x .* must be integer */ << y
+	x = x /* ERROR operand x .* must be integer */ >> y
+
+	z = z + 1
+	z = z + -1
+	z = z + 1.0
+	z = z + 1.1
+	z = z /* ERROR mismatched types */ + y
+	z = z /* ERROR mismatched types */ - y
+	z = z /* ERROR mismatched types */ * y
+	z = z /* ERROR mismatched types */ / y
+	z = z /* ERROR mismatched types */ % y
+	z = z /* ERROR operand z .* must be integer */ << y
+	z = z /* ERROR operand z .* must be integer */ >> y
+}
+
+type mystring string
+
+func _(x, y string, z mystring) {
+	x = x + "foo"
+	x = x /* ERROR not defined */ - "foo"
+	x = x + 1 // ERROR cannot convert
+	x = x + y
+	x = x /* ERROR not defined */ - y
+	x = x * 10 // ERROR cannot convert
+}
+
+func f() (a, b int) { return }
+
+func _(x int) {
+	_ = f /* ERROR 2-valued f */ () + 1
+	_ = x + f /* ERROR 2-valued f */ ()
+	_ = f /* ERROR 2-valued f */ () + f
+	_ = f /* ERROR 2-valued f */ () + f /* ERROR 2-valued f */ ()
+}
diff --git a/src/go/types/testdata/expr2.src b/src/go/types/testdata/expr2.src
new file mode 100644
index 0000000..0c959e8
--- /dev/null
+++ b/src/go/types/testdata/expr2.src
@@ -0,0 +1,260 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// comparisons
+
+package expr2
+
+func _bool() {
+	const t = true == true
+	const f = true == false
+	_ = t /* ERROR "cannot compare" */ < f
+	_ = 0 /* ERROR "cannot convert" */ == t
+	var b bool
+	var x, y float32
+	b = x < y
+	_ = b
+	_ = struct{b bool}{x < y}
+}
+
+// corner cases
+var (
+	v0 = nil /* ERROR "cannot compare" */ == nil
+)
+
+func arrays() {
+	// basics
+	var a, b [10]int
+	_ = a == b
+	_ = a != b
+	_ = a /* ERROR < not defined */ < b
+	_ = a == nil /* ERROR cannot convert */
+
+	type C [10]int
+	var c C
+	_ = a == c
+
+	type D [10]int
+	var d D
+	_ = c /* ERROR mismatched types */ == d
+
+	var e [10]func() int
+	_ = e /* ERROR == not defined */ == e
+}
+
+func structs() {
+	// basics
+	var s, t struct {
+		x int
+		a [10]float32
+		_ bool
+	}
+	_ = s == t
+	_ = s != t
+	_ = s /* ERROR < not defined */ < t
+	_ = s == nil /* ERROR cannot convert */
+
+	type S struct {
+		x int
+		a [10]float32
+		_ bool
+	}
+	type T struct {
+		x int
+		a [10]float32
+		_ bool
+	}
+	var ss S
+	var tt T
+	_ = s == ss
+	_ = ss /* ERROR mismatched types */ == tt
+
+	var u struct {
+		x int
+		a [10]map[string]int
+	}
+	_ = u /* ERROR cannot compare */ == u
+}
+
+func pointers() {
+	// nil
+	_ = nil /* ERROR == not defined */ == nil
+	_ = nil /* ERROR != not defined */ != nil
+	_ = nil /* ERROR < not defined */ < nil
+	_ = nil /* ERROR <= not defined */ <= nil
+	_ = nil /* ERROR > not defined */ > nil
+	_ = nil /* ERROR >= not defined */ >= nil
+
+	// basics
+	var p, q *int
+	_ = p == q
+	_ = p != q
+
+	_ = p == nil
+	_ = p != nil
+	_ = nil == q
+	_ = nil != q
+
+	_ = p /* ERROR < not defined */ < q
+	_ = p /* ERROR <= not defined */ <= q
+	_ = p /* ERROR > not defined */ > q
+	_ = p /* ERROR >= not defined */ >= q
+
+	// various element types
+	type (
+		S1 struct{}
+		S2 struct{}
+		P1 *S1
+		P2 *S2
+	)
+	var (
+		ps1 *S1
+		ps2 *S2
+		p1 P1
+		p2 P2
+	)
+	_ = ps1 == ps1
+	_ = ps1 /* ERROR mismatched types */ == ps2
+	_ = ps2 /* ERROR mismatched types */ == ps1
+
+	_ = p1 == p1
+	_ = p1 /* ERROR mismatched types */ == p2
+
+	_ = p1 == ps1
+}
+
+func channels() {
+	// basics
+	var c, d chan int
+	_ = c == d
+	_ = c != d
+	_ = c == nil
+	_ = c /* ERROR < not defined */ < d
+
+	// various element types (named types)
+	type (
+		C1 chan int
+		C1r <-chan int
+		C1s chan<- int
+		C2 chan float32
+	)
+	var (
+		c1 C1
+		c1r C1r
+		c1s C1s
+		c1a chan int
+		c2 C2
+	)
+	_ = c1 == c1
+	_ = c1 /* ERROR mismatched types */ == c1r
+	_ = c1 /* ERROR mismatched types */ == c1s
+	_ = c1r /* ERROR mismatched types */ == c1s
+	_ = c1 == c1a
+	_ = c1a == c1
+	_ = c1 /* ERROR mismatched types */ == c2
+	_ = c1a /* ERROR mismatched types */ == c2
+
+	// various element types (unnamed types)
+	var (
+		d1 chan int
+		d1r <-chan int
+		d1s chan<- int
+		d1a chan<- int
+		d2 chan float32
+	)
+	_ = d1 == d1
+	_ = d1 == d1r
+	_ = d1 == d1s
+	_ = d1r /* ERROR mismatched types */ == d1s
+	_ = d1 == d1a
+	_ = d1a == d1
+	_ = d1 /* ERROR mismatched types */ == d2
+	_ = d1a /* ERROR mismatched types */ == d2
+}
+
+// for interfaces test
+type S1 struct{}
+type S11 struct{}
+type S2 struct{}
+func (*S1) m() int
+func (*S11) m() int
+func (*S11) n()
+func (*S2) m() float32
+
+func interfaces() {
+	// basics
+	var i, j interface{ m() int }
+	_ = i == j
+	_ = i != j
+	_ = i == nil
+	_ = i /* ERROR < not defined */ < j
+
+	// various interfaces
+	var ii interface { m() int; n() }
+	var k interface { m() float32 }
+	_ = i == ii
+	_ = i /* ERROR mismatched types */ == k
+
+	// interfaces vs values
+	var s1 S1
+	var s11 S11
+	var s2 S2
+
+	_ = i == 0 /* ERROR cannot convert */
+	_ = i /* ERROR mismatched types */ == s1
+	_ = i == &s1
+	_ = i == &s11
+
+	_ = i /* ERROR mismatched types */ == s2
+	_ = i /* ERROR mismatched types */ == &s2
+
+	// issue #28164
+	// testcase from issue
+	_ = interface /* ERROR cannot compare */ {}(nil) == []int(nil)
+
+	// related cases
+	var e interface{}
+	var s []int
+	var x int
+	_ = e /* ERROR cannot compare */ == s
+	_ = s /* ERROR cannot compare */ == e
+	_ = e /* ERROR cannot compare */ < x
+	_ = x /* ERROR cannot compare */ < e
+}
+
+func slices() {
+	// basics
+	var s []int
+	_ = s == nil
+	_ = s != nil
+	_ = s /* ERROR < not defined */ < nil
+
+	// slices are not otherwise comparable
+	_ = s /* ERROR == not defined */ == s
+	_ = s /* ERROR < not defined */ < s
+}
+
+func maps() {
+	// basics
+	var m map[string]int
+	_ = m == nil
+	_ = m != nil
+	_ = m /* ERROR < not defined */ < nil
+
+	// maps are not otherwise comparable
+	_ = m /* ERROR == not defined */ == m
+	_ = m /* ERROR < not defined */ < m
+}
+
+func funcs() {
+	// basics
+	var f func(int) float32
+	_ = f == nil
+	_ = f != nil
+	_ = f /* ERROR < not defined */ < nil
+
+	// funcs are not otherwise comparable
+	_ = f /* ERROR == not defined */ == f
+	_ = f /* ERROR < not defined */ < f
+}
diff --git a/src/go/types/testdata/expr3.src b/src/go/types/testdata/expr3.src
new file mode 100644
index 0000000..e6777aa
--- /dev/null
+++ b/src/go/types/testdata/expr3.src
@@ -0,0 +1,563 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package expr3
+
+import "time"
+
+func indexes() {
+	_ = 1 /* ERROR "cannot index" */ [0]
+	_ = indexes /* ERROR "cannot index" */ [0]
+	_ = ( /* ERROR "cannot slice" */ 12 + 3)[1:2]
+
+	var a [10]int
+	_ = a[true /* ERROR "cannot convert" */ ]
+	_ = a["foo" /* ERROR "cannot convert" */ ]
+	_ = a[1.1 /* ERROR "truncated" */ ]
+	_ = a[1.0]
+	_ = a[- /* ERROR "negative" */ 1]
+	_ = a[- /* ERROR "negative" */ 1 :]
+	_ = a[: - /* ERROR "negative" */ 1]
+	_ = a[: /* ERROR "2nd index required" */ : /* ERROR "3rd index required" */ ]
+	_ = a[0: /* ERROR "2nd index required" */ : /* ERROR "3rd index required" */ ]
+	_ = a[0: /* ERROR "2nd index required" */ :10]
+	_ = a[:10:10]
+
+	var a0 int
+	a0 = a[0]
+	_ = a0
+	var a1 int32
+	a1 = a /* ERROR "cannot use .* in assignment" */ [1]
+	_ = a1
+
+	_ = a[9]
+	_ = a[10 /* ERROR "index .* out of bounds" */ ]
+	_ = a[1 /* ERROR "overflows" */ <<100]
+	_ = a[10:]
+	_ = a[:10]
+	_ = a[10:10]
+	_ = a[11 /* ERROR "index .* out of bounds" */ :]
+	_ = a[: 11 /* ERROR "index .* out of bounds" */ ]
+	_ = a[: 1 /* ERROR "overflows" */ <<100]
+	_ = a[:10:10]
+	_ = a[:11 /* ERROR "index .* out of bounds" */ :10]
+	_ = a[:10:11 /* ERROR "index .* out of bounds" */ ]
+	_ = a[10:0:10] /* ERROR swapped slice indices" */
+	_ = a[0:10:0] /* ERROR "swapped slice indices" */
+	_ = a[10:0:0] /* ERROR "swapped slice indices" */
+	_ = &a /* ERROR "cannot take address" */ [:10]
+
+	pa := &a
+	_ = pa[9]
+	_ = pa[10 /* ERROR "index .* out of bounds" */ ]
+	_ = pa[1 /* ERROR "overflows" */ <<100]
+	_ = pa[10:]
+	_ = pa[:10]
+	_ = pa[10:10]
+	_ = pa[11 /* ERROR "index .* out of bounds" */ :]
+	_ = pa[: 11 /* ERROR "index .* out of bounds" */ ]
+	_ = pa[: 1 /* ERROR "overflows" */ <<100]
+	_ = pa[:10:10]
+	_ = pa[:11 /* ERROR "index .* out of bounds" */ :10]
+	_ = pa[:10:11 /* ERROR "index .* out of bounds" */ ]
+	_ = pa[10:0:10] /* ERROR "swapped slice indices" */
+	_ = pa[0:10:0] /* ERROR "swapped slice indices" */
+	_ = pa[10:0:0] /* ERROR "swapped slice indices" */
+	_ = &pa /* ERROR "cannot take address" */ [:10]
+
+	var b [0]int
+	_ = b[0 /* ERROR "index .* out of bounds" */ ]
+	_ = b[:]
+	_ = b[0:]
+	_ = b[:0]
+	_ = b[0:0]
+	_ = b[0:0:0]
+	_ = b[1 /* ERROR "index .* out of bounds" */ :0:0]
+
+	var s []int
+	_ = s[- /* ERROR "negative" */ 1]
+	_ = s[- /* ERROR "negative" */ 1 :]
+	_ = s[: - /* ERROR "negative" */ 1]
+	_ = s[0]
+	_ = s[1:2]
+	_ = s[2:1] /* ERROR "swapped slice indices" */
+	_ = s[2:]
+	_ = s[: 1 /* ERROR "overflows" */ <<100]
+	_ = s[1 /* ERROR "overflows" */ <<100 :]
+	_ = s[1 /* ERROR "overflows" */ <<100 : 1 /* ERROR "overflows" */ <<100]
+	_ = s[: /* ERROR "2nd index required" */ :  /* ERROR "3rd index required" */ ]
+	_ = s[:10:10]
+	_ = s[10:0:10] /* ERROR "swapped slice indices" */
+	_ = s[0:10:0] /* ERROR "swapped slice indices" */
+	_ = s[10:0:0] /* ERROR "swapped slice indices" */
+	_ = &s /* ERROR "cannot take address" */ [:10]
+
+	var m map[string]int
+	_ = m[0 /* ERROR "cannot use .* in map index" */ ]
+	_ = m /* ERROR "cannot slice" */ ["foo" : "bar"]
+	_ = m["foo"]
+	// ok is of type bool
+	type mybool bool
+	var ok mybool
+	_, ok = m["bar"]
+	_ = ok
+	_ = m[0 /* ERROR "cannot use 0" */ ] + "foo" // ERROR "cannot convert"
+
+	var t string
+	_ = t[- /* ERROR "negative" */ 1]
+	_ = t[- /* ERROR "negative" */ 1 :]
+	_ = t[: - /* ERROR "negative" */ 1]
+	_ = t /* ERROR "3-index slice of string" */ [1:2:3]
+	_ = "foo" /* ERROR "3-index slice of string" */ [1:2:3]
+	var t0 byte
+	t0 = t[0]
+	_ = t0
+	var t1 rune
+	t1 = t /* ERROR "cannot use .* in assignment" */ [2]
+	_ = t1
+	_ = ("foo" + "bar")[5]
+	_ = ("foo" + "bar")[6 /* ERROR "index .* out of bounds" */ ]
+
+	const c = "foo"
+	_ = c[- /* ERROR "negative" */ 1]
+	_ = c[- /* ERROR "negative" */ 1 :]
+	_ = c[: - /* ERROR "negative" */ 1]
+	var c0 byte
+	c0 = c[0]
+	_ = c0
+	var c2 float32
+	c2 = c /* ERROR "cannot use .* in assignment" */ [2]
+	_ = c[3 /* ERROR "index .* out of bounds" */ ]
+	_ = ""[0 /* ERROR "index .* out of bounds" */ ]
+	_ = c2
+
+	_ = s[1<<30] // no compile-time error here
+
+	// issue 4913
+	type mystring string
+	var ss string
+	var ms mystring
+	var i, j int
+	ss = "foo"[1:2]
+	ss = "foo"[i:j]
+	ms = "foo" /* ERROR "cannot use .* in assignment" */ [1:2]
+	ms = "foo" /* ERROR "cannot use .* in assignment" */ [i:j]
+	_, _ = ss, ms
+}
+
+type T struct {
+	x int
+	y func()
+}
+
+func (*T) m() {}
+
+func method_expressions() {
+	_ = T.a /* ERROR "no field or method" */
+	_ = T.x /* ERROR "has no method" */
+	_ = T.m /* ERROR "cannot call pointer method m on T" */
+	_ = (*T).m
+
+	var f func(*T) = T.m /* ERROR "cannot call pointer method m on T" */
+	var g func(*T) = (*T).m
+	_, _ = f, g
+
+	_ = T.y /* ERROR "has no method" */
+	_ = (*T).y /* ERROR "has no method" */
+}
+
+func struct_literals() {
+	type T0 struct {
+		a, b, c int
+	}
+
+	type T1 struct {
+		T0
+		a, b int
+		u float64
+		s string
+	}
+
+	// keyed elements
+	_ = T1{}
+	_ = T1{a: 0, 1 /* ERROR "mixture of .* elements" */ }
+	_ = T1{aa /* ERROR "unknown field" */ : 0}
+	_ = T1{1 /* ERROR "invalid field name" */ : 0}
+	_ = T1{a: 0, s: "foo", u: 0, a /* ERROR "duplicate field" */: 10}
+	_ = T1{a: "foo" /* ERROR "cannot use .* in struct literal" */ }
+	_ = T1{c /* ERROR "unknown field" */ : 0}
+	_ = T1{T0: { /* ERROR "missing type" */ }} // struct literal element type may not be elided
+	_ = T1{T0: T0{}}
+	_ = T1{T0 /* ERROR "invalid field name" */ .a: 0}
+
+	// unkeyed elements
+	_ = T0{1, 2, 3}
+	_ = T0{1, b /* ERROR "mixture" */ : 2, 3}
+	_ = T0{1, 2} /* ERROR "too few values" */
+	_ = T0{1, 2, 3, 4  /* ERROR "too many values" */ }
+	_ = T0{1, "foo" /* ERROR "cannot use .* in struct literal" */, 3.4  /* ERROR "cannot use .*\(truncated\)" */}
+
+	// invalid type
+	type P *struct{
+		x int
+	}
+	_ = P /* ERROR "invalid composite literal type" */ {}
+
+	// unexported fields
+	_ = time.Time{}
+	_ = time.Time{sec /* ERROR "unknown field" */ : 0}
+	_ = time.Time{
+		0 /* ERROR implicit assignment to unexported field wall in time.Time literal */,
+		0 /* ERROR implicit assignment */ ,
+		nil /* ERROR implicit assignment */ ,
+	}
+}
+
+func array_literals() {
+	type A0 [0]int
+	_ = A0{}
+	_ = A0{0 /* ERROR "index .* out of bounds" */}
+	_ = A0{0 /* ERROR "index .* out of bounds" */ : 0}
+
+	type A1 [10]int
+	_ = A1{}
+	_ = A1{0, 1, 2}
+	_ = A1{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+	_ = A1{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 /* ERROR "index .* out of bounds" */ }
+	_ = A1{- /* ERROR "negative" */ 1: 0}
+	_ = A1{8: 8, 9}
+	_ = A1{8: 8, 9, 10 /* ERROR "index .* out of bounds" */ }
+	_ = A1{0, 1, 2, 0 /* ERROR "duplicate index" */ : 0, 3: 3, 4}
+	_ = A1{5: 5, 6, 7, 3: 3, 4}
+	_ = A1{5: 5, 6, 7, 3: 3, 4, 5 /* ERROR "duplicate index" */ }
+	_ = A1{10 /* ERROR "index .* out of bounds" */ : 10, 10 /* ERROR "index .* out of bounds" */ : 10}
+	_ = A1{5: 5, 6, 7, 3: 3, 1 /* ERROR "overflows" */ <<100: 4, 5 /* ERROR "duplicate index" */ }
+	_ = A1{5: 5, 6, 7, 4: 4, 1 /* ERROR "overflows" */ <<100: 4}
+	_ = A1{2.0}
+	_ = A1{2.1 /* ERROR "truncated" */ }
+	_ = A1{"foo" /* ERROR "cannot use .* in array or slice literal" */ }
+
+	// indices must be integer constants
+	i := 1
+	const f = 2.1
+	const s = "foo"
+	_ = A1{i /* ERROR "index i must be integer constant" */ : 0}
+	_ = A1{f /* ERROR "truncated" */ : 0}
+	_ = A1{s /* ERROR "cannot convert" */ : 0}
+
+	a0 := [...]int{}
+	assert(len(a0) == 0)
+
+	a1 := [...]int{0, 1, 2}
+	assert(len(a1) == 3)
+	var a13 [3]int
+	var a14 [4]int
+	a13 = a1
+	a14 = a1 /* ERROR "cannot use .* in assignment" */
+	_, _ = a13, a14
+
+	a2 := [...]int{- /* ERROR "negative" */ 1: 0}
+	_ = a2
+
+	a3 := [...]int{0, 1, 2, 0 /* ERROR "duplicate index" */ : 0, 3: 3, 4}
+	assert(len(a3) == 5) // somewhat arbitrary
+
+	a4 := [...]complex128{0, 1, 2, 1<<10-2: -1i, 1i, 400: 10, 12, 14}
+	assert(len(a4) == 1024)
+
+	// composite literal element types may be elided
+	type T []int
+	_ = [10]T{T{}, {}, 5: T{1, 2, 3}, 7: {1, 2, 3}}
+	a6 := [...]T{T{}, {}, 5: T{1, 2, 3}, 7: {1, 2, 3}}
+	assert(len(a6) == 8)
+
+	// recursively so
+	_ = [10][10]T{{}, [10]T{{}}, {{1, 2, 3}}}
+
+	// from the spec
+	type Point struct { x, y float32 }
+	_ = [...]Point{Point{1.5, -3.5}, Point{0, 0}}
+	_ = [...]Point{{1.5, -3.5}, {0, 0}}
+	_ = [][]int{[]int{1, 2, 3}, []int{4, 5}}
+	_ = [][]int{{1, 2, 3}, {4, 5}}
+	_ = [...]*Point{&Point{1.5, -3.5}, &Point{0, 0}}
+	_ = [...]*Point{{1.5, -3.5}, {0, 0}}
+}
+
+func slice_literals() {
+	type S0 []int
+	_ = S0{}
+	_ = S0{0, 1, 2}
+	_ = S0{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
+	_ = S0{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
+	_ = S0{- /* ERROR "negative" */ 1: 0}
+	_ = S0{8: 8, 9}
+	_ = S0{8: 8, 9, 10}
+	_ = S0{0, 1, 2, 0 /* ERROR "duplicate index" */ : 0, 3: 3, 4}
+	_ = S0{5: 5, 6, 7, 3: 3, 4}
+	_ = S0{5: 5, 6, 7, 3: 3, 4, 5 /* ERROR "duplicate index" */ }
+	_ = S0{10: 10, 10 /* ERROR "duplicate index" */ : 10}
+	_ = S0{5: 5, 6, 7, 3: 3, 1 /* ERROR "overflows" */ <<100: 4, 5 /* ERROR "duplicate index" */ }
+	_ = S0{5: 5, 6, 7, 4: 4, 1 /* ERROR "overflows" */ <<100: 4}
+	_ = S0{2.0}
+	_ = S0{2.1 /* ERROR "truncated" */ }
+	_ = S0{"foo" /* ERROR "cannot use .* in array or slice literal" */ }
+
+	// indices must be resolved correctly
+	const index1 = 1
+	_ = S0{index1: 1}
+	_ = S0{index2: 2}
+	_ = S0{index3 /* ERROR "undeclared name" */ : 3}
+
+	// indices must be integer constants
+	i := 1
+	const f = 2.1
+	const s = "foo"
+	_ = S0{i /* ERROR "index i must be integer constant" */ : 0}
+	_ = S0{f /* ERROR "truncated" */ : 0}
+	_ = S0{s /* ERROR "cannot convert" */ : 0}
+
+	// composite literal element types may be elided
+	type T []int
+	_ = []T{T{}, {}, 5: T{1, 2, 3}, 7: {1, 2, 3}}
+	_ = [][]int{{1, 2, 3}, {4, 5}}
+
+	// recursively so
+	_ = [][]T{{}, []T{{}}, {{1, 2, 3}}}
+
+	// issue 17954
+	type T0 *struct { s string }
+	_ = []T0{{}}
+	_ = []T0{{"foo"}}
+
+	type T1 *struct{ int }
+	_ = []T1{}
+	_ = []T1{{0}, {1}, {2}}
+
+	type T2 T1
+	_ = []T2{}
+	_ = []T2{{0}, {1}, {2}}
+
+	_ = map[T0]T2{}
+	_ = map[T0]T2{{}: {}}
+}
+
+const index2 int = 2
+
+type N int
+func (N) f() {}
+
+func map_literals() {
+	type M0 map[string]int
+	type M1 map[bool]int
+	type M2 map[*int]int
+
+	_ = M0{}
+	_ = M0{1 /* ERROR "missing key" */ }
+	_ = M0{1 /* ERROR "cannot use .* in map literal" */ : 2}
+	_ = M0{"foo": "bar" /* ERROR "cannot use .* in map literal" */ }
+	_ = M0{"foo": 1, "bar": 2, "foo" /* ERROR "duplicate key" */ : 3 }
+
+	_ = map[interface{}]int{2: 1, 2 /* ERROR "duplicate key" */ : 1}
+	_ = map[interface{}]int{int(2): 1, int16(2): 1}
+	_ = map[interface{}]int{int16(2): 1, int16 /* ERROR "duplicate key" */ (2): 1}
+
+	type S string
+
+	_ = map[interface{}]int{"a": 1, "a" /* ERROR "duplicate key" */ : 1}
+	_ = map[interface{}]int{"a": 1, S("a"): 1}
+	_ = map[interface{}]int{S("a"): 1, S /* ERROR "duplicate key" */ ("a"): 1}
+	_ = map[interface{}]int{1.0: 1, 1.0 /* ERROR "duplicate key" */: 1}
+	_ = map[interface{}]int{int64(-1): 1, int64 /* ERROR "duplicate key" */ (-1) : 1}
+	_ = map[interface{}]int{^uint64(0): 1, ^ /* ERROR "duplicate key" */ uint64(0): 1}
+	_ = map[interface{}]int{complex(1,2): 1, complex /* ERROR "duplicate key" */ (1,2) : 1}
+
+	type I interface {
+		f()
+	}
+
+	_ = map[I]int{N(0): 1, N(2): 1}
+	_ = map[I]int{N(2): 1, N /* ERROR "duplicate key" */ (2): 1}
+
+	// map keys must be resolved correctly
+	key1 := "foo"
+	_ = M0{key1: 1}
+	_ = M0{key2: 2}
+	_ = M0{key3 /* ERROR "undeclared name" */ : 2}
+
+	var value int
+	_ = M1{true: 1, false: 0}
+	_ = M2{nil: 0, &value: 1}
+
+	// composite literal element types may be elided
+	type T [2]int
+	_ = map[int]T{0: T{3, 4}, 1: {5, 6}}
+
+	// recursively so
+	_ = map[int][]T{0: {}, 1: {{}, T{1, 2}}}
+
+	// composite literal key types may be elided
+	_ = map[T]int{T{3, 4}: 0, {5, 6}: 1}
+
+	// recursively so
+	_ = map[[2]T]int{{}: 0, {{}}: 1, [2]T{{}}: 2, {T{1, 2}}: 3}
+
+	// composite literal element and key types may be elided
+	_ = map[T]T{{}: {}, {1, 2}: T{3, 4}, T{4, 5}: {}}
+	_ = map[T]M0{{} : {}, T{1, 2}: M0{"foo": 0}, {1, 3}: {"foo": 1}}
+
+	// recursively so
+	_ = map[[2]T][]T{{}: {}, {{}}: {{}, T{1, 2}}, [2]T{{}}: nil, {T{1, 2}}: {{}, {}}}
+
+	// from the spec
+	type Point struct { x, y float32 }
+	_ = map[string]Point{"orig": {0, 0}}
+	_ = map[*Point]string{{0, 0}: "orig"}
+
+	// issue 17954
+	type T0 *struct{ s string }
+	type T1 *struct{ int }
+	type T2 T1
+
+	_ = map[T0]T2{}
+	_ = map[T0]T2{{}: {}}
+}
+
+var key2 string = "bar"
+
+type I interface {
+	m()
+}
+
+type I2 interface {
+	m(int)
+}
+
+type T1 struct{}
+type T2 struct{}
+
+func (T2) m(int) {}
+
+type mybool bool
+
+func type_asserts() {
+	var x int
+	_ = x /* ERROR "not an interface" */ .(int)
+
+	var e interface{}
+	var ok bool
+	x, ok = e.(int)
+	_ = ok
+
+	// ok value is of type bool
+	var myok mybool
+	_, myok = e.(int)
+	_ = myok
+
+	var t I
+	_ = t /* ERROR "use of .* outside type switch" */ .(type)
+	_ = t /* ERROR "missing method m" */ .(T)
+	_ = t.(*T)
+	_ = t /* ERROR "missing method m" */ .(T1)
+	_ = t /* ERROR "wrong type for method m" */ .(T2)
+	_ = t /* STRICT "wrong type for method m" */ .(I2) // only an error in strict mode (issue 8561)
+
+	// e doesn't statically have an m, but may have one dynamically.
+	_ = e.(I2)
+}
+
+func f0() {}
+func f1(x int) {}
+func f2(u float32, s string) {}
+func fs(s []byte) {}
+func fv(x ...int) {}
+func fi(x ... interface{}) {}
+func (T) fm(x ...int)
+
+func g0() {}
+func g1() int { return 0}
+func g2() (u float32, s string) { return }
+func gs() []byte { return nil }
+
+func _calls() {
+	var x int
+	var y float32
+	var s []int
+
+	f0()
+	_ = f0 /* ERROR "used as value" */ ()
+	f0(g0 /* ERROR "too many arguments" */ )
+
+	f1(0)
+	f1(x)
+	f1(10.0)
+	f1() /* ERROR "too few arguments" */
+	f1(x, y /* ERROR "too many arguments" */ )
+	f1(s /* ERROR "cannot use .* in argument" */ )
+	f1(x ... /* ERROR "cannot use ..." */ )
+	f1(g0 /* ERROR "used as value" */ ())
+	f1(g1())
+	f1(g2 /* ERROR "cannot use g2" */ /* ERROR "too many arguments" */ ())
+
+	f2() /* ERROR "too few arguments" */
+	f2(3.14) /* ERROR "too few arguments" */
+	f2(3.14, "foo")
+	f2(x /* ERROR "cannot use .* in argument" */ , "foo")
+	f2(g0 /* ERROR "used as value" */ ())
+	f2(g1 /* ERROR "cannot use .* in argument" */ ()) /* ERROR "too few arguments" */
+	f2(g2())
+
+	fs() /* ERROR "too few arguments" */
+	fs(g0 /* ERROR "used as value" */ ())
+	fs(g1 /* ERROR "cannot use .* in argument" */ ())
+	fs(g2 /* ERROR "cannot use .* in argument" */ /* ERROR "too many arguments" */ ())
+	fs(gs())
+
+	fv()
+	fv(1, 2.0, x)
+	fv(s /* ERROR "cannot use .* in argument" */ )
+	fv(s...)
+	fv(x /* ERROR "cannot use" */ ...)
+	fv(1, s... /* ERROR "can only use ... with matching parameter" */ )
+	fv(gs /* ERROR "cannot use .* in argument" */ ())
+	fv(gs /* ERROR "cannot use .* in argument" */ ()...)
+
+	var t T
+	t.fm()
+	t.fm(1, 2.0, x)
+	t.fm(s /* ERROR "cannot use .* in argument" */ )
+	t.fm(g1())
+	t.fm(1, s... /* ERROR "can only use ... with matching parameter" */ )
+	t.fm(gs /* ERROR "cannot use .* in argument" */ ())
+	t.fm(gs /* ERROR "cannot use .* in argument" */ ()...)
+
+	T.fm(t, )
+	T.fm(t, 1, 2.0, x)
+	T.fm(t, s /* ERROR "cannot use .* in argument" */ )
+	T.fm(t, g1())
+	T.fm(t, 1, s... /* ERROR "can only use ... with matching parameter" */ )
+	T.fm(t, gs /* ERROR "cannot use .* in argument" */ ())
+	T.fm(t, gs /* ERROR "cannot use .* in argument" */ ()...)
+
+	var i interface{ fm(x ...int) } = t
+	i.fm()
+	i.fm(1, 2.0, x)
+	i.fm(s /* ERROR "cannot use .* in argument" */ )
+	i.fm(g1())
+	i.fm(1, s... /* ERROR "can only use ... with matching parameter" */ )
+	i.fm(gs /* ERROR "cannot use .* in argument" */ ())
+	i.fm(gs /* ERROR "cannot use .* in argument" */ ()...)
+
+	fi()
+	fi(1, 2.0, x, 3.14, "foo")
+	fi(g2())
+	fi(0, g2)
+	fi(0, g2 /* ERROR "2-valued g2" */ ())
+}
+
+func issue6344() {
+	type T []interface{}
+	var x T
+	fi(x...) // ... applies also to named slices
+}
diff --git a/src/go/types/testdata/gotos.src b/src/go/types/testdata/gotos.src
new file mode 100644
index 0000000..069a94b
--- /dev/null
+++ b/src/go/types/testdata/gotos.src
@@ -0,0 +1,560 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is a modified copy of $GOROOT/test/goto.go.
+
+package gotos
+
+var (
+	i, n int
+	x    []int
+	c    chan int
+	m    map[int]int
+	s    string
+)
+
+// goto after declaration okay
+func _() {
+	x := 1
+	goto L
+L:
+	_ = x
+}
+
+// goto before declaration okay
+func _() {
+	goto L
+L:
+	x := 1
+	_ = x
+}
+
+// goto across declaration not okay
+func _() {
+	goto L /* ERROR "goto L jumps over variable declaration at line 36" */
+	x := 1
+	_ = x
+L:
+}
+
+// goto across declaration in inner scope okay
+func _() {
+	goto L
+	{
+		x := 1
+		_ = x
+	}
+L:
+}
+
+// goto across declaration after inner scope not okay
+func _() {
+	goto L /* ERROR "goto L jumps over variable declaration at line 58" */
+	{
+		x := 1
+		_ = x
+	}
+	x := 1
+	_ = x
+L:
+}
+
+// goto across declaration in reverse okay
+func _() {
+L:
+	x := 1
+	_ = x
+	goto L
+}
+
+func _() {
+L: L1:
+	x := 1
+	_ = x
+	goto L
+	goto L1
+}
+
+// error shows first offending variable
+func _() {
+	goto L /* ERROR "goto L jumps over variable declaration at line 84" */
+	x := 1
+	_ = x
+	y := 1
+	_ = y
+L:
+}
+
+// goto not okay even if code path is dead
+func _() {
+	goto L /* ERROR "goto L jumps over variable declaration" */
+	x := 1
+	_ = x
+	y := 1
+	_ = y
+	return
+L:
+}
+
+// goto into outer block okay
+func _() {
+	{
+		goto L
+	}
+L:
+}
+
+func _() {
+	{
+		goto L
+		goto L1
+	}
+L: L1:
+}
+
+// goto backward into outer block okay
+func _() {
+L:
+	{
+		goto L
+	}
+}
+
+func _() {
+L: L1:
+	{
+		goto L
+		goto L1
+	}
+}
+
+// goto into inner block not okay
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	{
+	L:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	goto L1 /* ERROR "goto L1 jumps into block" */
+	{
+	L: L1:
+	}
+}
+
+// goto backward into inner block still not okay
+func _() {
+	{
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+func _() {
+	{
+	L: L1:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+	goto L1 /* ERROR "goto L1 jumps into block" */
+}
+
+// error shows first (outermost) offending block
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	{
+		{
+			{
+			L:
+			}
+		}
+	}
+}
+
+// error prefers block diagnostic over declaration diagnostic
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	x := 1
+	_ = x
+	{
+	L:
+	}
+}
+
+// many kinds of blocks, all invalid to jump into or among,
+// but valid to jump out of
+
+// if
+
+func _() {
+L:
+	if true {
+		goto L
+	}
+}
+
+func _() {
+L:
+	if true {
+		goto L
+	} else {
+	}
+}
+
+func _() {
+L:
+	if false {
+	} else {
+		goto L
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	if true {
+	L:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	if true {
+	L:
+	} else {
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	if true {
+	} else {
+	L:
+	}
+}
+
+func _() {
+	if false {
+	L:
+	} else {
+		goto L /* ERROR "goto L jumps into block" */
+	}
+}
+
+func _() {
+	if true {
+		goto L /* ERROR "goto L jumps into block" */
+	} else {
+	L:
+	}
+}
+
+func _() {
+	if true {
+		goto L /* ERROR "goto L jumps into block" */
+	} else if false {
+	L:
+	}
+}
+
+func _() {
+	if true {
+		goto L /* ERROR "goto L jumps into block" */
+	} else if false {
+	L:
+	} else {
+	}
+}
+
+func _() {
+	if true {
+		goto L /* ERROR "goto L jumps into block" */
+	} else if false {
+	} else {
+	L:
+	}
+}
+
+func _() {
+	if true {
+		goto L /* ERROR "goto L jumps into block" */
+	} else {
+		L:
+	}
+}
+
+func _() {
+	if true {
+		L:
+	} else {
+		goto L /* ERROR "goto L jumps into block" */
+	}
+}
+
+// for
+
+func _() {
+	for {
+		goto L
+	}
+L:
+}
+
+func _() {
+	for {
+		goto L
+	L:
+	}
+}
+
+func _() {
+	for {
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+func _() {
+	for {
+		goto L
+	L1:
+	}
+L:
+	goto L1 /* ERROR "goto L1 jumps into block" */
+}
+
+func _() {
+	for i < n {
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+func _() {
+	for i = 0; i < n; i++ {
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+func _() {
+	for i = range x {
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+func _() {
+	for i = range c {
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+func _() {
+	for i = range m {
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+func _() {
+	for i = range s {
+	L:
+	}
+	goto L /* ERROR "goto L jumps into block" */
+}
+
+// switch
+
+func _() {
+L:
+	switch i {
+	case 0:
+		goto L
+	}
+}
+
+func _() {
+L:
+	switch i {
+	case 0:
+
+	default:
+		goto L
+	}
+}
+
+func _() {
+	switch i {
+	case 0:
+
+	default:
+	L:
+		goto L
+	}
+}
+
+func _() {
+	switch i {
+	case 0:
+
+	default:
+		goto L
+	L:
+	}
+}
+
+func _() {
+	switch i {
+	case 0:
+		goto L
+	L:
+		;
+	default:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	switch i {
+	case 0:
+	L:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	switch i {
+	case 0:
+	L:
+		;
+	default:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	switch i {
+	case 0:
+	default:
+	L:
+	}
+}
+
+func _() {
+	switch i {
+	default:
+		goto L /* ERROR "goto L jumps into block" */
+	case 0:
+	L:
+	}
+}
+
+func _() {
+	switch i {
+	case 0:
+	L:
+		;
+	default:
+		goto L /* ERROR "goto L jumps into block" */
+	}
+}
+
+// select
+// different from switch.  the statement has no implicit block around it.
+
+func _() {
+L:
+	select {
+	case <-c:
+		goto L
+	}
+}
+
+func _() {
+L:
+	select {
+	case c <- 1:
+
+	default:
+		goto L
+	}
+}
+
+func _() {
+	select {
+	case <-c:
+
+	default:
+	L:
+		goto L
+	}
+}
+
+func _() {
+	select {
+	case c <- 1:
+
+	default:
+		goto L
+	L:
+	}
+}
+
+func _() {
+	select {
+	case <-c:
+		goto L
+	L:
+		;
+	default:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	select {
+	case c <- 1:
+	L:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	select {
+	case c <- 1:
+	L:
+		;
+	default:
+	}
+}
+
+func _() {
+	goto L /* ERROR "goto L jumps into block" */
+	select {
+	case <-c:
+	default:
+	L:
+	}
+}
+
+func _() {
+	select {
+	default:
+		goto L /* ERROR "goto L jumps into block" */
+	case <-c:
+	L:
+	}
+}
+
+func _() {
+	select {
+	case <-c:
+	L:
+		;
+	default:
+		goto L /* ERROR "goto L jumps into block" */
+	}
+}
diff --git a/src/go/types/testdata/importC.src b/src/go/types/testdata/importC.src
new file mode 100644
index 0000000..f55be2d
--- /dev/null
+++ b/src/go/types/testdata/importC.src
@@ -0,0 +1,54 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package importC
+
+import "C"
+import _ /* ERROR cannot rename import "C" */ "C"
+import foo /* ERROR cannot rename import "C" */ "C"
+import . /* ERROR cannot rename import "C" */ "C"
+
+// Test cases extracted from issue #22090.
+
+import "unsafe"
+
+const _ C.int = 0xff // no error due to invalid constant type
+
+type T struct {
+	Name    string
+	Ordinal int
+}
+
+func _(args []T) {
+	var s string
+	for i, v := range args {
+		cname := C.CString(v.Name)
+		args[i].Ordinal = int(C.sqlite3_bind_parameter_index(s, cname)) // no error due to i not being "used"
+		C.free(unsafe.Pointer(cname))
+	}
+}
+
+type CType C.Type
+
+const _ CType = C.X // no error due to invalid constant type
+const _ = C.X
+
+// Test cases extracted from issue #23712.
+
+func _() {
+	var a [C.ArrayLength]byte
+	_ = a[0] // no index out of bounds error here
+}
+
+// Additional tests to verify fix for #23712.
+
+func _() {
+	var a [C.ArrayLength1]byte
+	_ = 1 / len(a) // no division by zero error here and below
+	_ = 1 / cap(a)
+	_ = uint(unsafe.Sizeof(a)) // must not be negative
+
+	var b [C.ArrayLength2]byte
+	a = b // should be valid
+}
diff --git a/src/go/types/testdata/importdecl0a.src b/src/go/types/testdata/importdecl0a.src
new file mode 100644
index 0000000..e96fca3
--- /dev/null
+++ b/src/go/types/testdata/importdecl0a.src
@@ -0,0 +1,53 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package importdecl0
+
+import ()
+
+import (
+	// we can have multiple blank imports (was bug)
+	_ "math"
+	_ "net/rpc"
+	init /* ERROR "cannot declare init" */ "fmt"
+	// reflect defines a type "flag" which shows up in the gc export data
+	"reflect"
+	. /* ERROR "imported but not used" */ "reflect"
+)
+
+import "math" /* ERROR "imported but not used" */
+import m /* ERROR "imported but not used as m" */ "math"
+import _ "math"
+
+import (
+	"math/big" /* ERROR "imported but not used" */
+	b /* ERROR "imported but not used" */ "math/big"
+	_ "math/big"
+)
+
+import "fmt"
+import f1 "fmt"
+import f2 "fmt"
+
+// reflect.flag must not be visible in this package
+type flag int
+type _ reflect.flag /* ERROR "not exported" */
+
+// imported package name may conflict with local objects
+type reflect /* ERROR "reflect already declared" */ int
+
+// dot-imported exported objects may conflict with local objects
+type Value /* ERROR "Value already declared through dot-import of package reflect" */ struct{}
+
+var _ = fmt.Println // use "fmt"
+
+func _() {
+	f1.Println() // use "fmt"
+}
+
+func _() {
+	_ = func() {
+		f2.Println() // use "fmt"
+	}
+}
diff --git a/src/go/types/testdata/importdecl0b.src b/src/go/types/testdata/importdecl0b.src
new file mode 100644
index 0000000..6844e70
--- /dev/null
+++ b/src/go/types/testdata/importdecl0b.src
@@ -0,0 +1,33 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package importdecl0
+
+import "math"
+import m "math"
+
+import . "testing" // declares T in file scope
+import . /* ERROR "imported but not used" */ "unsafe"
+import . "fmt"     // declares Println in file scope
+
+import (
+	// TODO(gri) At the moment, 2 errors are reported because both go/parser
+	// and the type checker report it. Eventually, this test should not be
+	// done by the parser anymore.
+	"" /* ERROR invalid import path */ /* ERROR invalid import path */
+	"a!b" /* ERROR invalid import path */ /* ERROR invalid import path */
+	"abc\xffdef" /* ERROR invalid import path */ /* ERROR invalid import path */
+)
+
+// using "math" in this file doesn't affect its use in other files
+const Pi0 = math.Pi
+const Pi1 = m.Pi
+
+type _ T // use "testing"
+
+func _() func() interface{} {
+	return func() interface{} {
+		return Println // use "fmt"
+	}
+}
diff --git a/src/go/types/testdata/importdecl1a.src b/src/go/types/testdata/importdecl1a.src
new file mode 100644
index 0000000..d377c01
--- /dev/null
+++ b/src/go/types/testdata/importdecl1a.src
@@ -0,0 +1,22 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test case for issue 8969.
+
+package importdecl1
+
+import "go/ast"
+import . "unsafe"
+
+var _ Pointer // use dot-imported package unsafe
+
+// Test cases for issue 23914.
+
+type A interface {
+	// Methods m1, m2 must be type-checked in this file scope
+	// even when embedded in an interface in a different
+	// file of the same package.
+	m1() ast.Node
+	m2() Pointer
+}
diff --git a/src/go/types/testdata/importdecl1b.src b/src/go/types/testdata/importdecl1b.src
new file mode 100644
index 0000000..ee70bbd
--- /dev/null
+++ b/src/go/types/testdata/importdecl1b.src
@@ -0,0 +1,11 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package importdecl1
+
+import . /* ERROR "imported but not used" */ "unsafe"
+
+type B interface {
+	A
+}
diff --git a/src/go/types/testdata/init0.src b/src/go/types/testdata/init0.src
new file mode 100644
index 0000000..6e8746a
--- /dev/null
+++ b/src/go/types/testdata/init0.src
@@ -0,0 +1,106 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// initialization cycles
+
+package init0
+
+// initialization cycles (we don't know the types)
+const (
+	s0 /* ERROR initialization cycle */ = s0
+
+	x0 /* ERROR initialization cycle */ = y0
+	y0 = x0
+
+	a0 = b0
+	b0 /* ERROR initialization cycle */ = c0
+	c0 = d0
+	d0 = b0
+)
+
+var (
+	s1 /* ERROR initialization cycle */ = s1
+
+	x1 /* ERROR initialization cycle */ = y1
+	y1 = x1
+
+	a1 = b1
+	b1 /* ERROR initialization cycle */ = c1
+	c1 = d1
+	d1 = b1
+)
+
+// initialization cycles (we know the types)
+const (
+	s2 /* ERROR initialization cycle */ int = s2
+
+	x2 /* ERROR initialization cycle */ int = y2
+	y2 = x2
+
+	a2 = b2
+	b2 /* ERROR initialization cycle */ int = c2
+	c2 = d2
+	d2 = b2
+)
+
+var (
+	s3 /* ERROR initialization cycle */ int = s3
+
+	x3 /* ERROR initialization cycle */ int = y3
+	y3 = x3
+
+	a3 = b3
+	b3 /* ERROR initialization cycle */ int = c3
+	c3 = d3
+	d3 = b3
+)
+
+// cycles via struct fields
+
+type S1 struct {
+	f int
+}
+const cx3 S1 /* ERROR invalid constant type */ = S1{cx3.f}
+var vx3 /* ERROR initialization cycle */ S1 = S1{vx3.f}
+
+// cycles via functions
+
+var x4 = x5
+var x5 /* ERROR initialization cycle */ = f1()
+func f1() int { return x5*10 }
+
+var x6, x7 /* ERROR initialization cycle */ = f2()
+var x8 = x7
+func f2() (int, int) { return f3() + f3(), 0 }
+func f3() int { return x8 }
+
+// cycles via function literals
+
+var x9 /* ERROR initialization cycle */ = func() int { return x9 }()
+
+var x10 /* ERROR initialization cycle */ = f4()
+
+func f4() int {
+	_ = func() {
+		_ = x10
+	}
+	return 0
+}
+
+// cycles via method expressions
+
+type T1 struct{}
+
+func (T1) m() bool { _ = x11; return false }
+
+var x11 /* ERROR initialization cycle */ = T1.m(T1{})
+
+// cycles via method values
+
+type T2 struct{}
+
+func (T2) m() bool { _ = x12; return false }
+
+var t1 T2
+var x12 /* ERROR initialization cycle */ = t1.m
diff --git a/src/go/types/testdata/init1.src b/src/go/types/testdata/init1.src
new file mode 100644
index 0000000..39ca314
--- /dev/null
+++ b/src/go/types/testdata/init1.src
@@ -0,0 +1,97 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// initialization cycles
+
+package init1
+
+// issue 6683 (marked as WorkingAsIntended)
+
+type T0 struct{}
+
+func (T0) m() int { return y0 }
+
+var x0 = T0{}
+
+var y0 /* ERROR initialization cycle */ = x0.m()
+
+type T1 struct{}
+
+func (T1) m() int { return y1 }
+
+var x1 interface {
+	m() int
+} = T1{}
+
+var y1 = x1.m() // no cycle reported, x1 is of interface type
+
+// issue 6703 (modified)
+
+var x2 /* ERROR initialization cycle */ = T2.m
+
+var y2 = x2
+
+type T2 struct{}
+
+func (T2) m() int {
+	_ = y2
+	return 0
+}
+
+var x3 /* ERROR initialization cycle */ = T3.m(T3{}) // <<<< added (T3{})
+
+var y3 = x3
+
+type T3 struct{}
+
+func (T3) m() int {
+	_ = y3
+	return 0
+}
+
+var x4 /* ERROR initialization cycle */ = T4{}.m // <<<< added {}
+
+var y4 = x4
+
+type T4 struct{}
+
+func (T4) m() int {
+	_ = y4
+	return 0
+}
+
+var x5 /* ERROR initialization cycle */ = T5{}.m() // <<<< added ()
+
+var y5 = x5
+
+type T5 struct{}
+
+func (T5) m() int {
+	_ = y5
+	return 0
+}
+
+// issue 4847
+// simplified test case
+
+var x6 = f6
+var y6 /* ERROR initialization cycle */ = f6
+func f6() { _ = y6 }
+
+// full test case
+
+type (
+      E int
+      S int
+)
+
+type matcher func(s *S) E
+
+func matchList(s *S) E { return matcher(matchAnyFn)(s) }
+
+var foo = matcher(matchList)
+
+var matchAny /* ERROR initialization cycle */ = matcher(matchList)
+
+func matchAnyFn(s *S) (err E) { return matchAny(s) }
\ No newline at end of file
diff --git a/src/go/types/testdata/init2.src b/src/go/types/testdata/init2.src
new file mode 100644
index 0000000..614db6c
--- /dev/null
+++ b/src/go/types/testdata/init2.src
@@ -0,0 +1,139 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// initialization cycles
+
+package init2
+
+// cycles through functions
+
+func f1() int { _ = x1; return 0 }
+var x1 /* ERROR initialization cycle */ = f1
+
+func f2() int { _ = x2; return 0 }
+var x2 /* ERROR initialization cycle */ = f2()
+
+// cycles through method expressions
+
+type T3 int
+func (T3) m() int { _ = x3; return 0 }
+var x3 /* ERROR initialization cycle */ = T3.m
+
+type T4 int
+func (T4) m() int { _ = x4; return 0 }
+var x4 /* ERROR initialization cycle */ = T4.m(0)
+
+type T3p int
+func (*T3p) m() int { _ = x3p; return 0 }
+var x3p /* ERROR initialization cycle */ = (*T3p).m
+
+type T4p int
+func (*T4p) m() int { _ = x4p; return 0 }
+var x4p /* ERROR initialization cycle */ = (*T4p).m(nil)
+
+// cycles through method expressions of embedded methods
+
+type T5 struct { E5 }
+type E5 int
+func (E5) m() int { _ = x5; return 0 }
+var x5 /* ERROR initialization cycle */ = T5.m
+
+type T6 struct { E6 }
+type E6 int
+func (E6) m() int { _ = x6; return 0 }
+var x6 /* ERROR initialization cycle */ = T6.m(T6{0})
+
+type T5p struct { E5p }
+type E5p int
+func (*E5p) m() int { _ = x5p; return 0 }
+var x5p /* ERROR initialization cycle */ = (*T5p).m
+
+type T6p struct { E6p }
+type E6p int
+func (*E6p) m() int { _ = x6p; return 0 }
+var x6p /* ERROR initialization cycle */ = (*T6p).m(nil)
+
+// cycles through method values
+
+type T7 int
+func (T7) m() int { _ = x7; return 0 }
+var x7 /* ERROR initialization cycle */ = T7(0).m
+
+type T8 int
+func (T8) m() int { _ = x8; return 0 }
+var x8 /* ERROR initialization cycle */ = T8(0).m()
+
+type T7p int
+func (*T7p) m() int { _ = x7p; return 0 }
+var x7p /* ERROR initialization cycle */ = new(T7p).m
+
+type T8p int
+func (*T8p) m() int { _ = x8p; return 0 }
+var x8p /* ERROR initialization cycle */ = new(T8p).m()
+
+type T7v int
+func (T7v) m() int { _ = x7v; return 0 }
+var x7var T7v
+var x7v /* ERROR initialization cycle */ = x7var.m
+
+type T8v int
+func (T8v) m() int { _ = x8v; return 0 }
+var x8var T8v
+var x8v /* ERROR initialization cycle */ = x8var.m()
+
+type T7pv int
+func (*T7pv) m() int { _ = x7pv; return 0 }
+var x7pvar *T7pv
+var x7pv /* ERROR initialization cycle */ = x7pvar.m
+
+type T8pv int
+func (*T8pv) m() int { _ = x8pv; return 0 }
+var x8pvar *T8pv
+var x8pv /* ERROR initialization cycle */ = x8pvar.m()
+
+// cycles through method values of embedded methods
+
+type T9 struct { E9 }
+type E9 int
+func (E9) m() int { _ = x9; return 0 }
+var x9 /* ERROR initialization cycle */ = T9{0}.m
+
+type T10 struct { E10 }
+type E10 int
+func (E10) m() int { _ = x10; return 0 }
+var x10 /* ERROR initialization cycle */ = T10{0}.m()
+
+type T9p struct { E9p }
+type E9p int
+func (*E9p) m() int { _ = x9p; return 0 }
+var x9p /* ERROR initialization cycle */ = new(T9p).m
+
+type T10p struct { E10p }
+type E10p int
+func (*E10p) m() int { _ = x10p; return 0 }
+var x10p /* ERROR initialization cycle */ = new(T10p).m()
+
+type T9v struct { E9v }
+type E9v int
+func (E9v) m() int { _ = x9v; return 0 }
+var x9var T9v
+var x9v /* ERROR initialization cycle */ = x9var.m
+
+type T10v struct { E10v }
+type E10v int
+func (E10v) m() int { _ = x10v; return 0 }
+var x10var T10v
+var x10v /* ERROR initialization cycle */ = x10var.m()
+
+type T9pv struct { E9pv }
+type E9pv int
+func (*E9pv) m() int { _ = x9pv; return 0 }
+var x9pvar *T9pv
+var x9pv /* ERROR initialization cycle */ = x9pvar.m
+
+type T10pv struct { E10pv }
+type E10pv int
+func (*E10pv) m() int { _ = x10pv; return 0 }
+var x10pvar *T10pv
+var x10pv /* ERROR initialization cycle */ = x10pvar.m()
diff --git a/src/go/types/testdata/issue25008a.src b/src/go/types/testdata/issue25008a.src
new file mode 100644
index 0000000..cf71ca1
--- /dev/null
+++ b/src/go/types/testdata/issue25008a.src
@@ -0,0 +1,15 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+import "io"
+
+type A interface {
+        io.Reader
+}
+
+func f(a A) {
+        a.Read(nil)
+}
diff --git a/src/go/types/testdata/issue25008b.src b/src/go/types/testdata/issue25008b.src
new file mode 100644
index 0000000..f132b7f
--- /dev/null
+++ b/src/go/types/testdata/issue25008b.src
@@ -0,0 +1,9 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type B interface {
+    A
+}
diff --git a/src/go/types/testdata/issues.src b/src/go/types/testdata/issues.src
new file mode 100644
index 0000000..e0c5d7a
--- /dev/null
+++ b/src/go/types/testdata/issues.src
@@ -0,0 +1,365 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issues
+
+import (
+	"fmt"
+	syn "cmd/compile/internal/syntax"
+	t1 "text/template"
+	t2 "html/template"
+)
+
+func issue7035() {
+	type T struct{ X int }
+	_ = func() {
+		fmt.Println() // must refer to imported fmt rather than the fmt below
+	}
+	fmt := new(T)
+	_ = fmt.X
+}
+
+func issue8066() {
+	const (
+		_ = float32(340282356779733661637539395458142568447)
+		_ = float32(340282356779733661637539395458142568448 /* ERROR cannot convert */ )
+	)
+}
+
+// Check that a missing identifier doesn't lead to a spurious error cascade.
+func issue8799a() {
+	x, ok := missing /* ERROR undeclared */ ()
+	_ = !ok
+	_ = x
+}
+
+func issue8799b(x int, ok bool) {
+	x, ok = missing /* ERROR undeclared */ ()
+	_ = !ok
+	_ = x
+}
+
+func issue9182() {
+	type Point C /* ERROR undeclared */ .Point
+	// no error for composite literal based on unknown type
+	_ = Point{x: 1, y: 2}
+}
+
+func f0() (a []int)         { return }
+func f1() (a []int, b int)  { return }
+func f2() (a, b []int)      { return }
+
+func append_([]int, ...int) {}
+
+func issue9473(a []int, b ...int) {
+	// variadic builtin function
+	_ = append(f0())
+	_ = append(f0(), f0()...)
+	_ = append(f1())
+	_ = append(f2 /* ERROR cannot use .* in argument */ ())
+	_ = append(f2()... /* ERROR cannot use ... */ )
+	_ = append(f0(), f1 /* ERROR 2-valued f1 */ ())
+	_ = append(f0(), f2 /* ERROR 2-valued f2 */ ())
+	_ = append(f0(), f1 /* ERROR 2-valued f1 */ ()...)
+	_ = append(f0(), f2 /* ERROR 2-valued f2 */ ()...)
+
+	// variadic user-defined function
+	append_(f0())
+	append_(f0(), f0()...)
+	append_(f1())
+	append_(f2 /* ERROR cannot use .* in argument */ ())
+	append_(f2()... /* ERROR cannot use ... */ )
+	append_(f0(), f1 /* ERROR 2-valued f1 */ ())
+	append_(f0(), f2 /* ERROR 2-valued f2 */ ())
+	append_(f0(), f1 /* ERROR 2-valued f1 */ ()...)
+	append_(f0(), f2 /* ERROR 2-valued f2 */ ()...)
+}
+
+// Check that embedding a non-interface type in an interface results in a good error message.
+func issue10979() {
+	type _ interface {
+		int /* ERROR int is not an interface */
+	}
+	type T struct{}
+	type _ interface {
+		T /* ERROR T is not an interface */
+	}
+	type _ interface {
+		nosuchtype /* ERROR undeclared name: nosuchtype */
+	}
+	type _ interface {
+		fmt.Nosuchtype /* ERROR Nosuchtype not declared by package fmt */
+	}
+	type _ interface {
+		nosuchpkg /* ERROR undeclared name: nosuchpkg */ .Nosuchtype
+	}
+	type I interface {
+		I.m /* ERROR no field or method m */
+		m()
+	}
+}
+
+// issue11347
+// These should not crash.
+var a1, b1 /* ERROR cycle */ , c1 /* ERROR cycle */ b1 = 0 > 0<<""[""[c1]]>c1
+var a2, b2 /* ERROR cycle */ = 0 /* ERROR cannot initialize */ /* ERROR cannot initialize */ > 0<<""[b2]
+var a3, b3 /* ERROR cycle */ = int /* ERROR cannot initialize */ /* ERROR cannot initialize */ (1<<""[b3])
+
+// issue10260
+// Check that error messages explain reason for interface assignment failures.
+type (
+	I0 interface{}
+	I1 interface{ foo() }
+	I2 interface{ foo(x int) }
+	T0 struct{}
+	T1 struct{}
+	T2 struct{}
+)
+
+func (*T1) foo() {}
+func (*T2) foo(x int) {}
+
+func issue10260() {
+	var (
+		i0 I0
+		i1 I1
+		i2 I2
+		t0 *T0
+		t1 *T1
+		t2 *T2
+	)
+
+	var x I1
+	x = T1 /* ERROR cannot use .*: missing method foo \(foo has pointer receiver\) */ {}
+	_ = x /* ERROR .* cannot have dynamic type T1 \(missing method foo \(foo has pointer receiver\)\) */ .(T1)
+
+	T1{}.foo /* ERROR cannot call pointer method foo on T1 */ ()
+	x.Foo /* ERROR "x.Foo undefined \(type I1 has no field or method Foo, but does have foo\)" */ ()
+
+	_ = i2 /* ERROR i2 .* cannot have dynamic type \*T1 \(wrong type for method foo \(have func\(\), want func\(x int\)\)\) */ .(*T1)
+
+	i1 = i0 /* ERROR cannot use .* missing method foo */
+	i1 = t0 /* ERROR cannot use .* missing method foo */
+	i1 = i2 /* ERROR cannot use .* wrong type for method foo */
+	i1 = t2 /* ERROR cannot use .* wrong type for method foo */
+	i2 = i1 /* ERROR cannot use .* wrong type for method foo */
+	i2 = t1 /* ERROR cannot use .* wrong type for method foo */
+
+	_ = func() I1 { return i0 /* ERROR cannot use .* missing method foo */ }
+	_ = func() I1 { return t0 /* ERROR cannot use .* missing method foo */ }
+	_ = func() I1 { return i2 /* ERROR cannot use .* wrong type for method foo */ }
+	_ = func() I1 { return t2 /* ERROR cannot use .* wrong type for method foo */ }
+	_ = func() I2 { return i1 /* ERROR cannot use .* wrong type for method foo */ }
+	_ = func() I2 { return t1 /* ERROR cannot use .* wrong type for method foo */ }
+
+	// a few more - less exhaustive now
+
+	f := func(I1, I2){}
+	f(i0 /* ERROR cannot use .* missing method foo */ , i1 /* ERROR cannot use .* wrong type for method foo \(have func\(\), want func\(x int\)\) */ )
+
+	_ = [...]I1{i0 /* ERROR cannot use .* missing method foo */ }
+	_ = [...]I1{i2 /* ERROR cannot use .* wrong type for method foo */ }
+	_ = []I1{i0 /* ERROR cannot use .* missing method foo */ }
+	_ = []I1{i2 /* ERROR cannot use .* wrong type for method foo */ }
+	_ = map[int]I1{0: i0 /* ERROR cannot use .* missing method foo */ }
+	_ = map[int]I1{0: i2 /* ERROR cannot use .* wrong type for method foo */ }
+
+	make(chan I1) <- i0 /* ERROR cannot use .* in send: missing method foo */
+	make(chan I1) <- i2 /* ERROR cannot use .* in send: wrong type for method foo */
+}
+
+// Check that constants representable as integers are in integer form
+// before being used in operations that are only defined on integers.
+func issue14229() {
+	// from the issue
+	const _ = int64(-1<<63) % 1e6
+
+	// related
+	const (
+		a int = 3
+		b = 4.0
+		_ = a / b
+		_ = a % b
+		_ = b / a
+		_ = b % a
+	)
+}
+
+// Check that in a n:1 variable declaration with type and initialization
+// expression the type is distributed to all variables of the lhs before
+// the initialization expression assignment is checked.
+func issue15755() {
+	// from issue
+	var i interface{}
+	type b bool
+	var x, y b = i.(b)
+	_ = x == y
+
+	// related: we should see an error since the result of f1 is ([]int, int)
+	var u, v []int = f1 /* ERROR cannot use f1 */ ()
+	_ = u
+	_ = v
+}
+
+// Test that we don't get "declared but not used"
+// errors in the context of invalid/C objects.
+func issue20358() {
+	var F C /* ERROR "undeclared" */ .F
+	var A C /* ERROR "undeclared" */ .A
+	var S C /* ERROR "undeclared" */ .S
+	type T C /* ERROR "undeclared" */ .T
+	type P C /* ERROR "undeclared" */ .P
+
+	// these variables must be "used" even though
+	// the LHS expressions/types below in which
+	// context they are used are unknown/invalid
+	var f, a, s1, s2, s3, t, p int
+
+	_ = F(f)
+	_ = A[a]
+	_ = S[s1:s2:s3]
+	_ = T{t}
+	_ = P{f: p}
+}
+
+// Test that we don't declare lhs variables in short variable
+// declarations before we type-check function literals on the
+// rhs.
+func issue24026() {
+	f := func() int { f(0) /* must refer to outer f */; return 0 }
+	_ = f
+
+	_ = func() {
+		f := func() { _ = f() /* must refer to outer f */ }
+		_ = f
+	}
+
+	// b and c must not be visible inside function literal
+	a := 0
+	a, b, c := func() (int, int, int) {
+		return a, b /* ERROR undeclared */ , c /* ERROR undeclared */
+	}()
+	_, _ = b, c
+}
+
+func f(int) {} // for issue24026
+
+// Test that we don't report a "missing return statement" error
+// (due to incorrect context when type-checking interfaces).
+func issue24140(x interface{}) int {
+        switch x.(type) {
+        case interface{}:
+                return 0
+        default:
+                panic(0)
+        }
+}
+
+// Test that we don't crash when the 'if' condition is missing.
+func issue25438() {
+	if { /* ERROR missing condition */ }
+	if x := 0; /* ERROR missing condition */ { _ = x }
+	if
+	{ /* ERROR missing condition */ }
+}
+
+// Test that we can embed alias type names in interfaces.
+type issue25301 interface {
+	E
+}
+
+type E = interface {
+	m()
+}
+
+// Test case from issue.
+// cmd/compile reports a cycle as well.
+type issue25301b /* ERROR cycle */ = interface {
+	m() interface{ issue25301b }
+}
+
+type issue25301c interface {
+	notE // ERROR struct\{\} is not an interface
+}
+
+type notE = struct{}
+
+// Test that method declarations don't introduce artificial cycles
+// (issue #26124).
+const CC TT = 1
+type TT int
+func (TT) MM() [CC]TT
+
+// Reduced test case from issue #26124.
+const preloadLimit LNumber = 128
+type LNumber float64
+func (LNumber) assertFunction() *LFunction
+type LFunction struct {
+	GFunction LGFunction
+}
+type LGFunction func(*LState)
+type LState struct {
+	reg *registry
+}
+type registry struct {
+	alloc *allocator
+}
+type allocator struct {
+	_ [int(preloadLimit)]int
+}
+
+// Test that we don't crash when type-checking composite literals
+// containing errors in the type.
+var issue27346 = [][n /* ERROR undeclared */ ]int{
+	0: {},
+}
+
+var issue22467 = map[int][... /* ERROR invalid use of ... */ ]int{0: {}}
+
+// Test that invalid use of ... in parameter lists is recognized
+// (issue #28281).
+func issue28281a(int, int, ...int)
+func issue28281b(a, b int, c ...int)
+func issue28281c(a, b, c ... /* ERROR can only use ... with final parameter */ int)
+func issue28281d(... /* ERROR can only use ... with final parameter */ int, int)
+func issue28281e(a, b, c  ... /* ERROR can only use ... with final parameter */ int, d int)
+func issue28281f(... /* ERROR can only use ... with final parameter */ int, ... /* ERROR can only use ... with final parameter */ int, int)
+func (... /* ERROR expected type */ TT) f()
+func issue28281g() (... /* ERROR expected type */ TT)
+
+// Issue #26234: Make various field/method lookup errors easier to read by matching cmd/compile's output
+func issue26234a(f *syn.File) {
+	// The error message below should refer to the actual package name (syntax)
+	// not the local package name (syn).
+	f.foo /* ERROR f.foo undefined \(type \*syntax.File has no field or method foo\) */
+}
+
+type T struct {
+	x int
+	E1
+	E2
+}
+
+type E1 struct{ f int }
+type E2 struct{ f int }
+
+func issue26234b(x T) {
+	_ = x.f /* ERROR ambiguous selector x.f */
+}
+
+func issue26234c() {
+	T.x /* ERROR T.x undefined \(type T has no method x\) */ ()
+}
+
+func issue35895() {
+	// T is defined in this package, don't qualify its name with the package name.
+	var _ T = 0 // ERROR cannot use 0 \(untyped int constant\) as T
+
+	// There is only one package with name syntax imported, only use the (global) package name in error messages.
+	var _ *syn.File = 0 // ERROR cannot use 0 \(untyped int constant\) as \*syntax.File
+
+	// Because both t1 and t2 have the same global package name (template),
+	// qualify packages with full path name in this case.
+	var _ t1.Template = t2 /* ERROR cannot use .* \(value of type "html/template".Template\) as "text/template".Template */ .Template{}
+}
diff --git a/src/go/types/testdata/labels.src b/src/go/types/testdata/labels.src
new file mode 100644
index 0000000..9f42406
--- /dev/null
+++ b/src/go/types/testdata/labels.src
@@ -0,0 +1,207 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is a modified concatenation of the files
+// $GOROOT/test/label.go and $GOROOT/test/label1.go.
+
+package labels
+
+var x int
+
+func f0() {
+L1 /* ERROR "label L1 declared but not used" */ :
+	for {
+	}
+L2 /* ERROR "label L2 declared but not used" */ :
+	select {
+	}
+L3 /* ERROR "label L3 declared but not used" */ :
+	switch {
+	}
+L4 /* ERROR "label L4 declared but not used" */ :
+	if true {
+	}
+L5 /* ERROR "label L5 declared but not used" */ :
+	f0()
+L6:
+	f0()
+L6 /* ERROR "label L6 already declared" */ :
+	f0()
+	if x == 20 {
+		goto L6
+	}
+
+L7:
+	for {
+		break L7
+		break L8 /* ERROR "invalid break label L8" */
+	}
+
+// A label must be directly associated with a switch, select, or
+// for statement; it cannot be the label of a labeled statement.
+
+L7a /* ERROR "declared but not used" */ : L7b:
+	for {
+		break L7a /* ERROR "invalid break label L7a" */
+		continue L7a /* ERROR "invalid continue label L7a" */
+		continue L7b
+	}
+
+L8:
+	for {
+		if x == 21 {
+			continue L8
+			continue L7 /* ERROR "invalid continue label L7" */
+		}
+	}
+
+L9:
+	switch {
+	case true:
+		break L9
+	defalt /* ERROR "label defalt declared but not used" */ :
+	}
+
+L10:
+	select {
+	default:
+		break L10
+		break L9 /* ERROR "invalid break label L9" */
+	}
+
+	goto L10a
+L10a: L10b:
+	select {
+	default:
+		break L10a /* ERROR "invalid break label L10a" */
+		break L10b
+		continue L10b /* ERROR "invalid continue label L10b" */
+	}
+}
+
+func f1() {
+L1:
+	for {
+		if x == 0 {
+			break L1
+		}
+		if x == 1 {
+			continue L1
+		}
+		goto L1
+	}
+
+L2:
+	select {
+	default:
+		if x == 0 {
+			break L2
+		}
+		if x == 1 {
+			continue L2 /* ERROR "invalid continue label L2" */
+		}
+		goto L2
+	}
+
+L3:
+	switch {
+	case x > 10:
+		if x == 11 {
+			break L3
+		}
+		if x == 12 {
+			continue L3 /* ERROR "invalid continue label L3" */
+		}
+		goto L3
+	}
+
+L4:
+	if true {
+		if x == 13 {
+			break L4 /* ERROR "invalid break label L4" */
+		}
+		if x == 14 {
+			continue L4 /* ERROR "invalid continue label L4" */
+		}
+		if x == 15 {
+			goto L4
+		}
+	}
+
+L5:
+	f1()
+	if x == 16 {
+		break L5 /* ERROR "invalid break label L5" */
+	}
+	if x == 17 {
+		continue L5 /* ERROR "invalid continue label L5" */
+	}
+	if x == 18 {
+		goto L5
+	}
+
+	for {
+		if x == 19 {
+			break L1 /* ERROR "invalid break label L1" */
+		}
+		if x == 20 {
+			continue L1 /* ERROR "invalid continue label L1" */
+		}
+		if x == 21 {
+			goto L1
+		}
+	}
+}
+
+// Additional tests not in the original files.
+
+func f2() {
+L1 /* ERROR "label L1 declared but not used" */ :
+	if x == 0 {
+		for {
+			continue L1 /* ERROR "invalid continue label L1" */
+		}
+	}
+}
+
+func f3() {
+L1:
+L2:
+L3:
+	for {
+		break L1 /* ERROR "invalid break label L1" */
+		break L2 /* ERROR "invalid break label L2" */
+		break L3
+		continue L1 /* ERROR "invalid continue label L1" */
+		continue L2 /* ERROR "invalid continue label L2" */
+		continue L3
+		goto L1
+		goto L2
+		goto L3
+	}
+}
+
+// Blank labels are never declared.
+
+func f4() {
+_:
+_: // multiple blank labels are ok
+	goto _ /* ERROR "label _ not declared" */
+}
+
+func f5() {
+_:
+	for {
+		break _ /* ERROR "invalid break label _" */
+		continue _ /* ERROR "invalid continue label _" */
+	}
+}
+
+func f6() {
+_:
+	switch {
+	default:
+		break _ /* ERROR "invalid break label _" */
+	}
+}
diff --git a/src/go/types/testdata/literals.src b/src/go/types/testdata/literals.src
new file mode 100644
index 0000000..494a465
--- /dev/null
+++ b/src/go/types/testdata/literals.src
@@ -0,0 +1,111 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file tests various representations of literals
+// and compares them with literals or constant expressions
+// of equal values.
+
+package literals
+
+func _() {
+	// 0-octals
+	assert(0_123 == 0123)
+	assert(0123_456 == 0123456)
+
+	// decimals
+	assert(1_234 == 1234)
+	assert(1_234_567 == 1234567)
+
+	// hexadecimals
+	assert(0X_0 == 0)
+	assert(0X_1234 == 0x1234)
+	assert(0X_CAFE_f00d == 0xcafef00d)
+
+	// octals
+	assert(0o0 == 0)
+	assert(0o1234 == 01234)
+	assert(0o01234567 == 01234567)
+
+	assert(0O0 == 0)
+	assert(0O1234 == 01234)
+	assert(0O01234567 == 01234567)
+
+	assert(0o_0 == 0)
+	assert(0o_1234 == 01234)
+	assert(0o0123_4567 == 01234567)
+
+	assert(0O_0 == 0)
+	assert(0O_1234 == 01234)
+	assert(0O0123_4567 == 01234567)
+
+	// binaries
+	assert(0b0 == 0)
+	assert(0b1011 == 0xb)
+	assert(0b00101101 == 0x2d)
+
+	assert(0B0 == 0)
+	assert(0B1011 == 0xb)
+	assert(0B00101101 == 0x2d)
+
+	assert(0b_0 == 0)
+	assert(0b10_11 == 0xb)
+	assert(0b_0010_1101 == 0x2d)
+
+	// decimal floats
+	assert(1_2_3. == 123.)
+	assert(0_123. == 123.)
+
+	assert(0_0e0 == 0.)
+	assert(1_2_3e0 == 123.)
+	assert(0_123e0 == 123.)
+
+	assert(0e-0_0 == 0.)
+	assert(1_2_3E+0 == 123.)
+	assert(0123E1_2_3 == 123e123)
+
+	assert(0.e+1 == 0.)
+	assert(123.E-1_0 == 123e-10)
+	assert(01_23.e123 == 123e123)
+
+	assert(.0e-1 == .0)
+	assert(.123E+10 == .123e10)
+	assert(.0123E123 == .0123e123)
+
+	assert(1_2_3.123 == 123.123)
+	assert(0123.01_23 == 123.0123)
+
+	// hexadecimal floats
+	assert(0x0.p+0 == 0.)
+	assert(0Xdeadcafe.p-10 == 0xdeadcafe/1024.0)
+	assert(0x1234.P84 == 0x1234000000000000000000000)
+
+	assert(0x.1p-0 == 1./16)
+	assert(0X.deadcafep4 == 1.0*0xdeadcafe/0x10000000)
+	assert(0x.1234P+12 == 1.0*0x1234/0x10)
+
+	assert(0x0p0 == 0.)
+	assert(0Xdeadcafep+1 == 0x1bd5b95fc)
+	assert(0x1234P-10 == 0x1234/1024.0)
+
+	assert(0x0.0p0 == 0.)
+	assert(0Xdead.cafep+1 == 1.0*0x1bd5b95fc/0x10000)
+	assert(0x12.34P-10 == 1.0*0x1234/0x40000)
+
+	assert(0Xdead_cafep+1 == 0xdeadcafep+1)
+	assert(0x_1234P-10 == 0x1234p-10)
+
+	assert(0X_dead_cafe.p-10 == 0xdeadcafe.p-10)
+	assert(0x12_34.P1_2_3 == 0x1234.p123)
+
+	assert(1_234i == 1234i)
+	assert(1_234_567i == 1234567i)
+
+	assert(0.i == 0i)
+	assert(123.i == 123i)
+	assert(0123.i == 123i)
+
+	assert(0.e+1i == 0i)
+	assert(123.E-1_0i == 123e-10i)
+	assert(01_23.e123i == 123e123i)
+}
diff --git a/src/go/types/testdata/methodsets.src b/src/go/types/testdata/methodsets.src
new file mode 100644
index 0000000..9fb10de
--- /dev/null
+++ b/src/go/types/testdata/methodsets.src
@@ -0,0 +1,214 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package methodsets
+
+type T0 struct {}
+
+func (T0) v0() {}
+func (*T0) p0() {}
+
+type T1 struct {} // like T0 with different method names
+
+func (T1) v1() {}
+func (*T1) p1() {}
+
+type T2 interface {
+	v2()
+	p2()
+}
+
+type T3 struct {
+	T0
+	*T1
+	T2
+}
+
+// Method expressions
+func _() {
+	var (
+		_ func(T0) = T0.v0
+		_ = T0.p0 /* ERROR "cannot call pointer method p0 on T0" */
+
+		_ func (*T0) = (*T0).v0
+		_ func (*T0) = (*T0).p0
+
+		// T1 is like T0
+
+		_ func(T2) = T2.v2
+		_ func(T2) = T2.p2
+
+		_ func(T3) = T3.v0
+		_ func(T3) = T3.p0 /* ERROR "cannot call pointer method p0 on T3" */
+		_ func(T3) = T3.v1
+		_ func(T3) = T3.p1
+		_ func(T3) = T3.v2
+		_ func(T3) = T3.p2
+
+		_ func(*T3) = (*T3).v0
+		_ func(*T3) = (*T3).p0
+		_ func(*T3) = (*T3).v1
+		_ func(*T3) = (*T3).p1
+		_ func(*T3) = (*T3).v2
+		_ func(*T3) = (*T3).p2
+	)
+}
+
+// Method values with addressable receivers
+func _() {
+	var (
+		v0 T0
+		_ func() = v0.v0
+		_ func() = v0.p0
+	)
+
+	var (
+		p0 *T0
+		_ func() = p0.v0
+		_ func() = p0.p0
+	)
+
+	// T1 is like T0
+
+	var (
+		v2 T2
+		_ func() = v2.v2
+		_ func() = v2.p2
+	)
+
+	var (
+		v4 T3
+		_ func() = v4.v0
+		_ func() = v4.p0
+		_ func() = v4.v1
+		_ func() = v4.p1
+		_ func() = v4.v2
+		_ func() = v4.p2
+	)
+
+	var (
+		p4 *T3
+		_ func() = p4.v0
+		_ func() = p4.p0
+		_ func() = p4.v1
+		_ func() = p4.p1
+		_ func() = p4.v2
+		_ func() = p4.p2
+	)
+}
+
+// Method calls with addressable receivers
+func _() {
+	var v0 T0
+	v0.v0()
+	v0.p0()
+
+	var p0 *T0
+	p0.v0()
+	p0.p0()
+
+	// T1 is like T0
+
+	var v2 T2
+	v2.v2()
+	v2.p2()
+
+	var v4 T3
+	v4.v0()
+	v4.p0()
+	v4.v1()
+	v4.p1()
+	v4.v2()
+	v4.p2()
+
+	var p4 *T3
+	p4.v0()
+	p4.p0()
+	p4.v1()
+	p4.p1()
+	p4.v2()
+	p4.p2()
+}
+
+// Method values with value receivers
+func _() {
+	var (
+		_ func() = T0{}.v0
+		_ func() = T0{}.p0 /* ERROR "cannot call pointer method p0 on T0" */
+
+		_ func() = (&T0{}).v0
+		_ func() = (&T0{}).p0
+
+		// T1 is like T0
+
+		// no values for T2
+
+		_ func() = T3{}.v0
+		_ func() = T3{}.p0 /* ERROR "cannot call pointer method p0 on T3" */
+		_ func() = T3{}.v1
+		_ func() = T3{}.p1
+		_ func() = T3{}.v2
+		_ func() = T3{}.p2
+
+		_ func() = (&T3{}).v0
+		_ func() = (&T3{}).p0
+		_ func() = (&T3{}).v1
+		_ func() = (&T3{}).p1
+		_ func() = (&T3{}).v2
+		_ func() = (&T3{}).p2
+	)
+}
+
+// Method calls with value receivers
+func _() {
+	T0{}.v0()
+	T0{}.p0 /* ERROR "cannot call pointer method p0 on T0" */ ()
+
+	(&T0{}).v0()
+	(&T0{}).p0()
+
+	// T1 is like T0
+
+	// no values for T2
+
+	T3{}.v0()
+	T3{}.p0 /* ERROR "cannot call pointer method p0 on T3" */ ()
+	T3{}.v1()
+	T3{}.p1()
+	T3{}.v2()
+	T3{}.p2()
+
+	(&T3{}).v0()
+	(&T3{}).p0()
+	(&T3{}).v1()
+	(&T3{}).p1()
+	(&T3{}).v2()
+	(&T3{}).p2()
+}
+
+// *T has no methods if T is an interface type
+func issue5918() {
+	var (
+		err error
+		_ = err.Error()
+		_ func() string = err.Error
+		_ func(error) string = error.Error
+
+		perr = &err
+		_ = perr.Error /* ERROR "no field or method" */ ()
+		_ func() string = perr.Error /* ERROR "no field or method" */
+		_ func(*error) string = (*error).Error /* ERROR "no field or method" */
+	)
+
+	type T *interface{ m() int }
+	var (
+		x T
+		_ = (*x).m()
+		_ = (*x).m
+
+		_ = x.m /* ERROR "no field or method" */ ()
+		_ = x.m /* ERROR "no field or method" */
+		_ = T.m /* ERROR "no field or method" */
+	)
+}
diff --git a/src/go/types/testdata/shifts.src b/src/go/types/testdata/shifts.src
new file mode 100644
index 0000000..c9a38ae
--- /dev/null
+++ b/src/go/types/testdata/shifts.src
@@ -0,0 +1,395 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package shifts
+
+func shifts0() {
+	// basic constant shifts
+	const (
+		s = 10
+		_ = 0<<0
+		_ = 1<<s
+		_ = 1<<- /* ERROR "negative shift count" */ 1
+		// For the test below we may decide to convert to int
+		// rather than uint and then report a negative shift
+		// count instead, which might be a better error. The
+		// (minor) difference is that this would restrict the
+		// shift count range by half (from all uint values to
+		// the positive int values).
+		// This depends on the exact spec wording which is not
+		// done yet.
+		// TODO(gri) revisit and adjust when spec change is done
+		_ = 1<<- /* ERROR "truncated to uint" */ 1.0
+		_ = 1<<1075 /* ERROR "invalid shift" */
+		_ = 2.0<<1
+		_ = 1<<1.0
+		_ = 1<<(1+0i)
+
+		_ int = 2<<s
+		_ float32 = 2<<s
+		_ complex64 = 2<<s
+
+		_ int = 2.0<<s
+		_ float32 = 2.0<<s
+		_ complex64 = 2.0<<s
+
+		_ int = 'a'<<s
+		_ float32 = 'a'<<s
+		_ complex64 = 'a'<<s
+	)
+}
+
+func shifts1() {
+	// basic non-constant shifts
+	var (
+		i int
+		u uint
+
+		_ = 1<<0
+		_ = 1<<i
+		_ = 1<<u
+		_ = 1<<"foo" /* ERROR "cannot convert" */
+		_ = i<<0
+		_ = i<<- /* ERROR "negative shift count" */ 1
+		_ = i<<1.0
+		_ = 1<<(1+0i)
+		_ = 1 /* ERROR "overflows" */ <<100
+
+		_ uint = 1 << 0
+		_ uint = 1 << u
+		_ float32 = 1 /* ERROR "must be integer" */ << u
+
+		// for issue 14822
+		_ = 1<<( /* ERROR "invalid shift count" */ 1<<64-1)
+		_ = 1<<( /* ERROR "invalid shift count" */ 1<<64)
+		_ = u<<(1<<63) // valid
+		_ = u<<(1<<64) // valid
+	)
+}
+
+func shifts2() {
+	// from the spec
+	var (
+		s uint = 33
+		i = 1<<s           // 1 has type int
+		j int32 = 1<<s     // 1 has type int32; j == 0
+		k = uint64(1<<s)   // 1 has type uint64; k == 1<<33
+		m int = 1.0<<s     // 1.0 has type int
+		n = 1.0<<s != i    // 1.0 has type int; n == false if ints are 32bits in size
+		o = 1<<s == 2<<s   // 1 and 2 have type int; o == true if ints are 32bits in size
+		p = 1<<s == 1<<33  // illegal if ints are 32bits in size: 1 has type int, but 1<<33 overflows int
+		u = 1.0 /* ERROR "must be integer" */ <<s         // illegal: 1.0 has type float64, cannot shift
+		u1 = 1.0 /* ERROR "must be integer" */ <<s != 0   // illegal: 1.0 has type float64, cannot shift
+		u2 = 1 /* ERROR "must be integer" */ <<s != 1.0   // illegal: 1 has type float64, cannot shift
+		v float32 = 1 /* ERROR "must be integer" */ <<s   // illegal: 1 has type float32, cannot shift
+		w int64 = 1.0<<33  // 1.0<<33 is a constant shift expression
+	)
+	_, _, _, _, _, _, _, _, _, _, _, _ = i, j, k, m, n, o, p, u, u1, u2, v, w
+}
+
+func shifts3(a int16, b float32) {
+	// random tests
+	var (
+		s uint = 11
+		u = 1 /* ERROR "must be integer" */ <<s + 1.0
+		v complex128 = 1 /* ERROR "must be integer" */ << s + 1.0 /* ERROR "must be integer" */ << s + 1
+	)
+	x := 1.0 /* ERROR "must be integer" */ <<s + 1
+	shifts3(1.0 << s, 1 /* ERROR "must be integer" */ >> s)
+	_, _, _ = u, v, x
+}
+
+func shifts4() {
+	// shifts in comparisons w/ untyped operands
+	var s uint
+
+	_ = 1<<s == 1
+	_ = 1 /* ERROR "integer" */ <<s == 1.
+	_ = 1. /* ERROR "integer" */ <<s == 1
+	_ = 1. /* ERROR "integer" */ <<s == 1.
+
+	_ = 1<<s + 1 == 1
+	_ = 1 /* ERROR "integer" */ <<s + 1 == 1.
+	_ = 1 /* ERROR "integer" */ <<s + 1. == 1
+	_ = 1 /* ERROR "integer" */ <<s + 1. == 1.
+	_ = 1. /* ERROR "integer" */ <<s + 1 == 1
+	_ = 1. /* ERROR "integer" */ <<s + 1 == 1.
+	_ = 1. /* ERROR "integer" */ <<s + 1. == 1
+	_ = 1. /* ERROR "integer" */ <<s + 1. == 1.
+
+	_ = 1<<s == 1<<s
+	_ = 1 /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s
+
+	_ = 1<<s + 1<<s == 1
+	_ = 1 /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1.
+	_ = 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1
+	_ = 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1.
+	_ = 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1
+	_ = 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1.
+	_ = 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1
+	_ = 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1.
+
+	_ = 1<<s + 1<<s == 1<<s + 1<<s
+	_ = 1 /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+	_ = 1 /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s
+	_ = 1 /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+	_ = 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s
+	_ = 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+	_ = 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s
+	_ = 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1 /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1 /* ERROR "integer" */ <<s
+	_ = 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s == 1. /* ERROR "integer" */ <<s + 1. /* ERROR "integer" */ <<s
+}
+
+func shifts5() {
+	// shifts in comparisons w/ typed operands
+	var s uint
+	var x int
+
+	_ = 1<<s == x
+	_ = 1.<<s == x
+	_ = 1.1 /* ERROR "int" */ <<s == x
+
+	_ = 1<<s + x == 1
+	_ = 1<<s + x == 1.
+	_ = 1<<s + x == 1.1 /* ERROR "int" */
+	_ = 1.<<s + x == 1
+	_ = 1.<<s + x == 1.
+	_ = 1.<<s + x == 1.1 /* ERROR "int" */
+	_ = 1.1 /* ERROR "int" */ <<s + x == 1
+	_ = 1.1 /* ERROR "int" */ <<s + x == 1.
+	_ = 1.1 /* ERROR "int" */ <<s + x == 1.1
+
+	_ = 1<<s == x<<s
+	_ = 1.<<s == x<<s
+	_ = 1.1  /* ERROR "int" */ <<s == x<<s
+}
+
+func shifts6() {
+	// shifts as operands in non-arithmetic operations and as arguments
+	var a [10]int
+	var s uint
+
+	_ = a[1<<s]
+	_ = a[1.0]
+	_ = a[1.0<<s]
+
+	_ = make([]int, 1.0)
+	_ = make([]int, 1.0<<s)
+	_ = make([]int, 1.1 /* ERROR "must be integer" */ <<s)
+
+	_ = float32(1)
+	_ = float32(1 /* ERROR "must be integer" */ <<s)
+	_ = float32(1.0)
+	_ = float32(1.0 /* ERROR "must be integer" */ <<s)
+	_ = float32(1.1 /* ERROR "must be integer" */ <<s)
+
+	_ = int32(0x80000000 /* ERROR "overflows int32" */ << s)
+	// TODO(rfindley) Eliminate the redundant error here.
+	_ = int32(( /* ERROR "truncated to int32" */ 0x80000000 /* ERROR "truncated to int32" */ + 0i) << s)
+
+	_ = int(1+0i<<0)
+	_ = int((1+0i)<<s)
+	_ = int(1.0<<s)
+	_ = int(complex(1, 0)<<s)
+	_ = int(float32/* ERROR "must be integer" */(1.0) <<s)
+	_ = int(1.1 /* ERROR must be integer */ <<s)
+	_ = int(( /* ERROR "must be integer" */ 1+1i)  <<s)
+
+	_ = complex(1 /* ERROR "must be integer" */ <<s, 0)
+
+	var b []int
+	_ = append(b, 1<<s)
+	_ = append(b, 1.0<<s)
+	_ = append(b, (1+0i)<<s)
+	_ = append(b, 1.1 /* ERROR "must be integer" */ <<s)
+	_ = append(b, (1 + 0i) <<s)
+	_ = append(b, ( /* ERROR "must be integer" */ 1 + 1i)  <<s)
+
+	_ = complex(1.0 /* ERROR "must be integer" */ <<s, 0)
+	_ = complex(1.1 /* ERROR "must be integer" */ <<s, 0)
+	_ = complex(0, 1.0 /* ERROR "must be integer" */ <<s)
+	_ = complex(0, 1.1 /* ERROR "must be integer" */ <<s)
+
+	// TODO(gri) The delete below is not type-checked correctly yet.
+	// var m1 map[int]string
+	// delete(m1, 1<<s)
+}
+
+func shifts7() {
+	// shifts of shifts
+	var s uint
+	var x int
+	_ = x
+
+	_ = 1<<(1<<s)
+	_ = 1<<(1.<<s)
+	_ = 1. /* ERROR "integer" */ <<(1<<s)
+	_ = 1. /* ERROR "integer" */ <<(1.<<s)
+
+	x = 1<<(1<<s)
+	x = 1<<(1.<<s)
+	x = 1.<<(1<<s)
+	x = 1.<<(1.<<s)
+
+	_ = (1<<s)<<(1<<s)
+	_ = (1<<s)<<(1.<<s)
+	_ = ( /* ERROR "integer" */ 1.<<s)<<(1<<s)
+	_ = ( /* ERROR "integer" */ 1.<<s)<<(1.<<s)
+
+	x = (1<<s)<<(1<<s)
+	x = (1<<s)<<(1.<<s)
+	x = ( /* ERROR "integer" */ 1.<<s)<<(1<<s)
+	x = ( /* ERROR "integer" */ 1.<<s)<<(1.<<s)
+}
+
+func shifts8() {
+	// shift examples from shift discussion: better error messages
+	var s uint
+	_ = 1.0 /* ERROR "shifted operand 1.0 \(type float64\) must be integer" */ <<s == 1
+	_ = 1.0 /* ERROR "shifted operand 1.0 \(type float64\) must be integer" */ <<s == 1.0
+	_ = 1 /* ERROR "shifted operand 1 \(type float64\) must be integer" */ <<s == 1.0
+	_ = 1 /* ERROR "shifted operand 1 \(type float64\) must be integer" */ <<s + 1.0 == 1
+	_ = 1 /* ERROR "shifted operand 1 \(type float64\) must be integer" */ <<s + 1.1 == 1
+	_ = 1 /* ERROR "shifted operand 1 \(type float64\) must be integer" */ <<s + 1 == 1.0
+
+	// additional cases
+	_ = complex(1.0 /* ERROR "shifted operand 1.0 \(type float64\) must be integer" */ <<s, 1)
+	_ = complex(1.0, 1 /* ERROR "shifted operand 1 \(type float64\) must be integer" */ <<s)
+
+	_ = int(1.<<s)
+	_ = int(1.1 /* ERROR "shifted operand .* must be integer" */ <<s)
+	_ = float32(1 /* ERROR "shifted operand .* must be integer" */ <<s)
+	_ = float32(1. /* ERROR "shifted operand .* must be integer" */ <<s)
+	_ = float32(1.1 /* ERROR "shifted operand .* must be integer" */ <<s)
+	// TODO(gri) the error messages for these two are incorrect - disabled for now
+	// _ = complex64(1<<s)
+	// _ = complex64(1.<<s)
+	_ = complex64(1.1 /* ERROR "shifted operand .* must be integer" */ <<s)
+}
+
+func shifts9() {
+	// various originally failing snippets of code from the std library
+	// from src/compress/lzw/reader.go:90
+	{
+		var d struct {
+			bits     uint32
+			width    uint
+		}
+		_ = uint16(d.bits & (1<<d.width - 1))
+	}
+
+	// from src/debug/dwarf/buf.go:116
+	{
+		var ux uint64
+		var bits uint
+		x := int64(ux)
+		if x&(1<<(bits-1)) != 0 {}
+	}
+
+	// from src/encoding/asn1/asn1.go:160
+	{
+		var bytes []byte
+		if bytes[len(bytes)-1]&((1<<bytes[0])-1) != 0 {}
+	}
+
+	// from src/math/big/rat.go:140
+	{
+		var exp int
+		var mantissa uint64
+		shift := uint64(-1022 - (exp - 1)) // [1..53)
+		_ = mantissa & (1<<shift - 1)
+	}
+
+	// from src/net/interface.go:51
+	{
+		type Flags uint
+		var f Flags
+		var i int
+		if f&(1<<uint(i)) != 0 {}
+	}
+
+	// from src/runtime/softfloat64.go:234
+	{
+		var gm uint64
+		var shift uint
+		_ = gm & (1<<shift - 1)
+	}
+
+	// from src/strconv/atof.go:326
+	{
+		var mant uint64
+		var mantbits uint
+		if mant == 2<<mantbits {}
+	}
+
+	// from src/route_bsd.go:82
+	{
+		var Addrs int32
+		const rtaRtMask = 1
+		var i uint
+		if Addrs&rtaRtMask&(1<<i) == 0 {}
+	}
+
+	// from src/text/scanner/scanner.go:540
+	{
+		var s struct { Whitespace uint64 }
+		var ch rune
+		for s.Whitespace&(1<<uint(ch)) != 0 {}
+	}
+}
+
+func issue5895() {
+	var x = 'a' << 1 // type of x must be rune
+	var _ rune = x
+}
+
+func issue11325() {
+	var _ = 0 >> 1.1 /* ERROR "truncated to uint" */ // example from issue 11325
+	_ = 0 >> 1.1 /* ERROR "truncated to uint" */
+	_ = 0 << 1.1 /* ERROR "truncated to uint" */
+	_ = 0 >> 1.
+	_ = 1 >> 1.1 /* ERROR "truncated to uint" */
+	_ = 1 >> 1.
+	_ = 1. >> 1
+	_ = 1. >> 1.
+	_ = 1.1 /* ERROR "must be integer" */ >> 1
+}
+
+func issue11594() {
+	var _ = complex64 /* ERROR "must be integer" */ (1) << 2 // example from issue 11594
+	_ = float32 /* ERROR "must be integer" */ (0) << 1
+	_ = float64 /* ERROR "must be integer" */ (0) >> 2
+	_ = complex64 /* ERROR "must be integer" */ (0) << 3
+	_ = complex64 /* ERROR "must be integer" */ (0) >> 4
+}
+
+func issue21727() {
+	var s uint
+	var a = make([]int, 1<<s + 1.2 /* ERROR "truncated to int" */ )
+	var _ = a[1<<s - 2.3 /* ERROR "truncated to int" */ ]
+	var _ int = 1<<s + 3.4 /* ERROR "truncated to int" */
+	var _ = string(1 << s)
+	var _ = string(1.0 /* ERROR "cannot convert" */ << s)
+}
+
+func issue22969() {
+	var s uint
+	var a []byte
+	_ = a[0xffffffffffffffff /* ERROR "overflows int" */ <<s] // example from issue 22969
+	_ = make([]int, 0xffffffffffffffff /* ERROR "overflows int" */ << s)
+	_ = make([]int, 0, 0xffffffffffffffff /* ERROR "overflows int" */ << s)
+	var _ byte = 0x100 /* ERROR "overflows byte" */ << s
+	var _ int8 = 0xff /* ERROR "overflows int8" */ << s
+	var _ int16 = 0xffff /* ERROR "overflows int16" */ << s
+	var _ int32 = 0x80000000 /* ERROR "overflows int32" */ << s
+}
diff --git a/src/go/types/testdata/stmt0.src b/src/go/types/testdata/stmt0.src
new file mode 100644
index 0000000..1377729
--- /dev/null
+++ b/src/go/types/testdata/stmt0.src
@@ -0,0 +1,980 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// statements
+
+package stmt0
+
+func assignments0() (int, int) {
+	var a, b, c int
+	var ch chan int
+	f0 := func() {}
+	f1 := func() int { return 1 }
+	f2 := func() (int, int) { return 1, 2 }
+	f3 := func() (int, int, int) { return 1, 2, 3 }
+
+	a, b, c = 1, 2, 3
+	a, b, c = 1 /* ERROR "cannot assign [1-9]+ values to [1-9]+ variables" */ , 2
+	a, b, c = 1 /* ERROR "cannot assign [1-9]+ values to [1-9]+ variables" */ , 2, 3, 4
+	_, _, _ = a, b, c
+
+	a = f0 /* ERROR "used as value" */ ()
+	a = f1()
+	a = f2 /* ERROR "cannot assign [1-9]+ values to [1-9]+ variables" */ ()
+	a, b = f2()
+	a, b, c = f2 /* ERROR "cannot assign [1-9]+ values to [1-9]+ variables" */ ()
+	a, b, c = f3()
+	a, b = f3 /* ERROR "cannot assign [1-9]+ values to [1-9]+ variables" */ ()
+
+	a, b, c = <- /* ERROR "cannot assign [1-9]+ values to [1-9]+ variables" */ ch
+
+	return /* ERROR "wrong number of return values" */
+	return /* ERROR "wrong number of return values" */ 1
+	return 1, 2
+	return /* ERROR "wrong number of return values" */ 1, 2, 3
+}
+
+func assignments1() {
+	b, i, f, c, s := false, 1, 1.0, 1i, "foo"
+	b = i /* ERROR "cannot use .* in assignment" */
+	i = f /* ERROR "cannot use .* in assignment" */
+	f = c /* ERROR "cannot use .* in assignment" */
+	c = s /* ERROR "cannot use .* in assignment" */
+	s = b /* ERROR "cannot use .* in assignment" */
+
+	v0, v1, v2 := 1 /* ERROR "cannot initialize" */ , 2, 3, 4
+	_, _, _ = v0, v1, v2
+
+	b = true
+
+	i += 1
+	i += "foo" /* ERROR "cannot convert.*int" */
+
+	f -= 1
+	f /= 0
+	f = float32(0)/0 /* ERROR "division by zero" */
+	f -= "foo" /* ERROR "cannot convert.*float64" */
+
+	c *= 1
+	c /= 0
+
+	s += "bar"
+	s += 1 /* ERROR "cannot convert.*string" */
+
+	var u64 uint64
+	u64 += 1<<u64
+
+	undeclared /* ERROR "undeclared" */ = 991
+
+	// test cases for issue 5800
+	var (
+		_ int = nil /* ERROR "untyped nil value" */
+		_ [10]int = nil /* ERROR "untyped nil value" */
+		_ []byte = nil
+		_ struct{} = nil /* ERROR "untyped nil value" */
+		_ func() = nil
+		_ map[int]string = nil
+		_ chan int = nil
+	)
+
+	// test cases for issue 5500
+	_ = func() (int, bool) {
+		var m map[int]int
+		return /* ERROR "wrong number of return values" */ m[0]
+	}
+
+	g := func(int, bool){}
+	var m map[int]int
+	g(m[0]) /* ERROR "too few arguments" */
+
+	// assignments to _
+	_ = nil /* ERROR "use of untyped nil" */
+	_ = 1 /* ERROR overflow */ <<1000
+	(_) = 0
+}
+
+func assignments2() {
+	type mybool bool
+	var m map[string][]bool
+	var s []bool
+	var b bool
+	var d mybool
+	_ = s
+	_ = b
+	_ = d
+
+	// assignments to map index expressions are ok
+	s, b = m["foo"]
+	_, d = m["bar"]
+	m["foo"] = nil
+	m["foo"] = nil /* ERROR cannot assign [1-9]+ values to [1-9]+ variables */ , false
+	_ = append(m["foo"])
+	_ = append(m["foo"], true)
+
+	var c chan int
+	_, b = <-c
+	_, d = <-c
+	<- /* ERROR cannot assign */ c = 0
+	<-c = 0 /* ERROR cannot assign [1-9]+ values to [1-9]+ variables */ , false
+
+	var x interface{}
+	_, b = x.(int)
+	x /* ERROR cannot assign */ .(int) = 0
+	x.(int) = 0 /* ERROR cannot assign [1-9]+ values to [1-9]+ variables */ , false
+
+	assignments2 /* ERROR used as value */ () = nil
+	int /* ERROR not an expression */ = 0
+}
+
+func issue6487() {
+	type S struct{x int}
+	_ = &S /* ERROR "cannot take address" */ {}.x
+	_ = &( /* ERROR "cannot take address" */ S{}.x)
+	_ = (&S{}).x
+	S /* ERROR "cannot assign" */ {}.x = 0
+	(&S{}).x = 0
+
+	type M map[string]S
+	var m M
+	m /* ERROR "cannot assign to struct field" */ ["foo"].x = 0
+	_ = &( /* ERROR "cannot take address" */ m["foo"].x)
+	_ = &m /* ERROR "cannot take address" */ ["foo"].x
+}
+
+func issue6766a() {
+	a, a /* ERROR redeclared */ := 1, 2
+	_ = a
+	a, b, b /* ERROR redeclared */ := 1, 2, 3
+	_ = b
+	c, c /* ERROR redeclared */, b := 1, 2, 3
+	_ = c
+	a, b := /* ERROR no new variables */ 1, 2
+}
+
+func shortVarDecls1() {
+	const c = 0
+	type d int
+	a, b, c /* ERROR "cannot assign" */ , d /* ERROR "cannot assign" */  := 1, "zwei", 3.0, 4
+	var _ int = a // a is of type int
+	var _ string = b // b is of type string
+}
+
+func incdecs() {
+	const c = 3.14
+	c /* ERROR "cannot assign" */ ++
+	s := "foo"
+	s /* ERROR "invalid operation" */ --
+	3.14 /* ERROR "cannot assign" */ ++
+	var (
+		x int
+		y float32
+		z complex128
+	)
+	x++
+	y--
+	z++
+}
+
+func sends() {
+	var ch chan int
+	var rch <-chan int
+	var x int
+	x <- /* ERROR "cannot send" */ x
+	rch <- /* ERROR "cannot send" */ x
+	ch <- "foo" /* ERROR "cannot use .* in send" */
+	ch <- x
+}
+
+func selects() {
+	select {}
+	var (
+		ch chan int
+		sc chan <- bool
+	)
+	select {
+	case <-ch:
+	case (<-ch):
+	case t := <-ch:
+		_ = t
+	case t := (<-ch):
+		_ = t
+	case t, ok := <-ch:
+		_, _ = t, ok
+	case t, ok := (<-ch):
+		_, _ = t, ok
+	case <-sc /* ERROR "cannot receive from send-only channel" */ :
+	}
+	select {
+	default:
+	default /* ERROR "multiple defaults" */ :
+	}
+	select {
+	case a, b := <-ch:
+		_, b = a, b
+	case x /* ERROR send or receive */ :
+	case a /* ERROR send or receive */ := ch:
+	}
+
+	// test for issue 9570: ch2 in second case falsely resolved to
+	// ch2 declared in body of first case
+	ch1 := make(chan int)
+	ch2 := make(chan int)
+	select {
+	case <-ch1:
+		var ch2 /* ERROR ch2 declared but not used */ chan bool
+	case i := <-ch2:
+		print(i + 1)
+	}
+}
+
+func gos() {
+	go 1 /* ERROR HERE "function must be invoked" */
+	go int /* ERROR "go requires function call, not conversion" */ (0)
+	go gos()
+	var c chan int
+	go close(c)
+	go len /* ERROR "go discards result" */ (c)
+}
+
+func defers() {
+	defer 1 /* ERROR HERE "function must be invoked" */
+	defer int /* ERROR "defer requires function call, not conversion" */ (0)
+	defer defers()
+	var c chan int
+	defer close(c)
+	defer len /* ERROR "defer discards result" */ (c)
+}
+
+func breaks() {
+	var x, y int
+
+	break /* ERROR "break" */
+	{
+		break /* ERROR "break" */
+	}
+	if x < y {
+		break /* ERROR "break" */
+	}
+
+	switch x {
+	case 0:
+		break
+	case 1:
+		if x == y {
+			break
+		}
+	default:
+		break
+		break
+	}
+
+	var z interface{}
+	switch z.(type) {
+	case int:
+		break
+	}
+
+	for {
+		break
+	}
+
+	var a []int
+	for _ = range a {
+		break
+	}
+
+	for {
+		if x == y {
+			break
+		}
+	}
+
+	var ch chan int
+	select {
+	case <-ch:
+		break
+	}
+
+	select {
+	case <-ch:
+		if x == y {
+			break
+		}
+	default:
+		break
+	}
+}
+
+func continues() {
+	var x, y int
+
+	continue /* ERROR "continue" */
+	{
+		continue /* ERROR "continue" */
+	}
+
+	if x < y {
+		continue /* ERROR "continue" */
+	}
+
+	switch x {
+	case 0:
+		continue /* ERROR "continue" */
+	}
+
+	var z interface{}
+	switch z.(type) {
+	case int:
+		continue /* ERROR "continue" */
+	}
+
+	var ch chan int
+	select {
+	case <-ch:
+		continue /* ERROR "continue" */
+	}
+
+	for i := 0; i < 10; i++ {
+		continue
+		if x < y {
+			continue
+			break
+		}
+		switch x {
+		case y:
+			continue
+		default:
+			break
+		}
+		select {
+		case <-ch:
+			continue
+		}
+	}
+
+	var a []int
+	for _ = range a {
+		continue
+		if x < y {
+			continue
+			break
+		}
+		switch x {
+		case y:
+			continue
+		default:
+			break
+		}
+		select {
+		case <-ch:
+			continue
+		}
+	}
+}
+
+func returns0() {
+	return
+	return 0 /* ERROR no result values expected */
+}
+
+func returns1(x float64) (int, *float64) {
+	return 0, &x
+	return /* ERROR wrong number of return values */
+	return "foo" /* ERROR "cannot .* in return statement" */, x /* ERROR "cannot use .* in return statement" */
+	return /* ERROR wrong number of return values */ 0, &x, 1
+}
+
+func returns2() (a, b int) {
+	return
+	return 1, "foo" /* ERROR cannot use .* in return statement */
+	return /* ERROR wrong number of return values */ 1, 2, 3
+	{
+		type a int
+		return 1, 2
+		return /* ERROR a not in scope at return */
+	}
+}
+
+func returns3() (_ int) {
+	return
+	{
+		var _ int // blank (_) identifiers never shadow since they are in no scope
+		return
+	}
+}
+
+func switches0() {
+	var x int
+
+	switch x {
+	}
+
+	switch x {
+	default:
+	default /* ERROR "multiple defaults" */ :
+	}
+
+	switch {
+	case 1  /* ERROR "cannot convert" */ :
+	}
+
+	true := "false"
+	_ = true
+	// A tagless switch is equivalent to the bool
+        // constant true, not the identifier 'true'.
+	switch {
+	case "false" /* ERROR "cannot convert" */:
+	}
+
+	switch int32(x) {
+	case 1, 2:
+	case x /* ERROR "cannot compare" */ :
+	}
+
+	switch x {
+	case 1 /* ERROR "overflows" */ << 100:
+	}
+
+	switch x {
+	case 1:
+	case 1 /* ERROR "duplicate case" */ :
+	case ( /* ERROR "duplicate case" */ 1):
+	case 2, 3, 4:
+	case 5, 1 /* ERROR "duplicate case" */ :
+	}
+
+	switch uint64(x) {
+	case 1<<64 - 1:
+	case 1 /* ERROR duplicate case */ <<64 - 1:
+	case 2, 3, 4:
+	case 5, 1 /* ERROR duplicate case */ <<64 - 1:
+	}
+
+	var y32 float32
+	switch y32 {
+	case 1.1:
+	case 11/10: // integer division!
+	case 11. /* ERROR duplicate case */ /10:
+	case 2, 3.0, 4.1:
+	case 5.2, 1.10 /* ERROR duplicate case */ :
+	}
+
+	var y64 float64
+	switch y64 {
+	case 1.1:
+	case 11/10: // integer division!
+	case 11. /* ERROR duplicate case */ /10:
+	case 2, 3.0, 4.1:
+	case 5.2, 1.10 /* ERROR duplicate case */ :
+	}
+
+	var s string
+	switch s {
+	case "foo":
+	case "foo" /* ERROR duplicate case */ :
+	case "f" /* ERROR duplicate case */ + "oo":
+	case "abc", "def", "ghi":
+	case "jkl", "foo" /* ERROR duplicate case */ :
+	}
+
+	type T int
+	type F float64
+	type S string
+	type B bool
+	var i interface{}
+	switch i {
+	case nil:
+	case nil: // no duplicate detection
+	case (*int)(nil):
+	case (*int)(nil): // do duplicate detection
+	case 1:
+	case byte(1):
+	case int /* ERROR duplicate case */ (1):
+	case T(1):
+	case 1.0:
+	case F(1.0):
+	case F /* ERROR duplicate case */ (1.0):
+	case "hello":
+	case S("hello"):
+	case S /* ERROR duplicate case */ ("hello"):
+	case 1==1, B(false):
+	case false, B(2==2):
+	}
+
+	// switch on array
+	var a [3]int
+	switch a {
+	case [3]int{1, 2, 3}:
+	case [3]int{1, 2, 3}: // no duplicate detection
+	case [ /* ERROR "mismatched types */ 4]int{4, 5, 6}:
+	}
+
+	// switch on channel
+	var c1, c2 chan int
+	switch c1 {
+	case nil:
+	case c1:
+	case c2:
+	case c1, c2: // no duplicate detection
+	}
+}
+
+func switches1() {
+	fallthrough /* ERROR "fallthrough statement out of place" */
+
+	var x int
+	switch x {
+	case 0:
+		fallthrough /* ERROR "fallthrough statement out of place" */
+		break
+	case 1:
+		fallthrough
+	case 2:
+		fallthrough; ; ; // trailing empty statements are ok
+	case 3:
+	default:
+		fallthrough; ;
+	case 4:
+		fallthrough /* ERROR "cannot fallthrough final case in switch" */
+	}
+
+	var y interface{}
+	switch y.(type) {
+	case int:
+		fallthrough /* ERROR "fallthrough statement out of place" */ ; ; ;
+	default:
+	}
+
+	switch x {
+	case 0:
+		if x == 0 {
+			fallthrough /* ERROR "fallthrough statement out of place" */
+		}
+	}
+
+	switch x {
+	case 0:
+		goto L1
+		L1: fallthrough; ;
+	case 1:
+		goto L2
+		goto L3
+		goto L4
+		L2: L3: L4: fallthrough
+	default:
+	}
+
+	switch x {
+	case 0:
+		goto L5
+		L5: fallthrough
+	default:
+		goto L6
+		goto L7
+		goto L8
+		L6: L7: L8: fallthrough /* ERROR "cannot fallthrough final case in switch" */
+	}
+
+	switch x {
+	case 0:
+		fallthrough; ;
+	case 1:
+		{
+			fallthrough /* ERROR "fallthrough statement out of place" */
+		}
+	case 2:
+		fallthrough
+	case 3:
+		fallthrough /* ERROR "fallthrough statement out of place" */
+		{ /* empty block is not an empty statement */ }; ;
+	default:
+		fallthrough /* ERROR "cannot fallthrough final case in switch" */
+	}
+
+	switch x {
+	case 0:
+		{
+			fallthrough /* ERROR "fallthrough statement out of place" */
+		}
+	}
+}
+
+func switches2() {
+	// untyped nil is not permitted as switch expression
+	switch nil /* ERROR "use of untyped nil" */ {
+	case 1, 2, "foo": // don't report additional errors here
+	}
+
+	// untyped constants are converted to default types
+	switch 1<<63-1 {
+	}
+	switch 1 /* ERROR "cannot use .* as int value.*\(overflows\)" */ << 63 {
+	}
+	var x int
+	switch 1.0 {
+	case 1.0, 2.0, x /* ERROR "mismatched types int and float64" */ :
+	}
+	switch x {
+	case 1.0:
+	}
+
+	// untyped bools become of type bool
+	type B bool
+	var b B = true
+	switch x == x {
+	case b /* ERROR "mismatched types B and bool" */ :
+	}
+	switch {
+	case b /* ERROR "mismatched types B and bool" */ :
+	}
+}
+
+func issue11667() {
+	switch 9223372036854775808 /* ERROR "cannot use .* as int value.*\(overflows\)" */ {
+	}
+	switch 9223372036854775808 /* ERROR "cannot use .* as int value.*\(overflows\)" */ {
+	case 9223372036854775808:
+	}
+	var x int
+	switch x {
+	case 9223372036854775808 /* ERROR "overflows int" */ :
+	}
+	var y float64
+	switch y {
+	case 9223372036854775808:
+	}
+}
+
+func issue11687() {
+	f := func() (_, _ int) { return }
+	switch f /* ERROR "2-valued f" */ () {
+	}
+	var x int
+	switch f /* ERROR "2-valued f" */ () {
+	case x:
+	}
+	switch x {
+	case f /* ERROR "2-valued f" */ ():
+	}
+}
+
+type I interface {
+	m()
+}
+
+type I2 interface {
+	m(int)
+}
+
+type T struct{}
+type T1 struct{}
+type T2 struct{}
+
+func (T) m() {}
+func (T2) m(int) {}
+
+func typeswitches() {
+	var i int
+	var x interface{}
+
+	switch x.(type) {}
+	switch (x /* ERROR "outside type switch" */ .(type)) {}
+
+	switch x.(type) {
+	default:
+	default /* ERROR "multiple defaults" */ :
+	}
+
+	switch x /* ERROR "declared but not used" */ := x.(type) {}
+	switch _ /* ERROR "no new variable on left side of :=" */ := x.(type) {}
+
+	switch x := x.(type) {
+	case int:
+		var y int = x
+		_ = y
+	}
+
+	switch x := i /* ERROR "not an interface" */ .(type) {}
+
+	switch t := x.(type) {
+	case nil:
+		var v bool = t /* ERROR "cannot use .* in variable declaration" */
+		_ = v
+	case int:
+		var v int = t
+		_ = v
+	case float32, complex64:
+		var v float32 = t /* ERROR "cannot use .* in variable declaration" */
+		_ = v
+	default:
+		var v float32 = t /* ERROR "cannot use .* in variable declaration" */
+		_ = v
+	}
+
+	var t I
+	switch t.(type) {
+	case T:
+	case T1 /* ERROR "missing method m" */ :
+	case T2 /* ERROR "wrong type for method m" */ :
+	case I2 /* STRICT "wrong type for method m" */ : // only an error in strict mode (issue 8561)
+	}
+}
+
+// Test that each case clause uses the correct type of the variable
+// declared by the type switch (issue 5504).
+func typeswitch0() {
+	switch y := interface{}(nil).(type) {
+	case int:
+		func() int { return y + 0 }()
+	case float32:
+		func() float32 { return y }()
+	}
+}
+
+// Test correct scope setup.
+// (no redeclaration errors expected in the type switch)
+func typeswitch1() {
+	var t I
+	switch t := t; t := t.(type) {
+	case nil:
+		var _ I = t
+	case T:
+		var _ T = t
+	default:
+		var _ I = t
+	}
+}
+
+// Test correct typeswitch against interface types.
+type A interface { a() }
+type B interface { b() }
+type C interface { a(int) }
+
+func typeswitch2() {
+	switch A(nil).(type) {
+	case A:
+	case B:
+	case C /* STRICT "cannot have dynamic type" */: // only an error in strict mode (issue 8561)
+	}
+}
+
+func typeswitch3(x interface{}) {
+	switch x.(type) {
+	case int:
+	case float64:
+	case int /* ERROR duplicate case */ :
+	}
+
+	switch x.(type) {
+	case nil:
+	case int:
+	case nil /* ERROR duplicate case */ , nil /* ERROR duplicate case */ :
+	}
+
+	type F func(int)
+	switch x.(type) {
+	case nil:
+	case int, func(int):
+	case float32, func /* ERROR duplicate case */ (x int):
+	case F:
+	}
+}
+
+func fors1() {
+	for {}
+	var i string
+	_ = i
+	for i := 0; i < 10; i++ {}
+	for i := 0; i < 10; j /* ERROR cannot declare */ := 0 {}
+}
+
+func rangeloops1() {
+	var (
+		x int
+		a [10]float32
+		b []string
+		p *[10]complex128
+		pp **[10]complex128
+		s string
+		m map[int]bool
+		c chan int
+		sc chan<- int
+		rc <-chan int
+	)
+
+	for range x /* ERROR "cannot range over" */ {}
+	for _ = range x /* ERROR "cannot range over" */ {}
+	for i := range x /* ERROR "cannot range over" */ {}
+
+	for range a {}
+	for i := range a {
+		var ii int
+		ii = i
+		_ = ii
+	}
+	for i, x := range a {
+		var ii int
+		ii = i
+		_ = ii
+		var xx float64
+		xx = x /* ERROR "cannot use .* in assignment" */
+		_ = xx
+	}
+	var ii int
+	var xx float32
+	for ii, xx = range a {}
+	_, _ = ii, xx
+
+	for range b {}
+	for i := range b {
+		var ii int
+		ii = i
+		_ = ii
+	}
+	for i, x := range b {
+		var ii int
+		ii = i
+		_ = ii
+		var xx string
+		xx = x
+		_ = xx
+	}
+
+	for range s {}
+	for i := range s {
+		var ii int
+		ii = i
+		_ = ii
+	}
+	for i, x := range s {
+		var ii int
+		ii = i
+		_ = ii
+		var xx rune
+		xx = x
+		_ = xx
+	}
+
+	for range p {}
+	for _, x := range p {
+		var xx complex128
+		xx = x
+		_ = xx
+	}
+
+	for range pp /* ERROR "cannot range over" */ {}
+	for _, x := range pp /* ERROR "cannot range over" */ {}
+
+	for range m {}
+	for k := range m {
+		var kk int32
+		kk = k /* ERROR "cannot use .* in assignment" */
+		_ = kk
+	}
+	for k, v := range m {
+		var kk int
+		kk = k
+		_ = kk
+		if v {}
+	}
+
+	for range c {}
+	for _, _ /* ERROR "only one iteration variable" */ = range c {}
+	for e := range c {
+		var ee int
+		ee = e
+		_ = ee
+	}
+	for _ = range sc /* ERROR "cannot range over send-only channel" */ {}
+	for _ = range rc {}
+
+	// constant strings
+	const cs = "foo"
+	for range cs {}
+	for range "" {}
+	for i, x := range cs { _, _ = i, x }
+	for i, x := range "" {
+		var ii int
+		ii = i
+		_ = ii
+		var xx rune
+		xx = x
+		_ = xx
+	}
+}
+
+func rangeloops2() {
+	type I int
+	type R rune
+
+	var a [10]int
+	var i I
+	_ = i
+	for i /* ERROR cannot use .* in assignment */ = range a {}
+	for i /* ERROR cannot use .* in assignment */ = range &a {}
+	for i /* ERROR cannot use .* in assignment */ = range a[:] {}
+
+	var s string
+	var r R
+	_ = r
+	for i /* ERROR cannot use .* in assignment */ = range s {}
+	for i /* ERROR cannot use .* in assignment */ = range "foo" {}
+	for _, r /* ERROR cannot use .* in assignment */ = range s {}
+	for _, r /* ERROR cannot use .* in assignment */ = range "foo" {}
+}
+
+func issue6766b() {
+	for _ := /* ERROR no new variables */ range "" {}
+	for a, a /* ERROR redeclared */ := range "" { _ = a }
+	var a int
+	_ = a
+	for a, a /* ERROR redeclared */ := range []int{1, 2, 3} { _ = a }
+}
+
+// Test that despite errors in the range clause,
+// the loop body is still type-checked (and thus
+// errors reported).
+func issue10148() {
+	for y /* ERROR declared but not used */ := range "" {
+		_ = "" /* ERROR cannot convert */ + 1
+	}
+	for range 1 /* ERROR cannot range over 1 */ {
+		_ = "" /* ERROR cannot convert */ + 1
+	}
+	for y := range 1 /* ERROR cannot range over 1 */ {
+		_ = "" /* ERROR cannot convert */ + 1
+	}
+}
+
+func labels0() {
+	goto L0
+	goto L1
+	L0:
+	L1:
+	L1 /* ERROR "already declared" */ :
+	if true {
+		goto L2
+		L2:
+		L0 /* ERROR "already declared" */ :
+	}
+	_ = func() {
+		goto L0
+		goto L1
+		goto L2
+		L0:
+		L1:
+		L2:
+	}
+}
+
+func expression_statements(ch chan int) {
+	expression_statements(ch)
+	<-ch
+	println()
+
+	0 /* ERROR "not used" */
+	1 /* ERROR "not used" */ +2
+	cap /* ERROR "not used" */ (ch)
+	println /* ERROR "must be called" */
+}
diff --git a/src/go/types/testdata/stmt1.src b/src/go/types/testdata/stmt1.src
new file mode 100644
index 0000000..f79f920
--- /dev/null
+++ b/src/go/types/testdata/stmt1.src
@@ -0,0 +1,259 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// terminating statements
+
+package stmt1
+
+func _() {}
+
+func _() int {} /* ERROR "missing return" */
+
+func _() int { panic(0) }
+func _() int { (panic(0)) }
+
+// block statements
+func _(x, y int) (z int) {
+	{
+		return
+	}
+}
+
+func _(x, y int) (z int) {
+	{
+		return; ; ; // trailing empty statements are ok
+	}
+	; ; ;
+}
+
+func _(x, y int) (z int) {
+	{
+	}
+} /* ERROR "missing return" */
+
+func _(x, y int) (z int) {
+	{
+		; ; ;
+	}
+	; ; ;
+} /* ERROR "missing return" */
+
+// if statements
+func _(x, y int) (z int) {
+	if x < y { return }
+	return 1
+}
+
+func _(x, y int) (z int) {
+	if x < y { return; ; ; ; }
+	return 1
+}
+
+func _(x, y int) (z int) {
+	if x < y { return }
+	return 1; ;
+}
+
+func _(x, y int) (z int) {
+	if x < y { return }
+} /* ERROR "missing return" */
+
+func _(x, y int) (z int) {
+	if x < y {
+	} else { return 1
+	}
+} /* ERROR "missing return" */
+
+func _(x, y int) (z int) {
+	if x < y { return
+	} else { return
+	}
+}
+
+// for statements
+func _(x, y int) (z int) {
+	for x < y {
+		return
+	}
+} /* ERROR "missing return" */
+
+func _(x, y int) (z int) {
+	for {
+		return
+	}
+}
+
+func _(x, y int) (z int) {
+	for {
+		return; ; ; ;
+	}
+}
+
+func _(x, y int) (z int) {
+	for {
+		return
+		break
+	}
+	; ; ;
+} /* ERROR "missing return" */
+
+func _(x, y int) (z int) {
+	for {
+		for { break }
+		return
+	}
+}
+
+func _(x, y int) (z int) {
+	for {
+		for { break }
+		return ; ;
+	}
+	;
+}
+
+func _(x, y int) (z int) {
+L:	for {
+		for { break L }
+		return
+	}
+} /* ERROR "missing return" */
+
+// switch statements
+func _(x, y int) (z int) {
+	switch x {
+	case 0: return
+	default: return
+	}
+}
+
+func _(x, y int) (z int) {
+	switch x {
+	case 0: return;
+	default: return; ; ;
+	}
+}
+
+func _(x, y int) (z int) {
+	switch x {
+	case 0: return
+	}
+} /* ERROR "missing return" */
+
+func _(x, y int) (z int) {
+	switch x {
+	case 0: return
+	case 1: break
+	}
+} /* ERROR "missing return" */
+
+func _(x, y int) (z int) {
+	switch x {
+	case 0: return
+	default:
+		switch y {
+		case 0: break
+		}
+		panic(0)
+	}
+}
+
+func _(x, y int) (z int) {
+	switch x {
+	case 0: return
+	default:
+		switch y {
+		case 0: break
+		}
+		panic(0); ; ;
+	}
+	;
+}
+
+func _(x, y int) (z int) {
+L:	switch x {
+	case 0: return
+	default:
+		switch y {
+		case 0: break L
+		}
+		panic(0)
+	}
+} /* ERROR "missing return" */
+
+// select statements
+func _(ch chan int) (z int) {
+	select {}
+} // nice!
+
+func _(ch chan int) (z int) {
+	select {}
+	; ;
+}
+
+func _(ch chan int) (z int) {
+	select {
+	default: break
+	}
+} /* ERROR "missing return" */
+
+func _(ch chan int) (z int) {
+	select {
+	case <-ch: return
+	default: break
+	}
+} /* ERROR "missing return" */
+
+func _(ch chan int) (z int) {
+	select {
+	case <-ch: return
+	default:
+		for i := 0; i < 10; i++ {
+			break
+		}
+		return
+	}
+}
+
+func _(ch chan int) (z int) {
+	select {
+	case <-ch: return; ; ;
+	default:
+		for i := 0; i < 10; i++ {
+			break
+		}
+		return; ; ;
+	}
+	; ; ;
+}
+
+func _(ch chan int) (z int) {
+L:	select {
+	case <-ch: return
+	default:
+		for i := 0; i < 10; i++ {
+			break L
+		}
+		return
+	}
+	; ; ;
+} /* ERROR "missing return" */
+
+func parenPanic() int {
+	((((((panic)))(0))))
+}
+
+func issue23218a() int {
+	{
+		panic := func(interface{}){}
+		panic(0)
+	}
+} /* ERROR "missing return" */
+
+func issue23218b() int {
+	{
+		panic := func(interface{}){}
+		((((panic))))(0)
+	}
+} /* ERROR "missing return" */
diff --git a/src/go/types/testdata/vardecl.src b/src/go/types/testdata/vardecl.src
new file mode 100644
index 0000000..54f5ef1
--- /dev/null
+++ b/src/go/types/testdata/vardecl.src
@@ -0,0 +1,206 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package vardecl
+
+// Prerequisites.
+import "math"
+func f() {}
+func g() (x, y int) { return }
+var m map[string]int
+
+// Var decls must have a type or an initializer.
+var _ int
+var _, _ int
+
+// The first error message is produced by the parser.
+// In a real-world scenario, the type-checker would not be run
+// in this case and the 2nd error message would not appear.
+var _ /* ERROR "missing variable type" */ /* ERROR "missing type or init expr" */
+var _ /* ERROR "missing variable type" */ /* ERROR "missing type or init expr" */, _
+var _ /* ERROR "missing variable type" */ /* ERROR "missing type or init expr" */, _, _
+
+// The initializer must be an expression.
+var _ = int /* ERROR "not an expression" */
+var _ = f /* ERROR "used as value" */ ()
+
+// Identifier and expression arity must match.
+var _, _ = 1, 2
+var _ = 1, 2 /* ERROR "extra init expr 2" */
+var _, _ = 1 /* ERROR "cannot initialize [0-9]+ variables with [0-9]+ values" */
+var _, _, _ /* ERROR "missing init expr for _" */ = 1, 2
+
+var _ = g /* ERROR "2-valued g" */ ()
+var _, _ = g()
+var _, _, _ = g /* ERROR "cannot initialize [0-9]+ variables with [0-9]+ values" */ ()
+
+var _ = m["foo"]
+var _, _ = m["foo"]
+var _, _, _ = m  /* ERROR "cannot initialize [0-9]+ variables with [0-9]+ values" */ ["foo"]
+
+var _, _ int = 1, 2
+var _ int = 1, 2 /* ERROR "extra init expr 2" */
+var _, _ int = 1 /* ERROR "cannot initialize [0-9]+ variables with [0-9]+ values" */
+var _, _, _ /* ERROR "missing init expr for _" */ int = 1, 2
+
+var (
+	_, _ = 1, 2
+	_ = 1, 2 /* ERROR "extra init expr 2" */
+	_, _ = 1 /* ERROR "cannot initialize [0-9]+ variables with [0-9]+ values" */
+	_, _, _ /* ERROR "missing init expr for _" */ = 1, 2
+
+	_ = g /* ERROR "2-valued g" */ ()
+	_, _ = g()
+	_, _, _ = g /* ERROR "cannot initialize [0-9]+ variables with [0-9]+ values" */ ()
+
+	_ = m["foo"]
+	_, _ = m["foo"]
+	_, _, _ = m /* ERROR "cannot initialize [0-9]+ variables with [0-9]+ values" */ ["foo"]
+
+	_, _ int = 1, 2
+	_ int = 1, 2 /* ERROR "extra init expr 2" */
+	_, _ int = 1 /* ERROR "cannot initialize [0-9]+ variables with [0-9]+ values" */
+	_, _, _ /* ERROR "missing init expr for _" */ int = 1, 2
+)
+
+// Variables declared in function bodies must be 'used'.
+type T struct{}
+func (r T) _(a, b, c int) (u, v, w int) {
+	var x1 /* ERROR "declared but not used" */ int
+	var x2 /* ERROR "declared but not used" */ int
+	x1 = 1
+	(x2) = 2
+
+	y1 /* ERROR "declared but not used" */ := 1
+	y2 /* ERROR "declared but not used" */ := 2
+	y1 = 1
+	(y1) = 2
+
+	{
+		var x1 /* ERROR "declared but not used" */ int
+		var x2 /* ERROR "declared but not used" */ int
+		x1 = 1
+		(x2) = 2
+
+		y1 /* ERROR "declared but not used" */ := 1
+		y2 /* ERROR "declared but not used" */ := 2
+		y1 = 1
+		(y1) = 2
+	}
+
+	if x /* ERROR "declared but not used" */ := 0; a < b {}
+
+	switch x /* ERROR "declared but not used" */, y := 0, 1; a {
+	case 0:
+		_ = y
+	case 1:
+		x /* ERROR "declared but not used" */ := 0
+	}
+
+	var t interface{}
+	switch t /* ERROR "declared but not used" */ := t.(type) {}
+
+	switch t /* ERROR "declared but not used" */ := t.(type) {
+	case int:
+	}
+
+	switch t /* ERROR "declared but not used" */ := t.(type) {
+	case int:
+	case float32, complex64:
+		t = nil
+	}
+
+	switch t := t.(type) {
+	case int:
+	case float32, complex64:
+		_ = t
+	}
+
+	switch t := t.(type) {
+	case int:
+	case float32:
+	case string:
+		_ = func() string {
+			return t
+		}
+	}
+
+	switch t := t; t /* ERROR "declared but not used" */ := t.(type) {}
+
+	var z1 /* ERROR "declared but not used" */ int
+	var z2 int
+	_ = func(a, b, c int) (u, v, w int) {
+		z1 = a
+		(z1) = b
+		a = z2
+		return
+	}
+
+	var s []int
+	var i /* ERROR "declared but not used" */ , j int
+	for i, j = range s {
+		_ = j
+	}
+
+	for i, j /* ERROR "declared but not used" */ := range s {
+		_ = func() int {
+			return i
+		}
+	}
+	return
+}
+
+// Unused variables in function literals must lead to only one error (issue #22524).
+func _() {
+	_ = func() {
+		var x /* ERROR declared but not used */ int
+	}
+}
+
+// Invalid (unused) expressions must not lead to spurious "declared but not used errors"
+func _() {
+	var a, b, c int
+	var x, y int
+	x, y = a /* ERROR cannot assign [0-9]+ values to [0-9]+ variables */ , b, c
+	_ = x
+	_ = y
+}
+
+func _() {
+	var x int
+	return x /* ERROR no result values expected */
+	return math /* ERROR no result values expected */ .Sin(0)
+}
+
+func _() int {
+	var x, y int
+	return /* ERROR wrong number of return values */ x, y
+}
+
+// Short variable declarations must declare at least one new non-blank variable.
+func _() {
+	_ := /* ERROR no new variables */ 0
+	_, a := 0, 1
+	_, a := /* ERROR no new variables */ 0, 1
+	_, a, b := 0, 1, 2
+	_, _, _ := /* ERROR no new variables */ 0, 1, 2
+
+	_ = a
+	_ = b
+}
+
+// Test case for variables depending on function literals (see also #22992).
+var A /* ERROR initialization cycle */ = func() int { return A }()
+
+func _() {
+	// The function literal below must not see a.
+	var a = func() int { return a /* ERROR "undeclared name" */ }()
+	var _ = func() int { return a }()
+
+	// The function literal below must not see x, y, or z.
+	var x, y, z = 0, 1, func() int { return x /* ERROR "undeclared name" */ + y /* ERROR "undeclared name" */ + z /* ERROR "undeclared name" */ }()
+	_, _, _ = x, y, z
+}
+
+// TODO(gri) consolidate other var decl checks in this file
\ No newline at end of file
diff --git a/src/go/types/token_test.go b/src/go/types/token_test.go
new file mode 100644
index 0000000..705bb29
--- /dev/null
+++ b/src/go/types/token_test.go
@@ -0,0 +1,47 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file checks invariants of token.Token ordering that we rely on
+// since package go/token doesn't provide any guarantees at the moment.
+
+package types
+
+import (
+	"go/token"
+	"testing"
+)
+
+var assignOps = map[token.Token]token.Token{
+	token.ADD_ASSIGN:     token.ADD,
+	token.SUB_ASSIGN:     token.SUB,
+	token.MUL_ASSIGN:     token.MUL,
+	token.QUO_ASSIGN:     token.QUO,
+	token.REM_ASSIGN:     token.REM,
+	token.AND_ASSIGN:     token.AND,
+	token.OR_ASSIGN:      token.OR,
+	token.XOR_ASSIGN:     token.XOR,
+	token.SHL_ASSIGN:     token.SHL,
+	token.SHR_ASSIGN:     token.SHR,
+	token.AND_NOT_ASSIGN: token.AND_NOT,
+}
+
+func TestZeroTok(t *testing.T) {
+	// zero value for token.Token must be token.ILLEGAL
+	var zero token.Token
+	if token.ILLEGAL != zero {
+		t.Errorf("%s == %d; want 0", token.ILLEGAL, zero)
+	}
+}
+
+func TestAssignOp(t *testing.T) {
+	// there are fewer than 256 tokens
+	for i := 0; i < 256; i++ {
+		tok := token.Token(i)
+		got := assignOp(tok)
+		want := assignOps[tok]
+		if got != want {
+			t.Errorf("for assignOp(%s): got %s; want %s", tok, got, want)
+		}
+	}
+}
diff --git a/src/go/types/type.go b/src/go/types/type.go
new file mode 100644
index 0000000..087cda4
--- /dev/null
+++ b/src/go/types/type.go
@@ -0,0 +1,523 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types
+
+import "sort"
+
+// A Type represents a type of Go.
+// All types implement the Type interface.
+type Type interface {
+	// Underlying returns the underlying type of a type.
+	Underlying() Type
+
+	// String returns a string representation of a type.
+	String() string
+}
+
+// BasicKind describes the kind of basic type.
+type BasicKind int
+
+const (
+	Invalid BasicKind = iota // type is invalid
+
+	// predeclared types
+	Bool
+	Int
+	Int8
+	Int16
+	Int32
+	Int64
+	Uint
+	Uint8
+	Uint16
+	Uint32
+	Uint64
+	Uintptr
+	Float32
+	Float64
+	Complex64
+	Complex128
+	String
+	UnsafePointer
+
+	// types for untyped values
+	UntypedBool
+	UntypedInt
+	UntypedRune
+	UntypedFloat
+	UntypedComplex
+	UntypedString
+	UntypedNil
+
+	// aliases
+	Byte = Uint8
+	Rune = Int32
+)
+
+// BasicInfo is a set of flags describing properties of a basic type.
+type BasicInfo int
+
+// Properties of basic types.
+const (
+	IsBoolean BasicInfo = 1 << iota
+	IsInteger
+	IsUnsigned
+	IsFloat
+	IsComplex
+	IsString
+	IsUntyped
+
+	IsOrdered   = IsInteger | IsFloat | IsString
+	IsNumeric   = IsInteger | IsFloat | IsComplex
+	IsConstType = IsBoolean | IsNumeric | IsString
+)
+
+// A Basic represents a basic type.
+type Basic struct {
+	kind BasicKind
+	info BasicInfo
+	name string
+}
+
+// Kind returns the kind of basic type b.
+func (b *Basic) Kind() BasicKind { return b.kind }
+
+// Info returns information about properties of basic type b.
+func (b *Basic) Info() BasicInfo { return b.info }
+
+// Name returns the name of basic type b.
+func (b *Basic) Name() string { return b.name }
+
+// An Array represents an array type.
+type Array struct {
+	len  int64
+	elem Type
+}
+
+// NewArray returns a new array type for the given element type and length.
+// A negative length indicates an unknown length.
+func NewArray(elem Type, len int64) *Array { return &Array{len, elem} }
+
+// Len returns the length of array a.
+// A negative result indicates an unknown length.
+func (a *Array) Len() int64 { return a.len }
+
+// Elem returns element type of array a.
+func (a *Array) Elem() Type { return a.elem }
+
+// A Slice represents a slice type.
+type Slice struct {
+	elem Type
+}
+
+// NewSlice returns a new slice type for the given element type.
+func NewSlice(elem Type) *Slice { return &Slice{elem} }
+
+// Elem returns the element type of slice s.
+func (s *Slice) Elem() Type { return s.elem }
+
+// A Struct represents a struct type.
+type Struct struct {
+	fields []*Var
+	tags   []string // field tags; nil if there are no tags
+}
+
+// NewStruct returns a new struct with the given fields and corresponding field tags.
+// If a field with index i has a tag, tags[i] must be that tag, but len(tags) may be
+// only as long as required to hold the tag with the largest index i. Consequently,
+// if no field has a tag, tags may be nil.
+func NewStruct(fields []*Var, tags []string) *Struct {
+	var fset objset
+	for _, f := range fields {
+		if f.name != "_" && fset.insert(f) != nil {
+			panic("multiple fields with the same name")
+		}
+	}
+	if len(tags) > len(fields) {
+		panic("more tags than fields")
+	}
+	return &Struct{fields: fields, tags: tags}
+}
+
+// NumFields returns the number of fields in the struct (including blank and embedded fields).
+func (s *Struct) NumFields() int { return len(s.fields) }
+
+// Field returns the i'th field for 0 <= i < NumFields().
+func (s *Struct) Field(i int) *Var { return s.fields[i] }
+
+// Tag returns the i'th field tag for 0 <= i < NumFields().
+func (s *Struct) Tag(i int) string {
+	if i < len(s.tags) {
+		return s.tags[i]
+	}
+	return ""
+}
+
+// A Pointer represents a pointer type.
+type Pointer struct {
+	base Type // element type
+}
+
+// NewPointer returns a new pointer type for the given element (base) type.
+func NewPointer(elem Type) *Pointer { return &Pointer{base: elem} }
+
+// Elem returns the element type for the given pointer p.
+func (p *Pointer) Elem() Type { return p.base }
+
+// A Tuple represents an ordered list of variables; a nil *Tuple is a valid (empty) tuple.
+// Tuples are used as components of signatures and to represent the type of multiple
+// assignments; they are not first class types of Go.
+type Tuple struct {
+	vars []*Var
+}
+
+// NewTuple returns a new tuple for the given variables.
+func NewTuple(x ...*Var) *Tuple {
+	if len(x) > 0 {
+		return &Tuple{x}
+	}
+	return nil
+}
+
+// Len returns the number variables of tuple t.
+func (t *Tuple) Len() int {
+	if t != nil {
+		return len(t.vars)
+	}
+	return 0
+}
+
+// At returns the i'th variable of tuple t.
+func (t *Tuple) At(i int) *Var { return t.vars[i] }
+
+// A Signature represents a (non-builtin) function or method type.
+// The receiver is ignored when comparing signatures for identity.
+type Signature struct {
+	// We need to keep the scope in Signature (rather than passing it around
+	// and store it in the Func Object) because when type-checking a function
+	// literal we call the general type checker which returns a general Type.
+	// We then unpack the *Signature and use the scope for the literal body.
+	scope    *Scope // function scope, present for package-local signatures
+	recv     *Var   // nil if not a method
+	params   *Tuple // (incoming) parameters from left to right; or nil
+	results  *Tuple // (outgoing) results from left to right; or nil
+	variadic bool   // true if the last parameter's type is of the form ...T (or string, for append built-in only)
+}
+
+// NewSignature returns a new function type for the given receiver, parameters,
+// and results, either of which may be nil. If variadic is set, the function
+// is variadic, it must have at least one parameter, and the last parameter
+// must be of unnamed slice type.
+func NewSignature(recv *Var, params, results *Tuple, variadic bool) *Signature {
+	if variadic {
+		n := params.Len()
+		if n == 0 {
+			panic("types.NewSignature: variadic function must have at least one parameter")
+		}
+		if _, ok := params.At(n - 1).typ.(*Slice); !ok {
+			panic("types.NewSignature: variadic parameter must be of unnamed slice type")
+		}
+	}
+	return &Signature{nil, recv, params, results, variadic}
+}
+
+// Recv returns the receiver of signature s (if a method), or nil if a
+// function. It is ignored when comparing signatures for identity.
+//
+// For an abstract method, Recv returns the enclosing interface either
+// as a *Named or an *Interface. Due to embedding, an interface may
+// contain methods whose receiver type is a different interface.
+func (s *Signature) Recv() *Var { return s.recv }
+
+// Params returns the parameters of signature s, or nil.
+func (s *Signature) Params() *Tuple { return s.params }
+
+// Results returns the results of signature s, or nil.
+func (s *Signature) Results() *Tuple { return s.results }
+
+// Variadic reports whether the signature s is variadic.
+func (s *Signature) Variadic() bool { return s.variadic }
+
+// An Interface represents an interface type.
+type Interface struct {
+	methods   []*Func // ordered list of explicitly declared methods
+	embeddeds []Type  // ordered list of explicitly embedded types
+
+	allMethods []*Func // ordered list of methods declared with or embedded in this interface (TODO(gri): replace with mset)
+}
+
+// emptyInterface represents the empty (completed) interface
+var emptyInterface = Interface{allMethods: markComplete}
+
+// markComplete is used to mark an empty interface as completely
+// set up by setting the allMethods field to a non-nil empty slice.
+var markComplete = make([]*Func, 0)
+
+// NewInterface returns a new (incomplete) interface for the given methods and embedded types.
+// Each embedded type must have an underlying type of interface type.
+// NewInterface takes ownership of the provided methods and may modify their types by setting
+// missing receivers. To compute the method set of the interface, Complete must be called.
+//
+// Deprecated: Use NewInterfaceType instead which allows any (even non-defined) interface types
+// to be embedded. This is necessary for interfaces that embed alias type names referring to
+// non-defined (literal) interface types.
+func NewInterface(methods []*Func, embeddeds []*Named) *Interface {
+	tnames := make([]Type, len(embeddeds))
+	for i, t := range embeddeds {
+		tnames[i] = t
+	}
+	return NewInterfaceType(methods, tnames)
+}
+
+// NewInterfaceType returns a new (incomplete) interface for the given methods and embedded types.
+// Each embedded type must have an underlying type of interface type (this property is not
+// verified for defined types, which may be in the process of being set up and which don't
+// have a valid underlying type yet).
+// NewInterfaceType takes ownership of the provided methods and may modify their types by setting
+// missing receivers. To compute the method set of the interface, Complete must be called.
+func NewInterfaceType(methods []*Func, embeddeds []Type) *Interface {
+	if len(methods) == 0 && len(embeddeds) == 0 {
+		return &emptyInterface
+	}
+
+	// set method receivers if necessary
+	typ := new(Interface)
+	for _, m := range methods {
+		if sig := m.typ.(*Signature); sig.recv == nil {
+			sig.recv = NewVar(m.pos, m.pkg, "", typ)
+		}
+	}
+
+	// All embedded types should be interfaces; however, defined types
+	// may not yet be fully resolved. Only verify that non-defined types
+	// are interfaces. This matches the behavior of the code before the
+	// fix for #25301 (issue #25596).
+	for _, t := range embeddeds {
+		if _, ok := t.(*Named); !ok && !IsInterface(t) {
+			panic("embedded type is not an interface")
+		}
+	}
+
+	// sort for API stability
+	sort.Sort(byUniqueMethodName(methods))
+	sort.Stable(byUniqueTypeName(embeddeds))
+
+	typ.methods = methods
+	typ.embeddeds = embeddeds
+	return typ
+}
+
+// NumExplicitMethods returns the number of explicitly declared methods of interface t.
+func (t *Interface) NumExplicitMethods() int { return len(t.methods) }
+
+// ExplicitMethod returns the i'th explicitly declared method of interface t for 0 <= i < t.NumExplicitMethods().
+// The methods are ordered by their unique Id.
+func (t *Interface) ExplicitMethod(i int) *Func { return t.methods[i] }
+
+// NumEmbeddeds returns the number of embedded types in interface t.
+func (t *Interface) NumEmbeddeds() int { return len(t.embeddeds) }
+
+// Embedded returns the i'th embedded defined (*Named) type of interface t for 0 <= i < t.NumEmbeddeds().
+// The result is nil if the i'th embedded type is not a defined type.
+//
+// Deprecated: Use EmbeddedType which is not restricted to defined (*Named) types.
+func (t *Interface) Embedded(i int) *Named { tname, _ := t.embeddeds[i].(*Named); return tname }
+
+// EmbeddedType returns the i'th embedded type of interface t for 0 <= i < t.NumEmbeddeds().
+func (t *Interface) EmbeddedType(i int) Type { return t.embeddeds[i] }
+
+// NumMethods returns the total number of methods of interface t.
+// The interface must have been completed.
+func (t *Interface) NumMethods() int { t.assertCompleteness(); return len(t.allMethods) }
+
+func (t *Interface) assertCompleteness() {
+	if t.allMethods == nil {
+		panic("interface is incomplete")
+	}
+}
+
+// Method returns the i'th method of interface t for 0 <= i < t.NumMethods().
+// The methods are ordered by their unique Id.
+// The interface must have been completed.
+func (t *Interface) Method(i int) *Func { t.assertCompleteness(); return t.allMethods[i] }
+
+// Empty reports whether t is the empty interface.
+// The interface must have been completed.
+func (t *Interface) Empty() bool { t.assertCompleteness(); return len(t.allMethods) == 0 }
+
+// Complete computes the interface's method set. It must be called by users of
+// NewInterfaceType and NewInterface after the interface's embedded types are
+// fully defined and before using the interface type in any way other than to
+// form other types. The interface must not contain duplicate methods or a
+// panic occurs. Complete returns the receiver.
+func (t *Interface) Complete() *Interface {
+	// TODO(gri) consolidate this method with Checker.completeInterface
+	if t.allMethods != nil {
+		return t
+	}
+
+	t.allMethods = markComplete // avoid infinite recursion
+
+	var todo []*Func
+	var methods []*Func
+	var seen objset
+	addMethod := func(m *Func, explicit bool) {
+		switch other := seen.insert(m); {
+		case other == nil:
+			methods = append(methods, m)
+		case explicit:
+			panic("duplicate method " + m.name)
+		default:
+			// check method signatures after all locally embedded interfaces are computed
+			todo = append(todo, m, other.(*Func))
+		}
+	}
+
+	for _, m := range t.methods {
+		addMethod(m, true)
+	}
+
+	for _, typ := range t.embeddeds {
+		typ := typ.Underlying().(*Interface)
+		typ.Complete()
+		for _, m := range typ.allMethods {
+			addMethod(m, false)
+		}
+	}
+
+	for i := 0; i < len(todo); i += 2 {
+		m := todo[i]
+		other := todo[i+1]
+		if !Identical(m.typ, other.typ) {
+			panic("duplicate method " + m.name)
+		}
+	}
+
+	if methods != nil {
+		sort.Sort(byUniqueMethodName(methods))
+		t.allMethods = methods
+	}
+
+	return t
+}
+
+// A Map represents a map type.
+type Map struct {
+	key, elem Type
+}
+
+// NewMap returns a new map for the given key and element types.
+func NewMap(key, elem Type) *Map {
+	return &Map{key, elem}
+}
+
+// Key returns the key type of map m.
+func (m *Map) Key() Type { return m.key }
+
+// Elem returns the element type of map m.
+func (m *Map) Elem() Type { return m.elem }
+
+// A Chan represents a channel type.
+type Chan struct {
+	dir  ChanDir
+	elem Type
+}
+
+// A ChanDir value indicates a channel direction.
+type ChanDir int
+
+// The direction of a channel is indicated by one of these constants.
+const (
+	SendRecv ChanDir = iota
+	SendOnly
+	RecvOnly
+)
+
+// NewChan returns a new channel type for the given direction and element type.
+func NewChan(dir ChanDir, elem Type) *Chan {
+	return &Chan{dir, elem}
+}
+
+// Dir returns the direction of channel c.
+func (c *Chan) Dir() ChanDir { return c.dir }
+
+// Elem returns the element type of channel c.
+func (c *Chan) Elem() Type { return c.elem }
+
+// A Named represents a named type.
+type Named struct {
+	info       typeInfo  // for cycle detection
+	obj        *TypeName // corresponding declared object
+	orig       Type      // type (on RHS of declaration) this *Named type is derived of (for cycle reporting)
+	underlying Type      // possibly a *Named during setup; never a *Named once set up completely
+	methods    []*Func   // methods declared for this type (not the method set of this type); signatures are type-checked lazily
+}
+
+// NewNamed returns a new named type for the given type name, underlying type, and associated methods.
+// If the given type name obj doesn't have a type yet, its type is set to the returned named type.
+// The underlying type must not be a *Named.
+func NewNamed(obj *TypeName, underlying Type, methods []*Func) *Named {
+	if _, ok := underlying.(*Named); ok {
+		panic("types.NewNamed: underlying type must not be *Named")
+	}
+	typ := &Named{obj: obj, orig: underlying, underlying: underlying, methods: methods}
+	if obj.typ == nil {
+		obj.typ = typ
+	}
+	return typ
+}
+
+// Obj returns the type name for the named type t.
+func (t *Named) Obj() *TypeName { return t.obj }
+
+// NumMethods returns the number of explicit methods whose receiver is named type t.
+func (t *Named) NumMethods() int { return len(t.methods) }
+
+// Method returns the i'th method of named type t for 0 <= i < t.NumMethods().
+func (t *Named) Method(i int) *Func { return t.methods[i] }
+
+// SetUnderlying sets the underlying type and marks t as complete.
+func (t *Named) SetUnderlying(underlying Type) {
+	if underlying == nil {
+		panic("types.Named.SetUnderlying: underlying type must not be nil")
+	}
+	if _, ok := underlying.(*Named); ok {
+		panic("types.Named.SetUnderlying: underlying type must not be *Named")
+	}
+	t.underlying = underlying
+}
+
+// AddMethod adds method m unless it is already in the method list.
+func (t *Named) AddMethod(m *Func) {
+	if i, _ := lookupMethod(t.methods, m.pkg, m.name); i < 0 {
+		t.methods = append(t.methods, m)
+	}
+}
+
+// Implementations for Type methods.
+
+func (b *Basic) Underlying() Type     { return b }
+func (a *Array) Underlying() Type     { return a }
+func (s *Slice) Underlying() Type     { return s }
+func (s *Struct) Underlying() Type    { return s }
+func (p *Pointer) Underlying() Type   { return p }
+func (t *Tuple) Underlying() Type     { return t }
+func (s *Signature) Underlying() Type { return s }
+func (t *Interface) Underlying() Type { return t }
+func (m *Map) Underlying() Type       { return m }
+func (c *Chan) Underlying() Type      { return c }
+func (t *Named) Underlying() Type     { return t.underlying }
+
+func (b *Basic) String() string     { return TypeString(b, nil) }
+func (a *Array) String() string     { return TypeString(a, nil) }
+func (s *Slice) String() string     { return TypeString(s, nil) }
+func (s *Struct) String() string    { return TypeString(s, nil) }
+func (p *Pointer) String() string   { return TypeString(p, nil) }
+func (t *Tuple) String() string     { return TypeString(t, nil) }
+func (s *Signature) String() string { return TypeString(s, nil) }
+func (t *Interface) String() string { return TypeString(t, nil) }
+func (m *Map) String() string       { return TypeString(m, nil) }
+func (c *Chan) String() string      { return TypeString(c, nil) }
+func (t *Named) String() string     { return TypeString(t, nil) }
diff --git a/src/go/types/typestring.go b/src/go/types/typestring.go
new file mode 100644
index 0000000..31c572f
--- /dev/null
+++ b/src/go/types/typestring.go
@@ -0,0 +1,307 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements printing of types.
+
+package types
+
+import (
+	"bytes"
+	"fmt"
+)
+
+// A Qualifier controls how named package-level objects are printed in
+// calls to TypeString, ObjectString, and SelectionString.
+//
+// These three formatting routines call the Qualifier for each
+// package-level object O, and if the Qualifier returns a non-empty
+// string p, the object is printed in the form p.O.
+// If it returns an empty string, only the object name O is printed.
+//
+// Using a nil Qualifier is equivalent to using (*Package).Path: the
+// object is qualified by the import path, e.g., "encoding/json.Marshal".
+//
+type Qualifier func(*Package) string
+
+// RelativeTo returns a Qualifier that fully qualifies members of
+// all packages other than pkg.
+func RelativeTo(pkg *Package) Qualifier {
+	if pkg == nil {
+		return nil
+	}
+	return func(other *Package) string {
+		if pkg == other {
+			return "" // same package; unqualified
+		}
+		return other.Path()
+	}
+}
+
+// If gcCompatibilityMode is set, printing of types is modified
+// to match the representation of some types in the gc compiler:
+//
+//	- byte and rune lose their alias name and simply stand for
+//	  uint8 and int32 respectively
+//	- embedded interfaces get flattened (the embedding info is lost,
+//	  and certain recursive interface types cannot be printed anymore)
+//
+// This makes it easier to compare packages computed with the type-
+// checker vs packages imported from gc export data.
+//
+// Caution: This flag affects all uses of WriteType, globally.
+// It is only provided for testing in conjunction with
+// gc-generated data.
+//
+// This flag is exported in the x/tools/go/types package. We don't
+// need it at the moment in the std repo and so we don't export it
+// anymore. We should eventually try to remove it altogether.
+// TODO(gri) remove this
+var gcCompatibilityMode bool
+
+// TypeString returns the string representation of typ.
+// The Qualifier controls the printing of
+// package-level objects, and may be nil.
+func TypeString(typ Type, qf Qualifier) string {
+	var buf bytes.Buffer
+	WriteType(&buf, typ, qf)
+	return buf.String()
+}
+
+// WriteType writes the string representation of typ to buf.
+// The Qualifier controls the printing of
+// package-level objects, and may be nil.
+func WriteType(buf *bytes.Buffer, typ Type, qf Qualifier) {
+	writeType(buf, typ, qf, make([]Type, 0, 8))
+}
+
+func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
+	// Theoretically, this is a quadratic lookup algorithm, but in
+	// practice deeply nested composite types with unnamed component
+	// types are uncommon. This code is likely more efficient than
+	// using a map.
+	for _, t := range visited {
+		if t == typ {
+			fmt.Fprintf(buf, "○%T", typ) // cycle to typ
+			return
+		}
+	}
+	visited = append(visited, typ)
+
+	switch t := typ.(type) {
+	case nil:
+		buf.WriteString("<nil>")
+
+	case *Basic:
+		if t.kind == UnsafePointer {
+			buf.WriteString("unsafe.")
+		}
+		if gcCompatibilityMode {
+			// forget the alias names
+			switch t.kind {
+			case Byte:
+				t = Typ[Uint8]
+			case Rune:
+				t = Typ[Int32]
+			}
+		}
+		buf.WriteString(t.name)
+
+	case *Array:
+		fmt.Fprintf(buf, "[%d]", t.len)
+		writeType(buf, t.elem, qf, visited)
+
+	case *Slice:
+		buf.WriteString("[]")
+		writeType(buf, t.elem, qf, visited)
+
+	case *Struct:
+		buf.WriteString("struct{")
+		for i, f := range t.fields {
+			if i > 0 {
+				buf.WriteString("; ")
+			}
+			if !f.embedded {
+				buf.WriteString(f.name)
+				buf.WriteByte(' ')
+			}
+			writeType(buf, f.typ, qf, visited)
+			if tag := t.Tag(i); tag != "" {
+				fmt.Fprintf(buf, " %q", tag)
+			}
+		}
+		buf.WriteByte('}')
+
+	case *Pointer:
+		buf.WriteByte('*')
+		writeType(buf, t.base, qf, visited)
+
+	case *Tuple:
+		writeTuple(buf, t, false, qf, visited)
+
+	case *Signature:
+		buf.WriteString("func")
+		writeSignature(buf, t, qf, visited)
+
+	case *Interface:
+		// We write the source-level methods and embedded types rather
+		// than the actual method set since resolved method signatures
+		// may have non-printable cycles if parameters have embedded
+		// interface types that (directly or indirectly) embed the
+		// current interface. For instance, consider the result type
+		// of m:
+		//
+		//     type T interface{
+		//         m() interface{ T }
+		//     }
+		//
+		buf.WriteString("interface{")
+		empty := true
+		if gcCompatibilityMode {
+			// print flattened interface
+			// (useful to compare against gc-generated interfaces)
+			for i, m := range t.allMethods {
+				if i > 0 {
+					buf.WriteString("; ")
+				}
+				buf.WriteString(m.name)
+				writeSignature(buf, m.typ.(*Signature), qf, visited)
+				empty = false
+			}
+		} else {
+			// print explicit interface methods and embedded types
+			for i, m := range t.methods {
+				if i > 0 {
+					buf.WriteString("; ")
+				}
+				buf.WriteString(m.name)
+				writeSignature(buf, m.typ.(*Signature), qf, visited)
+				empty = false
+			}
+			for i, typ := range t.embeddeds {
+				if i > 0 || len(t.methods) > 0 {
+					buf.WriteString("; ")
+				}
+				writeType(buf, typ, qf, visited)
+				empty = false
+			}
+		}
+		if t.allMethods == nil || len(t.methods) > len(t.allMethods) {
+			if !empty {
+				buf.WriteByte(' ')
+			}
+			buf.WriteString("/* incomplete */")
+		}
+		buf.WriteByte('}')
+
+	case *Map:
+		buf.WriteString("map[")
+		writeType(buf, t.key, qf, visited)
+		buf.WriteByte(']')
+		writeType(buf, t.elem, qf, visited)
+
+	case *Chan:
+		var s string
+		var parens bool
+		switch t.dir {
+		case SendRecv:
+			s = "chan "
+			// chan (<-chan T) requires parentheses
+			if c, _ := t.elem.(*Chan); c != nil && c.dir == RecvOnly {
+				parens = true
+			}
+		case SendOnly:
+			s = "chan<- "
+		case RecvOnly:
+			s = "<-chan "
+		default:
+			panic("unreachable")
+		}
+		buf.WriteString(s)
+		if parens {
+			buf.WriteByte('(')
+		}
+		writeType(buf, t.elem, qf, visited)
+		if parens {
+			buf.WriteByte(')')
+		}
+
+	case *Named:
+		s := "<Named w/o object>"
+		if obj := t.obj; obj != nil {
+			if obj.pkg != nil {
+				writePackage(buf, obj.pkg, qf)
+			}
+			// TODO(gri): function-local named types should be displayed
+			// differently from named types at package level to avoid
+			// ambiguity.
+			s = obj.name
+		}
+		buf.WriteString(s)
+
+	default:
+		// For externally defined implementations of Type.
+		buf.WriteString(t.String())
+	}
+}
+
+func writeTuple(buf *bytes.Buffer, tup *Tuple, variadic bool, qf Qualifier, visited []Type) {
+	buf.WriteByte('(')
+	if tup != nil {
+		for i, v := range tup.vars {
+			if i > 0 {
+				buf.WriteString(", ")
+			}
+			if v.name != "" {
+				buf.WriteString(v.name)
+				buf.WriteByte(' ')
+			}
+			typ := v.typ
+			if variadic && i == len(tup.vars)-1 {
+				if s, ok := typ.(*Slice); ok {
+					buf.WriteString("...")
+					typ = s.elem
+				} else {
+					// special case:
+					// append(s, "foo"...) leads to signature func([]byte, string...)
+					if t, ok := typ.Underlying().(*Basic); !ok || t.kind != String {
+						panic("internal error: string type expected")
+					}
+					writeType(buf, typ, qf, visited)
+					buf.WriteString("...")
+					continue
+				}
+			}
+			writeType(buf, typ, qf, visited)
+		}
+	}
+	buf.WriteByte(')')
+}
+
+// WriteSignature writes the representation of the signature sig to buf,
+// without a leading "func" keyword.
+// The Qualifier controls the printing of
+// package-level objects, and may be nil.
+func WriteSignature(buf *bytes.Buffer, sig *Signature, qf Qualifier) {
+	writeSignature(buf, sig, qf, make([]Type, 0, 8))
+}
+
+func writeSignature(buf *bytes.Buffer, sig *Signature, qf Qualifier, visited []Type) {
+	writeTuple(buf, sig.params, sig.variadic, qf, visited)
+
+	n := sig.results.Len()
+	if n == 0 {
+		// no result
+		return
+	}
+
+	buf.WriteByte(' ')
+	if n == 1 && sig.results.vars[0].name == "" {
+		// single unnamed result
+		writeType(buf, sig.results.vars[0].typ, qf, visited)
+		return
+	}
+
+	// multiple or named result(s)
+	writeTuple(buf, sig.results, false, qf, visited)
+}
diff --git a/src/go/types/typestring_test.go b/src/go/types/typestring_test.go
new file mode 100644
index 0000000..5d9db39
--- /dev/null
+++ b/src/go/types/typestring_test.go
@@ -0,0 +1,221 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package types_test
+
+import (
+	"go/ast"
+	"go/importer"
+	"go/parser"
+	"go/token"
+	"internal/testenv"
+	"testing"
+
+	. "go/types"
+)
+
+const filename = "<src>"
+
+func makePkg(src string) (*Package, error) {
+	fset := token.NewFileSet()
+	file, err := parser.ParseFile(fset, filename, src, parser.DeclarationErrors)
+	if err != nil {
+		return nil, err
+	}
+	// use the package name as package path
+	conf := Config{Importer: importer.Default()}
+	return conf.Check(file.Name.Name, fset, []*ast.File{file}, nil)
+}
+
+type testEntry struct {
+	src, str string
+}
+
+// dup returns a testEntry where both src and str are the same.
+func dup(s string) testEntry {
+	return testEntry{s, s}
+}
+
+// types that don't depend on any other type declarations
+var independentTestTypes = []testEntry{
+	// basic types
+	dup("int"),
+	dup("float32"),
+	dup("string"),
+
+	// arrays
+	dup("[10]int"),
+
+	// slices
+	dup("[]int"),
+	dup("[][]int"),
+
+	// structs
+	dup("struct{}"),
+	dup("struct{x int}"),
+	{`struct {
+		x, y int
+		z float32 "foo"
+	}`, `struct{x int; y int; z float32 "foo"}`},
+	{`struct {
+		string
+		elems []complex128
+	}`, `struct{string; elems []complex128}`},
+
+	// pointers
+	dup("*int"),
+	dup("***struct{}"),
+	dup("*struct{a int; b float32}"),
+
+	// functions
+	dup("func()"),
+	dup("func(x int)"),
+	{"func(x, y int)", "func(x int, y int)"},
+	{"func(x, y int, z string)", "func(x int, y int, z string)"},
+	dup("func(int)"),
+	{"func(int, string, byte)", "func(int, string, byte)"},
+
+	dup("func() int"),
+	{"func() (string)", "func() string"},
+	dup("func() (u int)"),
+	{"func() (u, v int, w string)", "func() (u int, v int, w string)"},
+
+	dup("func(int) string"),
+	dup("func(x int) string"),
+	dup("func(x int) (u string)"),
+	{"func(x, y int) (u string)", "func(x int, y int) (u string)"},
+
+	dup("func(...int) string"),
+	dup("func(x ...int) string"),
+	dup("func(x ...int) (u string)"),
+	{"func(x int, y ...int) (u string)", "func(x int, y ...int) (u string)"},
+
+	// interfaces
+	dup("interface{}"),
+	dup("interface{m()}"),
+	dup(`interface{String() string; m(int) float32}`),
+
+	// maps
+	dup("map[string]int"),
+	{"map[struct{x, y int}][]byte", "map[struct{x int; y int}][]byte"},
+
+	// channels
+	dup("chan<- chan int"),
+	dup("chan<- <-chan int"),
+	dup("<-chan <-chan int"),
+	dup("chan (<-chan int)"),
+	dup("chan<- func()"),
+	dup("<-chan []func() int"),
+}
+
+// types that depend on other type declarations (src in TestTypes)
+var dependentTestTypes = []testEntry{
+	// interfaces
+	dup(`interface{io.Reader; io.Writer}`),
+	dup(`interface{m() int; io.Writer}`),
+	{`interface{m() interface{T}}`, `interface{m() interface{p.T}}`},
+}
+
+func TestTypeString(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	var tests []testEntry
+	tests = append(tests, independentTestTypes...)
+	tests = append(tests, dependentTestTypes...)
+
+	for _, test := range tests {
+		src := `package p; import "io"; type _ io.Writer; type T ` + test.src
+		pkg, err := makePkg(src)
+		if err != nil {
+			t.Errorf("%s: %s", src, err)
+			continue
+		}
+		typ := pkg.Scope().Lookup("T").Type().Underlying()
+		if got := typ.String(); got != test.str {
+			t.Errorf("%s: got %s, want %s", test.src, got, test.str)
+		}
+	}
+}
+
+func TestIncompleteInterfaces(t *testing.T) {
+	sig := NewSignature(nil, nil, nil, false)
+	m := NewFunc(token.NoPos, nil, "m", sig)
+	for _, test := range []struct {
+		typ  *Interface
+		want string
+	}{
+		{new(Interface), "interface{/* incomplete */}"},
+		{new(Interface).Complete(), "interface{}"},
+
+		{NewInterface(nil, nil), "interface{}"},
+		{NewInterface(nil, nil).Complete(), "interface{}"},
+		{NewInterface([]*Func{}, nil), "interface{}"},
+		{NewInterface([]*Func{}, nil).Complete(), "interface{}"},
+		{NewInterface(nil, []*Named{}), "interface{}"},
+		{NewInterface(nil, []*Named{}).Complete(), "interface{}"},
+		{NewInterface([]*Func{m}, nil), "interface{m() /* incomplete */}"},
+		{NewInterface([]*Func{m}, nil).Complete(), "interface{m()}"},
+		{NewInterface(nil, []*Named{newDefined(new(Interface).Complete())}), "interface{T /* incomplete */}"},
+		{NewInterface(nil, []*Named{newDefined(new(Interface).Complete())}).Complete(), "interface{T}"},
+		{NewInterface(nil, []*Named{newDefined(NewInterface([]*Func{m}, nil))}), "interface{T /* incomplete */}"},
+		{NewInterface(nil, []*Named{newDefined(NewInterface([]*Func{m}, nil).Complete())}), "interface{T /* incomplete */}"},
+		{NewInterface(nil, []*Named{newDefined(NewInterface([]*Func{m}, nil).Complete())}).Complete(), "interface{T}"},
+
+		{NewInterfaceType(nil, nil), "interface{}"},
+		{NewInterfaceType(nil, nil).Complete(), "interface{}"},
+		{NewInterfaceType([]*Func{}, nil), "interface{}"},
+		{NewInterfaceType([]*Func{}, nil).Complete(), "interface{}"},
+		{NewInterfaceType(nil, []Type{}), "interface{}"},
+		{NewInterfaceType(nil, []Type{}).Complete(), "interface{}"},
+		{NewInterfaceType([]*Func{m}, nil), "interface{m() /* incomplete */}"},
+		{NewInterfaceType([]*Func{m}, nil).Complete(), "interface{m()}"},
+		{NewInterfaceType(nil, []Type{new(Interface).Complete()}), "interface{interface{} /* incomplete */}"},
+		{NewInterfaceType(nil, []Type{new(Interface).Complete()}).Complete(), "interface{interface{}}"},
+		{NewInterfaceType(nil, []Type{NewInterfaceType([]*Func{m}, nil)}), "interface{interface{m() /* incomplete */} /* incomplete */}"},
+		{NewInterfaceType(nil, []Type{NewInterfaceType([]*Func{m}, nil).Complete()}), "interface{interface{m()} /* incomplete */}"},
+		{NewInterfaceType(nil, []Type{NewInterfaceType([]*Func{m}, nil).Complete()}).Complete(), "interface{interface{m()}}"},
+	} {
+		got := test.typ.String()
+		if got != test.want {
+			t.Errorf("got: %s, want: %s", got, test.want)
+		}
+	}
+}
+
+// newDefined creates a new defined type named T with the given underlying type.
+// Helper function for use with TestIncompleteInterfaces only.
+func newDefined(underlying Type) *Named {
+	tname := NewTypeName(token.NoPos, nil, "T", nil)
+	return NewNamed(tname, underlying, nil)
+}
+
+func TestQualifiedTypeString(t *testing.T) {
+	p, _ := pkgFor("p.go", "package p; type T int", nil)
+	q, _ := pkgFor("q.go", "package q", nil)
+
+	pT := p.Scope().Lookup("T").Type()
+	for _, test := range []struct {
+		typ  Type
+		this *Package
+		want string
+	}{
+		{nil, nil, "<nil>"},
+		{pT, nil, "p.T"},
+		{pT, p, "T"},
+		{pT, q, "p.T"},
+		{NewPointer(pT), p, "*T"},
+		{NewPointer(pT), q, "*p.T"},
+	} {
+		qualifier := func(pkg *Package) string {
+			if pkg != test.this {
+				return pkg.Name()
+			}
+			return ""
+		}
+		if got := TypeString(test.typ, qualifier); got != test.want {
+			t.Errorf("TypeString(%s, %s) = %s, want %s",
+				test.this, test.typ, got, test.want)
+		}
+	}
+}
diff --git a/src/go/types/typexpr.go b/src/go/types/typexpr.go
new file mode 100644
index 0000000..2b39801
--- /dev/null
+++ b/src/go/types/typexpr.go
@@ -0,0 +1,764 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file implements type-checking of identifiers and type expressions.
+
+package types
+
+import (
+	"go/ast"
+	"go/constant"
+	"go/token"
+	"sort"
+	"strconv"
+)
+
+// ident type-checks identifier e and initializes x with the value or type of e.
+// If an error occurred, x.mode is set to invalid.
+// For the meaning of def, see Checker.definedType, below.
+// If wantType is set, the identifier e is expected to denote a type.
+//
+func (check *Checker) ident(x *operand, e *ast.Ident, def *Named, wantType bool) {
+	x.mode = invalid
+	x.expr = e
+
+	// Note that we cannot use check.lookup here because the returned scope
+	// may be different from obj.Parent(). See also Scope.LookupParent doc.
+	scope, obj := check.scope.LookupParent(e.Name, check.pos)
+	if obj == nil {
+		if e.Name == "_" {
+			check.errorf(e, _InvalidBlank, "cannot use _ as value or type")
+		} else {
+			check.errorf(e, _UndeclaredName, "undeclared name: %s", e.Name)
+		}
+		return
+	}
+	check.recordUse(e, obj)
+
+	// Type-check the object.
+	// Only call Checker.objDecl if the object doesn't have a type yet
+	// (in which case we must actually determine it) or the object is a
+	// TypeName and we also want a type (in which case we might detect
+	// a cycle which needs to be reported). Otherwise we can skip the
+	// call and avoid a possible cycle error in favor of the more
+	// informative "not a type/value" error that this function's caller
+	// will issue (see issue #25790).
+	typ := obj.Type()
+	if _, gotType := obj.(*TypeName); typ == nil || gotType && wantType {
+		check.objDecl(obj, def)
+		typ = obj.Type() // type must have been assigned by Checker.objDecl
+	}
+	assert(typ != nil)
+
+	// The object may be dot-imported: If so, remove its package from
+	// the map of unused dot imports for the respective file scope.
+	// (This code is only needed for dot-imports. Without them,
+	// we only have to mark variables, see *Var case below).
+	if pkg := obj.Pkg(); pkg != check.pkg && pkg != nil {
+		delete(check.unusedDotImports[scope], pkg)
+	}
+
+	switch obj := obj.(type) {
+	case *PkgName:
+		check.errorf(e, _InvalidPkgUse, "use of package %s not in selector", obj.name)
+		return
+
+	case *Const:
+		check.addDeclDep(obj)
+		if typ == Typ[Invalid] {
+			return
+		}
+		if obj == universeIota {
+			if check.iota == nil {
+				check.errorf(e, _InvalidIota, "cannot use iota outside constant declaration")
+				return
+			}
+			x.val = check.iota
+		} else {
+			x.val = obj.val
+		}
+		assert(x.val != nil)
+		x.mode = constant_
+
+	case *TypeName:
+		x.mode = typexpr
+
+	case *Var:
+		// It's ok to mark non-local variables, but ignore variables
+		// from other packages to avoid potential race conditions with
+		// dot-imported variables.
+		if obj.pkg == check.pkg {
+			obj.used = true
+		}
+		check.addDeclDep(obj)
+		if typ == Typ[Invalid] {
+			return
+		}
+		x.mode = variable
+
+	case *Func:
+		check.addDeclDep(obj)
+		x.mode = value
+
+	case *Builtin:
+		x.id = obj.id
+		x.mode = builtin
+
+	case *Nil:
+		x.mode = value
+
+	default:
+		unreachable()
+	}
+
+	x.typ = typ
+}
+
+// typ type-checks the type expression e and returns its type, or Typ[Invalid].
+func (check *Checker) typ(e ast.Expr) Type {
+	return check.definedType(e, nil)
+}
+
+// definedType is like typ but also accepts a type name def.
+// If def != nil, e is the type specification for the defined type def, declared
+// in a type declaration, and def.underlying will be set to the type of e before
+// any components of e are type-checked.
+//
+func (check *Checker) definedType(e ast.Expr, def *Named) (T Type) {
+	if trace {
+		check.trace(e.Pos(), "%s", e)
+		check.indent++
+		defer func() {
+			check.indent--
+			check.trace(e.Pos(), "=> %s", T)
+		}()
+	}
+
+	T = check.typInternal(e, def)
+	assert(isTyped(T))
+	check.recordTypeAndValue(e, typexpr, T, nil)
+
+	return
+}
+
+// funcType type-checks a function or method type.
+func (check *Checker) funcType(sig *Signature, recvPar *ast.FieldList, ftyp *ast.FuncType) {
+	scope := NewScope(check.scope, token.NoPos, token.NoPos, "function")
+	scope.isFunc = true
+	check.recordScope(ftyp, scope)
+
+	recvList, _ := check.collectParams(scope, recvPar, false)
+	params, variadic := check.collectParams(scope, ftyp.Params, true)
+	results, _ := check.collectParams(scope, ftyp.Results, false)
+
+	if recvPar != nil {
+		// recv parameter list present (may be empty)
+		// spec: "The receiver is specified via an extra parameter section preceding the
+		// method name. That parameter section must declare a single parameter, the receiver."
+		var recv *Var
+		switch len(recvList) {
+		case 0:
+			check.error(recvPar, _BadRecv, "method is missing receiver")
+			recv = NewParam(0, nil, "", Typ[Invalid]) // ignore recv below
+		default:
+			// more than one receiver
+			check.error(recvList[len(recvList)-1], _BadRecv, "method must have exactly one receiver")
+			fallthrough // continue with first receiver
+		case 1:
+			recv = recvList[0]
+		}
+		// spec: "The receiver type must be of the form T or *T where T is a type name."
+		// (ignore invalid types - error was reported before)
+		if t, _ := deref(recv.typ); t != Typ[Invalid] {
+			var err string
+			if T, _ := t.(*Named); T != nil {
+				// spec: "The type denoted by T is called the receiver base type; it must not
+				// be a pointer or interface type and it must be declared in the same package
+				// as the method."
+				if T.obj.pkg != check.pkg {
+					err = "type not defined in this package"
+				} else {
+					// TODO(gri) This is not correct if the underlying type is unknown yet.
+					switch u := T.underlying.(type) {
+					case *Basic:
+						// unsafe.Pointer is treated like a regular pointer
+						if u.kind == UnsafePointer {
+							err = "unsafe.Pointer"
+						}
+					case *Pointer, *Interface:
+						err = "pointer or interface type"
+					}
+				}
+			} else {
+				err = "basic or unnamed type"
+			}
+			if err != "" {
+				check.errorf(recv, _InvalidRecv, "invalid receiver %s (%s)", recv.typ, err)
+				// ok to continue
+			}
+		}
+		sig.recv = recv
+	}
+
+	sig.scope = scope
+	sig.params = NewTuple(params...)
+	sig.results = NewTuple(results...)
+	sig.variadic = variadic
+}
+
+// typInternal drives type checking of types.
+// Must only be called by definedType.
+//
+func (check *Checker) typInternal(e ast.Expr, def *Named) Type {
+	switch e := e.(type) {
+	case *ast.BadExpr:
+		// ignore - error reported before
+
+	case *ast.Ident:
+		var x operand
+		check.ident(&x, e, def, true)
+
+		switch x.mode {
+		case typexpr:
+			typ := x.typ
+			def.setUnderlying(typ)
+			return typ
+		case invalid:
+			// ignore - error reported before
+		case novalue:
+			check.errorf(&x, _NotAType, "%s used as type", &x)
+		default:
+			check.errorf(&x, _NotAType, "%s is not a type", &x)
+		}
+
+	case *ast.SelectorExpr:
+		var x operand
+		check.selector(&x, e)
+
+		switch x.mode {
+		case typexpr:
+			typ := x.typ
+			def.setUnderlying(typ)
+			return typ
+		case invalid:
+			// ignore - error reported before
+		case novalue:
+			check.errorf(&x, _NotAType, "%s used as type", &x)
+		default:
+			check.errorf(&x, _NotAType, "%s is not a type", &x)
+		}
+
+	case *ast.ParenExpr:
+		return check.definedType(e.X, def)
+
+	case *ast.ArrayType:
+		if e.Len != nil {
+			typ := new(Array)
+			def.setUnderlying(typ)
+			typ.len = check.arrayLength(e.Len)
+			typ.elem = check.typ(e.Elt)
+			return typ
+
+		} else {
+			typ := new(Slice)
+			def.setUnderlying(typ)
+			typ.elem = check.typ(e.Elt)
+			return typ
+		}
+
+	case *ast.StructType:
+		typ := new(Struct)
+		def.setUnderlying(typ)
+		check.structType(typ, e)
+		return typ
+
+	case *ast.StarExpr:
+		typ := new(Pointer)
+		def.setUnderlying(typ)
+		typ.base = check.typ(e.X)
+		return typ
+
+	case *ast.FuncType:
+		typ := new(Signature)
+		def.setUnderlying(typ)
+		check.funcType(typ, nil, e)
+		return typ
+
+	case *ast.InterfaceType:
+		typ := new(Interface)
+		def.setUnderlying(typ)
+		check.interfaceType(typ, e, def)
+		return typ
+
+	case *ast.MapType:
+		typ := new(Map)
+		def.setUnderlying(typ)
+
+		typ.key = check.typ(e.Key)
+		typ.elem = check.typ(e.Value)
+
+		// spec: "The comparison operators == and != must be fully defined
+		// for operands of the key type; thus the key type must not be a
+		// function, map, or slice."
+		//
+		// Delay this check because it requires fully setup types;
+		// it is safe to continue in any case (was issue 6667).
+		check.atEnd(func() {
+			if !Comparable(typ.key) {
+				check.errorf(e.Key, _IncomparableMapKey, "incomparable map key type %s", typ.key)
+			}
+		})
+
+		return typ
+
+	case *ast.ChanType:
+		typ := new(Chan)
+		def.setUnderlying(typ)
+
+		dir := SendRecv
+		switch e.Dir {
+		case ast.SEND | ast.RECV:
+			// nothing to do
+		case ast.SEND:
+			dir = SendOnly
+		case ast.RECV:
+			dir = RecvOnly
+		default:
+			check.invalidAST(e, "unknown channel direction %d", e.Dir)
+			// ok to continue
+		}
+
+		typ.dir = dir
+		typ.elem = check.typ(e.Value)
+		return typ
+
+	default:
+		check.errorf(e, _NotAType, "%s is not a type", e)
+	}
+
+	typ := Typ[Invalid]
+	def.setUnderlying(typ)
+	return typ
+}
+
+// typeOrNil type-checks the type expression (or nil value) e
+// and returns the typ of e, or nil.
+// If e is neither a type nor nil, typOrNil returns Typ[Invalid].
+//
+func (check *Checker) typOrNil(e ast.Expr) Type {
+	var x operand
+	check.rawExpr(&x, e, nil)
+	switch x.mode {
+	case invalid:
+		// ignore - error reported before
+	case novalue:
+		check.errorf(&x, _NotAType, "%s used as type", &x)
+	case typexpr:
+		return x.typ
+	case value:
+		if x.isNil() {
+			return nil
+		}
+		fallthrough
+	default:
+		check.errorf(&x, _NotAType, "%s is not a type", &x)
+	}
+	return Typ[Invalid]
+}
+
+// arrayLength type-checks the array length expression e
+// and returns the constant length >= 0, or a value < 0
+// to indicate an error (and thus an unknown length).
+func (check *Checker) arrayLength(e ast.Expr) int64 {
+	var x operand
+	check.expr(&x, e)
+	if x.mode != constant_ {
+		if x.mode != invalid {
+			check.errorf(&x, _InvalidArrayLen, "array length %s must be constant", &x)
+		}
+		return -1
+	}
+	if isUntyped(x.typ) || isInteger(x.typ) {
+		if val := constant.ToInt(x.val); val.Kind() == constant.Int {
+			if representableConst(val, check, Typ[Int], nil) {
+				if n, ok := constant.Int64Val(val); ok && n >= 0 {
+					return n
+				}
+				check.errorf(&x, _InvalidArrayLen, "invalid array length %s", &x)
+				return -1
+			}
+		}
+	}
+	check.errorf(&x, _InvalidArrayLen, "array length %s must be integer", &x)
+	return -1
+}
+
+func (check *Checker) collectParams(scope *Scope, list *ast.FieldList, variadicOk bool) (params []*Var, variadic bool) {
+	if list == nil {
+		return
+	}
+
+	var named, anonymous bool
+	for i, field := range list.List {
+		ftype := field.Type
+		if t, _ := ftype.(*ast.Ellipsis); t != nil {
+			ftype = t.Elt
+			if variadicOk && i == len(list.List)-1 && len(field.Names) <= 1 {
+				variadic = true
+			} else {
+				check.softErrorf(t, _MisplacedDotDotDot, "can only use ... with final parameter in list")
+				// ignore ... and continue
+			}
+		}
+		typ := check.typ(ftype)
+		// The parser ensures that f.Tag is nil and we don't
+		// care if a constructed AST contains a non-nil tag.
+		if len(field.Names) > 0 {
+			// named parameter
+			for _, name := range field.Names {
+				if name.Name == "" {
+					check.invalidAST(name, "anonymous parameter")
+					// ok to continue
+				}
+				par := NewParam(name.Pos(), check.pkg, name.Name, typ)
+				check.declare(scope, name, par, scope.pos)
+				params = append(params, par)
+			}
+			named = true
+		} else {
+			// anonymous parameter
+			par := NewParam(ftype.Pos(), check.pkg, "", typ)
+			check.recordImplicit(field, par)
+			params = append(params, par)
+			anonymous = true
+		}
+	}
+
+	if named && anonymous {
+		check.invalidAST(list, "list contains both named and anonymous parameters")
+		// ok to continue
+	}
+
+	// For a variadic function, change the last parameter's type from T to []T.
+	// Since we type-checked T rather than ...T, we also need to retro-actively
+	// record the type for ...T.
+	if variadic {
+		last := params[len(params)-1]
+		last.typ = &Slice{elem: last.typ}
+		check.recordTypeAndValue(list.List[len(list.List)-1].Type, typexpr, last.typ, nil)
+	}
+
+	return
+}
+
+func (check *Checker) declareInSet(oset *objset, pos token.Pos, obj Object) bool {
+	if alt := oset.insert(obj); alt != nil {
+		check.errorf(atPos(pos), _DuplicateDecl, "%s redeclared", obj.Name())
+		check.reportAltDecl(alt)
+		return false
+	}
+	return true
+}
+
+func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, def *Named) {
+	for _, f := range iface.Methods.List {
+		if len(f.Names) > 0 {
+			// We have a method with name f.Names[0].
+			// (The parser ensures that there's only one method
+			// and we don't care if a constructed AST has more.)
+			name := f.Names[0]
+			if name.Name == "_" {
+				check.errorf(name, _BlankIfaceMethod, "invalid method name _")
+				continue // ignore
+			}
+
+			typ := check.typ(f.Type)
+			sig, _ := typ.(*Signature)
+			if sig == nil {
+				if typ != Typ[Invalid] {
+					check.invalidAST(f.Type, "%s is not a method signature", typ)
+				}
+				continue // ignore
+			}
+
+			// use named receiver type if available (for better error messages)
+			var recvTyp Type = ityp
+			if def != nil {
+				recvTyp = def
+			}
+			sig.recv = NewVar(name.Pos(), check.pkg, "", recvTyp)
+
+			m := NewFunc(name.Pos(), check.pkg, name.Name, sig)
+			check.recordDef(name, m)
+			ityp.methods = append(ityp.methods, m)
+		} else {
+			// We have an embedded interface and f.Type is its
+			// (possibly qualified) embedded type name. Collect
+			// it if it's a valid interface.
+			typ := check.typ(f.Type)
+
+			utyp := check.underlying(typ)
+			if _, ok := utyp.(*Interface); !ok {
+				if utyp != Typ[Invalid] {
+					check.errorf(f.Type, _InvalidIfaceEmbed, "%s is not an interface", typ)
+				}
+				continue
+			}
+
+			ityp.embeddeds = append(ityp.embeddeds, typ)
+			check.posMap[ityp] = append(check.posMap[ityp], f.Type.Pos())
+		}
+	}
+
+	if len(ityp.methods) == 0 && len(ityp.embeddeds) == 0 {
+		// empty interface
+		ityp.allMethods = markComplete
+		return
+	}
+
+	// sort for API stability
+	sort.Sort(byUniqueMethodName(ityp.methods))
+	sort.Stable(byUniqueTypeName(ityp.embeddeds))
+
+	check.later(func() { check.completeInterface(ityp) })
+}
+
+func (check *Checker) completeInterface(ityp *Interface) {
+	if ityp.allMethods != nil {
+		return
+	}
+
+	// completeInterface may be called via the LookupFieldOrMethod,
+	// MissingMethod, Identical, or IdenticalIgnoreTags external API
+	// in which case check will be nil. In this case, type-checking
+	// must be finished and all interfaces should have been completed.
+	if check == nil {
+		panic("internal error: incomplete interface")
+	}
+
+	if trace {
+		check.trace(token.NoPos, "complete %s", ityp)
+		check.indent++
+		defer func() {
+			check.indent--
+			check.trace(token.NoPos, "=> %s", ityp)
+		}()
+	}
+
+	// An infinitely expanding interface (due to a cycle) is detected
+	// elsewhere (Checker.validType), so here we simply assume we only
+	// have valid interfaces. Mark the interface as complete to avoid
+	// infinite recursion if the validType check occurs later for some
+	// reason.
+	ityp.allMethods = markComplete
+
+	// Methods of embedded interfaces are collected unchanged; i.e., the identity
+	// of a method I.m's Func Object of an interface I is the same as that of
+	// the method m in an interface that embeds interface I. On the other hand,
+	// if a method is embedded via multiple overlapping embedded interfaces, we
+	// don't provide a guarantee which "original m" got chosen for the embedding
+	// interface. See also issue #34421.
+	//
+	// If we don't care to provide this identity guarantee anymore, instead of
+	// reusing the original method in embeddings, we can clone the method's Func
+	// Object and give it the position of a corresponding embedded interface. Then
+	// we can get rid of the mpos map below and simply use the cloned method's
+	// position.
+
+	var seen objset
+	var methods []*Func
+	mpos := make(map[*Func]token.Pos) // method specification or method embedding position, for good error messages
+	addMethod := func(pos token.Pos, m *Func, explicit bool) {
+		switch other := seen.insert(m); {
+		case other == nil:
+			methods = append(methods, m)
+			mpos[m] = pos
+		case explicit:
+			check.errorf(atPos(pos), _DuplicateDecl, "duplicate method %s", m.name)
+			check.errorf(atPos(mpos[other.(*Func)]), _DuplicateDecl, "\tother declaration of %s", m.name) // secondary error, \t indented
+		default:
+			// check method signatures after all types are computed (issue #33656)
+			check.atEnd(func() {
+				if !check.identical(m.typ, other.Type()) {
+					check.errorf(atPos(pos), _DuplicateDecl, "duplicate method %s", m.name)
+					check.errorf(atPos(mpos[other.(*Func)]), _DuplicateDecl, "\tother declaration of %s", m.name) // secondary error, \t indented
+				}
+			})
+		}
+	}
+
+	for _, m := range ityp.methods {
+		addMethod(m.pos, m, true)
+	}
+
+	posList := check.posMap[ityp]
+	for i, typ := range ityp.embeddeds {
+		pos := posList[i] // embedding position
+		typ, ok := check.underlying(typ).(*Interface)
+		if !ok {
+			// An error was reported when collecting the embedded types.
+			// Ignore it.
+			continue
+		}
+		check.completeInterface(typ)
+		for _, m := range typ.allMethods {
+			addMethod(pos, m, false) // use embedding position pos rather than m.pos
+		}
+	}
+
+	if methods != nil {
+		sort.Sort(byUniqueMethodName(methods))
+		ityp.allMethods = methods
+	}
+}
+
+// byUniqueTypeName named type lists can be sorted by their unique type names.
+type byUniqueTypeName []Type
+
+func (a byUniqueTypeName) Len() int           { return len(a) }
+func (a byUniqueTypeName) Less(i, j int) bool { return sortName(a[i]) < sortName(a[j]) }
+func (a byUniqueTypeName) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+
+func sortName(t Type) string {
+	if named, _ := t.(*Named); named != nil {
+		return named.obj.Id()
+	}
+	return ""
+}
+
+// byUniqueMethodName method lists can be sorted by their unique method names.
+type byUniqueMethodName []*Func
+
+func (a byUniqueMethodName) Len() int           { return len(a) }
+func (a byUniqueMethodName) Less(i, j int) bool { return a[i].Id() < a[j].Id() }
+func (a byUniqueMethodName) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+
+func (check *Checker) tag(t *ast.BasicLit) string {
+	if t != nil {
+		if t.Kind == token.STRING {
+			if val, err := strconv.Unquote(t.Value); err == nil {
+				return val
+			}
+		}
+		check.invalidAST(t, "incorrect tag syntax: %q", t.Value)
+	}
+	return ""
+}
+
+func (check *Checker) structType(styp *Struct, e *ast.StructType) {
+	list := e.Fields
+	if list == nil {
+		return
+	}
+
+	// struct fields and tags
+	var fields []*Var
+	var tags []string
+
+	// for double-declaration checks
+	var fset objset
+
+	// current field typ and tag
+	var typ Type
+	var tag string
+	add := func(ident *ast.Ident, embedded bool, pos token.Pos) {
+		if tag != "" && tags == nil {
+			tags = make([]string, len(fields))
+		}
+		if tags != nil {
+			tags = append(tags, tag)
+		}
+
+		name := ident.Name
+		fld := NewField(pos, check.pkg, name, typ, embedded)
+		// spec: "Within a struct, non-blank field names must be unique."
+		if name == "_" || check.declareInSet(&fset, pos, fld) {
+			fields = append(fields, fld)
+			check.recordDef(ident, fld)
+		}
+	}
+
+	// addInvalid adds an embedded field of invalid type to the struct for
+	// fields with errors; this keeps the number of struct fields in sync
+	// with the source as long as the fields are _ or have different names
+	// (issue #25627).
+	addInvalid := func(ident *ast.Ident, pos token.Pos) {
+		typ = Typ[Invalid]
+		tag = ""
+		add(ident, true, pos)
+	}
+
+	for _, f := range list.List {
+		typ = check.typ(f.Type)
+		tag = check.tag(f.Tag)
+		if len(f.Names) > 0 {
+			// named fields
+			for _, name := range f.Names {
+				add(name, false, name.Pos())
+			}
+		} else {
+			// embedded field
+			// spec: "An embedded type must be specified as a type name T or as a pointer
+			// to a non-interface type name *T, and T itself may not be a pointer type."
+			pos := f.Type.Pos()
+			name := embeddedFieldIdent(f.Type)
+			if name == nil {
+				check.invalidAST(f.Type, "embedded field type %s has no name", f.Type)
+				name = ast.NewIdent("_")
+				name.NamePos = pos
+				addInvalid(name, pos)
+				continue
+			}
+			t, isPtr := deref(typ)
+			// Because we have a name, typ must be of the form T or *T, where T is the name
+			// of a (named or alias) type, and t (= deref(typ)) must be the type of T.
+			switch t := t.Underlying().(type) {
+			case *Basic:
+				if t == Typ[Invalid] {
+					// error was reported before
+					addInvalid(name, pos)
+					continue
+				}
+
+				// unsafe.Pointer is treated like a regular pointer
+				if t.kind == UnsafePointer {
+					check.errorf(f.Type, _InvalidPtrEmbed, "embedded field type cannot be unsafe.Pointer")
+					addInvalid(name, pos)
+					continue
+				}
+
+			case *Pointer:
+				check.errorf(f.Type, _InvalidPtrEmbed, "embedded field type cannot be a pointer")
+				addInvalid(name, pos)
+				continue
+
+			case *Interface:
+				if isPtr {
+					check.errorf(f.Type, _InvalidPtrEmbed, "embedded field type cannot be a pointer to an interface")
+					addInvalid(name, pos)
+					continue
+				}
+			}
+			add(name, true, pos)
+		}
+	}
+
+	styp.fields = fields
+	styp.tags = tags
+}
+
+func embeddedFieldIdent(e ast.Expr) *ast.Ident {
+	switch e := e.(type) {
+	case *ast.Ident:
+		return e
+	case *ast.StarExpr:
+		// *T is valid, but **T is not
+		if _, ok := e.X.(*ast.StarExpr); !ok {
+			return embeddedFieldIdent(e.X)
+		}
+	case *ast.SelectorExpr:
+		return e.Sel
+	}
+	return nil // invalid embedded field
+}
diff --git a/src/go/types/universe.go b/src/go/types/universe.go
new file mode 100644
index 0000000..ff5b891
--- /dev/null
+++ b/src/go/types/universe.go
@@ -0,0 +1,238 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file sets up the universe scope and the unsafe package.
+
+package types
+
+import (
+	"go/constant"
+	"go/token"
+	"strings"
+)
+
+// The Universe scope contains all predeclared objects of Go.
+// It is the outermost scope of any chain of nested scopes.
+var Universe *Scope
+
+// The Unsafe package is the package returned by an importer
+// for the import path "unsafe".
+var Unsafe *Package
+
+var (
+	universeIota  *Const
+	universeByte  *Basic // uint8 alias, but has name "byte"
+	universeRune  *Basic // int32 alias, but has name "rune"
+	universeError *Named
+)
+
+// Typ contains the predeclared *Basic types indexed by their
+// corresponding BasicKind.
+//
+// The *Basic type for Typ[Byte] will have the name "uint8".
+// Use Universe.Lookup("byte").Type() to obtain the specific
+// alias basic type named "byte" (and analogous for "rune").
+var Typ = []*Basic{
+	Invalid: {Invalid, 0, "invalid type"},
+
+	Bool:          {Bool, IsBoolean, "bool"},
+	Int:           {Int, IsInteger, "int"},
+	Int8:          {Int8, IsInteger, "int8"},
+	Int16:         {Int16, IsInteger, "int16"},
+	Int32:         {Int32, IsInteger, "int32"},
+	Int64:         {Int64, IsInteger, "int64"},
+	Uint:          {Uint, IsInteger | IsUnsigned, "uint"},
+	Uint8:         {Uint8, IsInteger | IsUnsigned, "uint8"},
+	Uint16:        {Uint16, IsInteger | IsUnsigned, "uint16"},
+	Uint32:        {Uint32, IsInteger | IsUnsigned, "uint32"},
+	Uint64:        {Uint64, IsInteger | IsUnsigned, "uint64"},
+	Uintptr:       {Uintptr, IsInteger | IsUnsigned, "uintptr"},
+	Float32:       {Float32, IsFloat, "float32"},
+	Float64:       {Float64, IsFloat, "float64"},
+	Complex64:     {Complex64, IsComplex, "complex64"},
+	Complex128:    {Complex128, IsComplex, "complex128"},
+	String:        {String, IsString, "string"},
+	UnsafePointer: {UnsafePointer, 0, "Pointer"},
+
+	UntypedBool:    {UntypedBool, IsBoolean | IsUntyped, "untyped bool"},
+	UntypedInt:     {UntypedInt, IsInteger | IsUntyped, "untyped int"},
+	UntypedRune:    {UntypedRune, IsInteger | IsUntyped, "untyped rune"},
+	UntypedFloat:   {UntypedFloat, IsFloat | IsUntyped, "untyped float"},
+	UntypedComplex: {UntypedComplex, IsComplex | IsUntyped, "untyped complex"},
+	UntypedString:  {UntypedString, IsString | IsUntyped, "untyped string"},
+	UntypedNil:     {UntypedNil, IsUntyped, "untyped nil"},
+}
+
+var aliases = [...]*Basic{
+	{Byte, IsInteger | IsUnsigned, "byte"},
+	{Rune, IsInteger, "rune"},
+}
+
+func defPredeclaredTypes() {
+	for _, t := range Typ {
+		def(NewTypeName(token.NoPos, nil, t.name, t))
+	}
+	for _, t := range aliases {
+		def(NewTypeName(token.NoPos, nil, t.name, t))
+	}
+
+	// Error has a nil package in its qualified name since it is in no package
+	res := NewVar(token.NoPos, nil, "", Typ[String])
+	sig := &Signature{results: NewTuple(res)}
+	err := NewFunc(token.NoPos, nil, "Error", sig)
+	typ := &Named{underlying: NewInterfaceType([]*Func{err}, nil).Complete()}
+	sig.recv = NewVar(token.NoPos, nil, "", typ)
+	def(NewTypeName(token.NoPos, nil, "error", typ))
+}
+
+var predeclaredConsts = [...]struct {
+	name string
+	kind BasicKind
+	val  constant.Value
+}{
+	{"true", UntypedBool, constant.MakeBool(true)},
+	{"false", UntypedBool, constant.MakeBool(false)},
+	{"iota", UntypedInt, constant.MakeInt64(0)},
+}
+
+func defPredeclaredConsts() {
+	for _, c := range predeclaredConsts {
+		def(NewConst(token.NoPos, nil, c.name, Typ[c.kind], c.val))
+	}
+}
+
+func defPredeclaredNil() {
+	def(&Nil{object{name: "nil", typ: Typ[UntypedNil], color_: black}})
+}
+
+// A builtinId is the id of a builtin function.
+type builtinId int
+
+const (
+	// universe scope
+	_Append builtinId = iota
+	_Cap
+	_Close
+	_Complex
+	_Copy
+	_Delete
+	_Imag
+	_Len
+	_Make
+	_New
+	_Panic
+	_Print
+	_Println
+	_Real
+	_Recover
+
+	// package unsafe
+	_Alignof
+	_Offsetof
+	_Sizeof
+
+	// testing support
+	_Assert
+	_Trace
+)
+
+var predeclaredFuncs = [...]struct {
+	name     string
+	nargs    int
+	variadic bool
+	kind     exprKind
+}{
+	_Append:  {"append", 1, true, expression},
+	_Cap:     {"cap", 1, false, expression},
+	_Close:   {"close", 1, false, statement},
+	_Complex: {"complex", 2, false, expression},
+	_Copy:    {"copy", 2, false, statement},
+	_Delete:  {"delete", 2, false, statement},
+	_Imag:    {"imag", 1, false, expression},
+	_Len:     {"len", 1, false, expression},
+	_Make:    {"make", 1, true, expression},
+	_New:     {"new", 1, false, expression},
+	_Panic:   {"panic", 1, false, statement},
+	_Print:   {"print", 0, true, statement},
+	_Println: {"println", 0, true, statement},
+	_Real:    {"real", 1, false, expression},
+	_Recover: {"recover", 0, false, statement},
+
+	_Alignof:  {"Alignof", 1, false, expression},
+	_Offsetof: {"Offsetof", 1, false, expression},
+	_Sizeof:   {"Sizeof", 1, false, expression},
+
+	_Assert: {"assert", 1, false, statement},
+	_Trace:  {"trace", 0, true, statement},
+}
+
+func defPredeclaredFuncs() {
+	for i := range predeclaredFuncs {
+		id := builtinId(i)
+		if id == _Assert || id == _Trace {
+			continue // only define these in testing environment
+		}
+		def(newBuiltin(id))
+	}
+}
+
+// DefPredeclaredTestFuncs defines the assert and trace built-ins.
+// These built-ins are intended for debugging and testing of this
+// package only.
+func DefPredeclaredTestFuncs() {
+	if Universe.Lookup("assert") != nil {
+		return // already defined
+	}
+	def(newBuiltin(_Assert))
+	def(newBuiltin(_Trace))
+}
+
+func init() {
+	Universe = NewScope(nil, token.NoPos, token.NoPos, "universe")
+	Unsafe = NewPackage("unsafe", "unsafe")
+	Unsafe.complete = true
+
+	defPredeclaredTypes()
+	defPredeclaredConsts()
+	defPredeclaredNil()
+	defPredeclaredFuncs()
+
+	universeIota = Universe.Lookup("iota").(*Const)
+	universeByte = Universe.Lookup("byte").(*TypeName).typ.(*Basic)
+	universeRune = Universe.Lookup("rune").(*TypeName).typ.(*Basic)
+	universeError = Universe.Lookup("error").(*TypeName).typ.(*Named)
+}
+
+// Objects with names containing blanks are internal and not entered into
+// a scope. Objects with exported names are inserted in the unsafe package
+// scope; other objects are inserted in the universe scope.
+//
+func def(obj Object) {
+	assert(obj.color() == black)
+	name := obj.Name()
+	if strings.Contains(name, " ") {
+		return // nothing to do
+	}
+	// fix Obj link for named types
+	if typ, ok := obj.Type().(*Named); ok {
+		typ.obj = obj.(*TypeName)
+	}
+	// exported identifiers go into package unsafe
+	scope := Universe
+	if obj.Exported() {
+		scope = Unsafe.scope
+		// set Pkg field
+		switch obj := obj.(type) {
+		case *TypeName:
+			obj.pkg = Unsafe
+		case *Builtin:
+			obj.pkg = Unsafe
+		default:
+			unreachable()
+		}
+	}
+	if scope.Insert(obj) != nil {
+		panic("internal error: double declaration")
+	}
+}