1 files changed, 78 insertions, 0 deletions
diff --git a/src/fmt/errors.go b/src/fmt/errors.go
new file mode 100644
index 0000000..1fbd39f
--- /dev/null
+++ b/src/fmt/errors.go
@@ -0,0 +1,78 @@
+// 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 fmt
+
+import (
+	"errors"
+	"sort"
+)
+
+// Errorf formats according to a format specifier and returns the string as a
+// value that satisfies error.
+//
+// If the format specifier includes a %w verb with an error operand,
+// the returned error will implement an Unwrap method returning the operand.
+// If there is more than one %w verb, the returned error will implement an
+// Unwrap method returning a []error containing all the %w operands in the
+// order they appear in the arguments.
+// It is invalid to supply the %w verb with an operand that does not implement
+// the error interface. The %w verb is otherwise a synonym for %v.
+func Errorf(format string, a ...any) error {
+	p := newPrinter()
+	p.wrapErrs = true
+	p.doPrintf(format, a)
+	s := string(p.buf)
+	var err error
+	switch len(p.wrappedErrs) {
+	case 0:
+		err = errors.New(s)
+	case 1:
+		w := &wrapError{msg: s}
+		w.err, _ = a[p.wrappedErrs[0]].(error)
+		err = w
+	default:
+		if p.reordered {
+			sort.Ints(p.wrappedErrs)
+		}
+		var errs []error
+		for i, argNum := range p.wrappedErrs {
+			if i > 0 && p.wrappedErrs[i-1] == argNum {
+				continue
+			}
+			if e, ok := a[argNum].(error); ok {
+				errs = append(errs, e)
+			}
+		}
+		err = &wrapErrors{s, errs}
+	}
+	p.free()
+	return err
+}
+
+type wrapError struct {
+	msg string
+	err error
+}
+
+func (e *wrapError) Error() string {
+	return e.msg
+}
+
+func (e *wrapError) Unwrap() error {
+	return e.err
+}
+
+type wrapErrors struct {
+	msg  string
+	errs []error
+}
+
+func (e *wrapErrors) Error() string {
+	return e.msg
+}
+
+func (e *wrapErrors) Unwrap() []error {
+	return e.errs
+}