1 files changed, 108 insertions, 0 deletions
diff --git a/test/typeparam/boundmethod.go b/test/typeparam/boundmethod.go
new file mode 100644
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--- /dev/null
+++ b/test/typeparam/boundmethod.go
@@ -0,0 +1,108 @@
+// run
+
+// Copyright 2021 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 test illustrates how a type bound method (String below) can be implemented
+// either by a concrete type (myint below) or a instantiated generic type
+// (StringInt[myint] below).
+
+package main
+
+import (
+	"fmt"
+	"reflect"
+	"strconv"
+)
+
+type myint int
+
+//go:noinline
+func (m myint) String() string {
+	return strconv.Itoa(int(m))
+}
+
+type Stringer interface {
+	String() string
+}
+
+func stringify[T Stringer](s []T) (ret []string) {
+	for _, v := range s {
+		// Test normal bounds method call on type param
+		x1 := v.String()
+
+		// Test converting type param to its bound interface first
+		v1 := Stringer(v)
+		x2 := v1.String()
+
+		// Test method expression with type param type
+		f1 := T.String
+		x3 := f1(v)
+
+		// Test creating and calling closure equivalent to the method expression
+		f2 := func(v1 T) string {
+			return Stringer(v1).String()
+		}
+		x4 := f2(v)
+
+		if x1 != x2 || x2 != x3 || x3 != x4 {
+			panic(fmt.Sprintf("Mismatched values %v, %v, %v, %v\n", x1, x2, x3, x4))
+		}
+
+		ret = append(ret, v.String())
+	}
+	return ret
+}
+
+type Ints interface {
+	~int32 | ~int
+}
+
+// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+// type StringInt[T Ints] T
+//
+// //go:noinline
+// func (m StringInt[T]) String() string {
+// 	return strconv.Itoa(int(m))
+// }
+
+type StringStruct[T Ints] struct {
+	f T
+}
+
+func (m StringStruct[T]) String() string {
+	return strconv.Itoa(int(m.f))
+}
+
+func main() {
+	x := []myint{myint(1), myint(2), myint(3)}
+
+	// stringify on a normal type, whose bound method is associated with the base type.
+	got := stringify(x)
+	want := []string{"1", "2", "3"}
+	if !reflect.DeepEqual(got, want) {
+		panic(fmt.Sprintf("got %s, want %s", got, want))
+	}
+
+	// For now, a lone type parameter is not permitted as RHS in a type declaration (issue #45639).
+	// x2 := []StringInt[myint]{StringInt[myint](5), StringInt[myint](7), StringInt[myint](6)}
+	//
+	// // stringify on an instantiated type, whose bound method is associated with
+	// // the generic type StringInt[T], which maps directly to T.
+	// got2 := stringify(x2)
+	// want2 := []string{"5", "7", "6"}
+	// if !reflect.DeepEqual(got2, want2) {
+	// 	panic(fmt.Sprintf("got %s, want %s", got2, want2))
+	// }
+
+	// stringify on an instantiated type, whose bound method is associated with
+	// the generic type StringStruct[T], which maps to a struct containing T.
+	x3 := []StringStruct[myint]{StringStruct[myint]{f: 11}, StringStruct[myint]{f: 10}, StringStruct[myint]{f: 9}}
+
+	got3 := stringify(x3)
+	want3 := []string{"11", "10", "9"}
+	if !reflect.DeepEqual(got3, want3) {
+		panic(fmt.Sprintf("got %s, want %s", got3, want3))
+	}
+}