go/types, types2: fix string to type parameter conversions

Converting an untyped constant to a type parameter results
in a non-constant value; but the constant must still be
representable by all specific types of the type parameter.

Adjust the special handling for constant-to-type parameter
conversions to also include string-to-[]byte and []rune
conversions, which are handled separately for conversions
to types that are not type parameters because those are not
constant conversions in non-generic code.

Fixes #51386.

Change-Id: I15e5a0fd281efd15af387280cd3dee320a1ac5e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/388254
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/conversions.go

@@ -49,11 +49,14 @@ func (check *Checker) conversion(x *operand, T Type) {
 		// have specific types, constant x cannot be
 		// converted.
 		ok = T.(*TypeParam).underIs(func(u Type) bool {
-			// t is nil if there are no specific type terms
+			// u is nil if there are no specific type terms
 			if u == nil {
 				cause = check.sprintf("%s does not contain specific types", T)
 				return false
 			}
+			if isString(x.typ) && isBytesOrRunes(u) {
+				return true
+			}
 			if !constConvertibleTo(u, nil) {
 				cause = check.sprintf("cannot convert %s to %s (in %s)", x, u, T)
 				return false

src/cmd/compile/internal/types2/testdata/fixedbugs/issue51386.go2

@@ -0,0 +1,17 @@
+// Copyright 2022 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 myString string
+
+func _[P ~string | ~[]byte | ~[]rune]() {
+	_ = P("")
+	const s myString = ""
+	_ = P(s)
+}
+
+func _[P myString]() {
+	_ = P("")
+}

src/go/types/conversions.go

@@ -48,11 +48,14 @@ func (check *Checker) conversion(x *operand, T Type) {
 		// have specific types, constant x cannot be
 		// converted.
 		ok = T.(*TypeParam).underIs(func(u Type) bool {
-			// t is nil if there are no specific type terms
+			// u is nil if there are no specific type terms
 			if u == nil {
 				cause = check.sprintf("%s does not contain specific types", T)
 				return false
 			}
+			if isString(x.typ) && isBytesOrRunes(u) {
+				return true
+			}
 			if !constConvertibleTo(u, nil) {
 				cause = check.sprintf("cannot convert %s to %s (in %s)", x, u, T)
 				return false

src/go/types/testdata/fixedbugs/issue51386.go2

@@ -0,0 +1,17 @@
+// Copyright 2022 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 myString string
+
+func _[P ~string | ~[]byte | ~[]rune]() {
+	_ = P("")
+	const s myString = ""
+	_ = P(s)
+}
+
+func _[P myString]() {
+	_ = P("")
+}