go/ssa/interp: support conversions to slices of named bytes

Support for conversions from string to slices of named byte and rune
types and vice versa.

Fixes golang/go#55115

Change-Id: Ie0fb94385f1bc89789fe299cc0c39063db676c21
Reviewed-on: https://go-review.googlesource.com/c/tools/+/501301
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Tim King <taking@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Tim King <taking@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

go/ssa/interp/interp_test.go

@@ -129,6 +129,7 @@ var testdataTests = []string{
 	"width32.go",
 
 	"fixedbugs/issue52342.go",
+	"fixedbugs/issue55115.go",
 }
 
 func init() {

go/ssa/interp/ops.go

@@ -1211,8 +1211,7 @@ func conv(t_dst, t_src types.Type, x value) value {
 
 	case *types.Slice:
 		// []byte or []rune -> string
-		// TODO(adonovan): fix: type B byte; conv([]B -> string).
-		switch ut_src.Elem().(*types.Basic).Kind() {
+		switch ut_src.Elem().Underlying().(*types.Basic).Kind() {
 		case types.Byte:
 			x := x.([]value)
 			b := make([]byte, 0, len(x))
@@ -1234,7 +1233,6 @@ func conv(t_dst, t_src types.Type, x value) value {
 		x = widen(x)
 
 		// integer -> string?
-		// TODO(adonovan): fix: test integer -> named alias of string.
 		if ut_src.Info()&types.IsInteger != 0 {
 			if ut_dst, ok := ut_dst.(*types.Basic); ok && ut_dst.Kind() == types.String {
 				return fmt.Sprintf("%c", x)
@@ -1246,8 +1244,7 @@ func conv(t_dst, t_src types.Type, x value) value {
 			switch ut_dst := ut_dst.(type) {
 			case *types.Slice:
 				var res []value
-				// TODO(adonovan): fix: test named alias of rune, byte.
-				switch ut_dst.Elem().(*types.Basic).Kind() {
+				switch ut_dst.Elem().Underlying().(*types.Basic).Kind() {
 				case types.Rune:
 					for _, r := range []rune(s) {
 						res = append(res, r)

go/ssa/interp/testdata/fixedbugs/issue55115.go

@@ -0,0 +1,38 @@
+// Copyright 2023 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 "reflect"
+
+func main() {
+	type MyByte byte
+	type MyRune rune
+	type MyString string
+
+	a := []MyByte{'a', 'b', 'c'}
+	if s := string(a); s != "abc" {
+		panic(s)
+	}
+
+	b := []MyRune{'五', '五'}
+	if s := string(b); s != "五五" {
+		panic(s)
+	}
+
+	c := []MyByte{'l', 'o', 'r', 'e', 'm'}
+	if s := MyString(c); s != MyString("lorem") {
+		panic(s)
+	}
+
+	d := "lorem"
+	if a := []MyByte(d); !reflect.DeepEqual(a, []MyByte{'l', 'o', 'r', 'e', 'm'}) {
+		panic(a)
+	}
+
+	e := 42
+	if s := MyString(e); s != "*" {
+		panic(s)
+	}
+}