reflect: fix test of whether structs are identical

The old code worked with gc, I assume because the linker
unified identical strings, but it failed with gccgo.

R=rsc
CC=gobot, golang-dev
https://golang.org/cl/6826063

src/pkg/reflect/type.go

@@ -1257,8 +1257,19 @@ func haveIdenticalUnderlyingType(T, V *commonType) bool {
 		for i := range t.fields {
 			tf := &t.fields[i]
 			vf := &v.fields[i]
-			if tf.name != vf.name || tf.pkgPath != vf.pkgPath ||
-				tf.typ != vf.typ || tf.tag != vf.tag || tf.offset != vf.offset {
+			if tf.name != vf.name && (tf.name == nil || vf.name == nil || *tf.name != *vf.name) {
+				return false
+			}
+			if tf.pkgPath != vf.pkgPath && (tf.pkgPath == nil || vf.pkgPath == nil || *tf.pkgPath != *vf.pkgPath) {
+				return false
+			}
+			if tf.typ != vf.typ {
+				return false
+			}
+			if tf.tag != vf.tag && (tf.tag == nil || vf.tag == nil || *tf.tag != *vf.tag) {
+				return false
+			}
+			if tf.offset != vf.offset {
 				return false
 			}
 		}

test/fixedbugs/bug468.dir/p1.go

+// 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 p1
+
+type S struct { X, Y int }

test/fixedbugs/bug468.dir/p2.go

+// 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 main
+
+import (
+	"reflect"
+
+	"./p1"
+)
+
+func main() {
+	var v1 = p1.S{1, 2}
+	var v2 = struct { X, Y int }{1, 2}
+	v1 = v2
+	t1 := reflect.TypeOf(v1)
+	t2 := reflect.TypeOf(v2)
+	if !t1.AssignableTo(t2) {
+		panic(0)
+	}
+	if !t2.AssignableTo(t1) {
+		panic(1)
+	}
+}

test/fixedbugs/bug468.go

+// rundir
+
+// 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 reflect package was not correctly checking field names
+// when checking for struct assignability.
+
+package ignored