gc: bug327

Fixes golang#1674.

R=ken2
CC=golang-dev
https://golang.org/cl/4368057

src/cmd/gc/subr.c

@@ -1889,8 +1889,9 @@ assignop(Type *src, Type *dst, char **why)
 		return OCONVNOP;
 
 	// 2. src and dst have identical underlying types
-	// and either src or dst is not a named type.
-	if(eqtype(src->orig, dst->orig) && (src->sym == S || dst->sym == S))
+	// and either src or dst is not a named type or
+	// both are interface types.
+	if(eqtype(src->orig, dst->orig) && (src->sym == S || dst->sym == S || src->etype == TINTER))
 		return OCONVNOP;
 
 	// 3. dst is an interface type and src implements dst.

test/fixedbugs/bug327.go

@@ -0,0 +1,18 @@
+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2011 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
+
+type (
+       a interface{}
+       b interface{}
+)
+
+func main() {
+       x := a(1)
+       z := b(x)
+       _ = z
+}