gc: fix copying of types

reset xmethod during copytype

Fixes #2497

R=rsc, dvyukov
CC=golang-dev
https://golang.org/cl/5441045

src/cmd/gc/typecheck.c

@@ -2530,6 +2530,7 @@ copytype(Node *n, Type *t)
 	t->vargen = n->vargen;
 	t->siggen = 0;
 	t->method = nil;
+	t->xmethod = nil;
 	t->nod = N;
 	t->printed = 0;
 	t->deferwidth = 0;

test/fixedbugs/bug378.go

+// $G $D/$F.go && $L $F.$A && ./$A.out || echo BUG: bug378
+
+// 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.
+
+// Issue 2497
+
+package main
+
+type Header struct{}
+func (h Header) Method() {}
+
+var _ interface{} = Header{}
+
+func main() {
+  	type X Header
+  	var _ interface{} = X{}
+}