cmd/gc: fix internal compiler error in struct compare

Fixes #9006.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/167800043

src/cmd/gc/walk.c

@@ -3157,7 +3157,7 @@ countfield(Type *t)
 static void
 walkcompare(Node **np, NodeList **init)
 {
-	Node *n, *l, *r, *call, *a, *li, *ri, *expr;
+	Node *n, *l, *r, *call, *a, *li, *ri, *expr, *cmpl, *cmpr;
 	int andor, i;
 	Type *t, *t1;
 	
@@ -3177,18 +3177,25 @@ walkcompare(Node **np, NodeList **init)
 		break;
 	}
 	
-	if(!islvalue(n->left) || !islvalue(n->right)) {
-		fatal("arguments of comparison must be lvalues");
+	cmpl = n->left;
+	while(cmpl != N && cmpl->op == OCONVNOP)
+		cmpl = cmpl->left;
+	cmpr = n->right;
+	while(cmpr != N && cmpr->op == OCONVNOP)
+		cmpr = cmpr->left;
+	
+	if(!islvalue(cmpl) || !islvalue(cmpr)) {
+		fatal("arguments of comparison must be lvalues - %N %N", cmpl, cmpr);
 	}
 
 	l = temp(ptrto(t));
-	a = nod(OAS, l, nod(OADDR, n->left, N));
+	a = nod(OAS, l, nod(OADDR, cmpl, N));
 	a->right->etype = 1;  // addr does not escape
 	typecheck(&a, Etop);
 	*init = list(*init, a);
 
 	r = temp(ptrto(t));
-	a = nod(OAS, r, nod(OADDR, n->right, N));
+	a = nod(OAS, r, nod(OADDR, cmpr, N));
 	a->right->etype = 1;  // addr does not escape
 	typecheck(&a, Etop);
 	*init = list(*init, a);

test/fixedbugs/issue9006.go

+// run
+
+// Copyright 2014 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 T1 struct {
+	X int
+}
+
+func NewT1(x int) T1 { return T1{x} }
+
+type T2 int
+
+func NewT2(x int) T2 { return T2(x) }
+
+func main() {
+	switch (T1{}) {
+	case NewT1(1):
+		panic("bad1")
+	case NewT1(0):
+		// ok
+	default:
+		panic("bad2")
+	}
+
+	switch T2(0) {
+	case NewT2(2):
+		panic("bad3")
+	case NewT2(0):
+		// ok
+	default:
+		panic("bad4")
+	}
+}