gc: handle invalid name in type switch

Fixes #1453.

R=ken2
CC=golang-dev
https://golang.org/cl/4125043

src/cmd/gc/go.y

@@ -422,11 +422,18 @@ simple_stmt:
 |	expr_list LCOLAS expr_list
 	{
 		if($3->n->op == OTYPESW) {
+			Node *n;
+			
+			n = N;
 			if($3->next != nil)
 				yyerror("expr.(type) must be alone in list");
-			else if($1->next != nil)
+			if($1->next != nil)
 				yyerror("argument count mismatch: %d = %d", count($1), 1);
-			$$ = nod(OTYPESW, $1->n, $3->n->right);
+			else if($1->n->op != ONAME && $1->n->op != OTYPE && $1->n->op != ONONAME)
+				yyerror("invalid variable name %#N in type switch", $1->n);
+			else
+				n = $1->n;
+			$$ = nod(OTYPESW, n, $3->n->right);
 			break;
 		}
 		$$ = colas($1, $3);

test/syntax/typesw.go

+// errchk $G -e $D/$F.go
+
+// 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
+
+func main() {
+	switch main() := interface{}(nil).(type) {	// ERROR "invalid variable name"
+	default:
+	}
+}