cmd/gc: Reject parenthesised .(type) expressions.

Fixes #4470.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6949073

src/cmd/gc/go.y

@@ -1020,6 +1020,7 @@ pexpr:
 		case OPACK:
 		case OTYPE:
 		case OLITERAL:
+		case OTYPESW:
 			$$ = nod(OPAREN, $$, N);
 		}
 	}

test/fixedbugs/issue4470.go

+// errorcheck
+
+// 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.
+
+// Issue 4470: parens are not allowed around .(type) "expressions"
+
+package main
+
+func main() {
+	var i interface{}
+	switch (i.(type)) { // ERROR "outside type switch"
+	default:
+	}
+}