gc: never include ( ) on singleton func return type

Fixes #749.

R=ken2
CC=golang-dev
https://golang.org/cl/963043

src/cmd/gc/subr.c

@@ -1227,14 +1227,10 @@ Tpretty(Fmt *fp, Type *t)
 				fmtprint(fp, " ?unknown-type?");
 				break;
 			}
-			if(t1->etype != TFIELD) {
+			if(t1->etype == TFIELD)
+				t1 = t1->type;
 			fmtprint(fp, " %T", t1);
 			break;
-			}
-			if(t1->sym == S) {
-				fmtprint(fp, " %T", t1->type);
-				break;
-			}
 		default:
 			t1 = getoutargx(t)->type;
 			fmtprint(fp, " (");

test/fixedbugs/bug269.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2010 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.
+
+// http://code.google.com/p/go/issues/detail?id=749
+
+package main
+
+func f() (ok bool) { return false }
+
+func main() {
+	var i interface{}
+	i = f
+	_ = i.(func()bool)
+	_ = i.(func()(bool))
+}