allow parens to disambiguate types.

examples:

	chan <- (chan int)
	chan (<- chan int)
	(map[string]func())("a": main)

R=ken
OCL=25151
CL=25151

src/cmd/gc/go.y

@@ -1004,6 +1004,10 @@ convtype:
 		$$->type = $5;
 	}
 |	structtype
+|	'(' type ')'
+	{
+		$$ = $2;
+	}
 
 /*
  * to avoid parsing conflicts, type is split into
@@ -1031,6 +1035,10 @@ Btype:
 |	Bchantype
 |	Bfntype
 |	Bothertype
+|	'(' type ')'
+	{
+		$$ = $2;
+	}
 
 non_name_type:
 	chantype
@@ -1052,6 +1060,10 @@ Bnon_chan_type:
 	nametype
 |	Bfntype
 |	Bothertype
+|	'(' Btype ')'
+	{
+		$$ = $2;
+	}
 
 Anon_fn_type:
 	Achantype
@@ -1062,7 +1074,6 @@ Bnon_fn_type:
 |	Bchantype
 |	Bothertype
 
-
 nametype:
 	LATYPE
 	{

test/parentype.go

+// $G $D/$F.go
+
+// Copyright 2009 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 f(interface{})
+func g() {}
+func main() {
+	f(map[string]string("a":"b","c":"d"));
+	f((map[string]string)("a":"b","c":"d"));
+	f((map[string]func())("a":g,"c":g));
+	f(make(chan(<-chan int)));
+	f(make(chan<-(chan int)));
+}