go/parser: permit parentheses in receiver types

Pending acceptance of CL 101500044
and adjustment of test/fixedbugs/bug299.go.

LGTM=adonovan
R=golang-codereviews, adonovan
CC=golang-codereviews
https://golang.org/cl/110160043

src/pkg/go/parser/parser.go

@@ -2310,9 +2310,9 @@ func (p *parser) parseReceiver(scope *ast.Scope) *ast.FieldList {
 		return par
 	}
 
-	// recv type must be of the form ["*"] identifier
+	// recv type must be of the form ["*"] identifier, possibly using parentheses
 	recv := par.List[0]
-	base := deref(recv.Type)
+	base := unparen(deref(unparen(recv.Type)))
 	if _, isIdent := base.(*ast.Ident); !isIdent {
 		if _, isBad := base.(*ast.BadExpr); !isBad {
 			// only report error if it's a new one

src/pkg/go/parser/short_test.go

@@ -35,6 +35,9 @@ var valids = []string{
 	`package p; func f() { for _ = range "foo" + "bar" {} };`,
 	`package p; func f() { var s []int; g(s[:], s[i:], s[:j], s[i:j], s[i:j:k], s[:j:k]) };`,
 	`package p; var ( _ = (struct {*T}).m; _ = (interface {T}).m )`,
+	`package p; func ((T),) m() {}`,
+	`package p; func ((*T),) m() {}`,
+	`package p; func (*(T),) m() {}`,
 }
 
 func TestValid(t *testing.T) {