bug300: literal types must not be parenthesized

( This CL is dependent on acceptance of
https://golang.org/cl/1913041/show )

R=go-dev
CC=golang-dev
https://golang.org/cl/1860045

test/bugs/bug300.go

+// errchk $G $D/$F.go
+
+// 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.
+
+package main
+
+type T struct {
+	x, y *T
+}
+
+func main() {
+	// legal composite literals
+	_ = struct{}{}
+	_ = [42]int{}
+	_ = [...]int{}
+	_ = []int{}
+	_ = map[int]int{}
+	_ = T{}
+
+	// illegal composite literals: parentheses not allowed around literal type
+	_ = (struct{}){}	// ERROR "xxx"
+	_ = ([42]int){}		// ERROR "xxx"
+	_ = ([...]int){}	// ERROR "xxx"
+	_ = ([]int){}		// ERROR "xxx"
+	_ = (map[int]int){}	// ERROR "xxx"
+	_ = (T){}		// ERROR "xxx"
+}

test/golden.out

@@ -186,3 +186,6 @@ errchk: bugs/bug299.go: unmatched error messages:
 ==================================================
 bugs/bug299.go:19: syntax error: unexpected }
 ==================================================
+
+=========== bugs/bug300.go
+BUG: errchk: command succeeded unexpectedly