add test case for enum disambiguation

src/test/compile-fail/enums-pats-not-idents.rs

+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//error-pattern:unresolved enum variant
+
+fn main() {
+    // a bug in the parser is allowing this:
+    let a() = 13;
+}