Fix empty lifetime list or one with trailing comma being rejected

Fixes #37733

src/libsyntax/parse/parser.rs

@@ -1943,12 +1943,12 @@ impl<'a> Parser<'a> {
                     if let Some(recv) = followed_by_ty_params {
                         assert!(recv.is_empty());
                         *recv = attrs;
-                    } else {
+                        debug!("parse_lifetime_defs ret {:?}", res);
+                        return Ok(res);
+                    } else if !attrs.is_empty() {
                         let msg = "trailing attribute after lifetime parameters";
                         return Err(self.fatal(msg));
                     }
-                    debug!("parse_lifetime_defs ret {:?}", res);
-                    return Ok(res);
                 }
             }
 

src/test/run-pass/issue-37733.rs

@@ -0,0 +1,15 @@
+// Copyright 2016 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.
+
+type A = for<> fn();
+
+type B = for<'a,> fn();
+
+pub fn main() {}