Add regression test for #20971. Fixes #20791.

Author: nikomatsakis

src/test/run-pass/issue-20971.rs

@@ -0,0 +1,32 @@
+// Copyright 2015 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.
+
+// Regression test for Issue #20971.
+
+pub trait Parser {
+    type Input;
+    fn parse(&mut self, input: <Self as Parser>::Input);
+}
+
+impl Parser for () {
+    type Input = ();
+    fn parse(&mut self, input: ()) {
+
+    }
+}
+
+pub fn many() -> Box<Parser<Input=<() as Parser>::Input> + 'static> {
+    panic!()
+}
+
+fn main() {
+    many()
+        .parse(());
+}