Add a regression test for issue #18809.

Closes #18809.

src/test/run-pass/issue-18809.rs

@@ -0,0 +1,21 @@
+// 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.
+
+trait Tup {
+    type T0;
+    type T1;
+}
+
+impl Tup for isize {
+    type T0 = f32;
+    type T1 = ();
+}
+
+fn main() {}