Add test for self-referencing pattern blocked by #20551. Fixes #20551.

Author: nikomatsakis

src/test/run-pass/associated-types-stream.rs

@@ -0,0 +1,47 @@
+// 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.
+
+// Test references to the trait `Stream` in the bounds for associated
+// types defined on `Stream`. Issue #20551.
+
+trait Stream {
+    type Car;
+    type Cdr: Stream;
+
+    fn car(&self) -> Self::Car;
+    fn cdr(self) -> Self::Cdr;
+}
+
+impl Stream for () {
+    type Car = ();
+    type Cdr = ();
+    fn car(&self) -> () { () }
+    fn cdr(self) -> () { self }
+}
+
+impl<T,U> Stream for (T, U)
+    where T : Clone, U : Stream
+{
+    type Car = T;
+    type Cdr = U;
+    fn car(&self) -> T { self.0.clone() }
+    fn cdr(self) -> U { self.1 }
+}
+
+fn main() {
+    let p = (22, (44, (66, ())));
+    assert_eq!(p.car(), 22);
+
+    let p = p.cdr();
+    assert_eq!(p.car(), 44);
+
+    let p = p.cdr();
+    assert_eq!(p.car(), 66);
+}