add tests for #20825

closes #20825

Author: apasel422

src/test/compile-fail/issue-20825.rs

@@ -0,0 +1,20 @@
+// 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.
+
+pub trait Subscriber {
+    type Input;
+}
+
+pub trait Processor: Subscriber<Input = Self::Input> {
+    //~^ ERROR unsupported cyclic reference between types/traits detected [E0391]
+    type Input;
+}
+
+fn main() {}

src/test/run-pass/issue-20825.rs

@@ -0,0 +1,19 @@
+// 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.
+
+pub trait Subscriber {
+    type Input;
+}
+
+pub trait Processor: Subscriber<Input = <Self as Processor>::Input> {
+    type Input;
+}
+
+fn main() {}