Add regression test for #21177

Closes #21177.

src/test/compile-fail/issue-21177.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.
+
+trait Trait {
+    type A;
+    type B;
+}
+
+fn foo<T: Trait<A = T::B>>() { }
+//~^ ERROR: unsupported cyclic reference between types/traits detected
+
+fn main() { }