Add regression test for #22037

Closes #22037.

src/test/compile-fail/issue-22037.rs

@@ -0,0 +1,27 @@
+// 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 A {
+    type Output;
+    fn a(&self) -> <Self as A>::X;
+//~^ ERROR: use of undeclared associated type `A::X`
+}
+
+impl A for u32 {
+    type Output = u32;
+    fn a(&self) -> u32 {
+        0
+    }
+}
+
+fn main() {
+    let a: u32 = 0;
+    let b: u32 = a.a();
+}