Add regression test for #21212. Fixes #21212.

src/test/run-pass/associated-types-issue-21212.rs

+// 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.
+
+// Regression test for #21212: an overflow occurred during trait
+// checking where normalizing `Self::Input` led to normalizing the
+// where clauses in the environment which in turn required normalizing
+// `Self::Input`.
+
+pub trait Parser {
+    type Input;
+
+    fn parse(input: <Self as Parser>::Input) {
+        panic!()
+    }
+}
+impl <P> Parser for P  {
+    type Input = ();
+}
+
+fn main() {
+}