Add regression test for #20174

Closes #20174.

Author: jooert

src/test/run-pass/issue-20174.rs

@@ -0,0 +1,16 @@
+// 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.
+
+struct GradFn<F: Fn() -> usize>(F);
+
+fn main() {
+    let GradFn(x_squared) : GradFn<_> = GradFn(|| -> usize { 2 });
+    let _  = x_squared();
+}