Add regression test for #21701

Closes #21701.

Author: jooert

src/test/compile-fail/issue-21701.rs

@@ -0,0 +1,25 @@
+// 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.
+
+fn foo<U>(t: U) {
+    let y = t();
+//~^ ERROR: expected function, found `U`
+}
+
+struct Bar;
+
+pub fn some_func() {
+    let f = Bar();
+//~^ ERROR: expected function, found `Bar`
+}
+
+fn main() {
+    foo(|| { 1 });
+}