Add regression test for #20862

Closes #20862.

Author: jooert

src/test/compile-fail/issue-20862.rs

@@ -0,0 +1,23 @@
+// 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(x: i32) {
+    |y| x + y
+//~^ ERROR: mismatched types:
+//~| expected `()`,
+//~|     found closure
+//~| (expected (),
+//~|     found closure) [E0308]
+}
+
+fn main() {
+    let x = foo(5)(2);
+//~^ ERROR: expected function, found `()`
+}