Add regression test for #20847.

src/test/run-pass/issue-20847.rs

@@ -0,0 +1,21 @@
+// Copyright 2016 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.
+
+#![feature(fn_traits)]
+
+use std::ops::Fn;
+
+fn say(x: u32, y: u32) {
+    println!("{} {}", x, y);
+}
+
+fn main() {
+    Fn::call(&say, (1, 2));
+}