testsuite: Add test for #4523

Author: catamorphism

src/test/compile-fail/issue-4523.rs

@@ -0,0 +1,17 @@
+// Copyright 2013 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 foopy() {}
+
+const f: fn() = foopy; //~ ERROR mismatched types: expected `&static/fn()`
+
+fn main () {
+    f();
+}