Add regression test for #20544

Closes #20544

Author: frewsxcv

src/test/run-pass/issue-20544.rs

@@ -0,0 +1,27 @@
+// 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.
+
+#![feature(unboxed_closures)]
+#![feature(core)]
+
+struct Fun<F>(F);
+
+impl<F, T> FnOnce<(T,)> for Fun<F> where F: Fn(T) -> T {
+    type Output = T;
+
+    extern "rust-call" fn call_once(self, (t,): (T,)) -> T {
+        (self.0)(t)
+    }
+}
+
+fn main() {
+    let fun = Fun(|i: isize| i * 2);
+    println!("{}", fun(3));
+}