Add regression test for #18711

Author: bkoropoff

src/test/auxiliary/issue-18711.rs

@@ -0,0 +1,16 @@
+// Copyright 2014 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, overloaded_calls)]
+#![crate_type = "rlib"]
+
+pub fn inner<F>(f: F) -> F {
+    (move |:| f)()
+}

src/test/run-pass/issue-18711.rs

@@ -0,0 +1,21 @@
+// Copyright 2014 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.
+
+// Test that we don't panic on a RefCell borrow conflict in certain
+// code paths involving unboxed closures.
+
+#![feature(unboxed_closures, overloaded_calls)]
+
+// aux-build:issue-18711.rs
+extern crate "issue-18711" as issue;
+
+fn main() {
+    (|:| issue::inner(()))();
+}