Add regression test for #18883

Author: bkoropoff

src/test/run-pass/issue-18883.rs

@@ -0,0 +1,19 @@
+// 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 ICE due to encountering unsubstituted type
+// parameters when untupling FnOnce parameters during translation of
+// an unboxing shim.
+
+#![feature(unboxed_closures)]
+
+fn main() {
+    let _: Box<FnOnce<(),()>> = box move |&mut:| {};
+}