Auto merge of #29261 - apasel422:issue-22403, r=alexcrichton

Closes #22403.

r? @pnkfelix

src/test/run-pass/issue-22403.rs

@@ -0,0 +1,15 @@
+// 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.
+
+fn main() {
+    let x = Box::new([1, 2, 3]);
+    let y = x as Box<[i32]>;
+    println!("y: {:?}", y);
+}