Permit `! as T` with test

src/librustc_typeck/check/coercion.rs

@@ -642,7 +642,10 @@ pub fn try_coerce(&self,
                 apply(&mut coerce, &|| Some(expr), source, target)?;
             if !adjustment.is_identity() {
                 debug!("Success, coerced with {:?}", adjustment);
-                assert!(!self.tables.borrow().adjustments.contains_key(&expr.id));
+                match self.tables.borrow().adjustments.get(&expr.id) {
+                    None | Some(&AdjustNeverToAny(..)) => (),
+                    _ => bug!("expr already has an adjustment on it!"),
+                };
                 self.write_adjustment(expr.id, adjustment);
             }
             Ok(ty)

src/test/run-fail/cast-never.rs

+// Copyright 2012 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(never_type)]
+
+// error-pattern:explicit
+fn main() {
+    let x: ! = panic!();
+    let y: u32 = x as u32;
+}
+