rustc: Fix an ICE with box-placement syntax

Closes #14084

src/librustc/middle/typeck/check/mod.rs

@@ -2767,7 +2767,8 @@ fn check_expr_with_unifier(fcx: &FnCtxt,
           if !checked {
               tcx.sess.span_err(expr.span,
                                 "only the managed heap and exchange heap are \
-                                 currently supported")
+                                 currently supported");
+              fcx.write_ty(id, ty::mk_err());
           }
       }
 

src/test/compile-fail/issue-14084.rs

@@ -0,0 +1,14 @@
+// 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.
+
+fn main() {
+    box ( () ) 0;
+    //~^ ERROR: only the managed heap and exchange heap are currently supported
+}