internal compiler error: cat_expr Errd when downcasting

``` rust
#![feature(box_syntax)]
use std::any::Any;
fn main()
{
    fn bar(x:i32) ->i32 { 3*x };
    let b:Box<Any> = box {bar as fn(_)->_};
    b.downcast_ref::<fn(_)->_>();
}
```
