Cannot call a unboxed-closure trait object with () syntax

``` rust
#![feature(unboxed_closures)]


fn main() {
    let f: &Fn<(), ()> = &|&:| ();

    f()
}
```

```
<anon>:7:5: 7:8 error: expected function, found `&core::ops::Fn<(),()>`
<anon>:7     f()
             ^~~
```
