<T as Trait>::X::Y fails to parse

``` rust
trait Foo {
    type X;
}
trait Bar {
    type Y;
}

fn foo<T>(_: <T as Foo>::X::Y) 
    where T: Foo,
          T::X: Bar,
{}
```

```
<anon>:8:27: 8:29 error: expected one of `+` or `,`, found `::`
<anon>:8 fn foo<T>(_: <T as Foo>::X::Y) 
                                   ^~
```
