Path to associated type via a struct's type parameter

```
#![feature(associated_types)]

trait Foo {
    type A;
}

struct B<X: Foo<A=int>>{
    x: X::A,
}

fn main() {
    let b = B { x: 42i };
    let _: int = b.x;
}
```

The struct definition is OK, but the struct initialiser fails to type check (which I think it should). We don't get as far as the second assignment.
