breakage due to incompletely constraining gat args

might be the cause behind the breakage in `deptypes`, but I am not sure about that

```rust
trait Trait {
    type Assoc<U>;
}

impl<T> Trait for T {
    type Assoc<U> = U;
}

fn ret<T: Trait, U>(x: U) -> <T as Trait>::Assoc<U> {
    loop {}
}

fn foo<T: Trait<Assoc<u32> = u32>, U>() {
    let inf = Default::default();
    let x = ret::<T, _>(inf);
    let _: i32 = inf;
}
```