we don't use item bounds of opaques when resolving associated item

[lcnr]

#![feature(type_alias_impl_trait)]
trait Trait {
    type Assoc;
    fn take_me(self);
}
impl<T> Trait for T {
    type Assoc = ();
    fn take_me(self) {}
}

type Tait = impl Trait;
#[define_opaque(Tait)]
fn bar() -> Tait::Assoc { // ERROR: this is ambig
    let _: Tait = ();
}