non-defining use in defining scope: projection goal with nested item bound

[lcnr]

// passes with old impl, ambiguous with the new one.
#![feature(type_alias_impl_trait)]
trait Trait {
    type Assoc;
}
impl<T: Copy> Trait for T {
    type Assoc = T;
}
fn needs_trait<T: Trait<Assoc = U>, U>() {}

type Tait = impl Copy;

fn define() -> Tait {}

fn check() {
    needs_trait::<Tait, Tait>();
    //[new]~^ ERROR type annotations needed
    //[new]~| NOTE cannot satisfy `<Tait as Trait>::Assoc == Tait`
}

fn main() {}

there are two candidates to prove <Tait as Trait>::Assoc = Tait:

• impl candidate
• ambig self infer type from trying to define the opaque

because it's a projection goal, we try to constrain the inference variable of the term, so we're unable to make progress and end up with ambiguity.

[lcnr]

resolved by rust-lang/rust#117861, needs test