Adding a equality constraint on an associated type can break the checks

[tomaka]

The following code works fine:

trait Upgr<S> {}

trait Proto {
    type S;
    type Upgr: Upgr<Self::S>;
}

struct Wrapper<P>(P);

impl<P: Proto> Proto for Wrapper<P> {
    type S = P::S;
    type Upgr = P::Upgr;
}

However if you change the last few lines for this:

impl<X, P: Proto<S = X>> Proto for Wrapper<P> {
    type S = P::S;
    type Upgr = P::Upgr;
}

Then you get:

error[E0277]: the trait bound <P as Proto>::Upgr: Upgr<X> is not satisfied

(link to playground)

Using type S = X; also doesn't work.

Rust version: nightly.
Apologize if an issue already exists. The vocabulary for this kind of problems is kind of tricky, which makes it hard to search for existing issues.

[Globidev]

The code from the playground seems to compile fine since rust 1.49.0
I can't pinpoint to the exact PR that solved it but this issue seems to be fixed 🙂

[tomaka]

Thanks!