Associated Type Equality Bounds are not properly resolved

[Jeff-A-Martin]

Hello!

Given the following definitions (minimized from my actual use case):

trait DeviceIdContext {
    type DeviceId;
}

trait BindingsContext<E> {}

trait EventContext<E> {
    type OtherEventRepr;
}

trait CoreContext<BC>: DeviceIdContext + EventContext<Self::DeviceId> {}

struct Foo<D> {
    d: D,
}

I'd like to be able to write the following implementation:

impl<D> Foo<D> {
    fn foo<BC, CC>(self, _core_ctx: CC, _bindings_ctx: BC)
    where 
        BC: BindingsContext<CC::OtherEventRepr>,
        CC: CoreContext<BC, DeviceId = D> {}
}

Compilation fails with error:

CC: EventContext<D> is not satisfied

However, that's incorrect. CC must implement EventContext. I've bounded its associated type DeviceId to equal D. Along with the compilation error, comes the suggestion to add an explicit EventContext<D> bound. That would look like the following:

impl<D> Foo<D> {
    fn foo<BC, CC>(self, _core_ctx: CC, _bindings_ctx: BC)
    where 
        BC: BindingsContext<CC::OtherEventRepr>,
        CC: CoreContext<BC, DeviceId = D> + EventContext<D> {}
}

That still fails to compile, this time for a different reason:

ambiguous associated type OtherEventRepr in bounds of CC
| ambiguous OtherEventRepr from EventContext<D>
| ambiguous OtherEventRepr from EventContext<<CC as DeviceIdContext>::DeviceId>

Interestingly, If I rework this implementation to be a free function, everything compiles:

fn foo<
    BC: BindingsContext<CC::OtherEventRepr>,
    CC: CoreContext<BC>
>(core_ctx: CC, bindings_ctx: BC, foo: Foo<CC::DeviceId>) {}

It seems to me that the DeviceId = D bound I've written isn't able to be reasoned about properly in examples 1 & 2. Checkout this Rust Playground with all the examples from above: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=91049b46781fb054e583994d7dde5966

I observe this issue on Stable, Beta & Nightly.

[QuineDot]

If you use a third generic parameter to represent CC::OtherEventRepr, it compiles.

impl<D> Foo<D> {
    fn foo1<BC, CC, EE>(self, _core_ctx: CC, _bindings_ctx: BC)
    where
        BC: BindingsContext<EE>,
        CC: CoreContext<BC, DeviceId = D, OtherEventRepr = EE>,
    {
    }
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=10d4002ad901a56a699023c284b9e165