Coherence issue when using crate-local traits with type parameters

[ghananigans]

When using crate-local traits with type parameters, it seems rust incorrectly thinks that downstream crates may implement the trait (which we know to be impossible as the traits are crate-local).

See this playground.

Also captured here for readability:

pub(crate) struct DummyCtx {}

pub(crate) trait FooContext<C> {}

trait LocalTrait {}

pub(crate) trait BarContext<C>: LocalTrait {}

impl<C> FooContext<C> for DummyCtx {}

// Doesn't work:
impl<C: BarContext<F>, F> FooContext<F> for C {}

// Works:
// impl<C: BarContext<F> + LocalTrait, F> FooContext<F> for C {}

Expected both impls to compile but only the impl that restricts C to LocalTrait compiles even though BarContext<F> is a supertrait over LocalTrait which (IIUC) makes the LocalTrait restriction in C: BarContext<F> + LocalTrait redundant.

Meta

Can observe failure on 2021 edition stable (see rust playground link)

This playground is another example of this but without a LocalTrait.

Note that using crate-local traits without any type parameters work just fine playground.

[ghananigans]

cc: @brunowonka @zeling