Conflicting implementation should not exist after generic trait paramters have been constrained to associated types

[ZhennanWu]

Consider this code. Currently it cannot compile.

pub trait Foo {}

pub trait ImplBy {
    type Impl;
}

pub trait FooBy<I> {}

impl<T> Foo for T
where
    T: ImplBy,
    T: FooBy<T::Impl>,
{
}

struct Bar;
impl ImplBy for Bar {
    type Impl = ();
}
impl Foo for Bar {}
// conflicting implementations of trait `Foo` for type `Bar`
// downstream crates may implement trait `FooBy<_>` for type `Bar`

The type solver assumed downstream can impl FooBy<Bar::Impl> for type Bar. But this is easy to reject with orphan rules. Bar::Impl must be in the same scope as Bar itself. Therefore FooBy<Bar::Impl> cannot be implemented on Bar outside its origin crate.