Associated item projections across crate boundaries slightly broken

[ebfull]

pub trait Foo<T> { }
pub trait Mirror {
        type Dual;
}

pub struct Eps;

pub struct A<T, U>(T, U);
impl<T, U: Mirror> Mirror for A<T, U> { type Dual = B<T, U::Dual>; }
pub struct B<T, U>(T, U);
impl<T, U: Mirror> Mirror for B<T, U> { type Dual = A<T, U::Dual>; }

impl Mirror for Eps { type Dual = Eps; }

extern crate whatever;
use whatever::*;

struct Dummy;

impl Foo<A<Eps, B<Eps, Eps>>> for Dummy {}

// doesn't work, causes "conflicting implementations for trait `whatever::Foo`"
// impl Foo<<A<Eps, B<Eps, Eps>> as Mirror>::Dual> for Dummy {}

// works just fine
// impl Foo<B<Eps, A<Eps, Eps>>> for Dummy {}

This despite the fact that <A<Eps, B<Eps, Eps>> as Mirror>::Dual and B<Eps, A<Eps, Eps>> are the same type. It works just fine when they're in the same crate.

[brson]

Needs repro.

[brson]

Is this a bug? P-?

[nikomatsakis]

I agree this is a bug. I think it falls under the general category of "failure to normalize" that I expect to basically go away as part of the "chalk overhaul" (rust-lang/rust-roadmap-2017#8).

[nikomatsakis]

triage: P-medium