Associated item projections across crate boundaries slightly broken

d123843
Opened by ebfull at 2024-09-03 08:57:45
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.

  1. 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.

    León Orell Valerian Liehr at 2024-09-03 08:55:39

  2. Needs repro.

    Brian Anderson at 2017-03-23 16:53:39

  3. Is this a bug? P-?

    Brian Anderson at 2017-03-23 16:54:09

  4. 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" (https://github.com/rust-lang/rust-roadmap/issues/8).

    Niko Matsakis at 2017-04-28 20:55:14

  5. triage: P-medium

    Niko Matsakis at 2017-04-28 20:55:25

  6. This bug report is still correct with rustc 1.44.1. I am commenting because the contribution guidelines say it is helpful to check whether old bug reports are still correct.

    Valentin at 2020-06-19 10:29:31