Trait aliases considers same type as not equal to itself if used in impl Trait return type

Example:

#![feature(trait_alias)]
#![feature(existential_type)]

trait Foo {
    type Bar: Baz<Self, Self>;

    fn bar(&self) -> Self::Bar;
}

struct X;

impl Foo for X {
    existential type Bar: Baz<Self, Self>;

    fn bar(&self) -> Self::Bar {
        |x: &X| x
    }
}

trait Baz<A, B> = Fn(&A) -> &B;

Workaround: Replace existential type Bar: Baz<Self, Self>; with existential type Bar: Fn(&X) -> &X;
Possibly related to #57611
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Reduced example

#![feature(trait_alias)]

struct X;

fn bar() -> impl Baz<X, X> {
    |x: &X| x
}

trait Baz<A, B> = Fn(&A) -> &B;

Not a bug anymore, Oli's example compiles now (with fixing some lifetime issues):

#![feature(trait_alias)]
#![feature(closure_lifetime_binder)]

struct X;

fn bar() -> impl Baz<X, X> {
    for<'a> |x: &'a X| -> &'a X { x }
}

trait Baz<A, B> = Fn(&A) -> &B;

Interestingly that moving HRTB into trait alias doesn't solve the lifetimes issue

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Trait aliases considers same type as not equal to itself if used in `impl Trait` return type #57612

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Trait aliases considers same type as not equal to itself if used in impl Trait return type #57612

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oli-obk commented on Mar 18, 2019

plazmoid commented on Aug 31, 2022

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Trait aliases considers same type as not equal to itself if used in `impl Trait` return type #57612