Trouble unifying function types involving HRTB

[Manishearth]

Spawned off of #84937 (comment)

Code example

trait MiniYokeable<'a> {
    type Output;
}

struct MiniYoke<Y: for<'a> MiniYokeable<'a>> {
    pub yokeable: Y,
}

fn map_project_broken<Y, P>(
    source: MiniYoke<Y>,
    f: impl for<'a> FnOnce(
        <Y as MiniYokeable<'a>>::Output,
        core::marker::PhantomData<&'a ()>,
    ) -> <P as MiniYokeable<'a>>::Output,
) -> MiniYoke<P>
where
    Y: for<'a> MiniYokeable<'a>,
    P: for<'a> MiniYokeable<'a>
{
    unimplemented!()
}

struct Bar<'a> {
    string_1: &'a str,
    string_2: &'a str,
}

impl<'a> MiniYokeable<'a> for Bar<'static> {
    type Output = Bar<'a>;
}

impl<'a> MiniYokeable<'a> for &'static str {
    type Output = &'a str;
}

fn demo_broken(bar: MiniYoke<Bar<'static>>) -> MiniYoke<&'static str> {
    map_project_broken(bar, |bar, _| bar.string_1)
}

(playpen)
throws an error of

51 |     map_project_broken(bar, |bar, _| bar.string_1)
   |     ^^^^^^^^^^^^^^^^^^      --------------------- found signature of `for<'r> fn(Bar<'r>, PhantomData<&'r ()>) -> _`
   |     |
   |     expected signature of `for<'a> fn(<Bar<'_> as MiniYokeable<'a>>::Output, PhantomData<&'a ()>) -> _`

This error is incorrect: <Bar<'_> as MiniYokeable<'a>>::Output is the same thing as Bar<'a>.

cc @sffc @jackh726

[jackh726]

I wonder if #88441 will fix this.

[Manishearth]

@jackh726 it changes the error to

error[E0271]: type mismatch resolving `for<'a> <&str as MiniYokeable<'_>>::Output == <&str as MiniYokeable<'a>>::Output`
  --> test.rs:51:5
   |
51 |     map_project_broken(bar, |bar, _| bar.string_1)
   |     ^^^^^^^^^^^^^^^^^^ expected associated type, found `&str`
   |
   = note: expected associated type `<&str as MiniYokeable<'a>>::Output`
                    found reference `&str`
   = help: consider constraining the associated type `<&str as MiniYokeable<'a>>::Output` to `&str` or calling a method that returns `<&str as MiniYokeable<'a>>::Output`
   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
note: required by a bound in `map_project_broken`
  --> test.rs:14:10
   |
9  | fn map_project_broken<Y, P>(
   |    ------------------ required by a bound in this
...
14 |     ) -> <P as MiniYokeable<'a>>::Output,
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `map_project_broken`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0271`.

[jackh726]

Hmm

[aliemjay]

Seems to be the same as #90875; adding type annotations fixes the problem (playground).

@Manishearth can you confirm this? You may want to close this in favor of #90875 which contains a minimal example otherwise I'm happy to close mine as a duplicate.

[Manishearth]

@aliemjay Ah, I see. Thanks!