Wrong `where Self: 'a` GAT bound suggested

[vandenheuvel]

Relates to #89970 and potentially to #90808.

#![feature(generic_associated_types)]

trait MyTrait {
    type AT<'a> where Self: 'a;
    
    fn selfreffing<'a>(&'a self) -> Self::AT<'a> { unimplemented!(); }
}

struct Inner;
impl MyTrait for Inner {
    type AT<'b> where Self: 'b = ();
}

struct Outer<'a, T>(&'a T);
impl<'a, T: MyTrait> MyTrait for Outer<'a, T> {
    type AT<'b> = T::AT<'b>;
    // type AT<'b> where T: 'b = T::AT<'b>; // suggested
    // type AT<'b> where Self: 'b = T::AT<'b>; // compiles
}

Playground

[vandenheuvel]

When switching to type AT<'b> = T::AT<'a>, the correct error is given, but the hint is not provided.

#![feature(generic_associated_types)]

trait MyTrait {
    type AT<'a> where Self: 'a;
    
    fn selfreffing<'a>(&'a self) -> Self::AT<'a> { unimplemented!(); }
}

struct Inner;
impl MyTrait for Inner {
    type AT<'b> where Self: 'b = ();
}

struct Outer<'a, T>(&'a T);
impl<'a, T: MyTrait> MyTrait for Outer<'a, T> {
    type AT<'b> = T::AT<'a>;
    // Should recommend type AT<'b> where Self: 'b = T::AT<'a>;
}

error[E0477]: the type `Outer<'a, T>` does not fulfill the required lifetime
  --> src/lib.rs:16:5
   |
16 |     type AT<'b> = T::AT<'a>;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: type must outlive the lifetime `'b` as defined here
  --> src/lib.rs:16:13
   |
16 |     type AT<'b> = T::AT<'a>;
   |             ^^

For more information about this error, try `rustc --explain E0477`.

Playground

[jackh726]

GATs issue triage: not blocking. I don't like that this diagnostic is just wrong. The "easy" fix here would be to disable the "consider adding a where clause" help if that where clause isn't on the trait, but a proper fix would be able to suggest Self: 'a. Either way, this is just poor diagnostics and doesn't miscompile.

[Dylan-DPC]

Current output:

error[[E0309]](https://doc.rust-lang.org/nightly/error_codes/E0309.html): the parameter type `T` may not live long enough
  --> src/lib.rs:16:19
   |
16 |     type AT<'b> = T::AT<'b>;
   |                   ^^^^^^^^^- help: consider adding a where clause: `where T: 'b`
   |                   |
   |                   ...so that the type `T` will meet its required lifetime bounds...
   |
note: ...that is required by this bound
  --> src/lib.rs:4:29
   |
4  |     type AT<'a> where Self: 'a;
   |                             ^^

For more information about this error, try `rustc --explain E0309`.