The type-checker assumes that Trait<'a>::AssociatedType may be outlived by 'a, even with "where AssociatedType: 'a"

[hiddenhare]

trait Trait<'a> {
    type Ty;
    fn method(ty_ref: &'a Self::Ty) where Self::Ty: 'a { }
}

fn caller<'a, T: Trait<'a>>(arg: &'a T::Ty) where T::Ty: 'a {
    T::method(arg)
}

Compilation fails on the playground (for stable 1.36.0 and nightly 1.38.0) with this error:

Compiling playground v0.0.1 (/playground)
error[E0309]: the associated type `<T as Trait<'_>>::Ty` may not live long enough
 --> src/lib.rs:7:9
  |
7 |         T::method(arg)
  |         ^^^^^^^^^
  |
  = help: consider adding an explicit lifetime bound `<T as Trait<'_>>::Ty: 'a`...
note: ...so that the type `<T as Trait<'_>>::Ty` will meet its required lifetime bounds
 --> src/lib.rs:7:9
  |
7 |         T::method(arg)
  |         ^^^^^^^^^

error: aborting due to previous error

Compilation succeeds if I make any of the following changes:

• Add a lifetime constraint to Ty at its definition site, type Ty: 'a;

• Move the lifetime parameter from the trait to the method, fn method<'a>(...)

Compilation fails with the same error message if I change method into a free function:

trait Trait<'a> {
    type Ty;
}

fn method<'a, T: Trait<'a>>(_arg: &'a T::Ty) where T::Ty: 'a { }

fn caller<'a, T: Trait<'a>>(arg: &'a T::Ty) where T::Ty: 'a {
    method::<T>(arg)
}

Even if this is intended behaviour, the error message could be improved.

[ocstl]

This seems to have been fixed by #87281.

[fmease]

Might or might not need a regression test depending on whether we already have a comparable test for this somewhere..

[zakrad]

@rustbot claim