Missing implied lifetime bounds for method with GAT in return position

[Pointerbender]

The following example fails to compile:

pub trait Foo<'foo>: 'foo {
    type Bar<'bar>
    where
        'foo: 'bar;

    fn baz(&self) -> Self::Bar<'_>;
}

impl<'foo, T, U> Foo<'foo> for T
where
    T: Deref<Target = U> + 'foo,
    U: Foo<'foo>,
{
    type Bar<'bar> = <U as Foo<'foo>>::Bar<'bar>
    where
        'foo: 'bar;

    fn baz(&self) -> Self::Bar<'_> {
        (**self).baz()
    }
}

The error message is:

error[E0478]: lifetime bound not satisfied
  --> examples/implicit.rs:69:5
   |
69 | /     fn baz(&self) -> Self::Bar<'_> {
70 | |         (**self).baz()
71 | |     }
   | |_____^
   |
note: lifetime parameter instantiated with the lifetime `'foo` as defined here
  --> examples/implicit.rs:60:6
   |
60 | impl<'foo, T, U> Foo<'foo> for T
   |      ^^^^
note: but lifetime parameter must outlive the anonymous lifetime defined here
  --> examples/implicit.rs:69:12
   |
69 |     fn baz(&self) -> Self::Bar<'_> {
   |            ^^^^^

For more information about this error, try `rustc --explain E0478`.

This error message is a bit surprising, because the lifetime bounds are specified on the generic associated type: type Bar<'bar> where 'foo: 'bar; . I had expected that Rust would be able to automatically infer these bounds from the GAT declaration instead of giving an error. By using the information that the GAT is used in the return position, these implicit lifetime bounds could be automatically applied.

If we explicitly spell out all the implicit bounds on `fn baz`, then it does compile:

pub trait Foo<'foo>: 'foo {
    type Bar<'bar>
    where
        'foo: 'bar;

    fn baz<'bar>(&'bar self) -> Self::Bar<'bar>
    where
        'foo: 'bar;
}

impl<'foo, T, U> Foo<'foo> for T
where
    T: Deref<Target = U> + 'foo,
    U: Foo<'foo>,
{
    type Bar<'bar> = <U as Foo<'foo>>::Bar<'bar>
    where
        'foo: 'bar;

    fn baz<'bar>(&'bar self) -> Self::Bar<'bar>
    where
        'foo: 'bar,
    {
        (**self).baz()
    }
}

p.s. I am on this Rust version

rustc 1.66.0-nightly (0ca356586 2022-10-06)
binary: rustc
commit-hash: 0ca356586fed56002b10920fd21ddf6fb12de797
commit-date: 2022-10-06
host: x86_64-unknown-linux-gnu
release: 1.66.0-nightly
LLVM version: 15.0.2

[Rageking8]

@rustbot label +T-types +C-bug +F-generic_associated_types