lifetime-parametric GATs cause ICEs (with feature(generic_associated_types))

[pnkfelix]

Many people playing with #[feature(generic_associated_types)] have noticed that they break very easily when you attempt to use a lifetime formal parameter.

Examples follow

ICE: Region parameter out of range

#62326 (play):

#![feature(generic_associated_types)]
trait Iterator {
    type Item<'a>: 'a;
}

#58694 (play)

#![feature(generic_associated_types)]
trait Foo {
  type PublicKey<'a> : From<&'a [u8]>;     
}

#49362 (play)

#![feature(generic_associated_types)]

trait Trait {
    type Associated<'a>;
}

impl Trait for () {
    type Associated<'a> = &'a ();
}

ICE: cannot convert ReEarlyBound(0, 'b) to a region vid

#60654 (play)

#![feature(generic_associated_types)]
struct Foo;

impl Iterator for Foo {
    type Item<'b> = &'b Foo;

    fn next(&mut self) -> Option<Self::Item> {
        None
    }
}

---

This obviously blocks stabilization of GATs (#44265).

[pnkfelix]

(also copying labels from the issues in the description)

[estebank]

CC #56445, seemingly same underlying issue, has longer thread.

[hellow554]

Not only an ICE will occur if you put Self::Item in the return type, but also as a parameter (take from #66734):

#![feature(generic_associated_types)]

trait Foo {
    type Bar<'a>;
    fn baz<'a>(_: Self::Bar) ;
}

impl Foo for () {
    type Bar<'a> = &'a ();
    fn baz<'a>(_: Self::Bar) {}
}

fn main () {}