GAT: `Self: 'a` bound cannot be added for types which store a generic T that is further constrained by the `impl`

[kahomayo]

Within an impl<T: Trait1> Trait2 for Struct<T::Foo>, a GAT bound of Self: 'a does not compile, asking me to add an explicit T::Foo: 'a bound.

Playground

#![feature(generic_associated_types)]
#![allow(non_camel_case_types)]

trait HasAssoc {
    type Assoc;
}

trait Iterate<S: HasAssoc> {
    type Iter<'a> where Self: 'a;
    fn iter<'a>(&'a self) -> Self::Iter<'a>;
}

struct KeySegment_Broken<T>{ key: T }
impl<S: HasAssoc> Iterate<S> for KeySegment_Broken<S::Assoc> {
    type Iter<'a> where Self: 'a = ();
    fn iter<'a>(&'a self) -> Self::Iter<'a> {
        todo!()
    }
}

This code results in two errors:

   Compiling playground v0.0.1 (/playground)
error: `impl` associated type signature for `Iter` doesn't match `trait` associated type signature
  --> src/lib.rs:17:5
   |
9  |     type Iter<'a> where Self: 'a;
   |     ----------------------------- expected
...
17 |     type Iter<'a> where Self: 'a = ();
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found

error[E0309]: the associated type `<S as HasAssoc>::Assoc` may not live long enough
  --> src/lib.rs:17:36
   |
17 |     type Iter<'a> where Self: 'a = ();
   |                                 -  ^^ ...so that the type `KeySegment_Broken<<S as HasAssoc>::Assoc>` will meet its required lifetime bounds
   |                                 |
   |                                 help: consider adding a where clause: `, <S as HasAssoc>::Assoc: 'a`

I believe that the E0309 error is a bug, as since key is known in this impl to be S::String, S::String must outlive Self and the suggested bound should thus already be implied by Self: 'a. Applying the suggested fix removes this error, but obviously cannot compile (as the GAT impl now has a stricter where than in the trait declaration). The first error seems incorrect too, as there is no visible difference between the highlighted lines other than = ():

struct KeySegment_SuggestedFix<T>{ key: T }
impl<S: HasAssoc> Iterate<S> for KeySegment_SuggestedFix<S::Assoc> {
    // error: `impl` associated type signature for `Iter` doesn't match `trait` associated type signature
    type Iter<'a>
    where
        Self: 'a,
        S::Assoc: 'a
    = ();
    
    fn iter<'a>(&'a self) -> Self::Iter<'a> {
        todo!()
    }
}

Adding the type bound on the struct declaration avoids these errors, but is less generic.

struct KeySegment<S: OnceTypeSystem> { key: S::String }
impl<S: OnceTypeSystem> PathSegment<S> for KeySegment<S> {
    type Iter<'a> where Self: 'a = ();
    fn iter<'a>(&'a self) -> Self::Iter<'a> {
        todo!()
    }
}

Meta

rustc --version --verbose:

rustc 1.59.0-nightly (475b00aa4 2021-12-24)
binary: rustc
commit-hash: 475b00aa4037461b506539a06d15ca6091b461a7
commit-date: 2021-12-24
host: x86_64-pc-windows-msvc
release: 1.59.0-nightly
LLVM version: 13.0.0

I have also encountered this bug on 1.59.0-nightly (e100ec5bc 2021-12-21).

[BGR360]

@rustbot label +F-generic_associated_types +T-compiler

[jackh726]

It's both rude and unconstructive to suggest that issues are not tracked. First and foremost, this is an unstable feature. Second, this issue was filed 15 days ago, right in the middle of holidays. Third, its labeled appropriately, and I'll either triage or fix it when I can.

I'm going to delete the above comment since it's not constructive at all.

[pro465]

@jackh726 I forgot what I said before, but if I said something bad, I'm really sorry for that 😞