Bad diagnostics for GATs in trait paths

[jackh726]

The diagnostics introduced by #77524 are wrong in some cases, e.g. in gat-trait-path-missing-lifetime we get the following stderr:

error[E0107]: missing generics for associated type `X::Y`
 --> src/test/ui/generic-associated-types/gat-trait-path-missing-lifetime.rs:5:8
  |
5 |   type Y<'a>;
  |        ^ expected 1 lifetime argument
  |
note: associated type defined here, with 1 lifetime parameter: `'a`
 --> src/test/ui/generic-associated-types/gat-trait-path-missing-lifetime.rs:5:8
  |
5 |   type Y<'a>;
  |        ^ --
help: use angle brackets to add missing lifetime argument
  |
5 |   type Y<'a><'a>;
  |         ^^^^

error: aborting due to previous error; 1 warning emitted

Originally posted by @b-naber in #79554 (comment)

[henryboisdequin]

The fix to this would be pointing to fn foo<'a, T1: X<Y = T1>>(t : T1) -> T1::Y<'a> and emmiting the error there right?

[b-naber]

The fix to this would be pointing to fn foo<'a, T1: X<Y = T1>>(t : T1) -> T1::Y<'a> and emmiting the error there right?

Yes.

[estebank]

Another case:

Given:

#![feature(generic_associated_types)]

// The cyclic dependency between trait A and B compiles as expected
trait A {
    type BType: B<AType = Self>;
}

trait B {
    type AType: A<BType = Self>;
}

// rustc crashes on the generic cyclic dependency between traits C and D
trait C {
    type DType<T>: D<T, CType = Self>;
}
trait D<T> {
    type CType: C<DType = Self>;
}

we currently emit

error[E0107]: missing generics for associated type `C::DType`
  --> src/lib.rs:14:10
   |
14 |     type DType<T>: D<T, CType = Self>;
   |          ^^^^^ expected 1 type argument
   |
note: associated type defined here, with 1 type parameter: `T`
  --> src/lib.rs:14:10
   |
14 |     type DType<T>: D<T, CType = Self>;
   |          ^^^^^ -
help: use angle brackets to add missing type argument
   |
14 |     type DType<T><T>: D<T, CType = Self>;
   |               ^^^

where the correct suggestion would be

error[E0107]: missing generics for associated type `C::DType`
  --> src/lib.rs:14:10
   |
17 |     type CType: C<DType = Self>;
   |                   ^^^^^ expected 1 type argument
   |
note: associated type defined here, with 1 type parameter: `T`
  --> src/lib.rs:14:10
   |
14 |     type DType<T>: D<T, CType = Self>;
   |          ^^^^^ -
help: use angle brackets to add missing type argument
   |
17 |     type CType: C<DType<T> = Self>;
   |                        ^^^

CC #81712