Associated types aren't normalized with impl cycles

[strega-nil]

trait Doop {
    type Shaboom;
    fn badoowop(self) -> Self::Shaboom;
}
trait BaDoop: Doop { }

impl<T: BaDoop> Doop for T {
    type Shaboom = T;
    fn badoowop(self) -> Self {
        self
    }
}

fn main() { }

errors with

error: method `badoowop` has an incompatible type for trait: 
 expected associated type,
 found type parameter

playground

A similar issue happens when you now return Self::Shaboom:

error: mismatched types: expected `<T as Doop>::Shaboom`,
    found `T`

playground

[strega-nil]

Honestly, it probably just needs a better error message.

[eddyb]

This seems related to #29859, but not... actually working?
AFAICT, <T as Doop> in the impl<T: Doop> of Doop for T cannot refer to the same impl.

cc @nikomatsakis

[strega-nil]

@eddyb looks like the forest may have fixed this? #30533

[oli-obk]

The error message seems reasonable to me now as we get

error[E0053]: method `badoowop` has an incompatible type for trait
 --> src/main.rs:9:26
  |
3 |     fn badoowop(self) -> Self::Shaboom;
  |                          ------------- type in trait
...
9 |     fn badoowop(self) -> Self {
  |                          ^^^^ expected associated type, found type parameter
  |
  = note: expected type `fn(T) -> <T as Doop>::Shaboom`
             found type `fn(T) -> T`
  = note: you might be missing a type parameter or trait bound

Please reopen if you disagree