E0207 when type parameter's associated type is used in `Self`.

[alex-ozdemir]

Not sure if this needs to be an error or not.

Per rustc --explain E0207, impl type parameter are acceptable if they appear in the Self type of the impl. Emphasis mine:

Any type parameter or lifetime parameter of an impl must meet at least one of
the following criteria:

• it appears in the implementing type of the impl, e.g. impl<T> Foo<T>
• for a trait impl, it appears in the implemented trait, e.g.
  impl<T> SomeTrait<T> for Foo
• it is bound as an associated type, e.g. impl<T, U> SomeTrait for T where T: AnotherTrait<AssocType=U>

Rustc currently generates an error even if an associated type of the parameter appears in the Self type: playground link:

trait Field {}

trait Vector {
    type MyField: Field;
}

struct BaseFieldPrinter<F: Field>;

impl<V: Vector> BaseFieldPrinter<V::MyField> {
}

It seems like this doesn't need to be an error, but perhaps I'm missing something?

[estebank]

I think this will need to be written as impl<V: Vector, T> BaseFieldPrinter<T> where T: V::MyField {} once associated type bounds lands.

[Centril]

It seems like this doesn't need to be an error, but perhaps I'm missing something?

Seems like "appears" needs to be clarified. In your snippet it appears in a projection which in turn appears in the implementing type but that's not the same thing.

I think this will need to be written as impl<V: Vector, T> BaseFieldPrinter<T> where T: V::MyField {} once associated type bounds lands.

T: V::MyField would be bounding T by an associated trait. This is entirely unrelated to associated_type_bounds (Trait<Assoc: Bound>) which are just a short-hand.

[estebank]

Current output:

error[E0207]: the type parameter `V` is not constrained by the impl trait, self type, or predicates
 --> src/lib.rs:9:6
  |
9 | impl<V: Vector> BaseFieldPrinter<V::MyField> {
  |      ^ unconstrained type parameter