Never types and inference

[canndrew]

@kennytm

!: Default isn’t the big issue if we disregard the internals of default() . A more fundamental problem is that you can’t simultaneously support

!: Iterator<Item = u8>,
!: Iterator<Item = u16>,

Allowing this breaks the assumption in the type system which the associated type is a one-to-one relationship, as mentioned in Never types and inference and also Draft RFC: default types for traits.

(Note: I don’t think default trait = type is the correct answer to this problem)

Well the other idea mentioned in that thread is to allow trait impls to be generic in their associate types. eg. allow impls like this:

// T not used on Future or !, just on the associated type.
impl<T> Future for ! {
    type Output = T;
    ...
}

Which would make <! as Future>::Output a type variable which can unify with anything. I don't know if this is sound though.