Generic type in a protocol

[Danny]

What's the state of higher kind types?

I think the feature you're looking for is generic associated types. Rust has this: 1598-generic_associated_types - The Rust RFC Book Swift allowing this some day is not outside the realm of possibility, but it's not completely trivial (the Rust folks spent several years designing and implementing this, AFAIK). It would require an overhaul of how the requirement machine builds a rewrite system from requirements, which currently makes a pretty fundamental assumption that associated types are una…