Annoying "where" clauses are required

[apt1002]

When writing something generic over <I: Index, J: Index> it is true that (I, J): Index but the type checker doesn't know that. The problem is actually with the Flatten trait (a super-trait of Index). The work-around is to add where clauses (I, J): Flatten, (I::Size, J::Size): Flatten.

This is annoying. It seems to be a limitation of Rust, which might go away in future. We would like to write

trait Flattenable: for<F: Flat> Flatten<F> {}

impl<T: for<F: Flat> Flatten<F>> Flattenable for T {}

and then use Flattenable instead of Flatten as the super-trait of Index. However, that's illegal.

[apt1002]

Ooh! I've found another approach: remove the supertrait Flatten from Index and Size. I wasn't relying on those bounds anyway.