Merging impls to be exhaustive

[lcnr]

What is this

This is a design document for const generics. Any discussions about its content should be on zulip. The conclusions of these discussions should then be edited back into this issue. Please do not post any comments directly in this issue.

Content

Whether we want to be able to unify different partial impls to consider them exhaustive.

trait ForBool<const B: bool> {}

impl ForBool<true> for u8 {}
impl ForBool<false> for u8 {}
// Does `for<const B: bool> u8: ForBool<B>` hold?