Proposal to add kind polymorphism to the language

Scala Improvement Process
15

I think the problem for AnyKind is that we did not define the meaning of Nothing properly.

Current implementation of type inference in Scala compiler 2.x internally use Nothing for any kind, which sometimes cause bugs.

We should distinguish Nothing (bottom type for any types that are child types of Any) from NothingKind (bottom type for any types that are child types of AnyKind).

So trait Bar[A <: AnyKind] is a shortcut of trait Bar[A >: NothingKind <: AnyKind].