Subtyping in associated types

[CrystDragon (Lincoln Wu)]

You cannot imply covariance for generic parameters by default:

Generic Covariance to improve API Usage

It's a useful feature but also really really hard. For value types with a covariant generic parameter, the upcast might involve a representation change (consider going from Optional<Int> to Optional<Any>). For reference types with a covariant generic parameter, you don't even have a chance to change the representation (since you need to preserve reference semantics), which rules out certain kinds of covariance. For non-ObjC generic reference types, the generic arguments are included in …