[swift-evolution] [Draft] Introducing StaticSelf, an Invariant Self

[Joe Groff]

> On May 13, 2016, at 8:18 AM, Matthew Johnson <matthew at anandabits.com> wrote:
> 
> When I write a class Base with non-final methods that return instances of Base I can choose whether to state the return type as Self (covariant) or Base (invariant, under this proposal StaticSelf would also be an alternative way to state this).  If I choose to specify Base as the return type derived classes *may* override the method but are not required to.  Further, if they *do* override the method they are allowed to choose whether their implementation returns Base or Derived.

`StaticSelf` requirements by themselves don't even save you from covariance. If Base conforms to a protocol (with Self == Base), Derived inherits that conformance and also conforms (with Self == Derived). If `StaticSelf` always refers to the conforming type, then it must also be bindable to Base and Derived, so a base class must still use a covariant-returning method to satisfy the `StaticSelf` requirement.

-Joe