[swift-evolution] Proposal: Sealed protocols

[Anandabits]

> 
> There are cases when the library isn't designed to support new conformances for the protocol, but the protocol is public because other public APIs are expressed in terms of it.
> 
> We have a case for it in the standard library, 'protocol AnyCollectionType'.  Foundation also has a use case -- property list types.

I have had cases for this as well.  Joe Groff's suggestion of adding more convenient syntax for sum types as an alternative may be an acceptable alternative for the cases I have seen, although I would need to have a better idea of what that might look like.  Joe, have you written a proposal for this?

Matthew