Enum cases as protocol witnesses

[Jumhyn (Frederick Kellison-Linn)]

@xwu

But I would argue that the best solution to this problem would be a way to designate certain static functions (or at least their requirements) as factories.

As a hypothetical, this could be a fully-fledged @implements feature which simply allows case statements with associated values to implement init requirements and static func requirements that return Self, without allowing bare case statements (without @implements to satisfy those static func requirements.