Sealed protocols

[xwu (Xiaodi Wu)]

@Karl

For example, if we extended the same rules regarding @_frozen to sealed protocols

What do you mean - like promising publicly that the set of conforming types is frozen forever?

@anandabits

I have a couple of places in my code that use the closed protocol hack and switch over the types with a fatalError in the default case. I would really like to get rid of the default case and get a compiler warning if the switch gets out of sync with the rest of the code. Would you by any chance be willing to consider pursuing this after you are finished with the current proposal?

I don't mean to suggest that sealed should imply @_frozen.

I'm pointing out that sealed, on its own, would not permit you to get rid of the default case when switching over, say, StringProtocol without a guarantee that the protocol is not just sealed but frozen, which would require extending @_frozen (an orthogonal feature) to protocols.