[Proposal:
https://github.com/apple/swift-evolution/blob/master/proposals/0194-derived-collection-of-enum-cases.md]
I think this is generally reasonable, and none of the names offend me enough to
weigh in on that discussion. I do think it's a little weird that @objc enums
defined in Swift cannot conform to ValueEnumerable, just because imported enums
won't. (But especially while knee-deep in SE-0192, I think it's correct that
imported enums won't. The exception could be C enums marked
`enum_extensibility(closed)`, but I'm not convinced we need that yet.)
The biggest problem I have is unavailable cases. An unavailable case must not
be instantiated—consider an enum where some cases are only available on iOS and
not macOS. (I bet we optimize based on this, which makes it all the more
important to get right.)
I think you should explicitly call out that the derived implementation only
kicks in when ValueEnumerable is declared on the enum itself, not an extension.
Or if that's not the case, it should be limited to extensions in the same
module as the enum. (You could add "unless the enum is '@frozen'", but that's
not really necessary.)
I don't think this should be implemented with a run-time function; compile-time
code generation makes more sense to me. But that's an implementation detail; it
doesn't change the language surface.
Jordan
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