[swift-evolution] ValueEnumerable protocol with derived implementation for enums

[Brent Royal-Gordon]

> To me, it seems that a "magic protocol" with no public requirements, producing an internal (non-resilient) static array, is a reasonable solution that might be able to fit into the Swift 3 timeframe, and I think that'd be a huge boon to language users. I also don't think it precludes further improvements.

On the other hand, since resilience has been deferred, this also weakens the case for not using a tight type now, like:

	protocol ValuesEnumerable {
		static var allValues: [Self]
	}

Or:

	protocol ValuesEnumerable {
		static var allValues: AnyForwardCollection<Self>
	}

And then loosening the requirement once we have the generic features we need to do it. The pull request for `where` clauses on associated types has already been submitted, so if we're lucky, we might even get it by Swift 3. <https://github.com/apple/swift-evolution/pull/284>

> Do others agree? Disagree? Is it definitely out of scope for Swift 3 regardless of the approach we choose?

I stopped thinking about this because I assumed it was now out of scope, but that is a mere assumption. If the core team thinks they can fit it in, I'm all for it.

-- 
Brent Royal-Gordon
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