Introduce an "ExpressibleByCase" protocol (to facilitate struct-based enums with payload)

[Charles_Constant]

@Ben_Cohen

can you expand a bit on what you mean by this?

Sure, by "disagreement over how enum payloads should work" I was thinking about various proposals to deal with if case let (caution: vulgar language). I don't have my finger on the Evolution pulse, though, so apologies if I mischaracterized the situation.

@Ben_Cohen

What are the problems you are trying to solve, that you believe Swift programmers face today, that justify adding new syntax to the core language? Can they be solved today in other ways, and how are those ways deficient?

There are, specifically, two ends:

1. Get at an enum payload with greater ease and flexibility than using a "switch" statement, or "if case let" statement. Afaic, let payload: (Int,Double) = mycase() would be a pretty welcome replacement.

2. Permit each enum case to be compared with or without arguments. Eg:

.foo      ==  .foo(123) // true
.foo(123) ==  .foo(456) // true
.bar      ==  .foo(123) // false

.foo(123) === .foo(123) // true
.foo(123) === .foo(456) // false

I probably should have led with that information, hey? These two problems have been my top two annoyances in Swift for the longest time. Since I don't see existing Swift enums ever adopting these solutions to them (source-breaking, and perhaps just my impression, previous proposals seem not ot go anywhere), I figure maybe we could just go with the building blocks to let people build their own?