[swift-evolution] [Draft] Change @noreturn to unconstructible return type

[Pyry Jahkola]

>>> I would prefer the type to be simple, and be implemented as a case-less enum (not a bottom value, as in Haskell).
>>> 
>>> None should be a usual enum, with no compiler magic except that functions returning None are equivalent to current @noreturn.
>> 
>> I think it would be more useful if the compiler allowed `Never` in every type context (i.e. whatever type `T` was expected, an expression of type `Never` would be allowed), making expressions like the following compile:
>> 
>>     let unwrapped: Int = optional ?? fatalError("explanation why this must not happen")
>> 
>> — Pyry
> 
> I dunno, I think @noreturn is clearer than any of these. It tells you that the function… won’t return. None, Never, etc. could be mistaken as a synonym for Void, whereas @noreturn is pretty hard to miss.

FWIW, in the rejection of SE-0097 <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160530/019879.html>, this was what the core team had to say about it:

1) For noreturn, the core team prefers to explore a solution where a function can be declared as returning an non-constructable “bottom” type (e.g. an enum with zero cases).  This would lead to something like:

	func abort() -> NoReturn { … }

This will require some new support in the compiler, but should flow better through the type system than @noreturn in function composition and other applications.  Joe Groff offered to write a proposal for this.

— Pyry

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