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

[Brent Royal-Gordon]

> Bringing up an old idea, we could rewrite `rethrows` using Never and throws type specification:
> 
> func call<T, E>(block: () throws E -> T) throws E -> T
> 
> But this requires some compiler magic: non-throwing functions should effectively become functions throwing Never.

This is very clever, and a very natural way to design things *if* Never is a proper bottom type, instead of just an empty enum. I don't even think you can really describe this as "compiler magic": Just as a function with no return type implicitly returns Void, so a function with no throw type implicitly throws Never.

(Incidentally, the `E` should be `E: ErrorProtocol`, I believe, since you can only throw ErrorProtocols. That's why Never needs to be a proper bottom type: it needs to conform to `ErrorProtocol`.)

-- 
Brent Royal-Gordon
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