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

[Brent Royal-Gordon]

> I disagree. We are discussing how to annotate a function in some way so that the compiler knows that the code following it will never be executed *and* so a human who reads the declaration knows that it does not return. “Never" is a poor choice for that. Never what? Never return? Never use this function? Never say never again? 

"Never return". That's why it's in the return type slot, right after the `->`. If you read it out loud, you'll read "returns Never", which is exactly correct.

NoReturn, on the other hand, does *not* read well in that slot: "returns NoReturn". Huh? I mean, I suppose you won't misunderstand it, but it makes no sense whatsoever *as a type name*.

> If you want bottom types for other uses, give them their own appropriate and self documenting names. 

The problem is, types flow through the type system. Use a NoReturn method with optional chaining, now you have an Optional<NoReturn>. flatMap over that Optional<NoReturn>, now you have a parameter of type NoReturn. What's a *parameter* of type NoReturn? You'd want it to be, say, a different bottom type named NoPass, but you can't—the type came from a NoReturn, and it's stuck being a NoReturn.

Never works pretty well—honestly, surprisingly well—in all of these contexts. The method returns Never, so optional chaining gives you an Optional<Never>, so flatMap has a Never parameter. I have yet to discover a case where it isn't easily interpreted to mean exactly what it really does mean.

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Brent Royal-Gordon
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