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

[Brent Royal-Gordon]

> A return type makes a lot of sense linguistically but does not cover all practical cases because you might not be the one deciding what the function's signature is. For example, you might implement a protocol method that is non-optional, but you never expect to be called. The only way to indicate that to the compiler is with an attribute.

If Swift permits return type covariance—that is, for an override or implementation to tighten the return type—then this will happen naturally: Never is a subtype of all types, so you can validly change any return type to Never and meet the supertype's requirements.

I believe Swift supports return type covariance for overridden methods, but not for protocol requirements. This is, in my opinion, an incorrect behavior which should be changed.

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Brent Royal-Gordon
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