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

[Charles Srstka]

> On Jun 5, 2016, at 1:49 PM, Pyry Jahkola via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On 05 Jun 2016, at 21:26, Антон Жилин via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> The following names were suggested: NoReturn, Bottom, None, Never.
>> I would pick None, because it looks like opposite to Any and fits nicely in generic types.
> 
> I would pick `Never` because `None` would overload the notion of a returnable nothingness with its antonym!
> 
> OTOH, "never" is quite unlikely to be a type name in user code, likely pretty googleable, and explains itself quite well in examples like `Result<Value, Never>`.
> 
>> 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.

Charles

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