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

[Charles Srstka]

> On Jun 7, 2016, at 3:12 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> On Tue, Jun 7, 2016 at 2:49 PM, Michael Peternell via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 
> > Am 07.06.2016 um 19:45 schrieb Charles Srstka via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>>:
> >
> >> On Jun 7, 2016, at 11:47 AM, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> >>
> >>> 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*.
> >
> > But it’s *not* a type. You’ll never have an instance of it.
> 
> That's the dictionary definition of a bottom type <https://en.wikipedia.org/wiki/Bottom_type>, though: a type which has no values.
>  
> Since it’s not a type name, it doesn’t make sense that it needs to look like one. What it is doing is telling you something about the behavior of the function itself, not its return value. Its return value, if there were one, is irrelevant, since the function, by its very nature, will never even get to the point where it would return it.
> 
> And that's the dictionary definition of a function where the return type is bottom. This isn't something being made up just for Swift...

What’s the dictionary definition of a do loop? Or a for loop? Or try/catch?

Swift doesn’t do things the same as other languages in lots of places. I don’t see why it has to be different here. @noreturn is a clearer description of what this construct does.

Charles

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