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

[Joe Groff]

`Never` seems reasonable to me too. I'll add that to the proposal as an alternative.

-Joe

> On Jun 5, 2016, at 11:37 AM, T.J. Usiyan via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I vote for Bottom or Never. None does not convey "this should not occur or be evaluated".
> 
> On Sun, Jun 5, 2016 at 2:26 PM, Антон Жилин <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 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.
> 
> Example 1.
> let x: None?
> // ...
> let y = x!
> 
> It will trap in runtime not because we discover scary bottom thing, as in Haskell, but because x had value Optional.none at that moment and we asserted otherwise.
> We could prove that it is always true in this case, but compiler must be stupid about this.
> 
> Example 2.
> Compiler should allow including None in structures. Error will show up in constructor, when we will not be able to initialize the field.
> 
> Example 3.
> None in an enum case makes that case never appear in values of such a type. But compiler can not know about that.
> 
> - Anton
> 
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