[swift-evolution] Proposal: Add "none" and simplify the language.

[Brent Royal-Gordon]

> var x:Int = none // uninitialized but not an optional
> 
> print(x) // run-time error as x is uninitialized
> 
> if x == nil { … } // compile time error… x can never be nil because it is not an optional
> 
> if x == none { x = 2 } // … but it can be uninitialized
> 
> Optionals can also be uninitialized:
> 
> var y:Int? = none // uninitialized and an optional
> 
> if y == nil { … } // run-time error as y is uninitialized
> 
> y = nil
> 
> if y == nil { … } // fine

I think there *may* be a case for this as a strictly static check. That is, setting a variable to `none` would be a way to explicitly mark the current state as invalid, and ask the compiler to prove that it's not used again until you've reinitialized it. But your example of `x == none` would not make any sense—`none` would not be a thing that existed at runtime. That's a very niche feature, though, and I'd need convincing that it's helpful.

I'm strongly opposed to this feature as a runtime construct. It's basically just reintroducing null references, which is an anti-goal.

-- 
Brent Royal-Gordon
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