[swift-evolution] Remove nil and NilLiteralConvertible

[John McCall]

> On Jun 8, 2016, at 1:41 PM, Антон Жилин via swift-evolution <swift-evolution at swift.org> wrote:
> (No joking)
> Points:
> 
> 1. When nil was added to the language, we could not infer enumeration type:
> if x != Optional.none { ... }
> 
> Now it looks like this:
> if x != .none { ... }
> 
> If at this point we had a proposal to add nil as a replacement for .none, would we accept it?
> 
> 2. nil is very generic, it only approximately allows to express the intentions.
> In case of Optional, .none is clearer. In case of JSON processing, .null is clearer. In case of a semantically nullable struct, NilLiteralConvertible usually goes to default constructor.
> 
> 3. Too many "empty" things: .none, nil; NSNull, Void, NoReturn types.
> 
> 4. There should be a single consistent terminology: no value in Swift equals none.

I know we're making a lot of source-compatibility breaks in Swift 3 and so these lines seem blurry, but I think the spelling of "nil" is quite solidly in the "that ship has sailed" category, along with "struct" and "func" and "var".

John.