[swift-evolution] Optional comparison operators

[David Sweeris]

Why not have them return a `Bool?`? If both arguments are non-nil, it can return the results of the comparison, otherwise it can return nil.

- Dave Sweeris

> On Jul 7, 2016, at 16:37, Jacob Bandes-Storch via swift-evolution <swift-evolution at swift.org> wrote:
> 
> These operators cause some potential for confusion:
> 
>     public func <<T : Comparable>(lhs: T?, rhs: T?) -> Bool
>     public func ><T : Comparable>(lhs: T?, rhs: T?) -> Bool
>     public func <=<T : Comparable>(lhs: T?, rhs: T?) -> Bool
>     public func >=<T : Comparable>(lhs: T?, rhs: T?) -> Bool
> 
> 1. The meaning of T? < T? is not immediately obvious (Why is nil < .some(x) for any x? Personally, my intuition says that Optional should only provide a partial order, with .none not being ordered w.r.t. .some(x).)
> 
> 2. Even if the meaning is understood, it can be surprising when the (T?, T?) -> Bool version is used instead of (T, T) -> Bool.
> 
> Prior discussion:
> - http://thread.gmane.org/gmane.comp.lang.swift.devel/2089
> - http://thread.gmane.org/gmane.comp.lang.swift.evolution/10095
> - rdar://16966712&22833869
> - Replies to https://twitter.com/jtbandes/status/646914031433871364
> 
> In the swift-dev thread from May, Chris said:
> 
>>> One of the ideas that Joe Pamer has been discussing is whether the implicit promotion from T to T? should be disabled when in an operator context.  Doing so would fix problems like this, but making the code invalid.
> 
> A change like this would be source-breaking, so if the core team has recommendations for how to handle these issues, now is probably the time to get it done.
> 
> Jacob
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