[swift-evolution] Optional comparison operators

[Jacob Bandes-Storch]

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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