[Pitch #2] Add `isKnownIdentical` Method for Quick Comparisons to `Equatable`

[allevato (Tony Allevato)]

@FlorianPircher

The main objection I have to lhs.isKnownIdentical(to: rhs) is that identity checking, like equality checking, should not treat the two sides differently. a.isKnownIdentical(to: b) should be the same as b.isKnownIdentical(to: a). And it is. But a?.isKnownIdentical(to: b) is not the same as b?.isKnownIdentical(to: a) and that is easy to miss.

On this note, it's also worth pointing out that the section on Argument Labels in the Swift API Design Guidelines calls out the situation you're talking about, saying

• Omit all labels when arguments can’t be usefully distinguished, e.g. min(number1, number2), zip(sequence1, sequence2).

While that does seem to focus specifically on the labels, I think there's an unstated implication that an API that doesn't distinguish between two (or more) arguments shouldn't privilege one of them by making it the receiver—and the prior art of functions like min and zip reinforce that.