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

[vanvoorden (Rick van Voorden)]

@xwu

We don't have the protocol hierarchy to identify "Spannable" types yet, but that seems like it will (or can, if we keep it in mind) fall out of the inevitable work ahead of us of creating a container hierarchy that accommodates noncopyable types.

I think this all composes very nicely and spares us the need for additional dedicated protocol requirements or APIs, or to introduce a named concept of "identity equality" to copyable types in an already overloaded (ha) space which has ==, ===, Identifiable, etc.

If what you are suggesting is that checking for identity equality can exist independently of the Equatable protocol this was part of the original pitch and is currently an "Alternative Considered". One reason we kept this inside Equatable is to prevent against needing extra generic specialization overloads. Where a product engineer needs:

• One generic specialization for a type that is Equatable and is Spannable.
• One generic specialization for a type that is Equatable and is not Spannable.
• One generic specialization for a type that is not Equatable and is Spannable.

Since we expect product engineers want isKnownIdentical in-place of (or accompanying) the result of a == value operator we think keeping that in Equatable is the better trade off for now.