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

[tera]

@vanvoorden

I am not sure I have enough context on this right now to give a good example. Maybe @lorentey can help with some more information on that?

Or it could probably be solved by not providing isKnownIdentical for non copyable types or always returning nil / false for those.

@vanvoorden

But my weak opinion on this topic is that Int should not expose a concept of identity equality.

I see. nil would be quite painful for optimisations like these, for such algorithms it would be better if such types were not conforming to the protocol in the first place, but I guess this is not on the table if isIdentical is coupled with Equatable.

BTW, what happens with this type with your proposal:

struct Storage: Equatable {
    var items: [String] 
    var count: Int
}

Would the default autogenerated isKnownIdentical of Storage be returning nil because one of its members says nil?

It's also not clear what Array<Something> should do when Something has a custom isKnownIdentical that returns false or nil. Or is it ok to have one Array identical to another even when its members are not?