Test if a type conforms to a non-existential protocol

[dabrahams (Dave Abrahams)]

@Joe_Groff

You can also make lastIndex a requirement of a new protocol, and provide default implementations:

protocol LastIndex: Collection {
  public func lastIndex() -> Index
}

extension LastIndex where Self: BidirectionalCollection {
  public func lastIndex() -> Index {
    print("BidirectionalCollection implementation")
    return index(endIndex, offsetBy: -1)
  }
}

extension LastIndex where Self: Collection {
  public func lastIndex() -> Index {
    print("Collection implementation")
    return indices.max()!
  }
}

Heh, there are at least two problems with this approach. The first is that you need to explicitly make every Collection conform to LastIndex. The second is that if your generic collection conditionally conforms to BidirectionalCollection, you'll get the O(N) implementation unconditionally from a generic context.

It would be nice if we had a mechanism for extending protocols with additional dispatched requirements.

+1; we had a pitch on these forums for such a feature recently but I can't find it.