Test if a type conforms to a non-existential protocol

[Joe_Groff]

@Paul_Cantrell

I wonder if that's an argument for:

protocol Collection {
  var isBidirectional: Bool { get }  // on the protocol itself, so virtual dispatch
}
extension Collection {
  var isBidirectional: Bool { false }
}
extension BidirectionalCollection {
  var isBidirectional: Bool { true }  // would be nice if Swift supported `final` here
}

…?

Yeah, or maybe some other way of opting into keeping cheap "dynamic cast" information inside a protocol hierarchy. This sort of reminds me of the problem @dabrahams discussed in this thread, where in order to be able to specialize a protocol requirement in a conditional conformance without resorting to global dynamic searches for the conformances, we might need some way to explicitly mark optional conformance requirements.