Protocol extensions open existentials

extension-opens-existential.swift

protocol HumanType {}
protocol HasName {}

func printHumanName<T: protocol<HumanType, HasName>>(x: T) {}

// It currently isn't possible to directly pass a protocol<> type value
// as a concrete value conforming to the protocol. However, a protocol
// extension method's Self type *is* a concrete type conforming to the
// extending protocol. By introducing extension methods we can "open"
// the protocol existential and use the opened Self type with generics.
//
// (Yes, this is a hacky workaround, and we should have first-class
// support for it in the language. Doug Gregor addresses this as
// a missing feature in the "Opening Existentials" section of his
// Generics Manifesto:
// https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md )

extension HumanType where Self: HasName {
  private func _printHumanName() {
    printHumanName(x: self)
  }
}

func tryToPrintUnknownData(unknownData: Any) {
  if let unknownData = unknownData as? protocol<HumanType, HasName> {
    // Compile Error: cannot invoke 'printHumanName'
    // with an argument list of type '(protocol<HasName, HumanType>)'
    /*printHumanName(unknownData)*/
    
    // However, we can use an extension method on the protocol value:
    unknownData._printHumanName()
  }
}