Test for conformance to non-existential protocol

IsNonExistentialTest.swift

protocol True {}
struct IsBidirectional<C> {}
extension IsBidirectional: True where C : BidirectionalCollection {  }
extension Collection {
  var isBidirectional: Bool {
    return IsBidirectional<Self>() is True
  }
}

func testIsBidirectional() {
  func check<C: Collection>(_ collection: C, isBidirectional: Bool) {
    assert(collection.isBidirectional == isBidirectional)
  }

  check([1, 2, 3], isBidirectional: true)
  check(AnyBidirectionalCollection([1, 2, 3]), isBidirectional: true)
  check(AnyCollection([1, 2, 3]), isBidirectional: false)
  check(Set([1, 2, 3]), isBidirectional: false) // Dang, Set still isn't bidirectional?! :(
}
testIsBidirectional()