func test<T: Equatable>(_ a: T, _ b: T) {}

enum Token: Equatable {

  case plus

  case invalidNumber(String)
  case invalidOperator(Character) // String is OK, Character not.

  static func tokenize(_ text: String) -> [Token] {
    return []
  }

  static func testX() {
    test(Token.tokenize(""), [.plus, .invalidOperator("&"), .invalidNumber("b")])
  }
}