Faux Equatable and Comparable Existentials

AnyComparable.swift

class AnyEquatableBase {
  func isEqual(to other: AnyEquatableBase) -> Bool {
    fatalError("overrideMe")
  }
}

class AnyEquatableBox<T: Equatable> : AnyEquatableBase {
  let value: T

  init(_ value: T) { self.value = value }
  
  override func isEqual(to other: AnyEquatableBase) -> Bool {
    if let otherSelf = other as? AnyEquatableBox<T> {
      return self.value == otherSelf.value
    }
    return false
  }
}

struct AnyEquatable : Equatable {
  let box: AnyEquatableBase
  init<T: Equatable>(_ x: T) {
    box = AnyEquatableBox(x)
  }
  static func == (l: AnyEquatable, r: AnyEquatable) -> Bool {
    return l.box.isEqual(to: r.box)
  }
}

class AnyComparableBase : AnyEquatableBase {
  func isLess(than other: AnyComparableBase) -> Bool {
    fatalError("overrideMe")
  }
}

class AnyComparableBox<T: Comparable> : AnyComparableBase {
  let value: T

  init(_ value: T) { self.value = value }
  
  override func isEqual(to other: AnyEquatableBase) -> Bool {
    if let otherSelf = other as? AnyComparableBox<T> {
      return self.value == otherSelf.value
    }
    return false
  }
  override func isLess(than other: AnyComparableBase) -> Bool {
    guard let otherSelf = other as? AnyComparableBox<T> else {
      fatalError("mixed type ordering comparison not supported")
    }
    return self.value < otherSelf.value
  }
}

struct AnyComparable : Comparable {
  let box: AnyComparableBase
  init<T: Comparable>(_ x: T) {
    box = AnyComparableBox(x)
  }
  static func == (l: AnyComparable, r: AnyComparable) -> Bool {
    return l.box.isEqual(to: r.box)
  }
  static func < (l: AnyComparable, r: AnyComparable) -> Bool {
    return l.box.isLess(than: r.box)
  }
}

Thanks, exactly what I needed!

Does this type have a swift evolution proposal number? I want to know why this isn't included in stdlib. Thanks.