Sample code for the question http://stackoverflow.com/questions/37365594/recommended-way-to-override-a-method-on-a-constrained-extension-in-swift-avoidi

Card.swift

enum Suit {}
enum Rank: Int {}

// In the example, let's say the Card is CustomStringConvertible so I can leverage all the external code
// that prints stuff, and depends on using the var description
struct Card : CustomStringConvertible {
  var rank: Rank
  var suit: Suit
  
  var description : String {
    return rank.simpleDescription() + suit.simpleDescription()
  }
}

// Define a protocol for specializing Arrays of Cards
protocol _CardType {}
extension Card: _CardType {}

Deck.swift

typealias Deck = [Card]

// Using swift's constrained extensions
extension Array where Element : _CardType {

  // This is the method that would not compile if I try to use override
	// override var description: String {
	var description: String {
    return "Deck: " + self.map { "\($0)" }.joinWithSeparator(",")
  }
}

DeckTests.swift

class CardsTests: XCTestCase {
  
  var deck : Deck!
  
  override func setUp() {
    super.setUp()
    deck = Deck.standard() // Create a standard poker deck
  }

  func testBasicDeck() {
    // Here you get the error Ambiguous use of description
  	print(deck.description)
  }

}