AnyEquatable.swift

protocol AnyEquatable {
    func equals(otherObject: AnyEquatable) -> Bool
}

extension AnyEquatable where Self : Equatable {
    // otherObject could also be 'Any'
    func equals(otherObject: AnyEquatable) -> Bool {
        if let otherAsSelf = otherObject as? Self {
            return otherAsSelf == self
        }
        return false
    }
}

extension Int : AnyEquatable { }
extension String : AnyEquatable { }

extension Array where Element : AnyEquatable {
    func indexOfElement(element : Element) -> Index? {
        return indexOf({ (currentElement : Element ) -> Bool in
            element.equals(currentElement)
        })
    }
}

let one = 1
let two = "two"
let three = 3
let array : [AnyEquatable] = [one, two, three]
array.indexOfElement(two) // ERROR: Using 'AnyEquatable' as a concrete type conforming to protocol 'AnyEquatable' is not supported