Protocol conformance in collections

CollectionTypes.swift

import Foundation

protocol UniqueIdentificableProtocol {
    var uid: String { get }
}

protocol UniqueIdentificableSubtypeProtocol: UniqueIdentificableProtocol {
    var c: String { get }
}


class UniqueIdentificableClass {
    var uid: String { return "uid" }
}

class UniqueIdentificableSubtypeClass: UniqueIdentificableClass {
    var c: String { return "c"}
}

let myCollectionClasses: [UniqueIdentificableSubtypeClass] = [UniqueIdentificableSubtypeClass]()
let myCastedCollectionClasses: [UniqueIdentificableClass] = myCollectionClasses // Everything ok with classes

let myCollectionProtocols = [UniqueIdentificableSubtypeProtocol]()
let myCastedCollectionProtocols: [UniqueIdentificableProtocol] = myCollectionProtocols // Compiler error