ConcreteType.swift

import Cocoa

class Object1: Protocol1, CustomStringConvertible {
    var description: String { return "<\(String(self.dynamicType)):\(unsafeAddressOf(self))>" }
}

protocol Protocol1: class {
}

let object1 = Object1()
let object2 = Object1()

public final class SMAnyObjectBox: CustomStringConvertible, Hashable {
    public init(_ value: AnyObject) { self.value = value }
    public let value: AnyObject
    public var description: String { return String(value) }
    public var hashValue: Int { return ObjectIdentifier(value).hashValue }
}

public func ==(lhs: SMAnyObjectBox, rhs: SMAnyObjectBox) -> Bool {
    return lhs.value === rhs.value
}

public extension Array where Element: AnyObject {
    public func uniqueElements() -> [Element] {
        let boxedElements = self.map({ SMAnyObjectBox($0) })
        let uniqueBoxedElements = Set(boxedElements)
        let uniqueElements = uniqueBoxedElements.flatMap({ $0.value as? Element })
        return uniqueElements
    }
}

let array1: [Object1] = [object1, object2, object1]
let uniqueArray1 = array1.uniqueElements()

let array2: [Protocol1] = [object1, object2, object1]
let uniqueArray2 = array2.uniqueElements() // <-  Using 'Protocol1' as a concrete type conforming to protocol 'AnyObject' is not supported