protocols+classes+structs.swift

//
// Swift version 2.2-dev (LLVM 3ebdbb2c7e, Clang f66c5bb67b, Swift 42591f7cba)
// Target: x86_64-unknown-linux-gnu
//
// [Swift IBM Sandbox](http://swiftlang.ng.bluemix.net/#/repl)
//

import Foundation

///
/// A little class
///
public class Message
{
    public var text: String
    public var sendDate: NSDate
    
    public init(_ text: String)
    {
        self.text = text
        self.sendDate = NSDate()
    }
}

///
/// A little struct
///
public struct Response
{
    public var text: String
    public var responseDate: NSDate
    
    public init(_ text: String)
    {
        self.text = text
        self.responseDate = NSDate()
    }
}

/**
    Change `:` to `==` in *where* clause works fine
*/
extension CollectionType where Generator.Element: Message
{
    func doSomeThingWithMessages() -> Void
    {
        print("There are \(self.count) message(s)") 
    }
}

/**
    Change `==` to `:` in *where* clause cause
    the following error
    
    **type 'Generator.Element' constrained to non-protocol type 'Response'**
*/
extension CollectionType where Generator.Element == Response
{
    func doSomeThingWithReponses() -> Void
    {
        print("There are \(self.count) response(s)") 
    }
}

//
// Go with example
//

let message: Message = Message("Where are you from?")
let response: Response = Response("I'm from Madrid")
let another_message: Message = Message("Great!")

let messages: [Message] = [ message, another_message ]
let responses: [Response] = [ response ]

messages.doSomeThingWithMessages()
//messages.doSomeThingWithReponses() // #Error
// Error: 'Response' is not convertible to 'Message'

responses.doSomeThingWithReponses()
//responses.doSomeThingWithMessages() // #Error
// Error: Response' is not a subtype of 'Message'