A swift protocol with associated type used as type parameter in generic function

ProtocolAssociatedType.swift

protocol Thing {
    typealias argType
    func doit(val:argType) -> argType
}

class IntThing : Thing {
    func doit(val: Int) -> Int {
        return val + 1
    }
}

func doThing<A:Thing>(thing:A, val:A.argType) -> A.argType {
    return thing.doit(val)
}

doThing(IntThing(), 2)

Update for Swift3:

protocol Thing {
    associatedtype argType
    func doit(_ val:argType) -> argType
}

class IntThing : Thing {
    typealias argType = Int
    func doit(_ val: Int) -> Int {
        return val + 1
    }
}

func doThing<A:Thing>(_ thing: A, _ val: A.argType) -> A.argType {
    return thing.doit(val)
}

doThing(IntThing(), 2)

How would you declare a delegate property of type Thing?

weak var delegate: Thing? would produce Xcode error:
Protocol 'Thing' can only be used as a generic constraint because it has Self or associated type requirements

@pejalo did you find the answer?

@pejalo, @ankits16 you should look Type Erasure to achieve this