Swift extensions for classes and structs

Extensions.swift

protocol ProtocolA {}
protocol ProtocolB: ProtocolA {}
protocol ProtocolC {
    func act()
}

protocol ProtocolD {
    typealias V
    func act()
}

struct StructA: ProtocolA {}

struct StructB: ProtocolB {}

struct StructC<T: ProtocolA>: ProtocolC {}

extension StructC {
    func act() {
        print("general")
    }
}

extension StructC where T: ProtocolB {
    func act() {
        print("special")
    }
}

struct StructD<T: ProtocolA>: ProtocolD {
    typealias V = T
}

extension ProtocolD {
    func act() {
        print("general")
    }
}

extension ProtocolD where V: ProtocolB {
    func act() {
        print("special")
    }
}

class ClassC<T: ProtocolA>: ProtocolC {
}

extension ClassC {
    func act() {
        print("general")
    }
}

extension ClassC where T: ProtocolB {
    func act() {
        print("special")
    }
}

let classCA = ClassC<StructA>()
let classCB = ClassC<StructB>()

//this works
classCA.act() //-> "general"
classCB.act() //-> "special"

let structCA = StructC<StructA>()
let structCB = StructC<StructB>()

//Does not even compile
structCA.act() 
structCB.act() // error: "Ambigous use of 'act()'"

let structDA = StructD<StructA>()
let structDB = StructD<StructB>()

//works again
structDA.act() //-> "general"
structDB.act() //-> "special"