protocol FirstProtocol {
    associatedtype Associatedtype

    var firstVar: Associatedtype? { get }
}

protocol SecondProtocol: FirstProtocol where Associatedtype == ThirdProtocol { }
protocol ThirdProtocol: class { }

// ----
final class FirstClass: SecondProtocol {

    weak var firstVar: ThirdProtocol?
}
// ----

final class ThirdClass {
    func firstFunc() {
        let firstClass = FirstClass()
        firstClass.firstVar = self
    }
}

extension ThirdClass: FirstClass.Associatedtype { }