ExtendableClass.swift

import UIKit

protocol CommonProtocol {
    func commonFunc()
}

protocol ProtocolA: CommonProtocol {
    func sameName()
}

protocol ProtocolB: CommonProtocol {
    func sameName()
}

class Buyer: ProtocolA {
    func sameName() {
        //
    }
    
    func commonFunc() {
        //
    }
}

class Seller: ProtocolB {
    func sameName() {
        //
    }
    
    func commonFunc() {
        //
    }
}

class Operation<T>
{
    var people: T?
}

extension Operation where T == Buyer
{
    func buySomething() {
        //
    }
}

extension Operation where T == Seller
{
    func sellSomething() {
        //
    }
}


class ViewController: UIViewController {

    var seller: Operation<Seller>?

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        seller?.sellSomething()
    }
}