title: @objc keyword extension subclass behaviour
body: <p>Can someone explain why <code>@objc</code> keyword is needed here to compile the code?</p>
  
  <p>As I understood this keyword is used in order to work ObjC message method dispatch. But this is not an <code>NSObject</code> instance.</p>
  
  <pre><code> class MyClass {
   }
  
   extension MyClass {
       @objc func extensionMethod() { /// THIS LINE
           print("A")
       }
   }
  
   class SubClass: MyClass {
       override func extensionMethod() {
           print("B")
       }
   }
  </code></pre>
  
  <p>Does <code>@objc</code> keyword enable message dispatch as well as <code>dynamic</code>? Or not?</p>