title: Swift: Cannot override mutable property with read-only property 'xxx'
body: <p>I don't understand the meaning of this error. This is what I am trying to do:</p>
  
  <pre><code>protocol P {
      var testTitle: String? { get }
  }
  
  class MyViewController: UIViewController, P {
      var testTitle: String? {
          didSet {
              self.title = testTitle
          }
      }
  }
  
  final class TestViewController: MyViewController {
      var testTitle: String? { // Error occurs here
          return "Test" 
      }
  }
  </code></pre>
  
  <p>And I'm getting:</p>
  
  <blockquote>
    <p>Cannot override mutable property with read-only property 'testTitle'</p>
  </blockquote>
  
  <p>Could you help me understanding what's going on? Maybe it's trivial for some people but not for me.</p>
  
  <p>Thanks for your help.</p>