title: How extend a parent class methods object literal return type?
body: <p>Im extending a class with a <code>serialize</code> method.
  It should do the same thing as its parent but append some keys.</p>
  <pre><code>export declare class Parent {
      serialize(): {
          x: number;
          y: number;
      };
  }
  </code></pre>
  <pre><code>export default class Child extends Parent {
      serialize() {
          return {
              ...super.serialize(),
              something: 'extra'
          };      
      }
  }
  </code></pre>
  <p>I would like a extract a type for my extension - something like <code>SerializedChild</code>. Is it possible in this case and if so how can I do that?</p>