title: Make typescript parameter keyof K and typeof FormGroup
body: <p>I want to pass in an object which is key of a class K and has the type of or extends FormGroup like:</p>
  <pre><code>  public async myMethod&lt;T, K&gt;(
      ...
      formGroupName: keyof K &amp; typeof FormGroup) { }
  </code></pre>
  <p>So wenn I call it then, the keyof K seems to work as I only get K members, but also others, i just want to receive from type of FormGroup!</p>
  <p>This would be the function call:</p>
  <pre><code>this.service.myMethod&lt;ComponentA, ComponentB&gt;(
    ...
    'form' // any member of ComponentB, which is of type formGroup
  );
  </code></pre>
  <p>Thank you</p>