title: Error Message: "An interface can only extend an object type or intersection of object types with statically known members"
body: <p>The following code:</p>
  
  <pre class="lang-js prettyprint-override"><code>export type Partial2DPoint = { x: number } | { y: number }
  export interface Partial3DPoint extends Partial2DPoint {
    z: number
  }
  </code></pre>
  
  <p>Fails with the following error:</p>
  
  <blockquote>
    <p>An interface can only extend an object type or intersection of object types with statically known members.</p>
  </blockquote>
  
  <p>Why is this happening?</p>