Error Message: "An interface can only extend an object type or intersection of object types with statically known members" : <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>
