this follows the normal narrowing rules. But <code>instanceof</code>
applies to subclasses too:</p>
<p><pre class="typescript-code"><span class="tm-kw">class</span> <span class="tm-typ"><span class="className">D</span></span> <span class="tm-kw">extends</span> <span class="error" tabindex="0" aria-describedby="errLabel-2304-16-1"><span class="error-highlight"></span>C<span id="errLabel-2304-16-1" role="tooltip" class="error-tooltip">Cannot find name 'C'.</span></span> { <span class="tm-vr">d</span>: <span c