title: Incorrect warning Converting null literal or possible null value to non-nullable type
body: <p>I try:</p>
  <pre><code>var err = (string)null;
  </code></pre>
  <p>(which I prefer to <code>string? err=null;</code>)
  <br><br><br>
  I get</p>
  <blockquote>
  <p>warning CS8600: Converting null literal or possible null value to non-nullable type.</p>
  </blockquote>
  <p>But the type of <code>err</code> is correctly set to a nullable string as is evident by hovering over it in visual studio.</p>
  <p>Is this warning a dud ?</p>