title: Extend a partial with excludes ignores the excluded properties
body: <p>I am not sure if I am misunderstanding how <code>Exclude</code> works but I have the following issue:</p>
  
  <pre><code>export interface Base {
      page: number;
      count: number;
  }
  
  export interface Sub extends Partial&lt;Exclude&lt;Base, 'count'&gt;&gt; { // error
      count?: 'nonzero'|'zero';
  }
  </code></pre>
  
  <p>The interface <code>Sub</code> is causing an error saying <code>Sub</code> incorrectly extends interface <code>Partial&lt;Base&gt;</code> however this is not what I was expecting. I was expecting that <code>Sub</code> extends <code>Partial&lt;{ page: number }&gt;</code> which is what I thought was the type of <code>Exclude&lt;Base, 'count'&gt;</code> however it does not seem to be the case.</p>
  
  <p><a href="https://www.typescriptlang.org/play/#src=export%20interface%20Base%20%7B%0D%0A%20%20%20%20page%3A%20number%3B%0D%0A%20%20%20%20count%3A%20number%3B%0D%0A%7D%0D%0A%0D%0Aexport%20interface%20Sub%20extends%20Partial%3CExclude%3CBase%2C%20&#39;count&#39;%3E%3E%20%7B%20%2F%2F%20error%0D%0A%20%20%20%20count%3F%3A%20&#39;nonzero&#39;%7C&#39;zero&#39;%3B%0D%0A%7D%0D%0A%0D%0A%0D%0Aexport%20interface%20Sub2%20extends%20Partial%3CPick%3CBase%2C%20&#39;page&#39;%3E%3E%20%7B%20%2F%2F%20works%0D%0A%20%20%20%20count%3F%3A%20&#39;nonzero&#39;%7C&#39;zero&#39;%3B%0D%0A%7D" rel="nofollow noreferrer">Playground</a></p>