title: Restrict index signature by specific key
body: <p>I have an external type that comes from the library:</p>
  <pre><code>type Anything = Record&lt;string, any&gt;; 
  </code></pre>
  <p>I want to override it and not allow usage of a specific key <code>c</code>.</p>
  <p>Results that I want to achieve:</p>
  <pre><code>type CustomAnything = Anything &amp; { c?: never } // This is what I was able to come to, but it does not completely forbid the usage of the `c`. Undefined is allowed in this case
  
  const a: CustomAnything = { a: 1, b: 2} // OK
  const b: CustomAnything = { a: 1, c: undefined } // Error, because I want to prevent the use of `c` key 
  const c: CustomAnything = { a: 1, c: 2 } // Error, because I want to prevent the use of `c` key 
  const d: CustomAnything = { a: 1} // OK
  </code></pre>
  <p>Here is a <a href="https://www.typescriptlang.org/play/#code/C4TwDgpgBAggdiYALAlnA5lAvFAShAYwHsAnAEwB4BnYEtdAGigEMEA+AbigHpuoBJYFBRUoEAB7AIJOMwA2UUJBZwyUAqwDkQgEbQAtkTIoAZighqAUJd6Lw0AMIBXGkX3xEqDNigB5fSjAFB7I9EyaBJpslkqOLsBuIV6YOEn0UABkUADe6gD8AFxQcBAAbtJQAL7WxHA0LEXOru4Iod44ucxFAIxMOkUATJU8fL4A0pa19f1QTQktnukdDVC96kVOqhBmJWrDtgCiJCSkk0R1QgSN8YmtyT6dPUxXUANVI1BHJyRnF1Bk12aaXaORW3X2ozGQA" rel="nofollow noreferrer">playground</a>.
  Any ideas how to achieve this?</p>