title: Typescript : index signature with numbers `in 1 | 2 | 3`
body: <pre><code>type A = {[K in '1' | '2']: string} // ok
  type B = {[K in 1 | 2]: string} // error
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/8iBoK.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/8iBoK.png" alt="enter image description here"></a></p>
  
  <p>Is there a decent reason, I didn't find an issue on github, but decided first to ask here?</p>