title: Why is this lambda function not an error in typescript?
body: <p>I imagined this code </p>
  
  <pre><code>let x: (a: { b: number }) =&gt; void = (a: { b: number, c: string }) =&gt; { alert(a.c) };
  x({ b: 123 });
  </code></pre>
  
  <p>should produce an error, since the lambda function requires an additional property on the <code>a</code> argument, so the signatures should not be compatible. But trying this in the latest typescript playground does not produce any errors! Why is that?</p>