title: Is default(CancellationToken) equivalent to CancellationToken.None?
body: <p>Looking at the implementation of <code>CancellationToken.None</code>, it is simply returning <code>default(CancellationToken)</code>. However, I see no reference in <code>CancellationToken</code>'s documentation that the two are equivalent.</p>
  
  <p>I'd like to offer an API like this but not until I'm sure it'll always work:</p>
  
  <pre><code>Task DoSomething(CancellationToken token = default(CancellationToken))
  </code></pre>
  
  <p>Is it defined behavior that <code>default(CancellationToken)</code> is the same as <code>CancellationToken.None</code>, or is this just an implementation detail?</p>