title: Xcode 10 code completion placeholders — what is the "T"?
body: <p>In Xcode 10 code completion, the text underlying the placeholder tokens has an extra <code>#T#</code> before it (to see that this is so, copy and paste the inserted code template into a different text editor):</p>
  
  <pre><code>let alert = UIAlertController(
      title: &lt;#T##String?#&gt;, message: &lt;#T##String?#&gt;, 
      preferredStyle: &lt;#T##UIAlertController.Style#&gt;)
  </code></pre>
  
  <p>What is that? Does "T" mean Type? What difference does it make in my usage of the placeholder?</p>