title: For "Box[+T]", can I say "The type Box is covariant", is it correct?
body: <p>Say I defined a type constructor <code>Box</code>:</p>
  
  <pre><code>trait Box[+T]
  </code></pre>
  
  <p>What is correct?</p>
  
  <ol>
  <li>type Box is covariant</li>
  <li>type constructor Box is covariant</li>
  <li>type parameter <code>T</code> is covariant</li>
  <li>type parameter <code>T</code> in type constructor <code>Box</code> is covariant</li>
  <li>If all of them are incorrect, what's the correct expression?</li>
  </ol>