title: instantiate mix traits is compiling while single trait is not
body: <p>I have two traits</p>
  
  <pre><code>trait Person
  trait Food
  </code></pre>
  
  <p>why this is compiling :</p>
  
  <pre><code>val pf = new Person with Food //OK
  </code></pre>
  
  <p>and this is not </p>
  
  <pre><code>val p = new Person //result  error: trait Person is abstract; cannot be instantiated
  </code></pre>