title: covariant type T occurs in contravariant position
body: <p>I know this question has been asked before, but either the answers don't apply to this case, or I don't understand them.</p>
  <p>Basically, why doesn't the following (simple example that recreates my problem) work ?</p>
  <pre><code>class Test[+T] {
      var list: List[T] = _
  }
  </code></pre>
  <p>The problem I am having is that I have an object, where I want to pass in an instance of <code>Test[Nothing]</code> (the empty <code>Test</code>), and this doesn't work unless I make <code>Test</code> co-variant in <code>T</code>.</p>