title: Scala upper bound for generic in field
body: <p>I have next field:</p>
  
  <pre><code>var operations = Map.empty[Long, _ &lt;: Operation]
  </code></pre>
  
  <p>I want to get second generic parameter upper bound extends <code>Operation</code> class. When I'm doing like above I have error <code>unbound wildcard type</code>.</p>
  
  <p>How can I achieve this?</p>