title: Cannot specialize a Scala method with specializable trait as return type
body: <pre><code>trait Eq[@specialized -X] {
    def eq(x: X, y: X): Boolean
  }
  
  trait Keyed[@specialized(Int) X] {
    def eqOnKey: Eq[X]
  }
  </code></pre>
  
  <p>The method <code>eqOnKey</code> is not specialized in the generated class <code>Keyed$mcI$sp</code>.</p>
  
  <p>How can I specialize this method, i.e. making the return type of method <code>eqOnKey$mcI$sp</code> in class <code>Keyed$mcI$sp</code> to be <code>Eq$mcI$sp</code>?</p>