title: impicit Ordering and covariance
body: <pre><code>class Foo
  class Bar extends Foo
  implicit val ord = new math.Ordering[Foo] {
    override defcompare(a: Foo, b: Foo) = 0
  }
  </code></pre>
  
  <p>When trying to summon the Ordering instance of Bar</p>
  
  <pre><code>implicitly[math.Ordering[Bar]]
  </code></pre>
  
  <p>it fails with </p>
  
  <pre><code>No implicit Ordering defined for Bar.
  </code></pre>
  
  <p>I expected it to pick up the instance I defined on Foo.</p>