title: Scala Option's collect method doesn't like my PartialFunction
body: <p>I think I'm missing something:</p>
  
  <pre><code>scala&gt; Some(1) collect ({ case n if n &gt; 0 =&gt; n + 1; case _ =&gt; 0})
  res0: Option[Int] = Some(2)
  
  scala&gt; None collect ({ case n if n &gt; 0 =&gt; n + 1; case _ =&gt; 0})   
  &lt;console&gt;:6: error: value &gt; is not a member of Nothing
         None collect ({ case n if n &gt; 0 =&gt; n + 1; case _ =&gt; 0})
                                   ^
  &lt;console&gt;:6: error: value + is not a member of Nothing
         None collect ({ case n if n &gt; 0 =&gt; n + 1; case _ =&gt; 0})
  </code></pre>
  
  <p>Why is this error happening? I think I'm misunderstanding how <code>collect</code> works...</p>