title: scala type mismatch error, GenTraversableOnce[?] required
body: <p>Why does this code result in the compilation error</p>
  
  <blockquote>
    <p>type mismatch; found : (Int, Char) required:
    scala.collection.GenTraversableOnce[?]</p>
  </blockquote>
  
  <p>?</p>
  
  <pre><code>val n = Map(1 -&gt; 'a', 4 -&gt; 'a')
  def f(i: Int, c: Char) = (i -&gt; c) 
  n.flatMap (e =&gt; f(e._1, e._2))
  </code></pre>