title: Scala: missing parameter type
body: <p>I typed the following into the Scala-REPL:</p>
  
  <pre><code>scala&gt; List(1, 2, 3).toSet.subsets(2).map(_.toList)
  res0: Iterator[List[Int]] = non-empty iterator
  
  scala&gt; List(1, 2, 3).toSet.subsets.map(_.toList)
  &lt;console&gt;:8: error: missing parameter type for expanded function ((x$1) =&gt; x$1.toList)
                List(1, 2, 3).toSet.subsets.map(_.toList)
  </code></pre>
  
  <p>Why am I getting an error for the second line? Is this a bug in the compiler or am I missing something?</p>