title: Why `trait X { def append[-](): Unit }` compiles?
body: <p>From the bottom of this answer: <a href="https://stackoverflow.com/a/23374938/342235">https://stackoverflow.com/a/23374938/342235</a>, there is some code which looks strange:</p>
  
  <pre><code>trait X { 
    def append[-](): Unit 
  }
  </code></pre>
  
  <p>Why it can be compiled? I mean the <code>[-]</code> is strange</p>