title: Warn about name shadowing in scala 2.11
body: <p>I'm using SBT to build a project and have in my <code>Build.scala</code>:</p>
  
  <pre><code>scalacOptions ++= Seq("-unchecked", "-deprecation", "-feature", "-Xfatal-warnings")
  </code></pre>
  
  <p>How can I configure the project to warn on name/variable shadowing? I've browsed the source <a href="https://github.com/scala/scala/blob/2.11.x/src/compiler/scala/tools/nsc/settings/StandardScalaSettings.scala" rel="nofollow noreferrer">here</a> and <a href="https://github.com/scala/scala/blob/2.11.x/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala" rel="nofollow noreferrer">here</a>, but don't see what I'm looking for among the many options. A comment on <a href="https://stackoverflow.com/questions/7031106/why-does-scala-support-shadow-variables">this old question</a> mentions a flag <code>-Ywarn-shadowing</code> which doesn't seem to exist.</p>