title: Where is the scaladoc for scala.language.existentials?
body: <p>When I tried to write a class that accepts any <code>Class[_]</code> as a parameter:</p>
  
  <pre><code>case class A(klass: Class[_])
  </code></pre>
  
  <p>I got this error:</p>
  
  <blockquote>
    <p>test.scala:1: warning: inferred existential type Option[Class[_$1]]
    forSome { type <em>$1 }, which cannot be expressed by wildcards,  should
    be enabled by making the implicit value scala.language.existentials
    visible. This can be achieved by adding the import clause 'import
    scala.language.existentials' or by setting the compiler option
    -language:existentials. See the Scala docs for value scala.language.existentials for a discussion why the feature should be
    explicitly enabled. case class A(klass: Class[</em>])
               ^ one warning found</p>
  </blockquote>
  
  <p>I'm willing to know why it doesn't work. But where is "the Scala docs for value scala.language.existentials"? I googled "scaladoc scala.language.existentials" but got some threads I can't understand.</p>
  
  <p>Clarification: I know the 'correct' way to implement such a class is:</p>
  
  <pre><code>case class A[T](klass: Class[T])
  </code></pre>
  
  <p>But I want to know what the warning message means.</p>