title: In Scala, should it be possible to instantiate an existential type?
body: <p><strong>Updated</strong> to remove bogus example.</p>
  
  <p>Should this (which compiles) be valid?</p>
  
  <pre><code>trait Foo[X]
  val foos: Seq[Foo[_]] = Seq()
  </code></pre>
  
  <p>These instantiated existentials seem to only lead to a downstream compiler errors, IMHO should not compile, and should instead be written as: </p>
  
  <pre><code>val foos: Seq[Foo[Any]] = ...
  </code></pre>
  
  <p>What am I missing?</p>
  
  <p>(A reaction to <a href="http://scabl.blogspot.com/2015/01/introduce-type-param-pattern.html" rel="nofollow">this blog post</a>.)</p>