"This is a scalac bug": when I say it, most often false; when scalac says it, guaranteed true!
the result in Scala version 2.11.0-20130322-102202-0acf87844e is the same.
2.9 says "class Y of type Y does not take type parameters", which is a correct-ish error to give though it doesn't help the naive user clear up the class vs. companion object confusion that probably made them write this. (what naive user? who, me? <looks around wildly>)