not-possible-cause.scala:2: error: type mismatch;
 found   : a.type (with underlying type A)
 required: AnyRef
Note that A is bounded only by Equals, which means AnyRef is not a known parent.
Such types can participate in value classes, but instances
cannot appear in singleton types or in reference comparisons.
  def foo[A <: Product](a: A) { type X = a.type }
                                         ^
one error found