Cannot derive generic for sealed trait family if base trait has a type member, even if bounded by another sealed trait

[TrustNoOne]

I can get a generic representation of my Base[T] if Base is sealed and T is bounded by another sealed trait:

  sealed trait Inner
  case class I1(i: Int) extends Inner
  case class I2(s: String) extends Inner

  sealed trait Base[T <: Inner]
  case class X[I <: Inner](x: I) extends Base[I]
  case class Y[I <: Inner](y: I) extends Base[I]

  Generic[Inner] // ok
  Generic[Base[_ <: Inner]] //ok

However, if T is a type member, I can't do it anymore:

  sealed trait Inner
  case class I1(i: Int) extends Inner
  case class I2(s: String) extends Inner

  sealed trait Base { type T <: Inner }
  object Base { type Aux[S <: Inner] = Base { type T = S } }
  case class X[S <: Inner](x: S) extends Base { final type T = S }
  case class Y[S <: Inner](y: S) extends Base { final type T = S }

  Generic[Base] // doesn't work
  Generic[Base { type T <: Inner }] // doesn't work
  Generic[Base.Aux[_ <: Inner]] // doesn't work

@milessabin already suggested in gitter that even if in theory this could be possible, the compiler doesn't allow it at the moment: in principle the match on the S values and the equation between S and T could be used to refine T, but the Scala compiler doesn't do that at the moment.