Overwrite specific subtype of ADT?

[fdietze]

I have sealed trait similar to this one:

sealed trait Expression
object Expression {
  case class Identifier(name: String)          extends Expression
  case class Num(x: Double)                    extends Expression
  case class Add(x: Expression, y: Expression) extends Expression
}

Now, I'd like to exhaustively generate an Arbitrary[Expression] with scalacheck-shapeless, which uses my custom implementation of Arbitrary[Expression.Identifier]. Is that possible somehow?

My problem is that I have certain requirements on the name: String field in Identifier, which throws away too many test cases when I just filter the Arbitrary[Expression]