Generic Case Class Migration, but explicitly setting missing values

GenericCaseClassMigration.scala

import shapeless._
import shapeless.ops.hlist._
import shapeless.syntax.singleton._

object GenericCaseClassMigration extends App {

  case class X(s1: String, s2: String)
  case class Y(s2: String, i: Int, s1: String)

  implicit class GenericCaseClassMigrator[A](f: A) {
    class PartiallyApplied[B] {
      def apply[ARepr <: HList, BRepr <: HList, Common <: HList, Added <: HList, Unaligned <: HList](added: Added)
                                                                                                    (implicit
                                                                                                     fromGen: LabelledGeneric.Aux[A, ARepr],
                                                                                                     toGen: LabelledGeneric.Aux[B, BRepr],
                                                                                         @typeHelper inter: Intersection.Aux[ARepr, BRepr, Common],
                                                                                         @typeHelper diff: Diff.Aux[BRepr, Common, Added],
                                                                                                     prepend: Prepend.Aux[Added, ARepr, Unaligned],
                                                                                                     align: Align[Unaligned, BRepr]
                                                                                                    ): B =
        toGen.from(align(prepend(added, fromGen.to(f))))
    }

    def migrateTo[B] = new PartiallyApplied[B]
  }

  val migratedY = X("s1", "s2").migrateTo[Y]('i ->> 42 :: HNil)

  assert(migratedY == Y("s2", 42, "s1"))
}

class typeHelper extends deprecated("not deprecated—this is only here to prevent the compiler from emitting unused param warnings", "")