Tuple.Map and upper bounds

[Adam-Vandervorst]

Compiler version

3.0.0-RC3

Minimized code

case class TBox[A <: Tuple](v: A)

@main def m =
    val t: TBox[EmptyTuple] = TBox(EmptyTuple) // works
    val tt: Tuple.Map[(EmptyTuple, EmptyTuple), TBox] = (TBox(EmptyTuple), TBox(EmptyTuple))

Output

Type argument TBox does not conform to upper bound [_$4] =>> Any
    val tt: Tuple.Map[(EmptyTuple, EmptyTuple), TBox] = (TBox(EmptyTuple), TBox(EmptyTuple))

Expectation

Since the first line of the main function and the snippet below work, I expected the above to work too.

case class ABox[A](v: A)

@main def main_f = {
    val a: ABox[Int] = ABox(1)
    val ta: Tuple.Map[(Int, Int), ABox] = (ABox(1), ABox(2))

[smarter]

The second parameter of Tuple.Map is F[_], meaning that one has to pass a type lambda that can be applied to any type parameter, this isn't the case of TBox which requires its type parameter to be a subtype of Tuple, hence the error message. It looks like this is fixable by changing that second parameter type to be upper-bounded by Union[Tup] instead:

object Test {
  type Map[Tup <: Tuple, F[_ <: Tuple.Union[Tup]]] <: Tuple = Tup match {
    case EmptyTuple => EmptyTuple
    case h *: t => F[h] *: Map[t, F]
  }
  case class TBox[A <: Tuple](v: A)
  val tt: Map[(EmptyTuple, EmptyTuple), TBox] = (TBox(EmptyTuple), TBox(EmptyTuple)) // now OK
}

The same change might be applicable to other match types in Tuple too /cc @nicolasstucki

[smarter]

To clarify, the fix also works when the second parameter upper-bound is not Tuple:

  case class IBox[A <: Int](v: A)
  val tt: Map[(1, 2), IBox] = (IBox(1), IBox(2)) // now OK