Type pattern matching and inference error in Scala 3

Question

I'm playing with type classes in Scala 3, and got to a compilation error that I can't explain.

Considering the following code:

trait Transformation[Input, Output <: Tuple]:
  def apply(x: Input): Output


trait ListOfTransformations[T[_, _] <: Transformation[_, _], Input <: Tuple, Output <: Tuple] extends Transformation[Input, Output]

object ListOfTransformations:
  given empty[T[_, _] <: Transformation[_, _]]: ListOfTransformations[T, EmptyTuple, EmptyTuple] with
    def apply(t: EmptyTuple): EmptyTuple = t

  given nonEmpty[T[_, _] <: Transformation[_, _], Head, Tail <: Tuple, HeadOutput <: Tuple, TailOutput <: Tuple](
    using
    ht: T[Head, HeadOutput],
    tt: ListOfTransformations[T, Tail, TailOutput]
  ): Transformation[Head *: Tail, Tuple.Concat[HeadOutput, TailOutput]] with
    def apply(x: Head *: Tail): Tuple.Concat[HeadOutput, TailOutput] = ht(x.head) ++ tt(x.tail)

I get:

Found:    Tuple.Head[Head² *: Tail]
Required: nonEmpty.this.ht.Input

where:    Head  is a type in object Tuple which is an alias of [X <: NonEmptyTuple] =>> 
  X match {
    case [x, _ <: Tuple] =>> scala.runtime.MatchCase[x *: _, x]
  }
          Head² is a type in class nonEmpty
          Tail  is a type in class nonEmpty with bounds <: Tuple

What am I missing?

Answer 1

When taking type constructors with bounds as type parameters, make sure you use actual arguments instead of wildcards. Using T[a, b] <: Transformation[a, b] instead of T[_, _] <: Transformation[_, _] lets it compile ( Scastie ). The former takes a type constructor that, when given two types, gives a type that is a subtype of Transformation[a, b] for some a and b that we don't know. By not using wildcards (and ignoring the actual parameters of T ), you're letting the compiler know precisely what a T[a, b] is a subtype of.