Regression in `apache/incubator-pekko-connectors`

[WojciechMazur]

Based on OpenCB build failure in apache/incubator-pekko-connectors - build logs

Compiler version

Last good release: 3.4.0-RC1-bin-20231212-97e4401-NIGHTLY
First bad release: 3.4.0-RC1-bin-20231213-9dce045-NIGHTLY
Present in 3.4.0-RC1
Bisect points to d6c7e18

Minimized code

case class Person(id: Int)

class GeodeContinuousSourceSpec {
  summon[PdxEncoder[Person]]
}

trait PdxEncoder[A] {
  def encode(a: A): Boolean
}

object PdxEncoder extends ObjectEncoder {
  implicit def intEncoder: PdxEncoder[Int] = ???
}

trait ObjectEncoder {
  given emptyTupleEncoder: PdxEncoder[EmptyTuple] = ???

  given tupleEncoder[K <: String, H, T <: Tuple](using
      m: ValueOf[K],
      hEncoder: PdxEncoder[H],
      tEncoder: PdxEncoder[T]
  ): PdxEncoder[FieldType[K, H] *: T] = ???

  given objectEncoder[A, Repr <: Tuple](using
      gen: LabelledGeneric.Aux[A, Repr],
      tupleEncoder: PdxEncoder[Repr]
  ): PdxEncoder[A] = ???
}

import scala.deriving.Mirror

private type FieldType[K, +V] = V & KeyTag[K, V]
private type KeyTag[K, +V]
private type ZipWith[T1 <: Tuple, T2 <: Tuple, F[_, _]] <: Tuple = (T1, T2) match {
  case (h1 *: t1, h2 *: t2) => F[h1, h2] *: ZipWith[t1, t2, F]
  case (EmptyTuple, ?)      => EmptyTuple
  case (?, EmptyTuple)      => EmptyTuple
  case _                    => Tuple
}

private trait LabelledGeneric[A] {
  type Repr
}

private object LabelledGeneric {
  type Aux[A, R] = LabelledGeneric[A] { type Repr = R }

  inline given productInst[A <: Product](using
      m: Mirror.ProductOf[A]
  ): LabelledGeneric.Aux[A, ZipWith[m.MirroredElemLabels, m.MirroredElemTypes, FieldType]] =
    new LabelledGeneric[A] {
      type Repr = Tuple & ZipWith[m.MirroredElemLabels, m.MirroredElemTypes, FieldType]
    }
}

Output

-- [E172] Type Error: /Users/wmazur/projects/sandbox/main.scala:4:28 -----------
4 |  summon[PdxEncoder[Person]]
  |                            ^
  |No given instance of type PdxEncoder[Person] was found for parameter x of method summon in object Predef.
  |I found:
  |
  |    PdxEncoder.objectEncoder[Person, Repr](
  |      LabelledGeneric.productInst[Person](
  |        Person.$asInstanceOf[
  |          
  |            scala.deriving.Mirror.Product{
  |              type MirroredMonoType = Person; type MirroredType = Person;
  |                type MirroredLabel = ("Person" : String);
  |                type MirroredElemTypes = Int *: EmptyTuple.type;
  |                type MirroredElemLabels = ("id" : String) *: EmptyTuple.type
  |            }
  |          
  |        ]
  |      ),
  |    ???)
  |
  |But given instance productInst in object LabelledGeneric does not match type LabelledGeneric.Aux[Person, Repr].
1 error found

Expectation

Not sure if it's false negative or not. Needs confirmation it works as expected or to provide a fix.

[bishabosha]

I don't think this has an issue with mirrors, the ZipWith match type also seems to work, now I'm checking again it was assigned to me because I made the change to add the constrain result fix in #19253

[bishabosha]

If you want to fix the compilation, then it compiles if LabelledGeneric.productInst is made transparent inline.

Maybe there's another way of redesigning the code to propagate the types if you want to keep it inline.

[dwijnand]

Changing what, how and when type vars are constraints can affect match type resolution.