kittens instance not found in 3.4.0 without -source:3.3

[joan38]

Compiler version

3.4.0

Minimized code

//> using scala 3.4.0
//> using dep org.typelevel::kittens:3.2.0

import cats.kernel.Monoid
import cats.derived.auto.monoid.given

case class B(a: Option[Int])
case class A(b: Option[B])

val _ = Monoid[A].empty

Output

[error] No given instance of type cats.derived.DerivedMonoid[scala$minus3$u002E4$minusbug$_.this.A] was found.

Expectation

Compiles fine in 3.3.3

[bishabosha]

workaround:

import scala.language.`3.3`

sounds like kittens has some inline defs that are affected by the type inference change, worth bringing this to them first, if there's no fix on their side, then we'll have to look again.

Here is the release notes of 3.4

Type inference has changed for inline methods in 3.4, which can cause a source incompatibility at the call site. The previous behaviour can be recovered in an affected file with

import scala.language.`3.3`

alternatively, the definition can be changed to transparent inline, but as this is a TASTy breaking change, it is not a default recommendation. (Also, inline should be preferred when possible over transparent inline for reduced binary size.)

[bishabosha]

I got this error anyway:

scala> import cats.kernel.Monoid
     | import cats.derived.auto.monoid.given
     |
     | case class B(a: Option[Int])
     | case class A(b: Option[B])
     |
     | val _ = Monoid[A].empty
-- [E172] Type Error: ----------------------------------------------------------
  7 |val _ = Monoid[A].empty
    |                 ^
    |No given instance of type cats.derived.DerivedMonoid[A] was found.
    |I found:
    |
    |    cats.derived.DerivedMonoid.given_DerivedMonoid_A[A](
    |      {
    |        val gen$proxy1:
    |
    |            scala.deriving.Mirror.Product{
    |              type Kind = shapeless3.deriving².K0.type; type MirroredType = A;
    |                type MirroredMonoType = A; type MirroredElemTypes <: Tuple
    |            }
    |           &
    |            scala.deriving.Mirror.Product{
    |              type MirroredMonoType = A; type MirroredType = A;
    |                type MirroredLabel = ("A" : String);
    |                type MirroredElemTypes = Option[B] *: EmptyTuple.type;
    |                type MirroredElemLabels = ("b" : String) *: EmptyTuple.type
    |            }
    |
    |         =
    |          A.$asInstanceOf[
    |
    |              scala.deriving.Mirror.Product{
    |                type Kind = shapeless3.deriving².K0.type; type MirroredType = A;
    |                  type MirroredMonoType = A; type MirroredElemTypes <: Tuple
    |              }
    |             &
    |              scala.deriving.Mirror.Product{
    |                type MirroredMonoType = A; type MirroredType = A;
    |                  type MirroredLabel = ("A" : String);
    |                  type MirroredElemTypes = Option[B] *: EmptyTuple.type;
    |                  type MirroredElemLabels = ("b" : String) *: EmptyTuple.type
    |              }
    |
    |          ]
    |        shapeless3.deriving².internals.ErasedProductInstances1.apply[
    |          shapeless3.deriving².K0.type,
    |          cats.derived.Derived.Or[cats.kernel.Monoid[A]]](gen$proxy1,
    |          scala.compiletime.summonInline[
    |            cats.derived.Derived.Or[cats.kernel.Monoid[Option[B]]]]:Any
    |        ):
    |          shapeless3.deriving².internals.ErasedProductInstances[
    |            shapeless3.deriving².K0.type,
    |            cats.derived.Derived.Or[cats.kernel.Monoid[A]]]
    |        :
    |          shapeless3.deriving².K0.ProductInstances[
    |            [A²] =>> cats.derived.Derived.Or[cats.kernel.Monoid[A²]], A]
    |      }
    |    )
    |
    |But given instance mkProductInstances in object K0 does not match type => shapeless3.deriving².K0.ProductInstances[cats.derived.DerivedMonoid.Or, A]
    |
    |where:    A         is a class in the empty package
    |          A²        is a type variable with constraint
    |          deriving  is a package in package scala
    |          deriving² is a package in package shapeless3
    |.
    |
    |The following import might make progress towards fixing the problem:
    |
    |  import cats.derived.DerivedMonoid.Strict.product
    |
    |---------------------------------------------------------------------------
    |Inline stack trace
    |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    |This location contains code that was inlined from DerivedMonoid.scala:18
    |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    |This location contains code that was inlined from DerivedMonoid.scala:18
     ---------------------------------------------------------------------------
1 error found

[joroKr21]

Type inference has changed how?

[bishabosha]

Type inference has changed how?

see here #19253

[joroKr21]

I really don't follow what's the issue.

Here is the definition of mkProductInstances

  inline given mkProductInstances[F[_], T](using gen: ProductGeneric[T]): ProductInstances[F, T] =
    ErasedProductInstances[K0.type, F[T], LiftP[F, gen.MirroredElemTypes]](gen)

This that it doesn't match is simply not true:

    |But given instance mkProductInstances in object K0 does not match type => shapeless3.deriving².K0.ProductInstances[cats.derived.DerivedMonoid.Or, A]
    |
    |where:    A         is a class in the empty package
    |          A²        is a type variable with constraint
    |          deriving  is a package in package scala
    |          deriving² is a package in package shapeless3

[joroKr21]

Btw, are there any instructions how to a project to the community build? I tried to do it but it didn't work.
It would be great to be notified sooner about these changes and also kittens is a good stress test for givens.

[bishabosha]

Btw, are there any instructions how to a project to the community build? I tried to do it but it didn't work.

maybe you can look at this PR which handles source dependencies: #10639

[SethTisue]

fwiw, there's been talk internally recently about making the open community build results easier for library maintainers to find and monitor. I hope that will happen

now that the open community build (https://github.com/VirtusLab/community-build3) is in such robust shape, it's unclear to me whether further additions to the small in-repo community build are desired. these repos are part of PR validation, which helps catch problems early, but just throwing projects in willy-nilly doesn't scale; not sure what the current happy medium is considered to be

@joroKr21 at minimum, you should check the kittens entry at https://github.com/VirtusLab/community-build3/blob/master/.github/workflows/buildConfig.json to see if it looks appropriate