UUIDGen Implicit Prioritization Issue on Scala 2.12

[bpholt]

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Given this code (written this way to make it work with scala-cli repl, which doesn't support kind-projector):

//> using scala 2.12.20
//> using dep org.typelevel::cats-effect:3.6.1
import cats.effect._, cats.effect.std._, cats.data._
import java.util.UUID
import scala.language.higherKinds

type KIOS[a] = Kleisli[IO, String, a]

def foo[F[_]: UUIDGen]: F[UUID] = UUIDGen[F].randomUUID

SecureRandom.javaSecuritySecureRandom[IO].flatMap { implicit random =>
  foo[KIOS].run("hello world")
}

the compiler prints

error: ambiguous implicit values:
 both method catsKleisliUUIDGen in object UUIDGen of type [F[_], R](implicit evidence$4: cats.effect.std.UUIDGen[F])cats.effect.std.UUIDGen[[γ$1$]cats.data.Kleisli[F,R,γ$1$]]
 and method fromSecureRandom in object UUIDGen of type [F[_]](implicit evidence$17: cats.Functor[F], implicit evidence$18: cats.effect.std.SecureRandom[F])cats.effect.std.UUIDGen[F]
 match expected type cats.effect.std.UUIDGen[KIOS]
         foo[KIOS].run("hello world")
            ^

The repro code above compiled with CE 3.5.7.

I think the same applies to all the transformers that have instances defined on SecureRandom, so EitherT, Kleisli, OptionT, IndexedStateT, WriterT, IorT, and IndexedReaderWriterStateT.

[bpholt]

I think the implicit paths follow these patterns on 2.12:

• SecureRandom[F]→ UUIDGen.fromSecureRandom[F] → UUIDGen.catsKleisliUUIDGen[F]
• SecureRandom[F]→ SecureRandom.catsKleisliSecureRandom[F] → UUIDGen.fromSecureRandom[Kleisli[…

Updating the example to use

SecureRandom.javaSecuritySecureRandom[IO].flatMap { implicit random =>
  IO(reflect.runtime.universe.reify(foo[KIOS].run("hello world")).tree)
}.unsafeRunSync()

gives

foo[KIOS](UUIDGen.catsKleisliUUIDGen(UUIDGen.fromSecureRandom(IO.asyncForIO, random))).run.apply("hello world")

on 2.13 (which is the first path I mentioned above).