Implicit resolution regression in 2.13.x

[Atry]

import scala.language.higherKinds
import scala.concurrent.Future

object Regression {
  
  trait Functor[F[_]]
  trait Monad[F[_]] extends Functor[F]
  trait MonadError[F[_], E] extends Monad[F]
  implicit def futureMonad: Monad[Future] = ???
  
  

  object OptionT extends OptionTInstances0 {
    implicit final def OptionTFunctor[F[_]](implicit F0: Functor[F]): Functor[({type T[A] = OptionT[F, A]})#T] = ???

  }

  sealed abstract class OptionTInstances0 {
    implicit final def OptionTMonadError[F[_]](implicit F0: Monad[F]): MonadError[({type T[A] = OptionT[F, A]})#T, Throwable] = ???

  }

  trait OptionT[F[_], A]

  def main(args: Array[String]): Unit = {

    def f = implicitly[Functor[({type T[A] = OptionT[Future, A]})#T]]

  }

}

• It compiles in Scala 2.12.6: https://scastie.scala-lang.org/ifY5zoUzQTO2qzmviNxv6g
• It does not compile in Scala 2.13.0-M4 https://scastie.scala-lang.org/5CenTZ5qRnSeE1tyhRUXfw

[SethTisue]

is it reproducible without outside dependencies?

[Atry]

The example is updated to remove outside dependencies.

[xuwei-k]

compile success if swap these implicit def.

@@ -11,12 +11,12 @@
object Regression {
   
 
   object OptionT extends OptionTInstances0 {
-    implicit final def OptionTFunctor[F[_]](implicit F0: Functor[F]): Functor[({type T[A] = OptionT[F, A]})#T] = ???
+    implicit final def OptionTMonadError[F[_]](implicit F0: Monad[F]): MonadError[({type T[A] = OptionT[F, A]})#T, Throwable] = ???
 
   }
 
   sealed abstract class OptionTInstances0 {
-    implicit final def OptionTMonadError[F[_]](implicit F0: Monad[F]): MonadError[({type T[A] = OptionT[F, A]})#T, Throwable] = ???
+    implicit final def OptionTFunctor[F[_]](implicit F0: Functor[F]): Functor[({type T[A] = OptionT[F, A]})#T] = ???
 
   }

Some libraries encountered the same error (since Scala 2.13.0-M3)

• scalaz/scalaz@2fdd887
• typelevel/kittens@04394a8#diff-3bca8c4f5e0332228ddac09f73898b24R54

[xuwei-k]

scalaz/scalaz#1629

[joroKr21]

MonadError is a subtype of Functor therefore this is not a bug but a bug fix! Granted, breaking change hence in 2.13

[barambani]

I think I ran into this as well. So I guess that the instance summoned by

implicitly[Functor[({type T[A] = OptionT[Future, A]})#T]]

will be MonadError, correct ?

[joroKr21]

will be MonadError, correct ?

This example doesn't make much sense in practice. E.g. why would we say that OptionT has an instance of MonadError for Throwable? That seems wrong to me. Maybe for Unit, but how useful is that?

Anyway in this example, given that Future has a Monad instance then yes, MonadError will be selected as the Functor implementation.

[adriaanm]

Closing since this is working as intended. Let us know if we should reopen.