Scala 2 regression: False ambiguous implicit error when supertype companion includes a polymorphic instance

Compiler version

3.2.1 and 3.2.2-RC1

Minimized code

https://scastie.scala-lang.org/rJ1TG8HeSgO8U2Amzt8aYg

trait Supertype {
  def id: Int
}
object Supertype extends SupertypeLowPriorityInstances {
  implicit def supertype: Subtype = Subtype(1)
}
trait SupertypeLowPriorityInstances {
  // deleting this instance fixes the ambiguity error
  implicit def supertypeIfCondition(implicit impossible: Some[Nothing]): Supertype = Subtype(9)
}

final case class Subtype(id: Int) extends Supertype
object Subtype {
  implicit def subtype: Subtype = Subtype(2)
}

object App0 extends App {
  println(implicitly[Subtype])
}

Output

Ambiguous given instances: both method subtype in object Subtype and method supertype in object Supertype match type Subtype of parameter e of method implicitly in object Predef

Expectation

Expected to work, as in Scala 2: https://scastie.scala-lang.org/bwBH5zecS9OWSOwE1bfsJg

In absence of Supertype.supertypeIfCondition there is no ambiguity error, but it’s unclear how could that instance interfere at all, given that Subtype.subtype is a more specific instance than both of the instances in Supertype companion no matter what.

The same bug also affects higher-kinded implicits.

This code is a minimized version of real library code in https://github.com/izumi/izumi which we’re trying to port to Scala 3

[Kordyjan]

It is even worse. Any implicit definition in any trait that is a supertype of the Supertype will cause this false error.

trait Unrelated
trait Supertype

object Supertype extends SupertypeLowPriorityInstances {
  implicit def supertype: Subtype = Subtype(1)
}
trait SupertypeLowPriorityInstances {
  implicit def unrelated: Unrelated = ??? // removing this line fixes the snippet
}

final case class Subtype(id: Int) extends Supertype
object Subtype {
  implicit def subtype: Subtype = Subtype(2)
}

object App0 extends App {
  println(implicitly[Subtype])
}

This is not compiling now. What’s even stranger is that this is a consistent behavior since 3.0.0.

[Kordyjan]

A and B are objects, B does not inherit any implicit members from base classes (*), and the companion class of A extends the companion class of B.

I remembered this rule requiring B not to inherit implicit members with the type that we are searching for. Now I see that it literally requires B not to inherit ANY implicit members.

I’ve learned something new.