Implicit resolution differs from Scala 2 in presence of GADTs

[abgruszecki]

dotty compiles following code:

object O {
  sealed trait Box[T] { def value: T }
  final case class IntBox(value: Int) extends Box[Int]

  implicit def s1[T](implicit box: Box[T]): String = "generic: " + box.value
  implicit def s2(implicit box: Box[Int]): String = "specific: " + box.value

  def test[T](implicit box: Box[T]): String = box match {
    case IntBox(_) => implicitly[String]
  }
}

The implicit resolved in test is s1 (the generic one). scalac refuses to compile the code, saying that the implicit is ambiguous.

[milessabin]

What are the chances of getting dotty to select s2 here?

[abgruszecki]

@milessabin I checked and the following code also emits ambiguous warning (in scalac):

@ object O {
    sealed trait Box[T] { def value: T }
    final case class IntBox(value: Int) extends Box[Int]
  
    implicit def s1[T](implicit box: Box[T]): String = "generic: " + box.value
    implicit def s2(implicit box: Box[Int]): String = "specific: " + box.value
  
    implicit val box: Box[Int] = new IntBox(0)
    val str = implicitly[String]
  } 
cmd3.sc:9: ambiguous implicit values:
 both method s2 in object O of type (implicit box: ammonite.$sess.cmd3.O.Box[Int])String
 and method s1 in object O of type [T](implicit box: ammonite.$sess.cmd3.O.Box[T])String
 match expected type String
  val str = implicitly[String]
                      ^
Compilation Failed

which would mean that selecting s2 is the same as changing implicit resolution algorithm - likely out of scope for this issue.

[milessabin]

Right, but surely that's because scalac isn't able to identify Box[Int] with IntBox?

[abgruszecki]

I'm not familiar enough with implicit resolution to answer that. Frankly, I'm surprised that scalac does not prefer s2 in the second example.

[milessabin]

Ahh ... I misread your example.

The reason scalac sees these as ambiguous is its odd (non-)handling of implicit arguments when computing the specificity of implicit methods ... see scala/bug#10526. I think this is a bug and I'd like to see it fixed: scala/scala#6139.