The following does not compile in Scala,

import scala.language.higherKinds

class D[T]

class C[F[_]](val i: Int)
object C {
  def apply[F[_]](implicit cf: C[F]): Int = cf.i

  implicit def c0[F[_]]: C[F] = new C[F](0)
  implicit def c1: C[D] = new C[D](1)
}

object Test extends App {
  assert(C[D] == 1) // Works in Dotty ...
}

because c0 and c1 are considered to be ambiguous. The spec and intuitions suggest that c1 is more specific than the quantified c0 and that the former should be selected over the latter. This is the behaviour in Dotty.

scalac gives the wrong result because in the specificity test the call isStrictlyMoreSpecific uses existentialAbstraction before comparing the result types of the two methods. The latter ought to take [F[_]]C[F] on to C[F] forSome { type F[_] } however due to a bug in the implementation of ContainsCollector (which backs Type#contains) it fails to identify the occurrence of F in C[F] as a variable requiring existential quantification. Neither C[D] nor C[F] is then a subtype of the other so neither is preferred over the other, hence ambiguity.

PR with a fix incoming ...