Compiling collections: subtyping failures for Factories

[DarkDimius]

while compiling scala.collection.immutable.Map

$ dotc Map.scala
warning: encountered F-bounded higher-kinded type parameters for type scala$collection$generic$GenMapFactory$$CC; assuming they are invariant
warning: encountered F-bounded higher-kinded type parameters for type scala$collection$generic$MapFactory$$CC; assuming they are invariant
warning: encountered F-bounded higher-kinded type parameters for type scala$collection$generic$ImmutableMapFactory$$CC; assuming they are invariant
Map.scala:79: error: Type argument [HK$0, +HK$1] => scala.collection.immutable.Map[HK$0, HK$1] does not conform to upper bound [HK$0, +HK$1] =>
   <:
    scala.collection.immutable.Map[HK$0, HK$1] &
      scala.collection.immutable.MapLike[HK$0, HK$1,
        LazyRef([HK$0, +HK$1] => scala.collection.immutable.Map[HK$0, HK$1])[
          HK$0
        , HK$1]
      ]
object Map extends ImmutableMapFactory[Map] {
                                       ^
three warnings found
one error found