implicit summon fails in dotty, works in 2.*

[drdozer]

The code below compiles and runs under 2.11.8 and 2.12.1, but fails under dotty with:

[info] Compiling 1 Scala source to /home/nmrp3/devel/turingatemyhamster/commandeer/target/scala-2.11/classes...
/home/nmrp3/devel/turingatemyhamster/commandeer/src/main/scala/commandeer/RunMe.scala:32: error: no implicit argument of type DSL found for parameter dsl of method apply in object CommandeerDSL
    val kevin = CommandeerDSL(null.asInstanceOf[Foo])
                                                     ^
/home/nmrp3/devel/turingatemyhamster/commandeer/src/main/scala/commandeer/RunMe.scala:34: error: value Bar is not a member of Nothing(kevin)
    val bar = kevin.Bar("bob", 3)
                    ^
two errors found
[error] (compile:compileIncremental) Compilation failed

I'm attempting to summon an implicit instance of a trait, but getting back type information specific to the instance that I've summoned, beyond that in the trait that I summoned it through. There may be a better way to do this with dotty, but I don't know it.

package commandeer


trait CommandeerDSL[Host] {
  trait Operation[T]
  type Op[T] <: Operation[T]
}

object CommandeerDSL {
  def apply[Host, DSL <: CommandeerDSL[Host]](host: Host)(implicit dsl: DSL): DSL = dsl
}
 
trait Foo {
  def bar(a: String, b: Int): Double
}
 
object Foo {
  implicit val fooDSL: FooDSL = new FooDSL {}
}
 
trait FooDSL extends CommandeerDSL[Foo] {
  sealed trait FooOperation[T] extends Operation[T]
  type Op[T] = FooOperation[T]

  case class Bar(a: String, b: Int) extends FooOperation[Double]
}
 
object RunMe {
  def main(args: Array[String]): Unit = {
    println("Hi Mum")

    val kevin = CommandeerDSL(null.asInstanceOf[Foo])
    println(s"Found DSL for Foo: $kevin")
    val bar = kevin.Bar("bob", 3)
    println(s"Made a bar: $bar")
  }
}

[odersky]

I think this can be fixed with a tweak to the inference rules, which was missing so far in dotty.