adapter.scala

// JsValue remembers Adapter
trait JsValue {
  type A
}
object JsValue {
  type Aux[A0] = JsValue { type A = A0}
}

// Adapter works with JsValue of type T
class Adapter[T] { self =>

  def get: JsValue.Aux[T] =
    new JsValue {
      type A = T
    }

  def set(v: JsValue.Aux[T]) = ()
}

class A
class B

object AdapterTest {

  val a1, a2 = new Adapter[A]
  val b1 = new Adapter[B]

  val js1 = a1.get
  val js2 = a2.get

  a1.set(js1) // compiles
  a1.set(js2) // compiles
  b1.set(js1) // do not compiles
}