Scala Play: how to define a "Json writable class" - json

Question

I would like to write a function that transforms case classes to Json:
import play.api.libs.json._
def myJson(cc: Product): JsValue = {
Json.toJson(cc) // simplified
}
Each case class has an implicit Writes[T], for example:
case class Test(a: Int)
object Test {
implicit val jsonWrites: Writes[Test] = Json.writes[Test]
}
It is possible to write Json.toJson(new Test(1)) individually, but the myJson function above does not compile because it never knows if cc has an implicit Writes defined.
[How can I write the function signature so that it takes only classes having a Writes implicit?]
Edit: How can I write the function input type so that it corresponds only to classes having a Writes implicit?
I tried this:
trait JsonWritableResult[T <: Product] {
implicit val jsonWrites: Writes[T]
}
case class Test(a: Int)
object Test extends JsonWritableResult[Test] {
implicit val jsonWrites: Writes[Test] = Json.writes[Test]
}
def myJson(cc: JsonWritableResult[_ <: Product]): JsValue = {
Json.toJson(cc)
}
But it says "No Json serializer found for type models.JsonWritableResult[_$2]".

Answer 1

Something like this seems to give you the behavior you want.
import play.api.libs.json.{JsValue, Json, Writes}
trait Product {}
case class Test(a: Int) extends Product
object Test {
implicit val jsonWrites: Writes[Test] = Json.writes[Test]
}
def myJson[T <: Product](cc: T)(implicit writes: Writes[T]): JsValue = {
Json.toJson(cc) // simplified
}
import Test._
myJson(Test(3))
This is not tested in the general case, but in a Worksheet it seems to work.

Answer 2

Instead of forcing the case class to have the implicit, it is more clever to force it to override a toJson method from a trait, using or not an implicit defined in the case class. Much simpler and it works. Then my factory can explicit this trait in its output type, and thus I can serialize whatever it outputs.
But since the other answer answered the question that I formulated wrong, I accept it ;)