playjson.scala

scala> import play.api.libs.json._
import play.api.libs.json._

scala> case class Test[A: Writes](a: A)
defined class Test

scala> implicit def testWrites[A: Writes] = Json.writes[Test[A]]
<console>:12: error: No apply function found matching unapply parameters
       implicit def testWrites[A: Writes] = Json.writes[Test[A]]
                                                       ^