ambiguous implicit values doing conversions with json types

Question

I am trying to make a generic mapper from a JValue (from json4s) to a Scala case class (with contains spire numeric existential type) and vice versa having the scala case class similar to :

case class SomeStat[T : Numeric](a:T, b:T)

for the generic mapper I have something like:

final class SynchronizerClass[B : Numeric, A <: SomeStat[B]](implicit manifest: Manifest[B], a:Manifest[A])
{format => (
  {case j:JValue => {
    implicit val formats = format
    def extractNumber(j:JValue) = {
      for {
        a <- scala.util.Try((j \ "a").extract[B]).toOption
        b <- scala.util.Try((j \ "b").extract[B]).toOption
      } yield {
        SomeStat(a,b).asInstanceOf[A]
      }
    }
    (extractNumber(j)) getOrElse {
      throw new MappingException("Invalid Number Type")
    }
  }},
  {case x:SomeStat[A] =>
    implicit val formats = format
    Map(
      "a" -> Extraction.decompose(x.a),
      "b" -> Extraction.decompose(x.b)
    )}
  )}

the problem is that spire implicit has two for Numeric, as an example I am trying to construct something like this

val j:JValue = ??? // Some Json Value
val someStat = Extraction.decompose(SomeStat[BigDecimal](j))