Scala Spray Templated Custom Routing Directive

Question

Okay so.... I have this:

  def convertPost  = extract {
    _.request.entity.asString.parseJson.convertTo[CustomClass]
  }

  private def myRoute: Route =
    (post & terminalPath("routeness")) {
      convertPost { req =>
        detach() {
           ThingHandler.getMyResults( req )
        }
      }
    }

but I want to template it, like this:

  def convertPost[T] = extract {
    _.request.entity.asString.parseJson.convertTo[T]
  }

  private def myRoute: Route =
    (post & terminalPath("routeness")) {
      convertPost[CustomClass] { req =>
        detach() {
           ThingHandler.getMyResults( req )
        }
      }
    }

But that doesn't work. I am using spray-json-shapeless. My error is

Error:(28, 50) Cannot find JsonReader or JsonFormat type class for T
    _.request.entity.asString.parseJson.convertTo[T]
                                                 ^

when I try:

  def getStuff[T] = extract {
    _.request.entity.asInstanceOf[T] // .convertTo[T]
  }

it gives:

spray.http.HttpEntity$NonEmpty cannot be cast to com.stuff.CustomClass