Using default type parameter to drive type inference and a type class instance resultion

defaults.scala

trait DefaultsTo[Type, Default]

object DefaultsTo {
  implicit def defaultDefaultsTo[T]: DefaultsTo[T, T] = null
  implicit def fallback[T, D]: DefaultsTo[T, D] = null  
}

trait Document

trait Reader[A]
object Reader {
  implicit def docReader: Reader[Document] =
    new Reader[Document] {}
}

class Storage[DocType](name: String)(implicit val r: Reader[DocType], default: DefaultsTo[DocType, Document])

/**
 * Inserting the above into the Scala repl, the result should be the following (Scala 2.11.7)
 */
scala> val mongo = new Storage("mongo")
mongo: Storage[Document] = Storage@221af3c0

// Resolved implicit value
scala> mongo.r 
res0: Reader[Document] = Reader$$anon$1@57d5872c