gistfile1.scala

// Define the following traits and companion object
// It's in Rapture Core (https://github.com/propensive/rapture-core) if you don't want to

trait LowPriorityDefaultsTo { implicit def fallback[T, S]: DefaultsTo[T, S] = null }
object DefaultsTo extends LowPriorityDefaultsTo { implicit def defaultDefaultsTo[T]: DefaultsTo[T, T] = null }
trait DefaultsTo[T, S]

// Then, assuming we want to specify a default for a type class like `Namer`,
case class Namer[T](name: String)

// where we have a couple of alternatives,
implicit val stringNamer = Namer[String]("string")
implicit val intNamer = Namer[Int]("int")

// we define the default one for a particular method like this:
def myMethod[T](implicit default: T DefaultsTo String, namer: Namer[T]) = namer.name

// Let's try it out in the REPL:
scala> myMethod
res0: String = string

scala> myMethod[Int]
res1: String = int

For documentation purpose, the link to a similar implementation in the mongo scala driver: https://github.com/mongodb/mongo-scala-driver/blob/76dad47c8dbfc98b59867355a966cf95ef72e556/bson/src/main/scala/org/mongodb/scala/bson/DefaultHelper.scala