scala> :paste
  class :=[T,Q]
  trait Default_:={
    /** Ignore default */
    implicit def useProvided[Provided,Default] = new :=[Provided,Default]
  }
  object := extends Default_:={
    /** Infer type argument to default */
    implicit def useDefault[Default] = new :=[Default,Default]
  }

scala> def foo[T](implicit default: T := Int, ct: reflect.ClassTag[T]) = ct.toString
scala> foo
res0: String = Int
scala> foo[String]
res1: String = java.lang.String