It’s easy to forget that a trait may extend a class. If you use a trait, you can postpone the decision of which constructor to call, like this:
trait Extended extends Base {
...
}
object Extended {
def apply(arg1: Int) = new Base(arg1) with Extended
def apply(arg2: String) = new Base(arg2) with Extended
def apply(arg3: Double) = new Base(arg3) with Extended
}
Scala 2 traits may not themselves have constructor parameters, but you can work around that by using abstract members instead.
(Scala 3 lets traits have constructor parameters.)