To satisfy your intent here (as clarified by your last comment to @AlexIv's answer) - keeping the field name of args
while allowing for a default value for the field - you can override the field, using a reference to the original value prefixed by super.
:
object Main extends App {
override val args: Array[String] = if (super.args.isEmpty) Array("default argument") else super.args
println("args(0) = " + args(0))
}