short form
 def toArray(arg0:Array[_]) : Array[_] = {
   new Array[_](0)
 }
results in:
class AllUsers needs to be abstract, since method toArray in trait Collection of type [T](Array[T])Array[T] is not defined

or my best guess at a long form:
 def toArray(arg0:Array[X] forSome {type X}) : Array[X] = {
   new Array[X](0)
 }
results in
not found: type X
not found: type X