4.3.2 Effects of variance and access modifiers

Overriding adheres to the rules of variance. That is, their argument types allow contravariance (less specific types) while their return type allows covariance (more specific types):

class Base {
  public function new() { }

class Child extends Base {
  private function method(obj:Child):Child {
    return obj;

class ChildChild extends Child {
  public override function method(obj:Base):ChildChild {
    return null;

class Main {
  static public function main() { }

Intuitively, this follows from the fact that arguments are "written to" the function and the return value is "read from" it.

The example also demonstrates how visibility may be changed: An overriding field may be public if the overridden field is private, but not the other way around.

It is not possible to override fields which are declared as inline. This is due to the conflicting concepts: While inlining is done at compile-time by replacing a call with the function body, overriding fields necessarily have to be resolved at runtime.