>If this restriction were to be encoded in the type system, GtkMenuShell would not be a true subtype of GtkContainer as it cannot be used in every place that a

>If this restriction were to be encoded in the type system, GtkMenuShell would not be a true subtype of GtkContainer as it cannot be used in every place that a GtkContainer could be used — specifically it cannot be the target of gtk_container_add(myButton). This is easily solvable by making the Container be a generic class "Container of <T extends Widget>". If the language supports such things of course, so not really talking about GTK specifically here. Then a Container<MenuItem> is not a subtype of Container<Widget> (so you're not violating LSP), yet you can still write code that operates on all kinds of containers without giving up any compile-time type checking. I agree with a lot of the other things in the article, just this was not a particularly good example, because it's more about the shortcomings of a specific language than the shortcomings of subtype polymorphism in general.