package methods are final? huh?

[Lodovico Giaretta via Digitalmars-d]

On Sunday, 2 October 2016 at 11:37:34 UTC, Manu wrote:
> [...]
>
> Why? I have a UI system with modules for each node type. UI 
> internal stuff is naturally 'package'. Virtuals are perfectly 
> reasonable within a package.

One reason I can think of: what happens if someone outside the 
package extends one of your classes?

===================================

module packA.something;

class Base
{
     abstract void foo();
}

module packB.something;

class Derived1: Base
{
     override void foo() {}     // Oops... should not know about 
this method,
                                //         but without it the 
class is abstract.
}
===================================

The fact is, package virtual is fine as long as the class cannot 
be extended outside the package, or can be extended outside the 
package but no subclass defined in this way may need a different 
definition of the method. Such a constraint is difficult to 
enforce in the language.