Adding a new design constraint to D

[The Zealot]

On Tuesday, 14 June 2022 at 10:50:56 UTC, bauss wrote:
> On Tuesday, 14 June 2022 at 10:21:34 UTC, The Zealot wrote:
>>
>> a) put the class in it's own module.
>>
>
> No you really can't and I already proved that with a couple 
> examples in the other discussions on this topic; there are 
> situations where you cannot do that, such as accessing private 
> fields on a parent class using a sub class type within the 
> parent class' module.
>
> That's why the module-level private is a lie in D or at least 
> incomplete.

The code you posted works exactly as it should. your function 
void handle(Bar child) does access the class Bar of another 
module, and the members of foo are not accessible.
and you can implement the desired behaviour like this:

import b;
void handle(Bar child) if(isDerivedFrom!(Foo, Bar))
{
   (cast(Foo)child)._c += child.c;
}