[Issue 21143] vtbl not accessible without typeid

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21143

--- Comment #4 from Andrei Alexandrescu <andrei at erdani.com> ---
Better yet, the vtbl would be NULL-terminated. That allows multiple
applications to add to it without stepping on each other's toes.

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