[Issue 23819] defining your own interface IUnknown messes up vtable without any warning

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23819

--- Comment #3 from Felix Hufnagel <puremagic at zoadian.de> ---
"A COM interface is defined as one that derives from the interface
core.sys.win­dows.com.IUnknown. A COM interface differs from a regular D
interface in that:

It derives from the interface core.sys.windows.com.IUnknown."

I never derived from core.sys.windows.com.IUnknown. 
I just named my own interface IUnknown, and the compiler did some magic.
if I don't define it, it even says it's undefined.

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