Surely if GCC's vtable layout isn't compatible with that used by MSVC then COM couldn't possibly work?
Not much of the rest of the Win32 API would care though, other than DirectX and possibly a few more modern additions I'm not aware of.
Luckily, GCC's vtable layout is compatible with COM, but that's because COM puts certain restrictions on how you write C++ classes. Most notably, you must not define a virtual destructor and you must not use overloaded virtual methods. It just so happens that the main difference between MSVC and GCC/Clang concern the implementation of virtual destructors (the former uses 1 entry, the latter uses 2 entries) and the vtable ordering of overloaded virtual methods. This means that COM is not affected!
