Help making testcase for CMake: pybind11 and vftable symbol

[mbechard]

Hey,
Recently I ran into an issue using pybind11 with a library that was built using CMake's CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS workflow, which automatically creates the .def file for MSVC to export symbols from a library that doesn't use the declspec(dllexport) explicit workflow. It does this by inspecting the .o files after compilation and building up a .def file from symbols it thinks are needed.
Specifically, I was getting a undefined symbol error for const DeviceBase::`vftable'
After looking at the source for CMake, I submitted a pull request to add the vftable as one of the symbols that get added to the auto-generated .def file.
https://gitlab.kitware.com/cmake/cmake/-/merge_requests/8201

But the missing piece is a testcase. How to make a simple testcase that would force a dependency on this symbol in some simple C++ code that doesn't use pybind11.
Does anyone know what code in pybind11 creates this dependency specifically? It seems to be related to the constructors. It occurs due to this call in this library:
https://github.com/luxonis/depthai-python/blob/7ef3f6f11117cc9b930bbfed952615a9a89d33e5/src/DeviceBindings.cpp#L476

Any ideas?
Thanks!

[mbechard]

I'll answer my own question, not related to pybind11 in the end. A delegating constructor in the header file causes a reference to the vftable.