Hello, I'm a member of Microsoft VCPKG, when I built this port in an internal version of Visual Studio, it failed with following errors:

mongocxx\instance.cpp(79): error C2338: Must be trivially constructible

Line79:                 static_assert(std::is_trivially_constructible<decltype(current_instance)>::value, "Must be trivially constructible");

Since there is a static_assert that std::atomic is trivially constructible, then the code need to be fixed to handle the adoption of http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0883r2.pdf which makes atomic have a constructor that value-initializes the contained T.