https://github.com/mapbox/vector-tile/blob/2.x/include/mapbox/vector_tile/builder.hpp#L96-L99

Values of type null_value_t, mapbox::util::recursive_wrapper<std::vector<value>>, and mapbox::util::recursive_wrapper<std::unordered_map<std::string, value>>> are simply ignored here. If this is correct, it needs to be documented.