Can #include'd files be missing?

[NoahAmsel]

My C++ code depends on an external library, so I add the flag -I${nlohmann_SOURCE_DIR}/include to ensure that #include can find the headers. Now #include doesn't really need to work for pybind11_mkdoc to do its job, and indeed, if I do not add the flag -I${EXTERNAL_SOURCE_DIR}/include, pybind11_mkdoc still runs successfully. However, I get the following error message

fatal error: 'nlohmann/json_fwd.hpp' file not found

at the line where this file is #included. Can we eliminate this error message and guarantee to the user that pybind11_mkdoc works even if included files can't be found, or should we require that all #includes succeed?