#include <filesystem> doesn't work with gcc-7 when `-std=c++17` is specified.

What is the issue you have?

GCC 7 (on Ubuntu 18.04) doesn't have <filesystem>, instead, it has <experimental/filesystem> with namespace std::experimental::filesystem. When -std=c++17 is specified on Ubuntu 18.04 with GCC-7, this library fails to compile, as

	#ifdef JSON_HAS_CPP_17
	#include <filesystem>
	#endif

doesn't work. The inclusion for <filesystem> requires more care with GCC.

If you experience a compilation error: can you compile and run the unit tests?

yes
no - please copy/paste the error message below

/home/runner/work/v6d/v6d/thirdparty/nlohmann-json/include/nlohmann/detail/conversions/from_json.hpp:23:14: fatal error: filesystem: No such file or directory
     #include <filesystem>
              ^~~~~~~~~~~~
compilation terminated.