feat: Add C++ modules support and update nlohmann::json

[mikomikotaishi]

This pull request adds support for C++20 modules.

In order to accomplish this, some constants were changed to using external linkage. I have also updated nlohmann/json to version 3.12.0, as the older version declares some symbols as internal.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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[Mishura4]

Thanks for the PR! Good stuff. Just gonna prefer the approach with including dpp.h in the module for maintainability, as discussed in https://discord.com/channels/825407338755653642/825411104208977952/1454456223952011427

[braindigitalis]

some module related unit tests would be good. not repeating a thousand lines of code in dpp.cppm is an absolute must, as @Mishura4 also said.

[mikomikotaishi]

I've looked at the src/unittest/ files but I'm not particularly familiar with how you've set up the unit testing framework. Are you just looking to run those same tests under import dpp; rather than #include <dpp/dpp.h>?

[braindigitalis]

I think that in this case you might want to make a new .cpp under the unittests folder which imports dpp instead of #including it. it can have some test functions that are exported and called via standard approaches from the main unittest file.

[mikomikotaishi]

I think this should fix the errors we're encountering. It's with the build system configuration

[braindigitalis]

you cant just do -DDPP_MODULES=ON for the entire CI matrix. Some CI's use older g++ (which we support) and some use C++17.

[mikomikotaishi]

Then is it possible to disable the modules unit tests for C++17 CI tests?

[mikomikotaishi]

@braindigitalis I think the modules CI should just be in a totally separate CI. It requires Ninja, as CMake can't build modules with Makefiles.

[braindigitalis]

unit tests only run in one g++, which is g++-12. This is the one which is packaged. we cant and dont run unit tests on them all, as the unit tests connect to discord and we cant do this concurrently.

[Jaskowicz1]

Updated Title to match code standards

[Jaskowicz1]

Since you're having issues with g++-14 (which makes no sense because I can build g++-14 modules fine locally), just use a clang version that's supported (pretty sure clang15 has modules, but use clang18).

Make sure you set the CXX for the env on both builds and then the apt install too 🫡

[mikomikotaishi]

I have no idea why this current error is happening seeing as it never happened on my end, though I built mine in Clang 21.

[Jaskowicz1]

Seems it's a clang18 issue 🙃 , change to clang20

[Mishura4]

Can't we set it up so that target_link_libraries(${testname} PUBLIC dpp) brings the module, as discussed on the discord?

[mikomikotaishi]

I think my current revision fixes this, but please review and let me know if it is acceptable.

[Jaskowicz1]

Im rather confused why the CI keeps saying

Manually-specified variables were not used by the project:

    DPP_MODULES