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

[mikomikotaishi]

Hmm, this problem with the standard library has to do with symbols having internal linkage. They fixed most of this in GCC 15, because that's when they shipped module std.

Let me try it with Ubuntu 25.10 and GCC 15

[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