CMake 4 on MacOS breaks compilation due to old compatibility requirement

[titoiride]

With MacOS homebrew moving to CMake 4, now a few deprecations become errors. For instance, I can't compile anymore because of a cmake compatibility requirement imposed by the Json library.

CMake Error at build/_deps/openpmd-src/share/openPMD/thirdParty/json/CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.

I can work around it setting the environmental variable, as recommended, but I was wondering if it makes sense to either make Json optional, not sure if it's possible, or maybe find an internal workaround?
In my case, I will need to set this flag for my entire code for which openPMD is built internally via cmake as a dependency, so I will need to set the flag for a dependency of a dependency.
I was just bringing this up to understand if you just recommend that I manually work around this in my cmake, which is easy to do, or if you see a long-term solution.

[titoiride]

I just saw that nlohmann json released the version 3.12.0 with support to CMake 4, perhaps it's possible to link to this newer version? I think the same goes for toml11, which is giving the same issue and which released a newer version which supports CMake 4. Unfortunately I don't seem to be able to compile even installing the toml11 version shipped with homebrew because it's version 4.4.0, while the code enforces a different one.

[franzpoeschel]

Hello Davide, we do not enforce any specific versions for those two dependencies. Our build script has specific default versions for nlohmann_json and toml11 that it downloads, but you can override that either by:

• Installing you own build of these two libraries and configuring openPMD with -DopenPMD_USE_INTERNAL_JSON=OFF -DopenPMD_USE_INTERNAL_TOML=OFF
• Setting -DopenPMD_json_tar=https://github.com/nlohmann/json/archive/refs/tags/v3.12.0.tar.gz -DopenPMD_json_tar_hash=SHA256=4b92eb0c06d10683f7447ce9406cb97cd4b453be18d7279320f7b2f025c10187 to change the downloaded release, similar for openPMD_toml11_tar and openPMD_toml11_tar_hash

Thank you for the info on CMake 4, I will open a PR to change the default versions.

[titoiride]

Thank you, I also realized that I was requesting openPMD 0.15.0 in my cmake, and links there were even older. Updating my own code and the requested version to 0.16.1 fixed toml11, but I still needed to manually install the nlohmann_json package via homebrew and use -DopenPMD_USE_INTERNAL_JSON=OFF. Thank you for updating!