CMake: ROOT & VTK JSON namespace conflicts

[2xB]

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Description

Since ROOT on Fedora 40 was updated from 6.32.08 to 6.34.02 a few days ago, our CIs (e.g. for https://github.com/KATRIN-Experiment/Kassiopeia) fail with the following CMake error:

CMake Error at /usr/lib64/cmake/vtk/VTK-targets.cmake:553 (set_target_properties):
  The link interface of target "VTK::nlohmannjson" contains:
    nlohmann_json::nlohmann_json
  but the target was not found.  Possible reasons include:
    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

Therefore there seems to be a namespace conflict between ROOT and VTK.

Reproducer

See https://github.com/KATRIN-Experiment/Kassiopeia for an exemplary project using both VTK and ROOT. It is set up so also in forks the CI can be used in the main branch or in Pull Requests to said fork. I also used said CI to test changes to VTK already, the code for which can be found here: KATRIN-Experiment/Kassiopeia@main...2xB:Kassiopeia:vtktest . Changes to ROOT should be testable in a similar way.

If that is not minimal enough, I can also try to create a minimal working example.

ROOT version

6.34.02

Installation method

Fedora Packages

Operating system

Fedora

Additional context

This was also reported to VTK here: https://discourse.vtk.org/t/cmake-error-vtk-nlohmannjson/15170

[linev]

Is ROOT build with builtin_nlohmannjson?

[2xB]

As far as I understand https://src.fedoraproject.org/rpms/root/blob/f40/f/root.spec , for the Fedora package that I use, builtin_nlohmannjson is set to OFF. (CC @ellert since they are the expert here)