VTK json clashes with local json

[davidscn]

Observed in the most recent vtk 9.4.1:

It seems that VTK now includes the same json as we do (nlohmann::json). However, vtk puts the json into their own namespace.
This leads to errors of the form

error: ‘vtknlohmann’ does not name a type; did you mean ‘nlohmann’?
   82 |   nlohmann::json SerializeValues();
      |   ^~~~~~~~
src/configreader.hpp:16:14: error: ‘vtknlohmann’ does not name a type; did you mean ‘nlohmann’?
   16 | using json = nlohmann::json;

My guess would be that the compiler then only includes one of the two json files. Requires some investigation. I didn't face this issue in VTK version 9.3.

[fsimonis]

This could be a packaging issue. Is this on Ubuntu or when compiling from source?

[davidscn]

This was compiling from source on ubuntu 24.04. Could be a make configuration issue as well

[r-walloner]

Can confirm. I faced the same issue when building VTK v9.4.2 from source on Fedora 42.

It seems, the VTK developers are aware and intend to resolve this issue in v9.5.
https://gitlab.kitware.com/vtk/vtk/-/merge_requests/11854

For now, the patch suggested here worked for me:
https://discourse.vtk.org/t/conflict-with-vtknlohmann-and-nlohmann/14977/6