conflict with installed nlohman/json

[KrisThielemans]

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Description

I have a compilation error in the case with an existing version of the nlohmann library

/home/runner/install/include/nlohmann/json.hpp:3435:8: error: redefinition of default argument for ‘class T’
 3435 | struct adl_serializer;
      |        ^~~~~~~~~~~~~~
In file included from /home/runner/work/SIRF-SuperBuild/SIRF-SuperBuild/build/sources/ROOT/graf3d/eve7/inc/ROOT/REveShape.hxx:15,
                 from /home/runner/work/SIRF-SuperBuild/SIRF-SuperBuild/build/sources/ROOT/graf3d/eve7/inc/ROOT/REveBox.hxx:15,
                 from /home/runner/work/SIRF-SuperBuild/SIRF-SuperBuild/build/sources/ROOT/graf3d/eve7/src/REveBox.cxx:12:
/home/runner/work/SIRF-SuperBuild/SIRF-SuperBuild/build/sources/ROOT/graf3d/eve7/inc/ROOT/REveElement.hxx:39:10: note: original definition appeared here
   39 | template<typename T = void, typename SFINAE = void>
      |          ^~~~~~~~
In file included from /home/runner/work/SIRF-SuperBuild/SIRF-SuperBuild/build/sources/ROOT/graf3d/eve7/src/REveBox.cxx:18:
/home/runner/install/include/nlohmann/json.hpp:3448:7: error: redefinition of default argument for ‘template<class U, class V, class ... Args> class ObjectType’
 3448 | class basic_json;

I believe this is cause by the fact that the installed version is v3.10.4 (which doesn't have json_fwd.h). Relevant bits of code:

• CMake code checks for version
• CMake defaults to "build-in" OFF
• REveElement.hxx contains a work-around if json_fwd.h is not found, i.e. it defines its own.

I believe the latter is causing the problem: somehow the existing (old) json.h has been read, and then creates a conflict with this definition.

I think this work-around needs to be removed. Possibly the easiest is to just require a more recent version.

Reproducer

install old version and try to build ROOT

ROOT version

I tried with v6-28-12 and 3-13.-3 with gcc-11.

Installation method

compile from source

Operating system

Ubuntu 22.04

Additional context

There is no problem with v6-26-10

[bellenot]

@linev might be interested, he fixed a couple of issue with nlohman/json in the past

[linev]

Just to understand - is ROOT compiled with external or builtin nlohmann/json.hpp?

Strange that v3.10.4 makes problem.
ROOT builtin version is 3.10.5 and definitions in REveElement.hxx should be compatible with it.

[KrisThielemans]

Just to understand - is ROOT compiled with external or builtin nlohmann/json.hpp?

I used default options, so builtin_nlohmannjson=OFF

Strange that v3.10.4 makes problem. ROOT builtin version is 3.10.5 and definitions in REveElement.hxx should be compatible with it.

yes, that's strange. I haven't checked the diff between 3.10.5 and 3.10.4.

[linev]

And seems to be, starting from llvm13 and root 6.30 use of the json_fwd.h is no longer necessary.
Before interpreter always had problems to correctly parse nlohmann/json.hpp

I will do several more checks and probably just remove it's usage from master and 6.30 branches.

[linev]

I remove usage of nlohmann/json_fwd.hpp in 6.30 branch.
Fix will be release with 6.30.06 version