Some slg libraries violates the C++ One Definition Rule on 2.8alpha1

[luyatshimbalanga]

The build system from Fedora reported a violation of C++ One Definition Rule within the SLG libraries. See
build.log with the following extract:

builddir/build/BUILD/LuxCore-luxcorerender_v2.8alpha1/redhat-linux-build/generated/luxparser/luxparse.hpp:139:34: warning: ‘luxcore_parserlxs_yylval’ violates the C++ One Definition Rule [-Wodr]
  139 | extern LUXCORE_PARSERLXS_YYSTYPE luxcore_parserlxs_yylval;
      |                                  ^
/builddir/build/BUILD/LuxCore-luxcorerender_v2.8alpha1/redhat-linux-build/generated/luxparser/luxparse.cpp:1728:9: note: type ‘union LUXCORE_PARSERLXS_YYSTYPE’ itself violates the C++ One Definition Rule
 1728 | YYSTYPE yylval;

Affected version: LuxCore 2.8 alpha1

Tested on development branch of Fedora running on GCC 13
root.log with built dependency

[CodeFHD]

I have tested this again on the branch for_v2.11, as the LuxCore build system changed significantly with v2.10.
I don't see this warning in the output anymore, though the code is still there and unchanged.

It seems that luxparse.hpp is not actually used:
https://github.com/LuxCoreRender/LuxCore/blob/54ee101247f51b12e666bcdb333e39138d78a41c/src/luxcore/luxparser/luxparse.cpp#L760C1-L760C63

So it seems to me there is not actually a problem with a double definition.
@howetuft seems to be a Bison/Yacc thing. Are you familiar with details?

[howetuft]

Yes I am!

Here's the explanation:
There is a small parser for LuxRender (nota bene: not LuxCore), written in Bison/Yacc. However, as Bison/Yacc may not be installed in all build environments, someone had also included an already processed version of the parser. This already processed version is the file luxparse.hpp. The build process falls back to it in case Bison/Yacc is not available.

This is this already processed version that raises the above warnings.

However, in the new build system, a working version of Bison/Yacc is always provided, in all environments, as a tool dependency, and the default luxparse.hpp is no more used. And the files processed by this version of B/Y do not raise the above warnings.