json.hpp forcibly defines GCC_VERSION

[polesapart]

While json.hpp defines CLANG_VERSION & GCC_VERSION where appropriate:

// exclude unsupported compilers
#if defined(__clang__)
    #define CLANG_VERSION (__clang_major__ * 10000 + __clang_minor__ * 100 +     __clang_patchlevel__)
    #if CLANG_VERSION < 30400
        #error "unsupported Clang version - see https://github.com/nlohmann/json#supported-compilers"
    #endif
#elif defined(__GNUC__)
    #define GCC_VERSION (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 +     __GNUC_PATCHLEVEL__)
    #if GCC_VERSION < 40900
        #error "unsupported GCC version - see https://github.com/nlohmann/json#supported-compilers"
    #endif
#endif

It fails to take a few facts into account:

1. At least GCC_VERSION is used elsewhere (several open source projects such as python, mysql, etc). Leaking it leads to errors such as this: components/third_party/json.hpp:66:0: error: GCC_VERSION" redefined [-Werror]

2. The macro is only used on the spot, so it seems like a better use would be something like that:

    #elif defined(__GNUC__)
        #define JSGCC_VERSION (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 +     __GNUC_PATCHLEVEL__)
       #if JSGCC_VERSION < 40900
       ...
       #undef JSGCC_VERSION
   #endif /* defined __GNUC__ */
   

Or something.

[nlohmann]

Good point!

[gregmarr]

This macro isn't even really needed:

#elif defined(__GNUC__)
    #if (__GNUC__ < 4) || ((__GNUC__ == 4) && (__GNUC_MINOR__ < 9))
        #error "unsupported GCC version - see https://github.com/nlohmann/json#supported-compilers"
    #endif
#endif /* defined __GNUC__ */

[nlohmann]

I changed the code so that no symbol is defined.