I'm experimentally rebuilding rawhide with the not-yet-released GCC 15 to see if anything breaks, and to help write the porting guide.  See https://fedoraproject.org/wiki/User:Dmalcolm/gcc-15

My test build with GCC 15 failed:
https://copr.fedorainfracloud.org/coprs/dmalcolm/gcc-15-smoketest-3.failed/build/8476064/

whereas my test build with GCC 14 succeeded:
https://copr.fedorainfracloud.org/coprs/dmalcolm/gcc-15-smoketest-3.failed.checker/build/8477649/

Looking at the failure logs e.g.
https://download.copr.fedorainfracloud.org/results/dmalcolm/gcc-15-smoketest-3.failed/fedora-rawhide-x86_64/08476064-adanaxisgpl/builder-live.log.gz

I see:

MushRuby/missing.h:163:12: error: conflicting types for 'vsnprintf'; have 'int(void)'
  163 | extern int vsnprintf _((char *, size_t n, char const *, va_list));
      |            ^~~~~~~~~
In file included from /usr/include/features.h:524,
                 from /usr/include/bits/libc-header-start.h:33,
                 from /usr/include/stdlib.h:26,
                 from MushRuby/ruby.h:52:
/usr/include/bits/stdio2.h:93:1: note: previous definition of 'vsnprintf' with type 'int(char *, long unsigned int,  const char *, __va_list_tag *)'
   93 | __NTH (vsnprintf (__fortify_clang_overload_arg (char *, __restrict, __s),
      | ^~~~~

This is probably due to GCC 15 now defaulting to -std=gnu23, whereas GCC 14 defaulted to -std=gnu17, and C23 is stricter about function prototypes than C17.  It's probably fixable by fixing the function prototypes (or by manually adding -std=gnu17 to the C build flags)



Reproducible: Always