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/build/8410922/

whereas my test build with GCC 14 succeeded:
  https://copr.fedorainfracloud.org/coprs/dmalcolm/gcc-15-smoketest-3.checker/build/8410980/

Looking at the failure:
https://download.copr.fedorainfracloud.org/results/dmalcolm/gcc-15-smoketest-3/fedora-rawhide-x86_64/08410922-6tunnel/builder-live.log.gz

I see e.g.:

      |                          ^~
6tunnel.c: In function ‘sighup’:
6tunnel.c:565:24: error: passing argument 2 of ‘signal’ from incompatible pointer type [-Wincompatible-pointer-types]
  565 |         signal(SIGHUP, sighup);
      |                        ^~~~~~
      |                        |
      |                        void (*)(void)
In file included from /usr/include/sys/wait.h:36,
                 from 6tunnel.c:33:
/usr/include/signal.h:88:57: note: expected ‘__sighandler_t’ {aka ‘void (*)(int)’} but argument is of type ‘void (*)(void)’
   88 | extern __sighandler_t signal (int __sig, __sighandler_t __handler)
      |                                          ~~~~~~~~~~~~~~~^~~~~~~~~


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