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/8476037/

whereas my test build with GCC 14 succeeded:
https://copr.fedorainfracloud.org/coprs/dmalcolm/gcc-15-smoketest-3.failed.checker/build/8477585/

Looking at the failure logs e.g.
https://download.copr.fedorainfracloud.org/results/dmalcolm/gcc-15-smoketest-3.failed/fedora-rawhide-x86_64/08476037-MagicPoint/builder-live.log.gz

I see:

mgp.c:114:14: error: conflicting types for 'expandPath'; have 'char *(char *)'
  114 | extern char *expandPath __P((char *));
      |              ^~~~~~~~~~
In file included from mgp.h:73,
                 from mgp.c:34:
image/xloadimage.h:79:7: note: previous declaration of 'expandPath' with type 'char *(void)'
   79 | char *expandPath(); /* path.c */
      |       ^~~~~~~~~~

Looks like C23 issues; see https://fedoraproject.org/wiki/User:Dmalcolm/gcc-15#C23:_Function_prototypes_with_empty_params_change_from_implicit_%22int%22_to_%22void%22

Reproducible: Always