[Bug middle-end/101415] New: [12 Regression] Bogus -Warray-bounds warning with stpcpy

["fw at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101415

            Bug ID: 101415
           Summary: [12 Regression] Bogus -Warray-bounds warning with
                    stpcpy
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fw at gcc dot gnu.org
  Target Milestone: ---

This (derived from the glibc function of the same name):

char *
nis_local_group (char *cptr)
{
  static char __nisgroup[1025];

  if (*cptr != '\0' && __builtin_strlen (cptr) < 1024)
    {
      char *cp = __builtin_stpcpy (__nisgroup, cptr);

      if (cp[-1] != '.')
        __builtin_abort ();
    }

  return __nisgroup;
}

results in 

/tmp/t.c: In function ‘nis_local_group’:
/tmp/t.c:10:13: error: array subscript -1 is outside array bounds of
‘char[1025]
’ [-Werror=array-bounds]
   10 |       if (cp[-1] != '.')
      |           ~~^~~~
/tmp/t.c:4:15: note: at offset -1 into object ‘__nisgroup’ of size 1025
    4 |   static char __nisgroup[1025];
      |               ^~~~~~~~~~

with -O2 -Wall on x86-64 gcc version 12.0.0 20210711 (experimental) [master
revision :97a8a2829:269256f33c51222167ad461f775d5468bb5ecaf5].

The warning is bogus because stpcpy returns a pointer to the NUL byte, which is
not at the first byte of __nisgroup after the stpcpy. The glibc original does
not have this check, so it is buggy, but the warning seems overly aggressive if
it cannot detect non-empty strings for stpcpy.