[Bug c/78155] New: missing warning on invalid isalpha et al.

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78155

            Bug ID: 78155
           Summary: missing warning on invalid isalpha et al.
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Isalpha and other character classification functions/macros defined in the
<ctype.h> header require their argument to be in range of unsigned char or EOF
and have undefined behavior otherwise.  On some implementations (such as
Glibc), calling them with an argument outside that range crashes the program
(see below).  To help detect this common error GCC should issue a warning when
the argument is known to be invalid (as in the test case below) or when its
value or range is unknown, when it its type is char that is subject to sign
extension.

$ cat b.c && /build/gcc-git/gcc/xgcc -B/build/gcc-git/gcc  -Wall -Wextra
-Wpedantic -fdump-tree-optimized=/dev/stdout b.c && ./a.out 
int main (void)
{
    __builtin_printf ("%i\n", __builtin_isalpha (999999));
}


;; Function main (main, funcdef_no=2, decl_uid=1965, cgraph_uid=2,
symbol_order=2)

main ()
{
  int D.1968;
  int _1;
  int _4;

  <bb 2>:
  _1 = __builtin_isalpha (999999);
  __builtin_printf ("%i\n", _1);
  _4 = 0;

<L0>:
  return _4;

}


Segmentation fault (core dumped)