[Bug c/95057] missing -Wunused-but-set-variable warning on multiple assignments, not all of them used

["vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95057

--- Comment #5 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
FYI, Clang 16 does not warn either on the testcases provided in comment 0 (bug
report).

But contrary to GCC (tested with master r14-1713-g6631fe419c6 - Debian
gcc-snapshot package 20230613-1), Clang 15 and 16 both warn on f and g below:

int h (void);

void f (void)
{
  int i = h ();
  i++;
}

void g (void)
{
  for (int i = 0 ;; i++)
    if (h ())
      break;
}

zira:~> clang-15 -c -Wunused-but-set-variable warn-inc.c
warn-inc.c:5:7: warning: variable 'i' set but not used
[-Wunused-but-set-variable]
  int i = h ();
      ^
warn-inc.c:11:12: warning: variable 'i' set but not used
[-Wunused-but-set-variable]
  for (int i = 0 ;; i++)
           ^
2 warnings generated.

Thanks to this detection, a test with Clang 16 found two issues in GNU MPFR
(one cosmetic about a useless loop variable and one important in the
testsuite). So it is useful to consider such particular cases.