With gcc (Debian 20201125-2) 11.0.0 20201125 (experimental) [master revision a4d9837ee4b:97273aee415:b13dacdfb315675803982ad5a3098f7b55e6357a]

double f1 (void)
{
  double d;
  int i;

  for (d = 2.0, i = 0; i < 5; i++, d *= d)
    ;

  return d;
}

double f2 (void)
{
  volatile double d;
  int i;

  for (d = 2.0, i = 0; i < 5; i++, d *= d)
    ;

  return d;
}

yields the following incorrect warning when using "-Wunused-value":

tst.c: In function 'f2':
tst.c:17:10: warning: right-hand operand of comma expression has no effect [-Wunused-value]
   17 |   for (d = 2.0, i = 0; i < 5; i++, d *= d)
      |        ~~^~~~~

Note that one gets a warning only for f2. The only difference is the "volatile".

I found this when testing GNU MPFR (a similar code occurs in tests/tset_ld.c).