[Bug c/112463] New: ternary operator / -Wsign-compare inconsistency

[Bug c/112463] New: ternary operator / -Wsign-compare inconsistency

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112463

            Bug ID: 112463
           Summary: ternary operator / -Wsign-compare inconsistency
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

-Wsign-compare is described in the man page as follows:

  -Wsign-compare
    Warn when a comparison between signed and unsigned  values  could
    produce an incorrect result when the signed value is converted to
    unsigned.   In C++, this warning is also enabled by -Wall.  In C,
    it is also enabled by -Wextra.

But it can emit a warning even in the absence of comparisons between signed and
unsigned values. For instance, it can appear due to the 2nd and 3rd operands of
the ternary operator (these operands are not compared, just selected from the
value of the first operand). This affects the warning output by -Wextra.

Consider the following C code:

#include <stdio.h>
int main (void)
{
  for (int c = -1; c <= 1; c++)
    {
      long long
        i = c == 0 ? 0LL : (c >= 0 ? 1U : -1),
        j = c >= 0 ? (c == 0 ? 0LL : 1U) : -1;
      printf ("i = %lld\nj = %lld\n", i, j);
    }
  return 0;
}

(which shows that the ternary operator is not associative due to type
conversions). With -Wextra, I get:

ternary-op.c: In function ‘main’:
ternary-op.c:7:43: warning: operand of ‘?:’ changes signedness from ‘int’ to
‘unsigned int’ due to unsignedness of other operand [-Wsign-compare]
    7 |         i = c == 0 ? 0LL : (c >= 0 ? 1U : -1),
      |                                           ^~

But the "-Wsign-compare" is incorrect as there are no comparisons between
signed and unsigned values. Only -Wsign-conversion should trigger a warning.

[Bug c/112463] ternary operator / -Wsign-compare inconsistency

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112463

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |egallager at gcc dot gnu.org
           Keywords|                            |diagnostic

--- Comment #1 from Eric Gallager <egallager at gcc dot gnu.org> ---
I think I've seen this bug before, but I can't seem to find the one I was
thinking it might be a dup of...

[Bug c/112463] ternary operator / -Wsign-compare inconsistency

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112463

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So digging through the history and I find this:
https://gcc.gnu.org/pipermail/gcc-patches/1999-December/024572.html

Which added the original place for the warning.
There has been improvements to it later on about enum types and constants but
the option has been that way since then.

One problem with -Wsign-conversion is that it is not enabled with -Wextra/-Wall
.

Maybe there should be a 3rd option added for this that gets enabled for -Wextra
(C)/-Wall(C++) .

[Bug c/112463] ternary operator / -Wsign-compare inconsistency

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112463

--- Comment #3 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Andrew Pinski from comment #2)
> One problem with -Wsign-conversion is that it is not enabled with
> -Wextra/-Wall .

However, I don't understand why -Wsign-compare is enabled by -Wextra but not
-Wsign-conversion, while these are similar warnings.

Note that when compiling GNU MPFR, *both* give lots of false positives (which
could be avoided, at least for most of them, if PR38470 were fixed).