[Bug c++/56289] New: Bad unused value warning with comma operator

[Bug c++/56289] New: Bad unused value warning with comma operator

[thomaseding at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56289

             Bug #: 56289
           Summary: Bad unused value warning with comma operator
    Classification: Unclassified
           Product: gcc
           Version: 4.6.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: thomaseding@gmail.com


Compile the following with -Wunused-value:

int main () {
    int x = 0;
    int y = 0;
    (void) x, y; /* warning: right operand of comma operator has no effect
[-Wunused-value] */
    return 0;
}


This is a bad warning, as casting to void should eliminate the warning for both
x and y.

[Bug c++/56289] Bad unused value warning with comma operator

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56289

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2013-02-11 18:32:58 UTC ---
 (void) x, y;
You are only casting x to void and not y.

[Bug c++/56289] Bad unused value warning with comma operator

[thomaseding at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56289

--- Comment #2 from Thomas <thomaseding at gmail dot com> 2013-02-11 18:35:29 UTC ---
But x was used inside the comma operator.

[Bug c++/56289] Bad unused value warning with comma operator

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56289

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> 2013-02-11 18:40:06 UTC ---
(In reply to comment #2)
> But x was used inside the comma operator.

It is complaining about the right hand side which is y.

[Bug c++/56289] Bad location for unused value warning with comma operator

[gayathri.gottumukkala.27 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56289

Gayathri Gottumukkala <gayathri.gottumukkala.27 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |gayathri.gottumukkala.27@gm
                   |                            |ail.com

--- Comment #5 from Gayathri Gottumukkala <gayathri.gottumukkala.27 at gmail dot com> ---
We cannot cast the two variables in single line using void. Instead of that we
have to use void for two variables separately.

[Bug c++/56289] Bad location for unused value warning with comma operator

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56289

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Gayathri Gottumukkala from comment #5)
> We cannot cast the two variables in single line using void.

That's not what the code is trying to do.

> Instead of that
> we have to use void for two variables separately.

Saying "if you don't write the code that triggers the bug, it doesn't trigger
the bug" is not very useful.