[Bug c/65446] Improve -Wformat-signedness

[Bug c/65446] Improve -Wformat-signedness

[burnus at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65446

--- Comment #1 from Tobias Burnus <burnus at gcc dot gnu.org> ---
(In reply to Tobias Burnus from comment #0)
> It doesn't warn but should warn for
>    printf("%u\n", _short);

Actually, it (correctly) does warn in this case (as short it promoted to int,
which is also unsigned).

[Bug c/65446] Improve -Wformat-signedness

[eblake at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65446

Eric Blake <eblake at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |eblake at redhat dot com

--- Comment #2 from Eric Blake <eblake at redhat dot com> ---
'unsigned short' promotes to 'int', not 'unsigned int', when passed through
var-args.  The warning is technically correct, but annoying, since all values
of 'unsigned short' are representable in both 'int' and in 'unsigned int',
therefore %d vs. %u won't ever have to deal with a negative value.

"%u" with signed 'short' should indeed warn, because that is a case where
promotion to 'int' differs from printing unsigned values.

"%x" with 1 should indeed warn, because that is passing 'int'.  Use "%x" with
1U if you want to avoid the warning.

[Bug c/65446] Improve -Wformat-signedness

[eblake at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65446

--- Comment #4 from Eric Blake <eblake at redhat dot com> ---
Arguably, "%u" with short should warn, while "%hu" with short should not.  On
the other hand, if I use "%hu" with int, it is unclear to me whether I should
get a warning (the fact that I'm using %h to intentionally truncate the value
before it is printed, even though %hu and %hd print different values, makes it
hard to discern whether I have a mismatch).  But this does mean that whatever
changes are made to -Wformat-signedness, it should take into account the use of
%h range-limiting vs. pre-integer-promotion types.