[Bug c/65040] [5 Regression] gcc-5 -Wformat broken

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65040

--- Comment #11 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
-funsigned-char is an option that shouldn't be used without serious
consideration.
That said, I think in that case the warning is intentional, you are indeed
passing a signed value to unsigned format.  If you don't care about that, don't
use -Wformat=2 or use -Wno-format-signedness.