[Bug c/65446] Improve -Wformat-signedness

["rnsanchez at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

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

Ricardo Nabinger Sanchez <rnsanchez at gmail dot com> changed:

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--- Comment #5 from Ricardo Nabinger Sanchez <rnsanchez at gmail dot com> ---
Created attachment 38666
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=38666&action=edit
Simple test case

I understand the reasoning behind the implicit promotions.

Here is my rationale, asking for the warning to be emitted nonetheless: suppose
the first test in the test case I supplied is used in code that emits assembly
for an assembler.  If the programmer mistakenly used %hu for a int16 type, and
that happened to generate a jump address, the implicit promotion would silently
hide an important error (possibly jumping to the opposite direction).  The
printed-out value is clearly incorrect.  It's not a compiler fault, but it just
so happens that the compiler is in the best position to warn the user that s/he
might run into problems.  Let the programmer decide whether this should be
looked into (fixing his/her code) or if s/he will ignore.

In my case, I noticed this issue (silent signedess-mismatch) because I was
getting negative ports (IPv4/IPv6) on someone else's code, which was only a
visual nuisance.  It would have been caught with the warning, but there was no
way I could get one.  Clang failed to generate such a warning as well (and in
fact produces the same output).

The same happens with %hhd/%hhu specifiers when given mismatching
uint8_t/int8_t.

In short: please warn.  Some cases will generate unexpected results (which are
not compilation errors), simply because the programmer used an incorrect format
specifier.