[Bug c/77970] inconsistent and unhelpful -Wformat warning for %lc

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77970

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Blocks|                            |125576
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=67479

--- Comment #3 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Martin Sebor from comment #0)
> The following snippet compiles with no warnings on popular 64-bit targets
> including powerpc64-*-linux, sparc-sun-solaris2.10, and x86_64-*-linux. 
> However, on a number of 32-bit targets the same code causes GCC to issue a
> warning about one or the other call pedantically complaining about either
> the literal zero not having the expected type of wint_t (e.g., where wint_t
> is an alias for long) even though the two have the same size and sign, or
> about the L'a' argument not having the wint_t type for the same reason (the
> type of L'a' is wchar_t).  (This is analogous to warnings issued for %li
> with an int argument, or for %i with a long argument, on ILP32.)
> 
>   printf ("%lc", 0);
>   printf ("%lc", L'a');
> 
> Unlike the warnings for ordinary integer directives which is issued
> consistently, issuing the warning for %lc inconsistently, based on the
> underlying type of wchar_t and wint_t, and only on uncommon targets and not
> on mainstream targets where most code is developed is unhelpful for
> portability and only serves to cause code to fail to compile when ported
> (see for example the thread around
> https://gcc.gnu.org/ml/gcc-patches/2016-10/msg00993.html for the effort it
> took to clean up these warnings in the builtin-sprintf*.c tests).
> 
> Issuing the warning when both the sign and the size of the argument are the
> same as those expected is useless entirely since objects of the two types
> are indistinguishable.  (The warning is not issued when the types only
> differ in signedness such as int and unsigned int unless the
> -Wformat-signedness option is explicitly specified.)
> 
> To be useful for portability, the warning should be issued consistently on
> all targets, especially those where most development is done (LP64 targets
> such powerpc64-*-linux and x86_64-*-linux.  The warning is also only useful
> if there exists a target where wint_t and wchar_t (for the L'a' case) or int
> (for the int argument case) are different sizes.  If there is no such target
> (I don't know of one and a survey of GCC config files didn't reveal one
> either) the warning isn't useful even if wint_t and wchar_t differ in sign
> because sign mismatches are handled separately under the -Wformat-signedness
> option.
> 
> As a data point, Clang stopped issuing this warning in 2011 in response to
> the following Clang bug:
> 
>   https://llvm.org/bugs/show_bug.cgi?id=7981
> 
> GCC should do the same and either suppress the warning altogether or issue
> it only with -Wpedantic.

...or maybe the -Wformat-pedantic flag requested in bug 67479?


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125576
[Bug 125576] [meta-bug] clang/LLVM compatibility