Re: ./lib/error.h:410:8: warning: ISO C forbids braced-groups within expressions [-Wpedantic]

[Pádraig Brady]

On 02/09/2023 18:54, Bruno Haible wrote:
> Bjarni Ingi Gislason wrote:
> > While compiling "groff" with gcc (Debian 13.2.0-2) 13.2.0) and "-std=gnu2x":
> >
> > ../lib/openat-die.c: In function 'openat_save_fail':
> > ./lib/error.h:410:8: warning: ISO C forbids braced-groups within expressions 
> > [-Wpedantic]
> >    410 |      : ({ \
> >        |        ^
> > ./lib/error.h:470:7: note: in expansion of macro '__gl_error_call'
> >    470 |       __gl_error_call (error, status, __VA_ARGS__)
> >        |       ^~~~~~~~~~~~~~~
> > ../lib/openat-die.c:37:3: note: in expansion of macro 'error'
> >     37 |   error (exit_failure, errnum,
> >        |   ^~~~~
> > ../lib/openat-die.c: In function 'openat_restore_fail':
> > ./lib/error.h:410:8: warning: ISO C forbids braced-groups within expressions 
> > [-Wpedantic]
> >    410 |      : ({ \
> >        |        ^
> > ./lib/error.h:470:7: note: in expansion of macro '__gl_error_call'
> >    470 |       __gl_error_call (error, status, __VA_ARGS__)
> >        |       ^~~~~~~~~~~~~~~
> > ../lib/openat-die.c:56:3: note: in expansion of macro 'error'
> >     56 |   error (exit_failure, errnum,
> >        |   ^~~~~
>
> The option "-std=gnu2x" is useful; however, the option "-Wpedantic" that you
> have enabled is generally much less useful.
>
> This code in lib/error.h exists for the purpose of providing actually
> useful warnings about the control flow, such as when a 'break;' statement
> in a switch statement has been forgotten. We will not revert or change
> this code, just for "-Wpedantic".
>
> GNU extensions like ({...}) have their purpose, and we use them because
> they are useful here.
>
> You asked for -Wpedantic warnings; you obtained them.

A recent gotcha I've noticed with -Wpedantic is that it may be ignored.
I.e. testing on gcc 13, and ever since commit:
https://git.sv.gnu.org/gitweb/?p=gnulib.git;a=commitdiff;h=4f6131a786
if the user specifies --Wpedantic, then it's ignored
(and that's not at all obvious as there are early warnings before the pragma).

I was trying to use -Wpedantic to check for standards conformance,
where some code compilable on gcc 13 by default, is no longer compilable on gcc 
<= 10.
A workaround for me for now is to use more specific gcc language conformance 
flags,
and filter to non gnulib code, like:

  $ make CFLAGS='-O2 -Wc99-c11-compat -Wc11-c2x-compat -Wno-error' 2>&1 |
    grep 'src/.*warning:' | grep -Ev '_Noreturn|_Static_assert'
  src/tail.c:2211:13: warning: a label can only be part of a statement \
  and a declaration is not a statement [-Wc11-c2x-compat]

cheers,
Pádraig