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From: "vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/105499] New: inconsistency between -Werror=c++-compat and
 g++ in __extension__ block
Date: Fri, 06 May 2022 00:12:51 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105499

            Bug ID: 105499
           Summary: inconsistency between -Werror=3Dc++-compat and g++ in
                    __extension__ block
           Product: gcc
           Version: 11.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

Consider

int *f (void *q)
{
  return __extension__ ({ int *p =3D q; p; });
}

With GCC 11.3.0 under Debian (Debian package), I get the following:

$ gcc -Werror=3Dc++-compat -c tst.c
$ g++ -c tst.c
tst.c: In function =E2=80=98int* f(void*)=E2=80=99:
tst.c:3:36: error: invalid conversion from =E2=80=98void*=E2=80=99 to =E2=
=80=98int*=E2=80=99 [-fpermissive]
    3 |   return __extension__ ({ int *p =3D q; p; });
      |                                    ^
      |                                    |
      |                                    void*

so no errors with "gcc -Werror=3Dc++-compat", but an error with g++. This i=
s not
consistent. Either this is regarded as a valid extension in C++ so that both
should succeed, or this is not valid C++ code even in __extension__ so that
both should fail.

Same issue with various GCC versions from 4.9 to 11.3.0.

AFAIK, the purpose of -Wc++-compat is to test whether code would still pass
when replacing C compilation by C++ (there might be false positives or false
negatives, but this should not be the case with the above example).

FYI, I got the above issue while testing GNU MPFR (tested with
-Werror=3Dc++-compat first, and with g++ a bit later in a more extensive te=
st).=