[Bug c++/105499] New: inconsistency between -Werror=c++-compat and g++ in __extension__ block

["vincent-gcc at vinc17 dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105499

            Bug ID: 105499
           Summary: inconsistency between -Werror=c++-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 = q; p; });
}

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

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

so no errors with "gcc -Werror=c++-compat", but an error with g++. This is 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=c++-compat first, and with g++ a bit later in a more extensive test).