[Bug c/37190] New: -Wredundant-decls -Wno-error=redundant-decls not working as expected

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

Consider this code:
 extern char *optarg;
 extern char *optarg;

$ gcc testcase-min.i -c  -Werror -Wredundant-decls -Wno-error=redundant-decls
-fdiagnostics-show-option
testcase-min.i:2: warning: redundant redeclaration of ?optarg?
[-Wredundant-decls]
cc1: warnings being treated as errors
testcase-min.i:1: error: previous declaration of ?optarg? was here

It does work if I don't use -Wredundant-decls at all, but I want to see the
warning:
$ gcc testcase-min.i -c  -Wall -Werror

$ gcc -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.3.1-9'
--with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.3
--program-suffix=-4.3 --enable-clocale=gnu --enable-libstdcxx-debug
--enable-objc-gc --enable-mpfr --enable-cld --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.3.1 (Debian 4.3.1-9)


-- 
           Summary: -Wredundant-decls -Wno-error=redundant-decls not working
                    as expected
           Product: gcc
           Version: 4.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: edwintorok at gmail dot com
 GCC build triplet: x86_64-linux-gnu
  GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37190