[Bug c++/64644] New: "warning: anonymous union with no members" should be an error with -pedantic-errors

["harald at gigawatt dot nl" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64644

            Bug ID: 64644
           Summary: "warning: anonymous union with no members" should be
                    an error with -pedantic-errors
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: harald at gigawatt dot nl

Compiling

    static union { };

with gcc -std=c++11 -pedantic-errors gives

    test.cc:1:16: warning: anonymous union with no members
     static union { };
                    ^

It's fine that it's a warning by default, but I think it should be an error
with -pedantic-errors, because it violates the last part of [dcl.dcl]p3: when
the init-declarator-list is omitted, "the decl-specifier-seq shall introduce
one or more names into the program, or shall redeclare a name introduced by a
previous declaration." No exception is made for anonymous unions, so the only
way an anonymous union can avoid violating this is by having members.

(FWIW, I cannot find any options to get any warning whatsoever with clang,
Intel or Sun, and MSVC only warns at the highest warning level, regardless of
whether language extensions are enabled. As far as I can tell, GCC's current
handling is already the best of these five.)