[Bug c++/52761] New: [C++11] Missing diagnostic for opaque unscoped enum declaration without base

["hstong at ca dot ibm.com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52761

             Bug #: 52761
           Summary: [C++11] Missing diagnostic for opaque unscoped enum
                    declaration without base
    Classification: Unclassified
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: hstong@ca.ibm.com
              Host: powerpc64-unknown-linux-gnu
            Target: powerpc64-unknown-linux-gnu


### Self-contained source:$ cat enumredecl.cc
enum E { X };
enum E;  // illegal


### Analysis:
For C++2011, subclause 7.2 [dcl.enum] paragraph 2 says:
An opaque-enum-declaration declaring an unscoped enumeration shall not omit the
enum-base.

The code is also invalid under C++03.
The "redeclaration" does not meet the requirements in C++03 clause 7 [dcl.dcl]
paragraph 3 for the omission of the init-declarator-list in a simple-
declaration. Namely, it is not a class definition or declaration and is not a
enum definition.

It also fails to requirements in subclause 7.1.5.3 [dcl.type.elab] paragraph 1
for an elaborated-type-specifier that is the sole constituent of a declaration.
Namely, it is neither an explicit specialization, explicit instantiation, class
declaration nor friend class declaration.


### Compiler invocations:
$ g++-4.6.0 --std='c++0x' -Wall -W -pedantic enumredecl.cc -c
$ g++-4.6.0 --std='c++98' -Wall -W -pedantic enumredecl.cc -c


### Compiler output (both cases):
(return code 0)


### g++ -v output:$ g++-4.6.0 -v
Using built-in specs.
COLLECT_GCC=g++-4.6.0
COLLECT_LTO_WRAPPER=/data/gcc/libexec/gcc/powerpc64-unknown-linux-gnu/4.6.0/lto-wrapper
Target: powerpc64-unknown-linux-gnu
Configured with: ./configure --prefix=/data/gcc --program-suffix=-4.6.0
--disable-libssp --disable-libgcj --enable-version-specific-runtime-libs
--with-cpu=default32 --enable-secureplt --with-long-double-128 --enable-shared
--enable-__cxa_atexit --enable-threads=posix --enable-languages=c,c++,fortran
--with-gmp=/usr/local
Thread model: posix
gcc version 4.6.0 (GCC)