[Bug c/51437] New: GCC should warn on the use of reserved identifier/macro names

["vanboxem.ruben at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51437

             Bug #: 51437
           Summary: GCC should warn on the use of reserved
                    identifier/macro names
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vanboxem.ruben@gmail.com


For C and C++, Clang should warn when user code uses identifiers or macros that
are implementation-reserved.

For C(99), this would mean names starting with an underscore followed by
another unerscore or capitalized letter.

For C++ a single underscore is also reserved for the global namespace.

I could have messed the above rules up, but these are the cause of a lot of
unexpected bugs, and a warning would be super-easy to implement.