[Bug c++/81513] New: __has_cpp_attribute returns non-zero in C++03 mode, but attributes don't work

["proski at gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81513

            Bug ID: 81513
           Summary: __has_cpp_attribute returns non-zero in C++03 mode,
                    but attributes don't work
           Product: gcc
           Version: 7.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: proski at gnu dot org
  Target Milestone: ---

Following compiles with "g++ -Wall -std=c++03 test.cpp -o test" and outputs
"200809 201603 199711"

#include <iostream>
int main()
{
  std::cout << __has_cpp_attribute(noreturn) << ' ' <<
    __has_cpp_attribute(maybe_unused) << ' ' << __cplusplus << '\n';
}

[[maybe_unused]] doesn't work in C++03 mode in g++. It does work in C++11 mode,
despite it being a C++17 feature. In C++11 mode, __cplusplus would be less than
__has_cpp_attribute(maybe_unused), yet [[maybe_unused]] would work.

With clang-4.0 in C++03 mode, the output is "0 0 199711". I believe GCC should
do the same. If attributes are not supported at all, __has_cpp_attribute()
should return 0.

I want to do following without special casing GCC or older standards:

#if defined(__has_cpp_attribute) && __has_cpp_attribute(maybe_unused)
#define __maybe_unused [[maybe_unused]]
#endif