Support for __VA_OPT__ in gcc

[Edward Diener]

On 9/30/2019 6:08 AM, Jonathan Wakely wrote:
> On Sat, 28 Sep 2019 at 04:29, Edward Diener wrote:
>>
>> Given this program:
>>
>> #define PP_THIRD_ARG(a,b,c,...) c
>> #define VA_OPT_SUPPORTED_I(...) PP_THIRD_ARG(__VA_OPT__(,),1,0,)
>> #define VA_OPT_SUPPORTED() VA_OPT_SUPPORTED_I(?)
>>
>> #include <iostream>
>>
>> int main()
>>        {
>>        int result = VA_OPT_SUPPORTED();
>>        std::cout << result;
>>        return 0;
>>        }
>>
>> as a test for __VA_OPT__ support in a C++ compiler ( taken from
>> https://stackoverflow.com/questions/48045470/portably-detect-va-opt-support
>> )
>> I have discovered that __VA_OPT__ support started with gcc-8.1.
> 
> You could also consult
> https://gcc.gnu.org/projects/cxx-status.html#cxx2a which notes that
> the support is incomplete.
> 
>> However
>> I have also discovered that the support occurs no matter what the C++
>> standard level is used for the compilation and not just when the option
>> is 'std=c++2a'. In other words I can compile the program with
>> 'std=c++03', link and run the program and the program will output 1,
>> showing __VA_OPT__ support, rather than 0, which shows that __VA_OPT__
>> is not supported.
>>
>> If I compile the above with any version of gcc lower than gcc-8.1 the
>> program will outpyt 0 no matter what -std mode I use.
>>
>> Is this intended, that the C++20 __VA_OPT__ support works in all modes
>> for gcc-8.1 and higher ?
> 
> Yes, it's available as a GNU extension. You can get a diagnostic with
> -std=c++14 -Wpedantic, but not with any -std=gnu++NN modes.

I would strongly argue that if it is available as a GNU extension 
outside of normal C++20 support in gcc 8.1 on up then it should be 
available when specifying '-std=gnu++nn' but not available when 
specifying '-std=c++nn' for any C++ level except for '-std=c++2a'. Isn't 
the idea of gnu extensions that they should only be available when the 
programmer is using a gnu compiler mode, but not available if a 
programmer is using a c++ compiler mode ?