Support for __VA_OPT__ in gcc

[Edward Diener]

On 9/30/2019 9:34 AM, Jonathan Wakely wrote:
> On Mon, 30 Sep 2019 at 14:20, Edward Diener
> <eldlistmailingz@tropicsoft.com> wrote:
>>
>> 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 ?
> 
> No, absolutely not. The manual seems clear on that point:
> 
> "The compiler can accept several base standards, such as ‘c90’ or
> ‘c++98’, and GNU dialects of those standards, such as ‘gnu90’ or
> ‘gnu++98’. When a base standard is specified, the compiler accepts all
> programs following that standard plus those using GNU extensions that
> do not contradict it. For example, -std=c90 turns off certain features
> of GCC that are incompatible with ISO C90, such as the asm and typeof
> keywords, but not other GNU extensions that do not have a meaning in
> ISO C90, such as omitting the middle term of a ?: expression."

So if a programmer compiles using "-std=c++11" gcc feels it has a 
perfect right to add to its C++11 implementation some feature of C++14 ( 
or any other C++ standard for that matter ) which does not have any 
meaning in C++11, as a GNU extension, without the programmer having to 
specify "-std=gnu++11"? I understand that this is what is specified in 
your manual but I think such an approach is wrong. If I as a programmer 
specify a version of the standard, that is what I expect to get, not 
some amalgam of other standard constructs which would normally have no 
meaning in the C++ standard I am using.

I am arguing that in principle that gcc's approach, in this regard, is 
wrong from the end-users point of view while fully conceding that it is 
what is specified in your manual from which you quote above.