Re: Support for __VA_OPT__ in gcc

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

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."

Since __VA_OPT__ has no meaning in C++ prior to C++20 (and is a
reserved name so can't conflict with anything else declared by users)
it clearly falls into the second category, "GNU extensions that do not
contradict it".