[Bug c++/98859] New: pedantic error on use of __VA_OPT__ before C++20 is unnecessary and counterproductive

["richard-gccbugzilla at metafoo dot co.uk" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98859

            Bug ID: 98859
           Summary: pedantic error on use of __VA_OPT__ before C++20 is
                    unnecessary and counterproductive
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: richard-gccbugzilla at metafoo dot co.uk
  Target Milestone: ---

There's no good way in ISO C or C++ to express what the GNU ,##__VA_ARGS__
extension does prior to the addition of __VA_OPT__. However, code targeting new
compilers (that doesn't want to use GNU C / GNU C++) cannot reliably use
__VA_OPT__ instead of the comma paste extension, because GCC's -pedantic-errors
mode rejects it outside C++20.

Such rejection is unnecessary: __VA_OPT__ is a reserved identifier in other
language modes, so there is no conformance reason to issue a diagnostic on its
use. I think it'd be useful for GCC to unconditionally allow using __VA_OPT__
in all language modes. (I'm changing Clang to do the same.)