[llvm-bugs] [Bug 40665] New: __VA_OPT__ is only supported by Clang's preprocessor in C++2a mode but GCC seems to support it in all language modes

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40665

            Bug ID: 40665
           Summary: __VA_OPT__ is only supported by Clang's preprocessor
                    in C++2a mode but GCC seems to support it in all
                    language modes
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: dan at su-root.co.uk
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

The attached program can be compiled by GCC 8.2.1 in various language modes

```
gcc -x c -std=gnu89 varargs.c
gcc -x c -std=gnu99 varargs.c
gcc -x c -std=gnu11 varargs.c
gcc -x c -std=gnu89 varargs.c
gcc -x c -std=gnu99 varargs.c
gcc -x c -std=gnu11 varargs.c
g++ -x c++ -std=c++98 varargs.c
g++ -x c++ -std=c++11 varargs.c
g++ -x c++ -std=c++14 varargs.c
g++ -x c++ -std=c++17 varargs.c
g++ -x c++ -std=c++2a varargs.c
g++ -x c++ -std=gnu++98 varargs.c
g++ -x c++ -std=gnu++11 varargs.c
g++ -x c++ -std=gnu++14 varargs.c
g++ -x c++ -std=gnu++17 varargs.c
g++ -x c++ -std=gnu++2a varargs.c
```

but Clang fails to compile it because the `__VA_OPT__()` macro does not get
expanded for all of the above language variants except for `c++2a` and
`gnu++2a`.

I believe `__VA_OPT_()` is a C++2a feature so Clang is probably more correct
here.

However according to https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html
`__VA_OPT_()` is supposed to be available for GNU C and GNU C++ too so we
probably ought to enable it when we are compiling for the GNU language variants
`-std=gnu*`.

The fact that GCC allows `__VA_OPT__()` in non-GNU language variants that isn't
`c++2a` seems like a bug but I am not currently in a position to report it.

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