Code gives compiler error when using VC++ new preprocessor

[eldiener]

The code:

`#include <boost/describe.hpp>

struct XXX { int x; }; struct YYY : public XXX { long y; };

BOOST_DESCRIBE_STRUCT(YYY,(XXX),(y))

int main() { return 0; }`

compiles without error for gcc-10.2 in C++14 mode, clang-11.0 in C++14 mode, in clang-win-11.0 in C++14 mode, in VC++14.2 in C++14 mode using the default preprocessor, and in VC++14.2 Preview in C++14 mode using the default preprocessor. Yet if I try to compile it with VC++14.2 or VC++14.2 Preview, using the new C++ standard preprocessor which is triggered with the /Zc:preprocessor switch, the code gives the error:

test_describe.cpp test_describe.cpp(12): error C2589: ';': illegal token on right side of '::' test_describe.cpp(12): error C2062: type 'unknown-type' unexpected

I would try to report this to Microsoft but they would have to get Describe, put it into a Boost directory structure, and then try to test it out that way, which might be onerous for them. Obviously gcc and clang have extremely standard C++ preprocessors, so why the new VC++ preprocessor gives an error while gcc and clang do not is the mystery.

[pdimov]

There's a MSVC-specific path in the code because of the use of ##__VA_ARGS__ which MSVC doesn't support and doesn't need: https://github.com/pdimov/describe/blob/eea288e04961374721020b5454707590815b2fcf/include/boost/describe/detail/members.hpp#L50

I suppose that the new preprocessor needs to take the conforming path.

[pdimov]

Or, since ##__VA_ARGS__ is a GCC extension, perhaps the new MSVC preprocessor needs __VA_OPT__(,) instead. I'll figure it out when I have time. :-)

[eldiener]

The old, default preprocessor is:

if !defined(_MSVC_TRADITIONAL) || _MSVC_TRADITIONAL

as opposed to the new preprocessor. I would not have any workarounds for the new preprocessor until there is some proof it needs something.

Identifying VC++ should be something like:

if defined(_MSC_VER) && !defined(__clang__)

You can look at the logic in the Preprocessor's config.hpp, where BOOST_PP_CONFIG_MSVC() identifies the old, default preprocessor and the new preprocessor is considered a strictly conforming C++ standard preprocessor, BOOST_PP_CONFIG_STRICT(). My testing has shown that the new preprocessor is extremely good, but of course I would not rule out any possible bugs which may still be there. But with the latest releases in both VS2019 or the VS2019 preview I have not found any bugs anymore.

[pdimov]

As I said ##__VA_ARGS__ is a non-conforming GCC extension. However, https://docs.microsoft.com/en-us/cpp/preprocessor/preprocessor-experimental-overview?view=msvc-160 says that the new MSVC preprocessor does support it, so using it should be fine. I'll change the condition to check _MSVC_TRADITIONAL after the review ends.

[pdimov]

This should be fixed now.

[eldiener]

It is. Bravo !