Fix feature detection of __VA_OPT__ for MSVC #345
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Demonstrate bug rollbear#343 - since MSVC offers individual options to control /std:c++20, /Zc:__cplusplus, and /Zc:preprocessor, it's possible to have a combination where the compiler is in C++20 mode (and says so), but the traditional preprocessor (which not support __VA_OPT__) is in use. This configuration is commonly trigged by Qt6, which requires c++17, suggests c++20, sets /Zc:__cpluspls (so it can #ifdef based on compiler), but doesn't care about the preprocessor. MSVC still doesn't enable the new /Zc:preprocessor by default, even with /std:c++20 or /permissive-.
Only the new conformant MSVC preprocessor (_MSVC_TRADITIONAL==0) ever has __VA_OPT__. This became avaialble with Visual Studio 16.6, but only if running in /std:c++20 or /std:c++latest mode (i.e. _MSVC_LANG > c++17). - https://devblogs.microsoft.com/cppblog/announcing-full-support-for-a-c-c-conformant-preprocessor-in-msvc/ Starting with Visual Studio 17.10, it is now enabled regardless of /std This was changed to better match the behavaior of GCC and clang, which also loosened up their checks: - https://developercommunity.visualstudio.com/t/Zc:preprocessor-__VA_OPT__-is-not-enabl/10205604 - https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98859 (GCC 13 and up allow __VA_OPT__ in -std=c2x as well as -std=c++20) - https://reviews.llvm.org/D91913 (clang 12 and up allow __VA_OPT__ regardless of c/c++ standard) The MSVC traditional preprocessor (_MSVC_TRADITIONAL defined and nonzero) never supports __VA_OPT__, regardless of MSVC or c++ standard version.
Codecov ReportAll modified and coverable lines are covered by tests ✅
Additional details and impacted files@@ Coverage Diff @@
## main #345 +/- ##
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Coverage 99.40% 99.41%
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Files 15 15
Lines 1344 1361 +17
Branches 21 22 +1
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+ Hits 1336 1353 +17
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__cplusplus >= 202002Lis combined with/Zc:preprocessor-(and fail to preprocess correctly).See https://github.com/puetzk/trompeloeil/actions/runs/14110751749 for an example where the new CI configurations (without the corresponding fix) fail to compile, because they try to use
__VA_OPT__in a configuration where the preprocessor does not support it.With the second commit (fixing the detection macro), all of the configurations now run, three of them with different behavior:
"Windows MSVC 2019 C++20 preprocessor" should detect that it does have
__VA_OPT__(because it's >= 16.6, /Zc:preprocessoris on, and/std:c++20was set, making_MSVC_TRADITIONAL=0and_MSVC_LANG=202002L."Windows MSVC 2019 C++20 __cppstd" and "Windows MSVC 2022 C++20 __cppstd" now see that they do not have
__VA_OPT__(because it's using the old preprocessor, despite being__cplusplus >= 202002LFixes #343