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So, besides missing #__VA_OPT__ patch for which I've posted patch last week, P1042R1 introduced some placemarker changes for __VA_OPT__, most notably the addition of before "removal of placemarker tokens," rescanning ... and the #define H4(X, ...) __VA_OPT__(a X ## X) ## b H4(, 1) // replaced by a b example mentioned there where we replace it currently with ab The following patch are the minimum changes (except for the __builtin_expect) that achieve the same preprocessing between current clang++ and patched gcc on all the testcases I've tried (i.e. gcc __VA_OPT__ testsuite in c-c++-common/cpp/va-opt* including the new test and the clang clang/test/Preprocessor/macro_va_opt* testcases). At one point I was trying to implement the __VA_OPT__(args) case as if for non-empty __VA_ARGS__ it expanded as if __VA_OPT__( and ) were missing, but from the tests it seems that is not how it should work, in particular if after (or before) we have some macro argument and it is not followed (or preceded) by ##, then it should be macro expanded even when __VA_OPT__ is after ## or ) is followed by ##. And it seems that not removing any padding tokens isn't possible either, because the expansion of the arguments typically has a padding token at the start and end and those at least according to the testsuite need to go. It is unclear if it would be enough to remove just one or if all padding tokens should be removed. Anyway, e.g. the previous removal of all padding tokens at the end of __VA_OPT__ is undesirable, as it e.g. eats also the padding tokens needed for the H4 example from the paper. 2021-09-01 Jakub Jelinek <jakub@redhat.com> PR preprocessor/101488 * macro.c (replace_args): Fix up handling of CPP_PADDING tokens at the start or end of __VA_OPT__ arguments when preceeded or followed by ##. * c-c++-common/cpp/va-opt-3.c: Adjust expected output. * c-c++-common/cpp/va-opt-7.c: New test.
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