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The following patch introduces a -Wkeyword-macro warning that clang has since 2014 to implement part of C++26 P2843R3 Preprocessing is never undefined paper. The relevant change in the paper is moving [macro.names]/2 paragraph to https://eel.is/c++draft/cpp.replace.general#9 : "A translation unit shall not #define or #undef names lexically identical to keywords, to the identifiers listed in Table 4, or to the attribute-tokens described in [dcl.attr], except that the names likely and unlikely may be defined as function-like macros." Now, my understanding of the paper is that in [macro.names] and surrounding sections the word shall bears different meaning from [cpp.replace.general], where only the latter location implies ill-formed, diagnostic required. The warning in clang when introduced diagnosed all #define/#undef directives on keywords, but shortly after introduction has been changed not to diagnose #undef at all (with "#undef a keyword is generally harmless but used often in configuration scripts" message) and later on even the #define part tweaked - not warn about say #define inline (or const, extern, static), or #define keyword keyword or #define keyword __keyword or #define keyword __keyword__ Later on the warning has been moved to be only pedantic diagnostic unless requested by users. Clearly some code in the wild does e.g. #define private public and similar games, or e.g. Linux kernel (sure, C) does #define inline __inline__ __attribute__((__always_inline__)) etc. Now, I believe at least with the current C++26 wording such exceptions aren't allowed (unless it is changed to IFNDR). But given that this is just pedantic stuff, the following patch makes the warning off by default for C and C++ before C++26 and even for C++26 it enables it by default only if -pedantic/-pedantic-errors (in that case it pedwarns, otherwise it warns). And it diagnoses both #define and #undef without exceptions. From what I can see, all the current NODE_WARN cases are macros starting with __ with one exception (_Pragma). As the NODE_* flags seem to be a limited resource, I chose to just use NODE_WARN as well and differentiate on the node names (if they don't start with __ or _P, they are considered to be -Wkeyword-macro registered ones, otherwise old NODE_WARN cases, typically builtin macros or __STDC* macros). 2025-08-07 Jakub Jelinek <jakub@redhat.com> PR preprocessor/120778 gcc/ * doc/invoke.texi (Wkeyword-macro): Document. gcc/c-family/ * c.opt (Wkeyword-macro): New option. * c.opt.urls: Regenerate. * c-common.h (cxx_dialect): Comment formatting fix. * c-opts.cc (c_common_post_options): Default to -Wkeyword-macro for C++26 if pedantic. gcc/c/ * c-decl.cc (c_init_decl_processing): Mark cpp nodes corresponding to keywords as NODE_WARN if warn_keyword_macro. gcc/cp/ * lex.cc (cxx_init): Mark cpp nodes corresponding to keywords, identifiers with special meaning and standard attribute identifiers as NODE_WARN if warn_keyword_macro. gcc/testsuite/ * gcc.dg/Wkeyword-macro-1.c: New test. * gcc.dg/Wkeyword-macro-2.c: New test. * gcc.dg/Wkeyword-macro-3.c: New test. * gcc.dg/Wkeyword-macro-4.c: New test. * gcc.dg/Wkeyword-macro-5.c: New test. * gcc.dg/Wkeyword-macro-6.c: New test. * gcc.dg/Wkeyword-macro-7.c: New test. * gcc.dg/Wkeyword-macro-8.c: New test. * gcc.dg/Wkeyword-macro-9.c: New test. * g++.dg/warn/Wkeyword-macro-1.C: New test. * g++.dg/warn/Wkeyword-macro-2.C: New test. * g++.dg/warn/Wkeyword-macro-3.C: New test. * g++.dg/warn/Wkeyword-macro-4.C: New test. * g++.dg/warn/Wkeyword-macro-5.C: New test. * g++.dg/warn/Wkeyword-macro-6.C: New test. * g++.dg/warn/Wkeyword-macro-7.C: New test. * g++.dg/warn/Wkeyword-macro-8.C: New test. * g++.dg/warn/Wkeyword-macro-9.C: New test. * g++.dg/warn/Wkeyword-macro-10.C: New test. * g++.dg/opt/pr82577.C: Don't #define register to nothing for C++17 and later. Instead define reg macro to nothing for C++17 and later or to register and use it instead of register. * g++.dg/modules/atom-preamble-3.C: Add -Wno-keyword-macro to dg-additional-options. * g++.dg/template/sfinae17.C (static_assert): Rename macro to ... (my_static_assert): ... this. (main): Use my_static_assert instead of static_assert. libcpp/ * include/cpplib.h (struct cpp_options): Add cpp_warn_keyword_macro. (enum cpp_warning_reason): Add CPP_W_KEYWORD_MACRO enumerator. (cpp_keyword_p): New inline function. * directives.cc (do_undef): Support -Wkeyword-macro diagnostics. * macro.cc (warn_of_redefinition): Ignore NODE_WARN flag on nodes registered for -Wkeyword-macro. (_cpp_create_definition): Support -Wkeyword-macro diagnostics. Formatting fixes.