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We already warn on #undef or pedwarn on #define (but not on #define after #undef) of some builtin macros mentioned in cpp.predefined. The C++26 P2843R3 paper changes it from (compile time) undefined behavior to ill-formed. The following patch arranges for warning (for #undef) and pedwarn (on #define) for the remaining cpp.predefined macros. __cpp_* feature test macros only for C++20 which added some of them to cpp.predefined, in earlier C++ versions it was just an extension and for pedantic diagnostic I think we don't need to diagnose anything, __STDCPP_* and __cplusplus macros for all C++ versions where they appeared. Like the earlier posted -Wkeyword-macro diagnostics (which is done regardless whether the identifier is defined as a macro or not, obviously most likely none of the keywords are defined as macros initially), this one also warns on #undef when a macro isn't defined or later #define after #undef. 2025-08-15 Jakub Jelinek <jakub@redhat.com> PR preprocessor/120778 PR target/121520 gcc/c-family/ * c-cppbuiltin.cc (c_cpp_builtins): Implement C++26 DR 2581. Add cpp_define_warn lambda and use it as well as cpp_warn where needed. In the if (c_dialect_cxx ()) block with __cpp_* predefinitions add cpp_define lambda. Formatting fixes. gcc/c/ * c-decl.cc (c_init_decl_processing): Use cpp_warn instead of cpp_lookup and NODE_WARN bit setting. gcc/cp/ * lex.cc (cxx_init): Remove warn_on lambda. Use cpp_warn instead of cpp_lookup and NODE_WARN bit setting or warn_on. gcc/testsuite/ * g++.dg/DRs/dr2581-1.C: New test. * g++.dg/DRs/dr2581-2.C: New test. * c-c++-common/cpp/pr92296-2.c: Expect warnings also on defining special macros after undefining them. libcpp/ * include/cpplib.h (struct cpp_options): Add suppress_builtin_macro_warnings member. (cpp_warn): New inline functions. * init.cc (cpp_create_reader): Clear suppress_builtin_macro_warnings. (cpp_init_builtins): Call cpp_warn on __cplusplus, __STDC__, __STDC_VERSION__, __STDC_MB_MIGHT_NEQ_WC__ and __STDCPP_STRICT_POINTER_SAFETY__ when appropriate. * directives.cc (do_undef): Warn on undefining NODE_WARN macros if not cpp_keyword_p. Don't emit any NODE_WARN related diagnostics if CPP_OPTION (pfile, suppress_builtin_macro_warnings). (cpp_define, _cpp_define_builtin, cpp_undef): Temporarily set CPP_OPTION (pfile, suppress_builtin_macro_warnings) around run_directive calls. * macro.cc (_cpp_create_definition): Warn on defining NODE_WARN macros if they weren't previously defined and not cpp_keyword_p. Ignore NODE_WARN for diagnostics if CPP_OPTION (pfile, suppress_builtin_macro_warnings).