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The C++26 https://wg21.link/P3348R4 C++26 should refer to C23 not C17 paper among other things changes va_start macro in the similar way how C23 has changed it. Now, unlike C17 and older, C++ has since forever allowed int (...) but just one wasn't able to use va_start/va_arg/va_end in such functions. With the current C++26 draft wording, we'd have to #define va_start(V, ...) __builtin_va_start (V, 0) like we've used for C23 before the PR107980 change. But Jonathan has kindly filed https://cplusplus.github.io/LWG/issue4388 which similarly to C23 will if accepted allow to define it as #define va_start(...) __builtin_c23_va_start(__VA_ARGS__) and let the compiler diagnose undesirable cases (see stdarg6.C testcase in the patch for what it can diagnose, basically anything that isn't either va_start (ap) or va_start (ap, i) where i is the last argument's identifier). This patch implements what assumes LWG4388 will pass. It also defines #define __STDC_VERSION_STDARG_H__ 202311L also for C++26. The hardest part is actually something different. C23 had to differentiate between C99 void foo (); i.e. unspecified arguments (but not stdarg) and the new C23 void bar (...); which is stdarg, but in both cases TYPE_ARG_TYPES (fntype) is NULL. This has been implemented through the new TYPE_NO_NAMED_ARGS_STDARG_P flag, fntypes with that flag set are considered stdarg_p and allow va_start in those, while fntypes with NULL TYPE_ARG_TYPES but the flag cleared are not stdarg_p, can accept any number of arguments but can't use va_start. So, I had to change various places in the C++ FE to pass true as the third argument to build_function_type for calls which are meant to be (...) so that one can actually use va_start in those. Done only for C++26 in order not to disturb older versions too much. And there is a problem with some of the builtins and #pragma weak which are using (...) declarations more in the sense of C17 unspecified arguments rather than this call has variable arguments. So, structural_comptypes now considers the non-C++26 (...) used for selected builtins and #pragma weak incompatible with C++26 (...) to avoid ICEs. 2025-10-09 Jakub Jelinek <jakub@redhat.com> gcc/ * ginclude/stdarg.h (va_start): Use __builtin_c23_va_start also for C++26. (__STDC_VERSION_STDARG_H__): Also define for C++26. gcc/c-family/ * c-common.h (D_CXX26): Define. * c-common.cc (c_common_resword): Add D_CXX26 to __builtin_c23_va_start flags, mention D_CXX26 in comment. gcc/cp/ * cp-tree.h (cp_build_function_type): Declare. * lex.cc: Implement va_start changes from P3348R4 - C++26 should refer to C23 not C17 paper. (init_reswords): Set D_CXX26 in mask for C++23 and older. * parser.cc (cp_parser_primary_expression): Handle RID_C23_VA_START. (cp_parser_builtin_c23_va_start): New function. * cp-objcp-common.cc (names_builtin_p): Likewise. * decl.cc (grokfndecl, check_function_type): Pass TYPE_NO_NAMED_ARGS_STDARG_P as last arg to build_function_type. (grokdeclarator, static_fn_type): Use cp_build_function_type instead of build_function_type. * typeck.cc (merge_types): Likewise. (structural_comptypes): Return false for TYPE_NO_NAMED_ARGS_STDARG_P differences. * lambda.cc (maybe_add_lambda_conv_op): Use cp_build_function_type instead of build_function_type. * tree.cc (cp_build_function_type): New function. (strip_typedefs): Pass TYPE_NO_NAMED_ARGS_STDARG_P as last arg to build_function_type. * name-lookup.cc (push_local_extern_decl_alias): Likewise. * module.cc (trees_in::tree_node): Use cp_build_function_type instead of build_function_type. * pt.cc (copy_default_args_to_explicit_spec, rebuild_function_or_method_type, build_deduction_guide): Likewise. (alias_ctad_tweaks): Pass TYPE_NO_NAMED_ARGS_STDARG_P as last arg to build_function_type. * decl2.cc (change_return_type, cp_reconstruct_complex_type): Likewise. gcc/testsuite/ * c-c++-common/cpp/has-builtin-4.c: Expect __has_builtin (__builtin_c23_va_start) == 1 also for C++26. * c-c++-common/Wvarargs.c (foo3): Don't expect undefined behavior warning for C++26. * g++.dg/cpp26/stdarg1.C: New test. * g++.dg/cpp26/stdarg2.C: New test. * g++.dg/cpp26/stdarg3.C: New test. * g++.dg/cpp26/stdarg4.C: New test. * g++.dg/cpp26/stdarg5.C: New test. * g++.dg/cpp26/stdarg6.C: New test. * g++.dg/cpp26/stdarg7.C: New test. * g++.dg/cpp26/stdarg8.C: New test. * g++.dg/cpp26/stdarg9.C: New test. * g++.dg/opt/pr60849.C (foo): Add explicit cast.
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