Apparently some programs in the wild use #if __has_attribute(musttail) __attribute__((musttail)) return foo (); #else return foo (); #endif clang supports musttail both as a standard attribute ([[clang::musttail]] which we also support for compatibility) and the above worked just fine with GCC 14 which had __has_attribute(musttail) 0. Now that it is 0, this doesn't compile anymore. So, either we need to ensure that __has_attribute(musttail) is 0 and just __has_c{,pp}_attribute({gnu,clang}::musttail) are non-zero, or IMHO better we just make it work in the attribute form, especially for C < C23 I can see why some projects would prefer that form. While [[gnu::musttail]] is rejected as an error in C11 etc. before GCC 15, rather than just handled as an unknown attribute. I view this as both a regression and compatibility issue. The patch handles it in similar spots to fallthrough/assume attributes inside of __attribute__ for C, and for C++ enables mixing of standard [[]] and GNU __attribute__(()) attributes at the start of statements in any order. While working on it, I've noticed we weren't diagnosing arguments to the clang::musttail attribute (fixed by the c-attribs.cc hunk) and newly on the __attribute__ form attribute (in that case the arguments aren't just skipped, they are always parsed and because we don't call decl_attributes etc., it wouldn't be diagnosed without a manual check). 2025-03-18 Jakub Jelinek <jakub@redhat.com> PR c/116545 gcc/ * doc/extend.texi (musttail statement attribute): Document that musttail GNU attribute can be used as well. gcc/c-family/ * c-attribs.cc (c_common_clang_attributes): Add musttail. gcc/c/ * c-parser.cc (c_parser_declaration_or_fndef): Parse __attribute__((musttail)) return. (c_parser_handle_musttail): Diagnose attribute arguments. (c_parser_statement_after_labels): Parse __attribute__((musttail)) return. gcc/cp/ * parser.cc (cp_parser_statement): Call cp_parser_attributes_opt rather than cp_parser_std_attribute_spec_seq. (cp_parser_jump_statement): Diagnose gnu::musttail attributes with no arguments. gcc/testsuite/ * c-c++-common/attr-fallthrough-2.c: Adjust expected diagnostics for C++. * c-c++-common/musttail15.c: New test. * c-c++-common/musttail16.c: New test. * c-c++-common/musttail17.c: New test. * c-c++-common/musttail18.c: New test. * c-c++-common/musttail19.c: New test. * c-c++-common/musttail20.c: New test. * c-c++-common/musttail21.c: New test. * c-c++-common/musttail22.c: New test. * c-c++-common/musttail23.c: New test. * c-c++-common/musttail24.c: New test. * g++.dg/musttail7.C: New test. * g++.dg/musttail8.C: New test. * g++.dg/musttail12.C: New test. * g++.dg/musttail13.C: New test. * g++.dg/musttail14.C: New test. * g++.dg/ext/pr116545.C: New test.