https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103401
--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The trunk branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>: https://gcc.gnu.org/g:6a071b2d40a1078b4029c2b77ef29ffca4e7050c commit r12-5860-g6a071b2d40a1078b4029c2b77ef29ffca4e7050c Author: Marek Polacek <pola...@redhat.com> Date: Thu Nov 25 09:08:03 2021 -0500 c++: Handle auto(x) in parameter-declaration-clause [PR103401] In C++23, auto(x) is valid, so decltype(auto(x)) should also be valid, so void f(decltype(auto(0))); should be just as void f(int); but currently, everytime we see 'auto' in a parameter-declaration-clause, we try to synthesize_implicit_template_parm for it, creating a new template parameter list. The code above actually has us calling s_i_t_p twice; once from cp_parser_decltype_expr -> cp_parser_postfix_expression which fails and then again from cp_parser_decltype_expr -> cp_parser_expression. So it looks like we have f<auto, auto> and we accept ill-formed code. This shows that we need to be more careful about synthesizing the implicit template parameter. [dcl.spec.auto.general] says that "A placeholder-type-specifier of the form type-constraintopt auto can be used as a decl-specifier of the decl-specifier-seq of a parameter-declaration of a function declaration or lambda-expression..." so this patch turns off auto_is_... after we've parsed the decl-specifier-seq. That doesn't quite cut it yet though, because we also need to handle an auto nested in the decl-specifier: void f(decltype(new auto{0})); therefore the cp_parser_decltype change. To accept "sizeof(auto{10})", the cp_parser_type_id_1 hunk only gives a hard error when we're not parsing tentatively. The cp_parser_parameter_declaration hunk broke lambda-generic-85713-2.C but I think the error we issue with this patch is in fact correct, and clang++ agrees. The r11-1913 change is OK: we need to make sure that we see '(auto)' after decltype to go ahead with 'decltype(auto)'. PR c++/103401 gcc/cp/ChangeLog: * parser.c (cp_parser_decltype): Clear auto_is_implicit_function_template_parm_p. (cp_parser_type_id_1): Give errors only when !cp_parser_simulate_error. (cp_parser_parameter_declaration): Clear auto_is_implicit_function_template_parm_p after parsing the decl-specifier-seq. (cp_parser_sizeof_operand): Clear auto_is_implicit_function_template_parm_p. gcc/testsuite/ChangeLog: * g++.dg/cpp1y/lambda-generic-85713-2.C: Add dg-error. * g++.dg/cpp1y/pr60054.C: Adjust dg-error. * g++.dg/cpp1y/pr60332.C: Likewise. * g++.dg/cpp2a/concepts-pr84979-2.C: Likewise. * g++.dg/cpp2a/concepts-pr84979-3.C: Likewise. * g++.dg/cpp2a/concepts-pr84979.C: Likewise. * g++.dg/cpp23/auto-fncast7.C: New test. * g++.dg/cpp23/auto-fncast8.C: New test. * g++.dg/cpp23/auto-fncast9.C: New test.
