[Bug c++/103401] [11/12 Regression] gcc accepts decltype(auto(0)) as the parameter of the function

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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 <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:6a071b2d40a1078b4029c2b77ef29ffca4e7050c

commit r12-5860-g6a071b2d40a1078b4029c2b77ef29ffca4e7050c
Author: Marek Polacek <polacek@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.