[Bug c++/102399] Cannot mix GCC and C++11 / C23 attribute syntax

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102399

--- Comment #2 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:aec868578d8515763d75693c1fdfbc30ff0a1e68

commit r13-991-gaec868578d8515763d75693c1fdfbc30ff0a1e68
Author: Marek Polacek <polacek@redhat.com>
Date:   Thu Jun 2 15:44:20 2022 -0400

    c++: Allow mixing GNU/std-style attributes [PR69585]

    cp_parser_attributes_opt doesn't accept GNU attributes followed by
    [[]] attributes and vice versa; only a sequence of attributes of the
    same kind.  That causes grief for code like:

      struct __attribute__ ((may_alias)) alignas (2) struct S { };

    or

      #define EXPORT __attribute__((visibility("default")))
      struct [[nodiscard]] EXPORT F { };

    It doesn't seem to a documented restriction, so this patch fixes the
    problem.

    However, the patch does not touch the C FE.  The C FE doesn't have
    a counterpart to C++'s cp_parser_attributes_opt -- it only has
    c_parser_transaction_attributes (which parses both GNU and [[]]
    attributes), but that's TM-specific.  The C FE seems to use either
    c_parser_gnu_attributes or c_parser_std_attribute_specifier_sequence.
    As a consequence, this works:

      [[maybe_unused]] __attribute__((deprecated)) void f2 ();

    but this doesn't:

      __attribute__((deprecated)) [[maybe_unused]] void f1 ();

    I'm not sure what, if anything, should be done about this.

            PR c++/102399
            PR c++/69585

    gcc/cp/ChangeLog:

            * parser.cc (cp_parser_attributes_opt): Accept GNU attributes
            followed by [[]] attributes and vice versa.

    gcc/testsuite/ChangeLog:

            * g++.dg/ext/attrib65.C: New test.
            * g++.dg/ext/attrib66.C: New test.
            * g++.dg/ext/attrib67.C: New test.