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

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

[dangelog at gmail dot com]

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

            Bug ID: 102399
           Summary: Cannot mix GCC and C++11 / C23 attribute syntax
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dangelog at gmail dot com
  Target Milestone: ---

Hello,

If one tries to mix GCC's attribute syntax (__attribute__) and with C++11 / C23
([[attribute]]), GCC rejects the code (no matter what the order of the
attributes is):


  #define EXPORT __attribute__((visibility("default")))

  struct [[nodiscard]] EXPORT Foo { Foo(); };


https://gcc.godbolt.org/z/W868xjeMq


While a workaround exists (by sticking to the same "kind" of attribute syntax),
it's not always possible or easy to achieve in big codebases, where for
compatibility/legacy reasons the attributes are expanded via macros and can
expand to a random combination of GCC or C++ attributes.

For comparison, Clang and MSVC allow mixed syntaxes.

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

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 69585.

*** This bug has been marked as a duplicate of bug 69585 ***

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

[cvs-commit at gcc dot 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.