[Bug c++/99032] New: GCC accepts attributes on friend declarations (not definitions)

[Bug c++/99032] New: GCC accepts attributes on friend declarations (not definitions)

[dangelog at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99032

            Bug ID: 99032
           Summary: GCC accepts attributes on friend declarations (not
                    definitions)
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dangelog at gmail dot com
  Target Milestone: ---

Hi,

The following code is accepted by GCC 10.2:

  struct S { [[deprecated]] friend void f(); };

No errors and no warnings are generated under -Wall -Wextra -pedantic.

The code is however ill-formed: an attribute cannot appear on a friend 
declaration which isn't also a definition:

https://eel.is/c++draft/dcl.attr#grammar-5.sentence-3

    If an attribute-specifier-seq appertains to a friend declaration
([class.friend]), that declaration shall be a definition.


For comparison, Clang rejects this, and MSVC accepts it (without warnings).

[Bug c++/99032] GCC accepts attributes on friend declarations (not definitions)

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99032

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-02-09
             Status|UNCONFIRMED                 |ASSIGNED
     Ever confirmed|0                           |1
           Keywords|                            |accepts-invalid
                 CC|                            |mpolacek at gcc dot gnu.org
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Mine for GCC 12.

[Bug c++/99032] GCC accepts attributes on friend declarations (not definitions)

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99032

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Current test:

class X { };
template<typename T1, typename T2>
void foo (T1, T2);

struct S {
  [[deprecated]] friend void f(); // error
  [[deprecated]] friend void f2() { }
  friend void f3 [[deprecated]] (); // error
  friend void f4 [[deprecated]] () { }
  [[deprecated]] friend void; // error
  [[deprecated]] friend X; // error
  [[deprecated]] friend class N; // error
  friend class [[deprecated]] N2; // error
  [[deprecated]] friend void foo<>(int, int); // error
  // FIXME: When PR100339 is resolved.
  //[[deprecated]] friend void ::foo(int, int); // error
};

template<typename T>
class node { };

template<typename T>
struct A {
  [[deprecated]] friend T; // error
  [[deprecated]] friend class node<T>; // we warn
  template<typename>
  [[deprecated]] friend class A; // we warn
  template<typename>
  [[deprecated]] friend void bar () { } // OK
  template<typename>
  [[deprecated]] friend void baz (); // error
};

[Bug c++/99032] GCC accepts attributes on friend declarations (not definitions)

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99032

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:149061188c7c6ddf27663c8c00b7574fc8d0fd23

commit r12-786-g149061188c7c6ddf27663c8c00b7574fc8d0fd23
Author: Marek Polacek <polacek@redhat.com>
Date:   Thu Apr 29 21:38:14 2021 -0400

    c++: Check attributes on friend declarations [PR99032]

    This patch implements [dcl.attr.grammar]/5: "If an attribute-specifier-seq
    appertains to a friend declaration ([class.friend]), that declaration shall
    be a definition."

    This restriction applies to C++11-style attributes as well as GNU
    attributes with the exception that we allow GNU attributes that require
    a type, such as vector_size to continue accepting code as in attrib63.C.
    There are various forms of friend declarations, we have friend
    templates, C++11 extended friend declarations, and so on.  In some cases
    we already ignore the attribute and warn that it was ignored.  But
    certain cases weren't diagnosed, and with this patch we'll give a hard
    error.  I tried hard not to emit both a warning and error and I think it
    worked out.

    Jason provided the cp_parser_decl_specifier_seq hunk to detect using
    standard attributes in the middle of decl-specifiers, which is invalid.

    Co-authored-by: Jason Merrill <jason@redhat.com>

    gcc/cp/ChangeLog:

            PR c++/99032
            * cp-tree.h (any_non_type_attribute_p): Declare.
            * decl.c (grokdeclarator): Diagnose when an attribute appertains to
            a friend declaration that is not a definition.
            * decl2.c (any_non_type_attribute_p): New.
            * parser.c (cp_parser_decl_specifier_seq): Diagnose standard
attributes
            in the middle of decl-specifiers.
            (cp_parser_elaborated_type_specifier): Diagnose when an attribute
            appertains to a friend declaration that is not a definition.
            (cp_parser_member_declaration): Likewise.

    gcc/testsuite/ChangeLog:

            PR c++/99032
            * g++.dg/cpp0x/friend7.C: New test.
            * g++.dg/cpp0x/gen-attrs-4.C: Add dg-error.
            * g++.dg/cpp0x/gen-attrs-39-1.C: Likewise.
            * g++.dg/cpp0x/gen-attrs-74.C: New test.
            * g++.dg/ext/attrib63.C: New test.

[Bug c++/99032] GCC accepts attributes on friend declarations (not definitions)

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99032

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed in GCC 12.