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List:       gcc-patches
Subject:    Re: [PATCH v2] c++: Check attributes on friend declarations [PR99032]
From:       Franz Sirl <franz.sirl-kernel () lauterbach ! com>
Date:       2021-05-18 17:35:52
Message-ID: ffa777b1-077f-c6ea-9999-d7a1ed28538a () lauterbach ! com
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Am 2021-05-14 um 00:08 schrieb Marek Polacek via Gcc-patches:
> On Wed, May 12, 2021 at 08:27:18PM -0400, Jason Merrill wrote:
>> On 5/12/21 8:03 PM, Marek Polacek wrote:
>>> diff --git a/gcc/cp/decl2.c b/gcc/cp/decl2.c
>>> index 89f874a32cc..2bcefb619aa 100644
>>> --- a/gcc/cp/decl2.c
>>> +++ b/gcc/cp/decl2.c
>>> @@ -1331,6 +1331,20 @@ any_dependent_type_attributes_p (tree attrs)
>>>      return false;
>>>    }
>>> +/* True if ATTRS contains any attribute that requires a type.  */
>>
>> Let's invert this to check if ATTRS contains any attribute that does *not*
>> require a type, and would therefore apply to the decl.
> 
> Sounds good, done.  Now I don't need to check *attrlist.
> I've also fixed up the xfail thing in my new test.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> -- >8 --
> 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.
> 

Hi,

I haven't investigated it in detail yet, but it seems this change breaks 
building Qt-based (tested with Qt-5.12.7) applications. Sample error 
output with trunk@r12+876 -std=gnu++14:

/usr/include/qt5/QtCore/qvariant.h:470:33: error: attribute appertains 
to a friend declaration that is not a definition
      friend Q_CORE_EXPORT QDebug operator<<(QDebug, const QVariant &);
                                  ^~~~~~~~

For GCC Q_CORE_EXPORT is defined (via Q_DECL_EXPORT) to 
__attribute__((visibility("default"))) AFAICS.

As this error seemingly cannot be turned into a warning, it's probably 
quite a problem for many people.

regards,
Franz
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