Re: [PATCH v2] c++: Fix C++11 attribute propagation [PR106712]

[Jason Merrill <jason@redhat.com>]

On 8/29/22 16:01, Marek Polacek wrote:
> On Mon, Aug 29, 2022 at 01:32:29PM -0400, Jason Merrill wrote:
>> On 8/26/22 19:01, Marek Polacek wrote:
>>> When we have
>>>
>>>     [[noreturn]] int fn1 [[nodiscard]](), fn2();
>>>
>>> "noreturn" should apply to both fn1 and fn2 but "nodiscard" only to fn1:
>>> [dcl.pre]/3: "The attribute-specifier-seq appertains to each of
>>> the entities declared by the declarators of the init-declarator-list."
>>> [dcl.spec.general]: "The attribute-specifier-seq affects the type
>>> only for the declaration it appears in, not other declarations involving
>>> the same type."
>>>
>>> As Ed Catmur correctly analyzed, this is because, for the test above,
>>> we call start_decl with prefix_attributes=noreturn, but this line:
>>>
>>>     attributes = attr_chainon (attributes, prefix_attributes);
>>>
>>> results in attributes == prefix_attributes, because chainon sees
>>> that attributes is null so it just returns prefix_attributes.  Then
>>> in grokdeclarator we reach
>>>
>>>     *attrlist = attr_chainon (*attrlist, declarator->std_attributes);
>>>
>>> which modifies prefix_attributes so now it's "noreturn, nodiscard"
>>> and so fn2 is wrongly marked nodiscard as well.  Fixed by copying
>>> prefix_attributes.
>>
>> How about reversing the order of arguments to the call in grokdeclarator, so
>> that the prefix attributes can remain shared at the end of the list?
> 
> Thanks, that seems like a cheaper solution.  It works because this way
> we tack the prefix attributes onto ->std_attributes, avoiding modifying
> prefix_attributes.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK.

> -- >8 --
> When we have
> 
>    [[noreturn]] int fn1 [[nodiscard]](), fn2();
> 
> "noreturn" should apply to both fn1 and fn2 but "nodiscard" only to fn1:
> [dcl.pre]/3: "The attribute-specifier-seq appertains to each of
> the entities declared by the declarators of the init-declarator-list."
> [dcl.spec.general]: "The attribute-specifier-seq affects the type
> only for the declaration it appears in, not other declarations involving
> the same type."
> 
> As Ed Catmur correctly analyzed, this is because, for the test above,
> we call start_decl with prefix_attributes=noreturn, but this line:
> 
>    attributes = attr_chainon (attributes, prefix_attributes);
> 
> results in attributes == prefix_attributes, because chainon sees
> that attributes is null so it just returns prefix_attributes.  Then
> in grokdeclarator we reach
> 
>    *attrlist = attr_chainon (*attrlist, declarator->std_attributes);
> 
> which modifies prefix_attributes so now it's "noreturn, nodiscard"
> and so fn2 is wrongly marked nodiscard as well.  Fixed by reversing
> the order of arguments to attr_chainon.  That way, we tack the prefix
> attributes onto ->std_attributes, avoiding modifying prefix_attributes.
> 
> 	PR c++/106712
> 
> gcc/cp/ChangeLog:
> 
> 	* decl.cc (grokdeclarator): Reverse the order of arguments to
> 	attr_chainon.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/gen-attrs-77.C: New test.
> ---
>   gcc/cp/decl.cc                            |  2 +-
>   gcc/testsuite/g++.dg/cpp0x/gen-attrs-77.C | 17 +++++++++++++++++
>   2 files changed, 18 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/gen-attrs-77.C
> 
> diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
> index d46a347a6c7..b72b2a8456b 100644
> --- a/gcc/cp/decl.cc
> +++ b/gcc/cp/decl.cc
> @@ -13474,7 +13474,7 @@ grokdeclarator (const cp_declarator *declarator,
>         /* [dcl.meaning]/1: The optional attribute-specifier-seq following
>   	 a declarator-id appertains to the entity that is declared.  */
>         if (declarator->std_attributes != error_mark_node)
> -	*attrlist = attr_chainon (*attrlist, declarator->std_attributes);
> +	*attrlist = attr_chainon (declarator->std_attributes, *attrlist);
>         else
>   	/* We should have already diagnosed the issue (c++/78344).  */
>   	gcc_assert (seen_error ());
> diff --git a/gcc/testsuite/g++.dg/cpp0x/gen-attrs-77.C b/gcc/testsuite/g++.dg/cpp0x/gen-attrs-77.C
> new file mode 100644
> index 00000000000..2c41c62f33b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/gen-attrs-77.C
> @@ -0,0 +1,17 @@
> +// PR c++/106712
> +// { dg-do compile { target c++11 } }
> +
> +[[noreturn]] int f1 [[nodiscard]](), f2 ();
> +[[nodiscard]] int f3 (), f4 ();
> +int f5 [[nodiscard]](), f6 ();
> +
> +int
> +main ()
> +{
> +  f1 (); // { dg-warning "ignoring" }
> +  f2 ();
> +  f3 (); // { dg-warning "ignoring" }
> +  f4 (); // { dg-warning "ignoring" }
> +  f5 (); // { dg-warning "ignoring" }
> +  f6 ();
> +}
> 
> base-commit: 60d1d296b42b22b08d4eaa38bea02100c07261ac