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

[Jason Merrill <jason@redhat.com>]

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?

> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> 	PR c++/106712
> 
> gcc/cp/ChangeLog:
> 
> 	* decl.cc (start_decl): Copy prefix_attributes.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp0x/gen-attrs-77.C: New test.
> ---
>   gcc/cp/decl.cc                            |  3 ++-
>   gcc/testsuite/g++.dg/cpp0x/gen-attrs-77.C | 17 +++++++++++++++++
>   2 files changed, 19 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..9fa80b926d5 100644
> --- a/gcc/cp/decl.cc
> +++ b/gcc/cp/decl.cc
> @@ -5566,7 +5566,8 @@ start_decl (const cp_declarator *declarator,
>     *pushed_scope_p = NULL_TREE;
>   
>     if (prefix_attributes != error_mark_node)
> -    attributes = attr_chainon (attributes, prefix_attributes);
> +    /* Don't let grokdeclarator modify prefix_attributes.  */
> +    attributes = attr_chainon (attributes, copy_list (prefix_attributes));
>   
>     decl = grokdeclarator (declarator, declspecs, NORMAL, initialized,
>   			 &attributes);
> 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: 390f94eee1ae694901f896ac45bfb148f8126baa