Re: [RFC PATCH] c++: Minimal handling of carries_dependency attribute

[Jakub Jelinek <jakub@redhat.com>]

On Tue, Nov 08, 2022 at 01:40:03PM -1000, Jason Merrill wrote:
> > A comment in D2552R1:
> > "The only questionable (but still conforming) case we found was
> > [[carries_dependency(some_argument)]] on GCC, where the emitted diagnostic said that the
> > carries_dependency attribute is not supported, but did not specifically call out the syntax error
> > in the argument clause."
> > made me try the following patch, where we'll error at least
> > for arguments to the attribute and for some uses of the attribute
> > appertaining to something not mentioned in the standard warn
> > with different diagnostics (or should that be an error?; clang++
> > does that, but I think we never do for any attribute, standard or not).
> > The diagnostics on toplevel attribute declaration is still an
> > attribute ignored warning and on empty statement different wording.
> > 
> > The paper additionally mentions
> > struct X { [[nodiscard]]; }; // no diagnostic on GCC
> > and 2 cases of missing diagnostics on [[fallthrough]] (guess I should
> > file a PR about those; one problem is that do { ... } while (0); there
> > is replaced during genericization just by ... and another that
> > [[fallthrough]] there is followed by a label, but not user/case/default
> > label, but an artificial one created from while loop genericization.
> > 
> > Thoughts on this?
> 
> LGTM.

Thanks, committed now.
Given CWG2538, I wonder whether we shouldn't at least pedwarn rather than
warning{,_at} for standard attributes that appertain to wrong entities
(and keep warning{,_at} for non-standard attributes including gnu variants
of standard attributes).
If yes, we'd need to differentiate between the standard attributes
and gnu variants thereof (I think the C FE does, but C++ FE has
      /* We used to treat C++11 noreturn attribute as equivalent to GNU's,
         but no longer: we have to be able to tell [[noreturn]] and
         __attribute__((noreturn)) apart.  */
      /* C++14 deprecated attribute is equivalent to GNU's.  */
      if (is_attribute_p ("deprecated", attr_id))
        TREE_PURPOSE (TREE_PURPOSE (attribute)) = gnu_identifier;
      /* C++17 fallthrough attribute is equivalent to GNU's.  */
      else if (is_attribute_p ("fallthrough", attr_id))
        TREE_PURPOSE (TREE_PURPOSE (attribute)) = gnu_identifier;
      /* C++23 assume attribute is equivalent to GNU's.  */
      else if (is_attribute_p ("assume", attr_id))
        TREE_PURPOSE (TREE_PURPOSE (attribute)) = gnu_identifier;
so we'd need to remove that and make sure those standard attributes
are handled.

	Jakub