Re: C++ PATCH for c++/86063, ICE with attribute with pack expansion

[Jason Merrill <jason@redhat.com>]

On Thu, Jun 14, 2018 at 3:40 PM, Marek Polacek <polacek@redhat.com> wrote:
> On Thu, Jun 14, 2018 at 12:59:00PM -0400, Jason Merrill wrote:
>> On Thu, Jun 14, 2018 at 12:51 PM, Marek Polacek <polacek@redhat.com> wrote:
>> > On Mon, Jun 11, 2018 at 03:00:04PM -0400, Jason Merrill wrote:
>> >> On Wed, Jun 6, 2018 at 5:18 PM, Marek Polacek <polacek@redhat.com> wrote:
>> >> > We crash on this testcase containing a bogus attribute, because
>> >> > cp_check_const_attributes accessed TREE_VALUE of a tree that happened to be
>> >> > expr_pack_expansion.  Since here we're merely trying to evaluate constexpr
>> >> > arguments, I thought we could skip such bogus arguments.
>> >>
>> >> Hmm, attributes should always be a TREE_LIST, lots of places assume
>> >> that.  Why isn't the pack expansion wrapped in a TREE_LIST?
>> >
>> > I believe you did that on purpose.  There pack comes from
>> > cp_parser_std_attribute_list. We could wrap it into a TREE_LIST, but then
>> > tsubst_attribute would have to be tweaked to handle the pack expansion
>> > correctly.
>>
>> How so?  tsubst_attribute expects to find a pack expansion in the
>> TREE_VALUE of a TREE_LIST.
>> And cp_parser_std_attribute_list puts the pack expansion in TREE_VALUE.
>
> Exactly.  But what tsubst_attribute gets currently is
>
>  <tree_list 0x7ffff001c280 tree_0
>     purpose <tree_list 0x7ffff0002f00
>         purpose <identifier_node 0x7fffefec7d80 gnu
>             normal local bindings <(nil)>>
>         value <identifier_node 0x7ffff0014f00 aligned
>             normal local bindings <(nil)>>>
>     value <expr_pack_expansion 0x7fffefeada20
>         arg:0 <tree_list 0x7ffff0002f50
>             value <alignof_expr 0x7ffff0015600 type <integer_type 0x7fffefecd7e0 long unsigned int>
>                 readonly tree_0 arg:0 <template_type_parm 0x7ffff001a1f8 T>
>                 alignas4.C:17:19 start: alignas4.C:17:19 finish: alignas4.C:17:29>>
>         arg:1 <tree_list 0x7ffff0002f78 value <template_type_parm 0x7ffff001a1f8 T>>>>
>
> so if I were to wrap the expr_pack_expansion in a TREE_LIST, I would have to adjust
> tsubst_attribute.  But cp_check_const_attributes doesn't expect that the TREE_VALUE
> of the above is a non-list.  Right?

Ah, of course, you're already looking at the arguments, I wasn't
reading closely enough.  The patch is OK.

Jason