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Date: Thu, 14 Jun 2018 19:40:00 -0000
From: Marek Polacek <polacek@redhat.com>
To: Jason Merrill <jason@redhat.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: C++ PATCH for c++/86063, ICE with attribute with pack expansion
Message-ID: <20180614194052.GH17989@redhat.com>
References: <20180606211835.GC28085@redhat.com> <CADzB+2mN+hNxXi6ofONGMSRw-SSzWFP1GenUF33o24EErD0qYA@mail.gmail.com> <20180614165107.GG17989@redhat.com> <CADzB+2=JOUByfME8McgDiKLvgLC+9c1Zv7zwmUOFgpCXrRnh3w@mail.gmail.com>
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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?

	Marek