From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id 3956E3858433; Mon, 25 Oct 2021 08:03:27 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 3956E3858433
From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/102921] error: modification of '<temporary>' is not a
 constant expression
Date: Mon, 25 Oct 2021 08:03:26 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 11.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: jakub at gcc dot gnu.org
X-Bugzilla-Status: NEW
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: bug_status cf_reconfirmed_on cc everconfirmed
Message-ID: <bug-102921-4-G9E8qVGpkn@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-102921-4@http.gcc.gnu.org/bugzilla/>
References: <bug-102921-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Mon, 25 Oct 2021 08:03:27 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102921

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-10-25
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |jason at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Testcase without headers:
namespace std
{
  typedef decltype (sizeof 0) size_t;
  template<class T>
  class initializer_list
  {
    const T *_M_array;
    size_t _M_len;
    constexpr initializer_list (const T &a, size_t l) : _M_array(a), _M_len=
(l)
{ }
  };
}

struct S
{
  constexpr S () noexcept { }
  int a =3D __builtin_is_constant_evaluated ();
};

constexpr std::initializer_list <S> a { { } };

The <temporary> is _ZGRL1a_[0] demangled reference temporary #0 for a, and =
the
expression that is being constant evaluated is (without cleanup points and
expr_stmt):
TARGET_EXPR <D.2197, {._M_array=3D (_ZGRL1a_[0] =3D TARGET_EXPR <D.2186, S:=
:S
(&D.2186)>;, (const struct S *) &_ZGRL1a_;, ._M_len=3D1}>;
No idea how to fix that though...
Doesn't seem to be a regression, behaved this way since=20
r9-2311-ge408261123697a82b5965c700fa2465999f0fd62 when
__builtin_is_constant_evaluated has been introduced.
Without the builtin, bet the ctor is folded into a CONSTRUCTOR already when
performing non-manifestly constant evaluation before.

The INIT_EXPR that stores into _ZGRL1a_[0] is created by
split_nonconstant_init_1 called indirectly from set_up_extended_ref_temp, s=
o at
that point the initializer clearly didn't go through manifestly constant
evaluation.  Should that be done somewhere if set_up_extended_ref_temp is
called for a constexpr variable?=