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 050FF3857004; Tue,  2 Feb 2021 14:16:33 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 050FF3857004
From: "markus.kuehni at triviso dot ch" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/98933] New: P0784R7 example support: constexpr new
Date: Tue, 02 Feb 2021 14:16:32 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: unknown
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: markus.kuehni at triviso dot ch
X-Bugzilla-Status: UNCONFIRMED
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_id short_desc product version bug_status
 bug_severity priority component assigned_to reporter target_milestone
Message-ID: <bug-98933-4@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: Tue, 02 Feb 2021 14:16:33 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98933

            Bug ID: 98933
           Summary: P0784R7 example support: constexpr new
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: markus.kuehni at triviso dot ch
  Target Milestone: ---

Trying to recreate the
[P0784R7](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0784r7.h=
tml)
code example.=20


<source>:13:10: error: 'mark_immutable_if_constexpr' is not a member of 'st=
d'
   13 |     std::mark_immutable_if_constexpr(this->ps);


Removed std::mark_immutable_if_constexpr(this->ps);


<source>:22:31: error: no matching function for call to 'S<char>::S(const c=
har
[7])'
   22 | constexpr S<char> str("Hello!");
      |                               ^


Added std::add_const<T>::type for constructor argument p.=20


<source>:22:31:   in 'constexpr' expansion of 'S<char>("Hello!")'
<source>:11:7: error: call to non-'constexpr' function 'void* operator
new(std::size_t, void*)'
   11 |       new(this->ps+k) T{p[k]};


Replaced with std::construct_at() and std::destroy_at() for symmetry, see
[91369](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D91369).=20


In file included from
/opt/compiler-explorer/gcc-trunk-20210202/include/c++/11.0.0/memory:64,
                 from <source>:1:
/opt/compiler-explorer/gcc-trunk-20210202/include/c++/11.0.0/bits/allocator=
.h:171:50:
error: 'S<char>("Hello!")' is not a constant expression because it refers t=
o a
result of 'operator new'
  171 |           return static_cast<_Tp*>(::operator new(__n * sizeof(_Tp)=
));


Now I'm stuck.=20

A cleaned-up version is here:

https://godbolt.org/z/KWMqna

```
#include <memory>
#include <new>

template<typename T> struct S : std::allocator<T>  {
        std::size_t sz;
        T *ps;

    template<std::size_t N>=20
    constexpr S(typename std::add_const<T>::type (&p)[N]) :
                        sz { N }, ps {this->allocate(N)} {
                for (std::size_t k =3D 0; k<N; ++k) {
                        std::construct_at(ps+k, p[k]);
                }
        }
        constexpr ~S() {
                for (std::size_t k =3D 0; k < this->sz; ++k) {
                        std::destroy_at(ps+k);
                }
                this->deallocate(this->ps, this->sz);
        }
};

constexpr S<char> str("Hello!");
// str ends up pointing to a static array
// containing the string "Hello!".

```=