From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <jwakely.gcc@gmail.com>
Received: from mail-wr1-x433.google.com (mail-wr1-x433.google.com
 [IPv6:2a00:1450:4864:20::433])
 by sourceware.org (Postfix) with ESMTPS id 45E30385800F
 for <libstdc++@gcc.gnu.org>; Mon, 28 Dec 2020 20:47:14 +0000 (GMT)
DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 45E30385800F
Received: by mail-wr1-x433.google.com with SMTP id q18so12522918wrn.1
 for <libstdc++@gcc.gnu.org>; Mon, 28 Dec 2020 12:47:14 -0800 (PST)
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
 d=1e100.net; s=20161025;
 h=x-gm-message-state:mime-version:references:in-reply-to:from:date
 :message-id:subject:to:cc;
 bh=Vl2wwaMkqKJLclvEbSfn+JuXIi/qmqFvYEc0enadQ6Q=;
 b=ZlKF0KvWum4nXjoQ/xh6DoZVWOsWrFTHDwNq7MTkkMg+ajdeq5bsfa2uhcSr5uN/XH
 PADaP7jkIDAmXJ9Oell+rVBpmNQs/kmsL3gQVUwEF3fZvkup79PC+Ui+9ri/HLVleGbc
 D9h1RXhASygkCoKHfFoyCNrARjjWVPaHR49DGNNwk1TroB2gO/ybfm/bycG9FOyFf3cP
 t9Hbi/UxyPjWpMZ7Amo2WVzSN5ybBzemj9XZQgKgwtI6qt7VyRbihgIWIvh1XsDszLzy
 pUZFEb08oYdFD+wXoviL7z72I7HmurlDt0hOX3IGG+Ye2vAm0Kobh9bpoFLTy5xf6nPE
 O3Gw==
X-Gm-Message-State: AOAM531bd3HheFATe/CdzuoBrRq6jqTpkIRfVjcEue6l4synJ6dbuN14
 ienYJSmSVMNTiAaB/Ukv5DD6HVfsGccwDmCYpu0=
X-Google-Smtp-Source: ABdhPJxfiPrEFnwdp5ybC9ASK1gNhYUZJ8ECf+xdD+rv+grXO6EnSDlyrnynoONmFQxHTGQKmn96Vs2Gp4Kbob/Wo3U=
X-Received: by 2002:a5d:4e8c:: with SMTP id e12mr51207187wru.321.1609188433332; 
 Mon, 28 Dec 2020 12:47:13 -0800 (PST)
MIME-Version: 1.0
References: <CAFkJGRf2q3kn8zD4mj3eWm-XTD9GH2=2WtC5VT6WZFTPE5dS+A@mail.gmail.com>
 <CAH6eHdTp18kG26jVOk2cxtNd9C3CzZiqt8-PvOX=diOwJORq2g@mail.gmail.com>
In-Reply-To: <CAH6eHdTp18kG26jVOk2cxtNd9C3CzZiqt8-PvOX=diOwJORq2g@mail.gmail.com>
From: Jonathan Wakely <jwakely.gcc@gmail.com>
Date: Mon, 28 Dec 2020 20:47:02 +0000
Message-ID: <CAH6eHdRUC-7zWXrVNYcBg5HO8XGfpK_TYerLd3RobV=miAD=Zw@mail.gmail.com>
Subject: Re: Should reinterpret_cast be constexpr?
To: Josh Marshall <joshua.r.marshall.1991@gmail.com>
Cc: "libstdc++" <libstdc++@gcc.gnu.org>
Content-Type: text/plain; charset="UTF-8"
X-Spam-Status: No, score=-8.0 required=5.0 tests=BAYES_00, DKIM_SIGNED,
 DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, GIT_PATCH_0,
 RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS,
 TXREP autolearn=ham autolearn_force=no version=3.4.2
X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on
 server2.sourceware.org
X-BeenThere: libstdc++@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Libstdc++ mailing list <libstdc++.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/libstdc++>,
 <mailto:libstdc++-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/libstdc++/>
List-Post: <mailto:libstdc++@gcc.gnu.org>
List-Help: <mailto:libstdc++-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/libstdc++>,
 <mailto:libstdc++-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Mon, 28 Dec 2020 20:47:15 -0000

On Mon, 28 Dec 2020 at 20:33, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>
> On Mon, 28 Dec 2020 at 19:51, Josh Marshall via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > Hello all,
> >
> > From the test error `libstdc++-v3/include/bits/stl_vector.h:1837: error:
> > 'reinterpret_cast' is not a constant expression` and the documentation at
> > https://en.cppreference.com/w/cpp/language/reinterpret_cast , is
> > `reinterpret_cast()` not constexpr in error?
>
> No, it's not an error. reinterpret_cast is not allowed in constant expressions.

This patch replaces the use of reinterpret_cast with an alternative
using C++20 features:

diff --git a/libstdc++-v3/include/bits/vector.tcc
b/libstdc++-v3/include/bits/vector.tcc
index 27e63388feb..93b2ccc02e3 100644
--- a/libstdc++-v3/include/bits/vector.tcc
+++ b/libstdc++-v3/include/bits/vector.tcc
@@ -516,9 +516,13 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
           {
#if __cplusplus < 201103L
             value_type __x_copy = __x;
-#else
+#elif ! __cpp_lib_make_obj_using_allocator
             _Temporary_value __tmp(this, __x);
             value_type& __x_copy = __tmp._M_val();
+#else
+             value_type __x_copy = std::make_from_tuple<value_type>(
+                 std::uses_allocator_construction_args<value_type>(
+                   _M_get_Tp_allocator(), __x));
#endif
             const size_type __elems_after = end() - __position;
             pointer __old_finish(this->_M_impl._M_finish);