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  <meta name="dcterms.date" content="2024-03-19" />
  <title>constexpr placement new</title>
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<h1 class="title" style="text-align:center"><code class="sourceCode cpp"><span class="kw">constexpr</span></code> placement new</h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P2747R2</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2024-03-19</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      EWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Barry Revzin<br>&lt;<a href="mailto:barry.revzin@gmail.com" class="email">barry.revzin@gmail.com</a>&gt;<br>
    </td>
  </tr>
</table>

</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#revision-history"><span class="toc-section-number">1</span> Revision History<span></span></a></li>
<li><a href="#introduction"><span class="toc-section-number">2</span> Introduction<span></span></a></li>
<li><a href="#wording"><span class="toc-section-number">3</span> Wording<span></span></a>
<ul>
<li><a href="#feature-test-macro"><span class="toc-section-number">3.1</span> Feature-Test Macro<span></span></a></li>
</ul></li>
<li><a href="#bibliography"><span class="toc-section-number">4</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="revision-history"><span class="header-section-number">1</span> Revision History<a href="#revision-history" class="self-link"></a></h1>
<p>R0 <span class="citation" data-cites="P2747R0">[<a href="#ref-P2747R0" role="doc-biblioref">P2747R0</a>]</span> of this paper proposed three related features:</p>
<ol type="1">
<li>Allowing casts from <code class="sourceCode cpp"><em>cv</em> <span class="dt">void</span><span class="op">*</span></code> to <code class="sourceCode cpp"><em>cv</em> T<span class="op">*</span></code> during constant evaluation</li>
<li>Allowing placement new during constant evaluation</li>
<li>Better handling an array of uninitialized objects</li>
</ol>
<p>Since then, <span class="citation" data-cites="P2738R1">[<a href="#ref-P2738R1" role="doc-biblioref">P2738R1</a>]</span> was adopted in Varna, which resolves problem #1. Separately, #3 is kind of a separate issue and there are ongoing conversations about how to handle this in order to make <code class="sourceCode cpp">inplace_vector</code> <span class="citation" data-cites="P0843R9">[<a href="#ref-P0843R9" role="doc-biblioref">P0843R9</a>]</span> actually during during constant evaluation for all types. So this paper refocuses to just solve problem #2 and has been renamed accordingly.</p>
<p>Since <span class="citation" data-cites="P2747R1">[<a href="#ref-P2747R1" role="doc-biblioref">P2747R1</a>]</span>, included support for arrays (yet another benefit over <code class="sourceCode cpp">std<span class="op">::</span>construct_at</code>).</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="introduction"><span class="header-section-number">2</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>Consider this implementation of <code class="sourceCode cpp">std<span class="op">::</span>uninitialized_copy</code>, partially adjusted from <a href="https://en.cppreference.com/w/cpp/memory/uninitialized_copy">cppreference</a>:</p>
<blockquote>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="kw">template</span> <span class="op">&lt;</span>input_iterator I, sentinel_for<span class="op">&lt;</span>I<span class="op">&gt;</span> S, nothrow_forward_iterator I2<span class="op">&gt;</span></span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="kw">constexpr</span> <span class="kw">auto</span> uninitialized_copy<span class="op">(</span>I first, S last, I2 d_first<span class="op">)</span> <span class="op">-&gt;</span> I2 <span class="op">{</span></span>
<span id="cb1-3"><a href="#cb1-3"></a>    <span class="kw">using</span> T <span class="op">=</span> iter_value_t<span class="op">&lt;</span>I2<span class="op">&gt;</span>;</span>
<span id="cb1-4"><a href="#cb1-4"></a>    I2 current <span class="op">=</span> d_first;</span>
<span id="cb1-5"><a href="#cb1-5"></a>    <span class="cf">try</span> <span class="op">{</span></span>
<span id="cb1-6"><a href="#cb1-6"></a>        <span class="cf">for</span> <span class="op">(</span>; first <span class="op">!=</span> last; <span class="op">++</span>first, <span class="op">(</span><span class="dt">void</span><span class="op">)++</span>current<span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-7"><a href="#cb1-7"></a>            <span class="op">::</span><span class="kw">new</span> <span class="op">(</span>std<span class="op">::</span>addressof<span class="op">(*</span>current<span class="op">))</span> T<span class="op">(*</span>first<span class="op">)</span>;</span>
<span id="cb1-8"><a href="#cb1-8"></a>        <span class="op">}</span></span>
<span id="cb1-9"><a href="#cb1-9"></a>    <span class="op">}</span> <span class="cf">catch</span> <span class="op">(...)</span> <span class="op">{</span></span>
<span id="cb1-10"><a href="#cb1-10"></a>        std<span class="op">::</span>destroy<span class="op">(</span>d_first, current<span class="op">)</span>;</span>
<span id="cb1-11"><a href="#cb1-11"></a>        <span class="cf">throw</span>;</span>
<span id="cb1-12"><a href="#cb1-12"></a>    <span class="op">}</span></span>
<span id="cb1-13"><a href="#cb1-13"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>This fails during constant evaluation today because placement new takes a <code class="sourceCode cpp"><span class="dt">void</span><span class="op">*</span></code>. But it takes a <code class="sourceCode cpp"><span class="dt">void</span><span class="op">*</span></code> that points to a <code class="sourceCode cpp">T</code> - we know that by construction. It’s just that we happen to lose that information along the way.</p>
<p>Moreover, that’s not <em>actually</em> how <code class="sourceCode cpp">uninitialized_copy</code> is specified, we actually do this:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb2"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb2-1"><a href="#cb2-1"></a><span class="st">- ::new (std::addressof(*current)) T(*first);</span></span>
<span id="cb2-2"><a href="#cb2-2"></a><span class="va">+ ::new (<em>voidify</em>(*current)) T(*first);</span></span></code></pre></div>
</div>
</blockquote>
<p>where:</p>
<blockquote>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb3-2"><a href="#cb3-2"></a>  <span class="kw">constexpr</span> <span class="dt">void</span><span class="op">*</span> <em>voidify</em><span class="op">(</span>T<span class="op">&amp;</span> obj<span class="op">)</span> <span class="kw">noexcept</span> <span class="op">{</span></span>
<span id="cb3-3"><a href="#cb3-3"></a>    <span class="cf">return</span> <span class="kw">const_cast</span><span class="op">&lt;</span><span class="dt">void</span><span class="op">*&gt;(</span><span class="kw">static_cast</span><span class="op">&lt;</span><span class="kw">const</span> <span class="kw">volatile</span> <span class="dt">void</span><span class="op">*&gt;(</span>std<span class="op">::</span>addressof<span class="op">(</span>obj<span class="op">)))</span>;</span>
<span id="cb3-4"><a href="#cb3-4"></a>  <span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Which exists to avoid users having written a global placement new that takes a <code class="sourceCode cpp">T<span class="op">*</span></code>.</p>
<p>The workaround, introduced by <span class="citation" data-cites="P0784R7">[<a href="#ref-P0784R7" role="doc-biblioref">P0784R7</a>]</span>, is a new library function:</p>
<blockquote>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> T, <span class="kw">class</span><span class="op">...</span> Args<span class="op">&gt;</span></span>
<span id="cb4-2"><a href="#cb4-2"></a><span class="kw">constexpr</span> T<span class="op">*</span> construct_at<span class="op">(</span> T<span class="op">*</span> p, Args<span class="op">&amp;&amp;...</span> args <span class="op">)</span>;</span></code></pre></div>
</blockquote>
<p>This is a magic library function that is specified to do the same <code class="sourceCode cpp"><em>voidify</em></code> dance, but which the language simply recognizes as an allowed thing to do. <code class="sourceCode cpp">std<span class="op">::</span>construct_at</code> is explicitly allowed in <a href="https://eel.is/c++draft/expr.const">[expr.const]/6</a>:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_1" id="pnum_1">6</a></span> […] Similarly, the evaluation of a call to <code class="sourceCode cpp">std​<span class="op">::</span>​construct_­at</code> or <code class="sourceCode cpp">std​<span class="op">::</span>​ranges​<span class="op">::</span>​construct_­at</code> ([specialized.construct]) does not disqualify <code class="sourceCode cpp">E</code> from being a core constant expression unless the first argument, of type <code class="sourceCode cpp">T<span class="op">*</span></code>, does not point to storage allocated with <code class="sourceCode cpp">std​<span class="op">::</span>​allocator<span class="op">&lt;</span>T<span class="op">&gt;</span></code> or to an object whose lifetime began within the evaluation of <code class="sourceCode cpp">E</code>, or the evaluation of the underlying constructor call disqualifies <code class="sourceCode cpp">E</code> from being a core constant expression.</p>
</blockquote>
<p>It’s good that we actually have a solution - we can make <code class="sourceCode cpp">uninitialized_copy</code> usable during constant evaluation simply by using <code class="sourceCode cpp">std<span class="op">::</span>construct_at</code>. There’s even a paper to do so (<span class="citation" data-cites="P2283R2">[<a href="#ref-P2283R2" role="doc-biblioref">P2283R2</a>]</span>). But that paper also had hinted at a larger problem: <code class="sourceCode cpp">std<span class="op">::</span>construct_at</code> is an <em>extremely</em> limited tool as compared to placement new.</p>
<p>Consider the different kinds of initialization we have in C++:</p>
<table>
<colgroup>
<col style="width: 20%"></col>
<col style="width: 20%"></col>
<col style="width: 60%"></col>
</colgroup>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>kind</strong>
</div></th>
<th><div style="text-align:center">
<strong>placement new</strong>
</div></th>
<th><div style="text-align:center">
<strong><code class="sourceCode cpp">std<span class="op">::</span>construct_at</code></strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td>value initialization</td>
<td><code class="sourceCode cpp"><span class="kw">new</span> <span class="op">(</span>p<span class="op">)</span> T<span class="op">(</span>args<span class="op">...)</span></code></td>
<td><code class="sourceCode cpp">std<span class="op">::</span>construct_at<span class="op">(</span>p, args<span class="op">...)</span></code></td>
</tr>
<tr class="even">
<td>default initialization</td>
<td><code class="sourceCode cpp"><span class="kw">new</span> <span class="op">(</span>p<span class="op">)</span> T</code></td>
<td>Not currently possible. <span class="citation" data-cites="P2283R1">[<a href="#ref-P2283R1" role="doc-biblioref">P2283R1</a>]</span> proposed <code class="sourceCode cpp">std<span class="op">::</span>default_construct_at</code></td>
</tr>
<tr class="odd">
<td>list initialization</td>
<td><code class="sourceCode cpp"><span class="kw">new</span> <span class="op">(</span>p<span class="op">)</span> T<span class="op">{</span>a, b<span class="op">}</span></code></td>
<td>Not currently possible, could be a new function?</td>
</tr>
<tr class="even">
<td>designated initialization</td>
<td><code class="sourceCode cpp"><span class="kw">new</span> <span class="op">(</span>p<span class="op">)</span> T<span class="op">{.</span>a<span class="op">=</span>a, <span class="op">.</span>b<span class="op">=</span>b<span class="op">}</span></code></td>
<td>Not possible to even write a function</td>
</tr>
</tbody>
</table>
<p>That’s already not a great outlook for <code class="sourceCode cpp">std<span class="op">::</span>construct_at</code>, but for use-cases like <code class="sourceCode cpp">uninitialized_copy</code>, we have to also consider the case of guaranteed copy elision:</p>
<blockquote>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1"></a><span class="kw">auto</span> get_object<span class="op">()</span> <span class="op">-&gt;</span> T;</span>
<span id="cb5-2"><a href="#cb5-2"></a></span>
<span id="cb5-3"><a href="#cb5-3"></a><span class="dt">void</span> construct_into<span class="op">(</span>T<span class="op">*</span> p<span class="op">)</span> <span class="op">{</span></span>
<span id="cb5-4"><a href="#cb5-4"></a>    <span class="co">// this definitely moves a T</span></span>
<span id="cb5-5"><a href="#cb5-5"></a>    std<span class="op">::</span>construct_at<span class="op">(</span>p, get_object<span class="op">())</span>;</span>
<span id="cb5-6"><a href="#cb5-6"></a></span>
<span id="cb5-7"><a href="#cb5-7"></a>    <span class="co">// this definitely does not move a T</span></span>
<span id="cb5-8"><a href="#cb5-8"></a>    <span class="op">:::</span><span class="kw">new</span> <span class="op">(</span>p<span class="op">)</span> T<span class="op">(</span>get_object<span class="op">())</span>;</span>
<span id="cb5-9"><a href="#cb5-9"></a></span>
<span id="cb5-10"><a href="#cb5-10"></a>    <span class="co">// this now also definitely does not move a T, but it isn&#39;t practical</span></span>
<span id="cb5-11"><a href="#cb5-11"></a>    <span class="co">// and you also have to deal with delightful edge cases - like what if</span></span>
<span id="cb5-12"><a href="#cb5-12"></a>    <span class="co">// T is actually constructible from defer?</span></span>
<span id="cb5-13"><a href="#cb5-13"></a>    <span class="kw">struct</span> defer <span class="op">{</span></span>
<span id="cb5-14"><a href="#cb5-14"></a>        <span class="kw">constexpr</span> <span class="kw">operator</span> T<span class="op">()</span> <span class="kw">const</span> <span class="op">{</span> <span class="cf">return</span> get_object<span class="op">()</span>; <span class="op">}</span></span>
<span id="cb5-15"><a href="#cb5-15"></a>    <span class="op">}</span>;</span>
<span id="cb5-16"><a href="#cb5-16"></a>    std<span class="op">::</span>construct_at<span class="op">(</span>p, defer<span class="op">{})</span>;</span>
<span id="cb5-17"><a href="#cb5-17"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Placement new is only unsafe because the language allows you to do practically anything - want to placement new a <code class="sourceCode cpp">std<span class="op">::</span>string</code> into a <code class="sourceCode cpp"><span class="dt">double</span><span class="op">*</span></code>? Sure, why not. But during constant evaluation we already have a way of limiting operations to those that make sense - we can require that the pointer we’re constructing into actually is a <code class="sourceCode cpp">T<span class="op">*</span></code>. The fact that we have to go through a <code class="sourceCode cpp"><span class="dt">void</span><span class="op">*</span></code> to get there doesn’t make it unsafe.</p>
<p>Now that we have support for <code class="sourceCode cpp"><span class="kw">static_cast</span><span class="op">&lt;</span>T<span class="op">*&gt;(</span><span class="kw">static_cast</span><span class="op">&lt;</span><span class="dt">void</span><span class="op">*&gt;(</span>p<span class="op">))</span></code>, we can adopt the same rules to make placement new work.</p>
<p>Additionally, <code class="sourceCode cpp">std<span class="op">::</span>construct_at</code> does not support arrays (see also <span class="citation" data-cites="LWG3436">[<a href="#ref-LWG3436" role="doc-biblioref">LWG3436</a>]</span>), but we can make it work with placement without much issue - another reason the language is simply better:</p>
<blockquote>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1"></a><span class="dt">int</span><span class="op">*</span> p <span class="op">=</span> std<span class="op">::</span>allocator<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;{}.</span>allocate<span class="op">(</span><span class="dv">3</span><span class="op">)</span>;</span>
<span id="cb6-2"><a href="#cb6-2"></a></span>
<span id="cb6-3"><a href="#cb6-3"></a><span class="kw">new</span> <span class="op">(</span>p<span class="op">)</span> <span class="dt">int</span><span class="op">[]{</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span><span class="op">}</span>;       <span class="co">// ok</span></span>
<span id="cb6-4"><a href="#cb6-4"></a><span class="kw">new</span> <span class="op">(</span>p <span class="op">+</span> <span class="dv">1</span><span class="op">)</span> <span class="dt">int</span><span class="op">[]{</span><span class="dv">2</span>, <span class="dv">3</span><span class="op">}</span>;      <span class="co">// error (in this paper)</span></span>
<span id="cb6-5"><a href="#cb6-5"></a><span class="kw">new</span> <span class="op">(</span>p<span class="op">)</span> <span class="dt">int</span><span class="op">[]{</span><span class="dv">1</span>, <span class="dv">2</span>, <span class="dv">3</span>, <span class="dv">4</span>, <span class="dv">5</span><span class="op">}</span>; <span class="co">// error</span></span></code></pre></div>
</blockquote>
<p>Note that the smaller array case could probably be made to work, but I’m not aware of a strong reason to want it - so it seems like a good start to allow the correct-size array case and reject the wrong-size array case for both directions of wrong size.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="wording"><span class="header-section-number">3</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Today, we have an exception for <code class="sourceCode cpp">std<span class="op">::</span>construct_at</code> and <code class="sourceCode cpp">std<span class="op">::</span>ranges<span class="op">::</span>construct_at</code> to avoid evaluating the placement new that they do internally. But once we allow placement new, we no longer need an exception for those cases - we simply need to move the lifetime requirement from the exception into the general rule for placement new.</p>
<p>Clarify that implicit object creation does not happen during constant evaluation in <span>6.7.2 <a href="https://wg21.link/intro.object">[intro.object]</a></span>/14:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_2" id="pnum_2">14</a></span> <span class="rm" style="color: #bf0303"><del>An</del></span> <span class="addu">Except during constant evaluation, an</span> operation that begins the lifetime of an array of <code class="sourceCode cpp"><span class="dt">unsigned</span> <span class="dt">char</span></code> or <code class="sourceCode cpp">std​<span class="op">::</span>​byte</code> implicitly creates objects within the region of storage occupied by the array.</p>
<p>[<em>Note 5</em>: The array object provides storage for these objects. — <em>end note</em>]</p>
<p><span class="rm" style="color: #bf0303"><del>Any</del></span> <span class="addu">Except during constant evaluation, any</span> implicit or explicit invocation of a function named <code class="sourceCode cpp"><span class="kw">operator</span> <span class="kw">new</span></code> or <code class="sourceCode cpp"><span class="kw">operator</span> <span class="kw">new</span><span class="op">[]</span></code> implicitly creates objects in the returned region of storage and returns a pointer to a suitable created object.</p>
<p>[<em>Note 6</em>: Some functions in the C++ standard library implicitly create objects ([obj.lifetime], [c.malloc], [mem.res.public], [bit.cast], [cstring.syn]). — <em>end note</em>]</p>
</blockquote>
<p>Change <span>7.6.2.8 <a href="https://wg21.link/expr.new">[expr.new]</a></span>/15:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">15</a></span> During an evaluation of a constant expression, a call to <span class="rm" style="color: #bf0303"><del>an</del></span> <span class="addu">a replaceable</span> allocation function is always omitted <span class="addu">([expr.const])</span>.</p>
<div class="rm" style="color: #bf0303">
<p><span class="note"><span>[ <em>Note 1:</em> </span> Only new-expressions that would otherwise result in a call to a replaceable global allocation function can be evaluated in constant expressions ([expr.const]).<span> — <em>end note</em> ]</span></span></p>
</div>
</blockquote>
<p>Change <span>7.7 <a href="https://wg21.link/expr.const">[expr.const]</a></span>/5.18 (paragraph 14 here for context was the <span class="citation" data-cites="P2738R1">[<a href="#ref-P2738R1" role="doc-biblioref">P2738R1</a>]</span> fix to allow converting from <code class="sourceCode cpp"><span class="dt">void</span><span class="op">*</span></code> to <code class="sourceCode cpp">T<span class="op">*</span></code> during constant evaluation, as adjusted by <span class="citation" data-cites="CWG2755">[<a href="#ref-CWG2755" role="doc-biblioref">CWG2755</a>]</span>):</p>
<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_4" id="pnum_4">(5.14)</a></span> a conversion from a prvalue <code class="sourceCode cpp">P</code> of type “pointer to cv <code class="sourceCode cpp"><span class="dt">void</span></code>” to a “<code class="sourceCode cpp"><em>cv1</em></code> pointer to <code class="sourceCode cpp">T</code>”, where <code class="sourceCode cpp">T</code> is not <code class="sourceCode cpp"><em>cv2</em> <span class="dt">void</span></code>, unless <code class="sourceCode cpp">P</code> points to an object whose type is similar to <code class="sourceCode cpp">T</code>;</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_5" id="pnum_5">(5.15)</a></span> …</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_6" id="pnum_6">(5.16)</a></span> …</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_7" id="pnum_7">(5.17)</a></span> …</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_8" id="pnum_8">(5.18)</a></span> a <em>new-expression</em> (<span>7.6.2.8 <a href="https://wg21.link/expr.new">[expr.new]</a></span>), unless <span class="addu">either</span>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_9" id="pnum_9">(5.18.1)</a></span> the selected allocation function is a replaceable global allocation function ([new.delete.single], [new.delete.array]) and the allocated storage is deallocated within the evaluation of <code class="sourceCode cpp">E</code><span class="addu">, or</span></li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_10" id="pnum_10">(5.18.2)</a></span> <span class="addu">the selected allocation function is a non-allocating form ([new.delete.placement]) with an allocated type <code class="sourceCode cpp">T</code>, where</span>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_11" id="pnum_11">(5.18.2.1)</a></span> <span class="addu">the placement argument to the <em>new-expression</em> points to an object that is pointer-interconvertible with an object of type <code class="sourceCode cpp">T</code> or, if <code class="sourceCode cpp">T</code> is an array type, with the first element of an object of type <code class="sourceCode cpp">T</code>, and</span></li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_12" id="pnum_12">(5.18.2.2)</a></span> <span class="addu">the placement argument points to storage whose duration began within the evaluation of <code class="sourceCode cpp">E</code></span>;</li>
</ul></li>
</ul></li>
</ul>
</blockquote>
<p>Remove the special case for <code class="sourceCode cpp">construct_at</code> in <span>7.7 <a href="https://wg21.link/expr.const">[expr.const]</a></span>/6:</p>
<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_13" id="pnum_13">6</a></span> For the purposes of determining whether an expression <code class="sourceCode cpp">E</code> is a core constant expression, the evaluation of the body of a member function of <code class="sourceCode cpp">std​<span class="op">::</span>​allocator<span class="op">&lt;</span>T<span class="op">&gt;</span></code> as defined in [allocator.members], where T is a literal type, is ignored. <span class="rm" style="color: #bf0303"><del>Similarly, the evaluation of the body of <span><code class="sourceCode default">std​::​construct_at</code></span> or <span><code class="sourceCode default">std​::​ranges​::​construct_at</code></span> is considered to include only the initialization of the <span><code class="sourceCode default">T</code></span> object if the first argument (of type <span><code class="sourceCode default">T*</code></span>) points to storage allocated with <span><code class="sourceCode default">std​::​allocator&lt;T&gt;</code></span> or to an object whose lifetime began within the evaluation of <span><code class="sourceCode default">E</code></span>.</del></span></li>
</ul>
</blockquote>
<p>Change <span>17.6.2 <a href="https://wg21.link/new.syn">[new.syn]</a></span> to mark the placement new functions <code class="sourceCode cpp"><span class="kw">constexpr</span></code>:</p>
<blockquote>
<div class="sourceCode" id="cb7"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb7-1"><a href="#cb7-1"></a>// all freestanding</span>
<span id="cb7-2"><a href="#cb7-2"></a>namespace std {</span>
<span id="cb7-3"><a href="#cb7-3"></a>  // [new.delete], storage allocation and deallocation</span>
<span id="cb7-4"><a href="#cb7-4"></a>  [[nodiscard]] <span class="addu">constexpr</span> void* operator new  (std::size_t size, void* ptr) noexcept;</span>
<span id="cb7-5"><a href="#cb7-5"></a>  [[nodiscard]] <span class="addu">constexpr</span> void* operator new[](std::size_t size, void* ptr) noexcept;</span>
<span id="cb7-6"><a href="#cb7-6"></a>}</span></code></pre></div>
</blockquote>
<p>And likewise in <span>17.6.3.4 <a href="https://wg21.link/new.delete.placement">[new.delete.placement]</a></span>:</p>
<blockquote>
<div class="sourceCode" id="cb8"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb8-1"><a href="#cb8-1"></a>[[nodiscard]] <span class="addu">constexpr</span> void* operator new(std::size_t size, void* ptr) noexcept;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_14" id="pnum_14">2</a></span> <em>Returns</em>: <code class="sourceCode cpp">ptr</code>.</p>
<p>…</p>
<div class="sourceCode" id="cb9"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb9-1"><a href="#cb9-1"></a>[[nodiscard]] <span class="addu">constexpr</span> void* operator new[](std::size_t size, void* ptr) noexcept;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_15" id="pnum_15">5</a></span> <em>Returns</em>: <code class="sourceCode cpp">ptr</code>.</p>
</blockquote>
<h2 data-number="3.1" id="feature-test-macro"><span class="header-section-number">3.1</span> Feature-Test Macro<a href="#feature-test-macro" class="self-link"></a></h2>
<p>Bump the value of <code class="sourceCode cpp">__cpp_constexpr</code> in <span>15.11 <a href="https://wg21.link/cpp.predefined">[cpp.predefined]</a></span>:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb10"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb10-1"><a href="#cb10-1"></a><span class="st">- __cpp_constexpr <span class="diffdel">202306L</span></span></span>
<span id="cb10-2"><a href="#cb10-2"></a><span class="va">+ __cpp_constexpr <span class="diffins">2024XXL</span></span></span></code></pre></div>
</div>
</blockquote>
<p>And add a new <code class="sourceCode cpp">__cpp_lib_constexpr_new</code> in <span>17.3.2 <a href="https://wg21.link/version.syn">[version.syn]</a></span>:</p>
<blockquote>
<div class="addu">
<div class="sourceCode" id="cb11"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb11-1"><a href="#cb11-1"></a>#define __cpp_lib_constexpr_new 2024XXL // freestanding, also in &lt;new&gt;</span></code></pre></div>
</div>
</blockquote>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="bibliography"><span class="header-section-number">4</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<div id="ref-CWG2755">
<p>[CWG2755] Jens Maurer. 2023-06-28. Incorrect wording applied by P2738R1. <br />
<a href="https://wg21.link/cwg2755">https://wg21.link/cwg2755</a></p>
</div>
<div id="ref-LWG3436">
<p>[LWG3436] Jonathan Wakely. std::construct_at should support arrays. <br />
<a href="https://wg21.link/lwg3436">https://wg21.link/lwg3436</a></p>
</div>
<div id="ref-P0784R7">
<p>[P0784R7] Daveed Vandevoorde, Peter Dimov,Louis Dionne, Nina Ranns, Richard Smith, Daveed Vandevoorde. 2019-07-22. More constexpr containers. <br />
<a href="https://wg21.link/p0784r7">https://wg21.link/p0784r7</a></p>
</div>
<div id="ref-P0843R9">
<p>[P0843R9] Gonzalo Brito Gadeschi, Timur Doumler, Nevin Liber, David Sankel. 2023-09-14. inplace_vector. <br />
<a href="https://wg21.link/p0843r9">https://wg21.link/p0843r9</a></p>
</div>
<div id="ref-P2283R1">
<p>[P2283R1] Michael Schellenberger Costa. 2021-04-19. constexpr for specialized memory algorithms. <br />
<a href="https://wg21.link/p2283r1">https://wg21.link/p2283r1</a></p>
</div>
<div id="ref-P2283R2">
<p>[P2283R2] Michael Schellenberger Costa. 2021-11-26. constexpr for specialized memory algorithms. <br />
<a href="https://wg21.link/p2283r2">https://wg21.link/p2283r2</a></p>
</div>
<div id="ref-P2738R1">
<p>[P2738R1] Corentin Jabot, David Ledger. 2023-02-13. constexpr cast from void*: towards constexpr type-erasure. <br />
<a href="https://wg21.link/p2738r1">https://wg21.link/p2738r1</a></p>
</div>
<div id="ref-P2747R0">
<p>[P2747R0] Barry Revzin. 2022-12-17. Limited support for constexpr void*. <br />
<a href="https://wg21.link/p2747r0">https://wg21.link/p2747r0</a></p>
</div>
<div id="ref-P2747R1">
<p>[P2747R1] Barry Revzin. 2023-12-10. constexpr placement new. <br />
<a href="https://wg21.link/p2747r1">https://wg21.link/p2747r1</a></p>
</div>
</div>
</div>
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