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  <title>Less transient constexpr allocation</title>
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<header id="title-block-header">
<h1 class="title" style="text-align:center">Less transient constexpr allocation</h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P3032R0</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2024-02-13</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="#introduction"><span class="toc-section-number">1</span> Introduction<span></span></a>
<ul>
<li><a href="#immediate-escalating-expressions"><span class="toc-section-number">1.1</span> Immediate-escalating expressions<span></span></a></li>
<li><a href="#transient-allocations"><span class="toc-section-number">1.2</span> Transient allocations<span></span></a></li>
</ul></li>
<li><a href="#proposal"><span class="toc-section-number">2</span> Proposal<span></span></a>
<ul>
<li><a href="#wording"><span class="toc-section-number">2.1</span> Wording<span></span></a></li>
</ul></li>
<li><a href="#acknowledgements"><span class="toc-section-number">3</span> Acknowledgements<span></span></a></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="introduction"><span class="header-section-number">1</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>C++20 introduced constexpr allocation, but in a limited form: any allocation must be deallocated during that constant evaluation.</p>
<p>The intent of the rule is that no constexpr allocation persists to runtime. For more on why we currently need to avoid that, see Jeff Snyder’s <span class="citation" data-cites="P1974R0">[<a href="#ref-P1974R0" role="doc-biblioref">P1974R0</a>]</span> and also <span class="citation" data-cites="P2670R1">[<a href="#ref-P2670R1" role="doc-biblioref">P2670R1</a>]</span>.</p>
<p>But the rule cited above does slightly more than prevent constexpr allocation to persist until runtime. The goal of this paper is to allow more examples of allocations that <em>do not</em> persist until runtime, that nevertheless are still rejected by the C++23 rules.</p>
<p>For the purposes of this paper, we’ll consider the example of wanting to get the number of enumerators of a given enumeration. While the specific example is using reflection (<span class="citation" data-cites="P2996R1">[<a href="#ref-P2996R1" role="doc-biblioref">P2996R1</a>]</span>), there isn’t anything particularly reflection-specific about the example - it just makes for a good example. All you need to know about reflection to understand the example is that <code class="sourceCode cpp"><span class="op">^</span>E</code> gives you an object of type <code class="sourceCode cpp">std<span class="op">::</span>meta<span class="op">::</span>info</code> and that this function exists:</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">namespace</span> std<span class="op">::</span>meta <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2"></a>  <span class="kw">using</span> info <span class="op">=</span> <span class="co">/* ... */</span>;</span>
<span id="cb1-3"><a href="#cb1-3"></a>  <span class="kw">consteval</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span> enumerators_of<span class="op">(</span>info<span class="op">)</span>;</span>
<span id="cb1-4"><a href="#cb1-4"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>With that, let’s go through several attempts at trying to get the <em>number</em> of enumerators of a given enumeration <code class="sourceCode cpp">E</code> as a constant:</p>
<table>
<tr>
<th></th>
<th>
Attempt
</th>
<th>
Result
</th>
</tr>
<tr>
<td style="text-align: center;vertical-align: middle">
1
</td>
<td>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span></span>
<span id="cb2-2"><a href="#cb2-2"></a>    <span class="kw">constexpr</span> <span class="dt">int</span> r1 <span class="op">=</span> enumerators_of<span class="op">(^</span>E<span class="op">).</span>size<span class="op">()</span>;</span>
<span id="cb2-3"><a href="#cb2-3"></a>    <span class="cf">return</span> r1;</span>
<span id="cb2-4"><a href="#cb2-4"></a><span class="op">}</span></span></code></pre></div>
</td>
<td>
✅. This one is valid - because <code class="sourceCode cpp">r1</code> is a <code class="sourceCode cpp"><span class="kw">constexpr</span></code> variable, it’s initializer starts a constant evaluation - which includes the entire expression. The temporary <code class="sourceCode cpp">vector</code> is destroyed at the end of that expression, so it doesn’t persist outside of any constant evaluation.
</td>
</tr>
<tr>
<td style="text-align: center;vertical-align: middle">
2
</td>
<td>
<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">constexpr</span> <span class="dt">int</span> f2<span class="op">()</span> <span class="op">{</span></span>
<span id="cb3-2"><a href="#cb3-2"></a>    <span class="cf">return</span> enumerators_of<span class="op">(^</span>E<span class="op">).</span>size<span class="op">()</span>;</span>
<span id="cb3-3"><a href="#cb3-3"></a><span class="op">}</span></span>
<span id="cb3-4"><a href="#cb3-4"></a></span>
<span id="cb3-5"><a href="#cb3-5"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span></span>
<span id="cb3-6"><a href="#cb3-6"></a>    <span class="kw">constexpr</span> <span class="dt">int</span> r2 <span class="op">=</span> f2<span class="op">()</span>;</span>
<span id="cb3-7"><a href="#cb3-7"></a>    <span class="cf">return</span> r2;</span>
<span id="cb3-8"><a href="#cb3-8"></a><span class="op">}</span></span></code></pre></div>
</td>
<td>
<p>❌. This one is invalid.</p>
<p>The same idea about initializing <code class="sourceCode cpp">r2</code> as mentioned in the previous example is valid - but because <code class="sourceCode cpp">f2</code> is a <code class="sourceCode cpp"><span class="kw">constexpr</span></code> function, the invocation of <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">)</span></code> is not in an immediate function context - so it needs to be a constant expression on its own. That is, we start a new constant evaluation within the original one - but this constant evaluation is just the expression <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">)</span></code>. It is not the full expression <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">).</span>size<span class="op">()</span></code>.</p>
As a result, the temporary vector returned by <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">)</span></code> persists outside of its constant expression in order to invoke <code class="sourceCode cpp"><span class="op">.</span>size<span class="op">()</span></code> on it, which is not allowed.
</td>
</tr>
<tr>
<td style="text-align: center;vertical-align: middle">
3
</td>
<td>
<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">consteval</span> <span class="dt">int</span> f3<span class="op">()</span> <span class="op">{</span></span>
<span id="cb4-2"><a href="#cb4-2"></a>    <span class="cf">return</span> enumerators_of<span class="op">(^</span>E<span class="op">).</span>size<span class="op">()</span>;</span>
<span id="cb4-3"><a href="#cb4-3"></a><span class="op">}</span></span>
<span id="cb4-4"><a href="#cb4-4"></a></span>
<span id="cb4-5"><a href="#cb4-5"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span></span>
<span id="cb4-6"><a href="#cb4-6"></a>    <span class="kw">constexpr</span> <span class="dt">int</span> r3 <span class="op">=</span> f<span class="op">()</span>;</span>
<span id="cb4-7"><a href="#cb4-7"></a>    <span class="cf">return</span> r3;</span>
<span id="cb4-8"><a href="#cb4-8"></a><span class="op">}</span></span></code></pre></div>
</td>
<td>
<p>✅. Both this row and the next row are subtle refinements of the second row that make it valid.</p>
The only difference between this and the previous row is that <code class="sourceCode cpp">f2</code> was <code class="sourceCode cpp"><span class="kw">constexpr</span></code> but <code class="sourceCode cpp">f3</code> is <code class="sourceCode cpp"><span class="kw">consteval</span></code>. This distinction matters, because now <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">)</span></code> is no longer an immediate invocation - it is now in an immediate function context. As a result, the only thing that matters is that the entire expression <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">).</span>size<span class="op">()</span></code> is constant - and the temporary <code class="sourceCode cpp">vector<span class="op">&lt;</span>info<span class="op">&gt;</span></code> does not persist past that.
</td>
</tr>
<tr>
<td style="text-align: center;vertical-align: middle">
4
</td>
<td>
<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">template</span><span class="op">&lt;</span><span class="kw">class</span> E<span class="op">&gt;</span> <span class="kw">constexpr</span> <span class="dt">int</span> f4<span class="op">()</span> <span class="op">{</span></span>
<span id="cb5-2"><a href="#cb5-2"></a>    <span class="cf">return</span> enumerators_of<span class="op">(^</span>E<span class="op">).</span>size<span class="op">()</span>;</span>
<span id="cb5-3"><a href="#cb5-3"></a><span class="op">}</span></span>
<span id="cb5-4"><a href="#cb5-4"></a></span>
<span id="cb5-5"><a href="#cb5-5"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span></span>
<span id="cb5-6"><a href="#cb5-6"></a>    <span class="kw">constexpr</span> <span class="dt">int</span> r4 <span class="op">=</span> f4<span class="op">&lt;</span>E<span class="op">&gt;()</span>;</span>
<span id="cb5-7"><a href="#cb5-7"></a>    <span class="cf">return</span> r4;</span>
<span id="cb5-8"><a href="#cb5-8"></a><span class="op">}</span></span></code></pre></div>
</td>
<td>
✅. Here <code class="sourceCode cpp">f4</code> is a <code class="sourceCode cpp"><span class="kw">constexpr</span></code> function <em>template</em>, whereas <code class="sourceCode cpp">f2</code> was a regular <code class="sourceCode cpp"><span class="kw">constexpr</span></code> function. This matters because of <span class="citation" data-cites="P2564R3">[<a href="#ref-P2564R3" role="doc-biblioref">P2564R3</a>]</span> - the fact that <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">).</span>size<span class="op">()</span></code> isn’t a constant expression now causes <code class="sourceCode cpp">f4</code> to become a <code class="sourceCode cpp"><span class="kw">consteval</span></code> function template - and thus we’re in the same state that we were in <code class="sourceCode cpp">f3</code>: it’s not <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">)</span></code> that needs to be a core constant expression but rather all of <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">).</span>size<span class="op">()</span></code>.
</td>
</tr>
<tr>
<td style="text-align: center;vertical-align: middle">
5
</td>
<td>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1"></a><span class="kw">consteval</span> <span class="dt">int</span> f5<span class="op">()</span> <span class="op">{</span></span>
<span id="cb6-2"><a href="#cb6-2"></a>    <span class="kw">constexpr</span> <span class="kw">auto</span> es <span class="op">=</span> enumerators_of<span class="op">(^</span>E<span class="op">)</span>;</span>
<span id="cb6-3"><a href="#cb6-3"></a>    <span class="cf">return</span> es<span class="op">.</span>size<span class="op">()</span>;</span>
<span id="cb6-4"><a href="#cb6-4"></a><span class="op">}</span></span>
<span id="cb6-5"><a href="#cb6-5"></a></span>
<span id="cb6-6"><a href="#cb6-6"></a><span class="dt">int</span> main<span class="op">()</span> <span class="op">{</span></span>
<span id="cb6-7"><a href="#cb6-7"></a>    <span class="kw">constexpr</span> <span class="dt">int</span> r5 <span class="op">=</span> f<span class="op">()</span>;</span>
<span id="cb6-8"><a href="#cb6-8"></a>    <span class="cf">return</span> r5;</span>
<span id="cb6-9"><a href="#cb6-9"></a><span class="op">}</span></span></code></pre></div>
</td>
<td>
❌. Even though <code class="sourceCode cpp">f5</code> is <code class="sourceCode cpp"><span class="kw">consteval</span></code>, we are still explicitly starting a new constant evaluation within <code class="sourceCode cpp">f5</code> by declaring <code class="sourceCode cpp">es</code> to be <code class="sourceCode cpp"><span class="kw">constexpr</span></code>. That allocation persists past that declaration - <em>even though</em> it does not persist past <code class="sourceCode cpp">f5</code>, which by being <code class="sourceCode cpp"><span class="kw">consteval</span></code> means that it does not persist until runtime.
</td>
</tr>
</table>
<p>Three of these rows are valid C++23 programs (modulo the fact that they’re using reflection), but <code class="sourceCode cpp"><span class="dv">2</span></code> and <code class="sourceCode cpp"><span class="dv">5</span></code> are invalid - albeit for different reasons:</p>
<ul>
<li>in the 2nd row, we are required to consider <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">)</span></code> as a constant expression all by itself - even if <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">).</span>size<span class="op">()</span></code> is definitely a constant expression.</li>
<li>in the 5th row, we are required to consider <code class="sourceCode cpp">es</code> as a non-transient constexpr allocation - even though it definitely does not persist until runtime, and thus does not actually cause any of the problems that non-transient constexpr allocation has to address.</li>
</ul>
<h2 data-number="1.1" id="immediate-escalating-expressions"><span class="header-section-number">1.1</span> Immediate-escalating expressions<a href="#immediate-escalating-expressions" class="self-link"></a></h2>
<p>The wording in <span class="citation" data-cites="P2564R3">[<a href="#ref-P2564R3" role="doc-biblioref">P2564R3</a>]</span> introduced the term <em>immediate-escalating expression</em> in <span>7.7 <a href="https://wg21.link/expr.const">[expr.const]</a></span>:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_1" id="pnum_1">17</a></span> An expression or conversion is <em>immediate-escalating</em> if it is not initially in an immediate function context and it is either</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_2" id="pnum_2">(17.1)</a></span> a potentially-evaluated <em>id-expression</em> that denotes an immediate function that is not a subexpression of an immediate invocation, or</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">(17.2)</a></span> an immediate invocation that is not a constant expression and is not a subexpression of an immediate invocation.</li>
</ul>
</blockquote>
<p>In the second example:</p>
<blockquote>
<div class="sourceCode" id="cb7"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb7-1"><a href="#cb7-1"></a><span class="kw">constexpr</span> <span class="dt">int</span> f2<span class="op">()</span> <span class="op">{</span></span>
<span id="cb7-2"><a href="#cb7-2"></a>    <span class="cf">return</span> enumerators_of<span class="op">(^</span>E<span class="op">).</span>size<span class="op">()</span>;</span>
<span id="cb7-3"><a href="#cb7-3"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>The expression <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">)</span></code> is immediate-escalating - it is an immediate invocation (<code class="sourceCode cpp">enumerators_of</code> is a <code class="sourceCode cpp"><span class="kw">consteval</span></code> function) that is not a constant expression (because the temporary vector persists outside of this expression). This is what causes <code class="sourceCode cpp">f4</code> to become a <code class="sourceCode cpp"><span class="kw">consteval</span></code> function template.</p>
<p>But <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">).</span>size<span class="op">()</span></code> is not an immediate invocation (it simply has a subexpression that is an immediate invocation). However, if we were to define it as an immediate invocation - then it would not be an immediate-escalating expression anymore because it is actually a constant expression. And that would be enough to fix this example (as well as <code class="sourceCode cpp">f4</code> which would then itself not escalate to <code class="sourceCode cpp"><span class="kw">consteval</span></code> since it wouldn’t need to).</p>
<p>Put differently: instead of escalating <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">)</span></code> up to the nearest function, which we then try to make <code class="sourceCode cpp"><span class="kw">consteval</span></code> (and fail in the case of <code class="sourceCode cpp">f2</code> because <code class="sourceCode cpp"><span class="kw">constexpr</span></code> functions are not <em>immediate-escalating</em>), we only need to escalate up to the nearest enclosing expression that could be a constant expression.</p>
<h2 data-number="1.2" id="transient-allocations"><span class="header-section-number">1.2</span> Transient allocations<a href="#transient-allocations" class="self-link"></a></h2>
<p>The wording in <span>7.7 <a href="https://wg21.link/expr.const">[expr.const]</a></span> for rejecting non-transient allocations rejects an expression <code class="sourceCode cpp">E</code> as being a core constant expressions if <code class="sourceCode cpp">E</code> evaluates:</p>
<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_4" id="pnum_4">(5.18)</a></span> a <em>new-expression</em> ([expr.new]), unless 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>;</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_5" id="pnum_5">(5.20)</a></span> a call to an instance of <code class="sourceCode cpp">std​<span class="op">::</span>​allocator<span class="op">&lt;</span>T<span class="op">&gt;</span>​<span class="op">::</span>​allocate</code> ([allocator.members]), unless the allocated storage is deallocated within the evaluation of <code class="sourceCode cpp">E</code>;</li>
</ul>
</blockquote>
<p>That is - an allocation within <code class="sourceCode cpp">E</code> has to be transient to <code class="sourceCode cpp">E</code>. However, the rule we really want is that a constant allocation is transient to constant evaluation. In the fifth example:</p>
<blockquote>
<div class="sourceCode" id="cb8"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1"></a><span class="kw">consteval</span> <span class="dt">int</span> f5<span class="op">()</span> <span class="op">{</span></span>
<span id="cb8-2"><a href="#cb8-2"></a>    <span class="kw">constexpr</span> <span class="kw">auto</span> es <span class="op">=</span> enumerators_of<span class="op">(^</span>E<span class="op">)</span>;</span>
<span id="cb8-3"><a href="#cb8-3"></a>    <span class="cf">return</span> es<span class="op">.</span>size<span class="op">()</span>;</span>
<span id="cb8-4"><a href="#cb8-4"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>The allocation in <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">)</span></code> isn’t transient to that expression, but it is definitely destroyed within <code class="sourceCode cpp">f5</code>, which is <code class="sourceCode cpp"><span class="kw">consteval</span></code>. That’s important: if <code class="sourceCode cpp">f5</code> were <code class="sourceCode cpp"><span class="kw">constexpr</span></code>, we’d have access to that allocation at runtime.</p>
<p>We can loosen the restriction such that an allocation within <code class="sourceCode cpp">E</code> must be deallocated within <code class="sourceCode cpp">E</code> or, if <code class="sourceCode cpp">E</code> is in an immediate function context, the end of that context. This would be the end of the <code class="sourceCode cpp"><span class="cf">if</span> <span class="kw">consteval</span> <span class="op">{</span> <span class="op">}</span></code> block or the end of the <code class="sourceCode cpp"><span class="kw">consteval</span></code> function. Such a loosening would allow <code class="sourceCode cpp">f5</code> above, but not if it’s <code class="sourceCode cpp"><span class="kw">constexpr</span></code>, and not if <code class="sourceCode cpp">es</code> were also declared <code class="sourceCode cpp"><span class="kw">static</span></code>.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="proposal"><span class="header-section-number">2</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>There are two separate potential changes here, that would each make one of the attempts above well-formed:</p>
<ol type="1">
<li>we could <a href="#immediate-escalating-expressions">escalate</a> expressions to larger expressions, so that <code class="sourceCode cpp">enumerators_of<span class="op">(^</span>E<span class="op">).</span>size<span class="op">()</span></code> becomes a constant expression, or</li>
<li>we could extend the notation of <a href="#transient-allocations">transient allocation</a> to include the full immediate context instead of just the constant evaluation</li>
</ol>
<p>The second of these is straightforward to word and provides a lot of value - since now particularly in the context of reflection you can declare a <code class="sourceCode cpp"><span class="kw">constexpr</span> vector<span class="op">&lt;</span>info<span class="op">&gt;</span></code> inside a <code class="sourceCode cpp"><span class="kw">consteval</span></code> function and use those contents as a constant expression. The first of these is complicated to word and does not provide as much value, as it is a limitation that is fairly easy to work around: either declare a local <code class="sourceCode cpp"><span class="kw">constexpr</span></code> variable, or change the function to be <code class="sourceCode cpp"><span class="kw">consteval</span></code> or a template.</p>
<p>As such, this paper only proposes extending the notion of transience.</p>
<h2 data-number="2.1" id="wording"><span class="header-section-number">2.1</span> Wording<a href="#wording" class="self-link"></a></h2>
<p>Change <span>7.7 <a href="https://wg21.link/expr.const">[expr.const]</a></span>/5:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_6" id="pnum_6">5</a></span> An expression E is a core constant expression unless the evaluation of E, following the rules of the abstract machine ([intro.execution]), would evaluate one of the following:</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_7" id="pnum_7">(5.1)</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> ([expr.new]), unless the selected allocation function is a replaceable global allocation function ([new.delete.single], [new.delete.array]) and the allocated storage is deallocated <span class="addu">either</span> within the evaluation of <code class="sourceCode cpp">E</code> <span class="addu">or, if <code class="sourceCode cpp">E</code> is in an immediate function content, within that context</span>;</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_9" id="pnum_9">(5.19)</a></span> a <em>delete-expression</em> ([expr.delete]), unless it deallocates a region of storage allocated <span class="addu">either</span> within the evaluation of <code class="sourceCode cpp">E</code> <span class="addu">or, if <code class="sourceCode cpp">E</code> is in an immediate function content, within that context</span>;</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_10" id="pnum_10">(5.20)</a></span> a call to an instance of <code class="sourceCode cpp">std​<span class="op">::</span>​allocator<span class="op">&lt;</span>T<span class="op">&gt;</span>​<span class="op">::</span>​allocate</code> ([allocator.members]), unless the allocated storage is deallocated <span class="addu">either</span> within the evaluation of <code class="sourceCode cpp">E</code> <span class="addu">or, if <code class="sourceCode cpp">E</code> is in an immediate function content, within that context</span>;</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_11" id="pnum_11">(5.21)</a></span> a call to an instance of <code class="sourceCode cpp">std​<span class="op">::</span>​allocator<span class="op">&lt;</span>T<span class="op">&gt;</span>​<span class="op">::</span>​deallocate</code> ([allocator.members]), unless it deallocates a region of storage allocated <span class="addu">either</span> within the evaluation of <code class="sourceCode cpp">E</code> <span class="addu">or, if <code class="sourceCode cpp">E</code> is in an immediate function content, within that context</span>;</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_12" id="pnum_12">(5.22)</a></span> […]</li>
</ul>
</blockquote>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="acknowledgements"><span class="header-section-number">3</span> Acknowledgements<a href="#acknowledgements" class="self-link"></a></h1>
<p>Thank you to Peter Dimov for being Peter Dimov and coming up with all of these examples.</p>
<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-P1974R0">
<p>[P1974R0] Jeff Snyder, Louis Dionne, Daveed Vandevoorde. 2020-05-15. Non-transient constexpr allocation using propconst. <br />
<a href="https://wg21.link/p1974r0">https://wg21.link/p1974r0</a></p>
</div>
<div id="ref-P2564R3">
<p>[P2564R3] Barry Revzin. 2022-11-11. consteval needs to propagate up. <br />
<a href="https://wg21.link/p2564r3">https://wg21.link/p2564r3</a></p>
</div>
<div id="ref-P2670R1">
<p>[P2670R1] Barry Revzin. 2023-02-03. Non-transient constexpr allocation. <br />
<a href="https://wg21.link/p2670r1">https://wg21.link/p2670r1</a></p>
</div>
<div id="ref-P2996R1">
<p>[P2996R1] Barry Revzin, Wyatt Childers, Peter Dimov, Andrew Sutton, Faisal Vali, Daveed Vandevoorde. 2023-12-18. Reflection for C++26. <br />
<a href="https://wg21.link/p2996r1">https://wg21.link/p2996r1</a></p>
</div>
</div>
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