<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang xml:lang>
<head>
  <meta charset="utf-8" />
  <meta name="generator" content="mpark/wg21" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
  <meta name="dcterms.date" content="2024-05-20" />
  <title>consteval blocks</title>
  <style>
      code{white-space: pre-wrap;}
      span.smallcaps{font-variant: small-caps;}
      span.underline{text-decoration: underline;}
      div.column{display: inline-block; vertical-align: top; width: 50%;}
      div.csl-block{margin-left: 1.5em;}
      ul.task-list{list-style: none;}
      pre > code.sourceCode { white-space: pre; position: relative; }
      pre > code.sourceCode > span { display: inline-block; line-height: 1.25; }
      pre > code.sourceCode > span:empty { height: 1.2em; }
      .sourceCode { overflow: visible; }
      code.sourceCode > span { color: inherit; text-decoration: inherit; }
      div.sourceCode { margin: 1em 0; }
      pre.sourceCode { margin: 0; }
      @media screen {
      div.sourceCode { overflow: auto; }
      }
      @media print {
      pre > code.sourceCode { white-space: pre-wrap; }
      pre > code.sourceCode > span { text-indent: -5em; padding-left: 5em; }
      }
      pre.numberSource code
        { counter-reset: source-line 0; }
      pre.numberSource code > span
        { position: relative; left: -4em; counter-increment: source-line; }
      pre.numberSource code > span > a:first-child::before
        { content: counter(source-line);
          position: relative; left: -1em; text-align: right; vertical-align: baseline;
          border: none; display: inline-block;
          -webkit-touch-callout: none; -webkit-user-select: none;
          -khtml-user-select: none; -moz-user-select: none;
          -ms-user-select: none; user-select: none;
          padding: 0 4px; width: 4em;
          color: #aaaaaa;
        }
      pre.numberSource { margin-left: 3em; border-left: 1px solid #aaaaaa;  padding-left: 4px; }
      div.sourceCode
        {  background-color: #f6f8fa; }
      @media screen {
      pre > code.sourceCode > span > a:first-child::before { text-decoration: underline; }
      }
      code span { } /* Normal */
      code span.al { color: #ff0000; } /* Alert */
      code span.an { } /* Annotation */
      code span.at { } /* Attribute */
      code span.bn { color: #9f6807; } /* BaseN */
      code span.bu { color: #9f6807; } /* BuiltIn */
      code span.cf { color: #00607c; } /* ControlFlow */
      code span.ch { color: #9f6807; } /* Char */
      code span.cn { } /* Constant */
      code span.co { color: #008000; font-style: italic; } /* Comment */
      code span.cv { color: #008000; font-style: italic; } /* CommentVar */
      code span.do { color: #008000; } /* Documentation */
      code span.dt { color: #00607c; } /* DataType */
      code span.dv { color: #9f6807; } /* DecVal */
      code span.er { color: #ff0000; font-weight: bold; } /* Error */
      code span.ex { } /* Extension */
      code span.fl { color: #9f6807; } /* Float */
      code span.fu { } /* Function */
      code span.im { } /* Import */
      code span.in { color: #008000; } /* Information */
      code span.kw { color: #00607c; } /* Keyword */
      code span.op { color: #af1915; } /* Operator */
      code span.ot { } /* Other */
      code span.pp { color: #6f4e37; } /* Preprocessor */
      code span.re { } /* RegionMarker */
      code span.sc { color: #9f6807; } /* SpecialChar */
      code span.ss { color: #9f6807; } /* SpecialString */
      code span.st { color: #9f6807; } /* String */
      code span.va { } /* Variable */
      code span.vs { color: #9f6807; } /* VerbatimString */
      code span.wa { color: #008000; font-weight: bold; } /* Warning */
      code.diff {color: #898887}
      code.diff span.va {color: #006e28}
      code.diff span.st {color: #bf0303}
  </style>
  <style type="text/css">
body {
margin: 5em;
font-family: serif;

hyphens: auto;
line-height: 1.35;
text-align: justify;
}
@media screen and (max-width: 30em) {
body {
margin: 1.5em;
}
}
div.wrapper {
max-width: 60em;
margin: auto;
}
ul {
list-style-type: none;
padding-left: 2em;
margin-top: -0.2em;
margin-bottom: -0.2em;
}
a {
text-decoration: none;
color: #4183C4;
}
a.hidden_link {
text-decoration: none;
color: inherit;
}
li {
margin-top: 0.6em;
margin-bottom: 0.6em;
}
h1, h2, h3, h4 {
position: relative;
line-height: 1;
}
a.self-link {
position: absolute;
top: 0;
left: calc(-1 * (3.5rem - 26px));
width: calc(3.5rem - 26px);
height: 2em;
text-align: center;
border: none;
transition: opacity .2s;
opacity: .5;
font-family: sans-serif;
font-weight: normal;
font-size: 83%;
}
a.self-link:hover { opacity: 1; }
a.self-link::before { content: "§"; }
ul > li:before {
content: "\2014";
position: absolute;
margin-left: -1.5em;
}
:target { background-color: #C9FBC9; }
:target .codeblock { background-color: #C9FBC9; }
:target ul { background-color: #C9FBC9; }
.abbr_ref { float: right; }
.folded_abbr_ref { float: right; }
:target .folded_abbr_ref { display: none; }
:target .unfolded_abbr_ref { float: right; display: inherit; }
.unfolded_abbr_ref { display: none; }
.secnum { display: inline-block; min-width: 35pt; }
.header-section-number { display: inline-block; min-width: 35pt; }
.annexnum { display: block; }
div.sourceLinkParent {
float: right;
}
a.sourceLink {
position: absolute;
opacity: 0;
margin-left: 10pt;
}
a.sourceLink:hover {
opacity: 1;
}
a.itemDeclLink {
position: absolute;
font-size: 75%;
text-align: right;
width: 5em;
opacity: 0;
}
a.itemDeclLink:hover { opacity: 1; }
span.marginalizedparent {
position: relative;
left: -5em;
}
li span.marginalizedparent { left: -7em; }
li ul > li span.marginalizedparent { left: -9em; }
li ul > li ul > li span.marginalizedparent { left: -11em; }
li ul > li ul > li ul > li span.marginalizedparent { left: -13em; }
div.footnoteNumberParent {
position: relative;
left: -4.7em;
}
a.marginalized {
position: absolute;
font-size: 75%;
text-align: right;
width: 5em;
}
a.enumerated_item_num {
position: relative;
left: -3.5em;
display: inline-block;
margin-right: -3em;
text-align: right;
width: 3em;
}
div.para { margin-bottom: 0.6em; margin-top: 0.6em; text-align: justify; }
div.section { text-align: justify; }
div.sentence { display: inline; }
span.indexparent {
display: inline;
position: relative;
float: right;
right: -1em;
}
a.index {
position: absolute;
display: none;
}
a.index:before { content: "⟵"; }

a.index:target {
display: inline;
}
.indexitems {
margin-left: 2em;
text-indent: -2em;
}
div.itemdescr {
margin-left: 3em;
}
.bnf {
font-family: serif;
margin-left: 40pt;
margin-top: 0.5em;
margin-bottom: 0.5em;
}
.ncbnf {
font-family: serif;
margin-top: 0.5em;
margin-bottom: 0.5em;
margin-left: 40pt;
}
.ncsimplebnf {
font-family: serif;
font-style: italic;
margin-top: 0.5em;
margin-bottom: 0.5em;
margin-left: 40pt;
background: inherit; 
}
span.textnormal {
font-style: normal;
font-family: serif;
white-space: normal;
display: inline-block;
}
span.rlap {
display: inline-block;
width: 0px;
}
span.descr { font-style: normal; font-family: serif; }
span.grammarterm { font-style: italic; }
span.term { font-style: italic; }
span.terminal { font-family: monospace; font-style: normal; }
span.nonterminal { font-style: italic; }
span.tcode { font-family: monospace; font-style: normal; }
span.textbf { font-weight: bold; }
span.textsc { font-variant: small-caps; }
a.nontermdef { font-style: italic; font-family: serif; }
span.emph { font-style: italic; }
span.techterm { font-style: italic; }
span.mathit { font-style: italic; }
span.mathsf { font-family: sans-serif; }
span.mathrm { font-family: serif; font-style: normal; }
span.textrm { font-family: serif; }
span.textsl { font-style: italic; }
span.mathtt { font-family: monospace; font-style: normal; }
span.mbox { font-family: serif; font-style: normal; }
span.ungap { display: inline-block; width: 2pt; }
span.textit { font-style: italic; }
span.texttt { font-family: monospace; }
span.tcode_in_codeblock { font-family: monospace; font-style: normal; }
span.phantom { color: white; }

span.math { font-style: normal; }
span.mathblock {
display: block;
margin-left: auto;
margin-right: auto;
margin-top: 1.2em;
margin-bottom: 1.2em;
text-align: center;
}
span.mathalpha {
font-style: italic;
}
span.synopsis {
font-weight: bold;
margin-top: 0.5em;
display: block;
}
span.definition {
font-weight: bold;
display: block;
}
.codeblock {
margin-left: 1.2em;
line-height: 127%;
}
.outputblock {
margin-left: 1.2em;
line-height: 127%;
}
div.itemdecl {
margin-top: 2ex;
}
code.itemdeclcode {
white-space: pre;
display: block;
}
span.textsuperscript {
vertical-align: super;
font-size: smaller;
line-height: 0;
}
.footnotenum { vertical-align: super; font-size: smaller; line-height: 0; }
.footnote {
font-size: small;
margin-left: 2em;
margin-right: 2em;
margin-top: 0.6em;
margin-bottom: 0.6em;
}
div.minipage {
display: inline-block;
margin-right: 3em;
}
div.numberedTable {
text-align: center;
margin: 2em;
}
div.figure {
text-align: center;
margin: 2em;
}
table {
border: 1px solid black;
border-collapse: collapse;
margin-left: auto;
margin-right: auto;
margin-top: 0.8em;
text-align: left;
hyphens: none; 
}
td, th {
padding-left: 1em;
padding-right: 1em;
vertical-align: top;
}
td.empty {
padding: 0px;
padding-left: 1px;
}
td.left {
text-align: left;
}
td.right {
text-align: right;
}
td.center {
text-align: center;
}
td.justify {
text-align: justify;
}
td.border {
border-left: 1px solid black;
}
tr.rowsep, td.cline {
border-top: 1px solid black;
}
tr.even, tr.odd {
border-bottom: 1px solid black;
}
tr.capsep {
border-top: 3px solid black;
border-top-style: double;
}
tr.header {
border-bottom: 3px solid black;
border-bottom-style: double;
}
th {
border-bottom: 1px solid black;
}
span.centry {
font-weight: bold;
}
div.table {
display: block;
margin-left: auto;
margin-right: auto;
text-align: center;
width: 90%;
}
span.indented {
display: block;
margin-left: 2em;
margin-bottom: 1em;
margin-top: 1em;
}
ol.enumeratea { list-style-type: none; background: inherit; }
ol.enumerate { list-style-type: none; background: inherit; }

code.sourceCode > span { display: inline; }
</style>
  <style type="text/css">a {
color : #4183C4;
text-decoration: underline;
}
a.marginalized {
text-decoration: none;
}
a.self-link {
text-decoration: none;
}
h1#toctitle {
border-bottom: 1px solid #cccccc;
}
#TOC li {
margin-top: 1px;
margin-bottom: 1px;
}
#TOC ul>li:before { display: none; }
h3.subtitle { margin-top: -15px; }
h1:target { background-color: transparent; }
h2:target { background-color: transparent; }
h3:target { background-color: transparent; }
h4:target { background-color: transparent; }
h5:target { background-color: transparent; }
h6:target { background-color: transparent; }
code span.co { font-family: monospace; }
table tr {
background-color: white;
}
table tr:nth-child(2n) {
background-color: #f6f8fa;
}
#title-block-header > table tr:nth-child(2n) {
background-color: white;
}
td > div.sourceCode {
background-color: inherit;
}
table {
border-collapse: collapse;
}
table td, table th {
border: 1px solid #cccccc;
}
table th {
border-bottom: 1px solid black;
text-align: center;
}
table tr:first-child th {
border-top: 0;
}
table tr:last-child td {
border-bottom: 0;
}
table tr td:first-child,
table tr th:first-child {
border-left: 0;
}
table tr td:last-child,
table tr th:last-child {
border-right: 0;
}
table tbody tr:first-child td {
border-top: 1px solid black;
}
#title-block-header td { border: 0; }
@media all {
body {
margin: 2em;
}
}
@media screen and (min-width: 480px) {
body {
margin: 5em;
}
}
#refs code{padding-left: 0px; text-indent: 0px;}
:root {
--diff-ins: #C9FBC9;
--diff-strongins: #acf2bd;
--diff-del: #FFC8EB;
--diff-strongdel: #ff8888;
}
span.diffins {
background-color: var(--diff-strongins);
}
span.diffdel {
background-color: var(--diff-strongdel);
}
div.rm { text-decoration: line-through; }
div.rm code.sourceCode { text-decoration: line-through; }
div.addu, span.addu {
color: #006e28;
background-color: var(--diff-ins);
}

div.rm pre, div.add pre { background-color: #f6f8fa; }
div.addu pre { background-color: var(--diff-ins); }
div.add, div.add pre { background-color: var(--diff-ins); }
div.addu blockquote {
border-left: 4px solid #00a000;
padding: 0 15px;
color: #006e28;
text-decoration: none;
}
div.addu blockquote code.sourceCode { text-decoration: none; }
div.addu blockquote pre { text-decoration: none; }
div.addu blockquote pre code { text-decoration: none; }
div.quote {
border-left: 7px solid #ccc;
background: #f9f9f9;
margin: 1.5em 10px;
padding-left: 20px;
}
code.diff span.va { color: #000000; background-color: var(--diff-ins); }
code.diff span.st { color: #000000; background-color: var(--diff-del); }
div.std blockquote { color: #000000; background-color: #F1F1F1;
border: 1px solid #D1D1D1;
padding-left: 0.5em; padding-right: 0.5em; }
div.std.ins blockquote {
color: #000000; background-color: #C8FFC8;
border: 1px solid #B3EBB3;
}
div.ins > div.example {
color: #000000; background-color: #C8FFC8;
border: 1px solid #B3EBB3;
}
div.std div.sourceCode { background-color: inherit; margin-left: 1em; }
div.std blockquote del { text-decoration: line-through;
color: #000000; background-color: var(--diff-del);
border: none; }
code del { border: 1px solid #ECB3C7; }
</style>
  <link 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<header id="title-block-header">
<h1 class="title" style="text-align:center"><code class="sourceCode cpp"><span class="kw">consteval</span></code>
blocks</h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P3289R0</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2024-05-20</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>
      Wyatt Childers<br>&lt;<a href="mailto:wcc@edg.com" class="email">wcc@edg.com</a>&gt;<br>
      Barry Revzin<br>&lt;<a href="mailto:barry.revzin@gmail.com" class="email">barry.revzin@gmail.com</a>&gt;<br>
      Daveed Vandevoorde<br>&lt;<a href="mailto:daveed@edg.com" class="email">daveed@edg.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" id="toc-introduction"><span class="toc-section-number">1</span> Introduction<span></span></a></li>
<li><a href="#proposal" id="toc-proposal"><span class="toc-section-number">2</span> Proposal<span></span></a></li>
<li><a href="#implementation-status" id="toc-implementation-status"><span class="toc-section-number">3</span>
Implementation Status<span></span></a></li>
<li><a href="#wording" id="toc-wording"><span class="toc-section-number">4</span> Wording<span></span></a>
<ul>
<li><a href="#feature-test-macro" id="toc-feature-test-macro"><span class="toc-section-number">4.1</span> Feature-Test
Macro<span></span></a></li>
</ul></li>
<li><a href="#bibliography" id="toc-bibliography"><span class="toc-section-number">5</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>Several proposals that produce side effects as part of constant
evaluation are in flight. That includes <span class="citation" data-cites="P2996R2">[<a href="https://wg21.link/p2996r2" role="doc-biblioref">P2996R2</a>]</span> (“Reflection for C++26”) and
<span class="citation" data-cites="P2758R2">[<a href="https://wg21.link/p2758r2" role="doc-biblioref">P2758R2</a>]</span> (“Emitting messages at compile
time”). Such a capability, in turn, quickly gives rise to the desire to
evaluate such constant expressions in declarative contexts.</p>
<p>Currently, this effect can be shoe-horned into <code class="sourceCode cpp"><span class="kw">static_assert</span></code>
declarations, but the result looks arcane. For example, P2996 contains
the following code in an example:</p>
<div class="std">
<blockquote>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#include </span><span class="im">&lt;meta&gt;</span></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">&gt;</span> <span class="kw">struct</span> Tuple <span class="op">{</span></span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a>  <span class="kw">struct</span> storage;</span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a>  <span class="kw">static_assert</span><span class="op">(</span></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a>    is_type<span class="op">(</span>define_class<span class="op">(^</span>storage, <span class="op">{</span>data_member_spec<span class="op">(^</span>Ts<span class="op">)...})))</span>;</span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a>  storage data;</span>
<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a>  Tuple<span class="op">():</span> data<span class="op">{}</span> <span class="op">{}</span></span>
<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a>  Tuple<span class="op">(</span>Ts <span class="kw">const</span><span class="op">&amp;</span> <span class="op">...</span>vs<span class="op">):</span> data<span class="op">{</span> vs<span class="op">...</span> <span class="op">}</span> <span class="op">{}</span></span>
<span id="cb1-11"><a href="#cb1-11" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>
</blockquote>
</div>
<p>Here, <code class="sourceCode cpp">define_class<span class="op">(...)</span></code>
is a constant expression with a side-effect that we want to evaluate
before parsing the member declaration that follows. It works, but it is
somewhat misleading: We’re not really trying to assert anything; we just
want to force that evaluation.</p>
<p>We therefore propose a simple, intuitive syntax to express that we
simply want to constant-evaluate a bit of code wherever a static_assert
declaration could appear by enclosing that code in <code class="sourceCode cpp"><span class="kw">consteval</span> <span class="op">{</span> <span class="op">...</span> <span class="op">}</span></code>
(a construct we’ll call a
<code class="sourceCode cpp"><span class="kw">consteval</span></code>
block).</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>Formally, we propose that a construct of the form</p>
<div class="std">
<blockquote>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="kw">consteval</span> <span class="op">{</span></span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a>    <em>statement-seq</em><sub>opt</sub></span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
</div>
<p>is equivalent to:</p>
<div class="std">
<blockquote>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="kw">static_assert</span><span class="op">(</span></span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a>  <span class="op">(</span></span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a>    <span class="op">[]()</span> <span class="op">-&gt;</span> <span class="dt">void</span> <span class="kw">consteval</span> <span class="op">{</span></span>
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a>      <em>statement-seq</em><sub>opt</sub></span>
<span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a>    <span class="op">}()</span>,</span>
<span id="cb3-6"><a href="#cb3-6" aria-hidden="true" tabindex="-1"></a>    <span class="kw">true</span></span>
<span id="cb3-7"><a href="#cb3-7" aria-hidden="true" tabindex="-1"></a>  <span class="op">)</span></span>
<span id="cb3-8"><a href="#cb3-8" aria-hidden="true" tabindex="-1"></a><span class="op">)</span>;</span></code></pre></div>
</blockquote>
</div>
<p>The <code class="sourceCode cpp"><span class="kw">static_assert</span></code>
condition is a <em>comma-expression</em> with the first operand an
immediately-invoked
<code class="sourceCode cpp"><span class="kw">consteval</span></code>
lambda.</p>
<p>Note that this allows a plain
<code class="sourceCode cpp"><span class="cf">return</span>;</code>
statement or even a <code class="sourceCode cpp"><span class="cf">return</span> f<span class="op">()</span>;</code>
statement where
<code class="sourceCode cpp">f<span class="op">()</span></code> has type
`void. We could go out of our way to disallow that, but we cannot find
any benefit in doing so.</p>
<p>With the feature as proposed, the example above becomes:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>Status Quo</strong>
</div></th>
<th><div style="text-align:center">
<strong>Proposed</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div>

<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#include </span><span class="im">&lt;meta&gt;</span></span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">&gt;</span> <span class="kw">struct</span> Tuple <span class="op">{</span></span>
<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a>  <span class="kw">struct</span> storage;</span>
<span id="cb4-5"><a href="#cb4-5" aria-hidden="true" tabindex="-1"></a>  <span class="kw">static_assert</span><span class="op">(</span></span>
<span id="cb4-6"><a href="#cb4-6" aria-hidden="true" tabindex="-1"></a>    is_type<span class="op">(</span>define_class<span class="op">(^</span>storage,</span>
<span id="cb4-7"><a href="#cb4-7" aria-hidden="true" tabindex="-1"></a>                         <span class="op">{</span>data_member_spec<span class="op">(^</span>Ts<span class="op">)...})))</span>;</span>
<span id="cb4-8"><a href="#cb4-8" aria-hidden="true" tabindex="-1"></a>  storage data;</span>
<span id="cb4-9"><a href="#cb4-9" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>

</div></td>
<td><div>

<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a><span class="pp">#include </span><span class="im">&lt;meta&gt;</span></span>
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">&gt;</span> <span class="kw">struct</span> Tuple <span class="op">{</span></span>
<span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></a>  <span class="kw">struct</span> storage;</span>
<span id="cb5-5"><a href="#cb5-5" aria-hidden="true" tabindex="-1"></a>  <span class="kw">consteval</span> <span class="op">{</span></span>
<span id="cb5-6"><a href="#cb5-6" aria-hidden="true" tabindex="-1"></a>    define_class<span class="op">(^</span>storage,</span>
<span id="cb5-7"><a href="#cb5-7" aria-hidden="true" tabindex="-1"></a>                 <span class="op">{</span>data_member_spec<span class="op">(^</span>Ts<span class="op">)...})</span>;</span>
<span id="cb5-8"><a href="#cb5-8" aria-hidden="true" tabindex="-1"></a>  <span class="op">}</span></span>
<span id="cb5-9"><a href="#cb5-9" aria-hidden="true" tabindex="-1"></a>  storage data;</span>
<span id="cb5-10"><a href="#cb5-10" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>

</div></td>
</tr>
</tbody>
</table>
<p>In this example, there is just a single expression statement being
evaluated. However, we are anticipating reflection code where more
complex statement sequences will be used (you can see eome examples in
previous papers, e.g. <span class="citation" data-cites="P1717R0">[<a href="https://wg21.link/p1717r0" role="doc-biblioref">P1717R0</a>]</span> <span class="citation" data-cites="P2237R0">[<a href="https://wg21.link/p2237r0" role="doc-biblioref">P2237R0</a>]</span>).</p>
<p>We did consider other syntax variations such as</p>
<ul>
<li><code class="sourceCode cpp"><span class="op">@</span>eval <em>expression</em>;</code></li>
<li><code class="sourceCode cpp"><span class="op">@</span> <em>expression</em></code>;</li>
<li><code class="sourceCode cpp"><span class="op">@</span> <em>statement</em></code>;</li>
</ul>
<p>but found those alternatives less general and not worth the slight
improvement in brevity.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="implementation-status"><span class="header-section-number">3</span>
Implementation Status<a href="#implementation-status" class="self-link"></a></h1>
<p>The Lock3 implementation of reflection facilities based on <span class="citation" data-cites="P1240R2">[<a href="https://wg21.link/p1240r2" role="doc-biblioref">P1240R2</a>]</span> (and other papers) includes
this feature. The EDG front end is expected to add this feature shortly
as part of its reflection extensions.</p>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="wording"><span class="header-section-number">4</span> Wording<a href="#wording" class="self-link"></a></h1>
<p><span class="ednote" style="color: #0000ff">[ Editor&#39;s note: The
simplest way to do the wording is to add a
<code class="sourceCode default">consteval</code> block as a
<em>kind</em> of
<code class="sourceCode default"><em>static_assert-declaration</em></code>.
That’s the minimal diff. However, it’s kind of weird to say that a
<code class="sourceCode default">consteval</code> block literally is a
<code class="sourceCode default">static_assert</code> - even if we
specify the former in terms of the latter. So we’d rather take a few
more words to get somewhere that feels more sensible. Plus this change
reduces a lot of duplication between
<code class="sourceCode default"><em>empty-declaration</em></code> and
<code class="sourceCode default"><em>static_assert-declaration</em></code>,
which are treated the same in a lot of places anyway. ]</span></p>
<p>Change <span>6.2 <a href="https://wg21.link/basic.def">[basic.def]</a></span>/2:</p>
<div class="std">
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_1" id="pnum_1">2</a></span>
Each entity declared by a
<code class="sourceCode cpp"><em>declaration</em></code> is also defined
by that <code class="sourceCode cpp"><em>declaration</em></code>
unless:</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_2" id="pnum_2">(2.1)</a></span>
it declares a function without specifying the function’s body
([dcl.fct.def]),</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">(2.2)</a></span>
[…]</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_4" id="pnum_4">(2.13)</a></span>
it is a <span class="rm" style="color: #bf0303"><del><span><code class="sourceCode default"><em>static_assert-declaration</em></code></span></del></span>
<span class="addu"><code class="sourceCode cpp"><em>vacant-declaration</em></code></span>
([dcl.pre]),</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_5" id="pnum_5">(2.14)</a></span>
it is an
<code class="sourceCode cpp"><em>attribute-declaration</em></code>
([dcl.pre]),</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_6" id="pnum_6">(2.15)</a></span>
<span class="rm" style="color: #bf0303"><del>it is an
<span><code class="sourceCode default"><em>empty-declaration</em></code></span>
([dcl.pre]),</del></span></li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_7" id="pnum_7">(2.16)</a></span>
[…]</li>
</ul>
</blockquote>
</div>
<p>Change <span>9.1 <a href="https://wg21.link/dcl.pre">[dcl.pre]</a></span>:</p>
<div class="std">
<blockquote>
<div>
<div class="sourceCode" id="cb6"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a>  <em>name-declaration</em>:</span>
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a>    <em>block-declaration</em></span>
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a>    <em>nodeclspec-function-declaration</em></span>
<span id="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a>    <em>function-definition</em></span>
<span id="cb6-5"><a href="#cb6-5" aria-hidden="true" tabindex="-1"></a>    <em>friend-type-declaration</em></span>
<span id="cb6-6"><a href="#cb6-6" aria-hidden="true" tabindex="-1"></a>    <em>template-declaration</em></span>
<span id="cb6-7"><a href="#cb6-7" aria-hidden="true" tabindex="-1"></a>    <em>deduction-guide</em></span>
<span id="cb6-8"><a href="#cb6-8" aria-hidden="true" tabindex="-1"></a>    <em>linkage-specification</em></span>
<span id="cb6-9"><a href="#cb6-9" aria-hidden="true" tabindex="-1"></a>    <em>namespace-definition</em></span>
<span id="cb6-10"><a href="#cb6-10" aria-hidden="true" tabindex="-1"></a><span class="st">-   <em>empty-declaration</em></span></span>
<span id="cb6-11"><a href="#cb6-11" aria-hidden="true" tabindex="-1"></a>    <em>attribute-declaration</em></span>
<span id="cb6-12"><a href="#cb6-12" aria-hidden="true" tabindex="-1"></a>    <em>module-import-declaration</em></span>
<span id="cb6-13"><a href="#cb6-13" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb6-14"><a href="#cb6-14" aria-hidden="true" tabindex="-1"></a>  <em>block-declaration</em>:</span>
<span id="cb6-15"><a href="#cb6-15" aria-hidden="true" tabindex="-1"></a>    <em>simple-declaration</em></span>
<span id="cb6-16"><a href="#cb6-16" aria-hidden="true" tabindex="-1"></a>    <em>asm-declaration</em></span>
<span id="cb6-17"><a href="#cb6-17" aria-hidden="true" tabindex="-1"></a>    <em>namespace-alias-definition</em></span>
<span id="cb6-18"><a href="#cb6-18" aria-hidden="true" tabindex="-1"></a>    <em>using-declaration</em></span>
<span id="cb6-19"><a href="#cb6-19" aria-hidden="true" tabindex="-1"></a>    <em>using-enum-declaration</em></span>
<span id="cb6-20"><a href="#cb6-20" aria-hidden="true" tabindex="-1"></a>    <em>using-directive</em></span>
<span id="cb6-21"><a href="#cb6-21" aria-hidden="true" tabindex="-1"></a><span class="st">-   <em>static_assert-declaration</em></span></span>
<span id="cb6-22"><a href="#cb6-22" aria-hidden="true" tabindex="-1"></a>    <em>alias-declaration</em></span>
<span id="cb6-23"><a href="#cb6-23" aria-hidden="true" tabindex="-1"></a>    <em>opaque-enum-declaration</em></span>
<span id="cb6-24"><a href="#cb6-24" aria-hidden="true" tabindex="-1"></a><span class="va">+   <em>vacant-declaration</em></span></span>
<span id="cb6-25"><a href="#cb6-25" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb6-26"><a href="#cb6-26" aria-hidden="true" tabindex="-1"></a><span class="va">+ <em>vacant-declaration</em>:</span></span>
<span id="cb6-27"><a href="#cb6-27" aria-hidden="true" tabindex="-1"></a><span class="va">+    <em>static_assert-declaration</em></span></span>
<span id="cb6-28"><a href="#cb6-28" aria-hidden="true" tabindex="-1"></a><span class="va">+    <em>empty-declaration</em></span></span>
<span id="cb6-29"><a href="#cb6-29" aria-hidden="true" tabindex="-1"></a><span class="va">+    <em>consteval-block-declaration</em></span></span>
<span id="cb6-30"><a href="#cb6-30" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb6-31"><a href="#cb6-31" aria-hidden="true" tabindex="-1"></a>  <em>static_assert-declaration</em>:</span>
<span id="cb6-32"><a href="#cb6-32" aria-hidden="true" tabindex="-1"></a>    static_assert ( <em>constant-expression</em> ) ;</span>
<span id="cb6-33"><a href="#cb6-33" aria-hidden="true" tabindex="-1"></a>    static_assert ( <em>constant-expression</em> , <em>static_assert-message</em> ) ;</span>
<span id="cb6-34"><a href="#cb6-34" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb6-35"><a href="#cb6-35" aria-hidden="true" tabindex="-1"></a><span class="va">+ <em>consteval-block-declaration</em>:</span></span>
<span id="cb6-36"><a href="#cb6-36" aria-hidden="true" tabindex="-1"></a><span class="va">+   consteval <em>compound-statement</em></span></span></code></pre></div>
</div>
</blockquote>
</div>
<p>And then after <span>9.1 <a href="https://wg21.link/dcl.pre">[dcl.pre]</a></span>/13:</p>
<div class="std">
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_8" id="pnum_8">13</a></span>
<em>Recommended practice</em>: When a
<code class="sourceCode cpp"><em>static_assert-declaration</em></code>
fails, […]</p>
<div class="addu">
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_9" id="pnum_9">*</a></span>
The
<code class="sourceCode cpp"><em>consteval-block-declaration</em></code></p>
<div class="sourceCode" id="cb7"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a><span class="kw">consteval</span> <em>compound-statement</em></span></code></pre></div>
<p>is equivalent to</p>
<div class="sourceCode" id="cb8"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a><span class="kw">static_assert</span><span class="op">(([]()</span> <span class="op">-&gt;</span> <span class="dt">void</span> <span class="kw">consteval</span> <em>compound-statement</em><span class="op">()</span>, <span class="kw">true</span><span class="op">))</span>;</span></code></pre></div>
<p><span class="note"><span>[ <em>Note 1:</em> </span>Such a
<code class="sourceCode cpp"><em>static_assert-declaration</em></code>
never fails.<span> — <em>end note</em> ]</span></span></p>
</div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_10" id="pnum_10">14</a></span>
An <code class="sourceCode cpp"><em>empty-declaration</em></code> has no
effect.</p>
</blockquote>
</div>
<p>Adjust the grammar in <span>11.4.1 <a href="https://wg21.link/class.mem.general">[class.mem.general]</a></span>
and the rule in p3:</p>
<div class="std">
<blockquote>
<div>
<div class="sourceCode" id="cb9"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb9-1"><a href="#cb9-1" aria-hidden="true" tabindex="-1"></a>  <em>member-declaration</em>:</span>
<span id="cb9-2"><a href="#cb9-2" aria-hidden="true" tabindex="-1"></a>    <em>attribute-specifier-seq</em><sub>opt</sub> <em>decl-specifier-seq</em><sub>opt</sub> <em>member-declarator-list</em><sub>opt</sub>;</span>
<span id="cb9-3"><a href="#cb9-3" aria-hidden="true" tabindex="-1"></a>    <em>function-definition</em></span>
<span id="cb9-4"><a href="#cb9-4" aria-hidden="true" tabindex="-1"></a>    <em>friend-type-declaration</em></span>
<span id="cb9-5"><a href="#cb9-5" aria-hidden="true" tabindex="-1"></a>    <em>using-declaration</em></span>
<span id="cb9-6"><a href="#cb9-6" aria-hidden="true" tabindex="-1"></a>    <em>using-enum-declaration</em></span>
<span id="cb9-7"><a href="#cb9-7" aria-hidden="true" tabindex="-1"></a><span class="st">-   <em>static_assert-declaration</em></span></span>
<span id="cb9-8"><a href="#cb9-8" aria-hidden="true" tabindex="-1"></a><span class="va">+   <em>vacant-declaration</em></span></span>
<span id="cb9-9"><a href="#cb9-9" aria-hidden="true" tabindex="-1"></a>    <em>template-declaration</em></span>
<span id="cb9-10"><a href="#cb9-10" aria-hidden="true" tabindex="-1"></a>    <em>explicit-specialization</em></span>
<span id="cb9-11"><a href="#cb9-11" aria-hidden="true" tabindex="-1"></a>    <em>deduction-guide</em></span>
<span id="cb9-12"><a href="#cb9-12" aria-hidden="true" tabindex="-1"></a>    <em>alias-declaration</em></span>
<span id="cb9-13"><a href="#cb9-13" aria-hidden="true" tabindex="-1"></a>    <em>opaque-enum-declaration</em></span>
<span id="cb9-14"><a href="#cb9-14" aria-hidden="true" tabindex="-1"></a><span class="st">-   <em>empty-declaration</em></span></span></code></pre></div>
</div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_11" id="pnum_11">3</a></span>
A <code class="sourceCode cpp"><em>member-declaration</em></code> does
not declare new members of the class if it is</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_12" id="pnum_12">(3.1)</a></span>
a friend declaration ([class.friend]),</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_13" id="pnum_13">(3.2)</a></span>
a <code class="sourceCode cpp"><em>deduction-guide</em></code>
([temp.deduct.guide]),</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_14" id="pnum_14">(3.3)</a></span>
a <code class="sourceCode cpp"><em>template-declaration</em></code>
whose declaration is one of the above,</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_15" id="pnum_15">(3.4)</a></span>
a <span class="rm" style="color: #bf0303"><del><span><code class="sourceCode default"><em>static_assert-declaration</em></code></span>,</del></span></li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_16" id="pnum_16">(3.5)</a></span>
a <code class="sourceCode cpp"><em>using-declaration</em></code>
([namespace.udecl]) , or</li>
<li><span class="marginalizedparent"><a class="marginalized" href="#pnum_17" id="pnum_17">(3.6)</a></span>
<span class="rm" style="color: #bf0303"><del>an
<span><code class="sourceCode default"><em>empty-declaration</em></code></span>.</del></span>
<span class="addu">a
<code class="sourceCode cpp"><em>vacant-declaration</em></code>.</span></li>
</ul>
</blockquote>
</div>
<p>And similar in <span>11.5.2 <a href="https://wg21.link/class.union.anon">[class.union.anon]</a></span>/1.
<span class="ednote" style="color: #0000ff">[ Editor&#39;s note: This
refactor allows putting in an
<code class="sourceCode default"><em>empty-declaration</em></code> into
an anonymous union, which is kind of a consistency drive by with other
classes. ]</span></p>
<div class="std">
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_18" id="pnum_18">1</a></span>
[…] Each <code class="sourceCode cpp"><em>member-declaration</em></code>
in the <code class="sourceCode cpp"><em>member-specification</em></code>
of an anonymous union shall either define one or more public non-static
data members or be a <span class="rm" style="color: #bf0303"><del><span><code class="sourceCode default"><em>static_assert-declaration</em></code></span></del></span>
<span class="addu"><code class="sourceCode cpp"><em>vacant-declaration</em></code></span>.
[…]</p>
</blockquote>
</div>
<h2 data-number="4.1" id="feature-test-macro"><span class="header-section-number">4.1</span> Feature-Test Macro<a href="#feature-test-macro" class="self-link"></a></h2>
<p>Add to the table in <span>15.11 <a href="https://wg21.link/cpp.predefined">[cpp.predefined]</a></span>:</p>
<div class="std">
<blockquote>
<div>
<div class="sourceCode" id="cb10"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a>  __cpp_consteval       202211L</span>
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a><span class="va">+ __cpp_consteval_block 2024XXL</span></span>
<span id="cb10-3"><a href="#cb10-3" aria-hidden="true" tabindex="-1"></a>  __cpp_constinit       201907L</span></code></pre></div>
</div>
</blockquote>
</div>
<h1 data-number="5" style="border-bottom:1px solid #cccccc" id="bibliography"><span class="header-section-number">5</span>
References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references csl-bib-body hanging-indent" data-entry-spacing="1" role="doc-bibliography">
<div id="ref-P1240R2" class="csl-entry" role="doc-biblioentry">
[P1240R2] Daveed Vandevoorde, Wyatt Childers, Andrew Sutton, Faisal
Vali. 2022-01-14. Scalable Reflection. <a href="https://wg21.link/p1240r2"><div class="csl-block">https://wg21.link/p1240r2</div></a>
</div>
<div id="ref-P1717R0" class="csl-entry" role="doc-biblioentry">
[P1717R0] Andrew Sutton, Wyatt Childers. 2019-06-17. Compile-time
Metaprogramming in C++. <a href="https://wg21.link/p1717r0"><div class="csl-block">https://wg21.link/p1717r0</div></a>
</div>
<div id="ref-P2237R0" class="csl-entry" role="doc-biblioentry">
[P2237R0] Andrew Sutton. 2020-10-15. Metaprogramming. <a href="https://wg21.link/p2237r0"><div class="csl-block">https://wg21.link/p2237r0</div></a>
</div>
<div id="ref-P2758R2" class="csl-entry" role="doc-biblioentry">
[P2758R2] Barry Revzin. 2024-02-15. Emitting messages at compile time.
<a href="https://wg21.link/p2758r2"><div class="csl-block">https://wg21.link/p2758r2</div></a>
</div>
<div id="ref-P2996R2" class="csl-entry" role="doc-biblioentry">
[P2996R2] Barry Revzin, Wyatt Childers, Peter Dimov, Andrew Sutton,
Faisal Vali, Daveed Vandevoorde, Dan Katz. 2024-02-15. Reflection for
C++26. <a href="https://wg21.link/p2996r2"><div class="csl-block">https://wg21.link/p2996r2</div></a>
</div>
</div>
</div>
</div>
</body>
</html>
