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<!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="2022-10-05" />
<title>Member visit</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.hanging-indent{margin-left: 1.5em; text-indent: -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; }
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 */
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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 */
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code span.er { color: #ff0000; font-weight: bold; } /* Error */
code span.ex { } /* Extension */
code span.fl { color: #9f6807; } /* Float */
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code span.im { } /* Import */
code span.in { color: #008000; } /* Information */
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code span.pp { color: #6f4e37; } /* Preprocessor */
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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>
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margin: auto;
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padding-left: 2em;
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margin-bottom: -0.2em;
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color: inherit;
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margin-bottom: 0.6em;
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line-height: 1;
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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;
}
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:target ul { background-color: #C9FBC9; }
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float: right;
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position: absolute;
font-size: 75%;
text-align: right;
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left: -5em;
}
li span.marginalizedparent { left: -7em; }
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li ul > li ul > li span.marginalizedparent { left: -11em; }
li ul > li ul > li ul > li span.marginalizedparent { left: -13em; }
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}
a.marginalized {
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font-size: 75%;
text-align: right;
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a.enumerated_item_num {
position: relative;
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display: inline-block;
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text-align: right;
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margin-bottom: 0.5em;
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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; }
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span.techterm { font-style: italic; }
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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;
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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;
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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: #e6ffed;
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}
span.diffins {
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}
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background-color: var(--diff-strongdel);
}
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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;
}
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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); }
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<h1 class="title" style="text-align:center">Member <code class="sourceCode cpp">visit</code></h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P2637R1</td>
</tr>
<tr>
<td>Date:</td>
<td>2022-10-05</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>
LEWG<br>
</td>
</tr>
<tr>
<td style="vertical-align:top">Reply-to:</td>
<td>
Barry Revzin<br><<a href="mailto:barry.revzin@gmail.com" class="email">barry.revzin@gmail.com</a>><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>
<ul>
<li><a href="#stdvisit"><span class="toc-section-number">2.1</span> <code class="sourceCode cpp">std<span class="op">::</span>visit</code><span></span></a></li>
<li><a href="#stdvisit_format_arg"><span class="toc-section-number">2.2</span> <code class="sourceCode cpp">std<span class="op">::</span>visit_format_arg</code><span></span></a></li>
<li><a href="#implementation"><span class="toc-section-number">2.3</span> Implementation<span></span></a></li>
</ul></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>Since <span class="citation" data-cites="P2637R0">[<a href="#ref-P2637R0" role="doc-biblioref">P2637R0</a>]</span>, dropped <code class="sourceCode cpp">apply</code>, added member <code class="sourceCode cpp">visit<span class="op"><</span>R<span class="op">></span></code> to <code class="sourceCode cpp">basic_format_arg</code>, and added support for types privately inheriting from <code class="sourceCode cpp">std<span class="op">::</span>variant</code> for member <code class="sourceCode cpp">visit</code> and <code class="sourceCode cpp">visit<span class="op"><</span>R<span class="op">></span></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>The standard library currently has two free function templates for variant visitation: <code class="sourceCode cpp">std<span class="op">::</span>visit</code> and <code class="sourceCode cpp">std<span class="op">::</span>visit_format_arg</code>. The goal of this paper is to add member function versions of each of them, simply for ergonomic reasons. This paper adds no new functionality that did not exist before.</p>
<h2 data-number="2.1" id="stdvisit"><span class="header-section-number">2.1</span> <code class="sourceCode cpp">std<span class="op">::</span>visit</code><a href="#stdvisit" class="self-link"></a></h2>
<p><code class="sourceCode cpp">std<span class="op">::</span>visit</code> is a variadic function template, which is the correct design since binary (and more) visitation is a useful and important piece of functionality. However, the common case is simply unary visitation. Even in that case, however, a non-member function was a superior implementation choice for forwarding const-ness and value category.<a href="#fn1" class="footnote-ref" id="fnref1" role="doc-noteref"><sup>1</sup></a></p>
<p>But this decision logic changes in C++23 with the introduction of deducing <code class="sourceCode cpp"><span class="kw">this</span></code> <span class="citation" data-cites="P0847R7">[<a href="#ref-P0847R7" role="doc-biblioref">P0847R7</a>]</span>. Now, it is possible to implement unary <code class="sourceCode cpp">visit</code> as a member function without any loss of functionality. We simply gain better syntax:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>Existing</strong>
</div></th>
<th><div style="text-align:center">
<strong>Proposed</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a>std<span class="op">::</span>visit<span class="op">(</span>overload<span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2"></a> <span class="op">[](</span><span class="dt">int</span> i<span class="op">){</span> std<span class="op">::</span>print<span class="op">(</span><span class="st">"i={}</span><span class="sc">\n</span><span class="st">"</span>, i<span class="op">)</span>; <span class="op">}</span>,</span>
<span id="cb1-3"><a href="#cb1-3"></a> <span class="op">[](</span>std<span class="op">::</span>string s<span class="op">){</span> std<span class="op">::</span>print<span class="op">(</span><span class="st">"s={:?}</span><span class="sc">\n</span><span class="st">"</span>, s<span class="op">)</span>; <span class="op">}</span></span>
<span id="cb1-4"><a href="#cb1-4"></a><span class="op">}</span>, value<span class="op">)</span>;</span></code></pre></div></td>
<td><div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a>value<span class="op">.</span>visit<span class="op">(</span>overload<span class="op">{</span></span>
<span id="cb2-2"><a href="#cb2-2"></a> <span class="op">[](</span><span class="dt">int</span> i<span class="op">){</span> std<span class="op">::</span>print<span class="op">(</span><span class="st">"i={}</span><span class="sc">\n</span><span class="st">"</span>, i<span class="op">)</span>; <span class="op">}</span>,</span>
<span id="cb2-3"><a href="#cb2-3"></a> <span class="op">[](</span>std<span class="op">::</span>string s<span class="op">){</span> std<span class="op">::</span>print<span class="op">(</span><span class="st">"s={:?}</span><span class="sc">\n</span><span class="st">"</span>, s<span class="op">)</span>; <span class="op">}</span></span>
<span id="cb2-4"><a href="#cb2-4"></a><span class="op">})</span>;</span></code></pre></div></td>
</tr>
</tbody>
</table>
<h2 data-number="2.2" id="stdvisit_format_arg"><span class="header-section-number">2.2</span> <code class="sourceCode cpp">std<span class="op">::</span>visit_format_arg</code><a href="#stdvisit_format_arg" class="self-link"></a></h2>
<p>One of the components of the format library is <code class="sourceCode cpp">basic_format_arg<span class="op"><</span>Context<span class="op">></span></code> (see <span>22.14.8.1 <a href="https://wg21.link/format.arg">[format.arg]</a></span>), which is basically a <code class="sourceCode cpp">std<span class="op">::</span>variant</code>. As such, it also needs to be visited in order to be used. To that end, the library provides:</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"><</span><span class="kw">class</span> Visitor, <span class="kw">class</span> Context<span class="op">></span></span>
<span id="cb3-2"><a href="#cb3-2"></a> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> visit_format_arg<span class="op">(</span>Visitor<span class="op">&&</span> vis, basic_format_arg<span class="op"><</span>Context<span class="op">></span> arg<span class="op">)</span>;</span></code></pre></div>
</blockquote>
<p>But here, the only reason <code class="sourceCode cpp">std<span class="op">::</span>visit_format_arg</code> is a non-member function was to mirror the interface for <code class="sourceCode cpp">std<span class="op">::</span>visit</code>. There is neither multiple visitation nor forwarding of value category or const-ness here. It could always have been a member function without any loss of functionality. With deducing <code class="sourceCode cpp"><span class="kw">this</span></code>, it can even be by-value member function.</p>
<p>This example is from the standard itself:</p>
<table>
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<tr class="header">
<th><div style="text-align:center">
<strong>Existing</strong>
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<strong>Proposed</strong>
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<tbody>
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<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">auto</span> format<span class="op">(</span>S s, format_context<span class="op">&</span> ctx<span class="op">)</span> <span class="op">{</span></span>
<span id="cb4-2"><a href="#cb4-2"></a> <span class="dt">int</span> width <span class="op">=</span> visit_format_arg<span class="op">([](</span><span class="kw">auto</span> value<span class="op">)</span> <span class="op">-></span> <span class="dt">int</span> <span class="op">{</span></span>
<span id="cb4-3"><a href="#cb4-3"></a> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(!</span>is_integral_v<span class="op"><</span><span class="kw">decltype</span><span class="op">(</span>value<span class="op">)>)</span></span>
<span id="cb4-4"><a href="#cb4-4"></a> <span class="cf">throw</span> format_error<span class="op">(</span><span class="st">"width is not integral"</span><span class="op">)</span>;</span>
<span id="cb4-5"><a href="#cb4-5"></a> <span class="cf">else</span> <span class="cf">if</span> <span class="op">(</span>value <span class="op"><</span> <span class="dv">0</span> <span class="op">||</span> value <span class="op">></span> numeric_limits<span class="op"><</span><span class="dt">int</span><span class="op">>::</span>max<span class="op">())</span></span>
<span id="cb4-6"><a href="#cb4-6"></a> <span class="cf">throw</span> format_error<span class="op">(</span><span class="st">"invalid width"</span><span class="op">)</span>;</span>
<span id="cb4-7"><a href="#cb4-7"></a> <span class="cf">else</span></span>
<span id="cb4-8"><a href="#cb4-8"></a> <span class="cf">return</span> value;</span>
<span id="cb4-9"><a href="#cb4-9"></a> <span class="op">}</span>, ctx<span class="op">.</span>arg<span class="op">(</span>width_arg_id<span class="op">))</span>;</span>
<span id="cb4-10"><a href="#cb4-10"></a> <span class="cf">return</span> format_to<span class="op">(</span>ctx<span class="op">.</span>out<span class="op">()</span>, <span class="st">"{0:x<{1}}"</span>, s<span class="op">.</span>value, width<span class="op">)</span>;</span>
<span id="cb4-11"><a href="#cb4-11"></a><span class="op">}</span></span></code></pre></div></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">auto</span> format<span class="op">(</span>S s, format_context<span class="op">&</span> ctx<span class="op">)</span> <span class="op">{</span></span>
<span id="cb5-2"><a href="#cb5-2"></a> <span class="dt">int</span> width <span class="op">=</span> ctx<span class="op">.</span>arg<span class="op">(</span>width_arg_id<span class="op">).</span>visit<span class="op">([](</span><span class="kw">auto</span> value<span class="op">)</span> <span class="op">-></span> <span class="dt">int</span> <span class="op">{</span></span>
<span id="cb5-3"><a href="#cb5-3"></a> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(!</span>is_integral_v<span class="op"><</span><span class="kw">decltype</span><span class="op">(</span>value<span class="op">)>)</span></span>
<span id="cb5-4"><a href="#cb5-4"></a> <span class="cf">throw</span> format_error<span class="op">(</span><span class="st">"width is not integral"</span><span class="op">)</span>;</span>
<span id="cb5-5"><a href="#cb5-5"></a> <span class="cf">else</span> <span class="cf">if</span> <span class="op">(</span>value <span class="op"><</span> <span class="dv">0</span> <span class="op">||</span> value <span class="op">></span> numeric_limits<span class="op"><</span><span class="dt">int</span><span class="op">>::</span>max<span class="op">())</span></span>
<span id="cb5-6"><a href="#cb5-6"></a> <span class="cf">throw</span> format_error<span class="op">(</span><span class="st">"invalid width"</span><span class="op">)</span>;</span>
<span id="cb5-7"><a href="#cb5-7"></a> <span class="cf">else</span></span>
<span id="cb5-8"><a href="#cb5-8"></a> <span class="cf">return</span> value;</span>
<span id="cb5-9"><a href="#cb5-9"></a> <span class="op">})</span>;</span>
<span id="cb5-10"><a href="#cb5-10"></a> <span class="cf">return</span> format_to<span class="op">(</span>ctx<span class="op">.</span>out<span class="op">()</span>, <span class="st">"{0:x<{1}}"</span>, s<span class="op">.</span>value, width<span class="op">)</span>;</span>
<span id="cb5-11"><a href="#cb5-11"></a><span class="op">}</span></span></code></pre></div></td>
</tr>
</tbody>
</table>
<p>The proposed name here is just <code class="sourceCode cpp">visit</code> (rather than <code class="sourceCode cpp">visit_format_arg</code>), since as a member function we don’t need the longer name for differentiation.</p>
<h2 data-number="2.3" id="implementation"><span class="header-section-number">2.3</span> Implementation<a href="#implementation" class="self-link"></a></h2>
<p>In each case, the implementation is simple: simply redirect to the corresponding non-member function. Member <code class="sourceCode cpp">visit</code>, for instance:</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="kw">template</span> <span class="op"><</span><span class="kw">class</span><span class="op">...</span> Types<span class="op">></span></span>
<span id="cb6-2"><a href="#cb6-2"></a><span class="kw">class</span> variant <span class="op">{</span></span>
<span id="cb6-3"><a href="#cb6-3"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb6-4"><a href="#cb6-4"></a> <span class="kw">template</span> <span class="op"><</span><span class="dt">int</span><span class="op">=</span><span class="dv">0</span>, <span class="kw">class</span> Self, <span class="kw">class</span> Visitor<span class="op">></span></span>
<span id="cb6-5"><a href="#cb6-5"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> visit<span class="op">(</span><span class="kw">this</span> Self<span class="op">&&</span> self, Visitor<span class="op">&&</span> vis<span class="op">)</span> <span class="op">-></span> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb6-6"><a href="#cb6-6"></a> <span class="cf">return</span> std<span class="op">::</span>visit<span class="op">(</span>std<span class="op">::</span>forward<span class="op"><</span>Visitor<span class="op">>(</span>vis<span class="op">)</span>, <span class="op">(</span>copy_cvref_t<span class="op"><</span>Self, variant<span class="op">>&&)</span>self<span class="op">)</span>;</span>
<span id="cb6-7"><a href="#cb6-7"></a> <span class="op">}</span></span>
<span id="cb6-8"><a href="#cb6-8"></a></span>
<span id="cb6-9"><a href="#cb6-9"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">class</span> R, <span class="kw">class</span> Self, <span class="kw">class</span> Visitor<span class="op">></span></span>
<span id="cb6-10"><a href="#cb6-10"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> visit<span class="op">(</span><span class="kw">this</span> Self<span class="op">&&</span> self, Visitor<span class="op">&&</span> vis<span class="op">)</span> <span class="op">-></span> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb6-11"><a href="#cb6-11"></a> <span class="cf">return</span> std<span class="op">::</span>visit<span class="op"><</span>R<span class="op">>(</span>std<span class="op">::</span>forward<span class="op"><</span>Visitor<span class="op">>(</span>vis<span class="op">)</span>, <span class="op">(</span>copy_cvref_t<span class="op"><</span>Self, variant<span class="op">>&&)</span>self<span class="op">)</span>;</span>
<span id="cb6-12"><a href="#cb6-12"></a> <span class="op">}</span></span>
<span id="cb6-13"><a href="#cb6-13"></a><span class="op">}</span>;</span></code></pre></div>
</blockquote>
<p><code class="sourceCode cpp">copy_cvref_t<span class="op"><</span>A, B<span class="op">></span></code> is a metafunction that simply pastes the const/ref qualifiers from <code class="sourceCode cpp">A</code> onto <code class="sourceCode cpp">B</code>. It will be added by <span class="citation" data-cites="P1450R3">[<a href="#ref-P1450R3" role="doc-biblioref">P1450R3</a>]</span>.</p>
<p>The C-style cast here is deliberate because <code class="sourceCode cpp">variant</code> might be a private base of <code class="sourceCode cpp">Self</code>. This is a case that <code class="sourceCode cpp">std<span class="op">::</span>visit</code> does not support, but LEWG preferred if member <code class="sourceCode cpp">visit</code> did.</p>
<p>There is also an extra leading <code class="sourceCode cpp"><span class="dt">int</span><span class="op">=</span><span class="dv">0</span></code> template parameter for the overload that just calls <code class="sourceCode cpp">std<span class="op">::</span>visit</code> (rather than <code class="sourceCode cpp">std<span class="op">::</span>visit<span class="op"><</span>R<span class="op">></span></code>). This is because, unlike with the non-member functions, an ambiguity would otherwise arise if you attempted to do:</p>
<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">using</span> State <span class="op">=</span> std<span class="op">::</span>variant<span class="op"><</span>A, B, C<span class="op">></span>;</span>
<span id="cb7-2"><a href="#cb7-2"></a></span>
<span id="cb7-3"><a href="#cb7-3"></a>State state <span class="op">=</span> <span class="co">/* ... */</span>;</span>
<span id="cb7-4"><a href="#cb7-4"></a>state <span class="op">=</span> std<span class="op">::</span>move<span class="op">(</span>state<span class="op">).</span>visit<span class="op"><</span>State<span class="op">>(</span>f<span class="op">)</span>;</span></code></pre></div>
<p>With non-member <code class="sourceCode cpp">visit</code>, there’s no real possible ambiguity because the function goes first. But here, unless we protect the non-return-type taking overload, <code class="sourceCode cpp">Self</code> could deduce as <code class="sourceCode cpp">State</code>, which would then be a perfectly valid overload. And this pattern isn’t rare either - so it’s important to support. The added <code class="sourceCode cpp"><span class="dt">int</span><span class="op">=</span><span class="dv">0</span></code> parameter ensures that only the first overload is viable for <code class="sourceCode cpp">v<span class="op">.</span>visit<span class="op">(</span>f<span class="op">)</span></code> and only the second is viable for <code class="sourceCode cpp">v<span class="op">.</span>visit<span class="op"><</span>R<span class="op">>(</span>f<span class="op">)</span></code>.</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>Add to <span>22.6.3.1 <a href="https://wg21.link/variant.variant.general">[variant.variant.general]</a></span>:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb8"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb8-1"><a href="#cb8-1"></a>namespace std {</span>
<span id="cb8-2"><a href="#cb8-2"></a> template<class... Types></span>
<span id="cb8-3"><a href="#cb8-3"></a> class variant {</span>
<span id="cb8-4"><a href="#cb8-4"></a> public:</span>
<span id="cb8-5"><a href="#cb8-5"></a> // ...</span>
<span id="cb8-6"><a href="#cb8-6"></a></span>
<span id="cb8-7"><a href="#cb8-7"></a> // [variant.status], value status</span>
<span id="cb8-8"><a href="#cb8-8"></a> constexpr bool valueless_by_exception() const noexcept;</span>
<span id="cb8-9"><a href="#cb8-9"></a> constexpr size_t index() const noexcept;</span>
<span id="cb8-10"><a href="#cb8-10"></a></span>
<span id="cb8-11"><a href="#cb8-11"></a> // [variant.swap], swap</span>
<span id="cb8-12"><a href="#cb8-12"></a> constexpr void swap(variant&) noexcept(see below);</span>
<span id="cb8-13"><a href="#cb8-13"></a></span>
<span id="cb8-14"><a href="#cb8-14"></a><span class="va">+ // [variant.visit], visitation</span></span>
<span id="cb8-15"><a href="#cb8-15"></a><span class="va">+ template<class Self, class Visitor></span></span>
<span id="cb8-16"><a href="#cb8-16"></a><span class="va">+ constexpr <em>see below</em> visit(this Self&&, Visitor&&);</span></span>
<span id="cb8-17"><a href="#cb8-17"></a><span class="va">+ template<class R, class Self, class Visitor></span></span>
<span id="cb8-18"><a href="#cb8-18"></a><span class="va">+ constexpr R visit(this Self&&, Visitor&&);</span></span>
<span id="cb8-19"><a href="#cb8-19"></a> };</span>
<span id="cb8-20"><a href="#cb8-20"></a>}</span></code></pre></div>
</div>
</blockquote>
<p>Add to <span>22.6.7 <a href="https://wg21.link/variant.visit">[variant.visit]</a></span>, after the definition of non-member <code class="sourceCode cpp">visit</code>:</p>
<blockquote>
<div class="addu">
<div class="sourceCode" id="cb9"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb9-1"><a href="#cb9-1"></a>template<class Self, class Visitor></span>
<span id="cb9-2"><a href="#cb9-2"></a> constexpr <em>see below</em> visit(this Self&& self, Visitor&& vis);</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_1" id="pnum_1">9</a></span> Let <code class="sourceCode cpp">V</code> be <code class="sourceCode cpp"><em>OVERRIDE_REF</em><span class="op">(</span>Self<span class="op">&&</span>, <em>COPY_CONST</em><span class="op">(</span>remove_reference_t<span class="op"><</span>Self<span class="op">></span>, variant<span class="op">))</span></code> ([forward]).</p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_2" id="pnum_2">10</a></span> <em>Constraints</em>: The call to <code class="sourceCode cpp">visit</code> does not use an explicit <code class="sourceCode cpp"><em>template-argument-list</em></code> that begins with a type <code class="sourceCode cpp"><em>template-argument</em></code>.</p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">11</a></span> <em>Effects</em>: Equivalent to <code class="sourceCode cpp"><span class="cf">return</span> std<span class="op">::</span>visit<span class="op">(</span>std<span class="op">::</span>forward<span class="op"><</span>Visitor<span class="op">>(</span>vis<span class="op">)</span>, <span class="op">(</span>V<span class="op">)</span>self<span class="op">)</span>;</code></p>
<div class="sourceCode" id="cb10"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb10-1"><a href="#cb10-1"></a>template<class R, class Self, class Visitor></span>
<span id="cb10-2"><a href="#cb10-2"></a> constexpr R visit(this Self&& self, Visitor&& vis);</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_4" id="pnum_4">12</a></span> Let <code class="sourceCode cpp">V</code> be <code class="sourceCode cpp"><em>OVERRIDE_REF</em><span class="op">(</span>Self<span class="op">&&</span>, <em>COPY_CONST</em><span class="op">(</span>remove_reference_t<span class="op"><</span>Self<span class="op">></span>, variant<span class="op">))</span></code> ([forward]).</p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_5" id="pnum_5">13</a></span> <em>Effects</em>: Equivalent to <code class="sourceCode cpp"><span class="cf">return</span> std<span class="op">::</span>visit<span class="op"><</span>R<span class="op">>(</span>std<span class="op">::</span>forward<span class="op"><</span>Visitor<span class="op">>(</span>vis<span class="op">)</span>, <span class="op">(</span>V<span class="op">)</span>self<span class="op">)</span>;</code></p>
</div>
</blockquote>
<p>Change the example in <span>22.14.6.6 <a href="https://wg21.link/format.context">[format.context]</a></span>/8:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb11"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb11-1"><a href="#cb11-1"></a>struct S { int value; };</span>
<span id="cb11-2"><a href="#cb11-2"></a></span>
<span id="cb11-3"><a href="#cb11-3"></a>template<> struct std::formatter<S> {</span>
<span id="cb11-4"><a href="#cb11-4"></a> size_t width_arg_id = 0;</span>
<span id="cb11-5"><a href="#cb11-5"></a></span>
<span id="cb11-6"><a href="#cb11-6"></a> // Parses a width argument id in the format { digit }.</span>
<span id="cb11-7"><a href="#cb11-7"></a> constexpr auto parse(format_parse_context& ctx) {</span>
<span id="cb11-8"><a href="#cb11-8"></a> auto iter = ctx.begin();</span>
<span id="cb11-9"><a href="#cb11-9"></a> auto get_char = [&]() { return iter != ctx.end() ? *iter : 0; };</span>
<span id="cb11-10"><a href="#cb11-10"></a> if (get_char() != '{')</span>
<span id="cb11-11"><a href="#cb11-11"></a> return iter;</span>
<span id="cb11-12"><a href="#cb11-12"></a> ++iter;</span>
<span id="cb11-13"><a href="#cb11-13"></a> char c = get_char();</span>
<span id="cb11-14"><a href="#cb11-14"></a> if (!isdigit(c) || (++iter, get_char()) != '}')</span>
<span id="cb11-15"><a href="#cb11-15"></a> throw format_error("invalid format");</span>
<span id="cb11-16"><a href="#cb11-16"></a> width_arg_id = c - '0';</span>
<span id="cb11-17"><a href="#cb11-17"></a> ctx.check_arg_id(width_arg_id);</span>
<span id="cb11-18"><a href="#cb11-18"></a> return ++iter;</span>
<span id="cb11-19"><a href="#cb11-19"></a> }</span>
<span id="cb11-20"><a href="#cb11-20"></a></span>
<span id="cb11-21"><a href="#cb11-21"></a> // Formats an S with width given by the argument width_arg_id.</span>
<span id="cb11-22"><a href="#cb11-22"></a> auto format(S s, format_context& ctx) {</span>
<span id="cb11-23"><a href="#cb11-23"></a><span class="st">- int width = visit_format_arg([](auto value) -> int {</span></span>
<span id="cb11-24"><a href="#cb11-24"></a><span class="va">+ int width = ctx.arg(width_arg_id).visit([](auto value) -> int {</span></span>
<span id="cb11-25"><a href="#cb11-25"></a> if constexpr (!is_integral_v<decltype(value)>)</span>
<span id="cb11-26"><a href="#cb11-26"></a> throw format_error("width is not integral");</span>
<span id="cb11-27"><a href="#cb11-27"></a> else if (value < 0 || value > numeric_limits<int>::max())</span>
<span id="cb11-28"><a href="#cb11-28"></a> throw format_error("invalid width");</span>
<span id="cb11-29"><a href="#cb11-29"></a> else</span>
<span id="cb11-30"><a href="#cb11-30"></a> return value;</span>
<span id="cb11-31"><a href="#cb11-31"></a><span class="st">- }, ctx.arg(width_arg_id));</span></span>
<span id="cb11-32"><a href="#cb11-32"></a><span class="va">+ });</span></span>
<span id="cb11-33"><a href="#cb11-33"></a> return format_to(ctx.out(), "{0:x<{1}}", s.value, width);</span>
<span id="cb11-34"><a href="#cb11-34"></a> }</span>
<span id="cb11-35"><a href="#cb11-35"></a>};</span>
<span id="cb11-36"><a href="#cb11-36"></a></span>
<span id="cb11-37"><a href="#cb11-37"></a>std::string s = std::format("{0:{1}}", S{42}, 10); // value of s is "xxxxxxxx42"</span></code></pre></div>
</div>
</blockquote>
<p>Add to <span>22.14.8.1 <a href="https://wg21.link/format.arg">[format.arg]</a></span>:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb12"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb12-1"><a href="#cb12-1"></a>namespace std {</span>
<span id="cb12-2"><a href="#cb12-2"></a> template<class Context></span>
<span id="cb12-3"><a href="#cb12-3"></a> class basic_format_arg {</span>
<span id="cb12-4"><a href="#cb12-4"></a> // ...</span>
<span id="cb12-5"><a href="#cb12-5"></a> public:</span>
<span id="cb12-6"><a href="#cb12-6"></a> basic_format_arg() noexcept;</span>
<span id="cb12-7"><a href="#cb12-7"></a></span>
<span id="cb12-8"><a href="#cb12-8"></a> explicit operator bool() const noexcept;</span>
<span id="cb12-9"><a href="#cb12-9"></a></span>
<span id="cb12-10"><a href="#cb12-10"></a><span class="va">+ template<class Visitor></span></span>
<span id="cb12-11"><a href="#cb12-11"></a><span class="va">+ decltype(auto) visit(this basic_format_arg arg, Visitor&& vis);</span></span>
<span id="cb12-12"><a href="#cb12-12"></a><span class="va">+ template<class R, class Visitor></span></span>
<span id="cb12-13"><a href="#cb12-13"></a><span class="va">+ R visit(this basic_format_arg arg, Visitor&& vis);</span></span>
<span id="cb12-14"><a href="#cb12-14"></a></span>
<span id="cb12-15"><a href="#cb12-15"></a> };</span>
<span id="cb12-16"><a href="#cb12-16"></a>}</span></code></pre></div>
</div>
</blockquote>
<p>And:</p>
<blockquote>
<div class="sourceCode" id="cb13"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb13-1"><a href="#cb13-1"></a>explicit operator bool() const noexcept;</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_6" id="pnum_6">15</a></span> <em>Returns</em>: <code class="sourceCode cpp"><span class="op">!</span>holds_alternative<span class="op"><</span>monostate<span class="op">>(</span>value<span class="op">)</span></code>.</p>
<div class="addu">
<div class="sourceCode" id="cb14"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb14-1"><a href="#cb14-1"></a>template<class Visitor></span>
<span id="cb14-2"><a href="#cb14-2"></a> decltype(auto) visit(this basic_format_arg arg, Visitor&& vis);</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_7" id="pnum_7">16</a></span> <em>Effects</em>: Equivalent to <code class="sourceCode cpp"><span class="cf">return</span> arg<span class="op">.</span>value<span class="op">.</span>visit<span class="op">(</span>std<span class="op">::</span>forward<span class="op"><</span>Visitor<span class="op">>(</span>vis<span class="op">))</span>;</code></p>
<div class="sourceCode" id="cb15"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb15-1"><a href="#cb15-1"></a>template<class R, class Visitor></span>
<span id="cb15-2"><a href="#cb15-2"></a> R visit(this basic_format_arg arg, Visitor&& vis);</span></code></pre></div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_8" id="pnum_8">17</a></span> <em>Effects</em>: Equivalent to <code class="sourceCode cpp"><span class="cf">return</span> arg<span class="op">.</span>value<span class="op">.</span>visit<span class="op"><</span>R<span class="op">>(</span>std<span class="op">::</span>forward<span class="op"><</span>Visitor<span class="op">>(</span>vis<span class="op">))</span>;</code></p>
</div>
</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>There isn’t much reason to provide one, since would anybody write this?</p>
<blockquote>
<div class="sourceCode" id="cb16"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb16-1"><a href="#cb16-1"></a><span class="kw">auto</span> result <span class="op">=</span></span>
<span id="cb16-2"><a href="#cb16-2"></a> <span class="pp">#ifdef __cpp_lib_member_visit </span><span class="co">// or whatever</span></span>
<span id="cb16-3"><a href="#cb16-3"></a> var<span class="op">.</span>visit<span class="op">(</span>f<span class="op">)</span>;</span>
<span id="cb16-4"><a href="#cb16-4"></a> <span class="pp">#else</span></span>
<span id="cb16-5"><a href="#cb16-5"></a> std<span class="op">::</span>visit<span class="op">(</span>f, var<span class="op">)</span>;</span>
<span id="cb16-6"><a href="#cb16-6"></a> <span class="pp">#endif</span></span></code></pre></div>
</blockquote>
<p>If you have to write the old code, the new code doesn’t give you any benefit. Moreover, a lot of visits are more complicated than just <code class="sourceCode cpp">f</code> - at the very least the’re a lambda, but potentially lots of lambdas.</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-P0847R7">
<p>[P0847R7] Barry Revzin, Gašper Ažman, Sy Brand, Ben Deane. 2021-07-14. Deducing this. <br />
<a href="https://wg21.link/p0847r7">https://wg21.link/p0847r7</a></p>
</div>
<div id="ref-P1450R3">
<p>[P1450R3] Vincent Reverdy. 2020-06-15. Enriching type modification traits. <br />
<a href="https://wg21.link/p1450r3">https://wg21.link/p1450r3</a></p>
</div>
<div id="ref-P2637R0">
<p>[P2637R0] Barry Revzin. 2022-09-17. Member <code class="sourceCode cpp">visit</code> and <code class="sourceCode cpp">apply</code>. <br />
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2637r0.html">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2637r0.html</a></p>
</div>
</div>
<section class="footnotes" role="doc-endnotes">
<hr />
<ol>
<li id="fn1" role="doc-endnote"><p>A single non-member function template is still superior to four member function overloads due to proper handling of certain edge cases. See the section on <a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p0847r7.html#sfinae-friendly-callables">SFINAE-friendly</a> for more information.<a href="#fnref1" class="footnote-back" role="doc-backlink">↩︎</a></p></li>
</ol>
</section>
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