<!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="2024-02-14" />
  <title>Specify Constructor of `std::nullopt_t`</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 { } 
code span.al { color: #ff0000; } 
code span.an { } 
code span.at { } 
code span.bn { color: #9f6807; } 
code span.bu { color: #9f6807; } 
code span.cf { color: #00607c; } 
code span.ch { color: #9f6807; } 
code span.cn { } 
code span.co { color: #008000; font-style: italic; } 
code span.cv { color: #008000; font-style: italic; } 
code span.do { color: #008000; } 
code span.dt { color: #00607c; } 
code span.dv { color: #9f6807; } 
code span.er { color: #ff0000; font-weight: bold; } 
code span.ex { } 
code span.fl { color: #9f6807; } 
code span.fu { } 
code span.im { } 
code span.in { color: #008000; } 
code span.kw { color: #00607c; } 
code span.op { color: #af1915; } 
code span.ot { } 
code span.pp { color: #6f4e37; } 
code span.re { } 
code span.sc { color: #9f6807; } 
code span.ss { color: #9f6807; } 
code span.st { color: #9f6807; } 
code span.va { } 
code span.vs { color: #9f6807; } 
code span.wa { color: #008000; font-weight: bold; } 
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: 2.5em;
margin-top: -0.2em;
margin-bottom: -0.2em;
}
ol {
padding-left: 2.5em;
}
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;
}

#TOC ul > li:before {
content: none;
}
#TOC > ul {
padding-left: 0;
}

.toc-section-number {
margin-right: 0.5em;
}
: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>
  <link href="data:image/x-icon;base64,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" rel="icon" />
  
  <!--[if lt IE 9]>
    <script src="//cdnjs.cloudflare.com/ajax/libs/html5shiv/3.7.3/html5shiv-printshiv.min.js"></script>
  <![endif]-->
</head>
<body>
<div class="wrapper">
<header id="title-block-header">
<h1 class="title" style="text-align:center">Specify Constructor of
<code class="sourceCode default">std::nullopt_t</code></h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P3112R0</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2024-02-14</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, LWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Brian Bi<br>&lt;<a href="mailto:bbi10@bloomberg.net" class="email">bbi10@bloomberg.net</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</a></li>
<li><a href="#why-does-this-problem-occur" id="toc-why-does-this-problem-occur"><span class="toc-section-number">2</span> Why does this problem
occur?</a></li>
<li><a href="#proposed-solution" id="toc-proposed-solution"><span class="toc-section-number">3</span> Proposed solution</a></li>
<li><a href="#wording" id="toc-wording"><span class="toc-section-number">4</span> Wording</a></li>
<li><a href="#bibliography" id="toc-bibliography"><span class="toc-section-number">5</span> References</a></li>
</ul>
</div>
<h1 data-number="1" id="introduction"><span class="header-section-number">1</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>Should the following code be well formed?</p>
<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;optional&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">struct</span> Widget <span class="op">{</span></span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a>    <span class="kw">class</span> PassKey <span class="op">{</span></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a>        PassKey<span class="op">()</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a>        <span class="kw">friend</span> <span class="kw">class</span> Widget;</span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a>    <span class="op">}</span>;</span>
<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a>    Widget<span class="op">(</span>PassKey<span class="op">)</span> <span class="op">{}</span></span>
<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a>    <span class="kw">static</span> std<span class="op">::</span>optional<span class="op">&lt;</span>Widget<span class="op">&gt;</span> make<span class="op">()</span> <span class="op">{</span></span>
<span id="cb1-11"><a href="#cb1-11" aria-hidden="true" tabindex="-1"></a>        std<span class="op">::</span>optional<span class="op">&lt;</span>Widget<span class="op">&gt;</span> result<span class="op">{{{}}}</span>;</span>
<span id="cb1-12"><a href="#cb1-12" aria-hidden="true" tabindex="-1"></a>        <span class="cf">return</span> result;</span>
<span id="cb1-13"><a href="#cb1-13" aria-hidden="true" tabindex="-1"></a>    <span class="op">}</span></span>
<span id="cb1-14"><a href="#cb1-14" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>
<p>Because <code class="sourceCode default">{{}}</code> can be
implicitly converted to <code class="sourceCode default">Widget</code>,
one would expect this code to initialize
<code class="sourceCode default">result</code> to hold a
<code class="sourceCode default">Widget</code> object whose value is
<code class="sourceCode default">Widget{{}}</code>. However, GCC 13.2
rejects this code, giving the following error message.</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a>&lt;source&gt;:12:42: error: converting to &#39;std::nullopt_t&#39; from initializer list would use explicit constructor &#39;constexpr std::nullopt_t::nullopt_t(_Construct)&#39;</span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a>   12 |         std::optional&lt;Widget&gt; result{{{}}};</span></code></pre></div>
<h1 data-number="2" id="why-does-this-problem-occur"><span class="header-section-number">2</span> Why does this problem occur?<a href="#why-does-this-problem-occur" class="self-link"></a></h1>
<p>An implementation detail has leaked from libstdc++: namely, that an
implicit conversion sequence exists from
<code class="sourceCode default">{{}}</code> to
<code class="sourceCode default">std::nullopt_t</code> in libstdc++’s
implementation of
<code class="sourceCode default">std::nullopt_t</code>, because
libstdc++ declares
<code class="sourceCode default">std::nullopt_t</code> to have a
constructor with a single parameter whose type is an
implementation-private scoped enumeration (and a scoped enumeration can
always be initialized from
<code class="sourceCode default">{}</code>).</p>
<p>The attempt to use this implicit conversion sequence renders the
program ill formed because the constructor that libstdc++ has declared
for <code class="sourceCode default">std::nullopt_t</code> is explicit.
This is not the behavior that most users would expect for the above
code, which is to call the value constructor of
<code class="sourceCode default">std::optional&lt;Widget&gt;</code>. The
constructor of
<code class="sourceCode default">std::optional&lt;Widget&gt;</code> that
takes <code class="sourceCode default">std::nullopt_t</code> has won
overload resolution because it is not a constructor template, while the
value constructor is (§<span>22.5.3.2
<a href="https://wg21.link/N4971#optional.ctor">[optional.ctor]</a><a href="#fn1" class="footnote-ref" id="fnref1" role="doc-noteref"><sup>1</sup></a></span>p23).</p>
<p>Clang and MSVC accept the above code. In libc++, the constructor of
<code class="sourceCode default">std::nullopt_t</code> takes two
arguments, not one. In both Clang and MSVC, the fact that
<code class="sourceCode default">std::nullopt_t</code> has an explicit
constructor prevents the formation of an implicit conversion sequence,
but it is unclear whether it is correct for it to do so; see <span class="citation" data-cites="CWG2525">[<a href="#ref-CWG2525" role="doc-biblioref">CWG2525</a>]</span>.</p>
<p>The standard has the following to say about how
<code class="sourceCode default">std::nullopt_t</code> can be
constructed (§<span>22.5.4
<a href="https://wg21.link/N4971#optional.nullopt">[optional.nullopt]</a></span>p2):</p>
<blockquote>
<p>Type <code class="sourceCode default">nullopt_t</code> shall not have
a default constructor or an initializer-list constructor, and shall not
be an aggregate.</p>
</blockquote>
<p>These restrictions are not strong enough to prevent the formation of
an implicit conversion sequence from a <em>braced-init-list</em> to
<code class="sourceCode default">std::nullopt_t</code>, because
list-initialization can call a non-initializer-list constructor.
libstdc++’s implementation, in which
<code class="sourceCode default">std::nullopt_t</code> has a constructor
taking a single object of a tag type that is itself
default-constructible, conforms to the standard. The following
implementation would also be conforming, even though it makes it even
easier for users to encounter the compilation error described above.</p>
<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">namespace</span> std <span class="op">{</span></span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> nullopt_t <span class="op">{</span></span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a>    nullopt_t<span class="op">(</span><span class="dt">int</span><span class="op">)</span> <span class="op">{}</span>  <span class="co">// not explicit</span></span>
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span>
<span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<h1 data-number="3" id="proposed-solution"><span class="header-section-number">3</span> Proposed solution<a href="#proposed-solution" class="self-link"></a></h1>
<p>We could choose to do nothing, and hope that the CWG will resolve
CWG2525 in favor of Clang and MSVC, which would make GCC, with
libstdc++’s current implementation of
<code class="sourceCode default">std::nullopt_t</code>, accept the code
shown in the introduction. On the other hand, if CWG2525 is resolved in
GCC’s favor, then the code will become ill formed with MSVC’s current
implementation of
<code class="sourceCode default">std::nullopt_t</code>, as well as when
Clang uses libstdc++.</p>
<p>Instead, I propose a simple solution that will completely specify how
<code class="sourceCode default">std::nullopt_t</code> is constructed
and, regardless of the disposition of CWG2525, guarantee that
<code class="sourceCode default">std::nullopt_t</code> will not
interfere when a user tries to construct
<code class="sourceCode default">std::optional&lt;T&gt;</code> from a
<em>braced-init-list</em>. We can declare a tag type from which
<code class="sourceCode default">std::nullopt_t</code> is constructed,
and make the actual constructor a template that accepts only that tag
type. Such a constructor will never be a candidate for construction from
a <em>braced-init-list</em>, because deduction can never succeed.</p>
<h1 data-number="4" id="wording"><span class="header-section-number">4</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>The proposed wording is relative to <span class="citation" data-cites="N4971">[<a href="#ref-N4971" role="doc-biblioref">N4971</a>]</span>.</p>
<p>In §<span>22.5.4
<a href="https://wg21.link/N4971#optional.nullopt">[optional.nullopt]</a></span>,
edit the code before p1 as follows.</p>
<blockquote>
<div class="rm" style="color: #bf0303">

<div class="sourceCode" id="cb4"><pre class="sourceCode default cpp"><code class="sourceCode default"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a>struct nullopt_t{<em>see below</em>};</span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a>inline constexpr nullopt_t nullopt(<em>unspecified</em>);</span></code></pre></div>

</div>
<div class="add" style="color: #006e28">

<div class="sourceCode" id="cb5"><pre class="sourceCode default cpp"><code class="sourceCode default"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a>struct nullopt_t {</span>
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a>  struct <em>nullopt-construct-tag</em> {};  // exposition only</span>
<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a>  constexpr explicit nullopt_t(same_as&lt;<em>nullopt-construct-tag</em>&gt; auto) {}</span>
<span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></a>};</span>
<span id="cb5-5"><a href="#cb5-5" aria-hidden="true" tabindex="-1"></a>inline constexpr nullopt_t nullopt(nullopt_t::<em>nullopt-construct-tag</em>());</span></code></pre></div>

</div>
</blockquote>
<p>Strike p2 in §<span>22.5.4
<a href="https://wg21.link/N4971#optional.nullopt">[optional.nullopt]</a></span>:</p>
<blockquote>
<ul>
<li><span class="rm" style="color: #bf0303"><del>Type
<span><code class="sourceCode default">nullopt_t</code></span> shall not
have a default constructor or an initializer-list constructor, and shall
not be an aggregate.</del></span></li>
</ul>
</blockquote>
<h1 data-number="5" 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" role="doc-bibliography">
<div id="ref-CWG2525" class="csl-entry" role="doc-biblioentry">
[CWG2525] Jim X. 2021-09-25. Incorrect definition of implicit conversion
sequence. <a href="https://wg21.link/cwg2525"><div class="csl-block">https://wg21.link/cwg2525</div></a>
</div>
<div id="ref-N4971" class="csl-entry" role="doc-biblioentry">
[N4971] Thomas Köppe. 2023-12-18. Working Draft, Programming Languages —
C++. <a href="https://wg21.link/n4971"><div class="csl-block">https://wg21.link/n4971</div></a>
</div>
</div>
<section id="footnotes" class="footnotes footnotes-end-of-document" role="doc-endnotes">
<hr />
<ol>
<li id="fn1"><p>All citations to the Standard are to working draft N4971
unless otherwise specified.<a href="#fnref1" class="footnote-back" role="doc-backlink">↩︎</a></p></li>
</ol>
</section>
</div>
</div>
</body>
</html>
