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<title>Missing `constexpr` in `std::optional` and `std::variant`</title>
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<h1 class="title" style="text-align:center">Missing <code class="sourceCode cpp"><span class="kw">constexpr</span></code> in <code class="sourceCode cpp">std<span class="op">::</span>optional</code> and <code class="sourceCode cpp">std<span class="op">::</span>variant</code></h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P2231R1</td>
</tr>
<tr>
<td>Date:</td>
<td>2021-02-11</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>
LWG<br>
</td>
</tr>
<tr>
<td style="vertical-align:top">Reply-to:</td>
<td>
Barry Revzin<br><<a href="mailto:[email protected]" class="email">[email protected]</a>><br>
</td>
</tr>
</table>
</header>
<div style="clear:both">
<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="P2231R0">[<a href="#ref-P2231R0" role="doc-biblioref">P2231R0</a>]</span>, added a section about feature-test macros for this change.</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>Each new language standard has increased the kinds of operations that we can do during constant evaluation time. C++20 was no different. With the adoption of <span class="citation" data-cites="P1330R0">[<a href="#ref-P1330R0" role="doc-biblioref">P1330R0</a>]</span>, C++20 added the ability to change the active member of a union inside constexpr (the paper specifically mentions <code class="sourceCode cpp">std<span class="op">::</span>optional</code>). And with the adoption of <span class="citation" data-cites="P0784R7">[<a href="#ref-P0784R7" role="doc-biblioref">P0784R7</a>]</span>, C++20 added the ability to do placement new inside constexpr (by way of <code class="sourceCode cpp">std<span class="op">::</span>construct_at</code>).</p>
<p>But even though the language provided the tools to make <code class="sourceCode cpp">std<span class="op">::</span>optional</code> and <code class="sourceCode cpp">std<span class="op">::</span>variant</code> completely constexpr-able, there was no such update to the library. This paper seeks to remedy that omission by simply adding <code class="sourceCode cpp"><span class="kw">constexpr</span></code> to all the relevant places.</p>
<h2 data-number="2.1" id="implementing-stdoptional"><span class="header-section-number">2.1</span> Implementing <code class="sourceCode cpp">std<span class="op">::</span>optional</code><a href="#implementing-stdoptional" class="self-link"></a></h2>
<p>I updated libstdc++’s implementation of <code class="sourceCode cpp">std<span class="op">::</span>optional</code> to add these <code class="sourceCode cpp"><span class="kw">constexpr</span></code>s (and replace the placement <code class="sourceCode cpp"><span class="kw">new</span></code> calls with calls to <code class="sourceCode cpp">std<span class="op">::</span>construct_at</code>) as follows, which compiles with both gcc and clang (<a href="https://godbolt.org/z/fE6aob">demo</a>):</p>
<div>
<div class="sourceCode" id="cb1"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb1-1"><a href="#cb1-1"></a><span class="dt">-13,6 +11,7</span></span>
<span id="cb1-2"><a href="#cb1-2"></a> #include <bits/exception_defines.h></span>
<span id="cb1-3"><a href="#cb1-3"></a> #include <bits/functional_hash.h></span>
<span id="cb1-4"><a href="#cb1-4"></a> #include <bits/enable_special_members.h></span>
<span id="cb1-5"><a href="#cb1-5"></a><span class="va">+#include <bits/stl_construct.h></span></span>
<span id="cb1-6"><a href="#cb1-6"></a> #if __cplusplus > 201703L</span>
<span id="cb1-7"><a href="#cb1-7"></a> # include <compare></span>
<span id="cb1-8"><a href="#cb1-8"></a> #endif</span>
<span id="cb1-9"><a href="#cb1-9"></a><span class="dt">-206,7 +205,7</span></span>
<span id="cb1-10"><a href="#cb1-10"></a> { }</span>
<span id="cb1-11"><a href="#cb1-11"></a> </span>
<span id="cb1-12"><a href="#cb1-12"></a> // User-provided destructor is needed when _Up has non-trivial dtor.</span>
<span id="cb1-13"><a href="#cb1-13"></a><span class="st">- ~_Storage() { }</span></span>
<span id="cb1-14"><a href="#cb1-14"></a><span class="va">+ constexpr ~_Storage() { }</span></span>
<span id="cb1-15"><a href="#cb1-15"></a> </span>
<span id="cb1-16"><a href="#cb1-16"></a> _Empty_byte _M_empty;</span>
<span id="cb1-17"><a href="#cb1-17"></a> _Up _M_value;</span>
<span id="cb1-18"><a href="#cb1-18"></a><span class="dt">-217,12 +216,12</span></span>
<span id="cb1-19"><a href="#cb1-19"></a> bool _M_engaged = false;</span>
<span id="cb1-20"><a href="#cb1-20"></a> </span>
<span id="cb1-21"><a href="#cb1-21"></a> template<typename... _Args></span>
<span id="cb1-22"><a href="#cb1-22"></a><span class="st">- void</span></span>
<span id="cb1-23"><a href="#cb1-23"></a><span class="va">+ constexpr void</span></span>
<span id="cb1-24"><a href="#cb1-24"></a> _M_construct(_Args&&... __args)</span>
<span id="cb1-25"><a href="#cb1-25"></a> noexcept(is_nothrow_constructible_v<_Stored_type, _Args...>)</span>
<span id="cb1-26"><a href="#cb1-26"></a> {</span>
<span id="cb1-27"><a href="#cb1-27"></a><span class="st">- ::new ((void *) std::__addressof(this->_M_payload))</span></span>
<span id="cb1-28"><a href="#cb1-28"></a><span class="st">- _Stored_type(std::forward<_Args>(__args)...);</span></span>
<span id="cb1-29"><a href="#cb1-29"></a><span class="va">+ std::construct_at(std::__addressof(this->_M_payload),</span></span>
<span id="cb1-30"><a href="#cb1-30"></a><span class="va">+ std::forward<_Args>(__args)...);</span></span>
<span id="cb1-31"><a href="#cb1-31"></a> this->_M_engaged = true;</span>
<span id="cb1-32"><a href="#cb1-32"></a> }</span>
<span id="cb1-33"><a href="#cb1-33"></a> </span>
<span id="cb1-34"><a href="#cb1-34"></a><span class="dt">-371,7 +370,7</span></span>
<span id="cb1-35"><a href="#cb1-35"></a> _Optional_payload& operator=(_Optional_payload&&) = default;</span>
<span id="cb1-36"><a href="#cb1-36"></a> </span>
<span id="cb1-37"><a href="#cb1-37"></a> // Destructor needs to destroy the contained value:</span>
<span id="cb1-38"><a href="#cb1-38"></a><span class="st">- ~_Optional_payload()</span></span>
<span id="cb1-39"><a href="#cb1-39"></a><span class="va">+ constexpr ~_Optional_payload()</span></span>
<span id="cb1-40"><a href="#cb1-40"></a> {</span>
<span id="cb1-41"><a href="#cb1-41"></a> this->_M_reset();</span>
<span id="cb1-42"><a href="#cb1-42"></a> }</span>
<span id="cb1-43"><a href="#cb1-43"></a><span class="dt">-388,17 +387,16</span></span>
<span id="cb1-44"><a href="#cb1-44"></a> // The _M_construct operation has !_M_engaged as a precondition</span>
<span id="cb1-45"><a href="#cb1-45"></a> // while _M_destruct has _M_engaged as a precondition.</span>
<span id="cb1-46"><a href="#cb1-46"></a> template<typename... _Args></span>
<span id="cb1-47"><a href="#cb1-47"></a><span class="st">- void</span></span>
<span id="cb1-48"><a href="#cb1-48"></a><span class="va">+ constexpr void</span></span>
<span id="cb1-49"><a href="#cb1-49"></a> _M_construct(_Args&&... __args)</span>
<span id="cb1-50"><a href="#cb1-50"></a> noexcept(is_nothrow_constructible_v<_Stored_type, _Args...>)</span>
<span id="cb1-51"><a href="#cb1-51"></a> {</span>
<span id="cb1-52"><a href="#cb1-52"></a><span class="st">- ::new</span></span>
<span id="cb1-53"><a href="#cb1-53"></a><span class="st">- (std::__addressof(static_cast<_Dp*>(this)->_M_payload._M_payload))</span></span>
<span id="cb1-54"><a href="#cb1-54"></a><span class="st">- _Stored_type(std::forward<_Args>(__args)...);</span></span>
<span id="cb1-55"><a href="#cb1-55"></a><span class="va">+ std::construct_at(std::__addressof(static_cast<_Dp*>(this)->_M_payload._M_payload._M_value),</span></span>
<span id="cb1-56"><a href="#cb1-56"></a><span class="va">+ std::forward<_Args>(__args)...);</span></span>
<span id="cb1-57"><a href="#cb1-57"></a> static_cast<_Dp*>(this)->_M_payload._M_engaged = true;</span>
<span id="cb1-58"><a href="#cb1-58"></a> }</span>
<span id="cb1-59"><a href="#cb1-59"></a> </span>
<span id="cb1-60"><a href="#cb1-60"></a><span class="st">- void</span></span>
<span id="cb1-61"><a href="#cb1-61"></a><span class="va">+ constexpr void</span></span>
<span id="cb1-62"><a href="#cb1-62"></a> _M_destruct() noexcept</span>
<span id="cb1-63"><a href="#cb1-63"></a> {</span>
<span id="cb1-64"><a href="#cb1-64"></a> static_cast<_Dp*>(this)->_M_payload._M_destroy();</span>
<span id="cb1-65"><a href="#cb1-65"></a><span class="dt">-754,7 +752,7</span></span>
<span id="cb1-66"><a href="#cb1-66"></a> : _Base(std::in_place, __il, std::forward<_Args>(__args)...) { }</span>
<span id="cb1-67"><a href="#cb1-67"></a> </span>
<span id="cb1-68"><a href="#cb1-68"></a> // Assignment operators.</span>
<span id="cb1-69"><a href="#cb1-69"></a><span class="st">- optional&</span></span>
<span id="cb1-70"><a href="#cb1-70"></a><span class="va">+ constexpr optional&</span></span>
<span id="cb1-71"><a href="#cb1-71"></a> operator=(nullopt_t) noexcept</span>
<span id="cb1-72"><a href="#cb1-72"></a> {</span>
<span id="cb1-73"><a href="#cb1-73"></a> this->_M_reset();</span>
<span id="cb1-74"><a href="#cb1-74"></a><span class="dt">-762,6 +760,7</span></span>
<span id="cb1-75"><a href="#cb1-75"></a> }</span>
<span id="cb1-76"><a href="#cb1-76"></a> </span>
<span id="cb1-77"><a href="#cb1-77"></a> template<typename _Up = _Tp></span>
<span id="cb1-78"><a href="#cb1-78"></a><span class="va">+ constexpr</span></span>
<span id="cb1-79"><a href="#cb1-79"></a> enable_if_t<__and_v<__not_self<_Up>,</span>
<span id="cb1-80"><a href="#cb1-80"></a> __not_<__and_<is_scalar<_Tp>,</span>
<span id="cb1-81"><a href="#cb1-81"></a> is_same<_Tp, decay_t<_Up>>>>,</span>
<span id="cb1-82"><a href="#cb1-82"></a><span class="dt">-780,6 +779,7</span></span>
<span id="cb1-83"><a href="#cb1-83"></a> }</span>
<span id="cb1-84"><a href="#cb1-84"></a> </span>
<span id="cb1-85"><a href="#cb1-85"></a> template<typename _Up></span>
<span id="cb1-86"><a href="#cb1-86"></a><span class="va">+ constexpr</span></span>
<span id="cb1-87"><a href="#cb1-87"></a> enable_if_t<__and_v<__not_<is_same<_Tp, _Up>>,</span>
<span id="cb1-88"><a href="#cb1-88"></a> is_constructible<_Tp, const _Up&>,</span>
<span id="cb1-89"><a href="#cb1-89"></a> is_assignable<_Tp&, _Up>,</span>
<span id="cb1-90"><a href="#cb1-90"></a><span class="dt">-801,6 +801,7</span></span>
<span id="cb1-91"><a href="#cb1-91"></a> }</span>
<span id="cb1-92"><a href="#cb1-92"></a> </span>
<span id="cb1-93"><a href="#cb1-93"></a> template<typename _Up></span>
<span id="cb1-94"><a href="#cb1-94"></a><span class="va">+ constexpr</span></span>
<span id="cb1-95"><a href="#cb1-95"></a> enable_if_t<__and_v<__not_<is_same<_Tp, _Up>>,</span>
<span id="cb1-96"><a href="#cb1-96"></a> is_constructible<_Tp, _Up>,</span>
<span id="cb1-97"><a href="#cb1-97"></a> is_assignable<_Tp&, _Up>,</span>
<span id="cb1-98"><a href="#cb1-98"></a><span class="dt">-823,6 +824,7</span></span>
<span id="cb1-99"><a href="#cb1-99"></a> }</span>
<span id="cb1-100"><a href="#cb1-100"></a> </span>
<span id="cb1-101"><a href="#cb1-101"></a> template<typename... _Args></span>
<span id="cb1-102"><a href="#cb1-102"></a><span class="va">+ constexpr</span></span>
<span id="cb1-103"><a href="#cb1-103"></a> enable_if_t<is_constructible_v<_Tp, _Args&&...>, _Tp&></span>
<span id="cb1-104"><a href="#cb1-104"></a> emplace(_Args&&... __args)</span>
<span id="cb1-105"><a href="#cb1-105"></a> {</span>
<span id="cb1-106"><a href="#cb1-106"></a><span class="dt">-832,6 +834,7</span></span>
<span id="cb1-107"><a href="#cb1-107"></a> }</span>
<span id="cb1-108"><a href="#cb1-108"></a> </span>
<span id="cb1-109"><a href="#cb1-109"></a> template<typename _Up, typename... _Args></span>
<span id="cb1-110"><a href="#cb1-110"></a><span class="va">+ constexpr</span></span>
<span id="cb1-111"><a href="#cb1-111"></a> enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&,</span>
<span id="cb1-112"><a href="#cb1-112"></a> _Args&&...>, _Tp&></span>
<span id="cb1-113"><a href="#cb1-113"></a> emplace(initializer_list<_Up> __il, _Args&&... __args)</span>
<span id="cb1-114"><a href="#cb1-114"></a><span class="dt">-844,7 +847,7</span></span>
<span id="cb1-115"><a href="#cb1-115"></a> // Destructor is implicit, implemented in _Optional_base.</span>
<span id="cb1-116"><a href="#cb1-116"></a> </span>
<span id="cb1-117"><a href="#cb1-117"></a> // Swap.</span>
<span id="cb1-118"><a href="#cb1-118"></a><span class="st">- void</span></span>
<span id="cb1-119"><a href="#cb1-119"></a><span class="va">+ constexpr void</span></span>
<span id="cb1-120"><a href="#cb1-120"></a> swap(optional& __other)</span>
<span id="cb1-121"><a href="#cb1-121"></a> noexcept(is_nothrow_move_constructible_v<_Tp></span>
<span id="cb1-122"><a href="#cb1-122"></a> && is_nothrow_swappable_v<_Tp>)</span>
<span id="cb1-123"><a href="#cb1-123"></a><span class="dt">-957,7 +960,7</span></span>
<span id="cb1-124"><a href="#cb1-124"></a> : static_cast<_Tp>(std::forward<_Up>(__u));</span>
<span id="cb1-125"><a href="#cb1-125"></a> }</span>
<span id="cb1-126"><a href="#cb1-126"></a> </span>
<span id="cb1-127"><a href="#cb1-127"></a><span class="st">- void reset() noexcept</span></span>
<span id="cb1-128"><a href="#cb1-128"></a><span class="va">+ constexpr void reset() noexcept</span></span>
<span id="cb1-129"><a href="#cb1-129"></a> {</span>
<span id="cb1-130"><a href="#cb1-130"></a> this->_M_reset();</span>
<span id="cb1-131"><a href="#cb1-131"></a> }</span>
<span id="cb1-132"><a href="#cb1-132"></a><span class="dt">-1187,7 +1190,7</span></span>
<span id="cb1-133"><a href="#cb1-133"></a> // _GLIBCXX_RESOLVE_LIB_DEFECTS</span>
<span id="cb1-134"><a href="#cb1-134"></a> // 2748. swappable traits for optionals</span>
<span id="cb1-135"><a href="#cb1-135"></a> template<typename _Tp></span>
<span id="cb1-136"><a href="#cb1-136"></a><span class="st">-inline enable_if_t<is_move_constructible_v<_Tp> && is_swappable_v<_Tp>></span></span>
<span id="cb1-137"><a href="#cb1-137"></a><span class="va">+constexpr inline enable_if_t<is_move_constructible_v<_Tp> && is_swappable_v<_Tp>></span></span>
<span id="cb1-138"><a href="#cb1-138"></a> swap(optional<_Tp>& __lhs, optional<_Tp>& __rhs)</span>
<span id="cb1-139"><a href="#cb1-139"></a> noexcept(noexcept(__lhs.swap(__rhs))) {</span>
<span id="cb1-140"><a href="#cb1-140"></a> __lhs.swap(__rhs);</span></code></pre></div>
</div>
<h2 data-number="2.2" id="implementing-stdvariant"><span class="header-section-number">2.2</span> Implementing <code class="sourceCode cpp">std<span class="op">::</span>variant</code><a href="#implementing-stdvariant" class="self-link"></a></h2>
<p>And likewise, here is a diff against libstdc++’s implementation of <code class="sourceCode cpp">std<span class="op">::</span>variant</code> for the changes proposed in this paper, which also compiles on both gcc and clang (<a href="https://godbolt.org/z/E5dcj9">demo</a>). This is slightly more complicated as we have to take more care with constructing and accessing the recursive union:</p>
<div>
<div class="sourceCode" id="cb2"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb2-1"><a href="#cb2-1"></a><span class="dt">-121,7 +121,7</span></span>
<span id="cb2-2"><a href="#cb2-2"></a> __do_visit(_Visitor&& __visitor, _Variants&&... __variants);</span>
<span id="cb2-3"><a href="#cb2-3"></a> </span>
<span id="cb2-4"><a href="#cb2-4"></a> template <typename... _Types, typename _Tp></span>
<span id="cb2-5"><a href="#cb2-5"></a><span class="st">-decltype(auto)</span></span>
<span id="cb2-6"><a href="#cb2-6"></a><span class="va">+constexpr decltype(auto)</span></span>
<span id="cb2-7"><a href="#cb2-7"></a> __variant_cast(_Tp&& __rhs) {</span>
<span id="cb2-8"><a href="#cb2-8"></a> if constexpr (is_lvalue_reference_v<_Tp>) {</span>
<span id="cb2-9"><a href="#cb2-9"></a> if constexpr (is_const_v<remove_reference_t<_Tp>>) {</span>
<span id="cb2-10"><a href="#cb2-10"></a><span class="dt">-329,6 +329,11</span></span>
<span id="cb2-11"><a href="#cb2-11"></a> : _M_rest(in_place_index<_Np-1>, std::forward<_Args>(__args)...)</span>
<span id="cb2-12"><a href="#cb2-12"></a> { }</span>
<span id="cb2-13"><a href="#cb2-13"></a> </span>
<span id="cb2-14"><a href="#cb2-14"></a><span class="va">+ constexpr ~_Variadic_union() {}</span></span>
<span id="cb2-15"><a href="#cb2-15"></a><span class="va">+ constexpr ~_Variadic_union() </span></span>
<span id="cb2-16"><a href="#cb2-16"></a><span class="va">+ requires (std::is_trivially_destructible_v<_First> && ... &&</span></span>
<span id="cb2-17"><a href="#cb2-17"></a><span class="va">+ std::is_trivially_destructible_v<_Rest>) = default;</span></span>
<span id="cb2-18"><a href="#cb2-18"></a><span class="va">+</span></span>
<span id="cb2-19"><a href="#cb2-19"></a> _Uninitialized<_First> _M_first;</span>
<span id="cb2-20"><a href="#cb2-20"></a> _Variadic_union<_Rest...> _M_rest;</span>
<span id="cb2-21"><a href="#cb2-21"></a> };</span>
<span id="cb2-22"><a href="#cb2-22"></a><span class="dt">-380,7 +385,7</span></span>
<span id="cb2-23"><a href="#cb2-23"></a> _M_index(_Np)</span>
<span id="cb2-24"><a href="#cb2-24"></a> { }</span>
<span id="cb2-25"><a href="#cb2-25"></a> </span>
<span id="cb2-26"><a href="#cb2-26"></a><span class="st">- void _M_reset()</span></span>
<span id="cb2-27"><a href="#cb2-27"></a><span class="va">+ constexpr void _M_reset()</span></span>
<span id="cb2-28"><a href="#cb2-28"></a> {</span>
<span id="cb2-29"><a href="#cb2-29"></a> if (!_M_valid()) [[unlikely]]</span>
<span id="cb2-30"><a href="#cb2-30"></a> return;</span>
<span id="cb2-31"><a href="#cb2-31"></a><span class="dt">-392,7 +397,7</span></span>
<span id="cb2-32"><a href="#cb2-32"></a> _M_index = variant_npos;</span>
<span id="cb2-33"><a href="#cb2-33"></a> }</span>
<span id="cb2-34"><a href="#cb2-34"></a> </span>
<span id="cb2-35"><a href="#cb2-35"></a><span class="st">- ~_Variant_storage()</span></span>
<span id="cb2-36"><a href="#cb2-36"></a><span class="va">+ constexpr ~_Variant_storage()</span></span>
<span id="cb2-37"><a href="#cb2-37"></a> {</span>
<span id="cb2-38"><a href="#cb2-38"></a> _M_reset();</span>
<span id="cb2-39"><a href="#cb2-39"></a> }</span>
<span id="cb2-40"><a href="#cb2-40"></a><span class="dt">-420,6 +425,10</span></span>
<span id="cb2-41"><a href="#cb2-41"></a> </span>
<span id="cb2-42"><a href="#cb2-42"></a> template<typename... _Types></span>
<span id="cb2-43"><a href="#cb2-43"></a> struct _Variant_storage<true, _Types...> {</span>
<span id="cb2-44"><a href="#cb2-44"></a><span class="va">+ template<typename _Tp></span></span>
<span id="cb2-45"><a href="#cb2-45"></a><span class="va">+ static constexpr size_t __index_of =</span></span>
<span id="cb2-46"><a href="#cb2-46"></a><span class="va">+ __detail::__variant::__index_of_v<_Tp, _Types...>;</span></span>
<span id="cb2-47"><a href="#cb2-47"></a><span class="va">+</span></span>
<span id="cb2-48"><a href="#cb2-48"></a> constexpr _Variant_storage() : _M_index(variant_npos) { }</span>
<span id="cb2-49"><a href="#cb2-49"></a> </span>
<span id="cb2-50"><a href="#cb2-50"></a> template<size_t _Np, typename... _Args></span>
<span id="cb2-51"><a href="#cb2-51"></a><span class="dt">-428,7 +437,7</span></span>
<span id="cb2-52"><a href="#cb2-52"></a> _M_index(_Np)</span>
<span id="cb2-53"><a href="#cb2-53"></a> { }</span>
<span id="cb2-54"><a href="#cb2-54"></a> </span>
<span id="cb2-55"><a href="#cb2-55"></a><span class="st">- void _M_reset() noexcept</span></span>
<span id="cb2-56"><a href="#cb2-56"></a><span class="va">+ constexpr void _M_reset() noexcept</span></span>
<span id="cb2-57"><a href="#cb2-57"></a> {</span>
<span id="cb2-58"><a href="#cb2-58"></a> _M_index = variant_npos;</span>
<span id="cb2-59"><a href="#cb2-59"></a> }</span>
<span id="cb2-60"><a href="#cb2-60"></a><span class="dt">-459,13 +468,16</span></span>
<span id="cb2-61"><a href="#cb2-61"></a> _Variant_storage<_Traits<_Types...>::_S_trivial_dtor, _Types...>;</span>
<span id="cb2-62"><a href="#cb2-62"></a> </span>
<span id="cb2-63"><a href="#cb2-63"></a> template<typename _Tp, typename _Up></span>
<span id="cb2-64"><a href="#cb2-64"></a><span class="st">-void __variant_construct_single(_Tp&& __lhs, _Up&& __rhs_mem)</span></span>
<span id="cb2-65"><a href="#cb2-65"></a><span class="va">+constexpr void __variant_construct_single(_Tp&& __lhs, _Up&& __rhs_mem)</span></span>
<span id="cb2-66"><a href="#cb2-66"></a> {</span>
<span id="cb2-67"><a href="#cb2-67"></a><span class="st">- void* __storage = std::addressof(__lhs._M_u);</span></span>
<span id="cb2-68"><a href="#cb2-68"></a> using _Type = remove_reference_t<decltype(__rhs_mem)>;</span>
<span id="cb2-69"><a href="#cb2-69"></a><span class="st">- if constexpr (!is_same_v<_Type, __variant_cookie>)</span></span>
<span id="cb2-70"><a href="#cb2-70"></a><span class="st">- ::new (__storage)</span></span>
<span id="cb2-71"><a href="#cb2-71"></a><span class="st">- _Type(std::forward<decltype(__rhs_mem)>(__rhs_mem));</span></span>
<span id="cb2-72"><a href="#cb2-72"></a><span class="va">+ constexpr auto index = remove_reference_t<_Tp>::template __index_of<_Type>;</span></span>
<span id="cb2-73"><a href="#cb2-73"></a><span class="va">+</span></span>
<span id="cb2-74"><a href="#cb2-74"></a><span class="va">+ if constexpr (!is_same_v<_Type, __variant_cookie>) {</span></span>
<span id="cb2-75"><a href="#cb2-75"></a><span class="va">+ std::construct_at(std::addressof(__lhs._M_u),</span></span>
<span id="cb2-76"><a href="#cb2-76"></a><span class="va">+ in_place_index<index>,</span></span>
<span id="cb2-77"><a href="#cb2-77"></a><span class="va">+ _Type(std::forward<decltype(__rhs_mem)>(__rhs_mem)));</span></span>
<span id="cb2-78"><a href="#cb2-78"></a><span class="va">+ }</span></span>
<span id="cb2-79"><a href="#cb2-79"></a> }</span>
<span id="cb2-80"><a href="#cb2-80"></a> </span>
<span id="cb2-81"><a href="#cb2-81"></a> template<typename... _Types, typename _Tp, typename _Up></span>
<span id="cb2-82"><a href="#cb2-82"></a><span class="dt">-519,7 +531,7</span></span>
<span id="cb2-83"><a href="#cb2-83"></a> }</span>
<span id="cb2-84"><a href="#cb2-84"></a> </span>
<span id="cb2-85"><a href="#cb2-85"></a> template<typename _Up></span>
<span id="cb2-86"><a href="#cb2-86"></a><span class="st">- void _M_destructive_move(unsigned short __rhs_index, _Up&& __rhs)</span></span>
<span id="cb2-87"><a href="#cb2-87"></a><span class="va">+ constexpr void _M_destructive_move(unsigned short __rhs_index, _Up&& __rhs)</span></span>
<span id="cb2-88"><a href="#cb2-88"></a> {</span>
<span id="cb2-89"><a href="#cb2-89"></a> this->_M_reset();</span>
<span id="cb2-90"><a href="#cb2-90"></a> __variant_construct_single(*this, std::forward<_Up>(__rhs));</span>
<span id="cb2-91"><a href="#cb2-91"></a><span class="dt">-527,7 +539,7</span></span>
<span id="cb2-92"><a href="#cb2-92"></a> }</span>
<span id="cb2-93"><a href="#cb2-93"></a> </span>
<span id="cb2-94"><a href="#cb2-94"></a> template<typename _Up></span>
<span id="cb2-95"><a href="#cb2-95"></a><span class="st">- void _M_destructive_copy(unsigned short __rhs_index, const _Up& __rhs)</span></span>
<span id="cb2-96"><a href="#cb2-96"></a><span class="va">+ constexpr void _M_destructive_copy(unsigned short __rhs_index, const _Up& __rhs)</span></span>
<span id="cb2-97"><a href="#cb2-97"></a> {</span>
<span id="cb2-98"><a href="#cb2-98"></a> this->_M_reset();</span>
<span id="cb2-99"><a href="#cb2-99"></a> __variant_construct_single(*this, __rhs);</span>
<span id="cb2-100"><a href="#cb2-100"></a><span class="dt">-545,7 +557,7</span></span>
<span id="cb2-101"><a href="#cb2-101"></a> using _Base::_Base;</span>
<span id="cb2-102"><a href="#cb2-102"></a> </span>
<span id="cb2-103"><a href="#cb2-103"></a> template<typename _Up></span>
<span id="cb2-104"><a href="#cb2-104"></a><span class="st">- void _M_destructive_move(unsigned short __rhs_index, _Up&& __rhs)</span></span>
<span id="cb2-105"><a href="#cb2-105"></a><span class="va">+ constexpr void _M_destructive_move(unsigned short __rhs_index, _Up&& __rhs)</span></span>
<span id="cb2-106"><a href="#cb2-106"></a> {</span>
<span id="cb2-107"><a href="#cb2-107"></a> this->_M_reset();</span>
<span id="cb2-108"><a href="#cb2-108"></a> __variant_construct_single(*this, std::forward<_Up>(__rhs));</span>
<span id="cb2-109"><a href="#cb2-109"></a><span class="dt">-553,7 +565,7</span></span>
<span id="cb2-110"><a href="#cb2-110"></a> }</span>
<span id="cb2-111"><a href="#cb2-111"></a> </span>
<span id="cb2-112"><a href="#cb2-112"></a> template<typename _Up></span>
<span id="cb2-113"><a href="#cb2-113"></a><span class="st">- void _M_destructive_copy(unsigned short __rhs_index, const _Up& __rhs)</span></span>
<span id="cb2-114"><a href="#cb2-114"></a><span class="va">+ constexpr void _M_destructive_copy(unsigned short __rhs_index, const _Up& __rhs)</span></span>
<span id="cb2-115"><a href="#cb2-115"></a> {</span>
<span id="cb2-116"><a href="#cb2-116"></a> this->_M_reset();</span>
<span id="cb2-117"><a href="#cb2-117"></a> __variant_construct_single(*this, __rhs);</span>
<span id="cb2-118"><a href="#cb2-118"></a><span class="dt">-570,7 +582,7</span></span>
<span id="cb2-119"><a href="#cb2-119"></a> using _Base = _Move_ctor_alias<_Types...>;</span>
<span id="cb2-120"><a href="#cb2-120"></a> using _Base::_Base;</span>
<span id="cb2-121"><a href="#cb2-121"></a> </span>
<span id="cb2-122"><a href="#cb2-122"></a><span class="st">- _Copy_assign_base&</span></span>
<span id="cb2-123"><a href="#cb2-123"></a><span class="va">+ constexpr _Copy_assign_base&</span></span>
<span id="cb2-124"><a href="#cb2-124"></a> operator=(const _Copy_assign_base& __rhs)</span>
<span id="cb2-125"><a href="#cb2-125"></a> noexcept(_Traits<_Types...>::_S_nothrow_copy_assign)</span>
<span id="cb2-126"><a href="#cb2-126"></a> {</span>
<span id="cb2-127"><a href="#cb2-127"></a><span class="dt">-625,7 +637,7</span></span>
<span id="cb2-128"><a href="#cb2-128"></a> using _Base = _Copy_assign_alias<_Types...>;</span>
<span id="cb2-129"><a href="#cb2-129"></a> using _Base::_Base;</span>
<span id="cb2-130"><a href="#cb2-130"></a> </span>
<span id="cb2-131"><a href="#cb2-131"></a><span class="st">- _Move_assign_base&</span></span>
<span id="cb2-132"><a href="#cb2-132"></a><span class="va">+ constexpr _Move_assign_base&</span></span>
<span id="cb2-133"><a href="#cb2-133"></a> operator=(_Move_assign_base&& __rhs)</span>
<span id="cb2-134"><a href="#cb2-134"></a> noexcept(_Traits<_Types...>::_S_nothrow_move_assign)</span>
<span id="cb2-135"><a href="#cb2-135"></a> {</span>
<span id="cb2-136"><a href="#cb2-136"></a><span class="dt">-1033,12 +1045,10</span></span>
<span id="cb2-137"><a href="#cb2-137"></a> } // namespace __detail</span>
<span id="cb2-138"><a href="#cb2-138"></a> </span>
<span id="cb2-139"><a href="#cb2-139"></a> template<size_t _Np, typename _Variant, typename... _Args></span>
<span id="cb2-140"><a href="#cb2-140"></a><span class="st">-void __variant_construct_by_index(_Variant& __v, _Args&&... __args) {</span></span>
<span id="cb2-141"><a href="#cb2-141"></a><span class="st">- __v._M_index = _Np;</span></span>
<span id="cb2-142"><a href="#cb2-142"></a><span class="st">- auto&& __storage = __detail::__variant::__get<_Np>(__v);</span></span>
<span id="cb2-143"><a href="#cb2-143"></a><span class="st">- ::new ((void*)std::addressof(__storage))</span></span>
<span id="cb2-144"><a href="#cb2-144"></a><span class="st">- remove_reference_t<decltype(__storage)></span></span>
<span id="cb2-145"><a href="#cb2-145"></a><span class="st">- (std::forward<_Args>(__args)...);</span></span>
<span id="cb2-146"><a href="#cb2-146"></a><span class="va">+constexpr void __variant_construct_by_index(_Variant& __v, _Args&&... __args) {</span></span>
<span id="cb2-147"><a href="#cb2-147"></a><span class="va">+ std::construct_at(std::addressof(__v), </span></span>
<span id="cb2-148"><a href="#cb2-148"></a><span class="va">+ in_place_index<_Np>,</span></span>
<span id="cb2-149"><a href="#cb2-149"></a><span class="va">+ std::forward<_Args>(__args)...);</span></span>
<span id="cb2-150"><a href="#cb2-150"></a> }</span>
<span id="cb2-151"><a href="#cb2-151"></a> </span>
<span id="cb2-152"><a href="#cb2-152"></a> template<typename _Tp, typename... _Types></span>
<span id="cb2-153"><a href="#cb2-153"></a><span class="dt">-1220,6 +1230,7</span></span>
<span id="cb2-154"><a href="#cb2-154"></a> constexpr decltype(auto) visit(_Visitor&&, _Variants&&...);</span>
<span id="cb2-155"><a href="#cb2-155"></a> </span>
<span id="cb2-156"><a href="#cb2-156"></a> template<typename... _Types></span>
<span id="cb2-157"><a href="#cb2-157"></a><span class="va">+constexpr</span></span>
<span id="cb2-158"><a href="#cb2-158"></a> inline enable_if_t<(is_move_constructible_v<_Types> && ...)</span>
<span id="cb2-159"><a href="#cb2-159"></a> && (is_swappable_v<_Types> && ...)></span>
<span id="cb2-160"><a href="#cb2-160"></a> swap(variant<_Types...>& __lhs, variant<_Types...>& __rhs)</span>
<span id="cb2-161"><a href="#cb2-161"></a><span class="dt">-1281,9 +1292,9</span></span>
<span id="cb2-162"><a href="#cb2-162"></a> {</span>
<span id="cb2-163"><a href="#cb2-163"></a> private:</span>
<span id="cb2-164"><a href="#cb2-164"></a> template <typename... _UTypes, typename _Tp></span>
<span id="cb2-165"><a href="#cb2-165"></a><span class="st">- friend decltype(auto) __variant_cast(_Tp&&);</span></span>
<span id="cb2-166"><a href="#cb2-166"></a><span class="va">+ friend constexpr decltype(auto) __variant_cast(_Tp&&);</span></span>
<span id="cb2-167"><a href="#cb2-167"></a> template<size_t _Np, typename _Variant, typename... _Args></span>
<span id="cb2-168"><a href="#cb2-168"></a><span class="st">- friend void __variant_construct_by_index(_Variant& __v,</span></span>
<span id="cb2-169"><a href="#cb2-169"></a><span class="va">+ friend constexpr void __variant_construct_by_index(_Variant& __v,</span></span>
<span id="cb2-170"><a href="#cb2-170"></a> _Args&&... __args);</span>
<span id="cb2-171"><a href="#cb2-171"></a> </span>
<span id="cb2-172"><a href="#cb2-172"></a> static_assert(sizeof...(_Types) > 0,</span>
<span id="cb2-173"><a href="#cb2-173"></a><span class="dt">-1397,6 +1408,7</span></span>
<span id="cb2-174"><a href="#cb2-174"></a> { }</span>
<span id="cb2-175"><a href="#cb2-175"></a> </span>
<span id="cb2-176"><a href="#cb2-176"></a> template<typename _Tp></span>
<span id="cb2-177"><a href="#cb2-177"></a><span class="va">+ constexpr</span></span>
<span id="cb2-178"><a href="#cb2-178"></a> enable_if_t<__exactly_once<__accepted_type<_Tp&&>></span>
<span id="cb2-179"><a href="#cb2-179"></a> && is_constructible_v<__accepted_type<_Tp&&>, _Tp></span>
<span id="cb2-180"><a href="#cb2-180"></a> && is_assignable_v<__accepted_type<_Tp&&>&, _Tp>,</span>
<span id="cb2-181"><a href="#cb2-181"></a><span class="dt">-1421,6 +1433,7</span></span>
<span id="cb2-182"><a href="#cb2-182"></a> }</span>
<span id="cb2-183"><a href="#cb2-183"></a> </span>
<span id="cb2-184"><a href="#cb2-184"></a> template<typename _Tp, typename... _Args></span>
<span id="cb2-185"><a href="#cb2-185"></a><span class="va">+ constexpr</span></span>
<span id="cb2-186"><a href="#cb2-186"></a> enable_if_t<is_constructible_v<_Tp, _Args...> && __exactly_once<_Tp>,</span>
<span id="cb2-187"><a href="#cb2-187"></a> _Tp&></span>
<span id="cb2-188"><a href="#cb2-188"></a> emplace(_Args&&... __args)</span>
<span id="cb2-189"><a href="#cb2-189"></a><span class="dt">-1430,6 +1443,7</span></span>
<span id="cb2-190"><a href="#cb2-190"></a> }</span>
<span id="cb2-191"><a href="#cb2-191"></a> </span>
<span id="cb2-192"><a href="#cb2-192"></a> template<typename _Tp, typename _Up, typename... _Args></span>
<span id="cb2-193"><a href="#cb2-193"></a><span class="va">+ constexpr</span></span>
<span id="cb2-194"><a href="#cb2-194"></a> enable_if_t<is_constructible_v<_Tp, initializer_list<_Up>&, _Args...></span>
<span id="cb2-195"><a href="#cb2-195"></a> && __exactly_once<_Tp>,</span>
<span id="cb2-196"><a href="#cb2-196"></a> _Tp&></span>
<span id="cb2-197"><a href="#cb2-197"></a><span class="dt">-1440,6 +1454,7</span></span>
<span id="cb2-198"><a href="#cb2-198"></a> }</span>
<span id="cb2-199"><a href="#cb2-199"></a> </span>
<span id="cb2-200"><a href="#cb2-200"></a> template<size_t _Np, typename... _Args></span>
<span id="cb2-201"><a href="#cb2-201"></a><span class="va">+ constexpr</span></span>
<span id="cb2-202"><a href="#cb2-202"></a> enable_if_t<is_constructible_v<variant_alternative_t<_Np, variant>,</span>
<span id="cb2-203"><a href="#cb2-203"></a> _Args...>,</span>
<span id="cb2-204"><a href="#cb2-204"></a> variant_alternative_t<_Np, variant>&></span>
<span id="cb2-205"><a href="#cb2-205"></a><span class="dt">-1484,6 +1499,7</span></span>
<span id="cb2-206"><a href="#cb2-206"></a> }</span>
<span id="cb2-207"><a href="#cb2-207"></a> </span>
<span id="cb2-208"><a href="#cb2-208"></a> template<size_t _Np, typename _Up, typename... _Args></span>
<span id="cb2-209"><a href="#cb2-209"></a><span class="va">+ constexpr</span></span>
<span id="cb2-210"><a href="#cb2-210"></a> enable_if_t<is_constructible_v<variant_alternative_t<_Np, variant>,</span>
<span id="cb2-211"><a href="#cb2-211"></a> initializer_list<_Up>&, _Args...>,</span>
<span id="cb2-212"><a href="#cb2-212"></a> variant_alternative_t<_Np, variant>&></span>
<span id="cb2-213"><a href="#cb2-213"></a><span class="dt">-1540,7 +1556,7</span></span>
<span id="cb2-214"><a href="#cb2-214"></a> }</span>
<span id="cb2-215"><a href="#cb2-215"></a> }</span>
<span id="cb2-216"><a href="#cb2-216"></a> </span>
<span id="cb2-217"><a href="#cb2-217"></a><span class="st">- void</span></span>
<span id="cb2-218"><a href="#cb2-218"></a><span class="va">+ constexpr void</span></span>
<span id="cb2-219"><a href="#cb2-219"></a> swap(variant& __rhs)</span>
<span id="cb2-220"><a href="#cb2-220"></a> noexcept((__is_nothrow_swappable<_Types>::value && ...)</span>
<span id="cb2-221"><a href="#cb2-221"></a> && is_nothrow_move_constructible_v<variant>)</span></code></pre></div>
</div>
<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><span class="ednote" style="color: #0000ff">[ Editor's note: The wording here just shows the added <code class="sourceCode default">constexpr</code>s in the synopses. They all need to be repeated in the specific wording for each function. ]</span></p>
<p>Add <code class="sourceCode cpp"><span class="kw">constexpr</span></code> to the <code class="sourceCode cpp">swap</code> in <span>20.6.2
<a href="https://wg21.link/optional.syn">[optional.syn]</a></span> (and in <span>20.6.9
<a href="https://wg21.link/optional.specalg">[optional.specalg]</a></span>):</p>
<div>
<div class="sourceCode" id="cb3"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb3-1"><a href="#cb3-1"></a>namespace std {</span>
<span id="cb3-2"><a href="#cb3-2"></a> // [optional.optional], class template optional</span>
<span id="cb3-3"><a href="#cb3-3"></a> template<class T></span>
<span id="cb3-4"><a href="#cb3-4"></a> class optional;</span>
<span id="cb3-5"><a href="#cb3-5"></a> </span>
<span id="cb3-6"><a href="#cb3-6"></a> [...]</span>
<span id="cb3-7"><a href="#cb3-7"></a> </span>
<span id="cb3-8"><a href="#cb3-8"></a> // [optional.specalg], specialized algorithms</span>
<span id="cb3-9"><a href="#cb3-9"></a> template<class T></span>
<span id="cb3-10"><a href="#cb3-10"></a><span class="st">- void swap(optional<T>&, optional<T>&) noexcept(<em>see below</em>); </span></span>
<span id="cb3-11"><a href="#cb3-11"></a><span class="va">+ <span class="diffins">constexpr</span> void swap(optional<T>&, optional<T>&) noexcept(<em>see below</em>); </span></span>
<span id="cb3-12"><a href="#cb3-12"></a></span>
<span id="cb3-13"><a href="#cb3-13"></a> [...]</span>
<span id="cb3-14"><a href="#cb3-14"></a></span>
<span id="cb3-15"><a href="#cb3-15"></a>}</span></code></pre></div>
</div>
<p>Add <code class="sourceCode cpp"><span class="kw">constexpr</span></code> to the rest of the functions in <span>20.6.3.1
<a href="https://wg21.link/optional.optional.general">[optional.optional.general]</a></span> (and likewise in <span>20.6.3.2
<a href="https://wg21.link/optional.ctor">[optional.ctor]</a></span>, <span>20.6.3.3
<a href="https://wg21.link/optional.dtor">[optional.dtor]</a></span>, <span>20.6.3.4
<a href="https://wg21.link/optional.assign">[optional.assign]</a></span>, <span>20.6.3.5
<a href="https://wg21.link/optional.swap">[optional.swap]</a></span>, and <span>20.6.3.7
<a href="https://wg21.link/optional.mod">[optional.mod]</a></span>):</p>
<div>
<div class="sourceCode" id="cb4"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb4-1"><a href="#cb4-1"></a>namespace std {</span>
<span id="cb4-2"><a href="#cb4-2"></a> template<class T></span>
<span id="cb4-3"><a href="#cb4-3"></a> class optional {</span>
<span id="cb4-4"><a href="#cb4-4"></a> public:</span>
<span id="cb4-5"><a href="#cb4-5"></a> using value_type = T;</span>
<span id="cb4-6"><a href="#cb4-6"></a></span>
<span id="cb4-7"><a href="#cb4-7"></a> // [optional.ctor], constructors</span>
<span id="cb4-8"><a href="#cb4-8"></a> constexpr optional() noexcept;</span>
<span id="cb4-9"><a href="#cb4-9"></a> constexpr optional(nullopt_t) noexcept;</span>
<span id="cb4-10"><a href="#cb4-10"></a> constexpr optional(const optional&);</span>
<span id="cb4-11"><a href="#cb4-11"></a> constexpr optional(optional&&) noexcept(<em>see below</em>);</span>
<span id="cb4-12"><a href="#cb4-12"></a> template<class... Args></span>
<span id="cb4-13"><a href="#cb4-13"></a> constexpr explicit optional(in_place_t, Args&&...);</span>
<span id="cb4-14"><a href="#cb4-14"></a> template<class U, class... Args></span>
<span id="cb4-15"><a href="#cb4-15"></a> constexpr explicit optional(in_place_t, initializer_list<U>, Args&&...);</span>
<span id="cb4-16"><a href="#cb4-16"></a> template<class U = T></span>
<span id="cb4-17"><a href="#cb4-17"></a> constexpr explicit(<em>see below</em>) optional(U&&);</span>
<span id="cb4-18"><a href="#cb4-18"></a> template<class U></span>
<span id="cb4-19"><a href="#cb4-19"></a><span class="st">- explicit(<em>see below</em>) optional(const optional<U>&);</span></span>
<span id="cb4-20"><a href="#cb4-20"></a><span class="va">+ <span class="diffins">constexpr</span> explicit(<em>see below</em>) optional(const optional<U>&);</span></span>
<span id="cb4-21"><a href="#cb4-21"></a> template<class U></span>
<span id="cb4-22"><a href="#cb4-22"></a><span class="st">- explicit(<em>see below</em>) optional(optional<U>&&);</span></span>
<span id="cb4-23"><a href="#cb4-23"></a><span class="va">+ <span class="diffins">constexpr</span> explicit(<em>see below</em>) optional(optional<U>&&);</span></span>
<span id="cb4-24"><a href="#cb4-24"></a></span>
<span id="cb4-25"><a href="#cb4-25"></a> // [optional.dtor], destructor</span>
<span id="cb4-26"><a href="#cb4-26"></a><span class="st">- ~optional();</span></span>
<span id="cb4-27"><a href="#cb4-27"></a><span class="va">+ <span class="diffins">constexpr</span> ~optional();</span></span>
<span id="cb4-28"><a href="#cb4-28"></a></span>
<span id="cb4-29"><a href="#cb4-29"></a> // [optional.assign], assignment</span>
<span id="cb4-30"><a href="#cb4-30"></a><span class="st">- optional& operator=(nullopt_t) noexcept;</span></span>
<span id="cb4-31"><a href="#cb4-31"></a><span class="va">+ <span class="diffins">constexpr</span> optional& operator=(nullopt_t) noexcept;</span></span>
<span id="cb4-32"><a href="#cb4-32"></a> constexpr optional& operator=(const optional&);</span>
<span id="cb4-33"><a href="#cb4-33"></a> constexpr optional& operator=(optional&&) noexcept(<em>see below</em>);</span>
<span id="cb4-34"><a href="#cb4-34"></a><span class="st">- template<class U = T> optional& operator=(U&&);</span></span>