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  <title>Inheriting from `std::variant`</title>
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<h1 class="title" style="text-align:center">Inheriting from <code class="sourceCode cpp">std<span class="op">::</span>variant</code></h1>
<h3 class="subtitle" style="text-align:center">Resolving LWG3052</h3>

<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P2162R1</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2020-08-13</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      LEWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Barry Revzin<br>&lt;<a href="mailto:barry.revzin@gmail.com" class="email">barry.revzin@gmail.com</a>&gt;<br>
    </td>
  </tr>
</table>

</header>
<div style="clear:both">
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="revision-history" data-number="1" style="border-bottom:1px solid #cccccc"><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="P2162R0">[<a href="#ref-P2162R0" role="doc-biblioref">P2162R0</a>]</span>, added more information in the implementation experience section.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="introduction" data-number="2" style="border-bottom:1px solid #cccccc"><span class="header-section-number">2</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p><span class="citation" data-cites="LWG3052">[<a href="#ref-LWG3052" role="doc-biblioref">LWG3052</a>]</span> describes an under-specification to <code class="sourceCode cpp">std<span class="op">::</span>visit</code>:</p>
<blockquote>
<p>the <em>Requires</em> element imposes no explicit requirements on the types in <code class="sourceCode cpp">Variant</code>s. Notably, the <code class="sourceCode cpp">Variant</code>s are not required to be <code class="sourceCode cpp">variant</code>s. This lack of constraints appears to be simply an oversight.</p>
</blockquote>
<p>The original proposal <span class="citation" data-cites="P0088R3">[<a href="#ref-P0088R3" role="doc-biblioref">P0088R3</a>]</span> makes no mention of other kinds of of variants besides <code class="sourceCode cpp">std<span class="op">::</span>variant</code>, and this does not appear to have been discussed in LEWG.</p>
<p>The proposed resolution in the library issue is to make <code class="sourceCode cpp">std<span class="op">::</span>visit</code> only work if all of the <code class="sourceCode cpp">Variant</code>s are, in fact, <code class="sourceCode cpp">std<span class="op">::</span>variant</code>s:</p>
<blockquote>
<p><em>Remarks</em>: This function shall not participate in overload resolution unless <code class="sourceCode cpp">remove_cvref_t<span class="op">&lt;</span>Variants<sub>i</sub><span class="op">&gt;</span></code> is a specialization of <code class="sourceCode cpp">variant</code> for all <code class="sourceCode cpp"><span class="dv">0</span> <span class="op">&lt;=</span> i <span class="op">&lt;</span> n</code>.</p>
</blockquote>
<p>This paper suggests a different direction. Instead of restricting to <em>just</em> <code class="sourceCode cpp">std<span class="op">::</span>variant</code> (and certainly not wanting to go all out and design a “variant-like” interface), this paper proposes to allow an additional category of useful types to be <code class="sourceCode cpp">std<span class="op">::</span>visit<span class="op">()</span></code>-ed: those that publicly and unambiguously inherit from a specialization of <code class="sourceCode cpp">std<span class="op">::</span>variant</code>.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="inheriting-from-variant" data-number="3" style="border-bottom:1px solid #cccccc"><span class="header-section-number">3</span> Inheriting from <code class="sourceCode cpp">variant</code><a href="#inheriting-from-variant" class="self-link"></a></h1>
<p>There are two primary motivators for inheriting from <code class="sourceCode cpp">std<span class="op">::</span>variant</code>.</p>
<p>One is to simply extend functionality. If we’re using <code class="sourceCode cpp">variant</code> to represent a state machine, we may want additional operations that are relevant to our state that <code class="sourceCode cpp">variant</code> doesn’t itself provide:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="kw">struct</span> State <span class="op">:</span> variant<span class="op">&lt;</span>Disconnected, Connecting, Connected<span class="op">&gt;</span></span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="op">{</span></span>
<span id="cb1-3"><a href="#cb1-3"></a>    <span class="kw">using</span> variant<span class="op">::</span>variant;</span>
<span id="cb1-4"><a href="#cb1-4"></a>    </span>
<span id="cb1-5"><a href="#cb1-5"></a>    <span class="dt">bool</span> is_connected<span class="op">()</span> <span class="kw">const</span> <span class="op">{</span></span>
<span id="cb1-6"><a href="#cb1-6"></a>        <span class="cf">return</span> std<span class="op">::</span>holds_alternative<span class="op">&lt;</span>Connected<span class="op">&gt;(*</span><span class="kw">this</span><span class="op">)</span>;</span>
<span id="cb1-7"><a href="#cb1-7"></a>    <span class="op">}</span></span>
<span id="cb1-8"><a href="#cb1-8"></a>    </span>
<span id="cb1-9"><a href="#cb1-9"></a>    <span class="kw">friend</span> std<span class="op">::</span>ostream<span class="op">&amp;</span> <span class="kw">operator</span><span class="op">&lt;&lt;(</span>std<span class="op">::</span>ostream<span class="op">&amp;</span>, State <span class="kw">const</span><span class="op">&amp;)</span> <span class="op">{</span></span>
<span id="cb1-10"><a href="#cb1-10"></a>        <span class="co">// ...</span></span>
<span id="cb1-11"><a href="#cb1-11"></a>    <span class="op">}</span></span>
<span id="cb1-12"><a href="#cb1-12"></a><span class="op">}</span>;</span></code></pre></div>
<p>Another may be to create a recursive variant, as in the example from <span class="citation" data-cites="P1371R2">[<a href="#ref-P1371R2" role="doc-biblioref">P1371R2</a>]</span>:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a><span class="kw">struct</span> Expr;</span>
<span id="cb2-2"><a href="#cb2-2"></a></span>
<span id="cb2-3"><a href="#cb2-3"></a><span class="kw">struct</span> Neg <span class="op">{</span></span>
<span id="cb2-4"><a href="#cb2-4"></a>    std<span class="op">::</span>shared_ptr<span class="op">&lt;</span>Expr<span class="op">&gt;</span> expr;</span>
<span id="cb2-5"><a href="#cb2-5"></a><span class="op">}</span>;</span>
<span id="cb2-6"><a href="#cb2-6"></a></span>
<span id="cb2-7"><a href="#cb2-7"></a><span class="kw">struct</span> Add <span class="op">{</span></span>
<span id="cb2-8"><a href="#cb2-8"></a>    std<span class="op">::</span>shared_ptr<span class="op">&lt;</span>Expr<span class="op">&gt;</span> lhs, rhs;</span>
<span id="cb2-9"><a href="#cb2-9"></a><span class="op">}</span>;</span>
<span id="cb2-10"><a href="#cb2-10"></a></span>
<span id="cb2-11"><a href="#cb2-11"></a><span class="kw">struct</span> Mul <span class="op">{</span></span>
<span id="cb2-12"><a href="#cb2-12"></a>    std<span class="op">::</span>shared_ptr<span class="op">&lt;</span>Expr<span class="op">&gt;</span> lhs, rhs;</span>
<span id="cb2-13"><a href="#cb2-13"></a><span class="op">}</span>;</span>
<span id="cb2-14"><a href="#cb2-14"></a></span>
<span id="cb2-15"><a href="#cb2-15"></a><span class="kw">struct</span> Expr <span class="op">:</span> std<span class="op">::</span>variant<span class="op">&lt;</span><span class="dt">int</span>, Neg, Add, Mul<span class="op">&gt;</span> <span class="op">{</span></span>
<span id="cb2-16"><a href="#cb2-16"></a>    <span class="kw">using</span> variant<span class="op">::</span>variant;</span>
<span id="cb2-17"><a href="#cb2-17"></a><span class="op">}</span>;</span>
<span id="cb2-18"><a href="#cb2-18"></a></span>
<span id="cb2-19"><a href="#cb2-19"></a><span class="kw">namespace</span> std <span class="op">{</span></span>
<span id="cb2-20"><a href="#cb2-20"></a>    <span class="kw">template</span> <span class="op">&lt;&gt;</span> <span class="kw">struct</span> variant_size<span class="op">&lt;</span>Expr<span class="op">&gt;</span> <span class="op">:</span> variant_size<span class="op">&lt;</span>Expr<span class="op">::</span>variant<span class="op">&gt;</span> <span class="op">{}</span>;</span>
<span id="cb2-21"><a href="#cb2-21"></a>    </span>
<span id="cb2-22"><a href="#cb2-22"></a>    <span class="kw">template</span> <span class="op">&lt;</span>std<span class="op">::</span><span class="dt">size_t</span> I<span class="op">&gt;</span> <span class="kw">struct</span> variant_alternative<span class="op">&lt;</span>I, Expr<span class="op">&gt;</span> <span class="op">:</span> variant_alternative<span class="op">&lt;</span>I, Expr<span class="op">::</span>variant<span class="op">&gt;</span> <span class="op">{}</span>;</span>
<span id="cb2-23"><a href="#cb2-23"></a><span class="op">}</span></span></code></pre></div>
<p>That paper even has an example of passing an <code class="sourceCode cpp">Expr</code> to <code class="sourceCode cpp">std<span class="op">::</span>visit<span class="op">()</span></code> directly, a use-case that this paper is seeking to properly specify. It would be pretty nice if that just worked.</p>
<p>Note also that the example includes an explicit specialization of <code class="sourceCode cpp">variant_size</code> and <code class="sourceCode cpp">variant_alternative</code> that just forward along to <code class="sourceCode cpp">Expr</code>’s base class. These specializations are pure boilerplate - they basically have to look the way they do, so they don’t really offer much in the way of adding value to the program.</p>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="implementation-approach" data-number="4" style="border-bottom:1px solid #cccccc"><span class="header-section-number">4</span> Implementation Approach<a href="#implementation-approach" class="self-link"></a></h1>
<p>The proposed resolution of LWG3052 is to, basically, add this constraint onto <code class="sourceCode cpp">std<span class="op">::</span>visit</code>:</p>
<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">&lt;</span><span class="kw">typename</span> Visitor, <span class="kw">typename</span><span class="op">...</span> Variants<span class="op">&gt;</span></span>
<span id="cb3-2"><a href="#cb3-2"></a>    <span class="kw">requires</span> <span class="op">(</span>is_specialization_of_v<span class="op">&lt;</span>remove_cvref_t<span class="op">&lt;</span>Variants<span class="op">&gt;</span>, variant<span class="op">&gt;</span> <span class="op">&amp;&amp;</span> <span class="op">...)</span></span>
<span id="cb3-3"><a href="#cb3-3"></a><span class="kw">constexpr</span> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> visit<span class="op">(</span>Visitor<span class="op">&amp;&amp;</span>, Variants<span class="op">&amp;&amp;)</span> <span class="op">{</span></span>
<span id="cb3-4"><a href="#cb3-4"></a>    <span class="co">// as today</span></span>
<span id="cb3-5"><a href="#cb3-5"></a><span class="op">}</span></span></code></pre></div>
<p>This paper proposes instead that <code class="sourceCode cpp">visit</code> conditionally upcasts all of its incoming variants to <code class="sourceCode cpp">std<span class="op">::</span>variant</code> specializations:</p>
<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">template</span> <span class="op">&lt;</span><span class="kw">typename</span> Visitor, <span class="kw">typename</span><span class="op">...</span> Variants<span class="op">&gt;</span></span>
<span id="cb4-2"><a href="#cb4-2"></a>    <span class="kw">requires</span> <span class="op">(</span>is_specialization_of_v<span class="op">&lt;</span>remove_cvref_t<span class="op">&lt;</span>Variants<span class="op">&gt;</span>, variant<span class="op">&gt;</span> <span class="op">&amp;&amp;</span> <span class="op">...)</span></span>
<span id="cb4-3"><a href="#cb4-3"></a><span class="kw">constexpr</span> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> visit<span class="op">(</span>Visitor<span class="op">&amp;&amp;</span>, Variants<span class="op">&amp;&amp;)</span> <span class="op">{</span></span>
<span id="cb4-4"><a href="#cb4-4"></a>    <span class="co">// as today</span></span>
<span id="cb4-5"><a href="#cb4-5"></a><span class="op">}</span></span>
<span id="cb4-6"><a href="#cb4-6"></a></span>
<span id="cb4-7"><a href="#cb4-7"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">&gt;</span></span>
<span id="cb4-8"><a href="#cb4-8"></a><span class="kw">constexpr</span> <span class="kw">auto</span> variant_cast<span class="op">(</span>std<span class="op">::</span>variant<span class="op">&lt;</span>Ts<span class="op">...&gt;&amp;</span> v<span class="op">)</span> <span class="op">-&gt;</span> std<span class="op">::</span>variant<span class="op">&lt;</span>Ts<span class="op">...&gt;&amp;</span> <span class="op">{</span></span>
<span id="cb4-9"><a href="#cb4-9"></a>    <span class="cf">return</span> v;</span>
<span id="cb4-10"><a href="#cb4-10"></a><span class="op">}</span></span>
<span id="cb4-11"><a href="#cb4-11"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">&gt;</span></span>
<span id="cb4-12"><a href="#cb4-12"></a><span class="kw">constexpr</span> <span class="kw">auto</span> variant_cast<span class="op">(</span>std<span class="op">::</span>variant<span class="op">&lt;</span>Ts<span class="op">...&gt;</span> <span class="kw">const</span><span class="op">&amp;</span> v<span class="op">)</span> <span class="op">-&gt;</span> std<span class="op">::</span>variant<span class="op">&lt;</span>Ts<span class="op">...&gt;</span> <span class="kw">const</span><span class="op">&amp;</span> <span class="op">{</span></span>
<span id="cb4-13"><a href="#cb4-13"></a>    <span class="cf">return</span> v;</span>
<span id="cb4-14"><a href="#cb4-14"></a><span class="op">}</span></span>
<span id="cb4-15"><a href="#cb4-15"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">&gt;</span></span>
<span id="cb4-16"><a href="#cb4-16"></a><span class="kw">constexpr</span> <span class="kw">auto</span> variant_cast<span class="op">(</span>std<span class="op">::</span>variant<span class="op">&lt;</span>Ts<span class="op">...&gt;&amp;&amp;</span> v<span class="op">)</span> <span class="op">-&gt;</span> std<span class="op">::</span>variant<span class="op">&lt;</span>Ts<span class="op">...&gt;&amp;&amp;</span> <span class="op">{</span></span>
<span id="cb4-17"><a href="#cb4-17"></a>    <span class="cf">return</span> std<span class="op">::</span>move<span class="op">(</span>v<span class="op">)</span>;</span>
<span id="cb4-18"><a href="#cb4-18"></a><span class="op">}</span></span>
<span id="cb4-19"><a href="#cb4-19"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span><span class="op">...</span> Ts<span class="op">&gt;</span></span>
<span id="cb4-20"><a href="#cb4-20"></a><span class="kw">constexpr</span> <span class="kw">auto</span> variant_cast<span class="op">(</span>std<span class="op">::</span>variant<span class="op">&lt;</span>Ts<span class="op">...&gt;</span> <span class="kw">const</span><span class="op">&amp;&amp;</span> v<span class="op">)</span> <span class="op">-&gt;</span> std<span class="op">::</span>variant<span class="op">&lt;</span>Ts<span class="op">...&gt;</span> <span class="kw">const</span><span class="op">&amp;&amp;</span> <span class="op">{</span></span>
<span id="cb4-21"><a href="#cb4-21"></a>    <span class="cf">return</span> std<span class="op">::</span>move<span class="op">(</span>v<span class="op">)</span>;</span>
<span id="cb4-22"><a href="#cb4-22"></a><span class="op">}</span></span>
<span id="cb4-23"><a href="#cb4-23"></a></span>
<span id="cb4-24"><a href="#cb4-24"></a><span class="kw">template</span> <span class="op">&lt;</span><span class="kw">typename</span> Visitor, <span class="kw">typename</span><span class="op">...</span> Variants<span class="op">&gt;</span></span>
<span id="cb4-25"><a href="#cb4-25"></a><span class="kw">constexpr</span> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> visit<span class="op">(</span>Visitor<span class="op">&amp;&amp;</span> vis, Variants<span class="op">&amp;&amp;...</span> vars<span class="op">)</span> <span class="op">{</span></span>
<span id="cb4-26"><a href="#cb4-26"></a>    <span class="cf">return</span> visit<span class="op">(</span>std<span class="op">::</span>forward<span class="op">&lt;</span>Visitor<span class="op">&gt;(</span>vis<span class="op">)</span>,</span>
<span id="cb4-27"><a href="#cb4-27"></a>        variant_cast<span class="op">(</span>std<span class="op">::</span>forward<span class="op">&lt;</span>Variants<span class="op">&gt;(</span>vars<span class="op">))...)</span>;</span>
<span id="cb4-28"><a href="#cb4-28"></a><span class="op">}</span></span></code></pre></div>
<p>This means the body of <code class="sourceCode cpp">std<span class="op">::</span>visit</code> for implementations can remain unchanged - it, as today, can just assume that all the variants are indeed <code class="sourceCode cpp">std<span class="op">::</span>variant</code>s. Such an implementation would allow visitation of the <code class="sourceCode cpp">State</code> and <code class="sourceCode cpp">Expr</code> examples provided earlier.</p>
<p>Now, this means we can <code class="sourceCode cpp">std<span class="op">::</span>visit</code> a variant that we can’t even invoke <code class="sourceCode cpp">std<span class="op">::</span>get</code> or <code class="sourceCode cpp">std<span class="op">::</span>get_if</code> on, such as with this delightful type courtesy of Tim Song:</p>
<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">struct</span> MyEvilVariantBase <span class="op">{</span></span>
<span id="cb5-2"><a href="#cb5-2"></a>    <span class="dt">int</span> index;</span>
<span id="cb5-3"><a href="#cb5-3"></a>    <span class="dt">char</span> valueless_by_exception;</span>
<span id="cb5-4"><a href="#cb5-4"></a><span class="op">}</span>;</span>
<span id="cb5-5"><a href="#cb5-5"></a></span>
<span id="cb5-6"><a href="#cb5-6"></a><span class="kw">struct</span> MyEvilVariant <span class="op">:</span> std<span class="op">::</span>variant<span class="op">&lt;</span><span class="dt">int</span>, <span class="dt">long</span><span class="op">&gt;</span>, std<span class="op">::</span>tuple<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;</span>, MyEvilVariantBase <span class="op">{</span> <span class="op">}</span>;</span></code></pre></div>
<p>But… who cares. Don’t write types like that.</p>
<h1 data-number="5" style="border-bottom:1px solid #cccccc" id="implementation-experience" data-number="5" style="border-bottom:1px solid #cccccc"><span class="header-section-number">5</span> Implementation Experience<a href="#implementation-experience" class="self-link"></a></h1>
<p>The Microsoft STL implementation already supports exactly this design <span class="citation" data-cites="stlstl">[<a href="#ref-stlstl" role="doc-biblioref">stlstl</a>]</span> since the first Visual Studio 2019 release in April 2019.</p>
<p>The libc++ implementation has supported <em>nearly</em> this design since day one <span class="citation" data-cites="libcpp">[<a href="#ref-libcpp" role="doc-biblioref">libcpp</a>]</span>. While the incoming variants to <code class="sourceCode cpp">visit</code> are upcast to specializations of <code class="sourceCode cpp">std<span class="op">::</span>variant</code>, the member function <code class="sourceCode cpp">valueless_by_exception<span class="op">()</span></code> is invoked directly on the arguments. The spirit of the implementation matches the intent of this paper, though it does technically break on Tim’s example (but does work fine on any types that inherit from <code class="sourceCode cpp">std<span class="op">::</span>variant</code> without touching <code class="sourceCode cpp">valueless_by_exception<span class="op">()</span></code> – and it’s just the <code class="sourceCode cpp">valueless_by_exception</code> member that causes the problem, the <code class="sourceCode cpp">index</code> member doesn’t).</p>
<p>When I pointed out to Tim that libc++’s variant only breaks for absurd types that do things like have a member named <code class="sourceCode cpp">valueless_by_exception</code>, he followed up by providing a different absurd type that instead breaks by inheriting from <code class="sourceCode cpp">std<span class="op">::</span>type_info</code>:</p>
<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">struct</span> MyEvilVariant <span class="op">:</span> std<span class="op">::</span>variant<span class="op">&lt;</span><span class="dt">int</span>, <span class="dt">long</span><span class="op">&gt;</span>, std<span class="op">::</span>type_info <span class="op">{</span> <span class="op">}</span>;</span>
<span id="cb6-2"><a href="#cb6-2"></a><span class="kw">using</span> x <span class="op">=</span> <span class="kw">decltype</span><span class="op">(</span>std<span class="op">::</span>visit<span class="op">([](</span><span class="kw">auto</span><span class="op">){}</span>,     <span class="co">// error for libc++</span></span>
<span id="cb6-3"><a href="#cb6-3"></a>    std<span class="op">::</span>declval<span class="op">&lt;</span>MyEvilVariant<span class="op">&gt;()))</span>;          <span class="co">// ambiguous look on __impl</span></span></code></pre></div>
<p>The libstdc++ implementation used to support visiting inheriting variants in gcc 8, but then stopped supporting them in gcc 9 - only because its check for whether the variant can be never valueless only works for <code class="sourceCode cpp">std<span class="op">::</span>variant</code> specializations directly <span class="citation" data-cites="libstdcpp">[<a href="#ref-libstdcpp" role="doc-biblioref">libstdcpp</a>]</span>. I filed a bug report <span class="citation" data-cites="gcc.90943">[<a href="#ref-gcc.90943" role="doc-biblioref">gcc.90943</a>]</span> to get them to start supporting again, but that bug report has been suspended pending the resolution of the library issue in question.</p>
<p>Boost.Variant supports visiting inherited <code class="sourceCode cpp">variant</code>s. Boost.Variant2 will start supporting visiting inherited <code class="sourceCode cpp">variant</code>s in Boost 1.74. <span class="citation" data-cites="boost.variant2">[<a href="#ref-boost.variant2" role="doc-biblioref">boost.variant2</a>]</span>.</p>
<h1 data-number="6" style="border-bottom:1px solid #cccccc" id="wording" data-number="6" style="border-bottom:1px solid #cccccc"><span class="header-section-number">6</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Change <span>20.7.7 <a href="https://wg21.link/variant.visit">[variant.visit]</a></span>:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb7"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb7-1"><a href="#cb7-1"></a>template&lt;class Visitor, class... Variants&gt;</span>
<span id="cb7-2"><a href="#cb7-2"></a>  constexpr see below visit(Visitor&amp;&amp; vis, Variants&amp;&amp;... vars);</span>
<span id="cb7-3"><a href="#cb7-3"></a>template&lt;class R, class Visitor, class... Variants&gt;</span>
<span id="cb7-4"><a href="#cb7-4"></a>  constexpr R visit(Visitor&amp;&amp; vis, Variants&amp;&amp;... vars);</span></code></pre></div>
</div>
<div class="addu">
<p><span class="marginalizedparent"><a class="marginalized">-2</a></span> Let <em><code class="sourceCode cpp">as<span class="op">-</span>variant</code></em> denote the exposition-only function template</p>
<div class="sourceCode" id="cb8"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span><span class="op">...</span> Ts<span class="op">&gt;</span></span>
<span id="cb8-2"><a href="#cb8-2"></a><span class="kw">const</span> variant<span class="op">&lt;</span>Ts<span class="op">...&gt;&amp;</span> <em>as-variant</em><span class="op">(</span><span class="kw">const</span> variant<span class="op">&lt;</span>Ts<span class="op">...&gt;&amp;</span> var<span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> var; <span class="op">}</span></span></code></pre></div>
<p>Let <code class="sourceCode cpp">n</code> be <code class="sourceCode cpp"><span class="kw">sizeof</span><span class="op">...(</span>Variants<span class="op">)</span></code>. For each <code class="sourceCode cpp"><span class="dv">0</span> <span class="op">&lt;=</span> i <span class="op">&lt;</span> n</code>, let <code class="sourceCode cpp">V<sub>i</sub></code> denote the the type <code class="sourceCode cpp">remove_cvref_t<span class="op">&lt;</span><span class="kw">decltype</span><span class="op">(</span><em>as-variant</em><span class="op">(</span>vars<sub>i</sub><span class="op">))&gt;</span></code>.</p>
<p><span class="marginalizedparent"><a class="marginalized">-1</a></span> <em>Constraints</em>: <code class="sourceCode cpp">V<sub>i</sub></code> is a valid type for all <code class="sourceCode cpp"><span class="dv">0</span> <span class="op">&lt;=</span> i <span class="op">&lt;</span> n</code>.</p>
<p><span class="marginalizedparent"><a class="marginalized">0</a></span> Let <code class="sourceCode cpp">VR<sub>i</sub></code> denote the type <code class="sourceCode cpp">V<sub>i</sub></code> with the addition of <code class="sourceCode cpp">Variant<sub>i</sub></code>’s cv and reference qualifiers. Let <code class="sourceCode cpp">VR</code> denote the pack of types <code class="sourceCode cpp">VR<sub>i</sub></code>.</p>
</div>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span> <span class="rm" style="color: #bf0303"><del>Let <span><code class="sourceCode default">n</code></span> be <span><code class="sourceCode default">sizeof...(Variants)</code></span>.</del></span> Let <code class="sourceCode cpp">m</code> be a pack of n values of type <code class="sourceCode cpp"><span class="dt">size_t</span></code>. Such a pack is called valid if <code class="sourceCode cpp"><span class="dv">0</span> <span class="op">&lt;=</span> m<sub>i</sub> <span class="op">&lt;</span> variant_size_v<span class="op">&lt;</span><span class="rm" style="color: #bf0303"><del>remove_reference_t&lt;Variants<sub>i</sub>&gt;</del></span> <span class="addu">V<sub>i</sub></span><span class="op">&gt;</span></code> for all <code class="sourceCode cpp"><span class="dv">0</span> <span class="op">&lt;=</span> i <span class="op">&lt;</span> n</code>. For each valid pack <code class="sourceCode cpp">m</code>, let <code class="sourceCode cpp">e<span class="op">(</span>m<span class="op">)</span></code> denote the expression:</p>
<div>
<div class="sourceCode" id="cb9"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb9-1"><a href="#cb9-1"></a><span class="st">- INVOKE(std::forward&lt;Visitor&gt;(vis), get&lt;m&gt;(std::forward&lt;Variants&gt;(vars))...) // see [func.require]</span></span>
<span id="cb9-2"><a href="#cb9-2"></a><span class="va">+ INVOKE(std::forward&lt;Visitor&gt;(vis), get&lt;m&gt;(std::forward&lt;VR&gt;(vars))...) // see [func.require]</span></span></code></pre></div>
</div>
<p>for the first form and</p>
<div>
<div class="sourceCode" id="cb10"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb10-1"><a href="#cb10-1"></a><span class="st">- INVOKE&lt;R&gt;(std::forward&lt;Visitor&gt;(vis), get&lt;m&gt;(std::forward&lt;Variants&gt;(vars))...) // see [func.require]</span></span>
<span id="cb10-2"><a href="#cb10-2"></a><span class="va">+ INVOKE&lt;R&gt;(std::forward&lt;Visitor&gt;(vis), get&lt;m&gt;(std::forward&lt;VR&gt;(vars))...) // see [func.require]</span></span></code></pre></div>
</div>
<p>for the second form.</p>
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> <em>Mandates</em>: For each valid pack <code class="sourceCode cpp">m</code>, <code class="sourceCode cpp">e<span class="op">(</span>m<span class="op">)</span></code> is a valid expression. All such expressions are of the same type and value category.</p>
<p><span class="marginalizedparent"><a class="marginalized">3</a></span> <em>Returns</em>: <code class="sourceCode cpp">e<span class="op">(</span>m<span class="op">)</span></code>, where <code class="sourceCode cpp">m</code> is the pack for which <code class="sourceCode cpp">m<sub>i</sub></code> is <span class="rm" style="color: #bf0303"><del><span><code class="sourceCode default">vars<sub>i</sub>.index()</code></span></del></span> <span class="addu"><code class="sourceCode cpp"><em>as-variant</em><span class="op">(</span>vars<sub>i</sub><span class="op">).</span>index<span class="op">()</span></code></span> for all <code class="sourceCode cpp"><span class="dv">0</span> <span class="op">&lt;=</span> i <span class="op">&lt;</span> n</code>. The return type is <code class="sourceCode cpp"><span class="kw">decltype</span><span class="op">(</span>e<span class="op">(</span>m<span class="op">))</span></code> for the first form.</p>
<p><span class="marginalizedparent"><a class="marginalized">4</a></span> <em>Throws</em>: <code class="sourceCode cpp">bad_variant_access</code> if <span class="rm" style="color: #bf0303"><del>any <span><code class="sourceCode default">variant</code></span> in <span><code class="sourceCode default">vars</code></span> is <span><code class="sourceCode default">valueless_by_exception()</code></span></del></span> if <span class="addu"><code class="sourceCode cpp"><span class="op">(</span><em>as-variant</em><span class="op">(</span>vars<span class="op">).</span>valueless_by_exception<span class="op">()</span> <span class="op">||</span> <span class="op">...)</span></code> is <code class="sourceCode cpp"><span class="kw">true</span></code></span>.</p>
<p><span class="marginalizedparent"><a class="marginalized">5</a></span> <em>Complexity</em>: For <code class="sourceCode cpp">n <span class="op">&lt;=</span> <span class="dv">1</span></code>, the invocation of the callable object is implemented in constant time, i.e., for <code class="sourceCode cpp">n<span class="op">=</span><span class="dv">1</span></code>, it does not depend on the number of alternative types of <span class="rm" style="color: #bf0303"><del><span><code class="sourceCode default">Variants<sub>0</sub></code></span></del></span> <span class="addu"><code class="sourceCode cpp">V<sub>0</sub></code></span>. For <code class="sourceCode cpp">n<span class="op">&gt;</span><span class="dv">1</span></code>, the invocation of the callable object has no complexity requirements.</p>
</blockquote>
<h2 data-number="6.1" id="feature-test-macro" data-number="6.1"><span class="header-section-number">6.1</span> Feature-test macro<a href="#feature-test-macro" class="self-link"></a></h2>
<p>This paper proposes to bump the value <code class="sourceCode cpp">__cpp_lib_variant</code>. The macro already exists, so this is, in a sense, free. And users can use the value of this macro to avoid having to specialize <code class="sourceCode cpp">variant_size</code> and <code class="sourceCode cpp">variant_alternative</code> for their inherited variants.</p>
<h1 data-number="7" style="border-bottom:1px solid #cccccc" id="acknowledgments" data-number="7" style="border-bottom:1px solid #cccccc"><span class="header-section-number">7</span> Acknowledgments<a href="#acknowledgments" class="self-link"></a></h1>
<p>Thanks to Casey Carter, Ville Voutilainen, and the unfortunately non-alliterative Tim Song for design discussion and help with the wording.</p>
<h1 data-number="8" style="border-bottom:1px solid #cccccc" id="bibliography" data-number="8" style="border-bottom:1px solid #cccccc"><span class="header-section-number">8</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<div id="ref-boost.variant2">
<p>[boost.variant2] Peter Dimov. 2020. Support derived types in visit. <br />
<a href="https://github.com/boostorg/variant2/commit/772ef0d312868a1bdb371e8f336d5abd41cc61b2">https://github.com/boostorg/variant2/commit/772ef0d312868a1bdb371e8f336d5abd41cc61b2</a></p>
</div>
<div id="ref-gcc.90943">
<p>[gcc.90943] Barry Revzin. 2019. Visiting inherited variants no longer works in 9.1. <br />
<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90943">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90943</a></p>
</div>
<div id="ref-libcpp">
<p>[libcpp] Michael Park. 2017. libc++ variant. <br />
<a href="https://github.com/llvm/llvm-project/blob/24b4965ce65b14ead595dcc68add22ba37533207/libcxx/include/variant#L455">https://github.com/llvm/llvm-project/blob/24b4965ce65b14ead595dcc68add22ba37533207/libcxx/include/variant#L455</a></p>
</div>
<div id="ref-libstdcpp">
<p>[libstdcpp] Tim Shen and Jonathan Wakely. 2018. libstdc++ variant. <br />
<a href="https://github.com/gcc-mirror/gcc/blob/ab2952c77d029c93fc813dec9760f8a517286e5e/libstdc%2B%2B-v3/include/std/variant#L798-L811">https://github.com/gcc-mirror/gcc/blob/ab2952c77d029c93fc813dec9760f8a517286e5e/libstdc%2B%2B-v3/include/std/variant#L798-L811</a></p>
</div>
<div id="ref-LWG3052">
<p>[LWG3052] Casey Carter. visit is underconstrained. <br />
<a href="https://wg21.link/lwg3052">https://wg21.link/lwg3052</a></p>
</div>
<div id="ref-P0088R3">
<p>[P0088R3] Axel Naumann. 2016. Variant: a type-safe union for C++17 (v8). <br />
<a href="https://wg21.link/p0088r3">https://wg21.link/p0088r3</a></p>
</div>
<div id="ref-P1371R2">
<p>[P1371R2] Sergei Murzin, Michael Park, David Sankel, Dan Sarginson. 2020. Pattern Matching. <br />
<a href="https://wg21.link/p1371r2">https://wg21.link/p1371r2</a></p>
</div>
<div id="ref-P2162R0">
<p>[P2162R0] Barry Revzin. 2020. Inheriting from std::variant (resolving LWG3052). <br />
<a href="https://wg21.link/p2162r0">https://wg21.link/p2162r0</a></p>
</div>
<div id="ref-stlstl">
<p>[stlstl] Microsoft. 2019. stlstl variant. <br />
<a href="https://github.com/microsoft/STL/blob/65d98ffabab3a95d79255f741daa1230692e8066/stl/inc/variant#L1638-L1657">https://github.com/microsoft/STL/blob/65d98ffabab3a95d79255f741daa1230692e8066/stl/inc/variant#L1638-L1657</a></p>
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