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<title>Issue 3052: visit is underconstrained</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#Resolved">Resolved</a> status.</em></p>
<h3 id="3052"><a href="lwg-defects.html#3052">3052</a>. <code>visit</code> is underconstrained</h3>
<p><b>Section:</b> 22.6.7 <a href="https://wg21.link/variant.visit">[variant.visit]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2018-01-23 <b>Last modified:</b> 2021-05-18</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#variant.visit">issues</a> in [variant.visit].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>std::visit</code> accepts a parameter pack of forwarding references named
<code>vars</code> whose types are the parameter pack <code>Variants</code>. Despite that:
<ol>
<li>the names of both packs are variants of "variant",</li>
<li>para 1 passes the types in <code>Variants</code> (modified) to <code>variant_size_v</code>,</li>
<li>para 3 uses the expression <code>vars<sub><i>i</i></sub>.index()</code>,</li>
<li>para 4 says "...if any <code>variant</code> in <code>vars</code> is <code>valueless_by_exception</code>,
and</li>
<li>para 5 mentions "..the number of alternative types of <code>Variants<sub>0</sub></code>"</li>
</ol>
the <i>Requires</i> element imposes no explicit requirements on the types in <code>Variants</code>.
Notably, the <code>Variants</code> are not required to be <code>variant</code>s. This lack of constraints
appears to be simply an oversight.
</p>

<p><i>[2018-01-24, Daniel comments]</i></p>

<p>
This issue should be reviewed in common with LWG <a href="lwg-defects.html#2970" title="Return type of std::visit misspecified (Status: C++20)">2970</a><sup><a href="https://cplusplus.github.io/LWG/issue2970" title="Latest snapshot">(i)</a></sup>.
</p>

<p><i>[2018-06-18 after reflector discussion]</i></p>

<p>Priority set to 2; status to LEWG</p>

<p><i>[2020-11-18; this will be resolved by <a href="https://wg21.link/p2162">P2162</a>.]</i></p>

<p><i>[2021-04-19 <a href="https://wg21.link/p2162r2">P2162R2</a> was adopted at February 2021 plenary. Status changed: Tentatively Resolved &rarr; Resolved.]</i></p>



<p id="res-3052"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4727">N4727</a>.</p>

<ol>
<li><p>Modify 22.6.7 <a href="https://wg21.link/variant.visit">[variant.visit]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class Visitor, class... Variants&gt;
  constexpr <i>see below</i> visit(Visitor&amp;&amp; vis, Variants&amp;&amp;... vars);
</pre>
<blockquote>
<p>[&hellip;]</p>
<p>
-4- <i>Throws:</i> <code>bad_variant_access</code> if <del>any <code>variant</code> in <code>vars</code> is
<code>valueless_by_exception()</code></del><ins><code>(vars.valueless_by_exception() || ...)</code> is
<code>true</code></ins>.
</p>
<p>-5- <i>Complexity:</i> [&hellip;]</p>
<p>
<ins>-?- <i>Remarks:</i> This function shall not participate in overload resolution unless
<code>remove_cvref_t&lt;Variants<sub><i>i</i></sub>&gt;</code> is a specialization of <code>variant</code>
for all <code>0 &lt;= i &lt; n</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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