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<title>Issue 2991: variant copy constructor missing noexcept(see below)</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#Open">Open</a> status.</em></p>
<h3 id="2991"><a href="lwg-active.html#2991">2991</a>. <code>variant</code> copy constructor missing <code>noexcept(<i>see below</i>)</code></h3>
<p><b>Section:</b> 22.6.3.2 <a href="https://wg21.link/variant.ctor">[variant.ctor]</a> <b>Status:</b> <a href="lwg-active.html#Open">Open</a>
 <b>Submitter:</b> Peter Dimov <b>Opened:</b> 2017-06-27 <b>Last modified:</b> 2024-11-19</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#variant.ctor">active issues</a> in [variant.ctor].</p>
<p><b>View all other</b> <a href="lwg-index.html#variant.ctor">issues</a> in [variant.ctor].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Open">Open</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The copy constructor of <code>std::variant</code> is not conditionally <code>noexcept</code> (I think
it was in the original proposal.)
<p/>
It should be, for two reasons: first, this would be consistent with the other three constructors
</p>
<blockquote><pre>
constexpr variant() noexcept(<i>see below</i>);

variant(variant&amp;&amp;) noexcept(<i>see below</i>);

template &lt;class T&gt;
constexpr variant(T&amp;&amp;) noexcept(<i>see below</i>);
</pre></blockquote>
<p>
and second, <code>variant</code> itself makes use of <code>is_nothrow_copy_constructible</code>, so
it's inconsistent for it to take a stance against it.
</p>

<p><i>[2017-07 Toronto Tuesday PM issue prioritization]</i></p>

<p>Status to LEWG</p>

<p><i>[Wrocław 2024-11-18; LEWG approves the direction]</i></p>

<p>
In <a href="https://wg21.link/P0088R1" title=" Variant: a type-safe union that is rarely invalid (v6)">P0088R1</a> the copy constructor was conditionally noexcept
in the synopsis, but not the detailed description. This was pointed out
during LWG review in Jacksonville.
The approved paper, <a href="https://wg21.link/P008R3">P008R3</a>, doesn't have it in either place.
</p>


<p id="res-2991"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4659">N4659</a>.
</p>

<ol>
<li><p>Edit 22.6.3 <a href="https://wg21.link/variant.variant">[variant.variant]</a>, class template <code>variant</code> synopsis, as indicated:</p>

<blockquote>
<pre>
template &lt;class... Types&gt;
  class variant {
  public:
    // 23.7.3.1, constructors
    constexpr variant() noexcept(<i>see below</i>);
    variant(const variant&amp;) <ins>noexcept(<i>see below</i>)</ins>;
    variant(variant&amp;&amp;) noexcept(<i>see below</i>);
    [&hellip;]
  };
</pre>
</blockquote>
</li>

<li><p>Edit 22.6.3.2 <a href="https://wg21.link/variant.ctor">[variant.ctor]</a> as indicated:</p>

<blockquote>
<pre>
variant(const variant&amp; w) <ins>noexcept(<i>see below</i>)</ins>;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-8- <i>Remarks:</i> This function shall not participate in overload resolution unless 
<code>is_copy_constructible_v&lt;T<sub><i>i</i></sub>&gt;</code> is <code>true</code> for all <code><i>i</i></code>.
<ins>The expression inside <code>noexcept</code> is equivalent to the logical AND of
<code>is_nothrow_copy_constructible_v&lt;T<sub><i>i</i></sub>&gt;</code> for all <code><i>i</i></code>.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>




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