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<title>Issue 3572: copyable-box should be fully constexpr</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#C++23">C++23</a> status.</em></p>
<h3 id="3572"><a href="lwg-defects.html#3572">3572</a>. <code><i>copyable-box</i></code> should be fully <code>constexpr</code></h3>
<p><b>Section:</b> 25.7.3 <a href="https://wg21.link/range.move.wrap">[range.move.wrap]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2021-06-19 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.move.wrap">issues</a> in [range.move.wrap].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<a href="https://wg21.link/P2231R1" title=" Add further constexpr support for optional/variant">P2231R1</a> made <code>optional</code> fully <code>constexpr</code>, but missed its cousin
<code><i>semiregular-box</i></code> (which was renamed to <code><i>copyable-box</i></code> by 
<a href="https://wg21.link/P2325R3" title=" Views should not be required to be default constructible">P2325R3</a>). Most operations of <code><i>copyable-box</i></code> are already 
<code>constexpr</code> simply because <code><i>copyable-box</i></code> is specified in terms of <code>optional</code>;
the only missing ones are the assignment operators layered on top when the wrapped type
isn't assignable itself.
</p>

<p><i>[2021-06-23; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after eight votes in favour during reflector poll.
</p>

<p><i>[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3572"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

<ol>
<li><p>Modify  [range.copy.wrap] as indicated:</p>

<blockquote>
<p>
-1- Many types in this subclause are specified in terms of an exposition-only class template <code><i>copyable-box</i></code>.
<code><i>copyable-box</i>&lt;T&gt;</code> behaves exactly like <code>optional&lt;T&gt;</code> with the following differences:
</p>
<ol style="list-style-type: none">
<li><p>(1.1) &mdash; [&hellip;]</p></li>
<li><p>(1.2) &mdash; [&hellip;]</p></li>
<li><p>(1.3) &mdash; If <code>copyable&lt;T&gt;</code> is not modeled, the copy assignment operator is equivalent to:</p>
<blockquote>
<pre>
<ins>constexpr</ins> <i>copyable-box</i>&amp; operator=(const <i>copyable-box</i>&amp; that)
  noexcept(is_nothrow_copy_constructible_v&lt;T&gt;) {
  if (this != addressof(that)) {
    if (that) emplace(*that);
    else reset();
  }
  return *this;
}
</pre>
</blockquote>
</li>
<li><p>(1.4) &mdash; If <code>movable&lt;T&gt;</code> is not modeled, the move assignment operator is equivalent to:</p>
<blockquote>
<pre>
<ins>constexpr</ins> <i>copyable-box</i>&amp; operator=(<i>copyable-box</i>&amp;&amp; that)
  noexcept(is_nothrow_move_constructible_v&lt;T&gt;) {
  if (this != addressof(that)) {
    if (that) emplace(std::move(*that));
    else reset();
  }
  return *this;
}
</pre>
</blockquote>
</li>
</ol>
</blockquote>
</li>

</ol>




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