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<title>Issue 3479: semiregular-box mishandles self-assignment</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="3479"><a href="lwg-defects.html#3479">3479</a>. <code><i>semiregular-box</i></code> mishandles self-assignment</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#Resolved">Resolved</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2020-08-25 <b>Last modified:</b> 2023-02-07</p>
<p><b>Priority: </b>3
</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#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The exposition-only wrapper type <code><i>semiregular-box</i></code> &mdash; specified in  [range.semi.wrap]
&mdash; layers behaviors onto <code>std::optional</code> so <code><i>semiregular-box</i>&lt;T&gt;</code> is
semiregular even when <code>T</code> is only copy constructible. It provides copy and move assignment
operators when <code>optional&lt;T&gt;</code>'s are deleted:
</p>
<blockquote><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>assignable_from&lt;T&amp;, const T&amp;&gt;</code>
is not modeled, the copy assignment operator is equivalent to:</p>
<pre>
<i>semiregular-box</i>&amp; operator=(const <i>semiregular-box</i>&amp; that)
  noexcept(is_nothrow_copy_constructible_v&lt;T&gt;)
{
  if (that) emplace(*that);
  else reset();
  return *this;
}
</pre>
</li>
<li><p>(1.4) &mdash; If <code>assignable_from&lt;T&amp;, T&gt;</code>
is not modeled, the move assignment operator is equivalent to:</p>
<pre>
<i>semiregular-box</i>&amp; operator=(<i>semiregular-box</i>&amp;&amp; that)
  noexcept(is_nothrow_move_constructible_v&lt;T&gt;)
{
  if (that) emplace(std::move(*that));
  else reset();
  return *this;
}
</pre>
</li>
</ol>
</p></blockquote>
<p>
How do these assignment operators handle self-assignment? When <code>*this</code> is empty, <code>that</code>
will test as <code>false</code> and <code>reset()</code> has no effect, so the result state of the object is
the same. No problems so far. When <code>*this</code> isn't empty, <code>that</code> will test as <code>true</code>,
and we evaluate <code>optional::emplace(**this)</code> (resp. <code>optional::emplace(std::move(**this)))</code>.
This outcome is not as pretty: <code>emplace</code> is specified in 22.5.3.4 <a href="https://wg21.link/optional.assign">[optional.assign]</a>/30:
"<i>Effects:</i> Calls <code>*this = nullopt</code>. Then initializes the contained value as if
direct-non-list-initializing an object of type <code>T</code> with the arguments
<code>std::forward&lt;Args&gt;(args)...</code>." When the sole argument is an lvalue (resp. xvalue) of type
<code>T</code> that denotes the <code>optional</code>'s stored value, <code>emplace</code> will destroy that
stored value and then try to copy/move construct a new object at the same address from the dead object
that used to live there resulting in undefined behavior. Mandatory undefined behavior does <em>not</em>
meet the semantic requirements for the <code>copyable</code> or <code>movable</code> concepts, we should do better.
</p>

<p><i>[2020-09-13; Reflector prioritization]</i></p>

<p>
Set priority to 3 during reflector discussions.
</p>
<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.
</p>

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

<blockquote>
<p>
-1- Many types in this subclause are specified in terms of an exposition-only class template
<code><i>semiregular-box</i></code>. <code><i>semiregular-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>assignable_from&lt;T&amp;, const T&amp;&gt;</code>
is not modeled, the copy assignment operator is equivalent to:</p>
<pre>
<i>semiregular-box</i>&amp; operator=(const <i>semiregular-box</i>&amp; that)
  noexcept(is_nothrow_copy_constructible_v&lt;T&gt;)
{
  <ins>if (this != addressof(that)) {</ins>
    if (that) emplace(*that);
    else reset();
  <ins>}</ins>
  return *this;
}
</pre>
</li>
<li><p>(1.4) &mdash; If <code>assignable_from&lt;T&amp;, T&gt;</code>
is not modeled, the move assignment operator is equivalent to:</p>
<pre>
<i>semiregular-box</i>&amp; operator=(<i>semiregular-box</i>&amp;&amp; that)
  noexcept(is_nothrow_move_constructible_v&lt;T&gt;)
{
  <ins>reset();</ins>
  if (that) emplace(std::move(*that));
  <del>else reset();</del>
  return *this;
}
</pre>
</li>
</ol>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2021-06-13 Resolved by the adoption of <a href="https://wg21.link/P2325R3" title=" Views should not be required to be default constructible">P2325R3</a> at the June 2021 plenary. Status changed: New &rarr; Resolved.]</i></p>



<p id="res-3479"><b>Proposed resolution:</b></p>





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