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<title>Issue 3627: Inconsistent specifications for std::make_optional overloads</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#New">New</a> status.</em></p>
<h3 id="3627"><a href="lwg-active.html#3627">3627</a>. Inconsistent specifications for <code>std::make_optional</code> overloads</h3>
<p><b>Section:</b> 22.5.10 <a href="https://wg21.link/optional.specalg">[optional.specalg]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2021-10-23 <b>Last modified:</b> 2022-01-29</p>
<p><b>Priority: </b>3
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Three <code>std::make_optional</code> overloads are specified in 22.5.10 <a href="https://wg21.link/optional.specalg">[optional.specalg]</a>. 
The first one is specified by "<i>Returns:</i>" and the other two are specified by "<i>Effects:</i> Equivalent to:".
According to 16.3.2.4 <a href="https://wg21.link/structure.specifications">[structure.specifications]</a>/4, such uses of "<i>Effects:</i> Equivalent to:" 
propagate the <i>Constraints</i> specified for constructors. As the selected constructor for the first 
overload has "<i>Constraints:</i>" (22.5.3.2 <a href="https://wg21.link/optional.ctor">[optional.ctor]</a>/22), it seems that inconsistency is introduced here.
<p/>
Existing implementations are inconsistent: libstdc++ constrains all three overloads, 
while libc++ and MSVC STL do not constrain any of them.
<p/>
IMO all three overloads should be constrained.
</p>

<p><i>[2022-01-29; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>



<p id="res-3627"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4901">N4901</a>.
</p>

<ol>
<li><p>Modify 22.5.10 <a href="https://wg21.link/optional.specalg">[optional.specalg]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class T&gt; constexpr optional&lt;decay_t&lt;T&gt;&gt; make_optional(T&amp;&amp; v);
</pre>
<blockquote>
<p>
-3- <i><del>Returns</del><ins>Effects</ins>:</i> <ins>Equivalent to: </ins><code><ins>return </ins>optional&lt;decay_t&lt;T&gt;&gt;(std::forward&lt;T&gt;(v))<ins>;</ins></code><del>.</del>
</p>
</blockquote>
</blockquote>
</li>
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