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<title>Issue 3196: std::optional&lt;T&gt; is ill-formed is T is an array</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++20">C++20</a> status.</em></p>
<h3 id="3196"><a href="lwg-defects.html#3196">3196</a>. <code>std::optional&lt;T&gt;</code> is ill-formed is <code>T</code> is an array</h3>
<p><b>Section:</b> 22.5.3 <a href="https://wg21.link/optional.optional">[optional.optional]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2019-03-18 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#optional.optional">issues</a> in [optional.optional].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
22.5.3 <a href="https://wg21.link/optional.optional">[optional.optional]</a> appears to allow arrays:
</p>
<blockquote><p>
<code>T</code> shall be an object type other than <i>cv</i> <code>in_place_t</code> or <i>cv</i> <code>nullopt_t</code> and shall 
satisfy the <i>Cpp17Destructible</i> requirements (Table 30).
</p></blockquote>
<p>
But instantiating it fails, because <code>value_or</code> attempts to return <code>T</code> by value, which isn't possible for an array type.
<p/>
Existing practice seems to be to reject array types. Libstdc++ and libc++ give an error for the signature of <code>value_or</code>, 
and MSVC fails a static assert saying the type needs to be destructible (which is misleading, because <code>int[2]</code> 
is destructible, but either way it's ill-formed).
</p>

<strong>Previous resolution [SUPERSEDED]:</strong>
<blockquote class="note">
<p>This wording is relative to <a href="https://wg21.link/n4810">N4810</a>.</p>

<ol>
<li><p>Modify 22.5.3 <a href="https://wg21.link/optional.optional">[optional.optional]</a> as indicated:</p>

<blockquote>
<p>
-3- <code>T</code> shall be a<del>n</del> <ins>non-array</ins> object type other than <i>cv</i> <code>in_place_t</code> 
or <i>cv</i> <code>nullopt_t</code> and shall satisfy the <i>Cpp17Destructible</i> requirements (Table 30).
</p>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2019-03-26 Marshall provides updated resolution based on reflector discussion]</i></p>

<p><i>[2019-06-16 Moved to "Tentatively Ready" based on five positive votes on the reflector]</i></p>



<p id="res-3196"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4810">N4810</a>.</p>

<ol>
<li><p>In 16.4.4.2 <a href="https://wg21.link/utility.arg.requirements">[utility.arg.requirements]</a>, edit Table 30 &mdash; "<i>Cpp17Destructible</i> requirements" as indicated:</p>

<blockquote>

<table border="1">
<caption>Table 30 &mdash; <i>Cpp17Destructible</i> requirements</caption>
<tr>
<th>Expression</th>
<th>Post-condition</th>
</tr>
<tr>
<td>
<code>u.~T()</code>
</td>
<td>
All resources owned by <code>u</code> are reclaimed, no exception is propagated.
</td>
</tr>
<tr>
<td colspan="2">
<ins>[<i>Note:</i> Array types and non-object types are not <i>Cpp17Destructible</i>. &mdash; <i>end note</i>]</ins>
</td>
</tr>
</table>
</blockquote>

</li>
</ol>

<ol>
<li><p>Modify 22.5.3 <a href="https://wg21.link/optional.optional">[optional.optional]</a> as indicated:</p>

<blockquote>
<p>
-3- <code>T</code> shall be a <del>object</del> type other than <i>cv</i> <code>in_place_t</code> 
or <i>cv</i> <code>nullopt_t</code> <del>and shall satisfy</del><ins>that meets</ins> the 
<i>Cpp17Destructible</i> requirements (Table 30).
</p>
</blockquote>
</li>
</ol>

<ol>
<li><p>Modify 22.6.3 <a href="https://wg21.link/variant.variant">[variant.variant]</a> as indicated:</p>

<blockquote>
<p>
-2-All types in <code>Types</code> shall <del>be (possibly cv-qualified) object types that 
are not arrays</del><ins>meet the <i>Cpp17Destructible</i> requirements (Table 30).</ins>
</p>
</blockquote>
</li>
</ol>




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