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<title>Issue 2902: variant should only support complete types</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#NAD">NAD</a> status.</em></p>
<h3 id="2902"><a href="lwg-closed.html#2902">2902</a>. <code>variant</code> should only support complete types</h3>
<p><b>Section:</b> 22.6.3 <a href="https://wg21.link/variant.variant">[variant.variant]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> United States <b>Opened:</b> 2017-02-03 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#variant.variant">issues</a> in [variant.variant].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<b>Addresses US 114</b>

<p><code>variant</code> needs to know the size of an object in order to compute the size of its internal buffer, so 
require that any <i>cv</i>-qualified object type in <code>Types...</code> be a complete type.</p>
<p>Proposed change: Add 'complete' in p2</p>

<p><i>[2017-07-01, Daniel comments]</i></p>

<p>
I recommend to close this issue as NAD: According to 16.4.5.8 <a href="https://wg21.link/res.on.functions">[res.on.functions]</a> p2 plus bullet (2.5):
</p>
<blockquote>
<p>
-2- In particular, the effects are undefined in the following cases:
</p>
<ol style="list-style-type: none">
<li><p>&mdash; [&hellip;]</p></li>
<li><p>(2.5) &mdash; if an incomplete type (6.9) is used as a template argument when instantiating a template component,
unless specifically allowed for that component.</p></li>
</ol>
</blockquote>
<p>
In other words: The standard currently already requires type completeness for all types in <code>Types...</code> of
<code>std::variant&lt;Types...&gt;</code>, we therefore should not repeat that again (Note that other templates &mdash; such as 
<code>shared_ptr</code> &mdash; <em>grant</em> type <b>in</b>completeness allowance for that reason).
<p/>
The criteria for the alternative resolution form <a href="lwg-active.html#Resolved">Resolved</a> do not apply for this case, 
because the quoted constraint do exist since C++98 (see 17.4.3.6 [lib.res.on.functions] p2 b5), so the issue was not 
even an issue at the point of the submission.
</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>This wording is relative to <a href="https://wg21.link/N4618">N4618</a>.</p>
<ol>
<li><p>Edit 22.6.3 <a href="https://wg21.link/variant.variant">[variant.variant]</a>/2 as indicated:</p>
<blockquote>
All types in <code>Types...</code> shall be (possibly <i>cv</i>-qualified)<ins> complete</ins> object types that are not arrays.
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2017-07-06 Moved to Tentatively NAD after 5 positive votes on c++std-lib.]</i></p>



<p id="res-2902"><b>Proposed resolution:</b></p>
<p>
The lack of the explicit constraint in 22.6.3 <a href="https://wg21.link/variant.variant">[variant.variant]</a> is not a defect, because the general requirements in 
16.4.5.8 <a href="https://wg21.link/res.on.functions">[res.on.functions]</a> p2 already impose the requested type completeness constraints.
</p>





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