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<title>Issue 3022: is_convertible&lt;derived*, base*&gt; may lead to ODR</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="3022"><a href="lwg-defects.html#3022">3022</a>. <code>is_convertible&lt;derived*, base*&gt;</code> may lead to ODR</h3>
<p><b>Section:</b> 21.3.8 <a href="https://wg21.link/meta.rel">[meta.rel]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2017-09-24 <b>Last modified:</b> 2018-11-12</p>
<p><b>Priority: </b>2
</p>
<p><b>View other</b> <a href="lwg-index-open.html#meta.rel">active issues</a> in [meta.rel].</p>
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<p><b>Discussion:</b></p>
<p>
Given two incomplete types, <code>base</code> and <code>derived</code>, that will have
the expected base/derived relationship when complete, the trait
<code>is_convertible</code> claims to support instantiation with pointers to
these types (as pointers to incomplete types are, themselves,
complete), yet will give a different answer when the types are
complete vs. when they are incomplete.
<p/>
We should require pointers (and pointers to pointers etc.)
point to a complete type, unless one is a pointer to
<i>cv</i>-<code>void</code>. We may also want some weasel-wording to
permit pointers to arrays-of-unknown-bound, and pointers
to <i>cv</i>-qualified variants of the same incomplete type.
</p>

<p><i>[2017-11 Albuquerque Wednesday night issues processing]</i></p>

<p>Priority set to 2</p>

<p><i>[2018-08 Batavia Monday issue discussion]</i></p>

<p>Issues <a href="lwg-defects.html#2797" title="Trait precondition violations (Status: Resolved)">2797</a><sup><a href="https://cplusplus.github.io/LWG/issue2797" title="Latest snapshot">(i)</a></sup>, <a href="lwg-active.html#2939" title="Some type-completeness constraints of traits are overspecified (Status: Open)">2939</a><sup><a href="https://cplusplus.github.io/LWG/issue2939" title="Latest snapshot">(i)</a></sup>, <a href="lwg-defects.html#3022" title="is_convertible&lt;derived*, base*&gt; may lead to ODR (Status: Resolved)">3022</a><sup><a href="https://cplusplus.github.io/LWG/issue3022" title="Latest snapshot">(i)</a></sup>, and <a href="lwg-active.html#3099" title="is_assignable&lt;Incomplete&amp;, Incomplete&amp;&gt; (Status: Open)">3099</a><sup><a href="https://cplusplus.github.io/LWG/issue3099" title="Latest snapshot">(i)</a></sup> are all closely related. Walter to write a paper resolving them.</p>

<p><i>[2018-11-11 Resolved by <a href="https://wg21.link/P1285R0">P1285R0</a>, adopted in San Diego.]</i></p>



<p id="res-3022"><b>Proposed resolution:</b></p>





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