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<title>Issue 3461: convertible_to's description mishandles cv-qualified void</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++23">C++23</a> status.</em></p>
<h3 id="3461"><a href="lwg-defects.html#3461">3461</a>. <code>convertible_to</code>'s description mishandles <i>cv</i>-qualified <code>void</code></h3>
<p><b>Section:</b> 18.4.4 <a href="https://wg21.link/concept.convertible">[concept.convertible]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2020-07-03 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>0
</p>
<p><b>View other</b> <a href="lwg-index-open.html#concept.convertible">active issues</a> in [concept.convertible].</p>
<p><b>View all other</b> <a href="lwg-index.html#concept.convertible">issues</a> in [concept.convertible].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
There are no expressions of type <i>cv</i>-qualified <code>void</code> because any such
expression must be prvalues and 7.2.2 <a href="https://wg21.link/expr.type">[expr.type]</a>/2 states:
</p>
<blockquote><p>
If a prvalue initially has the type "<i>cv</i> <code>T</code>", where <code>T</code> is a
<i>cv</i>-unqualified non-class, non-array type, the type of the expression is adjusted 
to <code>T</code> prior to any further analysis.
</p></blockquote>
<p>
However, 18.4.4 <a href="https://wg21.link/concept.convertible">[concept.convertible]</a> p1 states:
</p>
<blockquote><p>
Given types <code>From</code> and <code>To</code> and an expression <code>E</code> such that 
<code>decltype((E))</code> is <code>add_rvalue_reference_t&lt;From&gt;</code>, 
<code>convertible_to&lt;From, To&gt;</code> requires <code>E</code> to be both implicitly and 
explicitly convertible to type <code>To</code>.
</p></blockquote>
<p>
When <code>From</code> is <i>cv</i>-qualified <code>void</code>, <code>E</code> does not exist, yet we do want
<code>convertible_to&lt;const void, void&gt;</code> to be modeled.
</p>

<p><i>[2020-07-12; Reflector prioritization]</i></p>

<p>
Set priority to 0 and status to Tentatively Ready after five votes in favour during reflector discussions.
</p>
<p><i>[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3461"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>. 
</p>

<ol>
<li><p>Modify 18.4.4 <a href="https://wg21.link/concept.convertible">[concept.convertible]</a> as indicated:</p>

<blockquote>
<p>
-1- Given types <code>From</code> and <code>To</code> and an expression <code>E</code> <del>such that <code>decltype((E))</code> 
is <code>add_rvalue_reference_t&lt;From&gt;</code></del><ins>whose type and value category are the same as those of 
<code>declval&lt;From&gt;()</code></ins>, <code>convertible_to&lt;From, To&gt;</code> requires <code>E</code> 
to be both implicitly and explicitly convertible to type <code>To</code>. The implicit and explicit conversions 
are required to produce equal results.
</p>
</blockquote>
</li>
</ol>





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