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<title>Issue 3140: COMMON_REF is unimplementable as specified</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="3140"><a href="lwg-defects.html#3140">3140</a>. <code><i>COMMON_REF</i></code> is unimplementable as specified</h3>
<p><b>Section:</b> 21.3.9.7 <a href="https://wg21.link/meta.trans.other">[meta.trans.other]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2018-07-07 <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#meta.trans.other">issues</a> in [meta.trans.other].</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>
21.3.9.7 <a href="https://wg21.link/meta.trans.other">[meta.trans.other]</a>/3.2 states:
<blockquote>
[Let] <code><i>XREF</i>(A)</code> denote a unary class template <code>T</code> such that <code>T&lt;U&gt;</code>
denotes the same type as <code>U</code> with the addition of <code>A</code>’s cv and reference qualifiers,
for a non-reference cv-unqualified type <code>U</code>,
</blockquote>
which is nonsensical: a specialization of a <em>class</em> template cannot possibly be a cv-qualified
type or reference type. <code><i>XREF</i>(A)</code> must be a unary alias template.
</p>

<p><i>[2018-09-11; Status set to Tentatively Ready after five positive votes on the reflector]</i></p>

<p><i>[2018-11, Adopted in San Diego]</i></p>



<p id="res-3140"><b>Proposed resolution:</b></p>

<p>This wording is relative to <a href="https://wg21.link/n4762">N4762</a>.</p>

<p>Change 21.3.9.7 <a href="https://wg21.link/meta.trans.other">[meta.trans.other]</a> as indicated:</p>
<blockquote>
<p>
-3- Let:
</p>
<blockquote>
<p>
(3.1) &mdash; <code><i>CREF</i>(A)</code> be <code>add_lvalue_reference_t&lt;const remove_reference_t&lt;A&gt;&gt;</code>,
</p><p>
(3.2) &mdash; <code><i>XREF</i>(A)</code> denote a unary <del>class</del><ins>alias</ins> template <code>T</code>
such that <code>T&lt;U&gt;</code> denotes the same type as <code>U</code> with the addition of <code>A</code>’s
cv and reference qualifiers, for a non-reference cv-unqualified type <code>U</code>,
</p><p>
(3.3) &mdash; <code><i>COPYCV</i>(FROM, TO)</code> be an alias for type <code>TO</code> with the addition of
<code>FROM</code>’s top-level cv-qualifiers. [<i>Example</i>: <code><i>COPYCV</i>(const int, volatile short)</code>
is an alias for <code>const volatile short</code>. &mdash;<i>end example</i>]
</p>
</blockquote>
</blockquote>






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