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<title>Issue 3136: [fund.ts.v3] LFTSv3 awkward wording in propagate_const requirements</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="3136"><a href="lwg-defects.html#3136">3136</a>. [fund.ts.v3] LFTSv3 awkward wording in <code>propagate_const</code> requirements</h3>
<p><b>Section:</b> 6.1.2.3 <a href="https://cplusplus.github.io/fundamentals-ts/v3.html#propagate_const.class_type_requirements">[fund.ts.v3::propagate_const.class_type_requirements]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Thomas K&ouml;ppe <b>Opened:</b> 2018-07-02 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>3
</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><b>Addresses: fund.ts.v3</b></p>
<p>
In the LFTSv3 prospective-working-paper <a href="https://wg21.link/n4758">N4758</a>, [propagate_const.class_type_requirements] 
uses a strange turn of phrase:
</p>
<blockquote><p>
"In this sub-clause, <code>t</code> denotes a non-<code>const</code> lvalue of type <code>T</code>, <code>ct</code> is a <code>const T&amp;</code> 
bound to <code>t</code>, [&hellip;]"
</p></blockquote>
<p>
The last bit is strange: "<code>ct</code> is a <code>const T&amp;</code> bound to <code>t</code>" is not how we usually say things. 
The specification-variables usually denote values, and values can't be references. Perhaps we could just say, 
"<code>ct</code> is <code>as_const(t)</code>"?
</p>

<p><i>[2018-07-20 Priority set to 3 after reflector discussion]</i></p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/n4758">N4758</a>.
</p>

<ol>
<li>
<p>
Edit 6.1.2.3 <a href="https://cplusplus.github.io/fundamentals-ts/v3.html#propagate_const.class_type_requirements">[fund.ts.v3::propagate_const.class_type_requirements]</a> as indicated:
</p>
<blockquote>
<p>
-1- If <code>T</code> is class type then it shall satisfy the following requirements. In this sub-clause <code>t</code> denotes 
a non-<code>const</code> lvalue of type <code>T</code>, <code>ct</code> is <del>a 
<code>const T&amp;</code> bound to <code>t</code></del><ins><code>as_const(t)</code></ins>, <code>element_type</code> denotes an object type. 
</p>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2022-10-12; Jonathan provides improved wording]</i></p>


<p><i>[2022-10-19; Reflector poll]</i></p>

<p>
Set status to "Tentatively Ready" after eight votes in favour in reflector poll.
</p>

<p><i>[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3136"><b>Proposed resolution:</b></p>

<p>
This wording is relative to <a href="https://wg21.link/N4920" title=" Working Draft, C++ Extensions for Library Fundamentals, Version 3">N4920</a>.
</p>

<ol>
<li>
<p>
Edit 6.1.2.2 <a href="https://cplusplus.github.io/fundamentals-ts/v3.html#propagate_const.requirements">[fund.ts.v3::propagate_const.requirements]</a> as indicated:
</p>
<blockquote>
<p>-1-
<code>T</code> shall be
<ins>a cv-unqualified pointer-to-object type</ins>
<del>an object pointer type</del>
or a <ins>cv-unqualified</ins>
class type for which
<code>decltype(*declval&lt;T&amp;&gt;())</code> is an lvalue reference
<ins>to object type</ins>;
otherwise the program is ill-formed.
</p>
<p><del>-2-
If <code>T</code> is an array type, reference type, pointer to function type or
pointer to (possibly cv-qualified) <code>void</code>, then the program is
ill-formed.
</del>
</p>
<p>-3-
[<em>Note:</em>
<code>propagate_const&lt;const int*&gt;</code> is well-formed
<ins>but propagate_const&lt;int* const&gt; is not.</ins>
&mdash; <em>end note</em>]
</p>
</blockquote>
</li>

<li>
<p>
Edit 6.1.2.3 <a href="https://cplusplus.github.io/fundamentals-ts/v3.html#propagate_const.class_type_requirements">[fund.ts.v3::propagate_const.class_type_requirements]</a> as indicated:
</p>
<blockquote>
<p>-1-
If <code>T</code> is class type then it shall satisfy the following requirements.
In this sub-clause <code>t</code> denotes
<del>a non-<code>const</code></del><ins>an</ins>
lvalue of type <code>T</code>, <code>ct</code>
<del>is a <code>const T&amp;</code> bound to <code>t</code>, <code>element_type</code> denotes an object type.</del>
<ins>denotes <code>as_const(t)</code>.</ins>
</p>
</blockquote>
</li>
</ol>






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