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<title>Issue 3818: Exposition-only concepts are not described in library intro</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="3818"><a href="lwg-defects.html#3818">3818</a>. Exposition-only concepts are not described in library intro</h3>
<p><b>Section:</b> 16.3.3 <a href="https://wg21.link/conventions">[conventions]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2022-11-08 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</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>
This is the resolution for <b><a href="https://github.com/cplusplus/nbballot/issues/435">GB-074</a></b>.
</p>

<p>
The comment is:
</p>

<blockquote>
<p>
 [expos.only.func] introduces exposition-only function templates.  [expos.only.types] introduces
exposition-only types. 16.3.3.6 <a href="https://wg21.link/objects.within.classes">[objects.within.classes]</a> introduces exposition-only private members.
<p/>
There is nothing about exposition-only concepts, despite them being used extensively in the library clauses.
<p/>
The same problem exists for exposition-only variable templates.
</p>
</blockquote>

<p><i>[Kona 2022-11-08; Move to Immediate status]</i></p>


<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<ol>
<li><p>Modify  [expos.only.func] as indicated, changing the stable name:</p>
<blockquote>
<p>
<b>16.3.3.2 Exposition-only <del>functions</del><ins>entities</ins> [expos.only.<del>func</del><ins>entity</ins>]</b>
<p/>
-1- Several <del>function templates</del><ins>entities</ins> defined in 17 <a href="https://wg21.link/support">[support]</a> 
through 32 <a href="https://wg21.link/thread">[thread]</a> and D <a href="https://wg21.link/depr">[depr]</a> are only defined for the purpose of exposition. 
The declaration of such <del>a function</del><ins>an entity</ins> is followed by a comment ending in <i>exposition only</i>.
</p>
</blockquote>
</li>

<li><p>Strike  [expos.only.types] as redundant:</p>

<blockquote>
<p>
<del><b>16.3.3.3.2 Exposition-only types [expos.only.types]</b></del>
<p/>

<del>-1- Several types defined in 17 <a href="https://wg21.link/support">[support]</a> through 32 <a href="https://wg21.link/thread">[thread]</a> and D <a href="https://wg21.link/depr">[depr]</a> 
are defined for the purpose of exposition. The declaration of such a type is followed by a 
comment ending in <i>exposition only</i>.</del>
<p/>
<del><i>[Example 1:</i></del>
</p>
<blockquote>
<pre>
<del>namespace std {
  extern "C" using <i>some-handler</i> = int(int, void*, double);  // exposition only
}</del>
</pre>
</blockquote>

<p>
<del>The type placeholder <i><code>some-handler</code></i> can now be used to specify a 
function that takes a callback parameter with C language linkage. 
<i>&mdash; end example]</i></del>
</p>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2022-11-09 Tim reopens]</i></p>

<p>
During LWG review of <a href="lwg-defects.html#3753" title="Clarify entity vs. freestanding entity (Status: C++23)">3753</a><sup><a href="https://cplusplus.github.io/LWG/issue3753" title="Latest snapshot">(i)</a></sup>, it was pointed out that <i>typedef-name</i>s are not necessarily entities.
</p>

<p><i>[Kona 2022-11-11; Move to Immediate]</i></p>


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



<p id="res-3818"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>
<li><p>Modify  [expos.only.func] as indicated, changing the stable name:</p>

<blockquote>
<p>
<b>16.3.3.2 Exposition-only <del>functions</del><ins>entities, etc.</ins> [expos.only.<del>func</del><ins>entity</ins>]</b>
<p/>
-1- Several <del>function templates</del><ins>entities and <i>typedef-name</i>s</ins> defined in 17 <a href="https://wg21.link/support">[support]</a> 
through 32 <a href="https://wg21.link/thread">[thread]</a> and D <a href="https://wg21.link/depr">[depr]</a> are only defined for the purpose of exposition. 
The declaration of such <del>a function</del><ins>an entity or <i>typedef-name</i></ins> is followed by a comment ending in <i>exposition only</i>.
</p>
</blockquote>
</li>

<li><p>Strike  [expos.only.types] as redundant:</p>

<blockquote>
<p>
<del><b>16.3.3.3.2 Exposition-only types [expos.only.types]</b></del>
<p/>

<del>-1- Several types defined in 17 <a href="https://wg21.link/support">[support]</a> through 32 <a href="https://wg21.link/thread">[thread]</a> and D <a href="https://wg21.link/depr">[depr]</a> 
are defined for the purpose of exposition. The declaration of such a type is followed by a 
comment ending in <i>exposition only</i>.</del>
<p/>
<del><i>[Example 1:</i></del>
</p>
<blockquote>
<pre>
<del>namespace std {
  extern "C" using <i>some-handler</i> = int(int, void*, double);  // exposition only
}</del>
</pre>
</blockquote>

<p>
<del>The type placeholder <i><code>some-handler</code></i> can now be used to specify a 
function that takes a callback parameter with C language linkage. 
<i>&mdash; end example]</i></del>
</p>
</blockquote>
</li>
</ol>





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