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<title>Issue 3929: Preconditions for type traits should be Mandates</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#New">New</a> status.</em></p>
<h3 id="3929"><a href="lwg-active.html#3929">3929</a>. <i>Preconditions</i> for type traits should be <i>Mandates</i></h3>
<p><b>Section:</b> 21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a>, 21.3.8 <a href="https://wg21.link/meta.rel">[meta.rel]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2023-05-01 <b>Last modified:</b> 2023-06-12</p>
<p><b>Priority: </b>3
</p>
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<p><b>Discussion:</b></p>
<p>
Since we have adopted the <i>Constraints</i>/<i>Mandates</i>/<i>Preconditions</i> form of wording,
"preconditions" refer to runtime requirements, not compile-time.  As such, the
column labeled "preconditions" in Table 47: Type property predicates [tab:meta.unary.prop]
would be better labeled as "Mandates".
<p/>
This is an LWG issue and not editorial, and "Mandates" would require the library
to diagnose violations, but after reviewing all traits in this table, I believe that is
reasonable.
<p/>
Table 48: Type property queries [tab:meta.unary.prop.query] shows how <i>Mandates:</i>
elements can be integrated into the "Value" column if we preferred that approach, but for the
number of entries in the first table seems like an aggressive change for consistency.
<p/>
Similarly, for Table 49: Type relationship predicates [tab:meta.rel] the "Comments"
column serves as a "Mandates" feature without using that term, so I suggest changing
that column title too.
<p/>
The other tables and type traits wording already appear to be adapted to the Mandates
wording style.
</p>

<p><i>[2023-05-24; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>
<p>
See also issue <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>. We should not turn the preconditions into
<i>Mandates</i> without fixing them first.
</p>

<p><i>[2023-06-12; Varna]</i></p>

<p>During the review <a href="https://wg21.link/P2874R1" title=" Mandating Annex D">P2874R1</a> the group decided to not change the now decided for
<i>Preconditions:</i> element in D.13 <a href="https://wg21.link/depr.meta.types">[depr.meta.types]</a> p3 into a <i>Mandates:</i> element 
but would like to solve this by this issue.</p>


<p id="res-3929"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4944" title=" Working Draft, Standard for Programming Language C++">N4944</a>.
</p>

<ol>
<li>
<p>Change in 21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a>, Table 47: Type property predicates [tab:meta.unary.prop] 
the column title "<b>Preconditions</b>" to "<b>Mandates</b>".</p>
</li>

<li>
<p>Change in 21.3.8 <a href="https://wg21.link/meta.rel">[meta.rel]</a>, Table 49: Type relationship predicates [tab:meta.rel] the column 
title "<b>Comments</b>" to "<b>Mandates</b>".</p>
</li>

</ol>





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