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<title>Issue 3930: Simplify type trait wording</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#NAD">NAD</a> status.</em></p>
<h3 id="3930"><a href="lwg-closed.html#3930">3930</a>. Simplify type trait wording</h3>
<p><b>Section:</b> 21 <a href="https://wg21.link/meta">[meta]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2023-05-01 <b>Last modified:</b> 2024-06-24</p>
<p><b>Priority: </b>Not Prioritized
</p>
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<p><b>Discussion:</b></p>
<p>
There are many traits that have a requirement that they are instantiated only if
"<code>T</code> shall be a complete type, <i>cv</i> <code>void</code>, or an array of unknown bound."
<p/>
Breaking down what this means, by supporting <i>cv</i>-<code>void</code> and arrays of unknown
bound (almost) the only remaining type-category is incomplete class types.
<p/>
The remaining edge case is incomplete enumerations, but they are required to
have a known fixed-base, so act as complete types, they can be copied,
assigned, etc., without knowing the names of their enumerators.
<p/>
Hence, I suggest clearer wording would be:
"<code>T</code> shall not be an incomplete class type."
<p/>
This is easier to understand, as we do not need to mentally enumerate every
type against a list to check it qualifies; it is a simpler test for the library to
check if we were to mandate these restrictions.
<p/>
There are a very small number of traits with subtly different wording, where
incomplete unions are supported, or arrays of unknown bound are not a
concern due to invoking <code>remove_all_extents</code> first. The bulk of the changes
can be made to traits with only the precise wording above though, and then
we can review whether any of the remaining restrictions deserve a wording
update of their own.
</p>

<p><i>[2023-06-01; Reflector poll]</i></p>

<p>
Set status to Tentatively NAD after four votes in favour during reflector poll,
including a request to withdraw the issue from the submitter.
</p>
<p>
Incomplete enumeration types are found within the <i>enum-specifier</i>
of an enum without a fixed underlying type:
<pre><code>
enum E {
    A = sizeof(E) // error, E is incomplete at this point
};
</code></pre>
and we definitely can't provide an underlying type for this case.
</p>

<p><i>[St. Louis 2024-06-24 Status changed: Tentatively NAD &rarr; NAD.]</i></p>



<p id="res-3930"><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>
Throughout 21 <a href="https://wg21.link/meta">[meta]</a> replace all occurrences of
</p>
<blockquote><p>
<code>T</code> shall be a complete type, <i>cv</i> <code>void</code>, or an array of unknown bound.
</p></blockquote>
<p>
by
</p>
<blockquote><p>
<code>T</code> shall not be an incomplete class type.
</p></blockquote>

</li>

</ol>





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