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<title>Issue 3198: Bad constraint on std::span::span()</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="3198"><a href="lwg-defects.html#3198">3198</a>. Bad constraint on <code>std::span::span()</code></h3>
<p><b>Section:</b> 23.7.2.2.2 <a href="https://wg21.link/span.cons">[span.cons]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Lars Gullik Bj&oslash;nnes <b>Opened:</b> 2019-04-03 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#span.cons">issues</a> in [span.cons].</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>
It seems that this was left out of <a href="https://wg21.link/p1089">P1089</a>. The constraint on 
<code>span()</code> (23.7.2.2.2 <a href="https://wg21.link/span.cons">[span.cons]</a>) in the current draft is:
</p>
<blockquote><p>
<i>Constraints:</i> <code>Extent &lt;= 0</code> is <code>true</code>.
</p></blockquote>
<p>
This does not seem to make much sense.
<p/>
The proposal is to change the constraint to be:
</p>
<blockquote><p>
<i>Constraints:</i> <code>Extent == dynamic_extent || Extent == 0</code> is <code>true</code>.
</p></blockquote>

[2019-06-09; Priority to 0 and Status to Tentatively Ready after five positive votes on the reflector.]


<p id="res-3198"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4810">N4810</a>.</p>

<ol>
<li><p>Modify 23.7.2.2.2 <a href="https://wg21.link/span.cons">[span.cons]</a> as indicated:</p>

<blockquote><pre>
constexpr span() noexcept;
</pre>
<blockquote>
<p>
-1- <i>Constraints:</i> <code>Extent <del>&lt;</del><ins>== dynamic_extent || Extent =</ins>= 0</code> is <code>true</code>.
</p>
</blockquote>
</blockquote>
</li>
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