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<title>Issue 3277: Pre-increment on prvalues is not a requirement of weakly_incrementable</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="3277"><a href="lwg-defects.html#3277">3277</a>. Pre-increment on prvalues is not a requirement of <code>weakly_incrementable</code></h3>
<p><b>Section:</b> 24.3.4.13 <a href="https://wg21.link/iterator.concept.random.access">[iterator.concept.random.access]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Eric Niebler <b>Opened:</b> 2019-09-09 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#iterator.concept.random.access">issues</a> in [iterator.concept.random.access].</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>
See 24.3.4.13 <a href="https://wg21.link/iterator.concept.random.access">[iterator.concept.random.access]</a>/2.6, which shows <code>++</code> being applied to a prvalue iterator.
<p/>
A similar change has already been made to 25.6.4.2 <a href="https://wg21.link/range.iota.view">[range.iota.view]</a>/4.6.
<p/>
Suggest priority P0 or P1 because it effects the definition of a concept.
</p>

<p><i>[2019-09-24 Issue Prioritization]</i></p>

<p>Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.</p>


<p id="res-3277"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4830">N4830</a>.</p>

<ol>
<li><p>Modify 24.3.4.13 <a href="https://wg21.link/iterator.concept.random.access">[iterator.concept.random.access]</a> as indicated:</p>

<blockquote>
<p>
-2- Let <code>a</code> and <code>b</code> be valid iterators of type <code>I</code> such that <code>b</code> 
is reachable from <code>a</code> after <code>n</code> applications of <code>++a</code>, let <code>D</code> be
<code>iter_difference_t&lt;I&gt;</code>, and let <code>n</code> denote a value of type <code>D</code>. 
<code>I</code> models <code>random_access_iterator</code> only if
</p>
<ol style="list-style-type: none">
<li><p>(2.1) &mdash; <code>(a += n)</code> is equal to <code>b</code>.</p></li>
<li><p>[&hellip;]</p></li>
<li><p>(2.6) &mdash; If <code>(a + D(n - 1))</code> is valid, then <code>(a + n)</code> is equal to 
<code><del>++</del><ins>[](I c){ return ++c; }</ins>(a + D(n - 1))</code>.</p></li>
<li><p>[&hellip;]</p></li>
</ol>
</blockquote>
</li>
</ol>





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