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<title>Issue 3490: ranges::drop_while_view::begin() is missing a precondition</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="3490"><a href="lwg-defects.html#3490">3490</a>. <code>ranges::drop_while_view::begin()</code> is missing a precondition</h3>
<p><b>Section:</b> 25.7.13.2 <a href="https://wg21.link/range.drop.while.view">[range.drop.while.view]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Michael Schellenberger Costa <b>Opened:</b> 2020-10-13 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.drop.while.view">issues</a> in [range.drop.while.view].</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>
Similar to <code>ranges::filter_view</code> 25.7.8.2 <a href="https://wg21.link/range.filter.view">[range.filter.view]</a> p3, <code>ranges::drop_while_view</code> 
should have a precondition on its <code>begin()</code> method that the predicate is set.
<p/>
I propose to add as 25.7.13.2 <a href="https://wg21.link/range.drop.while.view">[range.drop.while.view]</a> p3:
</p>
<blockquote><pre>
<ins><i>Preconditions:</i> <code><i>pred_</i>.has_value()</code>.</ins>  
</pre></blockquote>


<p><i>[2020-11-07; Reflector prioritization]</i></p>

<p>
Set priority to 0 and status to Tentatively Ready after six votes in favour during reflector discussions.
</p>
<p><i>[2021-02-26 Approved at February 2021 virtual plenary. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3490"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4868">N4868</a>. 
</p>

<ol>
<li>
<p>
Modify 25.7.13.2 <a href="https://wg21.link/range.drop.while.view">[range.drop.while.view]</a> as indicated:
</p>

<blockquote class="note">
<p>
Since we usually don't rely on implicit <code>bool</code> conversion in <i>Preconditions:</i> elements 
an explicit "is <code>true</code>" has been added. Editorial fixes of the referenced paragraph
25.7.13.2 <a href="https://wg21.link/range.drop.while.view">[range.drop.while.view]</a> p3 and similar places have been requested separately.
</p>
</blockquote>

<blockquote>
<pre>
constexpr auto begin();
</pre>
<blockquote>
<p>
<ins>-?- <i>Preconditions:</i> <code><i>pred_</i>.has_value()</code> is <code>true</code>.</ins>
<p/>
-3- <i>Returns:</i> <code>ranges::find_if_not(<i>base_</i>, cref(*<i>pred_</i>))</code>.
<p/>
-4- [&hellip;]
</p>
</blockquote>
</blockquote>
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