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<title>Issue 3450: The const overloads of take_while_view::begin/end are underconstrained</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="3450"><a href="lwg-defects.html#3450">3450</a>. The <code>const</code> overloads of <code>take_while_view::begin/end</code> are underconstrained</h3>
<p><b>Section:</b> 25.7.11.2 <a href="https://wg21.link/range.take.while.view">[range.take.while.view]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2020-06-06 <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.take.while.view">issues</a> in [range.take.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>
The <code>const</code> overloads of <code>take_while_view::begin</code>
and <code>take_while_view::end</code> are underconstrained: they should require
the predicate to model <code>indirect_unary_predicate&lt;iterator_t&lt;const V&gt;&gt;</code>.
A simple example is
</p>
<blockquote><pre>
    #include &lt;ranges&gt;

    auto v = std::views::single(1) | std::views::take_while([](int&amp; x) { return true;});
    static_assert(std::ranges::range&lt;decltype(v)&gt;);
    static_assert(std::ranges::range&lt;decltype(v) const&gt;);
    bool b = std::ranges::cbegin(v) == std::ranges::cend(v);
</pre></blockquote>
<p>
Here, the <code>static_assert</code>s pass but the comparison fails to compile. The other
views using <code>indirect_unary_predicate</code> (<code>filter_view</code> and
<code>drop_while_view</code>) do not have this problem because they do not support
const-iteration.
</p>

<p><i>[2020-07-17; Moved to Ready in telecon]</i></p>

<p><i>[2020-11-09 Approved In November virtual meeting. Status changed: Ready &rarr; WP.]</i></p>



<p id="res-3450"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.</p>

<ol>
<li>
<p>Modify 25.7.11.2 <a href="https://wg21.link/range.take.while.view">[range.take.while.view]</a>, class template <code>take_while_view</code> synopsis, as indicated:</p>
<blockquote>
<blockquote>
<pre>
namespace std::ranges {
    template&lt;view V, class Pred&gt;
      requires input_range&lt;V&gt; &amp;&amp; is_object_v&lt;Pred&gt; &amp;&amp;
               indirect_unary_predicate&lt;const Pred, iterator_t&lt;V&gt;&gt;
    class take_while_view : public view_interface&lt;take_while_view&lt;V, Pred&gt;&gt; {
    [&hellip;]

    constexpr auto begin() requires (!<i>simple-view</i>&lt;V&gt;)
    { return ranges::begin(<i>base_</i>); }

    constexpr auto begin() const requires range&lt;const V&gt; <ins>&amp;&amp; indirect_unary_predicate&lt;const Pred, iterator_t&lt;const V&gt;&gt;</ins>
    { return ranges::begin(<i>base_</i>); }

    constexpr auto end() requires (!<i>simple-view</i>&lt;V&gt;)
    { return sentinel&lt;false&gt;(ranges::end(<i>base_</i>), addressof(*<i>pred_</i>)); }

    constexpr auto end() const requires range&lt;const V&gt; <ins>&amp;&amp; indirect_unary_predicate&lt;const Pred, iterator_t&lt;const V&gt;&gt;</ins>
    { return sentinel&lt;true&gt;(ranges::end(<i>base_</i>), addressof(*<i>pred_</i>)); }
    };
}
</pre>
</blockquote>
</blockquote>
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