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  <meta name="dcterms.date" content="2021-04-29" />
  <title>Remove misuses of list-initialization from Clause 24</title>
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<h1 class="title" style="text-align:center">Remove misuses of list-initialization from Clause 24</h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P2367R0</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2021-04-29</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      LWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Tim Song<br>&lt;<a href="mailto:t.canens.cpp@gmail.com" class="email">t.canens.cpp@gmail.com</a>&gt;<br>
    </td>
  </tr>
</table>

</header>
<div style="clear:both">
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="abstract"><span class="header-section-number">1</span> Abstract<a href="#abstract" class="self-link"></a></h1>
<p>This paper provides wording for <span class="citation" data-cites="LWG3524">[<a href="#ref-LWG3524" role="doc-biblioref">LWG3524</a>]</span> and resolves related issues caused by the erroneous use of list-initialization in ranges wording.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="discussion"><span class="header-section-number">2</span> Discussion<a href="#discussion" class="self-link"></a></h1>
<p>As discussed in <span class="citation" data-cites="LWG3524">[<a href="#ref-LWG3524" role="doc-biblioref">LWG3524</a>]</span>, the use of list-initialization in the ranges specification implies ordering guarantees that are unintended and unimplementable in ordinary C++, as well as narrowing checks that are unnecessary and sometimes unimplementable.</p>
<p>Other issues have also been reported against the misuse of list-initialization in ranges wording. See <span class="citation" data-cites="subrange">[<a href="#ref-subrange" role="doc-biblioref">subrange</a>]</span> and <span class="citation" data-cites="take.view">[<a href="#ref-take.view" role="doc-biblioref">take.view</a>]</span>.</p>
<p>The approach taken in this paper is to remove uses of list-initialization that are actively harmful, as well as any neighboring uses to maintain some level of local consistency. I have not attempted to remove all uses of list-initialization in the normative text of Clause 24.</p>
<p>During the drafting of this paper, two issues were discovered with <code class="sourceCode cpp">views<span class="op">::</span>single</code>:</p>
<ul>
<li>The implicit deduction guides fail to remove cv-qualification for const rvalues or decay array/function types. This is inconsistent with range-v3 and appears to be an oversight.</li>
<li><code class="sourceCode cpp">views<span class="op">::</span>single<span class="op">(</span>a_single_view<span class="op">)</span></code> unexpectedly copies instead of wraps, similar to <span class="citation" data-cites="LWG3474">[<a href="#ref-LWG3474" role="doc-biblioref">LWG3474</a>]</span>.</li>
</ul>
<p>The proposed wording below fixes these issues as well.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="wording"><span class="header-section-number">3</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>This wording is relative to <span class="citation" data-cites="N4885">[<a href="#ref-N4885" role="doc-biblioref">N4885</a>]</span>.</p>
<div>
<ol type="1">
<li>Edit <span>24.5.4.2 <a href="https://wg21.link/range.subrange.ctor">[range.subrange.ctor]</a></span> p6 as indicated:</li>
</ol>
<blockquote>
<div>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="kw">template</span><span class="op">&lt;</span><em>not-same-as</em><span class="op">&lt;</span>subrange<span class="op">&gt;</span> R<span class="op">&gt;</span></span>
<span id="cb1-2"><a href="#cb1-2"></a>  <span class="kw">requires</span> borrowed_range<span class="op">&lt;</span>R<span class="op">&gt;</span> <span class="op">&amp;&amp;</span></span>
<span id="cb1-3"><a href="#cb1-3"></a>           <em>convertible-to-non-slicing</em><span class="op">&lt;</span>iterator_t<span class="op">&lt;</span>R<span class="op">&gt;</span>, I<span class="op">&gt;</span> <span class="op">&amp;&amp;</span></span>
<span id="cb1-4"><a href="#cb1-4"></a>           convertible_to<span class="op">&lt;</span>sentinel_t<span class="op">&lt;</span>R<span class="op">&gt;</span>, S<span class="op">&gt;</span></span>
<span id="cb1-5"><a href="#cb1-5"></a><span class="kw">constexpr</span> subrange<span class="op">(</span>R<span class="op">&amp;&amp;</span> r<span class="op">)</span> <span class="kw">requires</span> <span class="op">(!</span><em>StoreSize</em> <span class="op">||</span> sized_range<span class="op">&lt;</span>R<span class="op">&gt;)</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">6</a></span> Effects: Equivalent to:</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(6.1)</a></span> If <em><code class="sourceCode cpp">StoreSize</code></em> is <code class="sourceCode cpp"><span class="kw">true</span></code>, <code class="sourceCode cpp">subrange<span class="diffdel">{</span><span class="diffins">(</span>r, ranges<span class="op">::</span>size<span class="op">(</span>r<span class="op">)</span><span class="diffdel">}</span><span class="diffins">)</span></code>.</li>
<li><span class="marginalizedparent"><a class="marginalized">(6.2)</a></span> Otherwise, <code class="sourceCode cpp">subrange<span class="diffdel">{</span><span class="diffins">(</span>ranges<span class="op">::</span>begin<span class="op">(</span>r<span class="op">)</span>, ranges<span class="op">::</span>end<span class="op">(</span>r<span class="op">)</span><span class="diffdel">}</span><span class="diffins">)</span></code>.</li>
</ul>
</blockquote>
</div>
</blockquote>
<ol start="2" type="1">
<li>Edit <span>24.6.3.1 <a href="https://wg21.link/range.single.overview">[range.single.overview]</a></span> p2 as indicated:</li>
</ol>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> The name <code class="sourceCode cpp">views<span class="op">::</span>single</code> denotes a customization point object (<span>16.3.3.3.6 <a href="https://wg21.link/customization.point.object">[customization.point.object]</a></span>). Given a subexpression <code class="sourceCode cpp">E</code>, the expression <code class="sourceCode cpp">views<span class="op">::</span>single<span class="op">(</span>E<span class="op">)</span></code> is expression-equivalent to <code class="sourceCode cpp">single_view<span class="diffins">&lt;decay_t&lt;decltype((E))&gt;&gt;</span><span class="diffdel">{</span><span class="diffins">(</span>E<span class="diffdel">}</span><span class="diffins">)</span></code>.</p>
</blockquote>
<ol start="3" type="1">
<li>Add a deduction guide to <span>24.6.3.2 <a href="https://wg21.link/range.single.view">[range.single.view]</a></span>:</li>
</ol>
<blockquote>
<div>
<div class="sourceCode" id="cb2"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb2-1"><a href="#cb2-1"></a> namespace std::ranges {</span>
<span id="cb2-2"><a href="#cb2-2"></a>   template&lt;copy_constructible T&gt;</span>
<span id="cb2-3"><a href="#cb2-3"></a>     requires is_object_v&lt;T&gt;</span>
<span id="cb2-4"><a href="#cb2-4"></a>   class single_view : public view_interface&lt;single_view&lt;T&gt;&gt; {</span>
<span id="cb2-5"><a href="#cb2-5"></a>     // [...]</span>
<span id="cb2-6"><a href="#cb2-6"></a>   };</span>
<span id="cb2-7"><a href="#cb2-7"></a></span>
<span id="cb2-8"><a href="#cb2-8"></a><span class="va">+  template&lt;class T&gt;</span></span>
<span id="cb2-9"><a href="#cb2-9"></a><span class="va">+    single_view(T) -&gt; single_view&lt;T&gt;;</span></span>
<span id="cb2-10"><a href="#cb2-10"></a> }</span></code></pre></div>
</div>
</blockquote>
<ol start="4" type="1">
<li>Edit <span>24.6.4.1 <a href="https://wg21.link/range.iota.overview">[range.iota.overview]</a></span> p2 as indicated:</li>
</ol>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> The name <code class="sourceCode cpp">views<span class="op">::</span>iota</code> denotes a customization point object (<span>16.3.3.3.6 <a href="https://wg21.link/customization.point.object">[customization.point.object]</a></span>). Given subexpressions <code class="sourceCode cpp">E</code> and <code class="sourceCode cpp">F</code>, the expressions <code class="sourceCode cpp">views<span class="op">::</span>iota<span class="op">(</span>E<span class="op">)</span></code> and <code class="sourceCode cpp">views<span class="op">::</span>iota<span class="op">(</span>E, F<span class="op">)</span></code> are expression-equivalent to <code class="sourceCode cpp">iota_view<span class="diffdel">{</span><span class="diffins">(</span>E<span class="diffdel">}</span><span class="diffins">)</span></code> and <code class="sourceCode cpp">iota_view<span class="diffdel">{</span><span class="diffins">(</span>E, F<span class="diffdel">}</span><span class="diffins">)</span></code>, respectively.</p>
</blockquote>
<ol start="5" type="1">
<li>Edit <span>24.7.5.1 <a href="https://wg21.link/range.filter.overview">[range.filter.overview]</a></span> p2 as indicated:</li>
</ol>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> The name <code class="sourceCode cpp">views<span class="op">::</span>filter</code> denotes a range adaptor object (<span>24.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a></span>). Given subexpressions <code class="sourceCode cpp">E</code> and <code class="sourceCode cpp">P</code>, the expression <code class="sourceCode cpp">views<span class="op">::</span>filter<span class="op">(</span>E, P<span class="op">)</span></code> is expression-equivalent to <code class="sourceCode cpp">filter_view<span class="diffdel">{</span><span class="diffins">(</span>E, P<span class="diffdel">}</span><span class="diffins">)</span></code>.</p>
</blockquote>
<ol start="6" type="1">
<li>Edit <span>24.7.6.1 <a href="https://wg21.link/range.transform.overview">[range.transform.overview]</a></span> p2 as indicated:</li>
</ol>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> The name <code class="sourceCode cpp">views<span class="op">::</span>transform</code> denotes a range adaptor object (<span>24.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a></span>). Given subexpressions <code class="sourceCode cpp">E</code> and <code class="sourceCode cpp">F</code>, the expression <code class="sourceCode cpp">views<span class="op">::</span>transform<span class="op">(</span>E, F<span class="op">)</span></code> is expression-equivalent to <code class="sourceCode cpp">transform_view<span class="diffdel">{</span><span class="diffins">(</span>E, F<span class="diffdel">}</span><span class="diffins">)</span></code>.</p>
</blockquote>
<ol start="7" type="1">
<li>Edit <span>24.7.7.1 <a href="https://wg21.link/range.take.overview">[range.take.overview]</a></span> p2 as indicated:</li>
</ol>
<blockquote>
<div class="dummy">
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> The name <code class="sourceCode cpp">views<span class="op">::</span>take</code> denotes a range adaptor object (<span>24.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a></span>). Let <code class="sourceCode cpp">E</code> and <code class="sourceCode cpp">F</code> be expressions, let <code class="sourceCode cpp">T</code> be <code class="sourceCode cpp">remove_cvref_t<span class="op">&lt;</span><span class="kw">decltype</span><span class="op">((</span>E<span class="op">))&gt;</span></code>, and let <code class="sourceCode cpp">D</code> be <code class="sourceCode cpp">range_difference_t<span class="op">&lt;</span><span class="kw">decltype</span><span class="op">((</span>E<span class="op">))&gt;</span></code>. If <code class="sourceCode cpp"><span class="kw">decltype</span><span class="op">((</span>F<span class="op">))</span></code> does not model <code class="sourceCode cpp">convertible_to<span class="op">&lt;</span>D<span class="op">&gt;</span></code>, <code class="sourceCode cpp">views<span class="op">::</span>take<span class="op">(</span>E, F<span class="op">)</span></code> is ill-formed. Otherwise, the expression <code class="sourceCode cpp">views<span class="op">::</span>take<span class="op">(</span>E, F<span class="op">)</span></code> is expression-equivalent to:</p>
<ul>
<li><p><span class="marginalizedparent"><a class="marginalized">(2.1)</a></span> If T is a specialization of <code class="sourceCode cpp">ranges<span class="op">::</span>empty_view</code> (<span>24.6.2.2 <a href="https://wg21.link/range.empty.view">[range.empty.view]</a></span>), then <code class="sourceCode cpp"><span class="op">((</span><span class="dt">void</span><span class="op">)</span> F, <em>decay-copy</em><span class="op">(</span>E<span class="op">))</span></code><span class="diffins">, except that the evaluations of <code class="sourceCode cpp">E</code> and <code class="sourceCode cpp">F</code> are indeterminately sequenced</span>.</p></li>
<li><p><span class="marginalizedparent"><a class="marginalized">(2.2)</a></span> Otherwise, if <code class="sourceCode cpp">T</code> models <code class="sourceCode cpp">random_access_range</code> and <code class="sourceCode cpp">sized_range</code> and is</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(2.2.1)</a></span> […]</li>
</ul>
<p>then <code class="sourceCode cpp">T<span class="diffdel">{</span><span class="diffins">(</span>ranges<span class="op">::</span>begin<span class="op">(</span>E<span class="op">)</span>, ranges<span class="op">::</span>begin<span class="op">(</span>E<span class="op">)</span> <span class="op">+</span> min<span class="op">&lt;</span>D<span class="op">&gt;(</span>ranges<span class="op">::</span>size<span class="op">(</span>E<span class="op">)</span>, F<span class="op">)</span><span class="diffdel">}</span><span class="diffins">)</span></code>, except that <code class="sourceCode cpp">E</code> is evaluated only once.</p></li>
<li><p><span class="marginalizedparent"><a class="marginalized">(2.3)</a></span> Otherwise, <code class="sourceCode cpp">ranges<span class="op">::</span>take_view<span class="diffdel">{</span><span class="diffins">(</span>E, F<span class="diffdel">}</span><span class="diffins">)</span></code>.</p></li>
</ul>
</div>
</blockquote>
<ol start="8" type="1">
<li>Edit <span>24.7.7.2 <a href="https://wg21.link/range.take.view">[range.take.view]</a></span> as indicated:</li>
</ol>
<blockquote>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="kw">namespace</span> std<span class="op">::</span>ranges <span class="op">{</span></span>
<span id="cb3-2"><a href="#cb3-2"></a>  <span class="kw">template</span><span class="op">&lt;</span>view V<span class="op">&gt;</span></span>
<span id="cb3-3"><a href="#cb3-3"></a>  <span class="kw">class</span> take_view <span class="op">:</span> <span class="kw">public</span> view_interface<span class="op">&lt;</span>take_view<span class="op">&lt;</span>V<span class="op">&gt;&gt;</span> <span class="op">{</span></span>
<span id="cb3-4"><a href="#cb3-4"></a></span>
<span id="cb3-5"><a href="#cb3-5"></a>    <span class="co">// [...]</span></span>
<span id="cb3-6"><a href="#cb3-6"></a></span>
<span id="cb3-7"><a href="#cb3-7"></a>    <span class="kw">constexpr</span> <span class="kw">auto</span> begin<span class="op">()</span> <span class="kw">requires</span> <span class="op">(!</span>simple<span class="op">-</span>view<span class="op">&lt;</span>V<span class="op">&gt;)</span> <span class="op">{</span></span>
<span id="cb3-8"><a href="#cb3-8"></a>      <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>sized_range<span class="op">&lt;</span>V<span class="op">&gt;)</span> <span class="op">{</span></span>
<span id="cb3-9"><a href="#cb3-9"></a>        <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>random_access_range<span class="op">&lt;</span>V<span class="op">&gt;)</span></span>
<span id="cb3-10"><a href="#cb3-10"></a>          <span class="cf">return</span> ranges<span class="op">::</span>begin<span class="op">(</span><em>base_</em><span class="op">)</span>;</span>
<span id="cb3-11"><a href="#cb3-11"></a>        <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb3-12"><a href="#cb3-12"></a>          <span class="kw">auto</span> sz <span class="op">=</span> size<span class="op">()</span>;</span>
<span id="cb3-13"><a href="#cb3-13"></a>          <span class="cf">return</span> counted_iterator<span class="diffdel">{</span><span class="diffins">(</span>ranges<span class="op">::</span>begin<span class="op">(</span><em>base_</em><span class="op">)</span>, sz<span class="diffdel">}</span><span class="diffins">)</span>;</span>
<span id="cb3-14"><a href="#cb3-14"></a>        <span class="op">}</span></span>
<span id="cb3-15"><a href="#cb3-15"></a>      <span class="op">}</span> <span class="cf">else</span></span>
<span id="cb3-16"><a href="#cb3-16"></a>        <span class="cf">return</span> counted_iterator<span class="diffdel">{</span><span class="diffins">(</span>ranges<span class="op">::</span>begin<span class="op">(</span><em>base_</em><span class="op">)</span>, <em>count_</em><span class="diffdel">}</span><span class="diffins">)</span>;</span>
<span id="cb3-17"><a href="#cb3-17"></a>    <span class="op">}</span></span>
<span id="cb3-18"><a href="#cb3-18"></a></span>
<span id="cb3-19"><a href="#cb3-19"></a>    <span class="kw">constexpr</span> <span class="kw">auto</span> begin<span class="op">()</span> <span class="kw">const</span> <span class="kw">requires</span> range<span class="op">&lt;</span><span class="kw">const</span> V<span class="op">&gt;</span> <span class="op">{</span></span>
<span id="cb3-20"><a href="#cb3-20"></a>      <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>sized_range<span class="op">&lt;</span><span class="kw">const</span> V<span class="op">&gt;)</span> <span class="op">{</span></span>
<span id="cb3-21"><a href="#cb3-21"></a>        <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>random_access_range<span class="op">&lt;</span><span class="kw">const</span> V<span class="op">&gt;)</span></span>
<span id="cb3-22"><a href="#cb3-22"></a>          <span class="cf">return</span> ranges<span class="op">::</span>begin<span class="op">(</span><em>base_</em><span class="op">)</span>;</span>
<span id="cb3-23"><a href="#cb3-23"></a>        <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb3-24"><a href="#cb3-24"></a>          <span class="kw">auto</span> sz <span class="op">=</span> size<span class="op">()</span>;</span>
<span id="cb3-25"><a href="#cb3-25"></a>          <span class="cf">return</span> counted_iterator<span class="diffdel">{</span><span class="diffins">(</span>ranges<span class="op">::</span>begin<span class="op">(</span><em>base_</em><span class="op">)</span>, sz<span class="diffdel">}</span><span class="diffins">)</span>;</span>
<span id="cb3-26"><a href="#cb3-26"></a>        <span class="op">}</span></span>
<span id="cb3-27"><a href="#cb3-27"></a>      <span class="op">}</span> <span class="cf">else</span></span>
<span id="cb3-28"><a href="#cb3-28"></a>        <span class="cf">return</span> counted_iterator<span class="diffdel">{</span><span class="diffins">(</span>ranges<span class="op">::</span>begin<span class="op">(</span><em>base_</em><span class="op">)</span>, <em>count_</em><span class="diffdel">}</span><span class="diffins">)</span>;</span>
<span id="cb3-29"><a href="#cb3-29"></a>    <span class="op">}</span></span>
<span id="cb3-30"><a href="#cb3-30"></a></span>
<span id="cb3-31"><a href="#cb3-31"></a>    <span class="co">// [...]</span></span>
<span id="cb3-32"><a href="#cb3-32"></a>  <span class="op">}</span>;</span>
<span id="cb3-33"><a href="#cb3-33"></a></span>
<span id="cb3-34"><a href="#cb3-34"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> R<span class="op">&gt;</span></span>
<span id="cb3-35"><a href="#cb3-35"></a>    take_view<span class="op">(</span>R<span class="op">&amp;&amp;</span>, range_difference_t<span class="op">&lt;</span>R<span class="op">&gt;)</span></span>
<span id="cb3-36"><a href="#cb3-36"></a>      <span class="op">-&gt;</span> take_view<span class="op">&lt;</span>views<span class="op">::</span>all_t<span class="op">&lt;</span>R<span class="op">&gt;&gt;</span>;</span>
<span id="cb3-37"><a href="#cb3-37"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<ol start="9" type="1">
<li>Edit <span>24.7.8.1 <a href="https://wg21.link/range.take.while.overview">[range.take.while.overview]</a></span> p2 as indicated:</li>
</ol>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> The name <code class="sourceCode cpp">views<span class="op">::</span>take_while</code> denotes a range adaptor object (<span>24.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a></span>). Given subexpressions <code class="sourceCode cpp">E</code> and <code class="sourceCode cpp">F</code>, the expression <code class="sourceCode cpp">views<span class="op">::</span>take_while<span class="op">(</span>E, F<span class="op">)</span></code> is expression-equivalent to <code class="sourceCode cpp">take_while_view<span class="diffdel">{</span><span class="diffins">(</span>E, F<span class="diffdel">}</span><span class="diffins">)</span></code>.</p>
</blockquote>
<ol start="10" type="1">
<li>Edit <span>24.7.9.1 <a href="https://wg21.link/range.drop.overview">[range.drop.overview]</a></span> p2 as indicated:</li>
</ol>
<blockquote>
<div class="dummy">
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> The name <code class="sourceCode cpp">views<span class="op">::</span>drop</code> denotes a range adaptor object (<span>24.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a></span>). Let <code class="sourceCode cpp">E</code> and <code class="sourceCode cpp">F</code> be expressions, let <code class="sourceCode cpp">T</code> be <code class="sourceCode cpp">remove_cvref_t<span class="op">&lt;</span><span class="kw">decltype</span><span class="op">((</span>E<span class="op">))&gt;</span></code>, and let <code class="sourceCode cpp">D</code> be <code class="sourceCode cpp">range_difference_t<span class="op">&lt;</span><span class="kw">decltype</span><span class="op">((</span>E<span class="op">))&gt;</span></code>. If <code class="sourceCode cpp"><span class="kw">decltype</span><span class="op">((</span>F<span class="op">))</span></code> does not model <code class="sourceCode cpp">convertible_to<span class="op">&lt;</span>D<span class="op">&gt;</span></code>, <code class="sourceCode cpp">views<span class="op">::</span>drop<span class="op">(</span>E, F<span class="op">)</span></code> is ill-formed. Otherwise, the expression <code class="sourceCode cpp">views<span class="op">::</span>drop<span class="op">(</span>E, F<span class="op">)</span></code> is expression-equivalent to:</p>
<ul>
<li><p><span class="marginalizedparent"><a class="marginalized">(2.1)</a></span> If T is a specialization of <code class="sourceCode cpp">ranges<span class="op">::</span>empty_view</code> (<span>24.6.2.2 <a href="https://wg21.link/range.empty.view">[range.empty.view]</a></span>), then <code class="sourceCode cpp"><span class="op">((</span><span class="dt">void</span><span class="op">)</span> F, <em>decay-copy</em><span class="op">(</span>E<span class="op">))</span></code><span class="diffins">, except that the evaluations of <code class="sourceCode cpp">E</code> and <code class="sourceCode cpp">F</code> are indeterminately sequenced</span>.</p></li>
<li><p><span class="marginalizedparent"><a class="marginalized">(2.2)</a></span> Otherwise, if <code class="sourceCode cpp">T</code> models <code class="sourceCode cpp">random_access_range</code> and <code class="sourceCode cpp">sized_range</code> and is</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(2.2.1)</a></span> […]</li>
</ul>
<p>then <code class="sourceCode cpp">T<span class="diffdel">{</span><span class="diffins">(</span>ranges<span class="op">::</span>begin<span class="op">(</span>E<span class="op">)</span> <span class="op">+</span> min<span class="op">&lt;</span>D<span class="op">&gt;(</span>ranges<span class="op">::</span>size<span class="op">(</span>E<span class="op">)</span>, F<span class="op">)</span>, ranges<span class="op">::</span>end<span class="op">(</span>E<span class="op">)</span><span class="diffdel">}</span><span class="diffins">)</span></code>, except that <code class="sourceCode cpp">E</code> is evaluated only once.</p></li>
<li><p><span class="marginalizedparent"><a class="marginalized">(2.3)</a></span> Otherwise, <code class="sourceCode cpp">ranges<span class="op">::</span>drop_view<span class="diffdel">{</span><span class="diffins">(</span>E, F<span class="diffdel">}</span><span class="diffins">)</span></code>.</p></li>
</ul>
</div>
</blockquote>
<ol start="11" type="1">
<li>Edit <span>24.7.10.1 <a href="https://wg21.link/range.drop.while.overview">[range.drop.while.overview]</a></span> p2 as indicated:</li>
</ol>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> The name <code class="sourceCode cpp">views<span class="op">::</span>drop_while</code> denotes a range adaptor object (<span>24.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a></span>). Given subexpressions <code class="sourceCode cpp">E</code> and <code class="sourceCode cpp">F</code>, the expression <code class="sourceCode cpp">views<span class="op">::</span>drop_while<span class="op">(</span>E, F<span class="op">)</span></code> is expression-equivalent to <code class="sourceCode cpp">drop_while_view<span class="diffdel">{</span><span class="diffins">(</span>E, F<span class="diffdel">}</span><span class="diffins">)</span></code>.</p>
</blockquote>
<ol start="12" type="1">
<li>Edit <span>24.7.12.1 <a href="https://wg21.link/range.split.overview">[range.split.overview]</a></span> p2 as indicated:</li>
</ol>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> The name <code class="sourceCode cpp">views<span class="op">::</span>split</code> denotes a range adaptor object (<span>24.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a></span>). Given subexpressions <code class="sourceCode cpp">E</code> and <code class="sourceCode cpp">F</code>, the expression <code class="sourceCode cpp">views<span class="op">::</span>split<span class="op">(</span>E, F<span class="op">)</span></code> is expression-equivalent to <code class="sourceCode cpp">split_view<span class="diffdel">{</span><span class="diffins">(</span>E, F<span class="diffdel">}</span><span class="diffins">)</span></code>.</p>
</blockquote>
<ol start="13" type="1">
<li>Edit <span>24.7.12.2 <a href="https://wg21.link/range.split.view">[range.split.view]</a></span> p2 as indicated:</li>
</ol>
<blockquote>
<div>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1"></a><span class="kw">template</span><span class="op">&lt;</span>input_range R<span class="op">&gt;</span></span>
<span id="cb4-2"><a href="#cb4-2"></a>  <span class="kw">requires</span> constructible_from<span class="op">&lt;</span>V, views<span class="op">::</span>all_t<span class="op">&lt;</span>R<span class="op">&gt;&gt;</span> <span class="op">&amp;&amp;</span></span>
<span id="cb4-3"><a href="#cb4-3"></a>           constructible_from<span class="op">&lt;</span>Pattern, single_view<span class="op">&lt;</span>range_value_t<span class="op">&lt;</span>R<span class="op">&gt;&gt;&gt;</span></span>
<span id="cb4-4"><a href="#cb4-4"></a><span class="kw">constexpr</span> split_view<span class="op">(</span>R<span class="op">&amp;&amp;</span> r, range_value_t<span class="op">&lt;</span>R<span class="op">&gt;</span> e<span class="op">)</span>;</span></code></pre></div>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> <em>Effects</em>: Initializes <em><code class="sourceCode cpp">base_</code></em> with <code class="sourceCode cpp">views<span class="op">::</span>all<span class="op">(</span>std<span class="op">::</span>forward<span class="op">&lt;</span>R<span class="op">&gt;(</span>r<span class="op">))</span></code>, and <em><code class="sourceCode cpp">pattern_</code></em> with <span class="diffdel"><code class="sourceCode cpp">single_view<span class="op">{</span>std<span class="op">::</span>move<span class="op">(</span>e<span class="op">)}</span></code></span> <span class="diffins"><code class="sourceCode cpp">views<span class="op">::</span>single<span class="op">(</span>std<span class="op">::</span>move<span class="op">(</span>e<span class="op">))</span></code></span>.</p>
</blockquote>
</div>
</blockquote>
<ol start="14" type="1">
<li>Edit <span>24.7.13 <a href="https://wg21.link/range.counted">[range.counted]</a></span> p2 as indicated:</li>
</ol>
<blockquote>
<div class="dummy">
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> The name <code class="sourceCode cpp">views<span class="op">::</span>counted</code> denotes a customization point object (<span>16.3.3.3.6 <a href="https://wg21.link/customization.point.object">[customization.point.object]</a></span>). Let <code class="sourceCode cpp">E</code> and <code class="sourceCode cpp">F</code> be expressions, let <code class="sourceCode cpp">T</code> be <code class="sourceCode cpp">decay_t<span class="op">&lt;</span><span class="kw">decltype</span><span class="op">((</span>E<span class="op">))&gt;</span></code>, and let <code class="sourceCode cpp">D</code> be <code class="sourceCode cpp">iter_difference_t<span class="op">&lt;</span>T<span class="op">&gt;</span></code>. If <code class="sourceCode cpp"><span class="kw">decltype</span><span class="op">((</span>F<span class="op">))</span></code> does not model <code class="sourceCode cpp">convertible_to<span class="op">&lt;</span>D<span class="op">&gt;</span></code>, <code class="sourceCode cpp">views<span class="op">::</span>counted<span class="op">(</span>E, F<span class="op">)</span></code> is ill-formed.</p>
<p><span class="note1"><span>[ <em>Note 1:</em> </span>This case can result in substitution failure when <code class="sourceCode cpp">views<span class="op">::</span>counted<span class="op">(</span>E, F<span class="op">)</span></code> appears in the immediate context of a template instantiation.<span> — <em>end note</em> ]</span></span></p>
<p>Otherwise, <code class="sourceCode cpp">views<span class="op">::</span>counted<span class="op">(</span>E, F<span class="op">)</span></code> is expression-equivalent to:</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(2.1)</a></span> If <code class="sourceCode cpp">T</code> models <code class="sourceCode cpp">contiguous_iterator</code>, then <code class="sourceCode cpp">span<span class="diffdel">{</span><span class="diffins">(</span>to_address<span class="op">(</span>E<span class="op">)</span>, <span class="kw">static_cast</span><span class="op">&lt;</span>D<span class="op">&gt;(</span>F<span class="op">)</span><span class="diffdel">}</span><span class="diffins">)</span></code>.</li>
<li><span class="marginalizedparent"><a class="marginalized">(2.2)</a></span> Otherwise, if <code class="sourceCode cpp">T</code> models <code class="sourceCode cpp">random_access_iterator</code>, then <code class="sourceCode cpp">subrange<span class="diffdel">{</span><span class="diffins">(</span>E, E <span class="op">+</span> <span class="kw">static_cast</span><span class="op">&lt;</span>D<span class="op">&gt;(</span>F<span class="op">)</span><span class="diffdel">}</span><span class="diffins">)</span></code>, except that <code class="sourceCode cpp">E</code> is evaluated only once.</li>
<li><span class="marginalizedparent"><a class="marginalized">(2.3)</a></span> Otherwise, <code class="sourceCode cpp">subrange<span class="diffdel">{</span><span class="diffins">(</span>counted_iterator<span class="diffdel">{</span><span class="diffins">(</span>E, F<span class="diffdel">}</span><span class="diffins">)</span>, default_sentinel<span class="diffdel">}</span><span class="diffins">)</span></code>.</li>
</ul>
</div>
</blockquote>
</div>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="bibliography"><span class="header-section-number">4</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<div id="ref-LWG3474">
<p>[LWG3474] Barry Revzin. Nesting join_views is broken because of CTAD. <br />
<a href="https://wg21.link/lwg3474">https://wg21.link/lwg3474</a></p>
</div>
<div id="ref-LWG3524">
<p>[LWG3524] Tim Song. Unimplementable narrowing and evaluation order requirements for range adaptors. <br />
<a href="https://wg21.link/lwg3524">https://wg21.link/lwg3524</a></p>
</div>
<div id="ref-N4885">
<p>[N4885] Thomas Köppe. 2021-03-17. Working Draft, Standard for Programming Language C++. <br />
<a href="https://wg21.link/n4885">https://wg21.link/n4885</a></p>
</div>
<div id="ref-subrange">
<p>[subrange] Jonathan Wakely. 2020. Yet Another Braced Init Bug. <br />
<a href="https://lists.isocpp.org/lib/2020/10/17806.php">https://lists.isocpp.org/lib/2020/10/17806.php</a></p>
</div>
<div id="ref-take.view">
<p>[take.view] Michael Schellenberger Costa. 2021. Fix error in <code class="sourceCode cpp">take_view<span class="op">::</span>begin</code> by casting to the appropriate size. <br />
<a href="https://github.com/microsoft/STL/pull/1844">https://github.com/microsoft/STL/pull/1844</a></p>
</div>
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