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<title>Issue 3811: views::as_const on ref_view&lt;T&gt; should return ref_view&lt;const T&gt;</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="3811"><a href="lwg-defects.html#3811">3811</a>. <code>views::as_const</code> on <code>ref_view&lt;T&gt;</code> should return <code>ref_view&lt;const T&gt;</code></h3>
<p><b>Section:</b> 25.7.22.1 <a href="https://wg21.link/range.as.const.overview">[range.as.const.overview]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tomasz Kami&nacute;ski <b>Opened:</b> 2022-11-03 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.as.const.overview">issues</a> in [range.as.const.overview].</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>
For <code>v</code> being a non-const lvalue of type <code>std::vector&lt;int&gt;</code>, <code>views::as_const(v)</code> 
produces <code>ref_view&lt;std::vector&lt;int&gt; const&gt;</code>. However, when <code>v</code> is converted to 
<code>ref_view</code> by using <code>views::all</code>, <code>views::as_const(views::all(v))</code> produces 
<code>as_const_view&lt;ref_view&lt;std::vector&lt;int&gt;&gt;&gt;</code>.
<p/>
Invoking <code>views::as_const</code> on <code>ref_view&lt;T&gt;</code> should produce <code>ref_view&lt;const T&gt;</code> when 
<code>const T</code> models a constant range. This will reduce the number of instantiations, and make a behavior 
of <code>views::as_const</code> consistent on references and <code>ref_view</code> to containers.
</p>

<p><i>[Kona 2022-11-08; Move to Ready]</i></p>


<p><i>[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3811"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>
<li><p>Modify 25.7.22.1 <a href="https://wg21.link/range.as.const.overview">[range.as.const.overview]</a> as indicated:</p>

<blockquote class="note">
<p>
[<i>Drafting note</i>: If we have <code>ref_view&lt;V&gt;</code>, when <code>V</code> is constant propagating view 
(<code>single_view</code>, <code>owning_view</code>), we still can (and should) produce <code>ref_view&lt;V const&gt;</code>. 
This wording achieves that.]
</p>
</blockquote>

<blockquote>
<p>
-2- The name <code>views::as_const</code> denotes a range adaptor object (25.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a>). 
Let <code>E</code> be an expression, let <code>T</code> be <code>decltype((E))</code>, and let <code>U</code> be 
<code>remove_cvref_t&lt;T&gt;</code>. The expression <code>views::as_const(E)</code> is expression-equivalent
to:
</p>
<ol style="list-style-type: none">
<li><p>(2.1) &mdash; If <code>views::all_t&lt;T&gt;</code> models <code>constant_range</code>, then <code>views::all(E)</code>.</p></li>
<li><p>(2.2) &mdash; Otherwise, if <code>U</code> denotes <code>span&lt;X, Extent&gt;</code> for some type <code>X</code> and some 
extent <code>Extent</code>, then <code>span&lt;const X, Extent&gt;(E)</code>.</p></li>
<li><p><ins>(2.?) &mdash; Otherwise, if <code>U</code> denotes <code>ref_view&lt;X&gt;</code> for some type <code>X</code> and 
<code>const X</code> models <code>constant_range</code>, then <code>ref_view(static_cast&lt;const X&amp;&gt;(E.base()))</code>.</ins></p></li>
<li><p>(2.3) &mdash; Otherwise, if <code>E</code> is an lvalue, <code>const U</code> models <code>constant_range</code>, and 
<code>U</code> does not model <code>view</code>, then <code>ref_view(static_cast&lt;const U&amp;&gt;(E))</code>.</p></li>
<li><p>(2.4) &mdash; Otherwise, <code>as_const_view(E)</code>.</p></li>
</ol>
</blockquote>
</li>

</ol>





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