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<title>Issue 3948: possibly-const-range and as-const-pointer should be noexcept</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#WP">WP</a> status.</em></p>
<h3 id="3948"><a href="lwg-defects.html#3948">3948</a>. <code><i>possibly-const-range</i></code> and <code><i>as-const-pointer</i></code> should be <code>noexcept</code></h3>
<p><b>Section:</b> 25.2 <a href="https://wg21.link/ranges.syn">[ranges.syn]</a>, 25.3.15 <a href="https://wg21.link/range.prim.cdata">[range.prim.cdata]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2023-06-20 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#ranges.syn">active issues</a> in [ranges.syn].</p>
<p><b>View all other</b> <a href="lwg-index.html#ranges.syn">issues</a> in [ranges.syn].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
As of <a href="https://wg21.link/P2278R4" title=" cbegin should always return a constant iterator">P2278R4</a>, several range access CPOs are specified with <code><i>possibly-const-range</i></code> 
and <code><i>as-const-pointer</i></code>. These helper functions never throw exceptions, but are not marked with 
<code>noexcept</code>. As a result, implementations are currently allowed to make a call to 
<code>ranges::c<i>cpo</i></code> potentially throwing while the underlying <code>ranges::<i>cpo</i></code> call is 
non-throwing, which doesn't seem to be intended.
</p>

<p><i>[2023-10-27; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after five votes in favour during reflector poll.
</p>

<p><i>[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3948"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4950" title=" Working Draft, Standard for Programming Language C++">N4950</a>.
</p>

<ol>
<li><p>Modify 25.2 <a href="https://wg21.link/ranges.syn">[ranges.syn]</a>, header <code>&lt;ranges&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
[&hellip;]
// <i>25.7.22 <a href="https://wg21.link/range.as.const">[range.as.const]</a>, as const view</i>
template&lt;input_range R&gt;
  constexpr auto&amp; <i>possibly-const-range</i>(R&amp; r) <ins>noexcept</ins> {      // <i>exposition only</i>
    if constexpr (constant_range&lt;const R&gt; &amp;&amp; !constant_range&lt;R&gt;) {
      return const_cast&lt;const R&amp;&gt;(r);
    } else {
      return r;
    }
  }
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 25.3.15 <a href="https://wg21.link/range.prim.cdata">[range.prim.cdata]</a> before p1 as indicated:</p>

<blockquote>
<pre>
template&lt;class T&gt;
constexpr auto <i>as-const-pointer</i>(const T* p) <ins>noexcept</ins> { return p; }    // <i>exposition only</i>
</pre>
</blockquote>
</li>

</ol>





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