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<title>Issue 3404: Finish removing subrange's conversions from pair-like</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="3404"><a href="lwg-defects.html#3404">3404</a>. Finish removing <code>subrange</code>'s conversions from <code><i>pair-like</i></code></h3>
<p><b>Section:</b> 25.5.4 <a href="https://wg21.link/range.subrange">[range.subrange]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2020-02-20 <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.subrange">issues</a> in [range.subrange].</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>
Both LWG <a href="lwg-defects.html#3281" title="Conversion from pair-like types to subrange is a silent semantic promotion (Status: C++20)">3281</a><sup><a href="https://cplusplus.github.io/LWG/issue3281" title="Latest snapshot">(i)</a></sup> "Conversion from <code><i>pair-like</i></code> types to <code>subrange</code> is a 
silent semantic promotion" and LWG <a href="lwg-defects.html#3282" title="subrange converting constructor should disallow derived to base conversions (Status: C++20)">3282</a><sup><a href="https://cplusplus.github.io/LWG/issue3282" title="Latest snapshot">(i)</a></sup> "<code>subrange</code> converting constructor should 
disallow derived to base conversions" removed <code>subrange</code>'s hated implicit conversions from 
<code><i>pair-like</i></code> types. Notably, neither issue removed the two "<code><i>iterator-sentinel-pair</i></code>" 
deduction guides which target the removed constructors nor the exposition-only <code><i>iterator-sentinel-pair</i></code> 
concept itself, all of which are now useless.
</p>

<p><i>[2020-03-11 Issue Prioritization]</i></p>
 
<p>
Status set to Tentatively Ready after seven positive votes on the reflector.
</p>
<p><i>[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3404"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4849">N4849</a>.
</p>

<ol>
<li><p>Modify 25.5.4 <a href="https://wg21.link/range.subrange">[range.subrange]</a> as indicated:</p>

<blockquote><pre>
[&hellip;]
template&lt;class T, class U, class V&gt;
  concept <i>pair-like-convertible-from</i> = <i>// exposition only</i>
    !range&lt;T&gt; &amp;&amp; <i>pair-like</i>&lt;T&gt; &amp;&amp; constructible_from&lt;T, U, V&gt;;

<del>template&lt;class T&gt;
  concept <i>iterator-sentinel-pair</i> = <i>// exposition only</i>
    !range&lt;T&gt; &amp;&amp; <i>pair-like</i>&lt;T&gt; &amp;&amp;
    sentinel_for&lt;tuple_element_t&lt;1, T&gt;, tuple_element_t&lt;0, T&gt;&gt;;</del>
    
[&hellip;]

<del>template&lt;<i>iterator-sentinel-pair</i> P&gt;
  subrange(P) -&gt; subrange&lt;tuple_element_t&lt;0, P&gt;, tuple_element_t&lt;1, P&gt;&gt;;</del>

<del>template&lt;<i>iterator-sentinel-pair</i> P&gt;
  subrange(P, <i>make-unsigned-like-t</i>(iter_difference_t&lt;tuple_element_t&lt;0, P&gt;&gt;)) -&gt;
    subrange&lt;uple_element_t&lt;0, P&gt;, tuple_element_t&lt;1, P&gt;, subrange_kind::sized&gt;;</del>
[&hellip;]
</pre></blockquote>
</li>
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