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<title>Issue 3405: common_view's converting constructor is bad, too</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="3405"><a href="lwg-defects.html#3405">3405</a>. <code>common_view</code>'s converting constructor is bad, too</h3>
<p><b>Section:</b> 25.7.20.2 <a href="https://wg21.link/range.common.view">[range.common.view]</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.common.view">issues</a> in [range.common.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>
LWG <a href="lwg-defects.html#3280" title="View converting constructors can cause constraint recursion and are unneeded (Status: C++20)">3280</a><sup><a href="https://cplusplus.github.io/LWG/issue3280" title="Latest snapshot">(i)</a></sup> struck the problematic/extraneous converting constructor templates from the 
<code><i>meow</i>_view</code> range adaptor types in the standard library with the exception of <code>common_view</code>. 
The omission of <code>common_view</code> seems to have been simply an oversight: its converting constructor 
template is no less problematic or extraneous. We should remove <code>common_view</code>'s converting 
constructor template as well to finish the task. Both <code>cmcstl2</code> and <code>range-v3</code> removed 
the converting constructor template from <code>common_view</code> when removing the other converting 
constructor templates, so we have implementation experience that this change is good as well as 
consistent with the general thrust of LWG <a href="lwg-defects.html#3280" title="View converting constructors can cause constraint recursion and are unneeded (Status: C++20)">3280</a><sup><a href="https://cplusplus.github.io/LWG/issue3280" title="Latest snapshot">(i)</a></sup>.
</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-3405"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4849">N4849</a>.
</p>

<ol>
<li><p>Modify 25.7.20.2 <a href="https://wg21.link/range.common.view">[range.common.view]</a>, class template <code>common_view</code> synopsis, as indicated:</p>

<blockquote>
<pre>
  
  [&hellip;]
  constexpr explicit common_view(V r);
  
  <del>template&lt;viewable_range R&gt;
    requires (!common_range&lt;R&gt; &amp;&amp; constructible_from&lt;V, all_view&lt;R&gt;&gt;)
  constexpr explicit common_view(R&amp;&amp; r);</del>
  
  constexpr V base() const&amp; requires copy_constructible&lt;V&gt; { return base_; }
  [&hellip;]
  
</pre>
[&hellip;]
<pre>
<del>template&lt;viewable_range R&gt;
  requires (!common_range&lt;R&gt; &amp;&amp; constructible_from&lt;V, all_view&lt;R&gt;&gt;)
constexpr explicit common_view(R&amp;&amp; r);</del>
</pre>
<blockquote>
<p>
<del>-2- <i>Effects:</i> Initializes <code>base_</code> with <code>views::all(std::forward&lt;R&gt;(r))</code>.</del>
</p>
</blockquote>
</blockquote>
</li>
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