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<title>Issue 3428: single_view's in place constructor should be explicit</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="3428"><a href="lwg-defects.html#3428">3428</a>. <code>single_view</code>'s in place constructor should be explicit</h3>
<p><b>Section:</b> 25.6.3.2 <a href="https://wg21.link/range.single.view">[range.single.view]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2020-04-07 <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.single.view">issues</a> in [range.single.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>
The <code>in_place_t</code> constructor template of <code>single_view</code> is not explicit:
</p>
<blockquote><pre>
template&lt;class... Args&gt;
  requires constructible_from&lt;T, Args...&gt;
constexpr single_view(in_place_t, Args&amp;&amp;... args);
</pre></blockquote>
<p>
so it defines an implicit conversion from <code>std::in_place_t</code> to
<code>single_view&lt;T&gt;</code> whenever <code>constructible_from&lt;T&gt;</code> is modeled, 
which seems unlikely to be the intent.
</p>

<p><i>[2020-04-18 Issue Prioritization]</i></p>
 
<p>
Status set to Tentatively Ready after six 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-3428"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.
</p>

<ol>
<li><p>Modify 25.6.3.2 <a href="https://wg21.link/range.single.view">[range.single.view]</a> as indicated:</p>

<blockquote>
<blockquote>
<pre>
namespace std::ranges {
  template&lt;copy_constructible T&gt;
    requires is_object_v&lt;T&gt;
  class single_view : public view_interface&lt;single_view&lt;T&gt;&gt; {
    [&hellip;]
  public:
    [&hellip;]
    template&lt;class... Args&gt;
      requires constructible_from&lt;T, Args...&gt;
    constexpr <ins>explicit</ins> single_view(in_place_t, Args&amp;&amp;... args);

    [&hellip;]
  };
}
</pre>
</blockquote>
[&hellip;]
<pre>
template&lt;class... Args&gt;
constexpr <ins>explicit</ins> single_view(in_place_t, Args&amp;&amp;... args);
</pre>

<blockquote>
<p>
-3- <i>Effects:</i> Initializes <code>value_</code> as if by <code>value_{in_place, std::forward&lt;Args&gt;(args)...}</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>




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