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<title>Issue 3724: decay-copy should be constrained</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="3724"><a href="lwg-defects.html#3724">3724</a>. <code><i>decay-copy</i></code> should be constrained</h3>
<p><b>Section:</b> 16.3.3.2 <a href="https://wg21.link/expos.only.entity">[expos.only.entity]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hui Xie <b>Opened:</b> 2022-06-23 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#expos.only.entity">issues</a> in [expos.only.entity].</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 spec of <code>views::all</code> 25.7.6.1 <a href="https://wg21.link/range.all.general">[range.all.general]</a> p2 says:
</p>
<blockquote><p>
Given a subexpression <code>E</code>, the expression <code>views::all(E)</code> is expression-equivalent to:
</p>
<ol style="list-style-type: none">
<li><p>(2.1) &mdash; <code><i>decay-copy</i>(E)</code> if the decayed type of E models <code>view</code>.</p></li>
<li><p>[&hellip;]</p></li>
</ol>
</blockquote>
<p>
If <code>E</code> is an lvalue move-only <code>view</code>, according to the spec, <code>views::all(E)</code> 
would be expression-equivalent to <code><i>decay-copy</i>(E)</code>.
<p/>
However,  [expos.only.func] p2 defines <code><i>decay-copy</i></code> as follows
</p>
<blockquote><pre>
template&lt;class T&gt; constexpr decay_t&lt;T&gt; <i>decay-copy</i>(T&amp;&amp; v)
    noexcept(is_nothrow_convertible_v&lt;T, decay_t&lt;T&gt;&gt;)         <i>// exposition only</i>
  { return std::forward&lt;T&gt;(v); }
</pre></blockquote>
<p>
It is unconstrained.  As a result, for the above example, <code>views::all(E)</code> is a well-formed 
expression and it would only error on the template instantiation of the function body of 
<code><i>decay-copy</i></code>, because <code>E</code> is an lvalue of move-only type and not copyable.
<p/>
I think this behaviour is wrong, instead, we should make <code><i>decay-copy(E)</i></code> ill-formed 
if it is not copyable, so that <code>views::all(E)</code> can be SFINAE friendly.
</p>

<p><i>[2022-07-08; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>


<p><i>[2022-07-11; Reflector poll]</i></p>

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

<p><i>[2022-07-15; LWG telecon: move to Ready]</i></p>


<p><i>[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready &rarr; WP.]</i></p>



<p id="res-3724"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>
<li><p>Modify  [expos.only.func] as indicated:</p>

<blockquote>
<p>
-2- The following are defined for exposition only to aid in the specification of the library:
</p>
<blockquote>
<pre>
namespace std {
  template&lt;class T&gt; 
    <ins>requires convertible_to&lt;T, decay_t&lt;T&gt;&gt;</ins>
      constexpr decay_t&lt;T&gt; decay-copy(T&amp;&amp; v)
        noexcept(is_nothrow_convertible_v&lt;T, decay_t&lt;T&gt;&gt;) <i>// exposition only</i>
      { return std::forward&lt;T&gt;(v); }
  [&hellip;]
}
</pre>
</blockquote>
</blockquote>
</li>

</ol>





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