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<title>Issue 3896: The definition of viewable_range is not quite right</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#New">New</a> status.</em></p>
<h3 id="3896"><a href="lwg-active.html#3896">3896</a>. The definition of <code>viewable_range</code> is not quite right</h3>
<p><b>Section:</b> 25.4.6 <a href="https://wg21.link/range.refinements">[range.refinements]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-02-27 <b>Last modified:</b> 2023-03-22</p>
<p><b>Priority: </b>4
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.refinements">issues</a> in [range.refinements].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The requirements of <code>viewable_range</code> for <code>view</code> type is <code>view&lt;remove_cvref_t&lt;T&gt;&gt; &amp;&amp; 
constructible_from&lt;remove_cvref_t&lt;T&gt;, T&gt;</code>, that is, when the decayed type of <code>T</code> models <code>view</code>, 
it must be constructible from <code>T</code>.
</p>
<p>
This part of the constraint corresponds to first bullet of <code>views::all</code> (25.7.6.1 <a href="https://wg21.link/range.all.general">[range.all.general]</a>), 
which returns <code><i>decay-copy</i>(E)</code> if the decayed type of <code>E</code> models <code>view</code>.
</p>
<p>
However, <code><i>decay-copy</i>(E)</code> constraints <code>convertible_to&lt;T, decay_t&lt;T&gt;&gt;</code> which is a 
stronger requirement than <code>constructible_from</code>, which is reflected in its rejection of types with explicit 
copy constructors.
</p>
<p>
This inconsistency is such that the following causes the range adapter to produce a hard error when invoked 
(<a href="https://godbolt.org/z/PxWfs1Yco">online example</a>):
</p>
<blockquote><pre>
#include &lt;ranges&gt;

struct View : std::ranges::view_base 
{
  View();
  explicit View(const View&amp;); // explicit copy constructor
  View&amp; operator=(const View&amp;);
  View(View&amp;&amp;);
  
  int* begin();
  int* end();
};

int main() 
{
  View v;
  auto r = std::views::take(v, 5); // <span style="color:red;font-weight:bolder">hard error</span>
}
</pre></blockquote>

<p><i>[2023-03-22; Reflector poll]</i></p>

<p>
Set priority to 4 after reflector poll.
"About as contrived as it gets."
"Add generic front matter telling users the library doesn't support types
with explicit copy constructors."
</p>



<p id="res-3896"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4928" title=" Working Draft, Standard for Programming Language C++">N4928</a>.
</p>

<ol>

<li><p>Modify 25.4.6 <a href="https://wg21.link/range.refinements">[range.refinements]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class T&gt;
  concept viewable_range =
    range&lt;T&gt; &amp;&amp;
    ((view&lt;remove_cvref_t&lt;T&gt;&gt; &amp;&amp; <ins>convertible_to&lt;T, remove_cvref_t&lt;T&gt;&gt;</ins><del>constructible_from&lt;remove_cvref_t&lt;T&gt;, T&gt;</del>) ||
     (!view&lt;remove_cvref_t&lt;T&gt;&gt; &amp;&amp;
      (is_lvalue_reference_v&lt;T&gt; || (movable&lt;remove_reference_t&lt;T&gt;&gt; &amp;&amp; !<i>is-initializer-list</i>&lt;T&gt;))));
</pre>
</blockquote>

</li>

</ol>





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