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<title>Issue 3323: has-tuple-element helper concept needs convertible_to</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++20">C++20</a> status.</em></p>
<h3 id="3323"><a href="lwg-defects.html#3323">3323</a>. <code><i>has-tuple-element</i></code> helper concept needs <code>convertible_to</code></h3>
<p><b>Section:</b> 25.7.23.2 <a href="https://wg21.link/range.elements.view">[range.elements.view]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Great Britain <b>Opened:</b> 2019-11-06 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.elements.view">issues</a> in [range.elements.view].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses <a href="https://github.com/cplusplus/nbballot/issues/295">GB 299</a></b></p>

<p>
<code><i>has-tuple-element</i></code> helper concept needs <code>convertible_to</code>
<p/>
The exposition-only <code><i>has-tuple-element</i></code> concept (for <code>elements_view</code>) is defined as
</p>
<blockquote><pre>
template&lt;class T, size_t N&gt;
concept <i>has-tuple-element</i> = <i>exposition only</i>
  requires(T t) {
    typename tuple_size&lt;T&gt;::type;
    requires N &lt; tuple_size_v&lt;T&gt;;
    typename tuple_element_t&lt;N, T&gt;;
    { get&lt;N&gt;(t) } -&gt; const tuple_element_t&lt;N, T&gt;&amp;;
  };
</pre></blockquote>
<p>
However, the return type constraint for <code>{ get&lt;N&gt;(t) }</code> is no longer valid 
under the latest concepts changes
<p/>
Proposed change:
</p>
<p>
Change to:
</p>
<blockquote><pre>
template&lt;class T, size_t N&gt;
concept <i>has-tuple-element</i> = <i>exposition only</i>
  requires(T t) {
    typename tuple_size&lt;T&gt;::type;
    requires N &lt; tuple_size_v&lt;T&gt;;
    typename tuple_element_t&lt;N, T&gt;;
    { get&lt;N&gt;(t) } -&gt; convertible_to&lt;const tuple_element_t&lt;N, T&gt;&amp;&gt;;
  };
</pre></blockquote>
<p>
<b>Jonathan Wakely:</b>
<p/>
The NB comment says "The return type constraint for <code>{ get(t) }</code> is no longer valid under 
the latest concepts changes." The changes referred to are those in <a href="https://wg21.link/p1452r2">P1452R2</a>. 
</p>

<p><i>[2019-11 Status to Ready during Wednesday night issue processing in Belfast.]</i></p>



<p id="res-3323"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4835">N4835</a>.</p>

<ol>
<li><p>Modify 25.7.23.2 <a href="https://wg21.link/range.elements.view">[range.elements.view]</a>, class template <code>elements_view</code> synopsis, 
as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  template&lt;class T, size_t N&gt;
  concept <i>has-tuple-element</i> = <i>// exposition only</i>
    requires(T t) {
      typename tuple_size&lt;T&gt;::type;
      requires N &lt; tuple_size_v&lt;T&gt;;
      typename tuple_element_t&lt;N, T&gt;;
      { get&lt;N&gt;(t) } -&gt; <ins>convertible_to&lt;</ins>const tuple_element_t&lt;N, T&gt;&amp;<ins>&gt;</ins>;
    };
[&hellip;]
}
</pre>
</blockquote>
</li>

</ol>





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