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<title>Issue 4122: Ill-formed operator&lt;=&gt; can cause hard error when instantiating std::inplace_vector</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="4122"><a href="lwg-active.html#4122">4122</a>. Ill-formed <code>operator&lt;=&gt;</code> can cause hard error when instantiating <code>std::inplace_vector</code></h3>
<p><b>Section:</b> 23.3.16.1 <a href="https://wg21.link/inplace.vector.overview">[inplace.vector.overview]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2024-07-20 <b>Last modified:</b> 2024-08-02</p>
<p><b>Priority: </b>2
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
This is almost the same problem as LWG <a href="lwg-defects.html#4071" title="reference_wrapper comparisons are not SFINAE-friendly (Status: WP)">4071</a><sup><a href="https://cplusplus.github.io/LWG/issue4071" title="Latest snapshot">(i)</a></sup> except that it happens to <code>std::inplace_vector</code>. 
As the <code>operator&lt;=&gt;</code> overload for <code>std::inplace_vector</code> is a non-template function whose return 
type (<code><i>synth-three-way-result</i>&lt;T&gt;</code>) is not deduced, when the return type is ill-formed, hard 
error occurs in the instantiation of the enclosing <code>std::inplace_vector&lt;T, N&gt;</code>.
</p>

<p><i>[2024-08-02; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
</p>



<p id="res-4122"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4986" title=" Working Draft, Programming Languages — C++">N4986</a>.
</p>

<ol>
<li><p>Modify 23.3.16.1 <a href="https://wg21.link/inplace.vector.overview">[inplace.vector.overview]</a>, class template <code>std::inplace_vector</code> synopsis, as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class T, size_t N&gt;
  class inplace_vector {
  public:
    [&hellip;]
    constexpr friend bool operator==(const inplace_vector&amp; x,
                                     const inplace_vector&amp; y);
    constexpr friend <del><i>synth-three-way-result</i>&lt;T&gt;</del><ins>auto</ins>
      operator&lt;=&gt;(const inplace_vector&amp; x, const inplace_vector&amp; y)<del>;</del><ins>
        requires requires (const T t) { <i>synth-three-way</i>(t, t); } 
      {
        return lexicographical_compare_three_way(x.begin(), x.end(), y.begin(), y.end(),
                                                 <i>synth-three-way</i>);
      }</ins>
    constexpr friend void swap(inplace_vector&amp; x, inplace_vector&amp; y)
      noexcept(N == 0 || (is_nothrow_swappable_v&lt;T&gt; &amp;&amp;
                          is_nothrow_move_constructible_v&lt;T&gt;))
    { x.swap(y); }
  };
};
</pre>
</blockquote>

</li>
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