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  <title>Allow defaulting comparisons by value</title>
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<h1 class="title" style="text-align:center">Allow defaulting comparisons by value</h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #: </td>
    <td>P1946R0</td>
  </tr>
  <tr>
    <td>Date: </td>
    <td>2019-11-08</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project: </td>
    <td>Programming Language C++<br>
      EWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to: </td>
    <td>
      Barry Revzin<br>&lt;<a href="mailto:barry.revzin@gmail.com" class="email">barry.revzin@gmail.com</a>&gt;<br>
      Casey Carter<br>&lt;<a href="mailto:casey@carter.com" class="email">casey@carter.com</a>&gt;<br>
    </td>
  </tr>
</table>

</header>
<div style="clear:both">
<h1 id="introduction-and-motivation" style="border-bottom:1px solid #cccccc"><span class="header-section-number">1</span> Introduction and Motivation<a href="#introduction-and-motivation" class="self-link"></a></h1>
<p>This is currently ill-formed:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="kw">struct</span> C <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2"></a>    <span class="dt">int</span> i;</span>
<span id="cb1-3"><a href="#cb1-3"></a>    <span class="kw">friend</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">==(</span>C, C<span class="op">)</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb1-4"><a href="#cb1-4"></a><span class="op">}</span>;</span></code></pre></div>
<p>Because the rule for defaulted comparisons in [class.compare.default] requires that a defaulted comparison for a class <code class="sourceCode cpp">C</code> be either:</p>
<blockquote>
<ul>
<li>a non-static const member of <code class="sourceCode cpp">C</code> having one parameter of type <code class="sourceCode cpp"><span class="kw">const</span> C<span class="op">&amp;</span></code>, or</li>
</ul>
</blockquote>
<blockquote>
<ul>
<li>a friend of <code class="sourceCode cpp">C</code> having two parameters of type <code class="sourceCode cpp"><span class="kw">const</span> C<span class="op">&amp;</span></code>.</li>
</ul>
</blockquote>
<p>There is no option for having two parameters of type <code class="sourceCode cpp">C</code>. This case appears not to have been previously discussed by Evolution, did not appear in any previous comparison papers, though was sort of mentioned in the Kona 2017 discussion of <span class="citation" data-cites="P0515R0">[<a href="#ref-P0515R0" role="doc-biblioref">P0515R0</a>]</span> that taking by-value was intended.</p>
<p>From Casey:</p>
<blockquote>
<p>I’ve been using C++20 <code class="sourceCode cpp"><span class="op">==</span></code> and <code class="sourceCode cpp"><span class="op">&lt;=&gt;</span></code> quite a bit implementing test cases for Ranges things, and I can say that it’s jarring to implement a class that’s passed by value everywhere <em>except</em> for defaulted <code class="sourceCode cpp"><span class="op">==</span></code> and/or <code class="sourceCode cpp"><span class="op">&lt;=&gt;</span></code>. I’d like to see [class.compare.default]/1.1 changed as well to allow “a non-static const member of <code class="sourceCode cpp">C</code> having one parameter of type <code class="sourceCode cpp"><span class="kw">const</span> C<span class="op">&amp;</span></code> <span class="addu">or <code class="sourceCode cpp">C</code></span>, or”. I can’t see any particular reason to forbid pass-by-value for member or non-member defaulted comparison operators.</p>
</blockquote>
<p>This seems like Core issue material to us, but just in case we’re writing this paper to fix this minor defect, as we would rather fix it in the language than have to deal with fixing it in the library (as in <span class="citation" data-cites="LWG3295">[<a href="#ref-LWG3295" role="doc-biblioref">LWG3295</a>]</span>) and the inevitable user questions that will come up when this doesn’t work.</p>
<p>There is a certain class of type that should be taken by value instead of by reference to const - and those situations surely include that type’s own comparison operators.</p>
<h1 id="evolution-direction" style="border-bottom:1px solid #cccccc"><span class="header-section-number">2</span> Evolution Direction<a href="#evolution-direction" class="self-link"></a></h1>
<p>A draft of this paper was presented to Evolution in Belfast, which decided that they wanted to allow the friend by-value case but none of the mixed-const cases. In other words, allow <code class="sourceCode cpp">A</code> but disallow <code class="sourceCode cpp">B</code> and <code class="sourceCode cpp">C</code> below:</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a><span class="kw">struct</span> A <span class="op">{</span></span>
<span id="cb2-2"><a href="#cb2-2"></a>    <span class="kw">friend</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">==(</span>A, A<span class="op">)</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb2-3"><a href="#cb2-3"></a><span class="op">}</span>;</span>
<span id="cb2-4"><a href="#cb2-4"></a></span>
<span id="cb2-5"><a href="#cb2-5"></a><span class="kw">struct</span> B <span class="op">{</span></span>
<span id="cb2-6"><a href="#cb2-6"></a>    <span class="dt">bool</span> <span class="kw">operator</span><span class="op">==(</span>B<span class="op">)</span> <span class="kw">const</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb2-7"><a href="#cb2-7"></a><span class="op">}</span>;</span>
<span id="cb2-8"><a href="#cb2-8"></a></span>
<span id="cb2-9"><a href="#cb2-9"></a><span class="kw">struct</span> C <span class="op">{</span></span>
<span id="cb2-10"><a href="#cb2-10"></a>    <span class="kw">friend</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">==(</span>C, C <span class="kw">const</span><span class="op">&amp;)</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb2-11"><a href="#cb2-11"></a><span class="op">}</span>;</span></code></pre></div>
<h1 id="wording" style="border-bottom:1px solid #cccccc"><span class="header-section-number">3</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Change 11.1.1 [class.compare.default], paragraph 1:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span> A defaulted comparison operator function ([expr.spaceship], [expr.rel], [expr.eq]) for some class <code class="sourceCode cpp">C</code> shall be a non-template function declared in the <em>member-specification</em> of <code class="sourceCode cpp">C</code> that is</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(1.1)</a></span> a non-static const member of <code class="sourceCode cpp">C</code> having one parameter of type <code class="sourceCode cpp"><span class="kw">const</span> C<span class="op">&amp;</span></code>, or</li>
<li><span class="marginalizedparent"><a class="marginalized">(1.2)</a></span> a friend of <code class="sourceCode cpp">C</code> having <span class="addu">either</span> two parameters of type <code class="sourceCode cpp"><span class="kw">const</span> C<span class="op">&amp;</span></code> <span class="addu">or two parameters of type <code class="sourceCode cpp">C</code></span>.</li>
</ul>
</blockquote>
<h1 id="references" style="border-bottom:1px solid #cccccc"><span class="header-section-number">4</span> References<a href="#references" class="self-link"></a></h1>

<div id="refs" role="doc-bibliography">
<div id="ref-LWG3295">
<p>[LWG3295] Barry Revzin. Comparison category operator== are mis-specified. <br />
<a href="https://wg21.link/lwg3295">https://wg21.link/lwg3295</a></p>
</div>
<div id="ref-P0515R0">
<p>[P0515R0] Herb Sutter. 2017. Consistent comparison. <br />
<a href="https://wg21.link/p0515r0">https://wg21.link/p0515r0</a></p>
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