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  <title>Wording for boolean-testable</title>
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<h1 class="title" style="text-align:center">Wording for <em><code class="sourceCode cpp">boolean<span class="op">-</span>testable</code></em></h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #: </td>
    <td>P1964R1</td>
  </tr>
  <tr>
    <td>Date: </td>
    <td>2020-01-11</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project: </td>
    <td>Programming Language C++<br>
      LWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to: </td>
    <td>
      Tim Song<br>&lt;<a href="mailto:t.canens.cpp@gmail.com" class="email">t.canens.cpp@gmail.com</a>&gt;<br>
    </td>
  </tr>
</table>

</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#introduction"><span class="toc-section-number">1</span> Introduction<span></span></a></li>
<li><a href="#lewg-belfast-vote"><span class="toc-section-number">2</span> LEWG Belfast vote<span></span></a></li>
<li><a href="#drafting-notes"><span class="toc-section-number">3</span> Drafting notes<span></span></a></li>
<li><a href="#wording"><span class="toc-section-number">4</span> Wording<span></span></a></li>
<li><a href="#references"><span class="toc-section-number">5</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="introduction"><span class="header-section-number">1</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>This paper provides wording for replacing <code class="sourceCode cpp">boolean</code> with the <em><code class="sourceCode cpp">boolean<span class="op">-</span>testable</code></em> exposition-only concept, as proposed in <span class="citation" data-cites="P1964R0">[<a href="#ref-P1964R0" role="doc-biblioref">P1964R0</a>]</span>. For detailed motivation and discussion, see that paper.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="lewg-belfast-vote"><span class="header-section-number">2</span> LEWG Belfast vote<a href="#lewg-belfast-vote" class="self-link"></a></h1>
<p>An early draft of <span class="citation" data-cites="P1964R0">[<a href="#ref-P1964R0" role="doc-biblioref">P1964R0</a>]</span> was presented to LEWG on Friday afternoon in Belfast. The following polls were taken:</p>
<blockquote>
<p>Introduce an exposition-only concept <code class="sourceCode cpp"><span class="er">$</span>NAME</code> as a replacement for the existing use of the <code class="sourceCode cpp">boolean</code> concept.</p>
<ul>
<li>subsume <code class="sourceCode cpp">convertible_to</code></li>
<li>add syntactic checking for <code class="sourceCode cpp"><span class="op">!</span></code></li>
<li>add semantic requirements for “there is no <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">||</span></code> nor <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">&amp;&amp;</span></code> that can be found in any use of T”</li>
</ul>
<p>In favor of the above design.</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>SF</strong>
</div></th>
<th><div style="text-align:center">
<strong>F</strong>
</div></th>
<th><div style="text-align:center">
<strong>N</strong>
</div></th>
<th><div style="text-align:center">
<strong>A</strong>
</div></th>
<th><div style="text-align:center">
<strong>SA</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td>3</td>
<td>11</td>
<td>2</td>
<td>0</td>
<td>0</td>
</tr>
</tbody>
</table>
<p>Name is <em><code class="sourceCode cpp">boolean<span class="op">-</span>testable</code></em></p>
<p>Unanimous consent</p>
<p>Tim and Casey will update P1964 with wording for the above design. We forward that paper to LWG for C++20.</p>
<p>Unanimous consent</p>
</blockquote>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="drafting-notes"><span class="header-section-number">3</span> Drafting notes<a href="#drafting-notes" class="self-link"></a></h1>
<p>Yes, this is a lot of words to specify what is basically “don’t be dumb”. Defining “dumb” with sufficient precision turns out to be complicated.</p>
<p>The basic requirement is that a <em><code class="sourceCode cpp">boolean<span class="op">-</span>testable</code></em> type must not contribute any potentially viable <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">&amp;&amp;</span></code> or <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">||</span></code> overload to the overload set. If so, then any two <em><code class="sourceCode cpp">boolean<span class="op">-</span>testable</code></em> types can be freely used with these operators, because there are no viable user-defined overloads and therefore they must have their built-in meaning.</p>
<p>There are three cases to consider:</p>
<ol type="1">
<li>Members. These are relatively straightforward: no <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">&amp;&amp;</span></code> or <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">||</span></code>, period.</li>
<li>Non-member functions. These are also simple: we know the concrete types, so they are either potentially viable (an implicit conversion sequence exists from the source expression to one of the parameter types) or they are not.</li>
<li>Non-member function templates. These are the hardest because we don’t know the parameter types at all and must therefore resort to template argument deduction.</li>
</ol>
<p>There are some wrinkles for this third case.</p>
<h3 class="unnumbered" data-number id="non-deduced-contexts">Non-deduced contexts<a href="#non-deduced-contexts" class="self-link"></a></h3>
<p>Consider the following example:</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">namespace</span> X <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2"></a>   <span class="kw">enum</span> A <span class="op">{}</span>;</span>
<span id="cb1-3"><a href="#cb1-3"></a></span>
<span id="cb1-4"><a href="#cb1-4"></a>   <span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span><span class="op">&gt;</span></span>
<span id="cb1-5"><a href="#cb1-5"></a>   <span class="kw">struct</span> trait <span class="op">{</span> <span class="kw">using</span> type <span class="op">=</span> A; <span class="op">}</span>;</span>
<span id="cb1-6"><a href="#cb1-6"></a></span>
<span id="cb1-7"><a href="#cb1-7"></a>   <span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb1-8"><a href="#cb1-8"></a>   <span class="dt">void</span> <span class="kw">operator</span><span class="op">&amp;&amp;(</span>T<span class="op">*</span>, <span class="kw">typename</span> trait<span class="op">&lt;</span>T<span class="op">&gt;::</span>type<span class="op">)</span>;</span>
<span id="cb1-9"><a href="#cb1-9"></a> <span class="op">}</span></span></code></pre></div>
<p>This <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">&amp;&amp;</span></code> will be picked up in an expression like <code class="sourceCode cpp"><span class="op">(</span><span class="dt">int</span><span class="op">*)</span> <span class="kw">nullptr</span> <span class="op">&amp;&amp;</span> X<span class="op">::</span>A<span class="op">()</span></code>. Since <code class="sourceCode cpp"><span class="dt">int</span><span class="op">*</span></code> should model <em><code class="sourceCode cpp">boolean<span class="op">-</span>testable</code></em> (and certainly isn’t responsible for this operator), we must make <code class="sourceCode cpp">X<span class="op">::</span>A</code> not model <em><code class="sourceCode cpp">boolean<span class="op">-</span>testable</code></em>. In other words, a non-deduced context, which can encode an arbitrary type transformation, must be considered to match everything.</p>
<h3 class="unnumbered" data-number id="stdvalarray"><code class="sourceCode cpp">std<span class="op">::</span>valarray</code><a href="#stdvalarray" class="self-link"></a></h3>
<p>As mentioned in <span class="citation" data-cites="P1964R0">[<a href="#ref-P1964R0" role="doc-biblioref">P1964R0</a>]</span>, the wording needs to avoid catching overloads like <code class="sourceCode cpp">std<span class="op">::</span>valarray</code>’s <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">&amp;&amp;</span></code> on unsuspecting types in <code class="sourceCode cpp"><span class="kw">namespace</span> std</code>. That is, we want declarations like</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">template</span><span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb2-2"><a href="#cb2-2"></a>valarray<span class="op">&lt;</span><span class="dt">bool</span><span class="op">&gt;</span> <span class="kw">operator</span><span class="op">&amp;&amp;(</span><span class="kw">const</span> valarray<span class="op">&lt;</span>T<span class="op">&gt;&amp;</span>, <span class="kw">const</span> <span class="kw">typename</span> valarray<span class="op">&lt;</span>T<span class="op">&gt;::</span>value_type<span class="op">&amp;)</span>;</span></code></pre></div>
<p>and its pre-<span class="citation" data-cites="LWG3074">[<a href="#ref-LWG3074" role="doc-biblioref">LWG3074</a>]</span> form</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb3-2"><a href="#cb3-2"></a>valarray<span class="op">&lt;</span><span class="dt">bool</span><span class="op">&gt;</span> <span class="kw">operator</span><span class="op">&amp;&amp;(</span><span class="kw">const</span> valarray<span class="op">&lt;</span>T<span class="op">&gt;&amp;</span>, <span class="kw">const</span> T<span class="op">&amp;)</span>;</span></code></pre></div>
<p>to disqualify specializations of <code class="sourceCode cpp">std<span class="op">::</span>valarray</code> (or any derived class that might have added a conversion to <code class="sourceCode cpp"><span class="dt">bool</span></code>), but not <code class="sourceCode cpp">std<span class="op">::</span>true_type</code>.</p>
<p>The distinguishing characteristics of these overloads are</p>
<ol type="1">
<li>They are part of the interface of some class template (e.g., <code class="sourceCode cpp">std<span class="op">::</span>valarray</code>);</li>
<li>They have a function parameter whose (uncvref’d) type is a specialization of that class template;</li>
<li>They are a member of the same namespace as that class template (so that they can be found by ADL)</li>
</ol>
<p>For these overloads, we can safely only consider the parameter(s) satisfying #2 above (called <em>key parameters</em> in the wording below). <span style="color:darkblue">This is because for template argument deduction to succeed on such a parameter, the type of the provided argument must be either a specialization of that class template or derived from it.</span> <span style="color:darkgreen">If so, then <em>that argument</em> must necessarily also bring this function template into the overload set.</span> As long as the wording excludes such arguments, then, there is no need to worry about other types that may happen to belong in the same namespace.</p>
<p>I have color-coded the two steps in this reasoning because there are corner cases involving each, which I’ll now discuss.</p>
<h4 class="unnumbered" data-number id="non-deduced-contexts-again">Non-deduced contexts, again<a href="#non-deduced-contexts-again" class="self-link"></a></h4>
<p>Consider this example:</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1"></a><span class="kw">namespace</span> Y <span class="op">{</span></span>
<span id="cb4-2"><a href="#cb4-2"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span><span class="op">&gt;</span></span>
<span id="cb4-3"><a href="#cb4-3"></a>  <span class="kw">struct</span> C <span class="op">{}</span>;</span>
<span id="cb4-4"><a href="#cb4-4"></a></span>
<span id="cb4-5"><a href="#cb4-5"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb4-6"><a href="#cb4-6"></a>  <span class="dt">void</span> <span class="kw">operator</span><span class="op">&amp;&amp;(</span>C<span class="op">&lt;</span>T<span class="op">&gt;</span> x, T y<span class="op">)</span>;</span>
<span id="cb4-7"><a href="#cb4-7"></a></span>
<span id="cb4-8"><a href="#cb4-8"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb4-9"><a href="#cb4-9"></a>  <span class="dt">void</span> <span class="kw">operator</span><span class="op">||(</span>C<span class="op">&lt;</span>std<span class="op">::</span>decay_t<span class="op">&lt;</span>T<span class="op">&gt;&gt;</span> x, T y<span class="op">)</span>;</span>
<span id="cb4-10"><a href="#cb4-10"></a></span>
<span id="cb4-11"><a href="#cb4-11"></a>  <span class="kw">enum</span> A <span class="op">{}</span>;</span>
<span id="cb4-12"><a href="#cb4-12"></a><span class="op">}</span></span>
<span id="cb4-13"><a href="#cb4-13"></a></span>
<span id="cb4-14"><a href="#cb4-14"></a><span class="kw">struct</span> B <span class="op">{</span> <span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span> <span class="kw">operator</span> T<span class="op">()</span>; <span class="op">}</span>;</span></code></pre></div>
<p>We don’t want the <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">&amp;&amp;</span></code> declaration to disqualify <code class="sourceCode cpp">Y<span class="op">::</span>A</code>; however the expression <code class="sourceCode cpp"><span class="op">::</span>B<span class="op">()</span> <span class="op">||</span> Y<span class="op">::</span>A<span class="op">()</span></code> will use the <code class="sourceCode cpp">Y<span class="op">::</span><span class="kw">operator</span><span class="op">||</span></code> overload, and therefore <code class="sourceCode cpp">Y<span class="op">::</span>A</code> cannot be <em><code class="sourceCode cpp">boolean<span class="op">-</span>testable</code></em>, even though its declaration might be superficially similar. The key difference here is that <code class="sourceCode cpp">C<span class="op">&lt;</span>std<span class="op">::</span>decay_t<span class="op">&lt;</span>T<span class="op">&gt;&gt;</span></code> doesn’t contain anything that participates in template argument deduction, and therefore no longer requires the argument to have any relation to <code class="sourceCode cpp">C</code>, contrary to the <span style="color:darkblue">first sentence of the reasoning above</span>.</p>
<p>To qualify as a <em>key parameter</em>, then, the type must contain at least one template parameter that participates in template argument deduction.</p>
<h4 class="unnumbered" data-number id="hidden-friends">Hidden friends<a href="#hidden-friends" class="self-link"></a></h4>
<p>Hidden friends strike at the <span style="color:darkgreen">second sentence of the reasoning above</span>. Consider:</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1"></a><span class="kw">namespace</span> Z <span class="op">{</span></span>
<span id="cb5-2"><a href="#cb5-2"></a>  <span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span><span class="op">&gt;</span></span>
<span id="cb5-3"><a href="#cb5-3"></a>  <span class="kw">struct</span> A <span class="op">{</span></span>
<span id="cb5-4"><a href="#cb5-4"></a>    <span class="kw">operator</span> <span class="dt">bool</span><span class="op">()</span>;</span>
<span id="cb5-5"><a href="#cb5-5"></a>  <span class="op">}</span>;</span>
<span id="cb5-6"><a href="#cb5-6"></a></span>
<span id="cb5-7"><a href="#cb5-7"></a>  <span class="kw">struct</span> B <span class="op">{</span></span>
<span id="cb5-8"><a href="#cb5-8"></a>    <span class="kw">operator</span> <span class="dt">bool</span><span class="op">()</span>;</span>
<span id="cb5-9"><a href="#cb5-9"></a>    <span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb5-10"><a href="#cb5-10"></a>    <span class="kw">friend</span> <span class="dt">void</span> <span class="kw">operator</span><span class="op">&amp;&amp;(</span>A<span class="op">&lt;</span>T<span class="op">&gt;</span>, B<span class="op">)</span>;</span>
<span id="cb5-11"><a href="#cb5-11"></a>  <span class="op">}</span>;</span>
<span id="cb5-12"><a href="#cb5-12"></a><span class="op">}</span></span></code></pre></div>
<p><code class="sourceCode cpp">Z<span class="op">::</span>A<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;()</span> <span class="op">&amp;&amp;</span> Z<span class="op">::</span>B<span class="op">()</span></code> will use the <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">&amp;&amp;</span></code> overload, but ADL for <code class="sourceCode cpp">Z<span class="op">::</span>A<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;()</span></code> alone will not even find the hidden friend overload (indeed, the author of <code class="sourceCode cpp">Z<span class="op">::</span>A</code> might have nothing to do with it). So we must disqualify <code class="sourceCode cpp">Z<span class="op">::</span>B</code> instead.</p>
<p>The problem here is that the hidden friend can be a friend of the wrong class. That means that the <span style="color:darkgreen">second sentence of the reasoning above</span> no longer applies, because we are no longer guaranteed that the argument related to <code class="sourceCode cpp">A</code> will bring the overload in.</p>
<p>The wording below excludes hidden friends from the <em>key parameter</em> special case: hidden friends disqualify a type from modeling <em><code class="sourceCode cpp">boolean<span class="op">-</span>testable</code></em> if template argument deduction for either parameter succeeds. Note that the concern motivating this special case doesn’t apply to hidden friends: if they are the hidden friend of the “right” class template, then ADL for other types in the namespace will not even find them, so they will not accidentally disqualify anything.</p>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="wording"><span class="header-section-number">4</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>This wording is relative to <span class="citation" data-cites="N4842">[<a href="#ref-N4842" role="doc-biblioref">N4842</a>]</span>.</p>
<p>Replace <span>18.5.2 <a href="https://wg21.link/concept.boolean">[concept.boolean]</a></span> with the following:</p>
<div class="add" style="color: #006e28">

<h3 class="unnumbered" data-number id="boolean-testability-concept.booleantestable">18.5.2 Boolean testability [concept.booleantestable]<a href="#boolean-testability-concept.booleantestable" class="self-link"></a></h3>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span> The exposition-only <em><code class="sourceCode cpp">boolean<span class="op">-</span>testable</code></em> concept specifies the requirements on expressions that are convertible to <code class="sourceCode cpp"><span class="dt">bool</span></code> and for which the logical operators (<span>7.6.14 <a href="https://wg21.link/expr.log.and">[expr.log.and]</a></span>, <span>7.6.15 <a href="https://wg21.link/expr.log.or">[expr.log.or]</a></span>, <span>7.6.2.1 <a href="https://wg21.link/expr.unary.op">[expr.unary.op]</a></span>) have the conventional semantics.</p>
<blockquote>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb6-2"><a href="#cb6-2"></a><span class="kw">concept</span> <em>boolean-testable-impl</em> <span class="op">=</span> convertible_to<span class="op">&lt;</span>T, <span class="dt">bool</span><span class="op">&gt;</span>;  <span class="co">// <em>exposition only</em></span></span></code></pre></div>
</blockquote>
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> Let <code class="sourceCode cpp">e</code> be an expression such that <code class="sourceCode cpp"><span class="kw">decltype</span><span class="op">((</span>e<span class="op">))</span></code> is <code class="sourceCode cpp">T</code>. <code class="sourceCode cpp">T</code> models <em><code class="sourceCode cpp">boolean<span class="op">-</span>testable<span class="op">-</span>impl</code></em> only if:</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(2.1)</a></span> either <code class="sourceCode cpp">remove_cvref_t<span class="op">&lt;</span>T<span class="op">&gt;</span></code> is not a class type, or name lookup for the names <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">&amp;&amp;</span></code> and <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">||</span></code> within the scope of <code class="sourceCode cpp">remove_cvref_t<span class="op">&lt;</span>T<span class="op">&gt;</span></code> as if by class member access lookup (<span>11.8 <a href="https://wg21.link/class.member.lookup">[class.member.lookup]</a></span>) results in an empty declaration set; and</li>
<li><span class="marginalizedparent"><a class="marginalized">(2.2)</a></span> name lookup for the names <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">&amp;&amp;</span></code> and <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">||</span></code> in the associated namespaces and entities of <code class="sourceCode cpp">T</code> (<span>6.5.2 <a href="https://wg21.link/basic.lookup.argdep">[basic.lookup.argdep]</a></span>) finds no disqualifying declaration (defined below).</li>
</ul>
<p><span class="marginalizedparent"><a class="marginalized">3</a></span> A <em>disqualifying parameter</em> is a function parameter whose declared type <code class="sourceCode cpp">P</code></p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(3.1)</a></span> is not dependent on a template parameter, and there exists an implicit conversion sequence (<span>12.4.3.1 <a href="https://wg21.link/over.best.ics">[over.best.ics]</a></span>) from <code class="sourceCode cpp">e</code> to <code class="sourceCode cpp">P</code>; or</li>
<li><span class="marginalizedparent"><a class="marginalized">(3.2)</a></span> is dependent on one or more template parameters, and either
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(3.2.1)</a></span> <code class="sourceCode cpp">P</code> contains no template parameter that participates in template argument deduction (<span>13.10.2.5 <a href="https://wg21.link/temp.deduct.type">[temp.deduct.type]</a></span>), or</li>
<li><span class="marginalizedparent"><a class="marginalized">(3.2.2)</a></span> template argument deduction using the rules for deducing template arguments in a function call (<span>13.10.2.1 <a href="https://wg21.link/temp.deduct.call">[temp.deduct.call]</a></span>) and the type of <code class="sourceCode cpp">e</code> as the argument type succeeds.</li>
</ul></li>
</ul>
<p><span class="marginalizedparent"><a class="marginalized">4</a></span> A <em>key parameter</em> of a function template <code class="sourceCode cpp">D</code> is a function parameter of type <em><code class="sourceCode cpp">cv</code></em> <code class="sourceCode cpp">X</code> or reference thereto, where <code class="sourceCode cpp">X</code> names a specialization of a class template that is a member of the same namespace as <code class="sourceCode cpp">D</code>, and <code class="sourceCode cpp">X</code> contains at least one template parameter that participates in template argument deduction.</p>
<div class="example">
<p><span>[ <em>Example:</em> </span>In</p>
<div class="sourceCode" id="cb7"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb7-1"><a href="#cb7-1"></a> <span class="kw">namespace</span> Z <span class="op">{</span></span>
<span id="cb7-2"><a href="#cb7-2"></a>   <span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span><span class="op">&gt;</span></span>
<span id="cb7-3"><a href="#cb7-3"></a>   <span class="kw">struct</span> C <span class="op">{}</span>;</span>
<span id="cb7-4"><a href="#cb7-4"></a></span>
<span id="cb7-5"><a href="#cb7-5"></a>   <span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb7-6"><a href="#cb7-6"></a>   <span class="dt">void</span> <span class="kw">operator</span><span class="op">&amp;&amp;(</span>C<span class="op">&lt;</span>T<span class="op">&gt;</span> x, T y<span class="op">)</span>;</span>
<span id="cb7-7"><a href="#cb7-7"></a></span>
<span id="cb7-8"><a href="#cb7-8"></a>   <span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb7-9"><a href="#cb7-9"></a>   <span class="dt">void</span> <span class="kw">operator</span><span class="op">||(</span>C<span class="op">&lt;</span>type_identity_t<span class="op">&lt;</span>T<span class="op">&gt;&gt;</span> x, T y<span class="op">)</span>;</span>
<span id="cb7-10"><a href="#cb7-10"></a> <span class="op">}</span></span></code></pre></div>
<p>the declaration of <code class="sourceCode cpp">Z<span class="op">::</span><span class="kw">operator</span><span class="op">&amp;&amp;</span></code> contains one key parameter, <code class="sourceCode cpp">C<span class="op">&lt;</span>T<span class="op">&gt;</span> x</code>, and the declaration of <code class="sourceCode cpp">Z<span class="op">::</span><span class="kw">operator</span><span class="op">||</span></code> contains no key parameters.<span> — <em>end example</em> ]</span></p>
</div>
<p><span class="marginalizedparent"><a class="marginalized">5</a></span> A <em>disqualifying declaration</em> is</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(5.1)</a></span> a (non-template) function declaration that contains at least one disqualifying parameter; or</li>
<li><span class="marginalizedparent"><a class="marginalized">(5.2)</a></span> a function template declaration that contains at least one disqualifying parameter, where
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(5.2.1)</a></span> at least one disqualifying parameter is a key parameter; or</li>
<li><span class="marginalizedparent"><a class="marginalized">(5.2.2)</a></span> the declaration contains no key parameters; or</li>
<li><span class="marginalizedparent"><a class="marginalized">(5.2.3)</a></span> the declaration declares a function template that is not visible in its namespace (<span>9.8.1.2 <a href="https://wg21.link/namespace.memdef">[namespace.memdef]</a></span>).</li>
</ul></li>
</ul>
<p><span class="marginalizedparent"><a class="marginalized">6</a></span> <span class="note"><span>[ <em>Note:</em> </span>The intention is to ensure that given two types <code class="sourceCode cpp">T1</code> and <code class="sourceCode cpp">T2</code> that each model <em><code class="sourceCode cpp">boolean<span class="op">-</span>testable<span class="op">-</span>impl</code></em>, the <code class="sourceCode cpp"><span class="op">&amp;&amp;</span></code> and <code class="sourceCode cpp"><span class="op">||</span></code> operators within the expressions <code class="sourceCode cpp">declval<span class="op">&lt;</span>T1<span class="op">&gt;()</span> <span class="op">&amp;&amp;</span> declval<span class="op">&lt;</span>T2<span class="op">&gt;()</span></code> and <code class="sourceCode cpp">declval<span class="op">&lt;</span>T1<span class="op">&gt;()</span> <span class="op">||</span> declval<span class="op">&lt;</span>T2<span class="op">&gt;()</span></code> resolve to the corresponding built-in operators.<span> — <em>end note</em> ]</span></span></p>
<blockquote>
<div class="sourceCode" id="cb8"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">class</span> T<span class="op">&gt;</span></span>
<span id="cb8-2"><a href="#cb8-2"></a><span class="kw">concept</span> <em>boolean-testable</em> <span class="op">=</span>                             <span class="co">// <em>exposition only</em></span></span>
<span id="cb8-3"><a href="#cb8-3"></a>    <em>boolean-testable-impl</em><span class="op">&lt;</span>T<span class="op">&gt;</span> <span class="op">&amp;&amp;</span> <span class="kw">requires</span> <span class="op">(</span>T<span class="op">&amp;&amp;</span> t<span class="op">)</span> <span class="op">{</span></span>
<span id="cb8-4"><a href="#cb8-4"></a>        <span class="op">{</span> <span class="op">!</span>std<span class="op">::</span>forward<span class="op">&lt;</span>T<span class="op">&gt;(</span>t<span class="op">)</span> <span class="op">}</span> <span class="op">-&gt;</span> <em>boolean-testable-impl</em></span>
<span id="cb8-5"><a href="#cb8-5"></a>    <span class="op">}</span>;</span></code></pre></div>
</blockquote>
<p><span class="marginalizedparent"><a class="marginalized">7</a></span> Let <code class="sourceCode cpp">e</code> be an expression such that <code class="sourceCode cpp"><span class="kw">decltype</span><span class="op">((</span>e<span class="op">))</span></code> is <code class="sourceCode cpp">T</code>. <code class="sourceCode cpp">T</code> models <em><code class="sourceCode cpp">boolean<span class="op">-</span>testable</code></em> only if <code class="sourceCode cpp"><span class="dt">bool</span><span class="op">(</span>e<span class="op">)</span> <span class="op">==</span> <span class="op">!</span><span class="dt">bool</span><span class="op">(!</span>e<span class="op">)</span></code>.</p>
<p><span class="marginalizedparent"><a class="marginalized">8</a></span> <span class="example"><span>[ <em>Example:</em> </span>The types <code class="sourceCode cpp"><span class="dt">bool</span></code>, <code class="sourceCode cpp">true_­type</code> (<span>20.15.2 <a href="https://wg21.link/meta.type.synop">[meta.type.synop]</a></span>), <code class="sourceCode cpp"><span class="dt">int</span><span class="op">*</span></code>, and <code class="sourceCode cpp">bitset<span class="op">&lt;</span>N<span class="op">&gt;</span>​<span class="op">::</span>​reference</code> (<span>20.9.2 <a href="https://wg21.link/template.bitset">[template.bitset]</a></span>) model <em><code class="sourceCode cpp">boolean<span class="op">-</span>testable</code></em>.<span> — <em>end example</em> ]</span></span></p>
</div>
<p>Edit <span>18.3 <a href="https://wg21.link/concepts.syn">[concepts.syn]</a></span> as indicated:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb9"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb9-1"><a href="#cb9-1"></a> namespace std {</span>
<span id="cb9-2"><a href="#cb9-2"></a>   […]</span>
<span id="cb9-3"><a href="#cb9-3"></a></span>
<span id="cb9-4"><a href="#cb9-4"></a>   // 18.5, comparison concepts</span>
<span id="cb9-5"><a href="#cb9-5"></a><span class="st">-  // 18.5.2, concept boolean</span></span>
<span id="cb9-6"><a href="#cb9-6"></a><span class="st">-  template&lt;class B&gt;</span></span>
<span id="cb9-7"><a href="#cb9-7"></a><span class="st">-  concept boolean = <em>see below</em> ;</span></span>
<span id="cb9-8"><a href="#cb9-8"></a></span>
<span id="cb9-9"><a href="#cb9-9"></a>   […]</span>
<span id="cb9-10"><a href="#cb9-10"></a> }</span></code></pre></div>
</div>
</blockquote>
<p>Replace all instances of <code class="sourceCode cpp">boolean</code> in <span>17.11.4 <a href="https://wg21.link/cmp.concept">[cmp.concept]</a></span>, <span>18.5.3 <a href="https://wg21.link/concept.equalitycomparable">[concept.equalitycomparable]</a></span>, <span>18.5.4 <a href="https://wg21.link/concept.totallyordered">[concept.totallyordered]</a></span> and <span>18.7.4 <a href="https://wg21.link/concept.predicate">[concept.predicate]</a></span> with <em><code class="sourceCode cpp">boolean<span class="op">-</span>testable</code></em>.</p>
<p>Edit <span>16.4.2.1 <a href="https://wg21.link/expos.only.func">[expos.only.func]</a></span> as indicated:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb10"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb10-1"><a href="#cb10-1"></a> constexpr auto synth-three-way =</span>
<span id="cb10-2"><a href="#cb10-2"></a>   []&lt;class T, class U&gt;(const T&amp; t, const U&amp; u)</span>
<span id="cb10-3"><a href="#cb10-3"></a>     requires requires {</span>
<span id="cb10-4"><a href="#cb10-4"></a><span class="st">-      { t &lt; u } -&gt; <span class="diffdel">convertible_to&lt;bool&gt;</span>;</span></span>
<span id="cb10-5"><a href="#cb10-5"></a><span class="st">-      { u &lt; t } -&gt; <span class="diffdel">convertible_to&lt;bool&gt;</span>;</span></span>
<span id="cb10-6"><a href="#cb10-6"></a><span class="va">+      { t &lt; u } -&gt; <em><span class="diffins">boolean-testable</span></em>;</span></span>
<span id="cb10-7"><a href="#cb10-7"></a><span class="va">+      { u &lt; t } -&gt; <em><span class="diffins">boolean-testable</span></em>;</span></span>
<span id="cb10-8"><a href="#cb10-8"></a>     }</span>
<span id="cb10-9"><a href="#cb10-9"></a>   {</span>
<span id="cb10-10"><a href="#cb10-10"></a>     if constexpr (three_way_comparable_with&lt;T, U&gt;) {</span>
<span id="cb10-11"><a href="#cb10-11"></a>       return t &lt;=&gt; u;</span>
<span id="cb10-12"><a href="#cb10-12"></a>     } else {</span>
<span id="cb10-13"><a href="#cb10-13"></a>       if (t &lt; u) return weak_ordering::less;</span>
<span id="cb10-14"><a href="#cb10-14"></a>       if (u &lt; t) return weak_ordering::greater;</span>
<span id="cb10-15"><a href="#cb10-15"></a>       return weak_ordering::equivalent;</span>
<span id="cb10-16"><a href="#cb10-16"></a>     }</span>
<span id="cb10-17"><a href="#cb10-17"></a>   };</span></code></pre></div>
</div>
</blockquote>
<p>Edit <span>25.5.5 <a href="https://wg21.link/alg.find">[alg.find]</a></span> p1 as indicated:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span> Let <em>E</em> be:</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(1.1)</a></span> <code class="sourceCode cpp"><span class="op">*</span>i <span class="op">==</span> value</code> for <code class="sourceCode cpp">find</code>,</li>
<li><span class="marginalizedparent"><a class="marginalized">(1.2)</a></span> <code class="sourceCode cpp">pred<span class="op">(*</span>i<span class="op">)</span> <span class="op">!=</span> <span class="kw">false</span></code> for <code class="sourceCode cpp">find_­if</code>,</li>
<li><span class="marginalizedparent"><a class="marginalized">(1.3)</a></span> <code class="sourceCode cpp">pred<span class="op">(*</span>i<span class="op">)</span> <span class="op">==</span> <span class="kw">false</span></code> for <code class="sourceCode cpp">find_­if_­not</code>,</li>
<li><span class="marginalizedparent"><a class="marginalized">(1.4)</a></span> <span class="diffins"><code class="sourceCode cpp"><span class="dt">bool</span><span class="op">(</span></code></span><code class="sourceCode cpp">invoke<span class="op">(</span>proj, <span class="op">*</span>i<span class="op">)</span> <span class="op">==</span> value</code><span class="diffins"><code class="sourceCode cpp"><span class="op">)</span></code></span> for ranges​::​find,</li>
<li><span class="marginalizedparent"><a class="marginalized">(1.5)</a></span> <span class="diffins"><code class="sourceCode cpp"><span class="dt">bool</span><span class="op">(</span></code></span><code class="sourceCode cpp">invoke<span class="op">(</span>pred, invoke<span class="op">(</span>proj, <span class="op">*</span>i<span class="op">))</span></code><span class="diffins"><code class="sourceCode cpp"><span class="op">)</span></code></span> <span class="diffdel"><code class="sourceCode cpp"><span class="op">!=</span> <span class="kw">false</span></code></span> for <code class="sourceCode cpp">ranges​<span class="op">::</span>​find_­if</code>,</li>
<li><span class="marginalizedparent"><a class="marginalized">(1.6)</a></span> <span class="diffins"><code class="sourceCode cpp"><span class="dt">bool</span><span class="op">(!</span></code></span><code class="sourceCode cpp">invoke<span class="op">(</span>pred, invoke<span class="op">(</span>proj, <span class="op">*</span>i<span class="op">))</span></code><span class="diffins"><code class="sourceCode cpp"><span class="op">)</span></code></span> <span class="diffdel"><code class="sourceCode cpp"><span class="op">==</span> <span class="kw">false</span></code></span> for <code class="sourceCode cpp">ranges​<span class="op">::</span>​find_­if_­not</code>.</li>
</ul>
</blockquote>
<p>Edit <span>25.5.7 <a href="https://wg21.link/alg.find.first.of">[alg.find.first.of]</a></span> p1 as indicated:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span> Let <em>E</em> be:</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(1.1)</a></span> <code class="sourceCode cpp"><span class="op">*</span>i <span class="op">==</span> <span class="op">*</span>j</code> for the overloads with no parameter <code class="sourceCode cpp">pred</code>,</li>
<li><span class="marginalizedparent"><a class="marginalized">(1.2)</a></span> <code class="sourceCode cpp">pred<span class="op">(*</span>i, <span class="op">*</span>j<span class="op">)</span> <span class="op">!=</span> <span class="kw">false</span></code> for the overloads with a parameter <code class="sourceCode cpp">pred</code> and no parameter <code class="sourceCode cpp">proj1</code>,</li>
<li><span class="marginalizedparent"><a class="marginalized">(1.3)</a></span> <span class="diffins"><code class="sourceCode cpp"><span class="dt">bool</span><span class="op">(</span></code></span><code class="sourceCode cpp">invoke<span class="op">(</span>pred, invoke<span class="op">(</span>proj1, <span class="op">*</span>i<span class="op">)</span>, invoke<span class="op">(</span>proj2, <span class="op">*</span>j<span class="op">))</span></code><span class="diffins"><code class="sourceCode cpp"><span class="op">)</span></code></span> <span class="diffdel"><code class="sourceCode cpp"><span class="op">!=</span> <span class="kw">false</span></code></span> for the overloads with parameters <code class="sourceCode cpp">pred</code> and <code class="sourceCode cpp">proj1</code>.</li>
</ul>
</blockquote>
<p>Edit <span>25.5.8 <a href="https://wg21.link/alg.adjacent.find">[alg.adjacent.find]</a></span> p1 as indicated:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span> Let <em>E</em> be:</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(1.1)</a></span> <code class="sourceCode cpp"><span class="op">*</span>i <span class="op">==</span> <span class="op">*(</span>i <span class="op">+</span> <span class="dv">1</span><span class="op">)</span></code> for the overloads with no parameter <code class="sourceCode cpp">pred</code>,</li>
<li><span class="marginalizedparent"><a class="marginalized">(1.2)</a></span> <code class="sourceCode cpp">pred<span class="op">(*</span>i, <span class="op">*(</span>i <span class="op">+</span> <span class="dv">1</span><span class="op">))</span> <span class="op">!=</span> <span class="kw">false</span></code> for the overloads with a parameter <code class="sourceCode cpp">pred</code> and no parameter <code class="sourceCode cpp">proj</code>,</li>
<li><span class="marginalizedparent"><a class="marginalized">(1.3)</a></span> <span class="diffins"><code class="sourceCode cpp"><span class="dt">bool</span><span class="op">(</span></code></span><code class="sourceCode cpp">invoke<span class="op">(</span>pred, invoke<span class="op">(</span>proj, <span class="op">*</span>i<span class="op">)</span>, invoke<span class="op">(</span>proj, <span class="op">*(</span>i <span class="op">+</span> <span class="dv">1</span><span class="op">)))</span></code><span class="diffins"><code class="sourceCode cpp"><span class="op">)</span></code></span> <span class="diffdel"><code class="sourceCode cpp"><span class="op">!=</span> <span class="kw">false</span></code></span> for the overloads with parameters <code class="sourceCode cpp">pred</code> and <code class="sourceCode cpp">proj</code>.</li>
</ul>
</blockquote>
<p>Edit <span>25.5.9 <a href="https://wg21.link/alg.count">[alg.count]</a></span> p1 as indicated:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span> Let <em>E</em> be:</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(1.1)</a></span> <code class="sourceCode cpp"><span class="op">*</span>i <span class="op">==</span> value</code> for the overloads with no parameter <code class="sourceCode cpp">pred</code> or <code class="sourceCode cpp">proj</code>,</li>
<li><span class="marginalizedparent"><a class="marginalized">(1.2)</a></span> <code class="sourceCode cpp">pred<span class="op">(*</span>i<span class="op">)</span> <span class="op">!=</span> <span class="kw">false</span></code> for the overloads with a parameter <code class="sourceCode cpp">pred</code> but no parameter <code class="sourceCode cpp">proj</code>,</li>
<li><span class="marginalizedparent"><a class="marginalized">(1.3)</a></span> <code class="sourceCode cpp">invoke<span class="op">(</span>proj, <span class="op">*</span>i<span class="op">)</span> <span class="op">==</span> value</code> for the overloads with a parameter <code class="sourceCode cpp">proj</code> but no parameter <code class="sourceCode cpp">pred</code>,</li>
<li><span class="marginalizedparent"><a class="marginalized">(1.4)</a></span> <span class="diffins"><code class="sourceCode cpp"><span class="dt">bool</span><span class="op">(</span></code></span><code class="sourceCode cpp">invoke<span class="op">(</span>pred, invoke<span class="op">(</span>proj, <span class="op">*</span>i<span class="op">))</span></code><span class="diffins"><code class="sourceCode cpp"><span class="op">)</span></code></span> <span class="diffdel"><code class="sourceCode cpp"><span class="op">!=</span> <span class="kw">false</span></code></span> for the overloads with both parameters <code class="sourceCode cpp">proj</code> and <code class="sourceCode cpp">pred</code>.</li>
</ul>
</blockquote>
<h1 data-number="5" style="border-bottom:1px solid #cccccc" id="references"><span class="header-section-number">5</span> References<a href="#references" class="self-link"></a></h1>
<div id="refs" class="references" role="doc-bibliography">
<div id="ref-LWG3074">
<p>[LWG3074] Jonathan Wakely. Non-member functions for valarray should only deduce from the valarray. <br />
<a href="https://wg21.link/lwg3074">https://wg21.link/lwg3074</a></p>
</div>
<div id="ref-N4842">
<p>[N4842] Richard Smith. 2019. Working Draft, Standard for Programming Language C++. <br />
<a href="https://wg21.link/n4842">https://wg21.link/n4842</a></p>
</div>
<div id="ref-P1964R0">
<p>[P1964R0] Tim Song. 2019. Casting convertible_to&lt;bool&gt; considered harmful. <br />
<a href="https://wg21.link/p1964r0">https://wg21.link/p1964r0</a></p>
</div>
</div>
</div>
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