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<title>Resolve lambda init-capture pack grammar (CWG2378)</title>
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<h1 class="title" style="text-align:center">Resolve lambda init-capture pack grammar (CWG2378)</h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P2095R0</td>
</tr>
<tr>
<td>Date:</td>
<td>2020-02-14</td>
</tr>
<tr>
<td style="vertical-align:top">Project:</td>
<td>Programming Language C++</td>
</tr>
<tr>
<td style="vertical-align:top">Audience:</td>
<td>
CWG<br>
</td>
</tr>
<tr>
<td style="vertical-align:top">Reply-to:</td>
<td>
Barry Revzin<br><<a href="mailto:barry.revzin@gmail.com" class="email">barry.revzin@gmail.com</a>><br>
</td>
</tr>
</table>
</header>
<div style="clear:both">
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="introduction" data-number="1" style="border-bottom:1px solid #cccccc"><span class="header-section-number">1</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>When <span class="citation" data-cites="P0780R2">[<a href="#ref-P0780R2" role="doc-biblioref">P0780R2</a>]</span> was adopted in Jacksonville in 2018, the paper contained a grammar rule that was inconsistent with other uses of packs. As pointed out by Richard Smith a few hours after plenary <span class="citation" data-cites="Smith.Core">[<a href="#ref-Smith.Core" role="doc-biblioref">Smith.Core</a>]</span>, the grammar requires a pack of references to be captured like:</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="op">[...&</span>x<span class="op">=</span>init<span class="op">]</span></span></code></pre></div>
<p>rather than how we write packs of references everywhere else.</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="op">[&...</span>x<span class="op">=</span>init<span class="op">]</span></span></code></pre></div>
<p>This is now <span class="citation" data-cites="CWG2378">[<a href="#ref-CWG2378" role="doc-biblioref">CWG2378</a>]</span>.</p>
<p>As of this writing, clang implements the Core Issue direction (with the <code class="sourceCode cpp"><span class="op">&</span></code> preceeding the <code class="sourceCode cpp"><span class="op">...</span></code>) and gcc implements the standard wording. I opened a gcc bug for this <span class="citation" data-cites="gcc.91847">[<a href="#ref-gcc.91847" role="doc-biblioref">gcc.91847</a>]</span>, but we should really fix the wording asap.</p>
<p>This paper exists to do that.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="wording" data-number="2" style="border-bottom:1px solid #cccccc"><span class="header-section-number">2</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Change the grammar in <span>7.5.5.2 <a href="https://wg21.link/expr.prim.lambda.capture">[expr.prim.lambda.capture]</a></span>:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb3"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb3-1"><a href="#cb3-1"></a> <em>capture</em>:</span>
<span id="cb3-2"><a href="#cb3-2"></a><span class="st">- <em>simple-capture</em> <span class="diffdel"><code class="x">...</code><sub>opt</sub></span></span></span>
<span id="cb3-3"><a href="#cb3-3"></a><span class="st">- <span class="diffdel"><code class="x">...</code><sub>opt</sub></span> <em>init-capture</em></span></span>
<span id="cb3-4"><a href="#cb3-4"></a><span class="va">+ <em>simple-capture</em></span></span>
<span id="cb3-5"><a href="#cb3-5"></a><span class="va">+ <em>init-capture</em></span></span>
<span id="cb3-6"><a href="#cb3-6"></a></span>
<span id="cb3-7"><a href="#cb3-7"></a> <em>simple-capture</em>:</span>
<span id="cb3-8"><a href="#cb3-8"></a><span class="st">- <em>identifier</em></span></span>
<span id="cb3-9"><a href="#cb3-9"></a><span class="st">- & <em>identifier</em></span></span>
<span id="cb3-10"><a href="#cb3-10"></a><span class="va">+ <em>identifier</em> <span class="diffins"><code class="x">...</code><sub>opt</sub></span></span></span>
<span id="cb3-11"><a href="#cb3-11"></a><span class="va">+ & <em>identifier</em> <span class="diffins"><code class="x">...</code><sub>opt</sub></span></span></span>
<span id="cb3-12"><a href="#cb3-12"></a> this</span>
<span id="cb3-13"><a href="#cb3-13"></a> * this</span>
<span id="cb3-14"><a href="#cb3-14"></a></span>
<span id="cb3-15"><a href="#cb3-15"></a> <em>init-capture</em>:</span>
<span id="cb3-16"><a href="#cb3-16"></a><span class="st">- <em>identifier</em> <em>initializer</em></span></span>
<span id="cb3-17"><a href="#cb3-17"></a><span class="st">- & <em>identifier</em> <em>initializer</em></span></span>
<span id="cb3-18"><a href="#cb3-18"></a><span class="va">+ <span class="diffins"><code class="x">...</code><sub>opt</sub></span> <em>identifier</em> <em>initializer</em></span></span>
<span id="cb3-19"><a href="#cb3-19"></a><span class="va">+ & <span class="diffins"><code class="x">...</code><sub>opt</sub></span> <em>identifier</em> <em>initializer</em></span></span></code></pre></div>
</div>
</blockquote>
<p>Change <span>7.5.5.2 <a href="https://wg21.link/expr.prim.lambda.capture">[expr.prim.lambda.capture]</a></span>/2:</p>
<blockquote>
<p>If a <em>lambda-capture</em> includes a <em>capture-default</em> that is <code class="sourceCode cpp"><span class="op">=</span></code>, each <em>simple-capture</em> of that <em>lambda-capture</em> shall be of the form “<code class="x">&</code> <span class="addu"><code class="x">...</code><sub>opt</sub></span> <em>identifier</em>”, “<code class="sourceCode cpp"><span class="kw">this</span></code>”, or “<code class="sourceCode cpp"><span class="op">*</span> <span class="kw">this</span></code>”.</p>
</blockquote>
<p>Change <span>7.5.5.2 <a href="https://wg21.link/expr.prim.lambda.capture">[expr.prim.lambda.capture]</a></span>/6:</p>
<blockquote>
<p>An <em>init-capture</em> <span class="addu">without ellipsis</span> behaves as if it declares and explicitly captures a variable of the form “<code class="x">auto init-capture ;</code>” whose declarative region is the <em>lambda-expression</em>’s compound-statement, except that:</p>
</blockquote>
<p>Change <span>7.5.5.2 <a href="https://wg21.link/expr.prim.lambda.capture">[expr.prim.lambda.capture]</a></span>/17:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">17</a></span> A <em>simple-capture</em> <span class="rm" style="color: #bf0303"><del>followed by</del></span> <span class="addu">containing</span> an ellipsis is a pack expansion ([temp.variadic]). An <em>init-capture</em> <span class="rm" style="color: #bf0303"><del>preceded by</del></span> <span class="addu">containing</span> an ellipsis is a pack expansion that introduces an <em>init-capture</em> pack ([temp.variadic]) whose declarative region is the <em>lambda-expression</em>’s <em>compound-statement</em>.</p>
</blockquote>
<p>Change <span>13.7.3 <a href="https://wg21.link/temp.variadic">[temp.variadic]</a></span>/5.10:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">5</a></span> […] Pack expansions can occur in the following contexts:</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(5.10)</a></span> In a <em>capture-list</em> ([expr.prim.lambda]); the pattern is <span class="rm" style="color: #bf0303"><del>a <em>capture</em></del></span> <span class="addu">the <em>capture</em> without the ellipses</span>.</li>
</ul>
</blockquote>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="bibliography" data-number="3" style="border-bottom:1px solid #cccccc"><span class="header-section-number">3</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<div id="ref-CWG2378">
<p>[CWG2378] Barry Revzin. 2018. Inconsistent grammar for reference init-capture of pack. <br />
<a href="https://wg21.link/cwg2378">https://wg21.link/cwg2378</a></p>
</div>
<div id="ref-gcc.91847">
<p>[gcc.91847] Barry Revzin. 2019. init-capture pack of references requires … on wrong side. <br />
<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91847">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91847</a></p>
</div>
<div id="ref-P0780R2">
<p>[P0780R2] Barry Revzin. 2018. Allow pack expansion in lambda init-capture. <br />
<a href="https://wg21.link/p0780r2">https://wg21.link/p0780r2</a></p>
</div>
<div id="ref-Smith.Core">
<p>[Smith.Core] Richard Smith. 2018. p0780r2 has wrong grammar for reference init-capture packs. <br />
<a href="https://lists.isocpp.org/core/2018/03/4095.php">https://lists.isocpp.org/core/2018/03/4095.php</a></p>
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