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<header id="title-block-header">
<h1 class="title" style="text-align:center">Missing feature test macros for C++20 core papers</h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P2493R0</td>
</tr>
<tr>
<td>Date:</td>
<td>2021-11-15</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, SG10<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">
<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="#proposal"><span class="toc-section-number">2</span> Proposal<span></span></a>
<ul>
<li><a href="#wording"><span class="toc-section-number">2.1</span> Wording<span></span></a></li>
</ul></li>
<li><a href="#bibliography"><span class="toc-section-number">3</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>As Jonathan Wakely pointed out on the <a href="https://lists.isocpp.org/sg10/2021/10/0810.php">SG10 mailing list</a>, neither <span class="citation" data-cites="P0848R3">[<a href="#ref-P0848R3" role="doc-biblioref">P0848R3</a>]</span> (Conditionally Trivial Special Member Functions) nor <span class="citation" data-cites="P1330R0">[<a href="#ref-P1330R0" role="doc-biblioref">P1330R0</a>]</span> ( Changing the active member of a union inside <code class="sourceCode cpp"><span class="kw">constexpr</span></code>) provided a feature-test macro.</p>
<p>In order to implement <span class="citation" data-cites="P2231R1">[<a href="#ref-P2231R1" role="doc-biblioref">P2231R1</a>]</span> (Missing constexpr in <code class="sourceCode cpp">std<span class="op">::</span>optional</code> and <code class="sourceCode cpp">std<span class="op">::</span>variant</code>), the standard library needs to know whether these features are available to know when it can use them in the library. And currently, there is no way of doing so.</p>
<p>Even though some compilers have implemented these features for a while, others haven’t, so it’s better to provide the feature-test macro now (when it will have some false negatives) than to not provide it at all (in which case libstdc++ needs to choose between never providing the library feature or causing unsupported compilers to fail even if users weren’t trying to use the new library feature).</p>
<p>As far as choice of value goes, P0848R3 was adopted in Cologne (201907) and P1330R0 in San Diego (201811), and there is already currently a <code class="sourceCode cpp"><span class="dv">201907</span><span class="bu">L</span></code> value in the history of <code class="sourceCode cpp">__cpp_constexpr</code> <span class="citation" data-cites="SD6">[<a href="#ref-SD6" role="doc-biblioref">SD6</a>]</span> (a value which includes <span class="citation" data-cites="P1331R2">[<a href="#ref-P1331R2" role="doc-biblioref">P1331R2</a>]</span> and <span class="citation" data-cites="P1668R1">[<a href="#ref-P1668R1" role="doc-biblioref">P1668R1</a>]</span>) and <code class="sourceCode cpp">__cpp_concepts</code> (including <span class="citation" data-cites="P1452R2">[<a href="#ref-P1452R2" role="doc-biblioref">P1452R2</a>]</span>). Richard Smith <a href="https://lists.isocpp.org/sg10/2021/10/0815.php">suggests</a> that we should either bump both macros to <code class="sourceCode cpp"><span class="dv">201908</span><span class="bu">L</span></code> (as just <em>some</em> larger value) or both to <code class="sourceCode cpp"><span class="dv">202002</span><span class="bu">L</span></code> (to mean the complete set of C++20 proposals).</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="proposal"><span class="header-section-number">2</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>This paper proposes Richard’s second suggestion: bump <code class="sourceCode cpp">__cpp_concepts</code> and <code class="sourceCode cpp">__cpp_constexpr</code> to <code class="sourceCode cpp"><span class="dv">202002</span><span class="bu">L</span></code> to include P0848R3 and P1330R0, respectively. For <code class="sourceCode cpp">__cpp_concepts</code>, this is a new value that requires a wording change. For<code class="sourceCode cpp">__cpp_constexpr</code>, there is already a later value as of <code class="sourceCode cpp"><span class="dv">202110</span><span class="bu">L</span></code> a result of <span class="citation" data-cites="P2242R3">[<a href="#ref-P2242R3" role="doc-biblioref">P2242R3</a>]</span> (gcc already implements this change and reports this new macro version, but gcc also already implements P1330R0 so this is okay. clang and msvc do not yet report this higher value).</p>
<h2 data-number="2.1" id="wording"><span class="header-section-number">2.1</span> Wording<a href="#wording" class="self-link"></a></h2>
<p>Change <span>15.11 <a href="https://wg21.link/cpp.predefined">[cpp.predefined]</a></span>, table 21:</p>
<blockquote>
<div>
<div class="sourceCode" id="cb1"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb1-1"><a href="#cb1-1"></a>__cpp_concepts <span class="diffdel">201907L</span> <span class="diffins">202002L</span></span></code></pre></div>
</div>
</blockquote>
<p>Note that no change to <code class="sourceCode cpp">__cpp_constexpr</code> is visible in the working draft, but it will appear in SD-FeatureTest <span class="citation" data-cites="SD6">[<a href="#ref-SD6" role="doc-biblioref">SD6</a>]</span>.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="bibliography"><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-P0848R3">
<p>[P0848R3] Barry Revzin, Casey Carter. 2019-07-28. Conditionally Trivial Special Member Functions. <br />
<a href="https://wg21.link/p0848r3">https://wg21.link/p0848r3</a></p>
</div>
<div id="ref-P1330R0">
<p>[P1330R0] Louis Dionne, David Vandevoorde. 2018-11-10. Changing the active member of a union inside constexpr. <br />
<a href="https://wg21.link/p1330r0">https://wg21.link/p1330r0</a></p>
</div>
<div id="ref-P1331R2">
<p>[P1331R2] CJ Johnson. 2019-07-26. Permitting trivial default initialization in constexpr contexts. <br />
<a href="https://wg21.link/p1331r2">https://wg21.link/p1331r2</a></p>
</div>
<div id="ref-P1452R2">
<p>[P1452R2] Hubert Tong. 2019-07-18. On the non-uniform semantics of return-type-requirements. <br />
<a href="https://wg21.link/p1452r2">https://wg21.link/p1452r2</a></p>
</div>
<div id="ref-P1668R1">
<p>[P1668R1] Erich Keane. 2019-07-17. Enabling constexpr Intrinsics By Permitting Unevaluated inline-assembly in constexpr Functions. <br />
<a href="https://wg21.link/p1668r1">https://wg21.link/p1668r1</a></p>
</div>
<div id="ref-P2231R1">
<p>[P2231R1] Barry Revzin. 2021-02-12. Add further constexpr support for optional/variant. <br />
<a href="https://wg21.link/p2231r1">https://wg21.link/p2231r1</a></p>
</div>
<div id="ref-P2242R3">
<p>[P2242R3] Ville Voutilainen. 2021-07-13. Non-literal variables (and labels and gotos) in constexpr functions. <br />
<a href="https://wg21.link/p2242r3">https://wg21.link/p2242r3</a></p>
</div>
<div id="ref-SD6">
<p>[SD6] SG10 Feature Test Recommendations. <br />
<a href="https://wg21.link/sd6">https://wg21.link/sd6</a></p>
</div>
</div>
</div>
</div>
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