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  <title>Relaxing some constexpr restrictions</title>
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<h1 class="title" style="text-align:center">Relaxing some <code class="sourceCode cpp"><span class="kw">constexpr</span></code> restrictions</h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P2448R0</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2021-10-13</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>&lt;<a href="mailto:barry.revzin@gmail.com" class="email">barry.revzin@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="#abstract"><span class="toc-section-number">1</span> Abstract<span></span></a></li>
<li><a href="#revision-history"><span class="toc-section-number">2</span> Revision History<span></span></a></li>
<li><a href="#maybe-not-now-but-soon"><span class="toc-section-number">3</span> Maybe Not Now, But Soon<span></span></a></li>
<li><a href="#sometimes-maybe-sometimes-always"><span class="toc-section-number">4</span> Sometimes Maybe, Sometimes Always<span></span></a></li>
<li><a href="#onwards-to-constexpr-classes"><span class="toc-section-number">5</span> Onwards to constexpr classes<span></span></a></li>
<li><a href="#proposal"><span class="toc-section-number">6</span> Proposal<span></span></a></li>
<li><a href="#bibliography"><span class="toc-section-number">7</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="abstract"><span class="header-section-number">1</span> Abstract<a href="#abstract" class="self-link"></a></h1>
<p>There are two rules about <code class="sourceCode cpp"><span class="kw">constexpr</span></code> programming that make code ill-formed or ill-formed (no diagnostic required) when functions or function templates are marked <code class="sourceCode cpp"><span class="kw">constexpr</span></code> that might never evaluate to a constant expression. But… so what if they don’t? The goal of this paper is to stop diagnosing problems that don’t exist.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="revision-history"><span class="header-section-number">2</span> Revision History<a href="#revision-history" class="self-link"></a></h1>
<p>A draft of the first revision of this paper was discussed in an <a href="https://wiki.edg.com/bin/view/Wg21telecons2021/EWG-2021-10-13">EWG telecon</a>, where the following poll was taken:</p>
<blockquote>
<p>send P2448 to electronic polling, targeting CWG for C++23.</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>10</td>
<td>8</td>
<td>1</td>
<td>0</td>
<td>0</td>
</tr>
</tbody>
</table>
</blockquote>
<p>This first published revision thus targets CWG.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="maybe-not-now-but-soon"><span class="header-section-number">3</span> Maybe Not Now, But Soon<a href="#maybe-not-now-but-soon" class="self-link"></a></h1>
<p><code class="sourceCode cpp"><span class="kw">constexpr</span></code> functions and function templates in C++ generally speaking mean <em>maybe</em> <code class="sourceCode cpp"><span class="kw">constexpr</span></code>. Not all instantiations or evaluations must be invocable at compile time, it’s just that there must be at least one set of function arguments in at least one instantiation that works.</p>
<p>And this isn’t just generally speaking, this is enshrined as a rule: <span>9.2.6 <a href="https://wg21.link/dcl.constexpr">[dcl.constexpr]</a></span>/6:</p>
<div class="quote">
<p>For a constexpr function or constexpr constructor that is neither defaulted nor a template, if no argument values exist such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression, or, for a constructor, an evaluated subexpression of the initialization full-expression of some constant-initialized object ([basic.start.static]), the program is ill-formed, no diagnostic required.</p>
</div>
<p>Here is an example of a program that violates this rule:</p>
<blockquote>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="dt">void</span> f<span class="op">(</span><span class="dt">int</span><span class="op">&amp;</span> i<span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2"></a>    i <span class="op">=</span> <span class="dv">0</span>;</span>
<span id="cb1-3"><a href="#cb1-3"></a><span class="op">}</span></span>
<span id="cb1-4"><a href="#cb1-4"></a></span>
<span id="cb1-5"><a href="#cb1-5"></a><span class="kw">constexpr</span> <span class="dt">void</span> g<span class="op">(</span><span class="dt">int</span><span class="op">&amp;</span> i<span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-6"><a href="#cb1-6"></a>    f<span class="op">(</span>i<span class="op">)</span>;</span>
<span id="cb1-7"><a href="#cb1-7"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p><code class="sourceCode cpp">g</code> unconditionally calls <code class="sourceCode cpp">f</code>, which is not a <code class="sourceCode cpp"><span class="kw">constexpr</span></code> function, so there does not exist any invocation that would be a constant expression. Ill-formed, no diagnostic required. gcc and msvc both diagnose this.</p>
<p>Now, one could argue that this diagnosis is a good thing: that <code class="sourceCode cpp"><span class="kw">constexpr</span></code> annotation on <code class="sourceCode cpp">g</code> makes no sense! It can’t be a constant expression, so having that specifier on the function is misleading. Diagnosing this error helps the programmer realize that they were mistaken and they can take steps to change this. That seems compelling enough.</p>
<p>Consider instead this example:</p>
<blockquote>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a><span class="pp">#include </span><span class="im">&lt;optional&gt;</span></span>
<span id="cb2-2"><a href="#cb2-2"></a></span>
<span id="cb2-3"><a href="#cb2-3"></a><span class="kw">constexpr</span> <span class="dt">void</span> h<span class="op">(</span>std<span class="op">::</span>optional<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;&amp;</span> o<span class="op">)</span> <span class="op">{</span></span>
<span id="cb2-4"><a href="#cb2-4"></a>    o<span class="op">.</span>reset<span class="op">()</span>;</span>
<span id="cb2-5"><a href="#cb2-5"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>Here, <code class="sourceCode cpp">h</code> is the exact same kind of function as <code class="sourceCode cpp">g</code>: we have a function unconditionally calling another non-<code class="sourceCode cpp"><span class="kw">constexpr</span></code> function. At least, that’s true for C++17. It’s won’t be true in C++23, and the answer for C++20 depends on how vendors choose to implement <span class="citation" data-cites="P2231R1">[<a href="#ref-P2231R1" role="doc-biblioref">P2231R1</a>]</span>. Here the answer depends: some functions can easily be <code class="sourceCode cpp"><span class="kw">constexpr</span></code> in one standard but not in earlier ones.</p>
<p>Now, the sanctioned way to fix this code is to write:</p>
<blockquote>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="pp">#include </span><span class="im">&lt;optional&gt;</span></span>
<span id="cb3-2"><a href="#cb3-2"></a></span>
<span id="cb3-3"><a href="#cb3-3"></a><span class="pp">#if __cpp_lib_optional &gt;= 202106</span></span>
<span id="cb3-4"><a href="#cb3-4"></a><span class="kw">constexpr</span></span>
<span id="cb3-5"><a href="#cb3-5"></a><span class="pp">#endif</span></span>
<span id="cb3-6"><a href="#cb3-6"></a><span class="dt">void</span> h<span class="op">(</span>std<span class="op">::</span>optional<span class="op">&lt;</span><span class="dt">int</span><span class="op">&gt;&amp;</span> o<span class="op">)</span> <span class="op">{</span></span>
<span id="cb3-7"><a href="#cb3-7"></a>    o<span class="op">.</span>reset<span class="op">()</span>;</span>
<span id="cb3-8"><a href="#cb3-8"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>This way, <code class="sourceCode cpp">h</code> will be <code class="sourceCode cpp"><span class="kw">constexpr</span></code> when the appropriate library changes are made, allowing it to be. But… is this better?</p>
<p>I would argue it’s not. The language rules for <code class="sourceCode cpp"><span class="kw">constexpr</span></code> have expanded in every language standard since <code class="sourceCode cpp"><span class="kw">constexpr</span></code> was introduced. Standard library support will always lag that. Third-party library support likely even more so. So the answer to the question “are there any arguments for which this function can be a constant expression?” can easily be No in C++N but Yes in C++N+1, for a wide variety of functions. Does forcing conscientious library authors to take painstaking care in conditionally marking function <code class="sourceCode cpp"><span class="kw">constexpr</span></code> provide value to the ecosystem? I’m skeptical that it does.</p>
<p>Moreover, while it’s possible to write the above for <code class="sourceCode cpp">std<span class="op">::</span>optional</code>, I’m not sure that it’s common for other library to provide macros that can be used to mark functions conditionally <code class="sourceCode cpp"><span class="kw">constexpr</span></code> like this. Or, indeed, if there is even another such example. So if I’m a consumer of a library that might have some functionality <code class="sourceCode cpp"><span class="kw">constexpr</span></code> in one version but more functionality <code class="sourceCode cpp"><span class="kw">constexpr</span></code> in the next, I always have to lag.</p>
<p>Such diagnosis may have made sense in the C++11 days, but now that we’re approaching C++23 where more and more things are <code class="sourceCode cpp"><span class="kw">constexpr</span></code> and more and more libraries will mark more of their functions <code class="sourceCode cpp"><span class="kw">constexpr</span></code> because they can be, it seems strictly better to just reserve diagnosing <code class="sourceCode cpp"><span class="kw">constexpr</span></code> violations to the place where we already have to diagnose them: when you write code that <em>must</em> be evaluated at compile time.</p>
<p>Put differently, the current rule is there must be some tuple (function arguments, template arguments) for which a <code class="sourceCode cpp"><span class="kw">constexpr</span></code> function invocation is a constant expression. But there’s really another input here: (function arguments, template arguments, version). The version here might be the language version, it might be a bunch of library versions. Ultimately, it’s a question of time. A function may not be <code class="sourceCode cpp"><span class="kw">constexpr</span></code> today, but it may be <code class="sourceCode cpp"><span class="kw">constexpr</span></code> soon. Diagnosing it as not being <code class="sourceCode cpp"><span class="kw">constexpr</span></code> <em>yet</em> seems harmful to the question of evolving code.</p>
<h1 data-number="4" style="border-bottom:1px solid #cccccc" id="sometimes-maybe-sometimes-always"><span class="header-section-number">4</span> Sometimes Maybe, Sometimes Always<a href="#sometimes-maybe-sometimes-always" class="self-link"></a></h1>
<p><code class="sourceCode cpp"><span class="kw">constexpr</span></code> usually means <em>maybe</em> <code class="sourceCode cpp"><span class="kw">constexpr</span></code>. But sometimes it actually means <em>always</em> <code class="sourceCode cpp"><span class="kw">constexpr</span></code>. That case comes up with explicitly defaulted functions. From <span>9.5.2 <a href="https://wg21.link/dcl.fct.def.default">[dcl.fct.def.default]</a></span>/3:</p>
<div class="quote">
<p>An explicitly-defaulted function that is not defined as deleted may be declared <code class="sourceCode cpp"><span class="kw">constexpr</span></code> or <code class="sourceCode cpp"><span class="kw">consteval</span></code> only if it is <code class="sourceCode cpp"><span class="kw">constexpr</span></code>-compatible ([special], [class.compare.default]). A function explicitly defaulted on its first declaration is implicitly inline ([dcl.inline]), and is implicitly constexpr ([dcl.constexpr]) if it is constexpr-compatible.</p>
</div>
<p>Let’s say I’m writing a wrapper class template that I’m intending to be usable during compile time:</p>
<blockquote>
<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">template</span> <span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span></span>
<span id="cb4-2"><a href="#cb4-2"></a><span class="kw">struct</span> Wrapper <span class="op">{</span></span>
<span id="cb4-3"><a href="#cb4-3"></a>    <span class="kw">constexpr</span> Wrapper<span class="op">()</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb4-4"><a href="#cb4-4"></a>    <span class="kw">constexpr</span> Wrapper<span class="op">(</span>Wrapper <span class="kw">const</span><span class="op">&amp;)</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb4-5"><a href="#cb4-5"></a>    <span class="kw">constexpr</span> Wrapper<span class="op">(</span>T <span class="kw">const</span><span class="op">&amp;</span> t<span class="op">)</span> <span class="op">:</span> t<span class="op">(</span>t<span class="op">)</span> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb4-6"><a href="#cb4-6"></a></span>
<span id="cb4-7"><a href="#cb4-7"></a>    <span class="kw">constexpr</span> T get<span class="op">()</span> <span class="kw">const</span> <span class="op">{</span> <span class="cf">return</span> t; <span class="op">}</span></span>
<span id="cb4-8"><a href="#cb4-8"></a>    <span class="kw">constexpr</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">==(</span>Wrapper <span class="kw">const</span><span class="op">&amp;)</span> <span class="kw">const</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb4-9"><a href="#cb4-9"></a><span class="kw">private</span><span class="op">:</span></span>
<span id="cb4-10"><a href="#cb4-10"></a>    T t;</span>
<span id="cb4-11"><a href="#cb4-11"></a><span class="op">}</span>;</span></code></pre></div>
</blockquote>
<p>I might take the strategy of just marking every function <code class="sourceCode cpp"><span class="kw">constexpr</span></code>. Both for consistency and also as a strategy to avoid forgetting to mark some functions <code class="sourceCode cpp"><span class="kw">constexpr</span></code>.</p>
<p>But then I try to use it:</p>
<blockquote>
<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">struct</span> X <span class="op">{</span></span>
<span id="cb5-2"><a href="#cb5-2"></a>    X<span class="op">()</span>;</span>
<span id="cb5-3"><a href="#cb5-3"></a>    <span class="dt">bool</span> <span class="kw">operator</span><span class="op">==(</span>X <span class="kw">const</span><span class="op">&amp;)</span> <span class="kw">const</span>;</span>
<span id="cb5-4"><a href="#cb5-4"></a><span class="op">}</span>;</span>
<span id="cb5-5"><a href="#cb5-5"></a></span>
<span id="cb5-6"><a href="#cb5-6"></a>Wrapper<span class="op">&lt;</span>X<span class="op">&gt;</span> x;</span></code></pre></div>
</blockquote>
<p>None of this code is trying to evaluate anything during constant evaluation, yet it is already ill-formed. gcc and clang already diagnose at this point. msvc and icc do not, but this rule is a mandatory diagnostic, so they are mistaken. Although, even here, none of the compilers care that I erroneously marked the default constructor <code class="sourceCode cpp"><span class="kw">constexpr</span></code>. And none of them care that I erroneously marked the copy constructor <code class="sourceCode cpp"><span class="kw">constexpr</span></code>. gcc and clang are only diagnosing <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">==</span></code> here (even though I’m not even using it).</p>
<p>But why do we even have this rule in the first place? Note that if I wrote my equality operator this way:</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">constexpr</span> <span class="dt">bool</span> <span class="kw">operator</span><span class="op">==(</span>Wrapper <span class="kw">const</span><span class="op">&amp;</span> rhs<span class="op">)</span> <span class="kw">const</span> <span class="op">{</span></span>
<span id="cb6-2"><a href="#cb6-2"></a>    <span class="cf">return</span> t <span class="op">==</span> rhs<span class="op">.</span>t;</span>
<span id="cb6-3"><a href="#cb6-3"></a><span class="op">}</span></span></code></pre></div>
</blockquote>
<p>That is, manually writing out what the defaulted version does, even with marking it <code class="sourceCode cpp"><span class="kw">constexpr</span></code>, this code is perfectly valid C++ code. It doesn’t matter that I’m marking this function <code class="sourceCode cpp"><span class="kw">constexpr</span></code>, because in this context, the only requirement is that <em>some</em> instantiation can be a constant expression. But because I want to <code class="sourceCode cpp"><span class="op">=</span> <span class="cf">default</span></code> it, suddenly <em>every</em> instantiation has to be a constant expression?</p>
<p>What this means is that the correct way to write my wrapper type is:</p>
<blockquote>
<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">template</span> <span class="op">&lt;</span><span class="kw">typename</span> T<span class="op">&gt;</span></span>
<span id="cb7-2"><a href="#cb7-2"></a><span class="kw">struct</span> Wrapper <span class="op">{</span></span>
<span id="cb7-3"><a href="#cb7-3"></a>    Wrapper<span class="op">()</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb7-4"><a href="#cb7-4"></a>    Wrapper<span class="op">(</span>Wrapper <span class="kw">const</span><span class="op">&amp;)</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb7-5"><a href="#cb7-5"></a>    <span class="kw">constexpr</span> Wrapper<span class="op">(</span>T <span class="kw">const</span><span class="op">&amp;</span> t<span class="op">)</span> <span class="op">:</span> t<span class="op">(</span>t<span class="op">)</span> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb7-6"><a href="#cb7-6"></a></span>
<span id="cb7-7"><a href="#cb7-7"></a>    <span class="kw">constexpr</span> T get<span class="op">()</span> <span class="kw">const</span> <span class="op">{</span> <span class="cf">return</span> t; <span class="op">}</span></span>
<span id="cb7-8"><a href="#cb7-8"></a>    <span class="dt">bool</span> <span class="kw">operator</span><span class="op">==(</span>Wrapper <span class="kw">const</span><span class="op">&amp;)</span> <span class="kw">const</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb7-9"><a href="#cb7-9"></a><span class="kw">private</span><span class="op">:</span></span>
<span id="cb7-10"><a href="#cb7-10"></a>    T t;</span>
<span id="cb7-11"><a href="#cb7-11"></a><span class="op">}</span>;</span></code></pre></div>
</blockquote>
<p>So I have some functions marked <code class="sourceCode cpp"><span class="kw">constexpr</span></code> and some not, but all of them are still usable during constant evaluation time (where appropriate based on <code class="sourceCode cpp">T</code>). The lack of consistency here is a bit jarring. <code class="sourceCode cpp"><span class="kw">constexpr</span></code> functions should always be <em>maybe</em> <code class="sourceCode cpp"><span class="kw">constexpr</span></code>, not <em>must be</em> <code class="sourceCode cpp"><span class="kw">constexpr</span></code>.</p>
<h1 data-number="5" style="border-bottom:1px solid #cccccc" id="onwards-to-constexpr-classes"><span class="header-section-number">5</span> Onwards to constexpr classes<a href="#onwards-to-constexpr-classes" class="self-link"></a></h1>
<p>We have a proposal in front of us to allow annotating the entire class as being <code class="sourceCode cpp"><span class="kw">constexpr</span></code>, to avoid all these extra annotations <span class="citation" data-cites="P2350R1">[<a href="#ref-P2350R1" role="doc-biblioref">P2350R1</a>]</span>. And that proposal currently runs into both issues:</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">typename</span> T<span class="op">&gt;</span></span>
<span id="cb8-2"><a href="#cb8-2"></a><span class="kw">struct</span> Wrapper <span class="kw">constexpr</span> <span class="op">{</span></span>
<span id="cb8-3"><a href="#cb8-3"></a>    Wrapper<span class="op">()</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb8-4"><a href="#cb8-4"></a>    Wrapper<span class="op">(</span>Wrapper <span class="kw">const</span><span class="op">&amp;)</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb8-5"><a href="#cb8-5"></a>    Wrapper<span class="op">(</span>T <span class="kw">const</span><span class="op">&amp;</span> t<span class="op">)</span> <span class="op">:</span> t<span class="op">(</span>t<span class="op">)</span> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb8-6"><a href="#cb8-6"></a></span>
<span id="cb8-7"><a href="#cb8-7"></a>    <span class="dt">void</span> reset<span class="op">()</span> <span class="op">{</span> t<span class="op">.</span>reset<span class="op">()</span>; <span class="op">}</span></span>
<span id="cb8-8"><a href="#cb8-8"></a>    <span class="dt">bool</span> <span class="kw">operator</span><span class="op">==(</span>Wrapper <span class="kw">const</span><span class="op">&amp;)</span> <span class="kw">const</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb8-9"><a href="#cb8-9"></a><span class="kw">private</span><span class="op">:</span></span>
<span id="cb8-10"><a href="#cb8-10"></a>    std<span class="op">::</span>optional<span class="op">&lt;</span>T<span class="op">&gt;</span> t;</span>
<span id="cb8-11"><a href="#cb8-11"></a><span class="op">}</span>;</span></code></pre></div>
</blockquote>
<p>Are all of these functions okay? I would argue that they <em>should</em> all be okay, but per the wording they’re currently not.</p>
<p>I want <code class="sourceCode cpp">Wrapper</code> to be entirely <code class="sourceCode cpp"><span class="kw">constexpr</span></code> where feasible. Some of those functions may not be <code class="sourceCode cpp"><span class="kw">constexpr</span></code> for all types, and that’s fine. Some of these functions may not be able to be <code class="sourceCode cpp"><span class="kw">constexpr</span></code> in C++N but may be later, and I don’t want to have to go back and either annotate against this (which I don’t think P2350 even allows room for) or stop using this feature and go back to manually marking and even more annotations.</p>
<h1 data-number="6" style="border-bottom:1px solid #cccccc" id="proposal"><span class="header-section-number">6</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>Strike <span>9.2.6 <a href="https://wg21.link/dcl.constexpr">[dcl.constexpr]</a></span>/6 and the example following it, in its entirety, along with the second sentence in /7:</p>
<blockquote>
<div class="rm" style="color: #bf0303">
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_1" id="pnum_1">6</a></span> For a constexpr function or constexpr constructor that is neither defaulted nor a template, if no argument values exist such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression, or, for a constructor, an evaluated subexpression of the initialization full-expression of some constant-initialized object ([basic.start.static]), the program is ill-formed, no diagnostic required.</p>
<p>[<em>Example 4</em>:</p>
<div class="sourceCode" id="cb9"><pre class="sourceCode default default"><code class="sourceCode default"><span id="cb9-1"><a href="#cb9-1"></a>constexpr int f(bool b)</span>
<span id="cb9-2"><a href="#cb9-2"></a>  { return b ? throw 0 : 0; }           // OK</span>
<span id="cb9-3"><a href="#cb9-3"></a>constexpr int f() { return f(true); }   // ill-formed, no diagnostic required</span>
<span id="cb9-4"><a href="#cb9-4"></a></span>
<span id="cb9-5"><a href="#cb9-5"></a>struct B {</span>
<span id="cb9-6"><a href="#cb9-6"></a>  constexpr B(int x) : i(0) { }         // x is unused</span>
<span id="cb9-7"><a href="#cb9-7"></a>  int i;</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>int global;</span>
<span id="cb9-11"><a href="#cb9-11"></a></span>
<span id="cb9-12"><a href="#cb9-12"></a>struct D : B {</span>
<span id="cb9-13"><a href="#cb9-13"></a>  constexpr D() : B(global) { }         // ill-formed, no diagnostic required</span>
<span id="cb9-14"><a href="#cb9-14"></a>                                        // lvalue-to-rvalue conversion on non-constant global</span>
<span id="cb9-15"><a href="#cb9-15"></a>};</span></code></pre></div>
<ul>
<li><em>end example</em>]</li>
</ul>

</div>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_2" id="pnum_2">7</a></span> If the instantiated template specialization of a constexpr function template or member function of a class template would fail to satisfy the requirements for a constexpr function, that specialization is still a constexpr function, even though a call to such a function cannot appear in a constant expression. <span class="rm" style="color: #bf0303"><del>If no specialization of the template would satisfy the requirements for a constexpr function when considered as a non-template function, the template is ill-formed, no diagnostic required.</del></span></p>
</blockquote>
<p>Strike part of <span>9.5.2 <a href="https://wg21.link/dcl.fct.def.default">[dcl.fct.def.default]</a></span>/3 and fix the example (which is already wrong at the moment, since default-initializing an <code class="sourceCode cpp"><span class="dt">int</span></code> during constant evaluation is ok):</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_3" id="pnum_3">3</a></span> An explicitly-defaulted function that is not defined as deleted may be declared <code class="sourceCode cpp"><span class="kw">constexpr</span></code> or <code class="sourceCode cpp"><span class="kw">consteval</span></code> <span class="rm" style="color: #bf0303"><del>only if it is constexpr-compatible ([special], [class.compare.default])</del></span>. A function explicitly defaulted on its first declaration is implicitly inline ([dcl.inline]), and is implicitly constexpr ([dcl.constexpr]) if it is constexpr-compatible.</p>
<p><span class="marginalizedparent"><a class="marginalized" href="#pnum_4" id="pnum_4">4</a></span> [<em>Example 1</em>:</p>
<div>
<div class="sourceCode" id="cb10"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb10-1"><a href="#cb10-1"></a>  struct S {</span>
<span id="cb10-2"><a href="#cb10-2"></a><span class="st">-   constexpr S() = default;              // error: implicit S() is not constexpr</span></span>
<span id="cb10-3"><a href="#cb10-3"></a>    S(int a = 0) = default;               // error: default argument</span>
<span id="cb10-4"><a href="#cb10-4"></a>    void operator=(const S&amp;) = default;   // error: non-matching return type</span>
<span id="cb10-5"><a href="#cb10-5"></a>    ~S() noexcept(false) = default;       // OK, despite mismatched exception specification</span>
<span id="cb10-6"><a href="#cb10-6"></a>  private:</span>
<span id="cb10-7"><a href="#cb10-7"></a>    int i;</span>
<span id="cb10-8"><a href="#cb10-8"></a>    S(S&amp;);                                // OK: private copy constructor</span>
<span id="cb10-9"><a href="#cb10-9"></a>  };</span>
<span id="cb10-10"><a href="#cb10-10"></a>  S::S(S&amp;) = default;                     // OK: defines copy constructor</span>
<span id="cb10-11"><a href="#cb10-11"></a></span>
<span id="cb10-12"><a href="#cb10-12"></a>  struct T {</span>
<span id="cb10-13"><a href="#cb10-13"></a>    T();</span>
<span id="cb10-14"><a href="#cb10-14"></a>    T(T &amp;&amp;) noexcept(false);</span>
<span id="cb10-15"><a href="#cb10-15"></a>  };</span>
<span id="cb10-16"><a href="#cb10-16"></a>  struct U {</span>
<span id="cb10-17"><a href="#cb10-17"></a>    T t;</span>
<span id="cb10-18"><a href="#cb10-18"></a>    U();</span>
<span id="cb10-19"><a href="#cb10-19"></a>    U(U &amp;&amp;) noexcept = default;</span>
<span id="cb10-20"><a href="#cb10-20"></a>  };</span>
<span id="cb10-21"><a href="#cb10-21"></a>  U u1;</span>
<span id="cb10-22"><a href="#cb10-22"></a>  U u2 = static_cast&lt;U&amp;&amp;&gt;(u1);            // OK, calls std​::​terminate if T​::​T(T&amp;&amp;) throws</span></code></pre></div>
</div>
<p>— <em>end example</em>]</p>
</blockquote>
<h1 data-number="7" style="border-bottom:1px solid #cccccc" id="bibliography"><span class="header-section-number">7</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<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-P2350R1">
<p>[P2350R1] Andreas Fertig. 2021-07-15. constexpr class. <br />
<a href="https://wg21.link/p2350r1">https://wg21.link/p2350r1</a></p>
</div>
</div>
</div>
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