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P1938R1__func_in_consteval_context__.html
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P1938R1__func_in_consteval_context__.html
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<h1 class="title" style="text-align:center">This is not P1938R1; there is no P1938R1 yet.</h1>
<h1 class="title" style="text-align:center"><code class="sourceCode cpp"><span class="cf">__func_in_consteval_context__</span></code></h1>
<h1 class="title" style="text-align:center">This is not P1938R1; there is no P1938R1 yet.</h1>
<table style="border:none;float:right">
<tr>
<td>Document #: </td>
<td>X1938X1</td>
</tr>
<tr>
<td>Date: </td>
<td>2019-11-06</td>
</tr>
<tr>
<td style="vertical-align:top">Project: </td>
<td>Programming Language C++<br>
EWG<br>
</td>
</tr>
<tr>
<td style="vertical-align:top">Reply-to: </td>
<td>
Barry Revzin<br><<a href="mailto:[email protected]" class="email">[email protected]</a>><br>
Richard Smith<br><<a href="mailto:[email protected]" class="email">[email protected]</a>><br>
Andrew Sutton<br><<a href="mailto:[email protected]" class="email">[email protected]</a>><br>
Daveed Vandevoorde<br><<a href="mailto:[email protected]" class="email">[email protected]</a>><br>
Jorg Brown<br><<a href="mailto:[email protected]" class="email">[email protected]</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="#problems-with-status-quo"><span class="toc-section-number">2</span> Problems with Status Quo<span></span></a></li>
<li><a href="#proposal"><span class="toc-section-number">3</span> Proposal<span></span></a></li>
<li><a href="#history"><span class="toc-section-number">4</span> History<span></span></a></li>
<li><a href="#wording"><span class="toc-section-number">5</span> Wording<span></span></a></li>
<li><a href="#acknowledgments"><span class="toc-section-number">6</span> Acknowledgments<span></span></a></li>
<li><a href="#references"><span class="toc-section-number">7</span> References<span></span></a></li>
</ul>
</div>
<h1 id="introduction" style="border-bottom:1px solid #cccccc"><span class="header-section-number">1</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<h1 class="title" style="text-align:center">This is not P1938R1; this is a suggestion for a change to P1938, in a form that resembles a paper.</h1>
<p>Despite this paper missing both our respective NB comment deadlines and the mailing deadline, we still believe it provides a significant enough improvement to the status quo that it should be considered for C++20.</p>
<p>C++20 will have several new features to aid programmers in writing code during constant evaluation. Two of these are <code class="sourceCode cpp">std<span class="op">::</span>is_constant_evaluated<span class="op">()</span></code> <span class="citation" data-cites="P0595R2">[<a href="#ref-P0595R2" role="doc-biblioref">P0595R2</a>]</span> and <code class="sourceCode cpp"><span class="kw">consteval</span></code> <span class="citation" data-cites="P1073R3">[<a href="#ref-P1073R3" role="doc-biblioref">P1073R3</a>]</span>, both adopted in San Diego 2018. <code class="sourceCode cpp"><span class="kw">consteval</span></code> is for functions that can only be invoked during constant evaluation. <code class="sourceCode cpp">is_constant_evaluated<span class="op">()</span></code> is a magic library function to check if the current evaluation is constant evaluation to provide, for instance, a valid implementation of an algorithm for constant evaluation time and a better implementation for runtime.</p>
<p>However, despite being adopted at the same meeting, these features interact poorly with each other and have other issues that make them ripe for confusion.</p>
<h1 id="problems-with-status-quo" style="border-bottom:1px solid #cccccc"><span class="header-section-number">2</span> Problems with Status Quo<a href="#problems-with-status-quo" class="self-link"></a></h1>
<p>There are two problems this paper wishes to address.</p>
<p>The first problem is the interplay between this magic library function and the new <code class="sourceCode cpp"><span class="kw">consteval</span></code>. Consider the 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">consteval</span> <span class="dt">int</span> f<span class="op">(</span><span class="dt">int</span> i<span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> i; <span class="op">}</span></span>
<span id="cb1-2"><a href="#cb1-2"></a></span>
<span id="cb1-3"><a href="#cb1-3"></a><span class="kw">constexpr</span> <span class="dt">int</span> g<span class="op">(</span><span class="dt">int</span> i<span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-4"><a href="#cb1-4"></a> <span class="cf">if</span> <span class="op">(</span>std<span class="op">::</span>is_constant_evaluated<span class="op">())</span> <span class="op">{</span></span>
<span id="cb1-5"><a href="#cb1-5"></a> <span class="cf">return</span> f<span class="op">(</span>i<span class="op">)</span> <span class="op">+</span> <span class="dv">1</span>; <span class="co">// <==</span></span>
<span id="cb1-6"><a href="#cb1-6"></a> <span class="op">}</span> <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb1-7"><a href="#cb1-7"></a> <span class="cf">return</span> <span class="dv">42</span>;</span>
<span id="cb1-8"><a href="#cb1-8"></a> <span class="op">}</span></span>
<span id="cb1-9"><a href="#cb1-9"></a><span class="op">}</span></span>
<span id="cb1-10"><a href="#cb1-10"></a></span>
<span id="cb1-11"><a href="#cb1-11"></a><span class="kw">consteval</span> <span class="dt">int</span> h<span class="op">(</span><span class="dt">int</span> i<span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-12"><a href="#cb1-12"></a> <span class="cf">return</span> f<span class="op">(</span>i<span class="op">)</span> <span class="op">+</span> <span class="dv">1</span>;</span>
<span id="cb1-13"><a href="#cb1-13"></a><span class="op">}</span></span></code></pre></div>
<p>The function <code class="sourceCode cpp">h</code> here is basically a lifted, constant-evaluation-only version of the function <code class="sourceCode cpp">g</code>. At constant evaluation time, they do the same thing, except that during runtime, you cannot call <code class="sourceCode cpp">h</code>, and <code class="sourceCode cpp">g</code> has this extra path. Maybe this code started with just <code class="sourceCode cpp">h</code> and someone decided a runtime version would also be useful and turned it into <code class="sourceCode cpp">g</code>.</p>
<p>Unfortunately, <code class="sourceCode cpp">h</code> is well-formed while <code class="sourceCode cpp">g</code> is ill-formed. You cannot make that call to <code class="sourceCode cpp">f</code> (that is ominously marked with an arrow) in that location. Even though that call will <em>only</em> happen during constant evaluation, that’s still not enough.</p>
<p>With specific terms, the call to <code class="sourceCode cpp">f<span class="op">()</span></code> inside of <code class="sourceCode cpp">g<span class="op">()</span></code> is an <em>immediate invocation</em> and needs to be a constant expression and it is not. Whereas the call to <code class="sourceCode cpp">f<span class="op">()</span></code> inside of <code class="sourceCode cpp">h<span class="op">()</span></code> is <em>not</em> considered an <em>immediate invocation</em> because it is in an <em>immediate function context</em> (i.e. it’s invoked from another immediate function), so it has a weaker set of restrictions that it needs to follow.</p>
<p>In other words, this kind of construction of conditionally invoking a <code class="sourceCode cpp"><span class="kw">consteval</span></code> function from a <code class="sourceCode cpp"><span class="kw">constexpr</span></code> function just Does Not Work (modulo the really trivial cases - one could call <code class="sourceCode cpp">f<span class="op">(</span><span class="dv">42</span><span class="op">)</span></code> for instance, just never <code class="sourceCode cpp">f<span class="op">(</span>i<span class="op">)</span></code>).</p>
<p>We find this lack of composability of features to be problematic and think it can be improved.</p>
<p>The second problem is specific to <code class="sourceCode cpp">is_constant_evaluated</code>. Once you learn what this magic function is for, the obvious usage of it is:</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="dt">size_t</span> strlen<span class="op">(</span><span class="dt">char</span> <span class="kw">const</span><span class="op">*</span> s<span class="op">)</span> <span class="op">{</span></span>
<span id="cb2-2"><a href="#cb2-2"></a> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>std<span class="op">::</span>is_constant_evaluated<span class="op">())</span> <span class="op">{</span></span>
<span id="cb2-3"><a href="#cb2-3"></a> <span class="cf">for</span> <span class="op">(</span><span class="kw">const</span> <span class="dt">char</span> <span class="op">*</span>p <span class="op">=</span> s; ; <span class="op">++</span>p<span class="op">)</span> <span class="op">{</span></span>
<span id="cb2-4"><a href="#cb2-4"></a> <span class="cf">if</span> <span class="op">(*</span>p <span class="op">==</span> <span class="ch">'</span><span class="sc">\0</span><span class="ch">'</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb2-5"><a href="#cb2-5"></a> <span class="cf">return</span> <span class="kw">static_cast</span><span class="op"><</span>std<span class="op">::</span><span class="dt">size_t</span><span class="op">>(</span>p <span class="op">-</span> s<span class="op">)</span>;</span>
<span id="cb2-6"><a href="#cb2-6"></a> <span class="op">}</span></span>
<span id="cb2-7"><a href="#cb2-7"></a> <span class="op">}</span> </span>
<span id="cb2-8"><a href="#cb2-8"></a> <span class="op">}</span> <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb2-9"><a href="#cb2-9"></a> <span class="ex">__asm__</span><span class="op">(</span><span class="st">"SSE 4.2 insanity"</span><span class="op">)</span>; </span>
<span id="cb2-10"><a href="#cb2-10"></a> <span class="op">}</span></span>
<span id="cb2-11"><a href="#cb2-11"></a><span class="op">}</span></span></code></pre></div>
<p>This example, inspired by <span class="citation" data-cites="P1045R0">[<a href="#ref-P1045R0" role="doc-biblioref">P1045R0</a>]</span>, has a bug: it uses <code class="sourceCode cpp"><span class="cf">if</span> <span class="kw">constexpr</span></code> to check the conditional <code class="sourceCode cpp">is_constant_evaluated<span class="op">()</span></code> rather than a simple <code class="sourceCode cpp"><span class="cf">if</span></code>. You have to really deeply understand a lot about how constant evaluation works in C++ to understand that this is in fact not only <em>not</em> “obviously correct” but is in fact “obviously incorrect,” for some definition of obvious. This is such a likely source of error that Barry submitted bugs to both <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91428">gcc</a> and <a href="https://bugs.llvm.org/show_bug.cgi?id=42977">clang</a> to encourage the compilers to warn on such improper usage. gcc 10.1 will provide a warning for the <a href="https://godbolt.org/z/LiiZoW">simple case</a>:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb3-1"><a href="#cb3-1"></a><source>: In function 'constexpr int f(int)':</span>
<span id="cb3-2"><a href="#cb3-2"></a><source>:4:45: warning: 'std::is_constant_evaluated' always evaluates to true in 'if constexpr' [-Wtautological-compare]</span>
<span id="cb3-3"><a href="#cb3-3"></a> 4 | if constexpr (std::is_constant_evaluated()) {</span>
<span id="cb3-4"><a href="#cb3-4"></a> | ~~~~~~~~~~~~~~~~~~~~~~~~~~^~</span></code></pre></div>
<p>But then people have to understand why this is a warning, and what this even means. Nevertheless, a compiler warning is substantially better than silently wrong code, but it is problematic to have an API in which many users are drawn to a usage that is tautologically incorrect.</p>
<h1 id="proposal" style="border-bottom:1px solid #cccccc"><span class="header-section-number">3</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>We propose a new function-local predefined variable:</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="cf">constexpr bool</span> __func_in_consteval_context__ <span class="op">=</span> true/false <span class="op">;</span></span></code></pre></div>
<p>If evaluation of this function occurs during constant evaluation, the value of this variable is <span class="kw">true</span>. Otherwise, its value is <span class="kw">false</span>.</p>
<p>This is exactly the same value as today’s:</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1"></a> std<span class="op">::</span>is_constant_evaluated<span class="op">())</span></span></code></pre></div>
<p>except with three differences:</p>
<ol type="1">
<li>No header include is necessary.</li>
<li>The syntax is different, which completely sidesteps the confusion over the proper way to check if we’re in constant evaluation. You simply cannot misuse the syntax.</li>
<li>We can use <code class="sourceCode cpp"><span class="cf">if</span> <span class="kw">constexpr</span><span class="op">(</span>__func_in_consteval_context__<span class="op">)</span></code> to allow invoking immediate functions.</li>
</ol>
<p>Additionally, such a feature would allow for an easy implementation of the original <code class="sourceCode cpp">std<span class="op">::</span>is_constant_evaluated<span class="op">()</span></code>:</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">constexpr</span> <span class="dt">bool</span> is_constant_evaluated<span class="op">()</span> <span class="op">{</span></span>
<span id="cb7-2"><a href="#cb7-2"></a> <span class="cf">return</span> __func_in_consteval_context__;</span>
<span id="cb7-7"><a href="#cb7-7"></a><span class="op">}</span></span></code></pre></div>
<p>Which in itself suggests that this is the more fundamental feature. As such, <code class="sourceCode cpp">std<span class="op">::</span>is_constant_evaluated<span class="op">()</span></code> may itself no longer be necessary. However, given the very late date of this proposal, we would more than happily keep it if it allows us this new language feature.</p>
<h1 id="history" style="border-bottom:1px solid #cccccc"><span class="header-section-number">4</span> History<a href="#history" class="self-link"></a></h1>
<p>The initial revision of this proposal <span class="citation" data-cites="P1938R0">[<a href="#ref-P1938R0" role="doc-biblioref">P1938R0</a>]</span> was actually targeted as a new syntax for if rather than a function-local variable. The original spelling was <code class="sourceCode cpp"><span class="cf">if</span> <span class="kw">consteval</span></code>. The paper was presented in Belfast on Monday and was received favorably for consideration in C++20 (28-6). It was also discussed on the EWG reflector, where many modifications were suggested.</p>
<p>The initial revision of the <code class="sourceCode cpp">std<span class="op">::</span>is_constant_evaluated<span class="op">()</span></code> proposal <span class="citation" data-cites="P0595R0">[<a href="#ref-P0595R0" role="doc-biblioref">P0595R0</a>]</span> was actually targeted as a language feature rather than a library feature. The original spelling was <code class="sourceCode cpp"><span class="cf">if</span> <span class="op">(</span><span class="kw">constexpr</span><span class="op">())</span></code>. The paper was presented in Kona 2017 and was received very favorably in the form it was presented (17-4). The poll to consider a magic library alternative was only marginally more preferred (17-3). We believe that in the two years since these polls were taken, having a dedicated language feature with an impossible-to-misuse API, that can coexist with the rest of the constant ecosystem, is the right direction.</p>
<h1 id="wording" style="border-bottom:1px solid #cccccc"><span class="header-section-number">5</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Add a new clause to Function definitions in general in 9.5.1 [dcl.fct.def.general]:</p>
<blockquote>
<div class="addu">
<p><span class="marginalizedparent"><a class="marginalized">9</a></span> The function-local predefined variable __func_in_consteval_context__ is defined as if a definition of the form:</p>
<div class="sourceCode" id="cb10"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb10-1"><a href="#cb10-1"></a>constexpr bool __func_in_consteval_context__ = <i>initial-value</i>;</span></code></pre>
</div>
had been provided, where initial-value is false, unless:</p>
<ul>
<li>The function being defined has been declared constexpr or consteval, and</span></li>
<li>the call of the function occurs within the evaluation of an expression or conversion that is manifestly constant-evaluated ([expr.const]). </li>
</ul>
</blockquote>
<h1 id="acknowledgments" style="border-bottom:1px solid #cccccc"><span class="header-section-number">6</span> Acknowledgments<a href="#acknowledgments" class="self-link"></a></h1>
<p>Thank you to David Stone and Tim Song for working through these examples.</p>
<p>Thank you to Andrew Tomazos for suggesting having an expression (under the name of consteval) which examines the context of its caller's caller, rather than merely its caller.</p>
<p>Thank you to Dawid Pilarski for suggesting std::is_function_constant_evaluated() as a better name for that expression.</p>
<h1 id="references" style="border-bottom:1px solid #cccccc"><span class="header-section-number">7</span> References<a href="#references" class="self-link"></a></h1>
<div id="refs" role="doc-bibliography">
<div id="ref-P0595R0">
<p>[P0595R0] Daveed Vandevoorde. 2017. The “constexpr” Operator. <br />
<a href="https://wg21.link/p0595r0">https://wg21.link/p0595r0</a></p>
</div>
<div id="ref-P0595R2">
<p>[P0595R2] Richard Smith, Andrew Sutton, Daveed Vandevoorde. 2018. <code class="sourceCode cpp">std<span class="op">::</span>is_constant_evaluated</code>. <br />
<a href="https://wg21.link/p0595r2">https://wg21.link/p0595r2</a></p>
</div>
<div id="ref-P1045R0">
<p>[P1045R0] David Stone. 2018. constexpr Function Parameters. <br />
<a href="https://wg21.link/p1045r0">https://wg21.link/p1045r0</a></p>
</div>
<div id="ref-P1073R3">
<p>[P1073R3] Richard Smith, Andrew Sutton, Daveed Vandevoorde. 2018. Immediate functions. <br />
<a href="https://wg21.link/p1073r3">https://wg21.link/p1073r3</a></p>
</div>
<div id="ref-P1938R0">
<p>[P1938R0] Barry Revzin, Richard Smith, Andrew Sutton, Daveed Vandevoorde. 2019. if consteval. <br />
<a href="https://wg21.link/p1938r0">https://wg21.link/p1938r0</a></p>
</div>
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