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  <meta name="dcterms.date" content="2025-09-01" />
  <title>Conditional noexcept specifiers in compound
requirements</title>
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<header id="title-block-header">
<h1 class="title" style="text-align:center">Conditional noexcept
specifiers in compound requirements</h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>
      P3822R0
      [<a href="https://wg21.link/P3822">Latest</a>]
      [<a href="https://wg21.link/P3822/status">Status</a>]
    </td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2025-09-01</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>
      EWGi<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Viacheslav Luchkin<br>&lt;<a href="mailto:someone12469@gmail.com" class="email">someone12469@gmail.com</a>&gt;<br>
      Gašper Ažman<br>&lt;<a href="mailto:gasper.azman@gmail.com" class="email">gasper.azman@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" id="toc-introduction"><span class="toc-section-number">1</span> Introduction</a></li>
<li><a href="#motivation" id="toc-motivation"><span class="toc-section-number">2</span> Motivation</a></li>
<li><a href="#proposal" id="toc-proposal"><span class="toc-section-number">3</span> Proposal</a></li>
<li><a href="#proposed-wording" id="toc-proposed-wording"><span class="toc-section-number">4</span> Proposed wording</a></li>
</ul>
</div>
<h1 data-number="1" id="introduction"><span class="header-section-number">1</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>This paper extends compound requirements to allow noexcept specifiers
to be applied conditionally. The proposed syntax for this is <code class="sourceCode cpp"><span class="kw">requires</span> <span class="op">{</span> <span class="op">{</span> <em>expression</em> <span class="op">}</span> <span class="kw">noexcept</span><span class="op">(</span><em>condition</em><span class="op">)</span> <span class="op">-&gt;</span> <em>return-type-constraint</em>; <span class="op">}</span></code>.</p>
<h1 data-number="2" id="motivation"><span class="header-section-number">2</span> Motivation<a href="#motivation" class="self-link"></a></h1>
<p>Requires-expressions can be used to assert that an expression is
non-throwing, but do not provide a way to do so conditionally, which is
sometimes needed in generic programming. Achieving this now usually
requires code duplication.</p>
<p>The lack of support for this syntax is also inconsistent with
function declarations, which have had conditional noexcept specifiers
since C++11.</p>
<p>Motivational example:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>Before</strong>
</div></th>
<th><div style="text-align:center">
<strong>After</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div>

<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> F, <span class="dt">bool</span> noexc<span class="op">&gt;</span></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a><span class="kw">concept</span> invocable <span class="op">=</span> noexc</span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a>  <span class="op">?</span> <span class="kw">requires</span><span class="op">(</span>F f<span class="op">)</span> <span class="op">{</span> <span class="op">{</span> f<span class="op">()</span> <span class="op">}</span> <span class="kw">noexcept</span>; <span class="op">}</span></span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a>  <span class="op">:</span> <span class="kw">requires</span><span class="op">(</span>F f<span class="op">)</span> <span class="op">{</span> f<span class="op">()</span>; <span class="op">}</span>;</span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="dt">bool</span> noexc<span class="op">&gt;</span></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> callable_ref <span class="op">{</span></span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a>  callable_ref<span class="op">(</span>invocable<span class="op">&lt;</span>noexc<span class="op">&gt;</span> <span class="kw">auto</span><span class="op">&amp;&amp;</span> fn<span class="op">)</span>;</span>
<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a>  <span class="op">[...]</span></span>
<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>

</div></td>
<td><div>

<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="kw">typename</span> F, <span class="dt">bool</span> noexc<span class="op">&gt;</span></span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="kw">concept</span> invocable <span class="op">=</span> <span class="kw">requires</span><span class="op">(</span>F f<span class="op">)</span> <span class="op">{</span></span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a>  <span class="op">{</span> f<span class="op">()</span> <span class="op">}</span> <span class="kw">noexcept</span><span class="op">(</span>noexc<span class="op">)</span>;</span>
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span>
<span id="cb2-5"><a href="#cb2-5" aria-hidden="true" tabindex="-1"></a><span class="kw">template</span><span class="op">&lt;</span><span class="dt">bool</span> noexc<span class="op">&gt;</span></span>
<span id="cb2-6"><a href="#cb2-6" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> callable_ref <span class="op">{</span></span>
<span id="cb2-7"><a href="#cb2-7" aria-hidden="true" tabindex="-1"></a>  callable_ref<span class="op">(</span>invocable<span class="op">&lt;</span>noexc<span class="op">&gt;</span> <span class="kw">auto</span><span class="op">&amp;&amp;</span> fn<span class="op">)</span>;</span>
<span id="cb2-8"><a href="#cb2-8" aria-hidden="true" tabindex="-1"></a>  <span class="op">[...]</span></span>
<span id="cb2-9"><a href="#cb2-9" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>

</div></td>
</tr>
</tbody>
</table>
<h1 data-number="3" id="proposal"><span class="header-section-number">3</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>Redefine the syntax for compound requirements as follows:</p>
<blockquote>
<div class="line-block"><em>compound-requirement:</em><br />
    <code class="sourceCode cpp"><span class="op">{</span></code>
<em>expression</em>
<code class="sourceCode cpp"><span class="op">}</span></code> <span class="rm" style="color: #bf0303"><del><span><code class="sourceCode default">noexcept</code></span><sub>opt</sub></del></span>
<span class="add" style="color: #006e28"><ins><em>noexcept-specifier<sub>opt</sub></em></ins></span>
<em>return-type-requirement<sub>opt</sub></em>
<code class="sourceCode cpp">;</code><br />
<em>noexcept-specifier:</em><br />
    <code class="sourceCode cpp"><span class="kw">noexcept</span><span class="op">(</span></code><em>condition</em><code class="sourceCode cpp"><span class="op">)</span></code><br />
    <code class="sourceCode cpp"><span class="kw">noexcept</span></code><br />
<em>return-type-requirement:</em><br />
    <code class="sourceCode cpp"><span class="op">-&gt;</span></code>
<em>type-constraint</em></div>
</blockquote>
<p>Where the <em>condition</em>, if supplied, shall be a contextually
converted constant expression of type
<code class="sourceCode cpp"><span class="dt">bool</span></code>. The
<em>noexcept-specifier</em>
<code class="sourceCode cpp"><span class="kw">noexcept</span></code>
without a <em>condition</em> is equivalent to <code class="sourceCode cpp"><span class="kw">noexcept</span><span class="op">(</span><span class="kw">true</span><span class="op">)</span></code>.</p>
<p>If <em>condition</em> evaluates to
<code class="sourceCode cpp"><span class="kw">true</span></code> and
<em>expression</em> is a potentially-throwing expression, the
requirement is not satisfied. If <em>condition</em> evaluates to
<code class="sourceCode cpp"><span class="kw">false</span></code> or the
<em>noexcept-specifier</em> is absent, <em>expression</em> may be
potentially-throwing.</p>
<p>If the conversion of <em>condition</em> to
<code class="sourceCode cpp"><span class="dt">bool</span></code> fails,
the requirement is not satisfied. This is not a hard error, because
requirements are often used in overload resolution and the expected
behavior for an unexpected input type is that other overloads are still
considered.</p>
<h1 data-number="4" id="proposed-wording"><span class="header-section-number">4</span> Proposed wording<a href="#proposed-wording" class="self-link"></a></h1>
<p>Recall that the grammar for <em>noexcept-specifier</em> is defined in
section <span>14.5
<a href="https://wg21.link/except.spec">[except.spec]</a></span> as
follows:</p>
<blockquote>
<div class="line-block"><em>noexcept-specifier:</em><br />
    <code class="sourceCode cpp"><span class="kw">noexcept</span><span class="op">(</span></code><em>constant-expression</em><code class="sourceCode cpp"><span class="op">)</span></code><br />
    <code class="sourceCode cpp"><span class="kw">noexcept</span></code></div>
</blockquote>
<p>Alter section <span>7.5.8.4
<a href="https://wg21.link/expr.prim.req.compound">[expr.prim.req.compound]</a></span>
as follows:</p>
<blockquote>
<div class="line-block"><em>compound-requirement:</em><br />
    <code class="sourceCode cpp"><span class="op">{</span></code>
<em>expression</em>
<code class="sourceCode cpp"><span class="op">}</span></code> <span class="rm" style="color: #bf0303"><del><span><code class="sourceCode default">noexcept</code></span><sub>opt</sub></del></span>
<span class="add" style="color: #006e28"><ins><em>noexcept-specifier<sub>opt</sub></em></ins></span>
<em>return-type-requirement<sub>opt</sub></em>
<code class="sourceCode cpp">;</code><br />
<em>return-type-requirement:</em><br />
    <code class="sourceCode cpp"><span class="op">-&gt;</span></code>
<em>type-constraint</em></div>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span>
A <em>compound-requirement</em> asserts properties of the
<em>expression</em> <em>E</em>. The <em>expression</em> is an
unevaluated operand. Substitution of template arguments (if any) and
verification of semantic properties proceed in the following order:</p>
</blockquote>
<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(1.1)</a></span>
Substitution of template arguments (if any) into the <em>expression</em>
is performed.</li>
</ul>
</blockquote>
<div class="rm" style="color: #bf0303">

<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(1.2)</a></span> If
the <code class="sourceCode default">noexcept</code> specifier is
present, <em>E</em> shall not be a potentially-throwing expression
(<span><a href="https://wg21.link/except.spec">[except.spec]</a></span>)</li>
</ul>
</blockquote>

</div>
<div class="add" style="color: #006e28">

<blockquote>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(1.2)</a></span> If
the <em>noexcept-specifier</em> (<span><a href="https://wg21.link/except.spec">[except.spec]</a></span>) is
present, then:
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(1.2.1)</a></span>
Substitution of template arguments (if any) into its associated
<em>constant-expression</em> is performed. The
<em>noexcept-specifier</em>
<code class="sourceCode default">noexcept</code> without a
<em>constant-expression</em> is equivalent to
<code class="sourceCode default">noexcept(true)</code>.</li>
<li><span class="marginalizedparent"><a class="marginalized">(1.2.2)</a></span>
The <em>constant-expression</em> shall be a contextually converted
constant expression of type <code class="sourceCode default">bool</code>
(<span><a href="https://wg21.link/expr.const">[expr.const]</a></span>).
If the <em>constant-expression</em> is true, <em>E</em> shall not be a
potentially-throwing expression (<span><a href="https://wg21.link/except.spec">[except.spec]</a></span>).</li>
</ul></li>
</ul>
</blockquote>

</div>
<p>Bump feature-test macro
<code class="sourceCode cpp">__cpp_concepts</code> in section
<span>15.12
<a href="https://wg21.link/cpp.predefined">[cpp.predefined]</a></span>:</p>
<div>
<div class="sourceCode" id="cb3"><pre class="sourceCode diff"><code class="sourceCode diff"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a>  __cpp_char8_t 202207L</span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a><span class="st">- __cpp_concepts 202002L</span></span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a><span class="va">+ __cpp_concepts 20XXXXL</span></span>
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a>  __cpp_conditional_explicit 201806L</span></code></pre></div>
</div>
</div>
</div>
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