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<!DOCTYPE html>
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<html lang="en">
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<title>Issue 4227: Missing noexcept operator in [exec.when.all]</title>
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<meta property="og:title" content="Issue 4227: Missing noexcept operator in [exec.when.all]">
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<meta property="og:description" content="C++ library issue. Status: New">
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p>
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<h3 id="4227"><a href="lwg-active.html#4227">4227</a>. Missing <code class='backtick'>noexcept</code> operator in [exec.when.all]</h3>
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<p><b>Section:</b> 33.9.12.11 <a href="https://wg21.link/exec.when.all">[exec.when.all]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Ian Petersen <b>Opened:</b> 2025-03-17 <b>Last modified:</b> 2025-03-22</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
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<p><b>Discussion:</b></p>
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<p>
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In 33.9.12.11 <a href="https://wg21.link/exec.when.all">[exec.when.all]</a> p7, the <code><i>impls-for</i>&lt;when_all_t&gt;::<i>get-state</i></code>
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member is defined to be equivalent to the following lambda:
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</p>
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<blockquote><pre>
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[]&lt;class Sndr, class Rcvr&gt;(Sndr&amp;&amp; sndr, Rcvr&amp; rcvr) noexcept(<i>e</i>) -> decltype(<i>e</i>) {
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return <i>e</i>;
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}
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</pre></blockquote>
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<p>
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and <code><i>e</i></code> is later defined to be:
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</p>
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<blockquote><pre>
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std::forward&lt;Sndr&gt;(sndr).apply(<i>make-state</i>&lt;Rcvr&gt;())
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</pre></blockquote>
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<p>
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Together, the two definitions imply that the <code class='backtick'>noexcept</code> clause on the provided lambda is:
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</p>
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<blockquote><pre>
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noexcept(std::forward&lt;Sndr&gt;(sndr).apply(<i>make-state</i>&lt;Rcvr&gt;()))
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</pre></blockquote>
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<p>
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which is invalid.
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<p/>
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Presumably, the lambda should be defined like so (with an extra <code class='backtick'>noexcept</code> operator in the <code class='backtick'>noexcept</code> clause):
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</p>
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<blockquote><pre>
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[]&lt;class Sndr, class Rcvr&gt;(Sndr&amp;&amp; sndr, Rcvr&amp; rcvr) noexcept(<ins>noexcept(</ins><i>e</i><ins>)</ins>) -&gt; decltype(<i>e</i>) {
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return <i>e</i>;
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}
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</pre></blockquote>
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<p id="res-4227"><b>Proposed resolution:</b></p>
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<p>
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This wording is relative to <a href="https://wg21.link/N5008">N5008</a>.
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</p>
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<ol>
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<li><p>Modify 33.9.12.11 <a href="https://wg21.link/exec.when.all">[exec.when.all]</a> as indicated:</p>
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<blockquote>
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<p>
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-7- The member <code><i>impls-for</i>&lt;when_all_t&gt;::<i>get-state</i></code> is initialized with a callable object
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equivalent to the following lambda expression:
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</p>
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<blockquote><pre>
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[]&lt;class Sndr, class Rcvr&gt;(Sndr&amp;&amp; sndr, Rcvr&amp; rcvr) noexcept(<ins>noexcept(</ins><i>e</i><ins>)</ins>) -&gt; decltype(<i>e</i>) {
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return <i>e</i>;
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}
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</pre></blockquote>
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</blockquote>
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</li>
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</ol>
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</body>
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</html>

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