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<!DOCTYPE html>
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<title>Issue 4208: Wording needs to ensure that in connect(sndr, rcvr) that rcvr expression is only evaluated once</title>
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<meta property="og:title" content="Issue 4208: Wording needs to ensure that in connect(sndr, rcvr) that rcvr expression is only evaluated once">
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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="4208"><a href="lwg-active.html#4208">4208</a>. Wording needs to ensure that in <code class='backtick'>connect(sndr, rcvr)</code> that <code class='backtick'>rcvr</code> expression is only evaluated once</h3>
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<p><b>Section:</b> 33.9.10 <a href="https://wg21.link/exec.connect">[exec.connect]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Eric Niebler <b>Opened:</b> 2025-02-07 <b>Last modified:</b> 2025-02-09</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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Imported from <a href="https://github.com/cplusplus/sender-receiver/issues/325">cplusplus/sender-receiver #325</a>.
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</p>
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<p>
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The current wording of <code class='backtick'>connect(sndr, rcvr)</code> defines the <code class='backtick'>new_sndr</code> expression as
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<code>transform_sender(decltype(<i>get-domain-late</i>(sndr, get_env(rcvr))){}, sndr, get_env(rcvr))</code>.
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<p/>
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It then defines <code class='backtick'>connect(sndr, rcvr)</code> as expression equivalent to <code class='backtick'>new_sndr.connect(rcvr)</code>.
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<p/>
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As currently worded, this requires evaluating the <code class='backtick'>rcvr</code> expression twice. Note that the first
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usage in the <code class='backtick'>new_sndr</code> expression is unevaluated, but the second usage in <code class='backtick'>get_env(rcvr)</code> is evaluated.
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<p/>
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I think we need to add an extra sentence at the end of this section saying "Where the expression
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<code class='backtick'>rcvr</code> is only evaluated once." or similar.
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</p>
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<p id="res-4208"><b>Proposed resolution:</b></p>
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This wording is relative to <a href="https://wg21.link/N5001">N5001</a>.
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</p>
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<ol>
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<p>Modify 33.9.10 <a href="https://wg21.link/exec.connect">[exec.connect]</a> as indicated:</p>
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<blockquote>
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-6- The expression <code class='backtick'>connect(sndr, rcvr)</code> is expression-equivalent to:
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</p>
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<ol style="list-style-type: none">
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<li><p>(6.1) &mdash; <code class='backtick'>new_sndr.connect(rcvr)</code> if that expression is well-formed.
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<p/>
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<i>Mandates</i>: The type of the expression above satisfies <code class='backtick'>operation_state</code>.
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</p>
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</li>
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<li><p>(6.2) &mdash; Otherwise, <code><i>connect-awaitable</i>(new_sndr, rcvr)</code>.</p></li>
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</ol>
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<ins>except that <code class='backtick'>rcvr</code> is evaluated only once.</ins>
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<p/>
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<i>Mandates</i>: <code>sender&lt;Sndr&gt; &amp;&amp; receiver&lt;Rcvr&gt;</code> is <code>true</code>.
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<p/>
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</html>

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