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‎issue4190.html‎

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@@ -67,6 +67,8 @@ <h3 id="4190"><a href="lwg-active.html#4190">4190</a>. Specification of <i>compl
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<b>Submitter:</b> Eric Niebler <b>Opened:</b> 2025-01-02 <b>Last modified:</b> 2025-02-07</p>
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<p><b>Priority: </b>2
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</p>
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<p><b>View other</b> <a href="lwg-index-open.html#exec.snd.expos">active issues</a> in [exec.snd.expos].</p>
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<p><b>View all other</b> <a href="lwg-index.html#exec.snd.expos">issues</a> in [exec.snd.expos].</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>

‎issue4248.html‎

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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="utf-8">
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<title>Issue 4248: Late domain-based dispatching of schedule_from and continues_on are flipped</title>
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<meta property="og:title" content="Issue 4248: Late domain-based dispatching of schedule_from and continues_on are flipped">
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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="4248"><a href="lwg-active.html#4248">4248</a>. Late domain-based dispatching of <code class='backtick'>schedule_from</code> and <code class='backtick'>continues_on</code> are flipped</h3>
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<p><b>Section:</b> 33.9.2 <a href="https://wg21.link/exec.snd.expos">[exec.snd.expos]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Eric Niebler <b>Opened:</b> 2025-04-26 <b>Last modified:</b> 2025-04-27</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View other</b> <a href="lwg-index-open.html#exec.snd.expos">active issues</a> in [exec.snd.expos].</p>
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<p><b>View all other</b> <a href="lwg-index.html#exec.snd.expos">issues</a> in [exec.snd.expos].</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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The reason for having two different customization points for transitioning between two execution contexts
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is described in 33.9.2 <a href="https://wg21.link/exec.snd.expos">[exec.snd.expos]</a> bullet (14.1) Note 1, to wit:
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</p>
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<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
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<p>
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[<i>Note 1</i>: The <code class='backtick'>continues_on</code> algorithm works in tandem with <code class='backtick'>schedule_from</code>
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(33.9.12.5 <a href="https://wg21.link/exec.schedule.from">[exec.schedule.from]</a>) to give scheduler authors a way to customize
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both how to transition onto (<code class='backtick'>continues_on</code>) and off of (<code class='backtick'>schedule_from</code>) a given
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execution context. Thus, <code class='backtick'>continues_on</code> ignores the domain of the predecessor and
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uses the domain of the destination scheduler to select a customization, a property
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that is unique to <code class='backtick'>continues_on</code>. That is why it is given special treatment here.
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&mdash; <i>end note</i>]
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</p>
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</blockquote>
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<p>
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The exposition-only <code><i>get-domain-late</i></code> function treats <code class='backtick'>continues_on</code>
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senders specially to make sure the correct domain (that of the destination scheduler)
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is used to find customizations at connect time.
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<p/>
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However, customizations are also searched for early when the sender is first constructed.
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And here the dispatching of <code class='backtick'>continues_on</code> and <code class='backtick'>schedule_from</code> are reversed.
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<p/>
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<code class='backtick'>continues_on(sndr, sch)</code> is defined as (33.9.12.4 <a href="https://wg21.link/exec.continues.on">[exec.continues.on]</a>):
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</p>
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<blockquote><pre>
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transform_sender(<i>get-domain-early</i>(sndr), <i>make-sender</i>(continues_on, sch, sndr))
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</pre></blockquote>
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<p>
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which is using the domain of the predecessor rather than ignoring it as 33.9.2 <a href="https://wg21.link/exec.snd.expos">[exec.snd.expos]</a> p14.1
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says it does. And <code class='backtick'>schedule_from(sch, sndr)</code> is currently defined as (33.9.12.5 <a href="https://wg21.link/exec.schedule.from">[exec.schedule.from]</a>):
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</p>
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<blockquote><pre>
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transform_sender(
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<i>query-or-default</i>(get_domain, sch, default_domain()),
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<i>make-sender</i>(schedule_from, sch, sndr))
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</pre></blockquote>
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<p>
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which is using the domain of the destination scheduler to find customizations. The logic for
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determining the domain to use for early customization of these two algorithms are opposite
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what they are for late customization. This is a bug. They should be consistent.
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<p/>
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"Lazy" customization (at connect time) was added to <a href="https://wg21.link/P2300" title=" `std::execution`">P2300</a> later in the process,
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and this inconsistency was a mistake on my part. The correct thing to do is to change
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<code><i>get-domain-late</i></code> to treat <code class='backtick'>schedule_from</code> as special, not <code class='backtick'>continues_on</code>.
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</p>
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<p id="res-4248"><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 33.9.2 <a href="https://wg21.link/exec.snd.expos">[exec.snd.expos]</a> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class Sndr, class Env&gt;
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constexpr auto <i>get-domain-late</i>(const Sndr&amp; sndr, const Env&amp; env) noexcept;
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</pre>
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<blockquote>
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<p>
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-14- <i>Effects</i>: Equivalent to:
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</p>
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<ol style="list-style-type: none">
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<li><p>(14.1) &mdash; If <code><i>sender-for</i>&lt;Sndr, <del>continues_on_t</del><ins>schedule_from_t</ins>&gt;</code> is <code class='backtick'>true</code>, then</p>
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<blockquote><pre>
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return Domain();
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</pre></blockquote>
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<p>
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where <code class='backtick'>Domain</code> is the type of the following expression:
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</p>
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<blockquote><pre>
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[] {
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auto [_, sch, _] = sndr;
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return <i>query-or-default</i>(get_domain, sch, default_domain());
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}();
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</pre></blockquote>
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<p>
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[<i>Note 1</i>: The <code><del>continues_on</del><ins>schedule_from</ins></code> algorithm
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works in tandem with <code><del>schedule_from</del><ins>continues_on</ins></code>
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(<del>33.9.12.5 <a href="https://wg21.link/exec.schedule.from">[exec.schedule.from]</a></del><ins>33.9.12.4 <a href="https://wg21.link/exec.continues.on">[exec.continues.on]</a></ins>)
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to give scheduler authors a way to customize both how to transition onto
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(<code><del>continues_on</del><ins>schedule_from</ins></code>) and off of
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(<code><del>schedule_from</del><ins>continues_on</ins></code>) a given execution context.
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Thus, <code><del>continues_on</del><ins>schedule_from</ins></code> ignores the domain of the
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predecessor and uses the domain of the destination scheduler to select a customization,
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a property that is unique to <code><del>continues_on</del><ins>schedule_from</ins></code>.
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That is why it is given special treatment here. &mdash; <i>end note</i>]
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</p>
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</li>
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<li><p>[&hellip;]</p></li>
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