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<!DOCTYPE html>
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<html lang="en">
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<meta charset="utf-8">
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<title>Issue 4209: default_domain::transform_env should be returning FWD-ENV(env)</title>
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<meta property="og:title" content="Issue 4209: default_domain::transform_env should be returning FWD-ENV(env)">
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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="4209"><a href="lwg-active.html#4209">4209</a>. <code>default_domain::transform_env</code> should be returning <code><i>FWD-ENV</i>(env)</code></h3>
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<p><b>Section:</b> 33.9.5 <a href="https://wg21.link/exec.domain.default">[exec.domain.default]</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/168">cplusplus/sender-receiver #168</a>.
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</p>
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<p>
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When writing a generic recursive sender transform, you need to ability to unpack an unknown sender
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<code class='backtick'>S</code> and recursively transform the children.
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<p/>
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For that, it can be useful to know the type of the environment that <code class='backtick'>S</code> will use when connecting its
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child senders, which is why <code class='backtick'>transform_env</code> exists.
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<p/>
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For an environment <code class='backtick'>E</code> and a sender <code class='backtick'>S</code> with tag <code class='backtick'>T</code> child <code class='backtick'>C</code>, the expression
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<code class='backtick'>default_domain().transform_env(S, E)</code> should return an environment <code class='backtick'>E2</code> that is identical to the
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environment of the receiver that <code class='backtick'>S</code> uses to connect <code class='backtick'>C</code>.
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<p/>
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<code class='backtick'>default_domain().transform_env(S, E)</code> will first check whether <code class='backtick'>T().transform_env(S, E)</code> is
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well-formed. If so, it will return that (e.g. <code class='backtick'>when_all_t</code> has a <code class='backtick'>transform_env</code> that adds a stop token
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to the environment).
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<p/>
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If <code class='backtick'>T().transform_env(S, E)</code> is not well-formed, what should <code class='backtick'>default_domain::transform_env</code> do? At
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present, it returns <code class='backtick'>E</code> unmodified.
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<p/>
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But 33.9.12.1 <a href="https://wg21.link/exec.adapt.general">[exec.adapt.general]</a> has this:
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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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[unless otherwise specified, when] a parent sender is connected to a receiver <code class='backtick'>rcvr</code>, any receiver used
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to connect a child sender has an associated environment equal to <code><i>FWD-ENV</i>(get_env(rcvr))</code>.
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</p>
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</blockquote>
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<p>
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So the correct thing for <code class='backtick'>default_domain::transform_env</code> to do is to return <code><i>FWD-ENV</i>(get_env(rcvr))</code>.
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</p>
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<p id="res-4209"><b>Proposed resolution:</b></p>
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<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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<li>
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<p>Modify 33.9.5 <a href="https://wg21.link/exec.domain.default">[exec.domain.default]</a> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;sender Sndr, <i>queryable</i> Env&gt;
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constexpr <i>queryable</i> decltype(auto) transform_env(Sndr&amp;&amp; sndr, Env&amp;&amp; env) noexcept;
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</pre>
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<blockquote>
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<p>
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-5- Let <code class='backtick'>e</code> be the expression
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</p>
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<blockquote><pre>
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tag_of_t&lt;Sndr&gt;().transform_env(std::forward&lt;Sndr&gt;(sndr), std::forward&lt;Env&gt;(env))
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</pre></blockquote>
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<p>
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if that expression is well-formed; otherwise, <code><del>static_cast&lt;Env&gt;</del><ins><i>FWD-ENV</i></ins>(std::forward&lt;Env&gt;(env))</code>.
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<p/>
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-6- <i>Mandates</i>: <code class='backtick'>noexcept(e)</code> is <code class='backtick'>true</code>.
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<p/>
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-7- <i>Returns</i>: <code class='backtick'>e</code>.
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</p>
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</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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