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<title>Issue 4209: default_domain::transform_env should be returning FWD-ENV(env)</title>
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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#WP">WP</a> status.</em></p>
<h3 id="4209"><a href="lwg-defects.html#4209">4209</a>. <code>default_domain::transform_env</code> should be returning <code><i>FWD-ENV</i>(env)</code></h3>
<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#WP">WP</a>
 <b>Submitter:</b> Eric Niebler <b>Opened:</b> 2025-02-07 <b>Last modified:</b> 2025-06-23</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#exec.domain.default">issues</a> in [exec.domain.default].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Imported from <a href="https://github.com/cplusplus/sender-receiver/issues/168">cplusplus/sender-receiver #168</a>.
</p>
<p>
When writing a generic recursive sender transform, you need to ability to unpack an unknown sender 
<code class='backtick'>S</code> and recursively transform the children.
<p/>
For that, it can be useful to know the type of the environment that <code class='backtick'>S</code> will use when connecting its 
child senders, which is why <code class='backtick'>transform_env</code> exists.
<p/>
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 
<code class='backtick'>default_domain().transform_env(S, E)</code> should return an environment <code class='backtick'>E2</code> that is identical to the 
environment of the receiver that <code class='backtick'>S</code> uses to connect <code class='backtick'>C</code>.
<p/>
<code class='backtick'>default_domain().transform_env(S, E)</code> will first check whether <code class='backtick'>T().transform_env(S, E)</code> is 
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 
to the environment).
<p/>
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 
present, it returns <code class='backtick'>E</code> unmodified.
<p/>
But 33.9.12.1 <a href="https://wg21.link/exec.adapt.general">[exec.adapt.general]</a> has this:
</p>
<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
<p>
[unless otherwise specified, when] a parent sender is connected to a receiver <code class='backtick'>rcvr</code>, any receiver used 
to connect a child sender has an associated environment equal to <code><i>FWD-ENV</i>(get_env(rcvr))</code>.
</p>
</blockquote>
<p>
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>.
</p>

<p><i>[Hagenberg 2025-02-11; move to Ready]</i></p>


<p><i>[Sofia 2025-06-21; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-4209"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5001" title=" Working Draft, Programming Languages — C++">N5001</a>.
</p>

<ol>
<li>
<p>Modify 33.9.5 <a href="https://wg21.link/exec.domain.default">[exec.domain.default]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;sender Sndr, <i>queryable</i> Env&gt;
  constexpr <i>queryable</i> decltype(auto) transform_env(Sndr&amp;&amp; sndr, Env&amp;&amp; env) noexcept;
</pre>
<blockquote>
<p>
-5- Let <code class='backtick'>e</code> be the expression
</p>
<blockquote><pre>
tag_of_t&lt;Sndr&gt;().transform_env(std::forward&lt;Sndr&gt;(sndr), std::forward&lt;Env&gt;(env))
</pre></blockquote>
<p>
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>.
<p/>
-6- <i>Mandates</i>: <code class='backtick'>noexcept(e)</code> is <code class='backtick'>true</code>.
<p/>
-7- <i>Returns</i>: <code class='backtick'>e</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>






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