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<title>Issue 4202: enable-sender should be a variable template</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="4202"><a href="lwg-defects.html#4202">4202</a>. <i><code>enable-sender</code></i> should be a variable template</h3>
<p><b>Section:</b> 33.9.3 <a href="https://wg21.link/exec.snd.concepts">[exec.snd.concepts]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Eric Niebler <b>Opened:</b> 2025-02-03 <b>Last modified:</b> 2025-06-23</p>
<p><b>Priority: </b>1
</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/305">cplusplus/sender-receiver #305</a> and
<a href="https://github.com/cplusplus/sender-receiver/issues/306">cplusplus/sender-receiver #306</a>.
</p>
<p>
We require an opt-in to satisfy the <code class='backtick'>sender</code> concept.
Making your type awaitable with an empty environment is one way to opt in.
If your awaitable requires an environment, you have two options:
<ul>
<li>give your awaitable type a nested <code class='backtick'>sender_concept</code> typedef, or</li>
<li>specialize <code class='backtick'>enable_sender</code> for your awaitable type.</li>
</ul>
The problem is that somewhere along the way,
the <code class='backtick'>enable_sender</code> variable template was turned into
an exposition-only <i><code>enable-sender</code></i> concept.
We should roll back that change.
</p>

<p><i>[2025-02-07; Reflector poll]</i></p>

<p>
Set priority to 1 after reflector poll.
</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-4202"><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>
Change 33.9.3 <a href="https://wg21.link/exec.snd.concepts">[exec.snd.concepts]</a> as indicated:
<blockquote>
<pre><code>
  template&lt;class Sndr&gt;
    concept <i>is-sender</i> =                                         <i>// exposition only</i>
      derived_from&lt;typename Sndr::sender_concept, sender_t&gt;;

  template&lt;class Sndr&gt;
    concept <i>enable-sender</i> =                                     <i>// exposition only</i>
      <i>is-sender</i>&lt;Sndr&gt; ||
      <i>is-awaitable</i>&lt;Sndr, <i>env-promise</i>&lt;empty_env&gt;&gt;;               <i>// [exec.awaitable]</i>
<ins>
  template&lt;class Sndr&gt;
    inline constexpr bool enable_sender = <i>enable-sender</i>&lt;Sndr&gt;;
</ins>
  template&lt;class Sndr&gt;
    concept sender =
      <del>bool(<i>enable-sender</i></del><ins>enable_sender</ins>&lt;remove_cvref_t&lt;Sndr&gt;&gt;<del>)</del> &amp;&amp;
      requires (const remove_cvref_t&lt;Sndr&gt;&amp; sndr) {
        { get_env(sndr) } -&gt; <i>queryable</i>;
      } &amp;&amp;
      move_constructible&lt;remove_cvref_t&lt;Sndr&gt;&gt; &amp;&amp;
      constructible_from&lt;remove_cvref_t&lt;Sndr&gt;, Sndr&gt;;
</code></pre>
<p>&hellip;</p>
<p>
-2-
Given a subexpression <code class='backtick'>sndr</code>, [&hellip;]
</p>
<p>
<ins>
-?-
<i>Remarks</i>:
Pursuant to 16.4.5.2.1 <a href="https://wg21.link/namespace.std">[namespace.std]</a>, users may specialize
<code class='backtick'>enable_sender</code> to <code class='backtick'>true</code> for cv-unqualified program-defined types that model
<code class='backtick'>sender</code>, and <code class='backtick'>false</code> for types that do not.
Such specializations shall be usable in constant expressions
(7.7 <a href="https://wg21.link/expr.const">[expr.const]</a>) and have type <code class='backtick'>const bool</code>.
</ins>
</p>
</blockquote>
</li>
</ol>





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