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  <title>Fixing Lazy Sender Algorithm Customization</title>
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<div class="wrapper">
<header id="title-block-header">
<h1 class="title" style="text-align:center">Fixing Lazy Sender Algorithm
Customization</h1>
<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P3303R0</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2024-05-21</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      LEWG Library Evolution<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Eric Niebler<br>&lt;<a href="mailto:eric.niebler@gmail.com" class="email">eric.niebler@gmail.com</a>&gt;<br>
    </td>
  </tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#introduction" id="toc-introduction"><span class="toc-section-number">1</span> Introduction<span></span></a></li>
<li><a href="#executive-summary" id="toc-executive-summary"><span class="toc-section-number">2</span> Executive
Summary<span></span></a></li>
<li><a href="#discussion" id="toc-discussion"><span class="toc-section-number">3</span> Discussion<span></span></a></li>
<li><a href="#proposed-wording" id="toc-proposed-wording"><span class="toc-section-number">4</span> Proposed
Wording<span></span></a></li>
<li><a href="#bibliography" id="toc-bibliography"><span class="toc-section-number">5</span> References<span></span></a></li>
</ul>
</div>
<div style="text-align: right;">
<p><em>“Experience is simply the name we give our mistakes.”</em><br /> —
Oscar Wilde</p>
<p><em>“To err is human, but to really foul things up you need a
computer.”</em><br /> — Paul R. Ehrlich</p>
</div>
<h1 data-number="1" id="introduction"><span class="header-section-number">1</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>The paper <span class="citation" data-cites="P2999R3">[<a href="#ref-P2999R3" role="doc-biblioref">P2999R3</a>]</span> “Sender
Algorithm Customization” proposed, among other things, to make sender
algorithms customizable lazily; that is, when their senders are
connected with receivers. LEWG agreed and forwarded P2999 to LWG. Due to
a gross oversight, however, P2999 didn’t propose the wording changes to
<code class="sourceCode default">connect</code> and
<code class="sourceCode default">get_completion_signatures</code> that
actually implement the design that LEWG approved. This paper corrects
the oversight by using the new
<code class="sourceCode default">transform_sender</code> utility in
<code class="sourceCode default">connect</code> and
<code class="sourceCode default">get_completion_signatures</code> as
P2999 promised and failed to do.</p>
<h1 data-number="2" id="executive-summary"><span class="header-section-number">2</span> Executive Summary<a href="#executive-summary" class="self-link"></a></h1>
<p>The changes this paper proposes are:</p>
<ol type="1">
<li><p><code class="sourceCode default">get_completion_signatures(sndr, env)</code>
first transforms <code class="sourceCode default">sndr</code> with <code class="sourceCode default">transform_sender(<em><code class="sourceCode default">get-domain-late</code></em>(sndr, env), sndr, env)</code>
and then uses the result in place of
<code class="sourceCode default">sndr</code>.</p></li>
<li><p><code class="sourceCode default">connect(sndr, rcvr)</code> first
transforms <code class="sourceCode default">sndr</code> with <code class="sourceCode default">transform_sender(<em><code class="sourceCode default">get-domain-late</code></em>(sndr, get_env(rcvr)), sndr, get_env(rcvr))</code>
and then uses the result in place of
<code class="sourceCode default">sndr</code>.</p></li>
</ol>
<h1 data-number="3" id="discussion"><span class="header-section-number">3</span> Discussion<a href="#discussion" class="self-link"></a></h1>
<p><a href="https://isocpp.org/files/papers/P2999R3.html#late-senderreceiver-connection-time-customization">Table
2</a> in <span class="citation" data-cites="P2999R3">[<a href="#ref-P2999R3" role="doc-biblioref">P2999R3</a>]</span> shows how
that paper proposed to change the
<code class="sourceCode default">connect</code> customization point. The
table is reproduced below. Note that P2999R3 was targetting a version of
P2300 that still employed
<code class="sourceCode default">tag_invoke</code>, which has since been
removed.</p>
<table style="caption-side: top; font-size: 80%;">
<caption>
Table 2 from P2999R3: The addition of
<code class="sourceCode default">transform_sender</code> to
<code class="sourceCode default">connect</code>
</caption>
<thead>
<tr class="header">
<th style="text-align: center;">
<strong>Before</strong>
</th>
<th style="text-align: center;">
<strong>After</strong>
</th>
</tr>
</thead>
<tbody>
<tr>
<td>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> connect_t <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>  <span class="kw">template</span> <span class="op">&lt;</span>receiver Receiver, sender_in<span class="op">&lt;</span>env_of_t<span class="op">&lt;</span>Receiver<span class="op">&gt;&gt;</span> Sender<span class="op">&gt;</span></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a>    <span class="kw">requires</span> <span class="co">/* ... */</span></span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a>  <span class="kw">auto</span> <span class="kw">operator</span><span class="op">()(</span>Sender<span class="op">&amp;&amp;</span> snd, Receiver<span class="op">&amp;&amp;</span> rcv<span class="op">)</span> <span class="kw">const</span> <span class="op">{</span></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb1-11"><a href="#cb1-11" aria-hidden="true" tabindex="-1"></a>    <span class="co">// First, look for a customization of tag_invoke:</span></span>
<span id="cb1-12"><a href="#cb1-12" aria-hidden="true" tabindex="-1"></a>    <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>tag_invocable<span class="op">&lt;</span>connect_t, Sender, Receiver<span class="op">&gt;)</span> <span class="op">{</span></span>
<span id="cb1-13"><a href="#cb1-13" aria-hidden="true" tabindex="-1"></a>      <span class="cf">return</span> tag_invoke<span class="op">(*</span><span class="kw">this</span>,</span>
<span id="cb1-14"><a href="#cb1-14" aria-hidden="true" tabindex="-1"></a>                        std<span class="op">::</span>forward<span class="op">&lt;</span>Sender<span class="op">&gt;(</span>snd<span class="op">)</span>,</span>
<span id="cb1-15"><a href="#cb1-15" aria-hidden="true" tabindex="-1"></a>                        std<span class="op">::</span>forward<span class="op">&lt;</span>Receiver<span class="op">&gt;(</span>rcv<span class="op">))</span>;</span>
<span id="cb1-16"><a href="#cb1-16" aria-hidden="true" tabindex="-1"></a>    <span class="op">}</span></span>
<span id="cb1-17"><a href="#cb1-17" aria-hidden="true" tabindex="-1"></a>    <span class="co">// Next, see if the sender is co_await-able:</span></span>
<span id="cb1-18"><a href="#cb1-18" aria-hidden="true" tabindex="-1"></a>    <span class="cf">else</span> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span><em>is-await-connectable</em><span class="op">&lt;</span>Sender, Receiver<span class="op">&gt;)</span> <span class="op">{</span></span>
<span id="cb1-19"><a href="#cb1-19" aria-hidden="true" tabindex="-1"></a>      <span class="co">/* ... */</span></span>
<span id="cb1-20"><a href="#cb1-20" aria-hidden="true" tabindex="-1"></a>    <span class="op">}</span></span>
<span id="cb1-21"><a href="#cb1-21" aria-hidden="true" tabindex="-1"></a>  <span class="op">}</span></span>
<span id="cb1-22"><a href="#cb1-22" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>
</td>
<td>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="kw">struct</span> connect_t <span class="op">{</span></span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a>  <span class="kw">template</span> <span class="op">&lt;</span>receiver Receiver, sender_in<span class="op">&lt;</span>env_of_t<span class="op">&lt;</span>Receiver<span class="op">&gt;&gt;</span> Sender<span class="op">&gt;</span></span>
<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a>    <span class="kw">requires</span> <span class="co">/* ... */</span></span>
<span id="cb2-4"><a href="#cb2-4" aria-hidden="true" tabindex="-1"></a>  <span class="kw">auto</span> <span class="kw">operator</span><span class="op">()(</span>Sender<span class="op">&amp;&amp;</span> snd, Receiver<span class="op">&amp;&amp;</span> rcv<span class="op">)</span> <span class="kw">const</span> <span class="op">{</span></span>
<span id="cb2-5"><a href="#cb2-5" aria-hidden="true" tabindex="-1"></a>    <mark><code class="sourceCode default">// Apply any sender tranformations using the receiver&#39;s domain:</code></mark></span>
<span id="cb2-6"><a href="#cb2-6" aria-hidden="true" tabindex="-1"></a>    <mark><code class="sourceCode default">auto&amp;&amp; snd2 = transform_sender(<em>get-domain-late</em>(snd, get_env(rcv)),</code></mark></span>
<span id="cb2-7"><a href="#cb2-7" aria-hidden="true" tabindex="-1"></a>                                   <mark><code class="sourceCode default">std::forward&lt;Sender&gt;(snd),</code></mark></span>
<span id="cb2-8"><a href="#cb2-8" aria-hidden="true" tabindex="-1"></a>                                   <mark><code class="sourceCode default">get_env(rcv));</code></mark></span>
<span id="cb2-9"><a href="#cb2-9" aria-hidden="true" tabindex="-1"></a>    <mark><code class="sourceCode default">using Sender2 = decltype(snd2);</code></mark></span>
<span id="cb2-10"><a href="#cb2-10" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb2-11"><a href="#cb2-11" aria-hidden="true" tabindex="-1"></a>    <span class="co">// First, look for a customization of tag_invoke:</span></span>
<span id="cb2-12"><a href="#cb2-12" aria-hidden="true" tabindex="-1"></a>    <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>tag_invocable<span class="op">&lt;</span>connect_t, <mark>Sender2</mark>, Receiver<span class="op">&gt;)</span> <span class="op">{</span></span>
<span id="cb2-13"><a href="#cb2-13" aria-hidden="true" tabindex="-1"></a>      <span class="cf">return</span> tag_invoke<span class="op">(*</span><span class="kw">this</span>,</span>
<span id="cb2-14"><a href="#cb2-14" aria-hidden="true" tabindex="-1"></a>                        std<span class="op">::</span>forward<span class="op">&lt;</span><mark>Sender2</mark><span class="op">&gt;(</span><mark>snd2</mark><span class="op">)</span>,</span>
<span id="cb2-15"><a href="#cb2-15" aria-hidden="true" tabindex="-1"></a>                        std<span class="op">::</span>forward<span class="op">&lt;</span>Receiver<span class="op">&gt;(</span>rcv<span class="op">))</span>;</span>
<span id="cb2-16"><a href="#cb2-16" aria-hidden="true" tabindex="-1"></a>    <span class="op">}</span></span>
<span id="cb2-17"><a href="#cb2-17" aria-hidden="true" tabindex="-1"></a>    <span class="co">// Next, see if the sender is co_await-able:</span></span>
<span id="cb2-18"><a href="#cb2-18" aria-hidden="true" tabindex="-1"></a>    <span class="cf">else</span> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span><em>is-await-connectable</em><span class="op">&lt;</span><mark>Sender2</mark>, Receiver<span class="op">&gt;)</span> <span class="op">{</span></span>
<span id="cb2-19"><a href="#cb2-19" aria-hidden="true" tabindex="-1"></a>      <span class="co">/* ... */</span></span>
<span id="cb2-20"><a href="#cb2-20" aria-hidden="true" tabindex="-1"></a>    <span class="op">}</span></span>
<span id="cb2-21"><a href="#cb2-21" aria-hidden="true" tabindex="-1"></a>  <span class="op">}</span></span>
<span id="cb2-22"><a href="#cb2-22" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>
</td>
</tr>
</tbody>
</table>
<p>The design shown above is the one that was discussed and voted on in
LEWG.</p>
<p>A glance at the <a href="https://isocpp.org/files/papers/P2999R3.html#proposed-wording">proposed
wording</a> from P2999R3 shows that no such change was ever made. This
<em>most critical</em> part of that paper’s design intent was
inadvertantly left out of the wording. Face, meet palm.</p>
<p>This paper proposes no design changes from those described in
P2999R3. It “merely” corrects the wording to agree with the design.</p>
<h1 data-number="4" id="proposed-wording"><span class="header-section-number">4</span> Proposed Wording<a href="#proposed-wording" class="self-link"></a></h1>
<p><span class="ednote" style="color: #0000ff">[ Editor&#39;s note: The
changes in this paper are relative to <span class="citation" data-cites="P2300R9">[<a href="#ref-P2300R9" role="doc-biblioref">P2300R9</a>]</span>. ]</span></p>
<p><span class="ednote" style="color: #0000ff">[ Editor&#39;s note: Change
[exec.getcomplsigs] as follows: ]</span></p>
<blockquote>
<p><b><code class="sourceCode default">execution::get_completion_signatures</code>
[exec.getcomplsigs]</b></p>
<ol type="1">
<li><p><code class="sourceCode default">get_completion_signatures</code>
is a customization point object. Let
<code class="sourceCode default">sndr</code> be an expression such that
<code class="sourceCode default">decltype((sndr))</code> is
<code class="sourceCode default">Sndr</code>, and let
<code class="sourceCode default">env</code> be an expression such that
<code class="sourceCode default">decltype((env))</code> is
<code class="sourceCode default">Env</code>. <span class="add" style="color: #006e28"><ins>Let
<span><code class="sourceCode default">new_sndr</code></span> be the
expression <span><code class="sourceCode default">transform_sender(<em><code class="sourceCode default">get-domain-late</code></em>(sndr, env), sndr, env)</code></span>,
and let <span><code class="sourceCode default">NewSndr</code></span> be
<span><code class="sourceCode default">decltype((new_sndr))</code></span>.</ins></span>
Then <code class="sourceCode default">get_completion_signatures(sndr, env)</code>
is expression-equivalent to:</p>
<ol type="1">
<li><p><code class="sourceCode default">decltype(<span class="add" style="color: #006e28"><ins><span><code class="sourceCode default">new_</code></span></ins></span>sndr.get_completion_signatures(env)){}</code>
if that expression is well-formed,</p></li>
<li><p>Otherwise, <code class="sourceCode default">remove_cvref_t&lt;<span class="add" style="color: #006e28"><ins>New</ins></span>Sndr&gt;::completion_signatures{}</code>
if that expression is well-formed,</p></li>
<li><p>Otherwise, if <code class="sourceCode default"><em><code class="sourceCode default">is-awaitable</code></em>&lt;<span class="add" style="color: #006e28"><ins>New</ins></span>Sndr, <em><code class="sourceCode default">env-promise</code></em>&lt;Env&gt;&gt;</code>
is <code class="sourceCode default">true</code>, then:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a>completion_signatures&lt;</span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a>  <em><code class="sourceCode default">SET-VALUE-SIG</code></em>(<em><code class="sourceCode default">await-result-type</code></em>&lt;<span class="add" style="color: #006e28"><ins>New</ins></span>Sndr, <em><code class="sourceCode default">env-promise</code></em>&lt;Env&gt;&gt;), <em><code class="sourceCode default">// see [exec.snd.concepts]</code></em></span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a>  set_error_t(exception_ptr),</span>
<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a>  set_stopped_t()&gt;{}</span></code></pre></div></li>
<li><p>Otherwise, <code class="sourceCode default">get_completion_signatures(sndr, env)</code>
is ill-formed.</p></li>
</ol></li>
<li><p>Let <code class="sourceCode default">rcvr</code> be an rvalue
receiver of type <code class="sourceCode default">Rcvr</code>, and let
<code class="sourceCode default">Sndr</code> be the type of a sender
such that <code class="sourceCode default">sender_in&lt;Sndr, env_of_t&lt;Rcvr&gt;&gt;</code>
is <code class="sourceCode default">true</code>. Let
<code class="sourceCode default">Sigs...</code> be the template
arguments of the
<code class="sourceCode default">completion_signatures</code>
specialization named by <code class="sourceCode default">completion_signatures_of_t&lt;Sndr, env_of_t&lt;Rcvr&gt;&gt;</code>.
Let <i><code class="sourceCode default">CSO</code></i> be a completion
function. If sender <code class="sourceCode default">Sndr</code> or its
operation state cause the expression
<code class="sourceCode default"><em><code class="sourceCode default">CSO</code></em>(rcvr, args...)</code>
to be potentially evaluated ([basic.def.odr]) then there shall be a
signature <code class="sourceCode default">Sig</code> in
<code class="sourceCode default">Sigs...</code> such that <code class="sourceCode default"><em><code class="sourceCode default">MATCHING-SIG</code></em>(<em><code class="sourceCode default">decayed-typeof</code></em>&lt;<em><code class="sourceCode default">CSO</code></em>&gt;(decltype(args)...), Sig)</code>
is <code class="sourceCode default">true</code>
([exec.general]).</p></li>
</ol>
</blockquote>
<p><span class="ednote" style="color: #0000ff">[ Editor&#39;s note: Change
[exec.connect] paragraphs 2 and 6 as shown below. Paragraphs 3-5 are
unchanged but are shown here in their entirety to give context to the
surrounding changes. ]</span></p>
<blockquote>
<p><b><code class="sourceCode default">execution::connect</code>
[exec.connect]</b></p>
<ol type="1">
<li><p><code class="sourceCode default">connect</code> connects
([async.ops]) a sender with a receiver.</p></li>
<li><p>The name <code class="sourceCode default">connect</code> denotes
a customization point object. For subexpressions
<code class="sourceCode default">sndr</code> and
<code class="sourceCode default">rcvr</code>, let
<code class="sourceCode default">Sndr</code> be
<code class="sourceCode default">decltype((sndr))</code> and
<code class="sourceCode default">Rcvr</code> be
<code class="sourceCode default">decltype((rcvr))</code>, <span class="add" style="color: #006e28"><ins>let
<span><code class="sourceCode default">new_sndr</code></span> be the
expression <span><code class="sourceCode default">transform_sender(<em><code class="sourceCode default">get-domain-late</code></em>(sndr, get_env(rcvr)), sndr, get_env(rcvr))</code></span>,
let <span><code class="sourceCode default">NewSndr</code></span> be
<span><code class="sourceCode default">decltype((new_sndr))</code></span>,</ins></span>,
and let <code class="sourceCode default">DS</code> and
<code class="sourceCode default">DR</code> be the decayed types of
<code class="sourceCode default"><span class="add" style="color: #006e28"><ins><span><code class="sourceCode default">New</code></span></ins></span>Sndr</code> and
<code class="sourceCode default">Rcvr</code>, respectively.</p></li>
<li><p>Let
<i><code class="sourceCode default">connect-awaitable-promise</code></i>
be the following class:</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a>namespace std::execution {</span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a>  struct <em><code class="sourceCode default">connect-awaitable-promise</code></em></span>
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a>    : <em><code class="sourceCode default">with-await-transform</code></em>&lt;<em><code class="sourceCode default">connect-awaitable-promise</code></em>&gt; {</span>
<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a>    DR&amp; <em><code class="sourceCode default">rcvr</code></em>; <em><code class="sourceCode default">// exposition only</code></em></span>
<span id="cb4-5"><a href="#cb4-5" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-6"><a href="#cb4-6" aria-hidden="true" tabindex="-1"></a>    <em><code class="sourceCode default">connect-awaitable-promise</code></em>(DS&amp;, DR&amp; rcvr) noexcept : <em><code class="sourceCode default">rcvr</code></em>(rcvr) {}</span>
<span id="cb4-7"><a href="#cb4-7" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-8"><a href="#cb4-8" aria-hidden="true" tabindex="-1"></a>    suspend_always initial_suspend() noexcept { return {}; }</span>
<span id="cb4-9"><a href="#cb4-9" aria-hidden="true" tabindex="-1"></a>    [[noreturn]] suspend_always final_suspend() noexcept { terminate(); }</span>
<span id="cb4-10"><a href="#cb4-10" aria-hidden="true" tabindex="-1"></a>    [[noreturn]] void unhandled_exception() noexcept { terminate(); }</span>
<span id="cb4-11"><a href="#cb4-11" aria-hidden="true" tabindex="-1"></a>    [[noreturn]] void return_void() noexcept { terminate(); }</span>
<span id="cb4-12"><a href="#cb4-12" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-13"><a href="#cb4-13" aria-hidden="true" tabindex="-1"></a>    coroutine_handle&lt;&gt; unhandled_stopped() noexcept {</span>
<span id="cb4-14"><a href="#cb4-14" aria-hidden="true" tabindex="-1"></a>      set_stopped((DR&amp;&amp;) <em><code class="sourceCode default">rcvr</code></em>);</span>
<span id="cb4-15"><a href="#cb4-15" aria-hidden="true" tabindex="-1"></a>      return noop_coroutine();</span>
<span id="cb4-16"><a href="#cb4-16" aria-hidden="true" tabindex="-1"></a>    }</span>
<span id="cb4-17"><a href="#cb4-17" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-18"><a href="#cb4-18" aria-hidden="true" tabindex="-1"></a>    <em><code class="sourceCode default">operation-state-task</code></em> get_return_object() noexcept {</span>
<span id="cb4-19"><a href="#cb4-19" aria-hidden="true" tabindex="-1"></a>      return <em><code class="sourceCode default">operation-state-task</code></em>{</span>
<span id="cb4-20"><a href="#cb4-20" aria-hidden="true" tabindex="-1"></a>        coroutine_handle&lt;<em><code class="sourceCode default">connect-awaitable-promise</code></em>&gt;::from_promise(*this)};</span>
<span id="cb4-21"><a href="#cb4-21" aria-hidden="true" tabindex="-1"></a>    }</span>
<span id="cb4-22"><a href="#cb4-22" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-23"><a href="#cb4-23" aria-hidden="true" tabindex="-1"></a>    env_of_t&lt;const DR&amp;&gt; get_env() const noexcept {</span>
<span id="cb4-24"><a href="#cb4-24" aria-hidden="true" tabindex="-1"></a>      return execution::get_env(<em><code class="sourceCode default">rcvr</code></em>);</span>
<span id="cb4-25"><a href="#cb4-25" aria-hidden="true" tabindex="-1"></a>    }</span>
<span id="cb4-26"><a href="#cb4-26" aria-hidden="true" tabindex="-1"></a>  };</span>
<span id="cb4-27"><a href="#cb4-27" aria-hidden="true" tabindex="-1"></a>}</span></code></pre></div></li>
<li><p>Let
<i><code class="sourceCode default">operation-state-task</code></i> be
the following class:</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a>namespace std::execution {</span>
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a>  struct <em><code class="sourceCode default">operation-state-task</code></em> {</span>
<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a>    using operation_state_concept = operation_state_t;</span>
<span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></a>    using promise_type = <em><code class="sourceCode default">connect-awaitable-promise</code></em>;</span>
<span id="cb5-5"><a href="#cb5-5" aria-hidden="true" tabindex="-1"></a>    coroutine_handle&lt;&gt; <em><code class="sourceCode default">coro</code></em>; <em><code class="sourceCode default">// exposition only</code></em></span>
<span id="cb5-6"><a href="#cb5-6" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb5-7"><a href="#cb5-7" aria-hidden="true" tabindex="-1"></a>    explicit <em><code class="sourceCode default">operation-state-task</code></em>(coroutine_handle&lt;&gt; h) noexcept : <em><code class="sourceCode default">coro</code></em>(h) {}</span>
<span id="cb5-8"><a href="#cb5-8" aria-hidden="true" tabindex="-1"></a>    <em><code class="sourceCode default">operation-state-task</code></em>(<em><code class="sourceCode default">operation-state-task</code></em>&amp;&amp; o) noexcept</span>
<span id="cb5-9"><a href="#cb5-9" aria-hidden="true" tabindex="-1"></a>      : <em><code class="sourceCode default">coro</code></em>(exchange(o.<em><code class="sourceCode default">coro</code></em>, {})) {}</span>
<span id="cb5-10"><a href="#cb5-10" aria-hidden="true" tabindex="-1"></a>    ~<em><code class="sourceCode default">operation-state-task</code></em>() { if (<em><code class="sourceCode default">coro</code></em>) <em><code class="sourceCode default">coro</code></em>.destroy(); }</span>
<span id="cb5-11"><a href="#cb5-11" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb5-12"><a href="#cb5-12" aria-hidden="true" tabindex="-1"></a>    void start() &amp; noexcept {</span>
<span id="cb5-13"><a href="#cb5-13" aria-hidden="true" tabindex="-1"></a>      <em><code class="sourceCode default">coro</code></em>.resume();</span>
<span id="cb5-14"><a href="#cb5-14" aria-hidden="true" tabindex="-1"></a>    }</span>
<span id="cb5-15"><a href="#cb5-15" aria-hidden="true" tabindex="-1"></a>  };</span>
<span id="cb5-16"><a href="#cb5-16" aria-hidden="true" tabindex="-1"></a>}</span></code></pre></div></li>
<li><p>Let <code class="sourceCode default">V</code> name the type <code class="sourceCode default"><em><code class="sourceCode default">await-result-type</code></em>&lt;DS, <em><code class="sourceCode default">connect-awaitable-promise</code></em>&gt;</code>,
let <code class="sourceCode default">Sigs</code> name the type:</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a>completion_signatures&lt;</span>
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a>  <em><code class="sourceCode default">SET-VALUE-SIG</code></em>(V), <em><code class="sourceCode default">// see [exec.snd.concepts]</code></em></span>
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a>  set_error_t(exception_ptr),</span>
<span id="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a>  set_stopped_t()&gt;</span></code></pre></div>
<p>and let
<em><code class="sourceCode default">connect-awaitable</code></em> be an
exposition-only coroutine defined as follows:</p>
<div class="sourceCode" id="cb7"><pre class="sourceCode default"><code class="sourceCode default"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a>namespace std::execution {</span>
<span id="cb7-2"><a href="#cb7-2" aria-hidden="true" tabindex="-1"></a>  template&lt;class Fun, class... Ts&gt;</span>
<span id="cb7-3"><a href="#cb7-3" aria-hidden="true" tabindex="-1"></a>  auto <em><code class="sourceCode default">suspend-complete</code></em>(Fun fun, Ts&amp;&amp;... as) noexcept { <em><code class="sourceCode default">// exposition only</code></em></span>
<span id="cb7-4"><a href="#cb7-4" aria-hidden="true" tabindex="-1"></a>    auto fn = [&amp;, fun]() noexcept { fun(std::forward&lt;Ts&gt;(as)...); };</span>
<span id="cb7-5"><a href="#cb7-5" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb7-6"><a href="#cb7-6" aria-hidden="true" tabindex="-1"></a>    struct awaiter {</span>
<span id="cb7-7"><a href="#cb7-7" aria-hidden="true" tabindex="-1"></a>      decltype(fn) <em><code class="sourceCode default">fn</code></em>;</span>
<span id="cb7-8"><a href="#cb7-8" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb7-9"><a href="#cb7-9" aria-hidden="true" tabindex="-1"></a>      static constexpr bool await_ready() noexcept { return false; }</span>
<span id="cb7-10"><a href="#cb7-10" aria-hidden="true" tabindex="-1"></a>      void await_suspend(coroutine_handle&lt;&gt;) noexcept { <em><code class="sourceCode default">fn</code></em>(); }</span>
<span id="cb7-11"><a href="#cb7-11" aria-hidden="true" tabindex="-1"></a>      [[noreturn]] void await_resume() noexcept { unreachable(); }</span>
<span id="cb7-12"><a href="#cb7-12" aria-hidden="true" tabindex="-1"></a>    };</span>
<span id="cb7-13"><a href="#cb7-13" aria-hidden="true" tabindex="-1"></a>    return awaiter{fn};</span>
<span id="cb7-14"><a href="#cb7-14" aria-hidden="true" tabindex="-1"></a>  };</span>
<span id="cb7-15"><a href="#cb7-15" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb7-16"><a href="#cb7-16" aria-hidden="true" tabindex="-1"></a>  <em><code class="sourceCode default">operation-state-task</code></em> <em><code class="sourceCode default">connect-awaitable</code></em>(DS sndr, DR rcvr) requires receiver_of&lt;DR, Sigs&gt; {</span>
<span id="cb7-17"><a href="#cb7-17" aria-hidden="true" tabindex="-1"></a>    exception_ptr ep;</span>
<span id="cb7-18"><a href="#cb7-18" aria-hidden="true" tabindex="-1"></a>    try {</span>
<span id="cb7-19"><a href="#cb7-19" aria-hidden="true" tabindex="-1"></a>      if constexpr (same_as&lt;V, void&gt;) {</span>
<span id="cb7-20"><a href="#cb7-20" aria-hidden="true" tabindex="-1"></a>        co_await std::move(sndr);</span>
<span id="cb7-21"><a href="#cb7-21" aria-hidden="true" tabindex="-1"></a>        co_await <em><code class="sourceCode default">suspend-complete</code></em>(set_value, std::move(rcvr));</span>
<span id="cb7-22"><a href="#cb7-22" aria-hidden="true" tabindex="-1"></a>      } else {</span>
<span id="cb7-23"><a href="#cb7-23" aria-hidden="true" tabindex="-1"></a>        co_await <em><code class="sourceCode default">suspend-complete</code></em>(set_value, std::move(rcvr), co_await std::move(sndr));</span>
<span id="cb7-24"><a href="#cb7-24" aria-hidden="true" tabindex="-1"></a>      }</span>
<span id="cb7-25"><a href="#cb7-25" aria-hidden="true" tabindex="-1"></a>    } catch(...) {</span>
<span id="cb7-26"><a href="#cb7-26" aria-hidden="true" tabindex="-1"></a>      ep = current_exception();</span>
<span id="cb7-27"><a href="#cb7-27" aria-hidden="true" tabindex="-1"></a>    }</span>
<span id="cb7-28"><a href="#cb7-28" aria-hidden="true" tabindex="-1"></a>    co_await <em><code class="sourceCode default">suspend-complete</code></em>(set_error, std::move(rcvr), std::move(ep));</span>
<span id="cb7-29"><a href="#cb7-29" aria-hidden="true" tabindex="-1"></a>  }</span>
<span id="cb7-30"><a href="#cb7-30" aria-hidden="true" tabindex="-1"></a>}</span></code></pre></div></li>
<li><p>If <code class="sourceCode default">Sndr</code> does not satisfy
<code class="sourceCode default">sender</code> or if
<code class="sourceCode default">Rcvr</code> does not satisfy
<code class="sourceCode default">receiver</code>,
<code class="sourceCode default">connect(sndr, rcvr)</code> is
ill-formed. Otherwise, the expression
<code class="sourceCode default">connect(sndr, rcvr)</code> is
expression-equivalent to:</p>
<ol type="1">
<li><p><code class="sourceCode default"><span class="add" style="color: #006e28"><ins><span><code class="sourceCode default">new_</code></span></ins></span>sndr.connect(rcvr)</code>
if that expression is well-formed.</p>
<ul>
<li><i>Mandates:</i> The type of the expression above satisfies
<code class="sourceCode default">operation_state</code>.</li>
</ul></li>
<li><p>Otherwise, <code class="sourceCode default"><em><code class="sourceCode default">connect-awaitable</code></em>(<span class="add" style="color: #006e28"><ins><span><code class="sourceCode default">new_</code></span></ins></span>sndr, rcvr)</code>
if that expression is well-formed.</p></li>
<li><p>Otherwise,
<code class="sourceCode default">connect(sndr, rcvr)</code> is
ill-formed.</p></li>
</ol></li>
</ol>
</blockquote>
<h1 data-number="5" id="bibliography"><span class="header-section-number">5</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references csl-bib-body hanging-indent" role="doc-bibliography">
<div id="ref-P2300R9" class="csl-entry" role="doc-biblioentry">
[P2300R9] Eric Niebler, Michał Dominiak, Georgy Evtushenko, Lewis Baker,
Lucian Radu Teodorescu, Lee Howes, Kirk Shoop, Michael Garland, Bryce
Adelstein Lelbach. 2024-04-02. `std::execution`. <a href="https://wg21.link/p2300r9"><div class="csl-block">https://wg21.link/p2300r9</div></a>
</div>
<div id="ref-P2999R3" class="csl-entry" role="doc-biblioentry">
[P2999R3] Eric Niebler. 2023-12-13. Sender Algorithm Customization. <a href="https://wg21.link/p2999r3"><div class="csl-block">https://wg21.link/p2999r3</div></a>
</div>
</div>
</div>
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