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<title>Issue 4369: check-types function for upon_error and upon_stopped is wrong</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#New">New</a> status.</em></p>
<h3 id="4369"><a href="lwg-active.html#4369">4369</a>. <code><i>check-types</i></code> function for <code class='backtick'>upon_error</code> and <code class='backtick'>upon_stopped</code> is wrong</h3>
<p><b>Section:</b> 33.9.12.9 <a href="https://wg21.link/exec.then">[exec.then]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Eric Niebler <b>Opened:</b> 2025-08-31 <b>Last modified:</b> 2025-09-15</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The following has been reported by Trevor Gray:
</p>
<p>
In 33.9.12.9 <a href="https://wg21.link/exec.then">[exec.then]</a> p5, the <code><i>impls-for</i>&lt;<i>decayed-typeof</i>&lt;<i>then-cpo</i>&gt;&gt;::<i>check-types</i></code> 
unction is specified as follows:
</p>
<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
<pre>
template&lt;class Sndr, class... Env&gt;
  static consteval void <i>check-types</i>();
</pre>
<blockquote>
<p>
<i>Effects</i>: Equivalent to:
</p>
<blockquote><pre>
auto cs = get_completion_signatures&lt;<i>child-type</i>&lt;Sndr&gt;, <i>FWD-ENV-T</i>(Env)...&gt;();
auto fn = []&lt;class... Ts&gt;(set_value_t(*)(Ts...)) {
  if constexpr (!invocable&lt;remove_cvref_t&lt;<i>data-type</i>&lt;Sndr&gt;&gt;, Ts...&gt;)
    throw <i>unspecified-exception</i>();
};
cs.<i>for-each</i>(<i>overload-set</i>{fn, [](auto){}});
</pre></blockquote>
<p>
where <code><i>unspecified-exception</i></code> is a type derived from <code class='backtick'>exception</code>.
</p>
</blockquote>
</blockquote>
<p>
The line <code>auto fn = []&lt;class... Ts&gt;(set_value_t(*)(Ts...)) {</code> 
is correct when <code><i>then-cpo</i></code> is <code class='backtick'>then</code> but not when it is <code class='backtick'>upon_error</code> or <code class='backtick'>upon_stopped</code>.
<p/>
For <code class='backtick'>upon_error</code> it should be:
</p>
<blockquote><pre>
auto fn = []&lt;class... Ts&gt;(set_error_t(*)(Ts...)) {
</pre></blockquote>
<p>
and for <code class='backtick'>upon_stopped</code> it should be:
</p>
<blockquote><pre>
auto fn = []&lt;class... Ts&gt;(set_stopped_t(*)(Ts...)) {
</pre></blockquote>
<p>
We can achieve that by replacing <code class='backtick'>set_value_t</code> in the problematic line with <code><i>decayed-typeof</i>&lt;<i>set-cpo</i>&gt;</code>.
</p>


<p id="res-4369"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5014">N5014</a>.
</p>

<ol>

<li><p>Modify 33.9.12.9 <a href="https://wg21.link/exec.then">[exec.then]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class Sndr, class... Env&gt;
  static consteval void <i>check-types</i>();
</pre>
<blockquote>
<p>
-5- <i>Effects:</i> Equivalent to:
</p>
<blockquote><pre>
auto cs = get_completion_signatures&lt;<i>child-type</i>&lt;Sndr&gt;, <i>FWD-ENV-T</i>(Env)...&gt;();
auto fn = []&lt;class... Ts&gt;(<del>set_value_t</del><ins><i>decayed-typeof</i>&lt;<i>set-cpo</i>&gt;</ins>(*)(Ts...)) {
  if constexpr (!invocable&lt;remove_cvref_t&lt;<i>data-type</i>&lt;Sndr&gt;&gt;, Ts...&gt;)
    throw <i>unspecified-exception</i>();
};
cs.<i>for-each</i>(<i>overload-set</i>{fn, [](auto){}});
</pre></blockquote>
<p>
where <code><i>unspecified-exception</i></code> is a type derived from <code class='backtick'>exception</code>.
</p>
</blockquote>
</blockquote>

</li>

</ol>




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