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<title>Issue 4119: generator::promise_type::yield_value(ranges::elements_of&lt;R, Alloc&gt;)'s nested generator may be ill-formed</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="4119"><a href="lwg-defects.html#4119">4119</a>. <code>generator::promise_type::yield_value(ranges::elements_of&lt;R, Alloc&gt;)</code>'s nested <code>generator</code> may be ill-formed</h3>
<p><b>Section:</b> 25.8.5 <a href="https://wg21.link/coro.generator.promise">[coro.generator.promise]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2024-07-11 <b>Last modified:</b> 2024-11-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#coro.generator.promise">issues</a> in [coro.generator.promise].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The nested coroutine is specified to return <code>generator&lt;yielded, ranges::range_value_t&lt;R&gt;, Alloc&gt;</code> 
which can be problematic as the value type of <code>R</code> is really irrelevant to <code>yielded</code>, 
unnecessarily violating the <code>generator</code>'s <i>Mandates</i> (<a href="https://godbolt.org/z/PzGP3G7rr">demo</a>):
</p>
<blockquote><pre>
#include &lt;generator&gt;
#include &lt;vector&gt;

std::generator&lt;std::span&lt;int&gt;&gt; f() {
  std::vector&lt;int&gt; v;
  co_yield v; // ok
}

std::generator&lt;std::span&lt;int&gt;&gt; g() {
  std::vector&lt;std::vector&lt;int&gt;&gt; v;
  co_yield std::ranges::elements_of(v); // <span style="color:red;font-weight:bolder">hard error</span>
}
</pre></blockquote>
<p>
This proposed resolution is to change the second template parameter from <code>range_value_t&lt;R&gt;</code> 
to <code>void</code> since that type doesn't matter to us.
</p>

<p><i>[2024-08-02; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after five votes in favour during reflector poll.
</p>

<p><i>[Wrocław 2024-11-23; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-4119"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4986" title=" Working Draft, Programming Languages — C++">N4986</a>.
</p>
<ol>
<li><p>Modify 25.8.5 <a href="https://wg21.link/coro.generator.promise">[coro.generator.promise]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;ranges::input_range R, class Alloc&gt;
  requires convertible_to&lt;ranges::range_reference_t&lt;R&gt;, yielded&gt;
  auto yield_value(ranges::elements_of&lt;R, Alloc&gt; r);
</pre>
<blockquote>
<p>
-13- <i>Effects</i>: Equivalent to:
</p>
<blockquote><pre>
auto nested = [](allocator_arg_t, Alloc, ranges::iterator_t&lt;R&gt; i, ranges::sentinel_t&lt;R&gt; s)
  -&gt; generator&lt;yielded, <del>ranges::range_value_t&lt;R&gt;</del><ins>void</ins>, Alloc&gt; {
    for (; i != s; ++i) {
      co_yield static_cast&lt;yielded&gt;(*i);
    }
  };
return yield_value(ranges::elements_of(nested(
  allocator_arg, r.allocator, ranges::begin(r.range), ranges::end(r.range))));
</pre></blockquote>
[&hellip;]
</blockquote>
</blockquote>
</li>
</ol>





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