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<title>Issue 3894: generator::promise_type::yield_value(ranges::elements_of&lt;Rng, Alloc&gt;) should not be noexcept</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="3894"><a href="lwg-defects.html#3894">3894</a>. <code>generator::promise_type::yield_value(ranges::elements_of&lt;Rng, Alloc&gt;)</code> should not be <code>noexcept</code></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> Tim Song <b>Opened:</b> 2023-02-25 <b>Last modified:</b> 2023-11-22</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 overload of <code>yield_value</code> for yielding elements of arbitrary ranges does so by creating a nested generator, 
but to do so it needs to:
</p>
<ul>
<li><p>call <code>ranges::begin</code>/<code>ranges::end</code> on the range  </p></li>
<li><p>allocate a new coroutine frame (unless the allocation is elided by the compiler, which isn't guaranteed)</p></li>
<li><p>copy/move the iterator and sentinel into the coroutine frame</p></li>
</ul>
<p>
All of these are allowed to throw, so this overload should not be <code>noexcept</code>.
</p>

<p><i>[2023-03-22; Reflector poll]</i></p>

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

<p><i>[2023-06-17 Approved at June 2023 meeting in Varna. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3894"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4928" title=" Working Draft, Standard for Programming Language C++">N4928</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>
<blockquote>
<pre>
namespace std {
  template&lt;class Ref, class V, class Allocator&gt;
  class generator&lt;Ref, V, Allocator&gt;::promise_type {
  public:
    [&hellip;]
    auto yield_value(const remove_reference_t&lt;yielded&gt;&amp; lval)
      requires is_rvalue_reference_v&lt;yielded&gt; &amp;&amp;
        constructible_from&lt;remove_cvref_t&lt;yielded&gt;, const remove_reference_t&lt;yielded&gt;&amp;&gt;;
    
    template&lt;class R2, class V2, class Alloc2, class Unused&gt;
      requires same_as&lt;typename generator&lt;R2, V2, Alloc2&gt;::yielded, yielded&gt;
        auto yield_value(ranges::elements_of&lt;generator&lt;R2, V2, Alloc2&gt;&amp;&amp;, Unused&gt; g) noexcept;
    
    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) <del>noexcept</del>;
    [&hellip;]
   };
}
</pre>
</blockquote>
[&hellip;]
<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) <del>noexcept</del>;
</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, ranges::range_value_t&lt;R&gt;, 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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