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<title>Issue 3899: co_yielding elements of an lvalue generator is unnecessarily inefficient</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="3899"><a href="lwg-defects.html#3899">3899</a>. <code>co_yield</code>ing elements of an lvalue <code>generator</code> is unnecessarily inefficient</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> 2023-03-04 <b>Last modified:</b> 2024-11-28</p>
<p><b>Priority: </b>3
</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>
Consider:
</p>
<blockquote>
<pre>
std::generator&lt;int&gt; f();
std::generator&lt;int&gt; g() {
    auto gen = f();
    auto gen2 = f();
    co_yield std::ranges::elements_of(std::move(gen));   // #1
    co_yield std::ranges::elements_of(gen2);             // #2
    // other stuff
}
</pre>
</blockquote>
<p>
Both #1 and #2 compile. The differences are:
</p>
<ul>
<li>
<p>
#2 is significantly less efficient (it uses the general overload of <code>yield_value</code>,
so it creates a new coroutine frame and doesn't do symmetric transfer into <code>gen2</code>'s coroutine)
</p>
</li>
<li>
<p>
the coroutine frame of <code>gen</code> and <code>gen2</code> are destroyed at different
times: <code>gen</code>'s frame is destroyed at the end of #1, but <code>gen2</code>'s is
not destroyed until the closing brace.
</p>
</li>
</ul>
<p>
But as far as the user is concerned, neither <code>gen</code> nor <code>gen2</code> is
usable after the <code>co_yield</code>. In both cases the only things you can do
with the objects are:
</p>
<ul>
<li><p>destroying them;</p></li>
<li><p>assigning to them;</p></li>
<li><p>call <code>end()</code> on them to get a copy of <code>default_sentinel</code>.</p></li>
</ul>
<p>
We could make #2 ill-formed, but that seems unnecessary: there is no meaningful
difference between <code>generator</code> and any other single-pass input range
(or a <code>generator</code> with a different yielded type that has to go through
the general overload) in this regard. We should just make #2 do the efficient
thing too.
</p>

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

<p>
Set priority to 3 after reflector poll.
</p>

<p><i>[St. Louis 2024-06-28; move to Ready]</i></p>


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



<p id="res-3899"><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;
<ins>    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;, Unused&gt; g) noexcept;</ins>

    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) noexcept;
    [&hellip;]
   };
}
</pre>
</blockquote>
[&hellip;]
<pre>
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;
<ins>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;, Unused&gt; g) noexcept;</ins>
</pre>
<blockquote>
<p>
-10- <i>Preconditions</i>: A handle referring to the coroutine whose promise object
is <code>*this</code> is at the top of <code>*<i>active_</i></code> of some generator
object <code>x</code>. The coroutine referred to by <code>g.range.<i>coroutine_</i></code>
is suspended at its initial suspend point.
<p/>
-11- <i>Returns:</i> An awaitable object of an unspecified type (7.6.2.4 <a href="https://wg21.link/expr.await">[expr.await]</a>)
into which <code>g.range</code> is moved, whose member <code>await_ready</code> returns <code>false</code>,
whose member <code>await_suspend</code> pushes <code>g.range.<i>coroutine_</i></code> into <code>*x.<i>active_</i></code>
and resumes execution of the coroutine referred to by <code>g.range.<i>coroutine_</i></code>,
and whose member <code>await_resume</code> evaluates <code>rethrow_exception(<i>except_</i>)</code>
if <code>bool(<i>except_</i>)</code> is <code>true</code>. If <code>bool(<i>except_</i>)</code> is <code>false</code>,
the <code>await_resume</code> member has no effects.
<p/>
-12- <i>Remarks</i>: A <i>yield-expression</i> that calls
<del>this function</del><ins>one of these functions</ins> has type
<code>void</code> (7.6.17 <a href="https://wg21.link/expr.yield">[expr.yield]</a>).
</p>
</blockquote>
</blockquote>

</li>


</ol>





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