[PATCH] libstdc++: Add lvalue overload for generator::yield_value

[Jonathan Wakely]

This was approved in Wrocław as LWG 3899.

libstdc++-v3/ChangeLog:

	* include/std/generator (generator::yield_value): Add overload
	taking lvalue element_of view, as per LWG 3899.
---

The issue suggests that this change avoids creating a new coroutine
frame, so I thought if I used a custom allocator I'd be able to see a
reduction in memory allocation after this fix. I was unable to see any
difference. I would welcome a testcase for it.

Tested x86_64-linux.

 libstdc++-v3/include/std/generator | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/libstdc++-v3/include/std/generator b/libstdc++-v3/include/std/generator
index bba85bd0aa4..807e724c0c8 100644
--- a/libstdc++-v3/include/std/generator
+++ b/libstdc++-v3/include/std/generator
@@ -153,6 +153,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  noexcept
 	{ return _Recursive_awaiter { std::move(__r.range) }; }
 
+	// _GLIBCXX_RESOLVE_LIB_DEFECTS
+	// 3899. co_yielding elements of an lvalue generator is
+	// unnecessarily inefficient
+	template<typename _R2, typename _V2, typename _A2, typename _U2>
+	requires std::same_as<_Yield2_t<_R2, _V2>, _Yielded>
+	auto
+	yield_value(ranges::elements_of<generator<_R2, _V2, _A2>&, _U2> __r)
+	  noexcept
+	{ return _Recursive_awaiter { std::move(__r.range) }; }
+
 	template<ranges::input_range _R, typename _Alloc>
 	requires convertible_to<ranges::range_reference_t<_R>, _Yielded>
 	auto
-- 
2.47.1