libstdc++: don't implicit-construct _Yielded_decvref [PR118022]

This overload requires

  constructible_from<remove_cvref_t<yielded>,
                     const remove_reference_t<yielded>&>

... but then tries to construct remove_cvref_t<yielded> implicitly,
which means it imposes an additional constraint not in the standard.

libstdc++-v3/ChangeLog:

	PR libstdc++/118022
	* include/std/generator
	(_Promise_erased::yield_value(const _Yielded_deref&)): Don't
	implicit-constuct _Yielded_decvref.
	* testsuite/24_iterators/range_generators/pr118022.cc: New test.

Author: ArsenArsen

libstdc++-v3/include/std/generator

@@ -144,7 +144,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  requires (is_rvalue_reference_v<_Yielded>
 		    && constructible_from<_Yielded_decvref,
 		    const _Yielded_deref&>)
-	{ return _Copy_awaiter(__val, _M_bottom_value()); }
+	{ return _Copy_awaiter(_Yielded_decvref(__val), _M_bottom_value()); }
 
 	template<typename _R2, typename _V2, typename _A2, typename _U2>
 	requires std::same_as<_Yield2_t<_R2, _V2>, _Yielded>

libstdc++-v3/testsuite/24_iterators/range_generators/pr118022.cc

@@ -0,0 +1,16 @@
+// { dg-do compile { target c++23 } }
+#include <generator>
+
+struct O {
+  O() = default;
+  explicit O(const O&) = default;
+};
+
+std::generator<O&&> gen() {
+  const O o;
+  co_yield o;
+}
+
+int
+main()
+{}