Implement LWG-4119 generator::promise_type::yield_value(ranges::elements_of<R, Alloc>)'s nested generator may be ill-formed (#5220)

Co-authored-by: Stephan T. Lavavej <[email protected]>

Author: frederick-vs-ja

stl/inc/generator

@@ -289,11 +289,10 @@ namespace _Gen_detail {
         template <_RANGES input_range _Rng, class _Alloc>
             requires convertible_to<_RANGES range_reference_t<_Rng>, _Yielded>
         _NODISCARD auto yield_value(_RANGES elements_of<_Rng, _Alloc> _Elem) {
-            using _Vty              = _RANGES range_value_t<_Rng>;
-            using _Nested_awaitable = _Nested_awaitable_provider<_Yielded, _Vty, _Alloc>::_Awaitable;
+            using _Nested_awaitable = _Nested_awaitable_provider<_Yielded, void, _Alloc>::_Awaitable;
 
             auto _Lambda = [](allocator_arg_t, _Alloc, _RANGES iterator_t<_Rng> _It,
-                               const _RANGES sentinel_t<_Rng> _Se) -> generator<_Yielded, _Vty, _Alloc> {
+                               const _RANGES sentinel_t<_Rng> _Se) -> generator<_Yielded, void, _Alloc> {
                 for (; _It != _Se; ++_It) {
                     co_yield static_cast<_Yielded>(*_It);
                 }

tests/std/tests/P2502R2_generator_promise/test.cpp

@@ -13,6 +13,7 @@
 #include <memory_resource>
 #include <new>
 #include <ranges>
+#include <span>
 #include <string>
 #include <type_traits>
 #include <utility>
@@ -226,6 +227,15 @@ struct Holder {
 struct Incomplete;
 #endif // ^^^ no workaround ^^^
 
+#if !(defined(__clang__) && defined(_M_IX86)) // TRANSITION, LLVM-56507
+// Also test LWG-4119:
+// "generator::promise_type::yield_value(ranges::elements_of<R, Alloc>)'s nested generator may be ill-formed"
+generator<span<int>> test_lwg_4119() { // COMPILE-ONLY
+    vector<vector<int>> v;
+    co_yield ranges::elements_of(v);
+}
+#endif // ^^^ no workaround ^^^
+
 int main() {
     test_with_type<int>();
     test_with_type<float>();