Implement execution::just_done factory

Author: miscco

stl/inc/execution

@@ -5511,6 +5511,35 @@ namespace execution {
         is_nothrow_constructible_v<remove_cvref_t<_Error>, _Error>) {
         return {_STD forward<_Error>(_Err)};
     }
+
+    struct _Just_done_sender {
+        template <template <class...> class _Tuple, template <class...> class _Variant>
+        using value_types = _Variant<>;
+
+        template <template <class...> class _Variant>
+        using error_types = _Variant<>;
+
+        static const constexpr auto sends_done = true;
+
+        template <receiver _Receiver>
+        struct operation_state {
+            _Receiver _Rec;
+
+            friend constexpr void tag_invoke(start_t, operation_state& _State) noexcept {
+                _EXEC set_done(_STD move(_State._Rec));
+            }
+        };
+
+        template <receiver _Receiver>
+        _NODISCARD friend constexpr auto tag_invoke(connect_t, const _Just_done_sender&, _Receiver&& _Rec) noexcept(
+            is_nothrow_constructible_v<remove_cvref_t<_Receiver>, _Receiver>) {
+            return operation_state<_Receiver>{_STD forward<_Receiver>(_Rec)};
+        }
+    };
+
+    _NODISCARD constexpr _Just_done_sender just_done() noexcept {
+        return {};
+    };
 } // namespace execution
 #endif // __cpp_lib_executors
 _STD_END

tests/std/tests/P2300R2_executors_factories/test.cpp

@@ -124,6 +124,27 @@ constexpr bool test_just_error() {
     return true;
 }
 
+constexpr bool test_just_done() {
+    {
+        Tracker tracker;
+        typed_sender auto sender   = execution::just_done();
+        operation_state auto state = connect(sender, nothrow_receiver<>{tracker});
+        assert(!tracker.set_done_called);
+        start(state);
+        assert(tracker.set_done_called);
+    }
+
+    {
+        Tracker tracker;
+        operation_state auto state = connect(execution::just_done(), nothrow_receiver<>{tracker});
+        assert(!tracker.set_done_called);
+        start(state);
+        assert(tracker.set_done_called);
+    }
+
+    return true;
+}
+
 int main() {
     test_just();
     static_assert(test_just());