Backported goodies to SelfList from Godot4.

core/containers/self_list.h

@@ -100,13 +100,78 @@ public:
 			p_elem->_root = nullptr;
 		}
 
+		void clear() {
+			while (_first) {
+				remove(_first);
+			}
+		}
+
+		void sort() {
+			sort_custom<Comparator<T>>();
+		}
+
+		template <class C>
+		void sort_custom() {
+			if (_first == _last) {
+				return;
+			}
+
+			SelfList<T> *from = _first;
+			SelfList<T> *current = from;
+			SelfList<T> *to = from;
+
+			while (current) {
+				SelfList<T> *next = current->_next;
+
+				if (from != current) {
+					current->_prev = nullptr;
+					current->_next = from;
+
+					SelfList<T> *find = from;
+					C less;
+					while (find && less(*find->_self, *current->_self)) {
+						current->_prev = find;
+						current->_next = find->_next;
+						find = find->_next;
+					}
+
+					if (current->_prev) {
+						current->_prev->_next = current;
+					} else {
+						from = current;
+					}
+
+					if (current->_next) {
+						current->_next->_prev = current;
+					} else {
+						to = current;
+					}
+				} else {
+					current->_prev = nullptr;
+					current->_next = nullptr;
+				}
+
+				current = next;
+			}
+			_first = from;
+			_last = to;
+		}
+
 		_FORCE_INLINE_ SelfList<T> *first() { return _first; }
 		_FORCE_INLINE_ const SelfList<T> *first() const { return _first; }
+
+		// Forbid copying, which has broken behavior.
+		void operator=(const List &) = delete;
+
 		_FORCE_INLINE_ List() {
 			_first = nullptr;
 			_last = nullptr;
 		}
-		_FORCE_INLINE_ ~List() { ERR_FAIL_COND(_first != nullptr); }
+
+		_FORCE_INLINE_ ~List() {
+			// A self list must be empty on destruction.
+			DEV_ASSERT(_first == nullptr);
+		}
 	};
 
 private:
@@ -128,6 +193,9 @@ public:
 	_FORCE_INLINE_ const SelfList<T> *prev() const { return _prev; }
 	_FORCE_INLINE_ T *self() const { return _self; }
 
+	// Forbid copying, which has broken behavior.
+	void operator=(const SelfList<T> &) = delete;
+
 	_FORCE_INLINE_ SelfList(T *p_self) {
 		_self = p_self;
 		_next = nullptr;