What elements or properties cannot be extended or supplemented using extensions?

In Kotlin, extensions allow adding new functions and properties to existing classes without inheritance. However, there are limitations:

• Cannot add new fields (properties with backing fields). Extension properties in Kotlin are essentially syntactic sugar for getters and setters; they cannot store state.

• Cannot override existing methods or properties of a class. Extensions do not modify the class but only provide external functions.

• Cannot extend class constructors.

• Cannot extend local or nested classes declared inside functions.

• Cannot extend elements that are not classes, such as primitive types directly (although Kotlin wraps them in classes).

Thus, extensions cannot change the internal state of a class or its behavior that requires access to private members.