What restrictions exist when developing extensions for data classes in programming languages?

In Kotlin, a data class automatically generates methods such as equals(), hashCode(), toString(), as well as componentN() and copy(). When developing extensions (extension functions or extension properties) for a data class, the following limitations exist:

• Extensions cannot override existing class methods. For example, you cannot change the behavior of equals() or copy().
• Extensions cannot add new fields to the class, as extensions work as static methods outside the class.
• Extensions cannot directly modify the internal state of an object unless it is exposed through a public API.

Thus, extensions for data classes are useful for adding helper functions but not for changing the core behavior or structure of the object.