3.5.1 Between Class/Interface

When defining unification behavior between classes, it is important to remember that unification is directional: we can assign a more specialized class to a generic class, but the reverse is not valid.

The following assignments are allowed:

• Child class to parent class.
• Class to implementing interface.
• Interface to base interface.

These rules are transitive, meaning that a child class can also be assigned to the base class of its base class, an interface its base class implements, the base interface of an implementing interface, and so on.

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