subtype_of_base_or_final_is_not_base_final_or_sealed

The mixin '{0}' must be 'base' because the supertype '{1}' is 'base'.

The mixin '{0}' must be 'base' because the supertype '{1}' is 'final'.

The type '{0}' must be 'base', 'final' or 'sealed' because the supertype '{1}' is 'base'.

The type '{0}' must be 'base', 'final' or 'sealed' because the supertype '{1}' is 'final'.

The analyzer produces this diagnostic when a class or mixin has a direct or indirect supertype that is either base or final, but the class or mixin itself isn't marked either base, final, or sealed.

The following code produces this diagnostic because the class B is a subtype of A, and A is a base class, but B is neither base, final or sealed:

base class A {}
class B extends A {}

Add either base, final or sealed to the class or mixin declaration:

base class A {}
final class B extends A {}