nullable_type_in_extends_clause

A class can't extend a nullable type.

The analyzer produces this diagnostic when a class declaration uses an extends clause to specify a superclass, and the superclass is followed by a ?.

It isn't valid to specify a nullable superclass because doing so would have no meaning; it wouldn't change either the interface or implementation being inherited by the class containing the extends clause.

Note, however, that it is valid to use a nullable type as a type argument to the superclass, such as class A extends B<C?> {}.

The following code produces this diagnostic because A? is a nullable type, and nullable types can't be used in an extends clause:

class A {}
class B extends A? {}

Remove the question mark from the type:

class A {}
class B extends A {}