nullable_type_in_on_clause

A mixin can't have a nullable type as a superclass constraint.

The analyzer produces this diagnostic when a mixin declaration uses an on clause to specify a superclass constraint, and the class that's specified is followed by a ?.

It isn't valid to specify a nullable superclass constraint because doing so would have no meaning; it wouldn't change the interface being depended on by the mixin containing the on clause.

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

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

class C {}
mixin M on C? {}

Remove the question mark from the type:

class C {}
mixin M on C {}