nullable_type_in_on_clause
A mixin can't have a nullable type as a superclass constraint.
Description
#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?> {}
.
Example
#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? {}
Common fixes
#Remove the question mark from the type:
class C {}
mixin M on C {}
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