A class or mixin can't mix in a nullable typeDRT-W1552

The analyzer produces this diagnostic when a class or mixin declaration has a with clause, and a mixin is followed by a ?.

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

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

Example

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

mixin M {}
class C with M? {}

Common fixes

Remove the question mark from the type:

mixin M {}
class C with M {}