mixin_class_declaration_extends_not_object

The class '{0}' can't be declared a mixin because it extends a class other than 'Object'.

The analyzer produces this diagnostic when a class that is marked with the mixin modifier extends a class other than Object. A mixin class can't have a superclass other than Object.

The following code produces this diagnostic because the class B, which has the modifier mixin, extends A:

class A {}
​
mixin class B extends A {}

If you want the class to be used as a mixin, then change the superclass to Object, either explicitly or by removing the extends clause:

class A {}
​
mixin class B {}

If the class needs to have a superclass other than Object, then remove the mixin modifier:

class A {}
​
class B extends A {}

If you need both a mixin and a subclass of a class other than Object, then move the members of the subclass to a new mixin, remove the mixin modifier from the subclass, and apply the new mixin to the subclass:

class A {}
​
class B extends A with M {}
​
mixin M {}

Depending on the members of the subclass this might require adding an on clause to the mixin.