class_used_as_mixin

The class '{0}' can't be used as a mixin because it's neither a mixin class nor a mixin.

The analyzer produces this diagnostic when a class that is neither a mixin class nor a mixin is used in a with clause.

The following code produces this diagnostic because the class M is being used as a mixin, but it isn't defined as a mixin class:

class M {}
class C with M {}

If the class can be a pure mixin, then change class to mixin:

mixin M {}
class C with M {}

If the class needs to be both a class and a mixin, then add mixin:

mixin class M {}
class C with M {}