mixin_modifier_mixin_application_class_with_multiple_mixins

The mixin application class '{0}' can only have a single mixin.

The analyzer produces this diagnostic when a mixin application class with the mixin modifier has more than one mixin. A mixin class application can only have a single mixin.

The following code produces this diagnostic because the mixin class C has multiple mixins:

mixin M1 {}
mixin M2 {}
​
mixin class C = Object with M1, M2;

If you need to use multiple mixins, remove the mixin modifier from the mixin application class:

mixin M1 {}
mixin M2 {}
​
class C = Object with M1, M2;

If you need the class to be a mixin class, then only use a single mixin:

mixin M1 {}
​
mixin class C = Object with M1;