mixin_class_declaration_with_clause

The class '{0}' can't be declared a mixin because it has a 'with' clause.

The analyzer produces this diagnostic when a class with the mixin modifier has a with clause. A mixin class declaration cannot have a with clause because that implies it extends a class other than Object.

The following code produces this diagnostic because the mixin class B has a with clause:

mixin M {}
​
mixin class B with M {}

If you want to use the class as a mixin, remove the with clause:

mixin M {}
​
mixin class B {}

If you need the class to have a with clause, then remove the mixin modifier:

mixin M {}
​
class B with M {}