mixin_class_declares_constructor

The class '{0}' can't be used as a mixin because it declares a constructor.

The analyzer produces this diagnostic when a class is used as a mixin and the mixed-in class defines a constructor.

The following code produces this diagnostic because the class A, which defines a constructor, is being used as a mixin:

//@dart=2.19
class A {
  A();
}

class B with A {}

If it's possible to convert the class to a mixin, then do so:

mixin A {
}

class B with A {}

If the class can't be a mixin and it's possible to remove the constructor, then do so:

//@dart=2.19
class A {
}

class B with A {}

If the class can't be a mixin and you can't remove the constructor, then try extending or implementing the class rather than mixing it in:

class A {
  A();
}

class B extends A {}