non_void_return_for_operator

The return type of the operator []= must be 'void'.

The analyzer produces this diagnostic when a declaration of the operator []= has a return type other than void.

The following code produces this diagnostic because the declaration of the operator []= has a return type of int:

class C {
  int operator []=(int index, int value) => 0;
}

Change the return type to void:

class C {
  void operator []=(int index, int value) => 0;
}