non_void_return_for_setter

The return type of the setter must be 'void' or absent.

The analyzer produces this diagnostic when a setter is defined with a return type other than void.

The following code produces this diagnostic because the setter p has a return type of int:

class C {
  int set p(int i) => 0;
}

Change the return type to void or omit the return type:

class C {
  set p(int i) => 0;
}