override.dart

// Function parameter types are in a "contravariant position", so allowing this
// override would break soundness. Consider:

class Base {
  final String Function(dynamic val) validator;
}

class Derived extends Base {
  String Function(String val) validator;
}

main() {
  // Upcast a Derived to a Base:
  Base b = Derived();

  // Call the function with wrong type:
  b.validator(1234); // Oops!
}

// Because function parameters are contravariant, the "reverse" the direction
// of variance that you are allowed to use. So an override cannot tighten a
// parameter type, but it can loosen it:

class Base {
  final String Function(int val) validator;
}

class Derived extends Base {
  String Function(num val) validator;
}

main() {
  // Upcast a Derived to a Base:
  Base b = Derived();

  // Always safe since the base class has the more precise type:
  b.validator(1234);
}