optional_parameter_in_operator

Optional parameters aren't allowed when defining an operator.

The analyzer produces this diagnostic when one or more of the parameters in an operator declaration are optional.

The following code produces this diagnostic because the parameter other is an optional parameter:

class C {
  C operator +([C? other]) => this;
}

Make all of the parameters be required parameters:

class C {
  C operator +(C other) => this;
}