argument_type_not_assignable_to_error_handler

The argument type '{0}' can't be assigned to the parameter type '{1} Function(Object)' or '{1} Function(Object, StackTrace)'.

The analyzer produces this diagnostic when an invocation of Future.catchError has an argument that is a function whose parameters aren't compatible with the arguments that will be passed to the function when it's invoked. The static type of the first argument to catchError is just Function, even though the function that is passed in is expected to have either a single parameter of type Object or two parameters of type Object and StackTrace.

The following code produces this diagnostic because the closure being passed to catchError doesn't take any parameters, but the function is required to take at least one parameter:

void f(Future<int> f) {
  f.catchError(() => 0);
}

The following code produces this diagnostic because the closure being passed to catchError takes three parameters, but it can't have more than two required parameters:

void f(Future<int> f) {
  f.catchError((one, two, three) => 0);
}

The following code produces this diagnostic because even though the closure being passed to catchError takes one parameter, the closure doesn't have a type that is compatible with Object:

void f(Future<int> f) {
  f.catchError((String error) => 0);
}

Change the function being passed to catchError so that it has either one or two required parameters, and the parameters have the required types:

void f(Future<int> f) {
  f.catchError((Object error) => 0);
}