How to pass a computed value to a super class?

[matanshukry]

I'm trying to do something like this: https://gist.github.com/matanshukry/d9e2d0a97a167d35cdc4caf20bcb3df8

but as I wrote in the comment of that gist too - the generated freezed file can't be run - it throw an error before even starting:

Error: A named parameter can't start with an underscore ('_').
@optionalTypeArgs TResult maybeMap<TResult extends Object?>({TResult ...

Specifically on the maybeMap generated code:

@optionalTypeArgs TResult maybeMap<TResult extends Object?>({TResult Function( CarCustom value)?  _custom,required TResult orElse(),}){
final _that = this;
switch (_that) {
case CarCustom() when _custom != null:
return _custom(_that);case _:
  return orElse();

}
}

How to fix this then?

Notes:

1. I want the id property to be based on the timestamp, and set when creating the object.
2. I want to have an actual type for that class, in this case CarCustom
3. I don't want users of this class to be able to provider their own id - hence the need for the private ctor