main.dart

void main() {
  var list1 = ['I', '💙', 'Flutter'];

  final list2 = list1;
  list2[2] = 'Dart';

// const list3 = list1;
}

class User {
  final String id;
  final String? subscriptionId;

  const User(
    this.subscriptionId, {
    required this.id,
  });
}

class Subscription {
  final String id;

  const Subscription({
    required this.id,
  });
}

Subscription getSubscription({
  required String id,
}) =>
    subscriptions.firstWhere((element) => element.id == id);

final subscriptions = [];

// final subscription = getSubscription(user.id);

extension type SubscriptionId(String id) {}

extension type UserId(String id) {}