class MapLike<K, V> {
  final Map<K, V> _map = {};
  V? get(Object key) => _map[key];
  V put(K key, V value) => _map[key] = value;
}

extension Extension<K, V> on MapLike<K, V> {
  V? operator [](Object key) => get(key);
  void operator []=(K key, V value) => put(key, value);
}

class MapLike2<K, V> {
  final Map<K, V> _map = {};
  final V defaultValue;
  MapLike2(this.defaultValue);
  V get(Object key) => _map[key] ?? defaultValue;
  V put(K key, V value) => _map[key] = value;
}

extension Extension2<K, V> on MapLike2<K, V> {
  V operator [](Object key) => get(key);
  void operator []=(K key, V value) => put(key, value);
}

main() {}
implicit() {}
explicitWithTypeArguments() {}
explicitInferredTypeArguments() {}
expect(expected, actual) {}