Dart cannot override function in generic class with subclass of generic

Question

I am writing an app which deals with both cryptos and stocks. Because they share many common characteristics such as symbol, price, name etc, I created a base class 'instrument' then subclasses for stock and crypto. I have a generic 'manager' class for both, as loading, saving etc are again very similar. However, there are some functions, such as fetching a current price, that need to be implemented differently, so I have extended my generic manager class and I am trying to override the relevant function, i.e. fetchCurrentPrice() in the example below. Dart complains that

'CryptoManager.fetchCurrentPrice' ('Future Function(Crypto)') isn't a valid override of 'Manager.fetchCurrentPrice' ('Future Function(Instrument)').

I don't understand this, as I have defined as type in the manager, and Crypto extends Instrument, so I am fulfilling this criterion. Any idea where I am going wrong?

abstract class Instrument {
  String name = 'Instrument';
  double currentPrice = 0.0;
  Instrument(this.name);
}

class Crypto extends Instrument {
  Crypto(super.name);
  String contractAddress = '0xBCCFF3FF6...';
}

abstract class Manager<T extends Instrument> {
  List<T> instruments = [];

  Future<double> fetchCurrentPrice(T instrument) async {
    print('Fetching price for generic instrument');
    return 12.80;
  }

}

class CryptoManager extends Manager {

  // this is causing the issue  
  @override
    Future<double> fetchCurrentPrice(Crypto instrument) async {
    print('Fetching price for crypto');
    return 12.80;
  }
}


void main(List<String> args) {
  CryptoManager cryptoManager = CryptoManager();
  var btcCrypto = Crypto('BTC');
  cryptoManager.fetchCurrentPrice(btcCrypto);
}

Answer 1

Your class CryptoManager needs to extend Manager<Crypto>, not just "some Manager, kinda".

class CryptoManager extends Manager<Crypto>