type problem in lists

main.dart

import 'dart:async';

class A<T>{
  A(this.val,this.dispose);
  T val;
  FutureOr Function(T) dispose;
}

List<A> allA = <A>[];

void main() {
  var instance = A<int>(42,(_){print('disposed int');});
  var instance2 = A<String>('42',(_){print('disposed string');});
  
  allA.add(instance);  
  allA.add(instance2);
  
  for(final a in allA){
    a.dispose(a.val);
  }

}