Overridden type mismatch
Reports the overridden methods (the methods with the same selector in the overridden classes) that have parameter or return types different from the base method's types. The return type is
covariant, which means that the overridden method can return any subtype of the base method's return type. Parameter types are
contravariant, which means that the overridden method can have parameters of any superclass of the base method's parameter types. Covariance and contravariance are applicable only to object types.