Full sample code for article "How to Achieve Dynamic Dispatch Using Generic Protocols in Swift 5.7" (https://swiftsenpai.com/uncategorized/dynamic-dispatch-with-generic-protocols)

dynamic-dispatch-generic-full.swift

protocol Vehicle {

    associatedtype FuelType: Fuel
    
    var name: String { get }

    func startEngin()
    func fillGasTank(with fuel: FuelType)
}

struct Car: Vehicle {

    let name: String

    func startEngin() {
        print("\(name) enjin started!")
    }
    
    func fillGasTank(with fuel: Gasoline) {
        print("Fill \(name) with \(fuel.name).")
    }
}

struct Bus: Vehicle {

    let name: String
    
    func startEngin() {
        print("\(name) enjin started!")
    }

    func fillGasTank(with fuel: Diesel) {
        print("Fill \(name) with \(fuel.name).")
    }
}

protocol Fuel {
    associatedtype FuelType where FuelType == Self
    static func purchase() -> FuelType
}

struct Gasoline: Fuel {
    
    let name = "gasoline"
    
    static func purchase() -> Gasoline {
        print("Purchase gasoline from gas station.")
        return Gasoline()
    }
}

struct Diesel: Fuel {
    
    let name = "diesel"
    
    static func purchase() -> Diesel {
        print("Purchase diesel from gas station.")
        return Diesel()
    }
}

// MARK: - Functions
func startAllEngin(for vehicles: [any Vehicle]) {
    for vehicle in vehicles {
        vehicle.startEngin()
    }
}

func fillAllGasTank(for vehicles: [any Vehicle]) {
    for vehicle in vehicles {
        fillGasTank(for: vehicle)
    }
}

func fillGasTank(for vehicle: some Vehicle) {

    let fuel = type(of: vehicle).FuelType.purchase()
    vehicle.fillGasTank(with: fuel)
}

// MARK: - Execution
let vehicles: [any Vehicle] = [
    Car(name: "Car_1"),
    Car(name: "Car_2"),
    Bus(name: "Bus_1"),
    Car(name: "Car_3"),
]

startAllEngin(for: vehicles)
fillAllGasTank(for: vehicles)

As of now (Swift 5.7), there is only 1 way to convert opaque type to existential type and vice versa, which is by using a function parameter. Thus using as will not work. Furthermore, in order for the conversion to work, the function parameter must be of type any or some, using [any] or [some] will not work.