protocol State {}

struct Transition<From: State, To: State> {
    let transition: () -> To
}

struct Machine<CurrentState: State> {

    var state: CurrentState

    func transition<To>(with transition: Transition<CurrentState, To>) -> Machine<To> where To: State {
        return Machine<To>(state: transition.transition())
    }
}

struct Pending: State {}
struct Loading: State {}
struct Loaded: State {}
struct Failed: State {}

let kickoff = Transition<Pending, Loading>(transition: { return Loading() })

let finishLoading = Transition<Loading, Loaded>(transition: { return Loaded() })

let machine = Machine<Pending>(state: Pending())

let newMachine = machine.transition(with: kickoff) // compiles
newMachine.transition(with: finishLoading)
machine.transition(with: finishLoading) // Cannot convert value of type 'Transition<Loading, Loaded>' to expected argument type 'Transition<Pending, _>'