struct Order: Equatable { let id: String }
enum SaveError: Error { case invalidID, unavailable }

protocol OrderStore {
    func insert(_ order: Order) throws
}

struct OrderService {
    let store: any OrderStore
    func save(_ order: Order) throws {
        guard !order.id.isEmpty else { throw SaveError.invalidID }
        try store.insert(order)
    }
}

final class RecordingStore: OrderStore {
    var calls: [Order] = []
    var shouldFail = false
    func insert(_ order: Order) throws {
        calls.append(order)
        if shouldFail { throw SaveError.unavailable }
    }
}

let store = RecordingStore()
let service = OrderService(store: store)
let order = Order(id: "order-42")
try service.save(order)
precondition(store.calls == [order])
do {
    try service.save(Order(id: ""))
    preconditionFailure("invalid input was accepted")
} catch SaveError.invalidID {}
precondition(store.calls.count == 1)
store.shouldFail = true
do {
    try service.save(order)
    preconditionFailure("storage failure was swallowed")
} catch SaveError.unavailable {}
precondition(store.calls.count == 2)
print("saved=order-42 invalid-input-writes=0 storage-error=propagated")