Swift optional generic parameters?

generics_playground.swift

protocol FactoryAType {
    typealias Product
}

protocol FactoryBType {
    typealias Product
}

struct CombinedFactory<T: FactoryAType, U: FactoryBType where T.Product == U.Product> {
    let factoryA: T
    let factoryB: U?

    init(factoryA: T, factoryB: U? = nil) {
        self.factoryA = factoryA
        self.factoryB = factoryB
    }
}

struct IntAFactory: FactoryAType {
    typealias Product = Int
}

struct IntBFactory: FactoryBType {
    typealias Product = Int
}

let fa = IntAFactory()
let fb = IntBFactory()

let allGood = CombinedFactory(factoryA: fa, factoryB: fb)

// I usually write this--it'd be declared this way as part of a type, anyway:
let combinedFactory: CombinedFactory<IntAFactory, IntBFactory> = CombinedFactory(factoryA: fa)