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)

// Does not work:
// Cannot invoke initializer for type 'CombinedFactory<_, _>' with an argument list of type '(factoryA: IntAFactory)'
let combinedFactory = CombinedFactory(factoryA: fa)

// This works:
// But this is awkward. I don't want to have to specify B if it is nil
let workingFactory = CombinedFactory<IntAFactory, IntBFactory>(factoryA: fa)

Updated my playing around here: https://gist.github.com/rnapier/44bf23300959cca9c43f

Thanks guys!

So the full context is https://github.com/jessesquires/JSQDataSourcesKit.

Specifically: https://github.com/jessesquires/JSQDataSourcesKit/blob/develop/JSQDataSourcesKit/JSQDataSourcesKit/CollectionViewDataSourceProvider.swift#L34

"Factory" is already sounding like something un-Swifty.

Ha -- probably because I'm trying to make Cocoa less cubersome. I hate that UICollectionView/UITableView delegates/dataSources are littered with @optional methods 😢