2つのOptional型の値を場合分けして処理する書き方（場合分け時の処理は自分の環境での例です）

original.swift

func logA(tag: String, param: [String: String]) {
	print("\(tag) : \(param)")
}

func logB(tag: String) {
	print("\(tag) : param なし")
}

func main0(param: [String: String]?, id: String?) {
	if var param = param {
		if let id = id {
			param["b"] = id
		}
		logA(tag: "main0", param: param)
	} else {
		if let id = id {
			let param = ["b": id]
			logA(tag: "main0", param: param)
		} else {
			logB(tag: "main0")
		}
	}
}

refactor1.swift

func main1(param: [String: String]?, id: String?) {
	switch (param, id) {
	case (var param?, let id):
		param["b"] = id
		logA(tag: "main1", param: param)
	case (let param?, nil):
		logA(tag: "main1", param: param)
	case (nil, let id?):
		let param = ["b": id]
		logA(tag: "main1", param: param)
	case (nil, nil):
		logB(tag: "main1")
	}
}

refactor2.swift

func main2(param: [String: String]?, id: String?) {
	if param == nil && id == nil {
		logB(tag: "main2")
		return
	}
	
	var param = param ?? [String: String]()
	param["b"] = id
	logA(tag: "main2", param: param)
}