@dynamicMemberCallable with KeyPath

Passthrough.swift

@dynamicMemberLookup
protocol Passthrough {
    associatedtype Previous
    var previous: Previous { get }
    subscript<T>(dynamicMember keyPath: KeyPath<Previous, T>) -> T { get }
}

extension Passthrough {
    subscript<T>(dynamicMember keyPath: KeyPath<Previous, T>) -> T {
        return previous[keyPath: keyPath]
    }
}

struct ObjectOne: Passthrough {
    var previous: Void
    var valueOne: String
}

struct ObjectTwo: Passthrough {
    var previous: ObjectOne
    let valueTwo: String
}

struct ObjectThree: Passthrough {
    let previous: ObjectTwo
    let valueThree: String
}

let objectOne = ObjectOne(previous: (), valueOne: "One")
let objectTwo = ObjectTwo(previous: objectOne, valueTwo: "Two")
let objectThree = ObjectThree(previous: objectTwo, valueThree: "Three")

objectThree.valueOne
objectThree.valueTwo
objectThree.valueThree