Patch models with dynamic member lookup

Patch.swift

@dynamicMemberLookup
struct Patch<T> {
    private var patches: [PartialKeyPath<T>: Any] = [:]

    subscript<U>(dynamicMember keyPath: WritableKeyPath<T, U>) -> U? {
        get { patches[keyPath] as? U }
        set { patches[keyPath] = newValue }
    }

    var modifiedFields: Set<PartialKeyPath<T>> {
        Set(patches.keys)
    }

    func hasChanges(in keyPath: PartialKeyPath<T>) -> Bool {
        patches[keyPath] != nil
    }

    func apply(to instance: T) -> T {
        var result = instance
        for (keyPath, value) in patches {
            if let writableKeyPath = keyPath as? WritableKeyPath<T, Any> {
                result[keyPath: writableKeyPath] = value
            }
        }
        return result
    }
}