2. Cast Types Back

This is the opposite of what we just did. Instead of removing types, we're adding types back:

AnyKeyPath as? WritableKeyPath<User, String>
PartialKeyPath<User> as? KeyPath<User, Bool>

Sometimes you might need to add generics in order to keep the necessary types around to perform the correct casting. An example of this was how we wrote

func editSettings<Provider: SettingsProvider>(provider: Provider)

instead of the simpler:

func editSettings(provider: SettingsProvider)

The second version would work just as fine, but it would not allow us to use Provider as the Root type in our WritableKeyPath<Provider, Bool> cast.