Want to use a name already taken by the standard library for a nested type? No problem - just use Swift. to disambiguate:

extension Command {
    enum Error: Swift.Error {
        case missing
        case invalid(String)
    }
}

Support Swift by Sundell by checking out this sponsor:

Raycast: Take the macOS Spotlight experience to the next level: Create Jira issues, manage GitHub pull requests and control other tools with a few keystrokes. Easily automate every-day tasks and boost your developer productivity by downloading Raycast for free.