Add completion for default subcommand to supercommand

[Samasaur1]

As it currently stands, although a command can designate its default subcommand and argument parsing works correctly, there is no completion for arguments, options, or flags of the subcommand unless the subcommand name is specified — which eliminates much of the convenience of having a default subcommand. I'd like it if the completion for the default subcommand was added to the list of all subcommands as the completion for the super command.

Here's an example¹:

@main
struct Greeter: ParsableCommand {
    static let configuration = CommandConfiguration(subcommands: [Informal.self, Formal.self], defaultSubcommand: Informal.self)

    struct Informal: ParsableCommand {
        @Option var firstName: String

        func run() throws {
            print("Hi, \(firstName)")
        }
    }

    struct Formal: ParsableCommand {
        @Option var fullName: String

        func run() throws {
            print("Good day, \(fullName)")
        }
    }
}

Here's what happens if you run it:

$ greeter informal --f<Tab> # successfully completes to the below line
$ greeter informal --firstName

$ greeter formal --f<Tab> # successfully completes to the below line
$ greeter formal --fullName

# here's the problem
$ greeter <Tab> # lists the following completions
--help    -h        formal    help      informal
$ greeter --f<Tab> # no completions

Instead, I would like greeter <Tab> to list all subcommands (as it currently does) and all the completions for its default command, so:

$ greeter <Tab>
--first-name  --help        -h            formal        help          informal

For backwards compatibility and user configurability, it might make sense to make this an optional parameter to the @Argument/@Option/@Flag initializer (or perhaps CompletionKind?)

Footnotes

1. I made up this code sample because the project I was working on when I found this error is more complex, but this should get the point across ↩