CommandParser ignores parent default options when parsing leaf command

[dspx-plcr]

Consider the following setup,

use "cli"

actor Main
  new create(env: Env) =>
    let spec = try make_spec()? else return end
    let cmd =
      match CommandParser(spec).parse(env.args, env.vars)
      | let c: Command => c
      else return
      end

    env.out.print("Value of arg: " + cmd.option("arg").string())

  fun make_spec(): CommandSpec? =>
    let root = CommandSpec.parent("cmd", "My cmd",
      [ OptionSpec.string("arg", "an arg" where default' = "foo") ])?
    let sub = CommandSpec.leaf("sub", "My subcmd")?

    root.add_command(sub)?
    root.add_help()?
    root

When running this, as ./cmd sub, Value of arg: is printed to the command line. I believe the problem is in command_parser.pony on line 101, where parsing a parent command recurses into parsing a sub-command and then returns. Filling in of default options would occur later, on line 147, but we never get that far.

Two possibilities which spring to mind are

1. when adding a spec to a parent, add a back-reference in the child so that when options() is called, it returns non-duplicated parent options
2. don't return from parsing a sub command, but keep track of the fact we've already parsed one sub-command

both suggestions raise questions around duplicated options between the parent and child, and how they should work.

As a motivating example, my use case is to have a config file for my command line utility which should apply to all sub-commands, but defaults to a sensible value if the user doesn't specify one.

Also, when debugging this issue, I found it would have been useful to know if an option was set, rather than having just a default fake option returned. There's a couple of places this could be done, but I think the easiest is to have an is_set or something similar on the Command class.