Can required options be inherited?

[remkop]

Ticket #649 provided support for inherited options.
This is a follow-up ticket for the following scenario.

Problem

Suppose we define a command with a required = true option that is also marked as scope = INHERIT, and a subcommand, like this:

@Command
class App {
    @Option(names = "-x", required = true, scope = INHERIT)
    int x;
    
    @Command void sub() {}
}

Now, if the end user invokes the command with the following parameters:

<cmd> sub -x=2

Currently, validation on the required options of the top-level command is done when the parser detects that a subcommand was invoked. Since at that point the required option has not been matched yet, the parser gives this error: Missing required option '-x=<x>'.

Solutions

1. One idea is to simply (in this release) prevent applications from combining required options from being INHERIT-ed. Picocli could throw an InitializationException at startup when it creates the model.
2. Alternatively, investigate if it is possible (how big a change it is) to postpone the validation of required inherited options. If unmatched required args are not inherited we can fail immediately like we do now. The inherited required args can be passed on to the subcommand; this subcommand should then only verify inherited required args if they are "passed on" (because they have not been matched yet on the parent command). Again, non-inherited required args can be validated unconditionally.