Pass None to default_value_if, or better adding default_value_if_not_set?

[runapp]

Please complete the following tasks

• I have searched the discussions
• I have searched the existing issues

Clap Version

2.33.3

Describe your use case

I found that I can't make an option that only has default value if another option (doesn't takes value) is not set. The closest way is like this:

    let matches = App::new("test")
        .arg(Arg::with_name("A").short("A"))
        .arg(Arg::with_name("B").short("B").default_value_if("A", None, "").default_value("A not set"))
        .get_matches();
    println!("A={} B={}",
        matches.value_of("A").unwrap_or("###"),
        matches.value_of("B").unwrap_or("###"),
    );

I was thinking of passing None to default_value_if might help emulate as if -B is never set (is_present("B")==false). But default_value_if only accepts &str rather than Option.

Describe the solution you'd like

I don't know which would be better. Modifying current signature of default_value_if seems painful, and might bring endless Some to most common cases. So would add default_value_if_not_set be a choice?

Alternatives, if applicable

No response

Additional Context

No response

[pksunkara]

This is fixed on v3/master through a PR. What search terms did you use when searching for issues like this as requested?