required_if and and default values

[kbknapp]

From #824

extern crate clap;

use clap::{Arg, App};

fn main() {
    let m = App::new("Test app")
        .version("1.0")
        .author("F0x06")
        .about("Arg test")
        .arg(Arg::with_name("target")
            .takes_value(true)
            .default_value("file")
            .possible_values(&["file", "stdout"])
            .long("target"))
        .arg(Arg::with_name("input")
            .takes_value(true)
            .required(true)
            .long("input"))
        .arg(Arg::with_name("output")
            .takes_value(true)
            .required_if("target", "file")
            .long("output"))
        .get_matches_from_safe(vec!["test", "--input", "some"]);

    assert!(m.is_err());
    assert_eq!(m.unwrap_err().kind, ErrorKind::MissingRequiredArgument);
}