clap: Combination of options causes clap to apply validator to both default and user-specified option

rustc version: rustc 1.12.0 (3191fbae9 2016-09-23) clap Version: 2.14.0

In one of my new projects, I noticed that it was dying with an error about the default argument value failing “path is readable” validation, even when I was trying to override it with an explicit argument.

I started working on a simplified testcase and found that this bug is very picky. If any of these conditions aren’t met, it will act properly and validate either the default or the user-provided value but not both:

• global(true) must be specified
• A default_value must be specified
• A validator must be provided
• A subcommand must be defined and passed on the command line

…but, if all of them are, it insists on validating both the default and the user-provided value.

It’s possible there’s more I could discover, but I’m sick and it’s getting late, so here’s how far I managed to minimize and pin down the test case so far:

extern crate clap;
use clap::{App,Arg,SubCommand};

const DEFAULT_INPATH: &'static str = "/dev/sr1";

fn main() {
    App::new("testcase")
        .arg(Arg::with_name("inpath")
            .global(true)
            .default_value(DEFAULT_INPATH)
            .validator(|x| {
                print!("Testing {}\n", x);
                if x == DEFAULT_INPATH {
                    print!("ERROR: Validator ran on default string\n");
                }
                Ok(())
            }))
        .subcommand(SubCommand::with_name("test"))
        .get_matches_from(&["testcase", "/dev/sr0", "test"]);
}

EDIT: I also just noticed that it’s validating the default after the user-provided value:

ssokolow@monolith testcase % cargo run            
    Finished debug [unoptimized + debuginfo] target(s) in 0.0 secs
     Running `target/debug/testcase`
Testing /dev/sr0
Testing /dev/sr1
ERROR: Validator ran on default string