default_value() and multiple(true) interact in surprising ways.

[Screwtapello]

Rust Version

rustc 1.20.0 (f3d6973f4 2017-08-27)

Affected Version of clap

clap 2.26.2

Steps to Reproduce the issue

I want to write a program that's a little bit like ssh: it does some setup, then runs the rest of the command-line arguments as a new process. If there are no other arguments, it just runs a shell. Looking at the clap documentation, I came up with the following pattern:

extern crate clap;

fn main() {
    let matches = clap::App::new("claptest")
        .setting(clap::AppSettings::TrailingVarArg)
        .arg(clap::Arg::with_name("command")
            .multiple(true)
            .default_value("/bin/sh")
        )
        .get_matches();

    let command: Vec<_> = matches.values_of_os("command").unwrap().collect();

    println!("Command to run: {:?}", command);
}                                                                               

If I build this and run it with no arguments, I get the default, just as I wanted:

$ ./target/debug/claptest 
Command to run: ["/bin/sh"]

If I give it a different command to run, I get the default appended to the command I gave:

$ ./target/debug/claptest foo bar
Command to run: ["foo", "bar", "/bin/sh"]

If there's an explicit value on the command-line, I'd expect that to replace the default, not extend it:

$ ./target/debug/claptest foo bar
Command to run: ["foo", "bar"]

Workarounds

If I don't use .default_value(), I can manually apply the default later, but of course then it doesn't show up in the --help output with all the other defaults.

.occurrences_of() returns 0 if only the default is present, 1 if there's one explicit argument, etc. I could do something like:

let command: Vec<_> = if matches.occurrences_of("command") > 0 {
    matches.values_of_os("command").unwrap().take(matches.occurrences_of("command"))
} else {
    matches.values_of_os("command").unwrap()
}.collect();

...but man, that's a mouthful.

[kbknapp]

This is a bug, thanks for reporting!

I'd be willing to mentor someone to fix this, which should be a super quick fix if someone wants an easy PR. Otherwise I can try and knock it out this week once I get a little caught up.

If anyone is interested, the code to potentially fix this issue is here. You'd simply need to ensure there are no values before adding the defaults.

[kbknapp]

Duplicate of #1050