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Code

use std::io::stdout;
use clap::{App, Arg, SubCommand, Shell};

fn app() -> App<'static, 'static> {
    App::new("completion-bug")
        .arg(
            Arg::with_name("foo")
                .short("f")
                .long("foo")
                .takes_value(true)
        )
        .subcommand(SubCommand::with_name("zsh"))
        .subcommand(SubCommand::with_name("bash"))
}

fn main() {
    let args = app().get_matches();
    eprintln!("{:?}", args);

    let shell = match args.subcommand() {
        ("zsh", _) => Shell::Zsh,
        ("bash", _) => Shell::Bash,
        _ => unreachable!(),
    };

    app().gen_completions_to("completion-bug", shell, &mut stdout());
}

Steps to reproduce the issue

cargo run -- zsh > ~/.zsh_completions/_completion-bug # you might need to tweak this for your setup
zsh

# this works
completion-bug z<tab>
# this doesn't
completion-bug -f x z<tab>
# this does something unexpected
completion-bug -f z<tab>

Version

• Rust: 1.42.0
• Clap: 2.33.0

Actual Behavior Summary

completion-bug -f x z<tab> doesn't complete anything.

This affects https://github.com/kpcyrd/sn0int because it has a lot of nested subcommands but it's very common to start the command with sn0int -w foo which breaks the completions for everything afterwards.

The bash completions work correctly.

Expected Behavior Summary

completion-bug -f x z<tab> should complete to completion-bug -f x zsh

Additional context

This bug was originally reported as TeXitoi/structopt#369 and then ported over to pure clap. The completions generated by the structopt program and the clap program are identical.