Arguments with takes_value(false) still accept "compact"-style arguments

[sfackler]

Rust Version

• 1.37.0

Affected Version of clap

• 2.33.0

Bug or Feature Request Summary

Arguments configured to not take a value still successfully parse from the --key=value form.

Expected Behavior Summary

The command line should fail to validate (or only accept --key=true or --key=false.

Actual Behavior Summary

The command line successfully validates as if it just contained --key.

Sample Code or Link to Sample Code

use clap::*;

fn main() {
    let matches = App::new("foo")
        .arg(Arg::with_name("foobar").long("foobar").takes_value(false).multiple(false))
        .get_matches_from(["foo", "--foobar=false"].iter());
    println!("{:?}", matches)
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=d08679674fa982b892dcc26d8ed8bb25

[haxpor]

Can we reopen this issue? I tested this from the tip of the git tree and the problem is still happening, also I faced the same issue on my own code using 3.1.6.

Modify tests/builder/multiple_occurrences.rs (just for the sake of quick dirty test)

#[cfg(feature = "env")]
#[test]
fn multiple_occurrences_of_before_env() {
    let cmd = Command::new("mo_before_env").arg(
        Arg::new("verbose")
            .env("VERBOSE")
            .short('v')
            .long("verbose")
            .takes_value(false)
            .multiple_occurrences(true),
    );

    let m = cmd.clone().try_get_matches_from(vec![""]);
    assert!(m.is_ok(), "{}", m.unwrap_err());
    assert_eq!(m.unwrap().occurrences_of("verbose"), 0);

    let m = cmd.clone().try_get_matches_from(vec!["", "-v"]);
    assert!(m.is_ok(), "{}", m.unwrap_err());
    assert_eq!(m.unwrap().occurrences_of("verbose"), 1);

    let m = cmd.clone().try_get_matches_from(vec!["", "-vv"]);
    assert!(m.is_ok(), "{}", m.unwrap_err());
    assert_eq!(m.unwrap().occurrences_of("verbose"), 2);
    let m = cmd.clone().try_get_matches_from(vec!["", "-vvv"]);
    assert!(m.is_ok(), "{}", m.unwrap_err());
    assert_eq!(m.unwrap().occurrences_of("verbose"), 3);

    // this chunk of code is added to test
    // notice `-vvv=false`, test result is fine, no error out
    let m = cmd.clone().try_get_matches_from(vec!["", "-vvv=false"]);
    assert!(m.is_ok(), "{}", m.unwrap_err());
    assert_eq!(m.unwrap().occurrences_of("verbose"), 3);
}

as well I didn't see relevant changes that fix this issue in #2619.
My apology if I'm wrong nor missing something.

[epage]

I just tried reproducing with the code sample you gave and it failed

thread 'multiple_occurrences::multiple_occurrences_of_before_env' panicked at 'error: Found argument '-=' which wasn't expected, or isn't valid in this context

        If you tried to supply `-=` as a value rather than a flag, use `-- -=`

USAGE:
    mo_before_env [OPTIONS]

For more information try --help
', tests/builder/multiple_occurrences.rs:128:5