`arg_required_else_help` with no arguments.

[Leandros]

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• [X] I have searched the discussions
• [X] I have searched the open and rejected issues

Rust Version

rustc 1.81.0 (eeb90cda1 2024-09-04)

Clap Version

4.5.21

Minimal reproducible code

use clap::Command;

fn main() {
    let cli = Command::new("repro")
        .subcommand(Command::new("start").arg_required_else_help(true))
        .arg_required_else_help(true);

    let matches = cli.get_matches();
    match matches.subcommand() {
        Some(("start", _args)) => {
            println!("start the engines!");
        }
        Some(_) | None => unreachable!(),
    }
}

Steps to reproduce the bug with the above code

cargo run -- start

Actual Behaviour

It prints the help.

Expected Behaviour

It should print start the engines!; because it executes the start sub-command without arguments, and because there are no required arguments, no arguments are missing.

Additional Context

The same issue happens even if there are non-required arguments. Below is another repro, which includes a non-required argument:

use clap::{Arg, ArgAction, Command};

fn main() {
    let cli = Command::new("repro")
        .subcommand(
            Command::new("start").arg_required_else_help(true).arg(
                Arg::new("verbose")
                    .short('v')
                    .long("verbose")
                    .action(ArgAction::SetTrue)
                    .required(false),
            ),
        )
        .arg_required_else_help(true);

    let matches = cli.get_matches();
    match matches.subcommand() {
        Some(("start", _args)) => {
            println!("start the engines!");
        }
        Some(_) | None => unreachable!(),
    }
}

It still inhibits this behavior.

Debug Output

     Running `target/debug/clap_repro start`
[clap_builder::builder::command]Command::_do_parse
[clap_builder::builder::command]Command::_build: name="repro"
[clap_builder::builder::command]Command::_propagate:repro
[clap_builder::builder::command]Command::_check_help_and_version:repro expand_help_tree=false
[clap_builder::builder::command]Command::long_help_exists
[clap_builder::builder::command]Command::_check_help_and_version: Building default --help
[clap_builder::builder::command]Command::_check_help_and_version: Building help subcommand
[clap_builder::builder::command]Command::_propagate_global_args:repro
[clap_builder::builder::debug_asserts]Command::_debug_asserts
[clap_builder::builder::debug_asserts]Arg::_debug_asserts:help
[clap_builder::builder::debug_asserts]Command::_verify_positionals
[clap_builder::parser::parser]Parser::get_matches_with
[clap_builder::parser::parser]Parser::parse
[clap_builder::parser::parser]Parser::get_matches_with: Begin parsing '"start"'
[clap_builder::parser::parser]Parser::possible_subcommand: arg=Ok("start")
[clap_builder::parser::parser]Parser::get_matches_with: sc=Some("start")
[clap_builder::parser::parser]Parser::parse_subcommand
[ clap_builder::output::usage]Usage::get_required_usage_from: incls=[], matcher=false, incl_last=true
[ clap_builder::output::usage]Usage::get_required_usage_from: unrolled_reqs=[]
[ clap_builder::output::usage]Usage::get_required_usage_from: ret_val=[]
[clap_builder::builder::command]Command::_build_subcommand Setting bin_name of start to "clap_repro start"
[clap_builder::builder::command]Command::_build_subcommand Setting display_name of start to "repro-start"
[clap_builder::builder::command]Command::_build: name="start"
[clap_builder::builder::command]Command::_propagate:start
[clap_builder::builder::command]Command::_check_help_and_version:start expand_help_tree=false
[clap_builder::builder::command]Command::long_help_exists
[clap_builder::builder::command]Command::_check_help_and_version: Building default --help
[clap_builder::builder::command]Command::_propagate_global_args:start
[clap_builder::builder::debug_asserts]Command::_debug_asserts
[clap_builder::builder::debug_asserts]Arg::_debug_asserts:verbose
[clap_builder::builder::debug_asserts]Arg::_debug_asserts:help
[clap_builder::builder::debug_asserts]Command::_verify_positionals
[clap_builder::parser::parser]Parser::parse_subcommand: About to parse sc=start
[clap_builder::parser::parser]Parser::get_matches_with
[clap_builder::parser::parser]Parser::parse
[clap_builder::parser::parser]Parser::add_defaults
[clap_builder::parser::parser]Parser::add_defaults:iter:verbose:
[clap_builder::parser::parser]Parser::add_default_value: doesn't have conditional defaults
[clap_builder::parser::parser]Parser::add_default_value:iter:verbose: has default vals
[clap_builder::parser::parser]Parser::add_default_value:iter:verbose: wasn't used
[clap_builder::parser::parser]Parser::react action=SetTrue, identifier=None, source=DefaultValue
[clap_builder::parser::arg_matcher]ArgMatcher::start_custom_arg: id="verbose", source=DefaultValue
[clap_builder::parser::parser]Parser::push_arg_values: ["false"]
[clap_builder::parser::parser]Parser::add_single_val_to_arg: cur_idx:=1
[clap_builder::parser::parser]Parser::add_defaults:iter:help:
[clap_builder::parser::parser]Parser::add_default_value: doesn't have conditional defaults
[clap_builder::parser::parser]Parser::add_default_value:iter:help: doesn't have default vals
[clap_builder::parser::validator]Validator::validate
[clap_builder::builder::command]Command::write_help_err: repro-start, use_long=false
[  clap_builder::output::help]write_help
[clap_builder::output::help_template]HelpTemplate::new cmd=start, use_long=false
[clap_builder::output::help_template]should_show_arg: use_long=false, arg=verbose
[clap_builder::output::help_template]HelpTemplate::write_templated_help
[clap_builder::output::help_template]HelpTemplate::write_before_help
[ clap_builder::output::usage]Usage::create_usage_no_title
[ clap_builder::output::usage]Usage::create_usage_no_title
[ clap_builder::output::usage]Usage::write_help_usage
[ clap_builder::output::usage]Usage::write_arg_usage; incl_reqs=true
[ clap_builder::output::usage]Usage::needs_options_tag
[ clap_builder::output::usage]Usage::needs_options_tag:iter: f=verbose
[clap_builder::builder::command]Command::groups_for_arg: id="verbose"
[ clap_builder::output::usage]Usage::needs_options_tag:iter: [OPTIONS] required
[ clap_builder::output::usage]Usage::write_args: incls=[]
[ clap_builder::output::usage]Usage::get_args: unrolled_reqs=[]
[ clap_builder::output::usage]Usage::write_subcommand_usage
[ clap_builder::output::usage]Usage::create_usage_no_title: usage=clap_repro start [OPTIONS]
[clap_builder::output::help_template]HelpTemplate::write_all_args
[clap_builder::output::help_template]should_show_arg: use_long=false, arg=verbose
[clap_builder::output::help_template]should_show_arg: use_long=false, arg=help
[clap_builder::output::help_template]HelpTemplate::write_args Options
[clap_builder::output::help_template]should_show_arg: use_long=false, arg=verbose
[clap_builder::output::help_template]HelpTemplate::write_args: arg="verbose" longest=9
[clap_builder::output::help_template]should_show_arg: use_long=false, arg=help
[clap_builder::output::help_template]HelpTemplate::write_args: arg="help" longest=9
[clap_builder::output::help_template]should_show_arg: use_long=false, arg=verbose
[clap_builder::output::help_template]HelpTemplate::spec_vals: a=--verbose
[clap_builder::output::help_template]should_show_arg: use_long=false, arg=help
[clap_builder::output::help_template]HelpTemplate::spec_vals: a=--help
[clap_builder::output::help_template]HelpTemplate::spec_vals: a=--verbose
[clap_builder::output::help_template]HelpTemplate::short
[clap_builder::output::help_template]HelpTemplate::long
[clap_builder::output::help_template]HelpTemplate::align_to_about: arg=verbose, next_line_help=false, longest=9
[clap_builder::output::help_template]HelpTemplate::align_to_about: positional=false arg_len=9, spaces=2
[clap_builder::output::help_template]HelpTemplate::help
[clap_builder::output::help_template]HelpTemplate::help: help_width=17, spaces=0, avail=83
[clap_builder::output::help_template]HelpTemplate::spec_vals: a=--help
[clap_builder::output::help_template]HelpTemplate::short
[clap_builder::output::help_template]HelpTemplate::long
[clap_builder::output::help_template]HelpTemplate::align_to_about: arg=help, next_line_help=false, longest=9
[clap_builder::output::help_template]HelpTemplate::align_to_about: positional=false arg_len=6, spaces=5
[clap_builder::output::help_template]HelpTemplate::help
[clap_builder::output::help_template]HelpTemplate::help: help_width=17, spaces=10, avail=83
[clap_builder::output::help_template]HelpTemplate::write_after_help
[clap_builder::builder::command]Command::color: Color setting...
[clap_builder::builder::command]Auto
[clap_builder::builder::command]Command::color: Color setting...
[clap_builder::builder::command]Auto
Usage: clap_repro start [OPTIONS]

Options:
  -v, --verbose
  -h, --help     Print help

[epage]

It should print start the engines!; because it executes the start sub-command without arguments, and because there are no required arguments, no arguments are missing.

I think there might be confusion with arg_required_else_help. It isn't saying "if an argument is required and missing, print the help" but "any argument is required and if one isn't present, print the help". See also the docs.

So in case given, start has arg_required_else_help(true) and with cargo run -- start, no arguments are passed to start, so help is printed. This is working as expected. Users need to determine whether arg_required_else_help is pertinent for their CLI. For example, clap_derive only sets it when a subcommand is required as that is the only time it knows for sure that an argument is required.

[Leandros]

Interesting, so it's working as expected. Somehow, for me personally, this wasn't clear from the one-liner in the documentation that you're referring to.

I'll close this.