#[clap(num_args = 2..)] is valid with 0 arguments

[Rignchen]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Rust Version

rustc 1.79.0 (129f3b996 2024-06-10)

Clap Version

4.5.9

Minimal reproducible code

use clap::Parser;

#[derive(Parser)]
struct Args {
    #[clap(num_args = 2..)]
    foo: Vec<String>
}

fn main() {
    let args = Args::parse();
    println!("{:?}", args.foo);
}

Steps to reproduce the bug with the above code

$ cargo run --
[]
$ cargo run -- bar
error: 2 values required by '[FOO] [FOO]...'; only 1 was provided
$ cargo run -- bar foobar
["bar", "foobar"]

Actual Behaviour

the program runs when no arguments are given

Expected Behaviour

the code should break when no arguments are given

Additional Context

No response

Debug Output

$ cargo run --
[clap_builder::builder::command]Command::_do_parse
[clap_builder::builder::command]Command::_build: name="clap"
[clap_builder::builder::command]Command::_propagate:clap
[clap_builder::builder::command]Command::_check_help_and_version:clap 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:clap
[clap_builder::builder::debug_asserts]Command::_debug_asserts
[clap_builder::builder::debug_asserts]Arg::_debug_asserts:foo
[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::add_defaults
[clap_builder::parser::parser]Parser::add_defaults:iter:foo:
[clap_builder::parser::parser]Parser::add_default_value: doesn't have conditional defaults
[clap_builder::parser::parser]Parser::add_default_value:iter:foo: doesn't have default vals
[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::parser::validator]Validator::validate_conflicts
[clap_builder::parser::validator]Validator::validate_exclusive
[clap_builder::parser::validator]Validator::validate_required: required=ChildGraph([])
[clap_builder::parser::validator]Validator::gather_requires
[clap_builder::parser::validator]Validator::validate_required: is_exclusive_present=false
[clap_builder::parser::arg_matcher]ArgMatcher::get_global_values: global_arg_vec=[]
[]

$ cargo run -- bar
[clap_builder::builder::command]Command::_do_parse
[clap_builder::builder::command]Command::_build: name="clap"
[clap_builder::builder::command]Command::_propagate:clap
[clap_builder::builder::command]Command::_check_help_and_version:clap 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:clap
[clap_builder::builder::debug_asserts]Command::_debug_asserts
[clap_builder::builder::debug_asserts]Arg::_debug_asserts:foo
[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::get_matches_with: Begin parsing '"bar"'
[clap_builder::parser::parser]Parser::possible_subcommand: arg=Ok("bar")
[clap_builder::parser::parser]Parser::get_matches_with: sc=None
[clap_builder::parser::parser]Parser::get_matches_with: Positional counter...1
[clap_builder::parser::parser]Parser::get_matches_with: Low index multiples...false
[clap_builder::parser::parser]Parser::resolve_pending: id="foo"
[clap_builder::parser::parser]Parser::react action=Append, identifier=Some(Index), source=CommandLine
[ clap_builder::output::usage]Usage::create_usage_with_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=help
[ clap_builder::output::usage]Usage::needs_options_tag:iter Option is built-in
[ clap_builder::output::usage]Usage::needs_options_tag: [OPTIONS] not 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_with_title: usage=Usage: clap [FOO] [FOO]...
[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
error: 2 values required by '[FOO] [FOO]...'; only 1 was provided

$ cargo run -- bar foobar
[clap_builder::builder::command]Command::_do_parse
[clap_builder::builder::command]Command::_build: name="clap"
[clap_builder::builder::command]Command::_propagate:clap
[clap_builder::builder::command]Command::_check_help_and_version:clap 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:clap
[clap_builder::builder::debug_asserts]Command::_debug_asserts
[clap_builder::builder::debug_asserts]Arg::_debug_asserts:foo
[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::get_matches_with: Begin parsing '"bar"'
[clap_builder::parser::parser]Parser::possible_subcommand: arg=Ok("bar")
[clap_builder::parser::parser]Parser::get_matches_with: sc=None
[clap_builder::parser::parser]Parser::get_matches_with: Positional counter...1
[clap_builder::parser::parser]Parser::get_matches_with: Low index multiples...false
[clap_builder::parser::parser]Parser::get_matches_with: Begin parsing '"foobar"'
[clap_builder::parser::parser]Parser::get_matches_with: Positional counter...1
[clap_builder::parser::parser]Parser::get_matches_with: Low index multiples...false
[clap_builder::parser::parser]Parser::resolve_pending: id="foo"
[clap_builder::parser::parser]Parser::react action=Append, identifier=Some(Index), source=CommandLine
[clap_builder::parser::parser]Parser::remove_overrides: id="foo"
[clap_builder::parser::arg_matcher]ArgMatcher::start_custom_arg: id="foo", source=CommandLine
[clap_builder::builder::command]Command::groups_for_arg: id="foo"
[clap_builder::parser::arg_matcher]ArgMatcher::start_custom_arg: id="Args", source=CommandLine
[clap_builder::parser::parser]Parser::push_arg_values: ["bar", "foobar"]
[clap_builder::parser::parser]Parser::add_single_val_to_arg: cur_idx:=1
[clap_builder::parser::parser]Parser::add_single_val_to_arg: cur_idx:=2
[clap_builder::parser::arg_matcher]ArgMatcher::needs_more_vals: o=foo, pending=0
[clap_builder::parser::arg_matcher]ArgMatcher::needs_more_vals: expected=2..=18446744073709551615, actual=0
[clap_builder::parser::parser]Parser::react not enough values passed in, leaving it to the validator to complain
[clap_builder::parser::parser]Parser::add_defaults
[clap_builder::parser::parser]Parser::add_defaults:iter:foo:
[clap_builder::parser::parser]Parser::add_default_value: doesn't have conditional defaults
[clap_builder::parser::parser]Parser::add_default_value:iter:foo: doesn't have default vals
[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::groups_for_arg: id="foo"
[clap_builder::parser::validator]Conflicts::gather_direct_conflicts id="foo", conflicts=[]
[clap_builder::parser::validator]Conflicts::gather_direct_conflicts id="Args", conflicts=[]
[clap_builder::parser::validator]Validator::validate_conflicts
[clap_builder::parser::validator]Validator::validate_exclusive
[clap_builder::parser::validator]Validator::validate_conflicts::iter: id="foo"
[clap_builder::parser::validator]Conflicts::gather_conflicts: arg="foo"
[clap_builder::parser::validator]Conflicts::gather_conflicts: conflicts=[]
[clap_builder::parser::validator]Validator::validate_required: required=ChildGraph([])
[clap_builder::parser::validator]Validator::gather_requires
[clap_builder::parser::validator]Validator::gather_requires:iter:"foo"
[clap_builder::parser::validator]Validator::gather_requires:iter:"Args"
[clap_builder::parser::validator]Validator::gather_requires:iter:"Args":group
[clap_builder::parser::validator]Validator::validate_required: is_exclusive_present=false
[clap_builder::parser::arg_matcher]ArgMatcher::get_global_values: global_arg_vec=[]
["bar", "foobar"]

[epage]

Taken from #5526:

With num_args set to 2.. and user gives 0, it does not produce an error as it should because the minimum number of args is 2

The behavior is correct. From the docs:

Specifies the number of arguments parsed per occurrence

This isn't as obvious for positionals but for options, the behavior is more clear.

use clap::Parser;

#[derive(Parser)]
struct Cli {
    /// Args
    #[arg(long, value_name = "ARG", num_args = 2..)]
    args: Vec<String>,
}

fn main() {
    let cli = Cli::parse();
    println!("{:#?}", cli.args);
}

$ # good: 2+ values
$ cmd --args asdf blah
$ # good: 2+ values
$ cmd --args asdf blah spam
$ # good: 2+ values per `--arg`
$ cmd --args asdf blah --arg spam pizza
$ # bad: too few values
$ cmd --args asdf
$ # bad: too few values for last `--arg`
$ cmd --args asdf blah --arg spam
$ # good:  no occurrences
$ cmd

What you are looking for is required = true. You also likely want to set action = ArgAction::Set because a Vec is automatically an Append, see https://docs.rs/clap/latest/clap/_derive/index.html#arg-types

use clap::Parser;
use clap::builder::ArgAction;

#[derive(Parser)]
struct Cli {
    /// Args
    #[arg(long, value_name = "ARG", num_args = 2.., required = true, action = ArgAction::Set)]
    args: Vec<String>,
}

fn main() {
    let cli = Cli::parse();
    println!("{:#?}", cli.args);
}

As this is expected behavior, I'm going to close this. If there is a reason we should reconsider, let us know!