ignore_errors makes boolean flag required in clap 4

[NicholasLYang]

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

Rust Version

rustc 1.66.0-nightly (e7119a030 2022-09-22)

Clap Version

4.0.15

Minimal reproducible code

use clap::Parser;

#[derive(Parser, Debug)]
#[clap(author, version, ignore_errors = true)]
struct Args {
    /// Force color usage in the terminal
    #[clap(long, global = true)]
    color: bool,
}

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

Steps to reproduce the bug with the above code

cargo run -- --help

Actual Behaviour

error: The following required argument was not provided: color

Usage: clap-test [OPTIONS]

For more information try '--help'

Expected Behaviour

false

Additional Context

The expected behavior occurs in version 3.2.22.

Debug Output

[ clap::builder::command] Command::_do_parse
[ clap::builder::command] Command::_build: name="clap-test"
[ clap::builder::command] Command::_propagate:clap-test
[ clap::builder::command] Command::_check_help_and_version:clap-test expand_help_tree=false
[ clap::builder::command] Command::long_help_exists
[ clap::builder::command] Command::_check_help_and_version: Building default --help
[ clap::builder::command] Command::_check_help_and_version: Building default --version
[ clap::builder::command] Command::_propagate_global_args:clap-test
[clap::builder::debug_asserts] Command::_debug_asserts
[clap::builder::debug_asserts] Arg::_debug_asserts:color
[clap::builder::debug_asserts] Arg::_debug_asserts:help
[clap::builder::debug_asserts] Arg::_debug_asserts:version
[clap::builder::debug_asserts] Command::_verify_positionals
[ clap::parser::parser] Parser::get_matches_with
[ clap::parser::parser] Parser::get_matches_with: Begin parsing 'RawOsStr("--help")' ([45, 45, 104, 101, 108, 112])
[ clap::parser::parser] Parser::possible_subcommand: arg=Ok("--help")
[ clap::parser::parser] Parser::get_matches_with: sc=None
[ clap::parser::parser] Parser::parse_long_arg
[ clap::parser::parser] Parser::parse_long_arg: Does it contain '='...
[ clap::parser::parser] Parser::parse_long_arg: Found valid arg or flag '--help'
[ clap::parser::parser] Parser::parse_long_arg("help"): Presence validated
[ clap::parser::parser] Parser::react action=Help, identifier=Some(Long), source=CommandLine
[ clap::parser::parser] Help: use_long=true
[ clap::builder::command] Command::long_help_exists: false
[ clap::builder::command] Command::write_help_err: clap-test, use_long=false
[ clap::builder::command] Command::long_help_exists: false
[ clap::output::help] write_help
[ clap::output::help_template] HelpTemplate::new cmd=clap-test, use_long=false
[ clap::output::help_template] should_show_arg: use_long=false, arg=color
[ clap::output::help_template] HelpTemplate::write_templated_help
[ clap::output::help_template] HelpTemplate::write_before_help
[ clap::output::usage] Usage::create_usage_no_title
[ clap::output::usage] Usage::create_help_usage; incl_reqs=true
[ clap::output::usage] Usage::needs_options_tag
[ clap::output::usage] Usage::needs_options_tag:iter: f=color
[ clap::builder::command] Command::groups_for_arg: id="color"
[ clap::output::usage] Usage::needs_options_tag:iter:iter: grp_s="Args"
[ clap::output::usage] Usage::needs_options_tag:iter: [OPTIONS] required
[ clap::output::usage] Usage::get_args: incls=[]
[ clap::output::usage] Usage::get_args: unrolled_reqs=[]
[ clap::output::usage] Usage::get_args: ret_val=[]
[ clap::output::usage] Usage::create_help_usage: usage=clap-test [OPTIONS]
[ clap::output::help_template] HelpTemplate::write_all_args
[ clap::output::help_template] should_show_arg: use_long=false, arg=color
[ clap::output::help_template] should_show_arg: use_long=false, arg=help
[ clap::output::help_template] should_show_arg: use_long=false, arg=version
[ clap::output::help_template] HelpTemplate::write_args Options
[ clap::output::help_template] should_show_arg: use_long=false, arg=color
[ clap::output::help_template] HelpTemplate::write_args: arg="color" longest=7
[ clap::output::help_template] should_show_arg: use_long=false, arg=help
[ clap::output::help_template] HelpTemplate::write_args: arg="help" longest=7
[ clap::output::help_template] should_show_arg: use_long=false, arg=version
[ clap::output::help_template] HelpTemplate::write_args: arg="version" longest=9
[ clap::output::help_template] should_show_arg: use_long=false, arg=color
[ clap::output::help_template] HelpTemplate::spec_vals: a=--color
[ clap::output::help_template] should_show_arg: use_long=false, arg=help
[ clap::output::help_template] HelpTemplate::spec_vals: a=--help
[ clap::output::help_template] should_show_arg: use_long=false, arg=version
[ clap::output::help_template] HelpTemplate::spec_vals: a=--version
[ clap::output::help_template] HelpTemplate::spec_vals: a=--color
[ clap::output::help_template] HelpTemplate::short
[ clap::output::help_template] HelpTemplate::long
[ clap::output::help_template] HelpTemplate::align_to_about: arg=color, next_line_help=false, longest=9
[ clap::output::help_template] HelpTemplate::align_to_about: positional=false arg_len=7, spaces=4
[ clap::output::help_template] HelpTemplate::help
[ clap::output::help_template] HelpTemplate::help: help_width=17, spaces=33, avail=83
[ clap::output::help_template] HelpTemplate::spec_vals: a=--help
[ clap::output::help_template] HelpTemplate::short
[ clap::output::help_template] HelpTemplate::long
[ clap::output::help_template] HelpTemplate::align_to_about: arg=help, next_line_help=false, longest=9
[ clap::output::help_template] HelpTemplate::align_to_about: positional=false arg_len=6, spaces=5
[ clap::output::help_template] HelpTemplate::help
[ clap::output::help_template] HelpTemplate::help: help_width=17, spaces=22, avail=83
[ clap::output::help_template] HelpTemplate::spec_vals: a=--version
[ clap::output::help_template] HelpTemplate::short
[ clap::output::help_template] HelpTemplate::long
[ clap::output::help_template] HelpTemplate::align_to_about: arg=version, next_line_help=false, longest=9
[ clap::output::help_template] HelpTemplate::align_to_about: positional=false arg_len=9, spaces=2
[ clap::output::help_template] HelpTemplate::help
[ clap::output::help_template] HelpTemplate::help: help_width=17, spaces=25, avail=83
[ clap::output::help_template] HelpTemplate::write_after_help
[ clap::builder::command] Command::color: Color setting...
[ clap::builder::command] Auto
[ clap::builder::command] Command::color: Color setting...
[ clap::builder::command] Auto
[ clap::builder::command] Command::_do_parse: ignoring error: Usage: clap-test [OPTIONS]

Options:
--color Force color usage in the terminal
-h, --help Print help information
-V, --version Print version information

Backtrace:
0: backtrace::backtrace::libunwind::trace
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.66/src/backtrace/libunwind.rs:93:5
backtrace::backtrace::trace_unsynchronized
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.66/src/backtrace/mod.rs:66:5
1: backtrace::backtrace::trace
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.66/src/backtrace/mod.rs:53:14
2: backtrace::capture::Backtrace::create
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.66/src/capture.rs:176:9
3: backtrace::capture::Backtrace::new
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/backtrace-0.3.66/src/capture.rs:140:22
4: clap::error::Backtrace::new
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/clap-4.0.15/src/error/mod.rs:835:19
5: clap::error::Error::new
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/clap-4.0.15/src/error/mod.rs:135:28
6: clap::error::Error::for_app
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/clap-4.0.15/src/error/mod.rs:277:9
7: clap::error::Error::display_help
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/clap-4.0.15/src/error/mod.rs:329:9
8: clap::parser::parser::Parser::help_err
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/clap-4.0.15/src/parser/parser.rs:1584:9
9: clap::parser::parser::Parser::react
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/clap-4.0.15/src/parser/parser.rs:1288:21
10: clap::parser::parser::Parser::parse_long_arg
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/clap-4.0.15/src/parser/parser.rs:815:17
11: clap::parser::parser::Parser::get_matches_with
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/clap-4.0.15/src/parser/parser.rs:132:44
12: clap::builder::command::Command::_do_parse
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/clap-4.0.15/src/builder/command.rs:3794:29
13: clap::builder::command::Command::try_get_matches_from_mut
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/clap-4.0.15/src/builder/command.rs:708:9
14: clap::builder::command::Command::get_matches_from
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/clap-4.0.15/src/builder/command.rs:578:9
15: clap::builder::command::Command::get_matches
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/clap-4.0.15/src/builder/command.rs:490:9
16: clap::derive::Parser::parse
at /Users/nicholas/.cargo/registry/src/git.luolix.top-1ecc6299db9ec823/clap-4.0.15/src/derive.rs:82:27
17: clap_test::main
at src/main.rs:12:16
18: core::ops::function::FnOnce::call_once
at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/core/src/ops/function.rs:248:5
19: std::sys_common::backtrace::__rust_begin_short_backtrace
at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/sys_common/backtrace.rs:122:18
20: std::rt::lang_start::{{closure}}
at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/rt.rs:145:18
21: core::ops::function::impls::<impl core::ops::function::FnOnce for &F>::call_once
at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/core/src/ops/function.rs:280:13
std::panicking::try::do_call
at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:492:40
std::panicking::try
at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:456:19
std::panic::catch_unwind
at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panic.rs:137:14
std::rt::lang_start_internal::{{closure}}
at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/rt.rs:128:48
std::panicking::try::do_call
at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:492:40
std::panicking::try
at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:456:19
std::panic::catch_unwind
at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panic.rs:137:14
std::rt::lang_start_internal
at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/rt.rs:128:20
22: std::rt::lang_start
at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/rt.rs:144:17
23: _main

[ clap::parser::arg_matcher] ArgMatcher::get_global_values: global_arg_vec=["color"]
[ clap::builder::command] Command::_build: name="clap-test"
[ clap::builder::command] Command::_propagate:clap-test
[ clap::builder::command] Command::_check_help_and_version:clap-test expand_help_tree=false
[ clap::builder::command] Command::long_help_exists
[ clap::builder::command] Command::_check_help_and_version: Building default --help
[ clap::builder::command] Command::_check_help_and_version: Building default --version
[ clap::builder::command] Command::_propagate_global_args:clap-test
[clap::builder::debug_asserts] Command::_debug_asserts
[clap::builder::debug_asserts] Arg::_debug_asserts:color
[clap::builder::debug_asserts] Arg::_debug_asserts:help
[clap::builder::debug_asserts] Arg::_debug_asserts:version
[clap::builder::debug_asserts] Command::_verify_positionals
[ clap::builder::command] Command::_build: name="clap-test"
[ clap::builder::command] Command::_build: already built
[ clap::output::usage] Usage::create_usage_with_title
[ clap::output::usage] Usage::create_usage_no_title
[ clap::output::usage] Usage::create_help_usage; incl_reqs=true
[ clap::output::usage] Usage::needs_options_tag
[ clap::output::usage] Usage::needs_options_tag:iter: f=color
[ clap::builder::command] Command::groups_for_arg: id="color"
[ clap::output::usage] Usage::needs_options_tag:iter:iter: grp_s="Args"
[ clap::output::usage] Usage::needs_options_tag:iter: [OPTIONS] required
[ clap::output::usage] Usage::get_args: incls=[]
[ clap::output::usage] Usage::get_args: unrolled_reqs=[]
[ clap::output::usage] Usage::get_args: ret_val=[]
[ clap::output::usage] Usage::create_help_usage: usage=clap-test [OPTIONS]
[ clap::builder::command] Command::color: Color setting...
[ clap::builder::command] Auto
[ clap::builder::command] Command::color: Color setting...
[ clap::builder::command] Auto
error: The following required argument was not provided: color

Usage: clap-test [OPTIONS]

For more information try '--help'

[epage]

You can work around this by switching to an Option<bool> but that will likely require also adding #[arg(action = ArgAction::SetTrue)].

I'm assuming you can reproduce this in clap v3 if you add #[clap(action)]. When we switched flags from ArgAction::IncOccurrences to ArgAction::SetTrue, we switched from just tracking of we had encountered the flag on the command line to inserting a true, defaulting to false This makes flags behave like everything else, making things more consistent / easier to use.

The workaround works because the error is coming from clap_derive when it is trying to fill in color, it has no value to put there so it errors.

My guess is that in at least some cases of ignore_errors, we are not processing defaults. This means the problem would be the same if you did #[arg(long, default_value = "-")] input: PathBuf

We should probably be making sure we populate defaults with ignore_errors. With the derive API, this turns a broken use case to a working one, so no compatibility concerns. With the builder API, it is a little more complicated.