Required arg with <TYPE> tag and default value always errors in debug build but never in release

[nbvdkamp]

Please complete the following tasks

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

Rust Version

rustc 1.59.0 (9d1b2106e 2022-02-23)

Clap Version

3.1.6

Minimal reproducible code

use clap::{arg, Command};

fn main() {
    let matches = Command::new("")
            .arg(arg!(--foo <FOO>)
                    .default_value("default")
            ).get_matches();
    
    println!("{}", matches.value_of("foo").unwrap());
}

Steps to reproduce the bug with the above code

These
cargo run -- --foo bar
cargo run --release -- --foo bar
give differing results

Actual Behaviour

Running a debug build cargo run -- --foo bar or cargo run results in:
thread 'main' panicked at 'Argument 'foo' is required and can't have a default value'

But running a release build with the flag set cargo run --release -- --foo bar results in:
bar
And leaving the flag out cargo run --release results in:
default

Expected Behaviour

When running with the flag it should never error. When leaving out the flag I am not certain what the correct behaviour is but it should be consistent between debug and release builds.

Additional Context

No response

Debug Output

For cargo run -- --foo bar:

[        clap::build::command]  App::_do_parse
[        clap::build::command]  App::_build
[        clap::build::command]  App::_propagate:
[        clap::build::command]  App::_check_help_and_version:
[        clap::build::command]  App::_check_help_and_version: Removing generated version
[        clap::build::command]  App::_propagate_global_args:
[        clap::build::command]  App::_derive_display_order:
[  clap::build::debug_asserts]  Command::_debug_asserts
[  clap::build::debug_asserts]  Arg::_debug_asserts:help
[  clap::build::debug_asserts]  Arg::_debug_asserts:foo
thread 'main' panicked at 'Argument 'foo' is required and can't have a default value'

For cargo run --release -- --foo bar:

[        clap::build::command]  App::_do_parse
[        clap::build::command]  App::_build
[        clap::build::command]  App::_propagate:
[        clap::build::command]  App::_check_help_and_version:
[        clap::build::command]  App::_check_help_and_version: Removing generated version
[        clap::build::command]  App::_propagate_global_args:
[        clap::build::command]  App::_derive_display_order:
[         clap::parse::parser]  Parser::get_matches_with
[         clap::parse::parser]  Parser::get_matches_with: Begin parsing 'RawOsString("--foo")' ([45, 45, 102, 111, 111])
[         clap::parse::parser]  Parser::get_matches_with: Positional counter...1
[         clap::parse::parser]  Parser::get_matches_with: Low index multiples...false
[         clap::parse::parser]  Parser::possible_subcommand: arg=RawOsStr("--foo")
[         clap::parse::parser]  Parser::get_matches_with: sc=None
[         clap::parse::parser]  Parser::parse_long_arg
[         clap::parse::parser]  Parser::parse_long_arg: cur_idx:=1
[         clap::parse::parser]  Parser::parse_long_arg: Does it contain '='...
[         clap::parse::parser]  No
[         clap::parse::parser]  Parser::parse_long_arg: Found valid opt or flag '--foo <FOO>'
[         clap::parse::parser]  Parser::parse_long_arg: Found an opt with value 'None'
[         clap::parse::parser]  Parser::parse_opt; opt=foo, val=None
[         clap::parse::parser]  Parser::parse_opt; opt.settings=ArgFlags(REQUIRED | TAKES_VAL)
[         clap::parse::parser]  Parser::parse_opt; Checking for val...
[         clap::parse::parser]  Parser::parse_opt: More arg vals required...
[         clap::parse::parser]  Parser::remove_overrides: id=[hash: 41CB8A11A30AC4B8]
[    clap::parse::arg_matcher]  ArgMatcher::inc_occurrence_of_arg: id=[hash: 41CB8A11A30AC4B8]
[        clap::build::command]  App::groups_for_arg: id=[hash: 41CB8A11A30AC4B8]
[        clap::build::command]  App::groups_for_arg: id=[hash: 41CB8A11A30AC4B8]
[         clap::parse::parser]  Parser::get_matches_with: After parse_long_arg Opt([hash: 41CB8A11A30AC4B8])
[         clap::parse::parser]  Parser::get_matches_with: Begin parsing 'RawOsString("bar")' ([98, 97, 114])
[         clap::parse::parser]  Parser::get_matches_with: Positional counter...1
[         clap::parse::parser]  Parser::get_matches_with: Low index multiples...false
[         clap::parse::parser]  Parser::add_val_to_arg; arg=foo, val=RawOsStr("bar")
[         clap::parse::parser]  Parser::add_val_to_arg; trailing_values=false, DontDelimTrailingVals=false
[         clap::parse::parser]  Parser::add_single_val_to_arg: adding val..."bar"
[         clap::parse::parser]  Parser::add_single_val_to_arg: cur_idx:=2
[        clap::build::command]  App::groups_for_arg: id=[hash: 41CB8A11A30AC4B8]
[    clap::parse::arg_matcher]  ArgMatcher::needs_more_vals: o=foo
[      clap::parse::validator]  Validator::validate
[         clap::parse::parser]  Parser::add_defaults
[         clap::parse::parser]  Parser::add_defaults:iter:foo:
[         clap::parse::parser]  Parser::add_value: doesn't have conditional defaults
[         clap::parse::parser]  Parser::add_value:iter:foo: has default vals
[         clap::parse::parser]  Parser::add_value:iter:foo: was used
[         clap::parse::parser]  Parser::add_value:iter:foo: doesn't have default missing vals
[      clap::parse::validator]  Validator::validate_conflicts
[      clap::parse::validator]  Validator::validate_exclusive
[      clap::parse::validator]  Validator::validate_exclusive:iter:[hash: 41CB8A11A30AC4B8]
[      clap::parse::validator]  Validator::validate_conflicts::iter: id=[hash: 41CB8A11A30AC4B8]
[      clap::parse::validator]  Conflicts::gather_conflicts
[      clap::parse::validator]  Validator::validate_required: required=ChildGraph([Child { id: [hash: 41CB8A11A30AC4B8], children: [] }])
[      clap::parse::validator]  Validator::gather_requires
[      clap::parse::validator]  Validator::gather_requires:iter:[hash: 41CB8A11A30AC4B8]
[      clap::parse::validator]  Validator::validate_required_unless
[      clap::parse::validator]  Validator::validate_matched_args
[      clap::parse::validator]  Validator::validate_matched_args:iter:[hash: 41CB8A11A30AC4B8]: vals=Flatten {
    inner: FlattenCompat {
        iter: Fuse {
            iter: Some(
                Iter(
                    [
                        [
                            "bar",
                        ],
                    ],
                ),
            ),
        },
        frontiter: None,
        backiter: None,
    },
}
[      clap::parse::validator]  Validator::validate_arg_num_vals
[      clap::parse::validator]  Validator::validate_arg_values: arg="foo"
[      clap::parse::validator]  Validator::validate_arg_num_occurs: "foo"=1
[    clap::parse::arg_matcher]  ArgMatcher::get_global_values: global_arg_vec=[[hash: 59636393CFFBFE5F]]
bar

[epage]

As long as the assert is happening in debug mode regardless of what flags get passed in, this is expected behavior.

Asserts are generally a technique for communicating a development error as opposed to a runtime error so long as they are exclusively a development error and don't get in the way of delivering mission critical software.

Debug-only asserts are something done to ensure the development-only assets do not impact end-user performance so long as testing is able to ensure they are never hit or that the release code can handle them not asserting.

Clap has been using debug asserts for a while but to help developers get quicker feedback on these, we added Command::debug_assert for running in tests for clap v3 and put it in our migration guide and in the tutorial.

As this is expected behavior, I'm going to go ahead and close. If my assumption earlier is wrong or if there is something I missed, feel free to let us know!

[nbvdkamp]

Oh I see now, I was developing my app in release builds because testing it is too slow in debug and assumed that this was a bug because it only showed up when I went to debug something and worked as I expected before. Thanks for the quick response.