Help text: flattening Args containing both positional arguments and named flags results in incorrect "usage" order

[jrose-signal]

Please complete the following tasks

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

Rust Version

1.80.0

Clap Version

4.5.11

Minimal reproducible code

use clap::*; // 4.5.11

#[derive(Parser)]
struct TopLevel {
    first: String,
    #[command(flatten)]
    second: Inner,
}

// *** Note: the user can specify *either* a positional argument *or* a named argument
#[derive(Args)]
#[group(required = true, multiple = false)]
struct Inner {
    value: Option<String>,
    #[arg(long)]
    path: Option<String>,
}

fn main() {
    let mut cmd = TopLevel::command();
    _ = cmd.print_long_help();

    let parsed = TopLevel::parse_from(["test", "a", "b"]);
    assert_eq!(parsed.first, "a");
    assert_eq!(parsed.second.value, "b");
}

Steps to reproduce the bug with the above code

Run on play.rust-lang.org

Actual Behaviour

Usage: playground <VALUE|--path <PATH>> <FIRST>

even though the assertions show that the FIRST argument is in fact getting parsed first, as desired.

Expected Behaviour

Usage: playground <FIRST> <VALUE|--path <PATH>>

It does work to put the flag argument after the first positional argument, so this isn't even wrong, even if it's a little unconventional.

Additional Context

I tried manually specifying index on the arguments, but it did not affect the help output.

Debug Output

No response

[epage]

The usage code generally assumes the mutually exclusive arguments are flags. It'll be a bit messy to try to integrate positional support.

In the mean time, override_usage can be used to manually specify something