#[command(flatten)] on optional field makes it required

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• I have searched the open and rejected issues

Rust Version

1.72

Clap Version

4.3.4

Minimal reproducible code

use clap::{Args, Parser};

fn main() {
    MainArgs::parse();
}

#[derive(Parser, Debug)]
pub struct MainArgs {
    #[arg(short = 'V', long)]
    pub version: bool,

    #[command(flatten)]
    pub specific_args: Option<SpecificArgs>,
}

#[derive(Args, Debug)]
pub struct SpecificArgs {
    pub positional: String,
}

Steps to reproduce the bug with the above code

cargo r -- --version

Actual Behaviour

error: the following required arguments were not provided:
  <POSITIONAL>

Usage: test-executable --version <POSITIONAL>

For more information, try '--help'.

Expected Behaviour

No error because the SpecificArgs is optional.

Additional Context

I want to use this to have a top level Parser with a few global args (e.g. --version but also some others), and I want to reuse this across different CLI tools, each with their own ConcreteArgs. The ConcreteArgs are usually written as mandatory in their corresponding #[derive(Args)] parser, but they should only be mandatory if none of the global flags are present. To do this, I made SpecificArgs optional in the #[command(flatten)] but that doesn't seem to have the intended effect.

Similar discussion: https://github.com/clap-rs/clap/discussions/4954

Debug Output

No response