Dead code in Subcommand::from_arg_matches

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Rust Version

rustc 1.53.0 (53cb7b09b 2021-06-17)

Clap Version

3.0.0-beta.2

Minimal reproducible code

#[derive(Clap)]
enum Parent {
  #[flatten]
  Child(Child)
}

#[derive(Clap)]
enum Child{
  One,
  Two,
}

Steps to reproduce the bug with the above code

Run cargo expand

Actual Behaviour

impl FromArgMatches for Parent will have a case for a command name child when no child command exists

Expected Behaviour

The child case doesn't exist

Additional Context

No response

Debug Output

No response