Function check_opt 

pub fn check_opt(
    constraint: &ConstConstraint,
    consts: &[ConstArg],
    index: usize,
    param_name: &str,
) -> Result<(), String>

Convenience: apply a single constraint to an optional positional argument. Absence is treated as “no value to check” (Ok) — the required flag on ParamSpec already handles mandatory-ness.