Function s7_method_signature 

fn s7_method_signature(
    class_name: &str,
    n_dispatch: usize,
    fallback: bool,
) -> String

The S7::method() signature: the class (or S7::class_any for a fallback) for the receiver, and S7::class_any for every other dispatch argument.

The other arguments are not narrowed. The method’s own argument checks and Rust conversions are the type check, and they report the parameter by name. No single S7 class matches what a parameter accepts in general (an ExternalPtr<T> takes the class object, a bare external pointer or a list with .ptr; coerce widens numeric parameters), so a narrower class would turn accepted values into “Can’t find method” errors. A more specific method registered in R still wins over this one.