Function s4_generic_guard 

pub(crate) fn s4_generic_guard(
    generic: &str,
    explicit: bool,
    receiver: &str,
) -> String

The line that defines an S4 instance method’s generic before its setMethod(), shared by the inherent and trait generators.

A generic of the package’s own (explicit false) is defined unless this namespace already has it. Unconditional setGeneric() would replace the generic object and clear the methods other types registered on it, and the check must be a namespace-local exists(): a bare isGeneric() searches S4 metadata globally, so with an installed copy of the package attached, load_all() would skip setGeneric() while setMethod() still can’t see the generic from the namespace being loaded, and isGeneric(where =) resolves the namespace to a package name, which fails mid-install (#1158).

An explicit generic = "..." may name a generic that lives elsewhere: methods::show (an S4 generic exists() misses when methods is not attached), the primitive length, base’s S3 summary. It is defined only when no function of that name is visible and methods::isGeneric() finds no generic of another package, with receiver as its dispatch formal; otherwise setMethod() attaches to the existing one instead of masking it with a function(x, ...) generic of this package (#1677). A generic whose @package is this namespace’s name is an installed copy’s, the #1158 case, so it is defined anyway. environmentName() is a plain lookup, unlike isGeneric(where =). A name nothing defines is created, but gets no generated page: whether it is the package’s own is only known at load time (#2039).