Function is_a 

pub fn is_a(h: u64, dest: ClassTemplateId) -> bool

$cast’s runtime check (6.24.2/8.16): whether h’s dynamic class is dest or one of dest’s descendants/implementors in the sense that matters for a cast — concretely, whether dest is in h’s dynamic template’s own registered ancestor-or-self closure (register_ancestors). A null/stale handle, or a dynamic template with nothing registered for it yet, is never is_a anything (including itself) — an unregistered template legitimately means “no class/interface hierarchy information is available yet”, not “matches everything”.