Python is the canonical mainstream example: __init_subclass__/metaclasses (definition-time hooks) + __subclasses__(), mro(), __dict__ (introspection). CLOS (Common Lisp Object System) also qualifies: defclass/initialize-instance/defmethod hooks plus runtime introspection of classes, generic functions, and method combinations. Smalltalk similarly: class creation executes code, and everything is introspectable at runtime. Languages that lack definition-time hooks (Java, C#, Go, Rust, TypeScript/TS-era JS) or lack sufficient runtime introspection for structural facts don’t meet both requirements without extra tooling.