[steveklabnik]

"untagged union" usually means "no discriminant" not "runtime discriminant."

(Rust has both tagged (enum) and untagged (union) unions, but untagged ones are unsafe and therefore mostly used for C interop and similar cases.)

[ameliaquining]

People often call Python/TypeScript unions "untagged unions" even though they're a very different creature from C/Rust unions. Ideally there'd be a term specifically for them but I'm not aware of one.