[ameliaquining]

That's more a consequence of Rust needing its tagged unions declared up front so it can lay them out consistently in memory without runtime type information. Python and TypeScript have untagged unions (that are discriminated at runtime by the RTTI attached to all objects in the underlying dynamic language); they don't happen to have an equivalent of Rust's ? operator, but if they did it'd work like you're describing.

[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.