Because the proposal is about several kinds of unions: - closed hierarchies of reference types that the compiler will verify at build time - value types that

Because the proposal is about several kinds of unions: - closed hierarchies of reference types that the compiler will verify at build time - value types that use compiler tricks to behave like closed hierarchies (tagged unions) - custom types that can have any implementation but can hook into the same compiler mechanisms to behave like a union of types - ad-hoc unions of existing types