This is only a small piece of the story for what people say when they want tagged unions. Without all of the ancillary support in the language, like exhaustive pattern matching, it really doesn't count.

You can also add support for exhaustive switches on tags.

Just as a linter config though?

No, by leveraging unkeyed struct initializers as the switch case pattern, which the Go compiler does exhaustiveness checking on.

That is a LOT of code (very ugly code, I would add) that could be replaced by `type Float = float32 | float64` in a language with actual support for union types.

Tagged unions are not union types. A union type is a supertype for any arbitrary collection of types, but a tagged union aka sum type is a single type with multiple data constructors, and does not require subtyping to be implemented.

The C++ committee said the same thing, and gave us std::variant. They are painful to work with and do not really deliver most of the benefits people want.