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The important detail that has bogged down almost every union type discussion is "zero values". What would be the zero value of a union type? If you've written Go you know the entire language is built around zero values, disabling it for some types is not an option.

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You could use the first entry, or require an explicit annotation. That doesn't seem like a big issue.

You would be wrong. Any use annotations complicates the language and compiler. As for first entry, this has been discussed many times without ever reaching any consensus, see - https://github.com/golang/go/issues/19412

Couldn't it be just the first enum item? Zero values are somewhat arbitrary anyway.

I don't think this is a good idea. Because zero value changes when you reorder the fields.

Why would it be just the first enum item? How do you even determine how the enum is ordered?

Source order, presumably?

That's what eg Haskell does, eg when you derive comparisons for sum tyes, they compare by source order.

As does Pascal, Modula-2, Ada, among plenty of others.

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