At this point I've skimmed a number of relatively serious union type proposals for Go (which is to say, people who were serious enough to write something substantial in the Go issue tracker). One of the feelings I've wound up with as a result of this is that any such union type proposal should probably start out by describing what its objectives are, not what its proposed syntax is.

There are a number of different things union types in Go could do, ranging from creating an interface type that only allows a limited range of concrete types to (potentially) reducing the amount of space needed to store one of several types of values to requiring people to do specific things to extract an interior value from a union value, implicitly forcing them to check for errors (or panic). Some of these objectives will be more or less complete by themselves, but some others will really want (or at least benefit from) additional changes to things like 'go vet', or in some cases be incomplete without other language changes.

Proposals that bury their objectives (or don't clearly spell them out) invite people to guess at what they really want, and then argue about it. Such proposals are also harder to evaluate, since a reader can't judge whether the proposal would have problems achieving its objectives, or even to what extent the objectives are reasonable for Go. Without objectives, people are left to discuss what the proposal does and doesn't achieve without understanding which of those things are important (and need to be kept in any changes) and which aren't (and could be altered or removed).

Presumably the proposer had an objective in mind and so can write it down. If they haven't actually considered their objectives, they should definitely start with that, not with the syntax or language semantics. I won't say that objectives are the most important thing about a change to Go, because the syntax and semantics matter too, but a clearly expressed objective is (in my view) necessary. And if you convince people that the objective is a good one, you have a much better chance of having that objective achieved somehow, even if it happens in a different way than you thought of.

(One reason that people may be reluctant to write down their objective is for fear that others won't like it and will be against the proposal as a result. One of my views is that if you can't persuade people of the objectives, your proposal should fail, and trying to sneak it in is a bad look.)

PS: One of the reasons I'm writing this down is for myself, because I was briefly tempted to write up a straw-person union types idea here on Wandering Thoughts. Had I done so, it definitely wouldn't have started from objectives, but from semantics and some effects. It's easy to have a clever idea (or at least a clever looking idea, one that sounds good for long enough for me to write a blog entry about it), but mere clever ideas are at best a distraction.