[TwentyPosts]

They're 'laughably simple' on a conceptual level. Adding them to the language in retrospect might genuinely be a bad idea at this point.

The thing which I am personally salty about is that they weren't there to begin with, and that the language wasn't built with them in mind.

[xyzzy_plugh]

Go development began circa 2007, I do not believe sum types were common place nor do I believe the average developer was aware of them at the time. I'm not saying you're wrong, but given even Scala didn't add support until 2010 they do not seem like something sorely obvious that should have been there from the beginning. In hindsight, sure.