I’ve played around with the untagged-union package a bit, and it seems to me quite fantastic! Are there features missing from that package that you’re hoping to get compiler support for? Or does that package just need to be more “canonical”?
Another concern I have of adding language-level support for untagged unions is I’m not sure all backends could support untagged unions. At least if it’s a library, you could build the support for that library for whichever backends support runtime type checks, but still be able to use PureScript from other backends that wouldn’t support it. A language-level change would require excluding any backends that couldn’t support runtime type checks. (I definitely could be proven wrong on this point).