It wouldn't be hard — rather easy, even — to write a static analyzer for that if you constrained your expectations to cases where sum types could practically be used. No reason to not do it right now!

But sum types don't really solve for a lot of cases. Even in Go you can add additional constraints to errors to get something approaching sum types. You don't have to use a naked `error`. But you are bound to start feeling pain down the road if you try. There is good reason why even the languages that support defined error sets have trended back to using open-ended constructs.

It does sound great in theory, no doubt, but reality is not so kind.