From the readme:

> The go-sumtype command recognizes this pattern, but it needs a small amount of help to recognize which interfaces should be treated as sum types

Why is this? Have you tried detecting sum types automatically? Maybe some heuristics like: package-level declaration of the interface, has an unexported method, has more than one type that implements it, maybe also the interface itself is exported.

It seems unwise to do exhaustiveness checks automatically on an interface that "might" be a sum type. The declaration is a single line and has the benefit of being completely unambiguous.