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Well you are right of course, I just wanted to explain what they wanted to show. Of course the type would be wrong if the second entry in itself is an empty list. I just wanted to explain the reasoning what they tried to accomplish

They could’ve done the Either type which would’ve been more correct or maybe EitherT (if the latter is even possible)

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I don't think they were trying to accomplish the same thing as the Scala/Haskell version; these are just two completely different things that happen to share a name because the blog post gave the example a name that is confusing when read literally. The purpose of the Cats version is “there is always a head element”. The purpose of the union in the blog post is more like “this can be a collection, but many callers will be thinking of it as a single element, so don't put the burden on them to convert it”. I do think it's a weak case for them in a type theory sense (I would tend to position that kind of implicit conversion elsewhere in the language), but I can also see it being motivating to a large class of developers…

… wait, I've made a different mistake here while trying to explain the difference, haven't I? I was describing it as a sum type, but it's not really a sum type, it's really just set-theoretic union, right?

Which also means OneOrMore is unsound in a different way because it doesn't guarantee that T and IEnumerable<T> are disjoint; OneOrMore<object> initialized from [x] will always return [[x]] from AsEnumerable, won't it? If I'm interpreting the switch expression correctly and the first case predominates, since a list is-an object? I don't have a test setup handy; someone with actual C# experience, please tell me whether that's correct or whether the compiler signals an error here or something…

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