I was planning on mentioning it as soon as I saw this post as well. I think most people who try using a functional programming language that encourages you to use sum types (at least when you're beginning) run into this in some way, but most people probably aren't aware that it has an actual name.

Here's a post on solving it using type classes in Haskell, if people are curious! http://koerbitz.me/posts/Solving-the-Expression-Problem-in-H...