You have foo in data and computed property foo too.
Just rename it: https://jsfiddle.net/hjdk9e5u/7/

Yes, I know Vue will not pollute the raw data, but the error message is really weird, I think it is better to throw a much more friendly error message with accurate reason. Some beginners always make mistakes like this, but they do not know how to debug it.

You are referencing a computed property inside itself - what else do you expect other than an infinite loop? The best we can do is warn you when you define a computed property that clashes with existing properties.

In case someone else comes here from Google, I was hitting something similar running unit tests but not when running the app normally.

I had created a normal computed property (no circular references), but also a a watch created for that computed property. As soon as I commented out the watch, the error went away and unit tests passed.

Something like: https://jsfiddle.net/x1jork9c/