Your post got me curious and I did some googling. I think this explains it, https://softwareengineering.stackexchange.com/questions/157684/what-limitations-doe

Your post got me curious and I did some googling. I think this explains it, https://softwareengineering.stackexchange.com/questions/157684/what-limitations-does-the-jvm-impose-on-tail-call-optimization https://softwareengineering.stackexchange.com/questions/1576... Looks like it's a scalac optimization that doesn't always work hence the need for the annotation to give the developer feedback. There is also a link to a Closure discussion board exploring why Clojure doesn't have the annotation, by design. I didn't read it as I'm not that interested.