I really don’t remember, sorry. I just scanned through https://github.com/Microsoft/TypeScript/wiki/Breaking-Change... and nothing jogged my memory; I’m wondering if it actually was something to do with decorators, though I still think probably not. I investigated a bit at the time because I was looking into using TypeScript on a certain project and the changes would probably affect me, but I wasn’t actually using TypeScript at the time, and it didn’t stick in my memory. Something about some change being spread over a few versions with deprecation followed by a silent breaking change in the generated code, but I can’t remember what.