My bigger issue with tail call optimization is that you really want it to be enforceable since if you accidentally deoptimize it for some reason then you can end up blowing up your stack at runtime. Usually failure to optimize some pattern doesn’t have such a drastic effect - normally code just runs more slowly. So tail call is one of those special optimizations you want a language annotation for so that if it fails you get a compiler error (and similarly you may want it applied even in debug builds).