Right, because any destructor would break your ability to do TCO assuming that it runs before the tail call... So as soon as you use an object mechanism that will invoke a destructor at some point your ability to use TCO is gone.

Interesting, isn't there a possible transformation where you would be able to move the constructor/destructor pairs out of the generated loop construct without breaking the function?

After all a tail call is just a fancy go-to under the hood.