In my own library, I use inject instead of adding Array#sum. But, I know that's sidestepping the issue, not solving it.

Honestly, I'm not sure how Rails uses #sum. I'm not sure whether they really need it or not. I suspect not.

I'm not a big fan of a lot of non-standard additions to Ruby. For example, all the #try, or #returning, or even your #andand. I don't consider any of those to have enough intrinsic value to be worth the risk of collisions with someone else's subtly different implementation.