How about throwing a TypeError? Seems about the same level of breaking change, but potentially less confusing.

that's what i was going to do at first, but after writing a bunch of tests for previously indeterminate behavior, almost everything returned "''" when passed to this function - numbers, boolean, date, object. The only problem is with things with a .length property which aren't strings (namely: array). It actually kinda feels more "escapey" to me to just turn your "not a string" into an empty string in the query if you're concatenating in there. But I was definitely only the fence.

Now that you mention those examples... how about casting everything to String? pg 9?

yeah that isn't a good idea - def breakage, though subtle. tbh whenever I, very rarely, use these functions to concatenate some form of user-input into a query directly I do a ton of external sanitization first. Check types, if its a number make sure its in an expected range, etc. It's scary & should be a last resort in most cases.