Lucas, I largely agree, but it is documented that the various combinatorial generators emit items in a particular lexicographic order.  So that is documented, and programs definitely rely on it.

That's why, in an earlier comment, Terry suggested that perhaps `product()` could make a special case of its (and only its) first argument (and only when repeat=1).  Each element of the first iterable is needed only once (although it may copied into any number of outputs), so there's no actual need to make a tuple of it first.  The implementation is just simpler and clearer by treating all arguments alike.

Which is a good enough reason for me - and the "use cases" for an unbounded first argument look exceptionally weak to me.