[1, [2, 3]].respond_to?(:sum) => false

I elided a discussion about this case from the post. For some arrays, a recursive sum is semantically valid. For other arrays a recursive sum is not semantically valid, and it is not a simple case of knowing whether all of the leaf elements respond to :+. So again, trying to implement this in the Array class doesn't work because containers are implementation rather than interface.

If you are going to use an Array class, I think the array's client is the one responsible for knowing whether its elements can be summed and if so, whether recursive summing is valid. The other approach is to subclass or otherwise create special-case arrays that know about their semantically valid operations like sum.