Could you share a use case that requires to depend on size() specifically rather than the iterator category?
Given an arbitrary range r, determine its size by the fasest means possible. If we use std::distance, we can do it in O(1) for std::vector, O(n) for std::map, and O(n) for std::forward_list. If we can detect whether "r.size()" is a valid expression, and use that if available, and std::distance otherwise, then we have O(1) for std::vector, O(1) for std::map, and O(n) for std::forward_list. Notice how that's an improvement for std::map. However, if detecting whether "r.size()" is a valid expression cannot be done reliably (for example, if we determine that for iterator_range<I> where I is not random-access it is a valid expression, but then that static-asserts on us), then we can't use this approach. Regards, Nate