Why does std::forward_list have no size() member function?

I have been using C++11's forward_list as a container for fast insertions, without much memory overhead, since it is a singly linked list.

After realizing that forward_list does not have a size() method, I'm a bit confused about the reasoning behind that. Couldn't it just maintain a private field keeping track of nodes inserted and removed, hence, implementing an O(1) size() operation?

N2543 is the proposal, and it has a detailed discussion about size().

The choice between Option 3 [not providing size()] and Option 2 [providing a O(1) size()] is more a matter of judgment. I have chosen Option 3 for the same reason that I chose insert-after instead of insert-before: Option 3 is more consistent with the goal of zero overhead compared to a hand-written C-style linked list. Maintaining a count doubles the size of a forward_list object (one word for the list head and one for the count), and it slows down every operation that changes the number of nodes. In most cases this isn't a change in asymptotic complexity (the one change in asymptotic complexity is in one of the forms of splice), but it is nonzero overhead. It's a cost that all users would have to pay for, whether they need this feature or not, and, for users who care about maintaining a count, it's just as easy to maintain it outside the list, by incrementing the count with every insert and decrementing it with every erase, as it is to maintain the count within the list.