Hi there! I hope, this message will go to where it's expected to go, since I'm not really familiar with e-mail threads.
I was the one who brought https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81806 issue about sub-optimal implementation of split function in pbds. The reason why I did so is clearly described on this comment: https://codeforces.com/blog/entry/10355?#comment-157883 So, a bit of story and context. Five years ago at codeforces.com (competitive programming website) someone eventually pointed out that there is order-statistics tree in SGI library. It turned out to be very useful in competitions since it is quite common type of queries to count number of elements less than k in set and unfortunately regular std::set doesn't provide such possibility although it would be extremely useful in competitions. I made two posts about pbds on codeforces to introduce them to community: https://codeforces.com/blog/entry/11080 and https://codeforces.com/blog/entry/13279. First one introduces structures in general and second one describes how to modify them so they support custom queries. Second one was not quite as popular, perhaps because it's not much easier to comprehend and remember concept than simply write something like cartesian tree on live contest. But the first one is pretty much alive, most recent comment was only 8 days ago. There was also another post (https://codeforces.com/blog/entry/60737) considering hash_map from pb_ds as a replacement for unordered_map since hash_map clearly outperform unordered_map. This one is also quite popular and well-known in competitive programming community. So speaking about "Do you actually use these containers?" I would say that I often use tree_order_statistics_node_update in competitions, and in general specifically tree_order_statistics_node_update and hash_map are pretty popular in competitive programming community. Deprecating policy based data structures will deal much pain to some competitors because problems in which it's possible to use pbds instead of custom balanced binary trees occur quite often and so now we'll have to implement bbst instead of using something out of the box. Not sure if you would consider this usage case as something "serious", but well, I was asked, so I answered. Regards, Oleksandr Kulkov
