I winter camp every year, -25° is my personal coldest. A solution to a full bladder is a urinal, not to bad on the rainy nights too.
Back a few years, my buddy and I winter camped on a ridge at 7,800 feet, next to an old dead pine we could salvage dead limbs of for a morning fire. With double sleeping bags, night before we achieved a snoot full of a number 7 dark label beverage. It was -30 in town 8 miles from our location. Buddy says: it's really cold. He's the mountain man type. I quickly hop out of the two bags, still wearing my fancy synthetic thinsulate bibs, and do the task, he wasn't even close to the meteorological scale with "really cold". The largest hardship was 2 feet of ice, no water in the creek, have to have fire to melt snow, we became very dehydrated in just the first hour of travel the night before. The morning cold was bearable, the dehydration was dangerous.
Later on, I learned to bring the small bottles of water and save the bottles. They make great temporary urinals for not having to leave the tent, or sleeping bags ...if you're skillful. 15 layers of zippers and cold hands, never wait too long to get the task started-it may take more time than one would anticipate. Nano seconds count sometimes.