I thought he was saying the `end` in range begin/end was now allowed to be some empty signifier to tell things to look for the null pointer (null is a sentinel, but `end` itself is now empty). That may not be right, I don't know enough about the new concepts stuff and range seems to be part of that.

edit: looking at this it maybe covers it:

https://foonathan.net/2020/03/iterator-sentinel/

Yes but I'm saying this is kind of moot when you're writing a struct that needs to store the iterators, because it can't get away with just storing the 'begin' -- you can't guarantee the second iterator you're storing will actually be the end. Meaning this iteration model isn't efficiently composable.

Why would you need a union?

Write me some code that shows me how to compose iterator-based ranges with just 1 iterator's worth of space in each range.

It is not so clear to me what you mean. Maybe you can point to a better version (maybe what some other language does), and I can explain how I think the same can be done with C++20 ranges. I believe ranges are strictly more powerful than what other languages provide, and often they can be slimmer/more efficient for the common case.