The changes from the One Ranges Proposal amount to adding:
- a constructor that takes a `default_sentinel_t` and is equivalent to
the default constructor;
- an `operator==` that compares the iterator to `default_sentinel_t`.
The original proposal defined two overloads for `operator==` (different
argument order) as well as `operator!=`. This has been removed by
[P1614](https://wg21.link/p1614).
Differential Revision: https://reviews.llvm.org/D119620
The macro that opts out of `std::ranges::` functionality is called
`_LIBCPP_HAS_NO_INCOMPLETE_RANGES`, and is unrelated to this macro
which is specifically about _compiler_ support for the _syntax_.
The only non-mechanical diff here is in `<__config>`.
Differential Revision: https://reviews.llvm.org/D118507
Konstantin Varlamov