Several parts of P1612R2 are already implemented:
- common_iterator (new in C++20) https://reviews.llvm.org/D103335
- unreachable_sentinel_t (new in C++20) https://reviews.llvm.org/D107920
- istream_iterator (removed) https://reviews.llvm.org/D119620
- istreambuf_iterator (removed) https://reviews.llvm.org/D119620