Extend the memcpy fast path in iterator_input_adapter to sized
sentinels of a different type, not just same-type iterator pairs.
std::counted_iterator paired with std::default_sentinel_t already
satisfies std::contiguous_iterator and std::sized_sentinel_for, so
std::ranges::distance (C++20) lets that combination reach the fast
path too, instead of silently falling back to the byte-by-byte path.
- iterator_is_contiguous now also allows std::sized_sentinel_for<SentinelType,
IteratorType> under C++20, gated the same way as the existing
std::contiguous_iterator detection.
- get_elements_impl's fast path uses std::ranges::distance under C++20
(works for both same-type and sized-sentinel pairs) and falls back to
std::distance pre-C++20, where SentinelType is always IteratorType.
- Add a C++20-only test exercising json::parse/accept with
std::counted_iterator + std::default_sentinel_t.
- Document std::default_sentinel_t + std::counted_iterator as a
SentinelType example across parse.md, accept.md, sax_parse.md, and
the five from_*.md pages, replacing an earlier ambiguously worded
bullet.
Addresses review feedback: https://github.com/nlohmann/json/pull/5265#discussion_r3564237584
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Niels Lohmann
0e03ecee10