Extend memcpy fast path to sized sentinels (e.g. std::counted_iterator)

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>

docs/mkdocs/docs/api/basic_json/accept.md

@@ -49,7 +49,7 @@ Unlike the [`parse()`](parse.md) function, this function neither throws an excep
 :   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance.
 
     - a custom sentinel type for C++20 ranges
-    - `std::counted_iterator` with a different sentinel type
+    - `std::default_sentinel_t`, when `IteratorType` is `std::counted_iterator`
 
 ## Parameters
 

docs/mkdocs/docs/api/basic_json/from_bjdata.md

@@ -36,8 +36,10 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   a compatible iterator type
 
 `SentinelType`
-:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance a
-    custom sentinel type for C++20 ranges
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance.
+
+    - a custom sentinel type for C++20 ranges
+    - `std::default_sentinel_t`, when `IteratorType` is `std::counted_iterator`
 
 ## Parameters
 

docs/mkdocs/docs/api/basic_json/from_bson.md

@@ -36,8 +36,10 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   a compatible iterator type
 
 `SentinelType`
-:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance a
-    custom sentinel type for C++20 ranges
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance.
+
+    - a custom sentinel type for C++20 ranges
+    - `std::default_sentinel_t`, when `IteratorType` is `std::counted_iterator`
 
 ## Parameters
 

docs/mkdocs/docs/api/basic_json/from_cbor.md

@@ -39,8 +39,10 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   a compatible iterator type
 
 `SentinelType`
-:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance a
-    custom sentinel type for C++20 ranges
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance.
+
+    - a custom sentinel type for C++20 ranges
+    - `std::default_sentinel_t`, when `IteratorType` is `std::counted_iterator`
 
 ## Parameters
 

docs/mkdocs/docs/api/basic_json/from_msgpack.md

@@ -36,8 +36,10 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   a compatible iterator type
 
 `SentinelType`
-:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance a
-    custom sentinel type for C++20 ranges
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance.
+
+    - a custom sentinel type for C++20 ranges
+    - `std::default_sentinel_t`, when `IteratorType` is `std::counted_iterator`
 
 ## Parameters
 

docs/mkdocs/docs/api/basic_json/from_ubjson.md

@@ -36,8 +36,10 @@ The exact mapping and its limitations are described on a [dedicated page](../../
 :   a compatible iterator type
 
 `SentinelType`
-:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance a
-    custom sentinel type for C++20 ranges
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance.
+
+    - a custom sentinel type for C++20 ranges
+    - `std::default_sentinel_t`, when `IteratorType` is `std::counted_iterator`
 
 ## Parameters
 

docs/mkdocs/docs/api/basic_json/parse.md

@@ -48,7 +48,7 @@ static basic_json parse(IteratorType first, SentinelType last,
 :   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for instance.
 
     - a custom sentinel type for C++20 ranges
-    - `std::counted_iterator` with a different sentinel type
+    - `std::default_sentinel_t`, when `IteratorType` is `std::counted_iterator`
 
 ## Parameters
 

docs/mkdocs/docs/api/basic_json/sax_parse.md

@@ -48,7 +48,10 @@ The SAX event lister must follow the interface of [`json_sax`](../json_sax/index
     with a size of 1, 2, or 4 bytes (interpreted respectively as UTF-8, UTF-16, and UTF-32)
 
 `SentinelType`
-:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for overload (2)
+:   defaults to `IteratorType`; may be a different type comparable to `IteratorType` via `operator!=`, for overload (2), for instance.
+
+    - a custom sentinel type for C++20 ranges
+    - `std::default_sentinel_t`, when `IteratorType` is `std::counted_iterator`
 
 `SAX`
 :   a class fulfilling the SAX event listener interface; see [`json_sax`](../json_sax/index.md)