Gate the C++20 iterator classification on JSON_HAS_RANGES

Making supports_bulk_scan depend on iterator_is_contiguous meant the trait
is now instantiated for every adapter, not only when get_elements() is
called. On standard libraries with an incomplete <ranges> that is fatal:
libstdc++ 10 evaluates std::contiguous_iterator<std::counted_iterator<T*>>
by calling std::to_address, which needs an operator-> its counted_iterator
does not have, so satisfaction checking is a hard error rather than false.
Reported by clang 14 + libstdc++ 10.

JSON_HAS_RANGES already encodes exactly this ("libstdc++ < 11 has incomplete
C++20 ranges", #4440), so require it for the C++20 branch. Affected
toolchains fall back to the pointer-only test and the byte-at-a-time
scanner, which parses identically, just without the bulk fast paths.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-user_defined_input.cpp

@@ -235,9 +235,14 @@ TEST_CASE("std::counted_iterator reaches the contiguous fast paths")
     // std::counted_iterator over a contiguous iterator must reach the same bulk
     // string/number scanners as a plain pointer - not just the byte-at-a-time
     // fallback (see #5268 for the equivalent memcpy fast path).
+#if JSON_HAS_RANGES
+    // JSON_HAS_RANGES is 0 on standard libraries with an incomplete <ranges>
+    // (libstdc++ < 11, libc++ < 16), where the adapter deliberately falls back
+    // to the byte-at-a-time scanner; everything below still has to work there.
     using adapter_type = nlohmann::detail::iterator_input_adapter<std::counted_iterator<const char*>, std::default_sentinel_t>;
     CHECK(adapter_type::supports_bulk_scan);
     CHECK(adapter_type::supports_seek);
+#endif
 
     // exercise every fast path: long ASCII run, multibyte UTF-8, escapes, and
     // integer/floating-point numbers