Fix stale doc comments and include lists in the input headers

input_adapters.hpp included <memory> and <numeric> for the removed
shared_ptr-based adapter design but used neither; it called
(std::min) without including <algorithm>. json_sax.hpp used
std::numeric_limits without including <limits>. Also corrected
comments that no longer matched the code: input_stream_adapter does
not skip the input's BOM (the lexer's skip_bom() does), the
span_input_adapter comment named the no-longer-existing
input_buffer_adapter type, lexer::get_string() does not reset the
token, binary_reader's get_number() doc opened with /* instead of
/*! (so Doxygen skipped it) and omitted BON8 from its endianness
note, and the UBJSON-binary-types note did not mention that BJData
'B' arrays are read as binary.

Left out: the lgtm suppression on lexer.hpp's scan_number() (in
#5616's hunk) and the "-1 if unknown" wording in json_sax.hpp's
start_object/start_array docs (in draft #5267's hunk), per the
verdict's conflict list.

Part of #5712

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

include/nlohmann/detail/input/binary_reader.hpp

@@ -3344,8 +3344,8 @@ class binary_reader
         return enter_object(detail::unknown_size());
     }
 
-    // Note, no reader for UBJSON binary types is implemented because they do
-    // not exist
+    // Note, UBJSON has no binary type of its own; BJData, which shares this
+    // reader, decodes optimized 'B' arrays as binary in get_ubjson_array().
 
     bool get_ubjson_high_precision_number()
     {
@@ -4052,7 +4052,7 @@ class binary_reader
 #endif
     }
 
-    /*
+    /*!
     @brief read a number from the input
 
     @tparam NumberType the type of the number
@@ -4062,10 +4062,10 @@ class binary_reader
     @return whether conversion completed
 
     @note This function needs to respect the system's endianness, because
-          bytes in CBOR, MessagePack, and UBJSON are stored in network order
-          (big endian) and therefore need reordering on little endian systems.
-          On the other hand, BSON and BJData use little endian and should reorder
-          on big endian systems.
+          bytes in CBOR, MessagePack, UBJSON, and BON8 are stored in network
+          order (big endian) and therefore need reordering on little endian
+          systems. On the other hand, BSON and BJData use little endian and
+          should reorder on big endian systems.
     */
     template<typename NumberType, bool InputIsLittleEndian = false>
     bool get_number(const input_format_t format, NumberType& result)

include/nlohmann/detail/input/input_adapters.hpp

@@ -8,12 +8,11 @@
 
 #pragma once
 
+#include <algorithm> // min
 #include <array> // array
 #include <cstddef> // size_t
 #include <cstring> // strlen
 #include <iterator> // begin, end, iterator_traits, random_access_iterator_tag, distance, next
-#include <memory> // shared_ptr, make_shared, addressof
-#include <numeric> // accumulate
 #include <streambuf> // streambuf
 #include <string> // string, char_traits
 #include <type_traits> // enable_if, is_base_of, is_pointer, is_integral, remove_pointer
@@ -82,8 +81,9 @@ class file_input_adapter
 };
 
 /*!
-Input adapter for a (caching) istream. Ignores a UFT Byte Order Mark at
-beginning of input. Does not support changing the underlying std::streambuf
+Input adapter for a (caching) istream. Does not skip a UTF Byte Order Mark
+itself; that is done by the lexer's skip_bom(). Does not support changing
+the underlying std::streambuf
 in mid-input. Maintains underlying std::istream and std::streambuf to support
 subsequent use of standard std::istream operations to process any input
 characters following those used in parsing the JSON input.  Clears the
@@ -862,9 +862,9 @@ auto input_adapter(T (&array)[N]) -> decltype(input_adapter(array, array + N)) /
     return input_adapter(array, array + N);
 }
 
-// This class only handles inputs of input_buffer_adapter type.
-// It's required so that expressions like {ptr, len} can be implicitly cast
-// to the correct adapter.
+// This class only handles inputs that construct a contiguous_bytes_input_adapter
+// (e.g. span_input_adapter). It's required so that expressions like {ptr, len}
+// can be implicitly cast to the correct adapter.
 class span_input_adapter
 {
   public:

include/nlohmann/detail/input/json_sax.hpp

@@ -10,6 +10,7 @@
 
 #include <algorithm> // find_if, min
 #include <cstddef>
+#include <limits> // numeric_limits
 #include <string> // string
 #include <type_traits> // enable_if_t
 #include <utility> // move, pair
@@ -175,6 +176,88 @@ template<typename ArrayType>
 inline void reserve_array(ArrayType& /*arr*/, std::size_t /*len*/, priority_tag<0> /*unused*/)
 {}
 
+#if JSON_DIAGNOSTIC_POSITIONS
+/*!
+@brief set the diagnostic positions of a value the DOM SAX parsers just stored
+
+Shared by json_sax_dom_parser and json_sax_dom_callback_parser. basic_json
+befriends this struct, as the position members are private.
+*/
+struct diagnostic_positions
+{
+    /*!
+    @param[in,out] v  the value that was just parsed
+    @param[in] lexer  the lexer that read it, or nullptr to leave @a v alone
+    */
+    template<typename BasicJsonType, typename LexerType>
+    static void set_from_lexer(BasicJsonType& v, LexerType* lexer)
+    {
+        if (lexer)
+        {
+            // Lexer has read past the current field value, so set the end position to the current position.
+            // The start position will be set below based on the length of the string representation
+            // of the value.
+            v.end_position = lexer->get_position();
+
+            switch (v.type())
+            {
+                case value_t::boolean:
+                {
+                    // 4 and 5 are the string length of "true" and "false"
+                    v.start_position = v.end_position - (v.m_data.m_value.boolean ? 4 : 5);
+                    break;
+                }
+
+                case value_t::null:
+                {
+                    // 4 is the string length of "null"
+                    v.start_position = v.end_position - 4;
+                    break;
+                }
+
+                case value_t::string:
+                {
+                    // escape sequences make the token longer than the value it
+                    // parses to, so the start position cannot be derived from
+                    // the value; use the offset the lexer recorded instead
+                    v.start_position = lexer->get_token_start_position();
+                    break;
+                }
+
+                case value_t::discarded:
+                {
+                    // an object or array the callback of
+                    // json_sax_dom_callback_parser rejected has no position
+                    v.end_position = std::string::npos;
+                    v.start_position = v.end_position;
+                    break;
+                }
+
+                case value_t::binary:
+                case value_t::number_integer:
+                case value_t::number_unsigned:
+                case value_t::number_float:
+                {
+                    v.start_position = v.end_position - lexer->get_string().size();
+                    break;
+                }
+
+                case value_t::object:
+                case value_t::array:
+                {
+                    // object and array are handled in start_object() and start_array() handlers
+                    // skip setting the values here.
+                    break;
+                }
+                default: // LCOV_EXCL_LINE
+                    // Handle all possible types discretely, default handler should never be reached.
+                    JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
+            }
+        }
+    }
+};
+#endif
+
 /*!
 @brief SAX implementation to create a JSON value from SAX events
 
@@ -376,76 +459,6 @@ class json_sax_dom_parser
 
   private:
 
-#if JSON_DIAGNOSTIC_POSITIONS
-    void handle_diagnostic_positions_for_json_value(BasicJsonType& v)
-    {
-        if (m_lexer_ref)
-        {
-            // Lexer has read past the current field value, so set the end position to the current position.
-            // The start position will be set below based on the length of the string representation
-            // of the value.
-            v.end_position = m_lexer_ref->get_position();
-
-            switch (v.type())
-            {
-                case value_t::boolean:
-                {
-                    // 4 and 5 are the string length of "true" and "false"
-                    v.start_position = v.end_position - (v.m_data.m_value.boolean ? 4 : 5);
-                    break;
-                }
-
-                case value_t::null:
-                {
-                    // 4 is the string length of "null"
-                    v.start_position = v.end_position - 4;
-                    break;
-                }
-
-                case value_t::string:
-                {
-                    // escape sequences make the token longer than the value it
-                    // parses to, so the start position cannot be derived from
-                    // the value; use the offset the lexer recorded instead
-                    v.start_position = m_lexer_ref->get_token_start_position();
-                    break;
-                }
-
-                // As we handle the start and end positions for values created during parsing,
-                // we do not expect the following value type to be called. Regardless, set the positions
-                // in case this is created manually or through a different constructor. Exclude from lcov
-                // since the exact condition of this switch is esoteric.
-                // LCOV_EXCL_START
-                case value_t::discarded:
-                {
-                    v.end_position = std::string::npos;
-                    v.start_position = v.end_position;
-                    break;
-                }
-                // LCOV_EXCL_STOP
-                case value_t::binary:
-                case value_t::number_integer:
-                case value_t::number_unsigned:
-                case value_t::number_float:
-                {
-                    v.start_position = v.end_position - m_lexer_ref->get_string().size();
-                    break;
-                }
-                case value_t::object:
-                case value_t::array:
-                {
-                    // object and array are handled in start_object() and start_array() handlers
-                    // skip setting the values here.
-                    break;
-                }
-                default: // LCOV_EXCL_LINE
-                    // Handle all possible types discretely, default handler should never be reached.
-                    JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert,-warnings-as-errors) LCOV_EXCL_LINE
-            }
-        }
-    }
-#endif
-
     /*!
     @invariant If the ref stack is empty, then the passed value will be the new
                root.
@@ -461,7 +474,7 @@ class json_sax_dom_parser
             root = BasicJsonType(std::forward<Value>(v));
 
 #if JSON_DIAGNOSTIC_POSITIONS
-            handle_diagnostic_positions_for_json_value(root);
+            diagnostic_positions::set_from_lexer(root, m_lexer_ref);
 #endif
 
             return &root;
@@ -474,7 +487,7 @@ class json_sax_dom_parser
             ref_stack.back()->m_data.m_value.array->emplace_back(std::forward<Value>(v));
 
 #if JSON_DIAGNOSTIC_POSITIONS
-            handle_diagnostic_positions_for_json_value(ref_stack.back()->m_data.m_value.array->back());
+            diagnostic_positions::set_from_lexer(ref_stack.back()->m_data.m_value.array->back(), m_lexer_ref);
 #endif
 
             return &(ref_stack.back()->m_data.m_value.array->back());
@@ -485,7 +498,7 @@ class json_sax_dom_parser
         *object_element = BasicJsonType(std::forward<Value>(v));
 
 #if JSON_DIAGNOSTIC_POSITIONS
-        handle_diagnostic_positions_for_json_value(*object_element);
+        diagnostic_positions::set_from_lexer(*object_element, m_lexer_ref);
 #endif
 
         return object_element;
@@ -662,7 +675,7 @@ class json_sax_dom_callback_parser
 
 #if JSON_DIAGNOSTIC_POSITIONS
                     // Set start/end positions for discarded object.
-                    handle_diagnostic_positions_for_json_value(*ref_stack.back());
+                    diagnostic_positions::set_from_lexer(*ref_stack.back(), m_lexer_ref);
 #endif
                 }
             }
@@ -778,7 +791,7 @@ class json_sax_dom_callback_parser
 
 #if JSON_DIAGNOSTIC_POSITIONS
                     // Set start/end positions for discarded array.
-                    handle_diagnostic_positions_for_json_value(*ref_stack.back());
+                    diagnostic_positions::set_from_lexer(*ref_stack.back(), m_lexer_ref);
 #endif
                 }
             }
@@ -831,72 +844,6 @@ class json_sax_dom_callback_parser
 
   private:
 
-#if JSON_DIAGNOSTIC_POSITIONS
-    void handle_diagnostic_positions_for_json_value(BasicJsonType& v)
-    {
-        if (m_lexer_ref)
-        {
-            // Lexer has read past the current field value, so set the end position to the current position.
-            // The start position will be set below based on the length of the string representation
-            // of the value.
-            v.end_position = m_lexer_ref->get_position();
-
-            switch (v.type())
-            {
-                case value_t::boolean:
-                {
-                    // 4 and 5 are the string length of "true" and "false"
-                    v.start_position = v.end_position - (v.m_data.m_value.boolean ? 4 : 5);
-                    break;
-                }
-
-                case value_t::null:
-                {
-                    // 4 is the string length of "null"
-                    v.start_position = v.end_position - 4;
-                    break;
-                }
-
-                case value_t::string:
-                {
-                    // escape sequences make the token longer than the value it
-                    // parses to, so the start position cannot be derived from
-                    // the value; use the offset the lexer recorded instead
-                    v.start_position = m_lexer_ref->get_token_start_position();
-                    break;
-                }
-
-                case value_t::discarded:
-                {
-                    v.end_position = std::string::npos;
-                    v.start_position = v.end_position;
-                    break;
-                }
-
-                case value_t::binary:
-                case value_t::number_integer:
-                case value_t::number_unsigned:
-                case value_t::number_float:
-                {
-                    v.start_position = v.end_position - m_lexer_ref->get_string().size();
-                    break;
-                }
-
-                case value_t::object:
-                case value_t::array:
-                {
-                    // object and array are handled in start_object() and start_array() handlers
-                    // skip setting the values here.
-                    break;
-                }
-                default: // LCOV_EXCL_LINE
-                    // Handle all possible types discretely, default handler should never be reached.
-                    JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert,-warnings-as-errors) LCOV_EXCL_LINE
-            }
-        }
-    }
-#endif
-
     /// if there is a pending duplicate-key stash entry for this exact slot,
     /// remove it from the stash; if restore_value is true, the stashed
     /// previous value is moved back into the slot first (use this when the
@@ -1018,7 +965,7 @@ class json_sax_dom_callback_parser
         auto value = BasicJsonType(std::forward<Value>(v));
 
 #if JSON_DIAGNOSTIC_POSITIONS
-        handle_diagnostic_positions_for_json_value(value);
+        diagnostic_positions::set_from_lexer(value, m_lexer_ref);
 #endif
 
         // check callback

include/nlohmann/detail/input/lexer.hpp

@@ -1494,45 +1494,30 @@ scan_number_done:
     */
     token_type convert_number(token_type number_type, std::size_t mantissa_end)
     {
-        // If the caller does not need the converted value (only whether the
-        // input is syntactically valid; see json_sax_acceptor/accept()), an
-        // unsigned/integer token can be reported without calling
-        // strtoull()/strtoll() at all, *provided* we can already tell from
-        // the digit count alone that the conversion cannot overflow 64 bits.
-        // Such tokens are always finite and are accepted unconditionally by
-        // the parser regardless of their actual value (parser::sax_parse_internal()
-        // never checks finiteness for value_unsigned/value_integer), so the
-        // classification below is all that is needed.
+        // accept() only needs to know whether the input is valid, so it sets
+        // discard_number_values (see json.hpp), and an integer token whose
+        // digit count shows that it fits is reported without calling
+        // convert_integer(). A number with up to 18 digits always fits into
+        // both std::uint64_t and std::int64_t (18 nines is about 1e18, below
+        // INT64_MAX, which is about 9.2e18). Longer tokens take the exact path
+        // below, including the fallback to floating point when the value does
+        // not fit.
         //
-        // A decimal number with up to 18 digits is always representable in
-        // both std::uint64_t and std::int64_t (18 nines is ~1e18, well below
-        // both UINT64_MAX ~1.8e19 and INT64_MAX ~9.2e18), so strtoull()/strtoll()
-        // could not have set errno to ERANGE for it. Numbers with more digits
-        // (rare in practice) fall through to the exact code below, unchanged,
-        // so their handling -- including reclassification to value_float when
-        // the value overflows 64 bits, and rejection when it is not even
-        // finite as a double -- is bit-for-bit identical to before this
-        // optimization.
+        // With a narrower number_unsigned_t/number_integer_t (e.g.
+        // std::uint32_t), the exact path would reclassify some of these tokens
+        // as (finite) floats, while this check reports integers. That does not
+        // change the result of accept(): it always parses through
+        // json_sax_acceptor, whose number callbacks discard their argument and
+        // return true, and the parser rejects neither integers nor finite
+        // floats. value_unsigned/value_integer are left unset here, so a caller
+        // that reads the converted value must not set discard_number_values.
         //
-        // Note this reasons about std::uint64_t/std::int64_t, not about
-        // number_unsigned_t/number_integer_t (BasicJsonType's own, possibly
-        // narrower, template parameters -- e.g. std::uint32_t). That is fine
-        // *only* because discard_number_values is exclusively set by
-        // accept() (see json.hpp), and accept() always parses through the
-        // library's own json_sax_acceptor -- never a user-supplied SAX
-        // consumer -- whose number_unsigned()/number_integer()/number_float()
-        // callbacks unconditionally discard their argument and return true.
-        // So for every caller that can reach this branch, neither the token
-        // classification below nor the eventual (possibly narrowed, and on
-        // this fast path left stale/unset) value_unsigned/value_integer is
-        // ever consulted -- an unsigned/integer token is accepted outright,
-        // and even a >18-digit token that this fast path deliberately falls
-        // through for is, once reclassified to value_float, still finite
-        // (and thus accepted) for any digit count that fits in number_unsigned_t
-        // or number_integer_t regardless of that type's width. If this
-        // function is ever taught to run with discard_number_values true for
-        // a caller that *does* read the converted value, this reasoning (and
-        // the fast path below) would need to be revisited.
+        // On contiguous input, scan_number_bulk_contiguous() converts integer
+        // tokens itself and does not pass them to this function, unless
+        // JSON_DIAGNOSTIC_POSITIONS is enabled. This check is therefore only
+        // reached for input without bulk access (e.g. streams), with
+        // JSON_DIAGNOSTIC_POSITIONS, or when scan_number_bulk_contiguous()
+        // falls back to scan_number().
         if (discard_number_values)
         {
             constexpr std::size_t safe_digit_count = 18;
@@ -2023,7 +2008,7 @@ scan_number_done:
         return value_float;
     }
 
-    /// return current string value (implicitly resets the token; useful only once)
+    /// return current string value
     string_t& get_string()
     {
         // a number token holds '.' regardless of the locale (#4084)
@@ -2325,11 +2310,11 @@ scan_number_done:
     /// the position of the decimal point in token_buffer
     std::size_t decimal_point_position = std::string::npos;
 
-    /// whether the caller (e.g. accept()/json_sax_acceptor) only needs the
-    /// token classification and never looks at the converted numeric value;
-    /// when set, scan_number() may skip strtoull()/strtoll() for
-    /// value_unsigned/value_integer tokens whose digit count guarantees they
-    /// fit into 64 bits (see scan_number())
+    /// whether the caller only needs the token types and never looks at the
+    /// converted numeric values; set only by accept(), which parses through
+    /// json_sax_acceptor. When set, convert_number() skips converting integer
+    /// tokens whose digit count guarantees that they fit into 64 bits (see
+    /// there)
     const bool discard_number_values = false;
 };
 

include/nlohmann/detail/input/parser.hpp

@@ -54,7 +54,8 @@ using parser_callback_t =
 /*!
 @brief syntax analysis
 
-This class implements a recursive descent parser.
+This class implements an iterative parser that keeps the open containers on
+an explicit stack and reports what it reads as SAX events.
 */
 template<typename BasicJsonType, typename InputAdapterType>
 class parser
@@ -439,8 +440,9 @@ class parser
 
                     // We are done with this array. Before we can parse a
                     // new value, we need to evaluate the new state first.
-                    // By setting skip_to_state_evaluation to false, we
-                    // are effectively jumping to the beginning of this if.
+                    // By setting skip_to_state_evaluation to true, the next
+                    // iteration skips parsing a value and evaluates the
+                    // enclosing state directly.
                     JSON_ASSERT(!states.empty());
                     states.pop_back();
                     skip_to_state_evaluation = true;
@@ -500,8 +502,9 @@ class parser
 
                 // We are done with this object. Before we can parse a
                 // new value, we need to evaluate the new state first.
-                // By setting skip_to_state_evaluation to false, we
-                // are effectively jumping to the beginning of this if.
+                // By setting skip_to_state_evaluation to true, the next
+                // iteration skips parsing a value and evaluates the
+                // enclosing state directly.
                 JSON_ASSERT(!states.empty());
                 states.pop_back();
                 skip_to_state_evaluation = true;

include/nlohmann/detail/string_utils.hpp

@@ -51,11 +51,23 @@ This is a single-byte step of a "shift-based" UTF-8 decoder originally
 written by Björn Hoehrmann. See
 http://bjoern.hoehrmann.de/utf-8/decoder/dfa/ for details.
 
-This decoder is the single source of truth for UTF-8 validation in this
-library: it is used both by the serializer (to escape and, in strict mode,
-reject ill-formed UTF-8 when dumping a string) and by the binary readers
-(to reject ill-formed UTF-8 in CBOR/MessagePack/BSON/UBJSON text strings at
-decode time; see @ref is_valid_utf8 below).
+The library checks UTF-8 well-formedness (RFC 3629, section 4) in four
+places, which differ in speed, diagnostics, and how they read the input:
+
+- decode() and @ref is_valid_utf8 below: the serializer (to escape and, in
+  strict mode, reject ill-formed UTF-8 when dumping a string) and the CBOR,
+  MessagePack, BSON, UBJSON and BJData readers (to reject ill-formed UTF-8 in
+  text strings at decode time).
+- the per-lead-byte switch in lexer::scan_string(): JSON text, with a
+  diagnostic for each kind of error.
+- validate_one_utf8() and valid_utf8_prefix() in string_scan.hpp: the lexer's
+  bulk string scan, the bulk path of the BON8 reader, and the BON8 writer.
+  They must accept exactly what the lexer's switch accepts.
+- the byte path of binary_reader::get_bon8_string(): BON8 input without bulk
+  access, and the bytes the bulk path leaves to it.
+
+All four must accept the same set of sequences, so a change to one needs a
+matching change to the others.
 
 @param[in,out] state  the current decoder state
 @param[in,out] codep  codepoint (valid only if resulting state is UTF8_ACCEPT)

include/nlohmann/json.hpp

@@ -149,6 +149,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     friend class ::nlohmann::detail::json_sax_dom_parser;
     template<typename BasicJsonType, typename InputAdapterType>
     friend class ::nlohmann::detail::json_sax_dom_callback_parser;
+#if JSON_DIAGNOSTIC_POSITIONS
+    friend struct ::nlohmann::detail::diagnostic_positions;
+#endif
     friend class ::nlohmann::detail::exception;
 
     /// workaround type for MSVC
@@ -1307,65 +1310,15 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     */
     template<bool Ordered>
     static compare_result compare_leaves(const_reference lhs, const_reference rhs) noexcept
-    {
-        return compare_leaves(lhs, rhs, std::integral_constant<bool, Ordered> {});
-    }
-
-    /// @brief compare two leaves that are only being checked for equality
-    static compare_result compare_leaves(const_reference lhs, const_reference rhs, std::false_type /*ordered*/) noexcept
     {
         if (lhs == rhs)
         {
             return compare_result::equal;
         }
 
-        return order_leaves(lhs, rhs, std::false_type {});
+        return order_leaves(lhs, rhs, std::integral_constant<bool, Ordered> {});
     }
 
-#if JSON_HAS_THREE_WAY_COMPARISON
-    /*!
-    @brief compare two leaves that are being ordered, for operator<=>
-
-    Reached only from operator<=>, so the leaves must be classified exactly
-    as operator<=> classifies them - which is not the same as asking
-    == and then order_leaves(), the way the other overload does it. The two
-    disagree on a binary value: == also compares the subtype, but <=> compares
-    only the bytes, through std::vector<std::uint8_t>::operator<=>. Using <=>
-    itself here keeps a leaf pair classified the same way regardless of how
-    deep it is nested - == first would again call operator<=> a level down
-    through order_leaves(), but call it after a mismatching == already ended
-    the comparison for a pair that <=> alone would still call equivalent.
-    */
-    static compare_result compare_leaves(const_reference lhs, const_reference rhs, std::true_type /*ordered*/) noexcept
-    {
-        const std::partial_ordering order = lhs <=> rhs; // *NOPAD*
-        if (order == 0)
-        {
-            return compare_result::equal;
-        }
-        if (order < 0)
-        {
-            return compare_result::less;
-        }
-        if (order > 0)
-        {
-            return compare_result::greater;
-        }
-        return compare_result::unordered;
-    }
-#else
-    /// @brief compare two leaves that are being ordered, for operator<
-    static compare_result compare_leaves(const_reference lhs, const_reference rhs, std::true_type /*ordered*/) noexcept
-    {
-        if (lhs == rhs)
-        {
-            return compare_result::equal;
-        }
-
-        return order_leaves(lhs, rhs, std::true_type {});
-    }
-#endif
-
     /*!
     @brief compare two object keys
 

single_include/nlohmann/json.hpp

@@ -6249,11 +6249,23 @@ This is a single-byte step of a "shift-based" UTF-8 decoder originally
 written by Björn Hoehrmann. See
 http://bjoern.hoehrmann.de/utf-8/decoder/dfa/ for details.
 
-This decoder is the single source of truth for UTF-8 validation in this
-library: it is used both by the serializer (to escape and, in strict mode,
-reject ill-formed UTF-8 when dumping a string) and by the binary readers
-(to reject ill-formed UTF-8 in CBOR/MessagePack/BSON/UBJSON text strings at
-decode time; see @ref is_valid_utf8 below).
+The library checks UTF-8 well-formedness (RFC 3629, section 4) in four
+places, which differ in speed, diagnostics, and how they read the input:
+
+- decode() and @ref is_valid_utf8 below: the serializer (to escape and, in
+  strict mode, reject ill-formed UTF-8 when dumping a string) and the CBOR,
+  MessagePack, BSON, UBJSON and BJData readers (to reject ill-formed UTF-8 in
+  text strings at decode time).
+- the per-lead-byte switch in lexer::scan_string(): JSON text, with a
+  diagnostic for each kind of error.
+- validate_one_utf8() and valid_utf8_prefix() in string_scan.hpp: the lexer's
+  bulk string scan, the bulk path of the BON8 reader, and the BON8 writer.
+  They must accept exactly what the lexer's switch accepts.
+- the byte path of binary_reader::get_bon8_string(): BON8 input without bulk
+  access, and the bytes the bulk path leaves to it.
+
+All four must accept the same set of sequences, so a change to one needs a
+matching change to the others.
 
 @param[in,out] state  the current decoder state
 @param[in,out] codep  codepoint (valid only if resulting state is UTF8_ACCEPT)
@@ -7548,12 +7560,11 @@ NLOHMANN_JSON_NAMESPACE_END
 
 
 
+#include <algorithm> // min
 #include <array> // array
 #include <cstddef> // size_t
 #include <cstring> // strlen
 #include <iterator> // begin, end, iterator_traits, random_access_iterator_tag, distance, next
-#include <memory> // shared_ptr, make_shared, addressof
-#include <numeric> // accumulate
 #include <streambuf> // streambuf
 #include <string> // string, char_traits
 #include <type_traits> // enable_if, is_base_of, is_pointer, is_integral, remove_pointer
@@ -7626,8 +7637,9 @@ class file_input_adapter
 };
 
 /*!
-Input adapter for a (caching) istream. Ignores a UFT Byte Order Mark at
-beginning of input. Does not support changing the underlying std::streambuf
+Input adapter for a (caching) istream. Does not skip a UTF Byte Order Mark
+itself; that is done by the lexer's skip_bom(). Does not support changing
+the underlying std::streambuf
 in mid-input. Maintains underlying std::istream and std::streambuf to support
 subsequent use of standard std::istream operations to process any input
 characters following those used in parsing the JSON input.  Clears the
@@ -8406,9 +8418,9 @@ auto input_adapter(T (&array)[N]) -> decltype(input_adapter(array, array + N)) /
     return input_adapter(array, array + N);
 }
 
-// This class only handles inputs of input_buffer_adapter type.
-// It's required so that expressions like {ptr, len} can be implicitly cast
-// to the correct adapter.
+// This class only handles inputs that construct a contiguous_bytes_input_adapter
+// (e.g. span_input_adapter). It's required so that expressions like {ptr, len}
+// can be implicitly cast to the correct adapter.
 class span_input_adapter
 {
   public:
@@ -8453,6 +8465,7 @@ NLOHMANN_JSON_NAMESPACE_END
 
 #include <algorithm> // find_if, min
 #include <cstddef>
+#include <limits> // numeric_limits
 #include <string> // string
 #include <type_traits> // enable_if_t
 #include <utility> // move, pair
@@ -10586,45 +10599,30 @@ scan_number_done:
     */
     token_type convert_number(token_type number_type, std::size_t mantissa_end)
     {
-        // If the caller does not need the converted value (only whether the
-        // input is syntactically valid; see json_sax_acceptor/accept()), an
-        // unsigned/integer token can be reported without calling
-        // strtoull()/strtoll() at all, *provided* we can already tell from
-        // the digit count alone that the conversion cannot overflow 64 bits.
-        // Such tokens are always finite and are accepted unconditionally by
-        // the parser regardless of their actual value (parser::sax_parse_internal()
-        // never checks finiteness for value_unsigned/value_integer), so the
-        // classification below is all that is needed.
+        // accept() only needs to know whether the input is valid, so it sets
+        // discard_number_values (see json.hpp), and an integer token whose
+        // digit count shows that it fits is reported without calling
+        // convert_integer(). A number with up to 18 digits always fits into
+        // both std::uint64_t and std::int64_t (18 nines is about 1e18, below
+        // INT64_MAX, which is about 9.2e18). Longer tokens take the exact path
+        // below, including the fallback to floating point when the value does
+        // not fit.
         //
-        // A decimal number with up to 18 digits is always representable in
-        // both std::uint64_t and std::int64_t (18 nines is ~1e18, well below
-        // both UINT64_MAX ~1.8e19 and INT64_MAX ~9.2e18), so strtoull()/strtoll()
-        // could not have set errno to ERANGE for it. Numbers with more digits
-        // (rare in practice) fall through to the exact code below, unchanged,
-        // so their handling -- including reclassification to value_float when
-        // the value overflows 64 bits, and rejection when it is not even
-        // finite as a double -- is bit-for-bit identical to before this
-        // optimization.
+        // With a narrower number_unsigned_t/number_integer_t (e.g.
+        // std::uint32_t), the exact path would reclassify some of these tokens
+        // as (finite) floats, while this check reports integers. That does not
+        // change the result of accept(): it always parses through
+        // json_sax_acceptor, whose number callbacks discard their argument and
+        // return true, and the parser rejects neither integers nor finite
+        // floats. value_unsigned/value_integer are left unset here, so a caller
+        // that reads the converted value must not set discard_number_values.
         //
-        // Note this reasons about std::uint64_t/std::int64_t, not about
-        // number_unsigned_t/number_integer_t (BasicJsonType's own, possibly
-        // narrower, template parameters -- e.g. std::uint32_t). That is fine
-        // *only* because discard_number_values is exclusively set by
-        // accept() (see json.hpp), and accept() always parses through the
-        // library's own json_sax_acceptor -- never a user-supplied SAX
-        // consumer -- whose number_unsigned()/number_integer()/number_float()
-        // callbacks unconditionally discard their argument and return true.
-        // So for every caller that can reach this branch, neither the token
-        // classification below nor the eventual (possibly narrowed, and on
-        // this fast path left stale/unset) value_unsigned/value_integer is
-        // ever consulted -- an unsigned/integer token is accepted outright,
-        // and even a >18-digit token that this fast path deliberately falls
-        // through for is, once reclassified to value_float, still finite
-        // (and thus accepted) for any digit count that fits in number_unsigned_t
-        // or number_integer_t regardless of that type's width. If this
-        // function is ever taught to run with discard_number_values true for
-        // a caller that *does* read the converted value, this reasoning (and
-        // the fast path below) would need to be revisited.
+        // On contiguous input, scan_number_bulk_contiguous() converts integer
+        // tokens itself and does not pass them to this function, unless
+        // JSON_DIAGNOSTIC_POSITIONS is enabled. This check is therefore only
+        // reached for input without bulk access (e.g. streams), with
+        // JSON_DIAGNOSTIC_POSITIONS, or when scan_number_bulk_contiguous()
+        // falls back to scan_number().
         if (discard_number_values)
         {
             constexpr std::size_t safe_digit_count = 18;
@@ -11115,7 +11113,7 @@ scan_number_done:
         return value_float;
     }
 
-    /// return current string value (implicitly resets the token; useful only once)
+    /// return current string value
     string_t& get_string()
     {
         // a number token holds '.' regardless of the locale (#4084)
@@ -11417,11 +11415,11 @@ scan_number_done:
     /// the position of the decimal point in token_buffer
     std::size_t decimal_point_position = std::string::npos;
 
-    /// whether the caller (e.g. accept()/json_sax_acceptor) only needs the
-    /// token classification and never looks at the converted numeric value;
-    /// when set, scan_number() may skip strtoull()/strtoll() for
-    /// value_unsigned/value_integer tokens whose digit count guarantees they
-    /// fit into 64 bits (see scan_number())
+    /// whether the caller only needs the token types and never looks at the
+    /// converted numeric values; set only by accept(), which parses through
+    /// json_sax_acceptor. When set, convert_number() skips converting integer
+    /// tokens whose digit count guarantees that they fit into 64 bits (see
+    /// there)
     const bool discard_number_values = false;
 };
 
@@ -11589,6 +11587,88 @@ template<typename ArrayType>
 inline void reserve_array(ArrayType& /*arr*/, std::size_t /*len*/, priority_tag<0> /*unused*/)
 {}
 
+#if JSON_DIAGNOSTIC_POSITIONS
+/*!
+@brief set the diagnostic positions of a value the DOM SAX parsers just stored
+
+Shared by json_sax_dom_parser and json_sax_dom_callback_parser. basic_json
+befriends this struct, as the position members are private.
+*/
+struct diagnostic_positions
+{
+    /*!
+    @param[in,out] v  the value that was just parsed
+    @param[in] lexer  the lexer that read it, or nullptr to leave @a v alone
+    */
+    template<typename BasicJsonType, typename LexerType>
+    static void set_from_lexer(BasicJsonType& v, LexerType* lexer)
+    {
+        if (lexer)
+        {
+            // Lexer has read past the current field value, so set the end position to the current position.
+            // The start position will be set below based on the length of the string representation
+            // of the value.
+            v.end_position = lexer->get_position();
+
+            switch (v.type())
+            {
+                case value_t::boolean:
+                {
+                    // 4 and 5 are the string length of "true" and "false"
+                    v.start_position = v.end_position - (v.m_data.m_value.boolean ? 4 : 5);
+                    break;
+                }
+
+                case value_t::null:
+                {
+                    // 4 is the string length of "null"
+                    v.start_position = v.end_position - 4;
+                    break;
+                }
+
+                case value_t::string:
+                {
+                    // escape sequences make the token longer than the value it
+                    // parses to, so the start position cannot be derived from
+                    // the value; use the offset the lexer recorded instead
+                    v.start_position = lexer->get_token_start_position();
+                    break;
+                }
+
+                case value_t::discarded:
+                {
+                    // an object or array the callback of
+                    // json_sax_dom_callback_parser rejected has no position
+                    v.end_position = std::string::npos;
+                    v.start_position = v.end_position;
+                    break;
+                }
+
+                case value_t::binary:
+                case value_t::number_integer:
+                case value_t::number_unsigned:
+                case value_t::number_float:
+                {
+                    v.start_position = v.end_position - lexer->get_string().size();
+                    break;
+                }
+
+                case value_t::object:
+                case value_t::array:
+                {
+                    // object and array are handled in start_object() and start_array() handlers
+                    // skip setting the values here.
+                    break;
+                }
+                default: // LCOV_EXCL_LINE
+                    // Handle all possible types discretely, default handler should never be reached.
+                    JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
+            }
+        }
+    }
+};
+#endif
+
 /*!
 @brief SAX implementation to create a JSON value from SAX events
 
@@ -11790,76 +11870,6 @@ class json_sax_dom_parser
 
   private:
 
-#if JSON_DIAGNOSTIC_POSITIONS
-    void handle_diagnostic_positions_for_json_value(BasicJsonType& v)
-    {
-        if (m_lexer_ref)
-        {
-            // Lexer has read past the current field value, so set the end position to the current position.
-            // The start position will be set below based on the length of the string representation
-            // of the value.
-            v.end_position = m_lexer_ref->get_position();
-
-            switch (v.type())
-            {
-                case value_t::boolean:
-                {
-                    // 4 and 5 are the string length of "true" and "false"
-                    v.start_position = v.end_position - (v.m_data.m_value.boolean ? 4 : 5);
-                    break;
-                }
-
-                case value_t::null:
-                {
-                    // 4 is the string length of "null"
-                    v.start_position = v.end_position - 4;
-                    break;
-                }
-
-                case value_t::string:
-                {
-                    // escape sequences make the token longer than the value it
-                    // parses to, so the start position cannot be derived from
-                    // the value; use the offset the lexer recorded instead
-                    v.start_position = m_lexer_ref->get_token_start_position();
-                    break;
-                }
-
-                // As we handle the start and end positions for values created during parsing,
-                // we do not expect the following value type to be called. Regardless, set the positions
-                // in case this is created manually or through a different constructor. Exclude from lcov
-                // since the exact condition of this switch is esoteric.
-                // LCOV_EXCL_START
-                case value_t::discarded:
-                {
-                    v.end_position = std::string::npos;
-                    v.start_position = v.end_position;
-                    break;
-                }
-                // LCOV_EXCL_STOP
-                case value_t::binary:
-                case value_t::number_integer:
-                case value_t::number_unsigned:
-                case value_t::number_float:
-                {
-                    v.start_position = v.end_position - m_lexer_ref->get_string().size();
-                    break;
-                }
-                case value_t::object:
-                case value_t::array:
-                {
-                    // object and array are handled in start_object() and start_array() handlers
-                    // skip setting the values here.
-                    break;
-                }
-                default: // LCOV_EXCL_LINE
-                    // Handle all possible types discretely, default handler should never be reached.
-                    JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert,-warnings-as-errors) LCOV_EXCL_LINE
-            }
-        }
-    }
-#endif
-
     /*!
     @invariant If the ref stack is empty, then the passed value will be the new
                root.
@@ -11875,7 +11885,7 @@ class json_sax_dom_parser
             root = BasicJsonType(std::forward<Value>(v));
 
 #if JSON_DIAGNOSTIC_POSITIONS
-            handle_diagnostic_positions_for_json_value(root);
+            diagnostic_positions::set_from_lexer(root, m_lexer_ref);
 #endif
 
             return &root;
@@ -11888,7 +11898,7 @@ class json_sax_dom_parser
             ref_stack.back()->m_data.m_value.array->emplace_back(std::forward<Value>(v));
 
 #if JSON_DIAGNOSTIC_POSITIONS
-            handle_diagnostic_positions_for_json_value(ref_stack.back()->m_data.m_value.array->back());
+            diagnostic_positions::set_from_lexer(ref_stack.back()->m_data.m_value.array->back(), m_lexer_ref);
 #endif
 
             return &(ref_stack.back()->m_data.m_value.array->back());
@@ -11899,7 +11909,7 @@ class json_sax_dom_parser
         *object_element = BasicJsonType(std::forward<Value>(v));
 
 #if JSON_DIAGNOSTIC_POSITIONS
-        handle_diagnostic_positions_for_json_value(*object_element);
+        diagnostic_positions::set_from_lexer(*object_element, m_lexer_ref);
 #endif
 
         return object_element;
@@ -12076,7 +12086,7 @@ class json_sax_dom_callback_parser
 
 #if JSON_DIAGNOSTIC_POSITIONS
                     // Set start/end positions for discarded object.
-                    handle_diagnostic_positions_for_json_value(*ref_stack.back());
+                    diagnostic_positions::set_from_lexer(*ref_stack.back(), m_lexer_ref);
 #endif
                 }
             }
@@ -12192,7 +12202,7 @@ class json_sax_dom_callback_parser
 
 #if JSON_DIAGNOSTIC_POSITIONS
                     // Set start/end positions for discarded array.
-                    handle_diagnostic_positions_for_json_value(*ref_stack.back());
+                    diagnostic_positions::set_from_lexer(*ref_stack.back(), m_lexer_ref);
 #endif
                 }
             }
@@ -12245,72 +12255,6 @@ class json_sax_dom_callback_parser
 
   private:
 
-#if JSON_DIAGNOSTIC_POSITIONS
-    void handle_diagnostic_positions_for_json_value(BasicJsonType& v)
-    {
-        if (m_lexer_ref)
-        {
-            // Lexer has read past the current field value, so set the end position to the current position.
-            // The start position will be set below based on the length of the string representation
-            // of the value.
-            v.end_position = m_lexer_ref->get_position();
-
-            switch (v.type())
-            {
-                case value_t::boolean:
-                {
-                    // 4 and 5 are the string length of "true" and "false"
-                    v.start_position = v.end_position - (v.m_data.m_value.boolean ? 4 : 5);
-                    break;
-                }
-
-                case value_t::null:
-                {
-                    // 4 is the string length of "null"
-                    v.start_position = v.end_position - 4;
-                    break;
-                }
-
-                case value_t::string:
-                {
-                    // escape sequences make the token longer than the value it
-                    // parses to, so the start position cannot be derived from
-                    // the value; use the offset the lexer recorded instead
-                    v.start_position = m_lexer_ref->get_token_start_position();
-                    break;
-                }
-
-                case value_t::discarded:
-                {
-                    v.end_position = std::string::npos;
-                    v.start_position = v.end_position;
-                    break;
-                }
-
-                case value_t::binary:
-                case value_t::number_integer:
-                case value_t::number_unsigned:
-                case value_t::number_float:
-                {
-                    v.start_position = v.end_position - m_lexer_ref->get_string().size();
-                    break;
-                }
-
-                case value_t::object:
-                case value_t::array:
-                {
-                    // object and array are handled in start_object() and start_array() handlers
-                    // skip setting the values here.
-                    break;
-                }
-                default: // LCOV_EXCL_LINE
-                    // Handle all possible types discretely, default handler should never be reached.
-                    JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert,-warnings-as-errors) LCOV_EXCL_LINE
-            }
-        }
-    }
-#endif
-
     /// if there is a pending duplicate-key stash entry for this exact slot,
     /// remove it from the stash; if restore_value is true, the stashed
     /// previous value is moved back into the slot first (use this when the
@@ -12432,7 +12376,7 @@ class json_sax_dom_callback_parser
         auto value = BasicJsonType(std::forward<Value>(v));
 
 #if JSON_DIAGNOSTIC_POSITIONS
-        handle_diagnostic_positions_for_json_value(value);
+        diagnostic_positions::set_from_lexer(value, m_lexer_ref);
 #endif
 
         // check callback
@@ -16111,8 +16055,8 @@ class binary_reader
         return enter_object(detail::unknown_size());
     }
 
-    // Note, no reader for UBJSON binary types is implemented because they do
-    // not exist
+    // Note, UBJSON has no binary type of its own; BJData, which shares this
+    // reader, decodes optimized 'B' arrays as binary in get_ubjson_array().
 
     bool get_ubjson_high_precision_number()
     {
@@ -16819,7 +16763,7 @@ class binary_reader
 #endif
     }
 
-    /*
+    /*!
     @brief read a number from the input
 
     @tparam NumberType the type of the number
@@ -16829,10 +16773,10 @@ class binary_reader
     @return whether conversion completed
 
     @note This function needs to respect the system's endianness, because
-          bytes in CBOR, MessagePack, and UBJSON are stored in network order
-          (big endian) and therefore need reordering on little endian systems.
-          On the other hand, BSON and BJData use little endian and should reorder
-          on big endian systems.
+          bytes in CBOR, MessagePack, UBJSON, and BON8 are stored in network
+          order (big endian) and therefore need reordering on little endian
+          systems. On the other hand, BSON and BJData use little endian and
+          should reorder on big endian systems.
     */
     template<typename NumberType, bool InputIsLittleEndian = false>
     bool get_number(const input_format_t format, NumberType& result)
@@ -17186,7 +17130,8 @@ using parser_callback_t =
 /*!
 @brief syntax analysis
 
-This class implements a recursive descent parser.
+This class implements an iterative parser that keeps the open containers on
+an explicit stack and reports what it reads as SAX events.
 */
 template<typename BasicJsonType, typename InputAdapterType>
 class parser
@@ -17571,8 +17516,9 @@ class parser
 
                     // We are done with this array. Before we can parse a
                     // new value, we need to evaluate the new state first.
-                    // By setting skip_to_state_evaluation to false, we
-                    // are effectively jumping to the beginning of this if.
+                    // By setting skip_to_state_evaluation to true, the next
+                    // iteration skips parsing a value and evaluates the
+                    // enclosing state directly.
                     JSON_ASSERT(!states.empty());
                     states.pop_back();
                     skip_to_state_evaluation = true;
@@ -17632,8 +17578,9 @@ class parser
 
                 // We are done with this object. Before we can parse a
                 // new value, we need to evaluate the new state first.
-                // By setting skip_to_state_evaluation to false, we
-                // are effectively jumping to the beginning of this if.
+                // By setting skip_to_state_evaluation to true, the next
+                // iteration skips parsing a value and evaluates the
+                // enclosing state directly.
                 JSON_ASSERT(!states.empty());
                 states.pop_back();
                 skip_to_state_evaluation = true;
@@ -26230,6 +26177,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     friend class ::nlohmann::detail::json_sax_dom_parser;
     template<typename BasicJsonType, typename InputAdapterType>
     friend class ::nlohmann::detail::json_sax_dom_callback_parser;
+#if JSON_DIAGNOSTIC_POSITIONS
+    friend struct ::nlohmann::detail::diagnostic_positions;
+#endif
     friend class ::nlohmann::detail::exception;
 
     /// workaround type for MSVC
@@ -27388,65 +27338,15 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     */
     template<bool Ordered>
     static compare_result compare_leaves(const_reference lhs, const_reference rhs) noexcept
-    {
-        return compare_leaves(lhs, rhs, std::integral_constant<bool, Ordered> {});
-    }
-
-    /// @brief compare two leaves that are only being checked for equality
-    static compare_result compare_leaves(const_reference lhs, const_reference rhs, std::false_type /*ordered*/) noexcept
     {
         if (lhs == rhs)
         {
             return compare_result::equal;
         }
 
-        return order_leaves(lhs, rhs, std::false_type {});
+        return order_leaves(lhs, rhs, std::integral_constant<bool, Ordered> {});
     }
 
-#if JSON_HAS_THREE_WAY_COMPARISON
-    /*!
-    @brief compare two leaves that are being ordered, for operator<=>
-
-    Reached only from operator<=>, so the leaves must be classified exactly
-    as operator<=> classifies them - which is not the same as asking
-    == and then order_leaves(), the way the other overload does it. The two
-    disagree on a binary value: == also compares the subtype, but <=> compares
-    only the bytes, through std::vector<std::uint8_t>::operator<=>. Using <=>
-    itself here keeps a leaf pair classified the same way regardless of how
-    deep it is nested - == first would again call operator<=> a level down
-    through order_leaves(), but call it after a mismatching == already ended
-    the comparison for a pair that <=> alone would still call equivalent.
-    */
-    static compare_result compare_leaves(const_reference lhs, const_reference rhs, std::true_type /*ordered*/) noexcept
-    {
-        const std::partial_ordering order = lhs <=> rhs; // *NOPAD*
-        if (order == 0)
-        {
-            return compare_result::equal;
-        }
-        if (order < 0)
-        {
-            return compare_result::less;
-        }
-        if (order > 0)
-        {
-            return compare_result::greater;
-        }
-        return compare_result::unordered;
-    }
-#else
-    /// @brief compare two leaves that are being ordered, for operator<
-    static compare_result compare_leaves(const_reference lhs, const_reference rhs, std::true_type /*ordered*/) noexcept
-    {
-        if (lhs == rhs)
-        {
-            return compare_result::equal;
-        }
-
-        return order_leaves(lhs, rhs, std::true_type {});
-    }
-#endif
-
     /*!
     @brief compare two object keys
 

tests/src/unit-comparison.cpp

@@ -952,51 +952,3 @@ TEST_CASE("containers are compared element by element")
         }
     }
 }
-
-#if JSON_HAS_THREE_WAY_COMPARISON
-// JSON_HAS_CPP_20 (do not remove; see note at top of file)
-TEST_CASE("operator<=> of binary values with a different subtype does not depend on nesting depth")
-{
-    // #5654: std::vector<std::uint8_t>::operator<=>, which the binary type's
-    // own operator<=> uses, ignores the subtype that operator== checks. So a
-    // pair of binary values with the same bytes but a different subtype is
-    // unequal, yet <=>-equivalent - the same inconsistency between == and <=>
-    // that a NaN has. Within the nesting bound, an array compares itself
-    // with std::vector's own operator<=>, which treats an equivalent pair as
-    // undecided and lets the next element decide, same as
-    // std::lexicographical_compare_three_way does. Past the bound,
-    // compare_iteratively<true>() takes over and must classify the pair the
-    // same way, or the result of operator<=> - and of <, which C++20 derives
-    // from it - depends on how deeply the values are nested.
-    const json a = json::array({json::binary({1}, 1), 1});
-    const json b = json::array({json::binary({1}, 2), 2});
-
-    // the root inconsistency: unequal, yet <=>-equivalent
-    CHECK_FALSE(a[0] == b[0]);
-    CHECK((a[0] <=> b[0]) == std::partial_ordering::equivalent); // *NOPAD*
-
-    const auto deep = [](const json & j, const std::size_t depth)
-    {
-        json result = j;
-        for (std::size_t i = 0; i < depth; ++i)
-        {
-            result = json::array({std::move(result)});
-        }
-        return result;
-    };
-
-    // 127 levels stay within nesting_depth_limit() (128); 128 and 200 do not,
-    // and must still agree with the levels that do
-    for (const std::size_t depth : std::vector<std::size_t> {0, 127, 128, 200})
-    {
-        CAPTURE(depth);
-        const json x = deep(a, depth);
-        const json y = deep(b, depth);
-        CHECK((x <=> y) == std::partial_ordering::less); // *NOPAD*
-        CHECK((y <=> x) == std::partial_ordering::greater); // *NOPAD*
-        CHECK(x < y);
-        CHECK(y > x);
-        CHECK_FALSE(y < x);
-    }
-}
-#endif