unit-class_parser.cpp

//     __ _____ _____ _____
//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
// |  |  |__   |  |  | | | |  version 3.12.0
// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
//
// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
// SPDX-License-Identifier: MIT

#include "doctest_compatibility.h"

// capture whether JSON_STRICT_NUL_HANDLING was enabled on the command line
// (e.g. -DJSON_STRICT_NUL_HANDLING=1) *before* including json.hpp, since the
// library #undefs JSON_STRICT_NUL_HANDLING itself once the header has been
// fully processed unless JSON_TEST_KEEP_MACROS is defined (see
// include/nlohmann/detail/macro_unscope.hpp)
#if defined(JSON_STRICT_NUL_HANDLING) && (JSON_STRICT_NUL_HANDLING == 1)
    #define JSON_TEST_STRICT_NUL_HANDLING_ENABLED 1
#endif

#define JSON_TEST_STRINGIZE_EX(x) #x
#define JSON_TEST_STRINGIZE(x) JSON_TEST_STRINGIZE_EX(x)

#define JSON_TESTS_PRIVATE
#include <nlohmann/json.hpp>
using nlohmann::json;
#ifdef JSON_TEST_NO_GLOBAL_UDLS
    using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
#endif

#include <valarray>
#include <algorithm>
#include <cstdio>
#include <fstream>
#include <list>
#include <sstream>
#include <string>
#include <utility>
#include <vector>

#include "sax_countdown.hpp"
#include "sax_event_loggers.hpp"
#include "test_utils.hpp"

using utils::SaxCountdown;
using utils::SaxEventLogger;

namespace
{
json parser_helper(const std::string& s);
bool accept_helper(const std::string& s);
void comments_helper(const std::string& s);
void trailing_comma_helper(const std::string& s);

json parser_helper(const std::string& s)
{
    json j;
    json::parser(nlohmann::detail::input_adapter(s)).parse(true, j);

    // if this line was reached, no exception occurred
    // -> check if result is the same without exceptions
    json j_nothrow;
    CHECK_NOTHROW(json::parser(nlohmann::detail::input_adapter(s), nullptr, false).parse(true, j_nothrow));
    CHECK(j_nothrow == j);

    json j_sax;
    nlohmann::detail::json_sax_dom_parser<json, nlohmann::detail::string_input_adapter_type> sdp(j_sax);
    json::sax_parse(s, &sdp);
    CHECK(j_sax == j);

    comments_helper(s);

    trailing_comma_helper(s);

    return j;
}

bool accept_helper(const std::string& s)
{
    CAPTURE(s)

    // 1. parse s without exceptions
    json j;
    CHECK_NOTHROW(json::parser(nlohmann::detail::input_adapter(s), nullptr, false).parse(true, j));
    const bool ok_noexcept = !j.is_discarded();

    // 2. accept s
    const bool ok_accept = json::parser(nlohmann::detail::input_adapter(s)).accept(true);

    // 3. check if both approaches come to the same result
    CHECK(ok_noexcept == ok_accept);

    // 4. parse with SAX (compare with relaxed accept result)
    SaxEventLogger el;
    CHECK_NOTHROW(json::sax_parse(s, &el, json::input_format_t::json, false));
    CHECK(json::parser(nlohmann::detail::input_adapter(s)).accept(false) == !el.errored);

    // 5. parse with simple callback
    json::parser_callback_t const cb = [](int /*unused*/, json::parse_event_t /*unused*/, json& /*unused*/) noexcept
    {
        return true;
    };
    json const j_cb = json::parse(s, cb, false);
    const bool ok_noexcept_cb = !j_cb.is_discarded();

    // 6. check if this approach came to the same result
    CHECK(ok_noexcept == ok_noexcept_cb);

    // 7. check if comments or trailing commas are properly ignored
    if (ok_accept)
    {
        comments_helper(s);
        trailing_comma_helper(s);
    }

    // 8. return result
    return ok_accept;
}

void comments_helper(const std::string& s)
{
    json _;

    // parse/accept with default parser
    CHECK_NOTHROW(_ = json::parse(s));
    CHECK(json::accept(s));

    // parse/accept while skipping comments
    CHECK_NOTHROW(_ = json::parse(s, nullptr, false, true));
    CHECK(json::accept(s, true));

    std::vector<std::string> json_with_comments;

    // start with a comment
    json_with_comments.push_back(std::string("// this is a comment\n") + s);
    json_with_comments.push_back(std::string("/* this is a comment */") + s);
    // end with a comment
    json_with_comments.push_back(s + "// this is a comment");
    json_with_comments.push_back(s + "/* this is a comment */");

    // check all strings
    for (const auto& json_with_comment : json_with_comments)
    {
        CAPTURE(json_with_comment)
        CHECK_THROWS_AS(_ = json::parse(json_with_comment), json::parse_error);
        CHECK(!json::accept(json_with_comment));

        CHECK_NOTHROW(_ = json::parse(json_with_comment, nullptr, true, true));
        CHECK(json::accept(json_with_comment, true));
    }
}

void trailing_comma_helper(const std::string& s)
{
    json _;

    // parse/accept with default parser
    CHECK_NOTHROW(_ = json::parse(s));
    CHECK(json::accept(s));

    // parse/accept while allowing trailing commas
    CHECK_NOTHROW(_ = json::parse(s, nullptr, false, false, true));
    CHECK(json::accept(s, false, true));

    // note: [,] and {,} are not allowed
    if (s.size() > 1 && (s.back() == ']' || s.back() == '}') && !_.empty())
    {
        std::vector<std::string> json_with_trailing_commas;
        json_with_trailing_commas.push_back(s.substr(0, s.size() - 1) + " ," + s.back());
        json_with_trailing_commas.push_back(s.substr(0, s.size() - 1) + "," + s.back());
        json_with_trailing_commas.push_back(s.substr(0, s.size() - 1) + ", " + s.back());

        for (const auto& json_with_trailing_comma : json_with_trailing_commas)
        {
            CAPTURE(json_with_trailing_comma)
            CHECK_THROWS_AS(_ = json::parse(json_with_trailing_comma), json::parse_error);
            CHECK(!json::accept(json_with_trailing_comma));

            CHECK_NOTHROW(_ = json::parse(json_with_trailing_comma, nullptr, true, false, true));
            CHECK(json::accept(json_with_trailing_comma, false, true));
        }
    }
}

#if JSON_DIAGNOSTIC_POSITIONS
/**
 * Validates that the generated JSON object is the same as expected
 * Validates that the start position and end position match the start and end of the string
 *
 * This check assumes that there is no whitespace around the json object in the original string.
 */
void validate_generated_json_and_start_end_pos_helper(const std::string& original_string, const json& j, const json& check)
{
    CHECK(j == check);
    CHECK(j.start_pos() == 0);
    CHECK(j.end_pos() == original_string.size());
}

/**
 * Parses the root object from the given root string and validates that the start and end positions for the nested object are correct.
 *
 * This checks that whitespace around the nested object is included in the start and end positions of the root object.
 */
void validate_start_end_pos_for_nested_obj_helper(const std::string& nested_type_json_str, const std::string& root_type_json_str, const json& expected_json, const json::parser_callback_t& cb = nullptr)
{
    json j;

    // 1. If callback is provided, use callback version of parse()
    if (cb)
    {
        j = json::parse(root_type_json_str, cb);
    }
    else
    {
        j = json::parse(root_type_json_str);
    }

    // 2. Check if the generated JSON is as expected
    // Assumptions: The root_type_json_str does not have any whitespace around the json object
    validate_generated_json_and_start_end_pos_helper(root_type_json_str, j, expected_json);

    // 3. Get the nested object
    const auto& nested = j["nested"];
    // 4. Check if the start and end positions are generated correctly for nested objects and arrays
    CHECK(nested_type_json_str == root_type_json_str.substr(nested.start_pos(), nested.end_pos() - nested.start_pos()));
}
#endif

} // namespace

TEST_CASE("parser class")
{
    SECTION("parse")
    {
        SECTION("null")
        {
            CHECK(parser_helper("null") == json(nullptr));
        }

        SECTION("true")
        {
            CHECK(parser_helper("true") == json(true));
        }

        SECTION("false")
        {
            CHECK(parser_helper("false") == json(false));
        }

        SECTION("array")
        {
            SECTION("empty array")
            {
                CHECK(parser_helper("[]") == json(json::value_t::array));
                CHECK(parser_helper("[ ]") == json(json::value_t::array));
            }

            SECTION("nonempty array")
            {
                CHECK(parser_helper("[true, false, null]") == json({true, false, nullptr}));
            }
        }

        SECTION("object")
        {
            SECTION("empty object")
            {
                CHECK(parser_helper("{}") == json(json::value_t::object));
                CHECK(parser_helper("{ }") == json(json::value_t::object));
            }

            SECTION("nonempty object")
            {
                CHECK(parser_helper("{\"\": true, \"one\": 1, \"two\": null}") == json({{"", true}, {"one", 1}, {"two", nullptr}}));
            }
        }

        SECTION("string")
        {
            // empty string
            CHECK(parser_helper("\"\"") == json(json::value_t::string));

            SECTION("errors")
            {
                // error: tab in string
                CHECK_THROWS_WITH_AS(parser_helper("\"\t\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0009 (HT) must be escaped to \\u0009 or \\t; last read: '\"<U+0009>'", json::parse_error&);
                // error: newline in string
                CHECK_THROWS_WITH_AS(parser_helper("\"\n\""), "[json.exception.parse_error.101] parse error at line 2, column 0: syntax error while parsing value - invalid string: control character U+000A (LF) must be escaped to \\u000A or \\n; last read: '\"<U+000A>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\r\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+000D (CR) must be escaped to \\u000D or \\r; last read: '\"<U+000D>'", json::parse_error&);
                // error: backspace in string
                CHECK_THROWS_WITH_AS(parser_helper("\"\b\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0008 (BS) must be escaped to \\u0008 or \\b; last read: '\"<U+0008>'", json::parse_error&);
                // improve code coverage
                CHECK_THROWS_AS(parser_helper("\uFF01"), json::parse_error&);
                CHECK_THROWS_AS(parser_helper("[-4:1,]"), json::parse_error&);
                // unescaped control characters
                CHECK_THROWS_WITH_AS(parser_helper("\"\x00\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: missing closing quote; last read: '\"'", json::parse_error&); // NOLINT(bugprone-string-literal-with-embedded-nul)
                CHECK_THROWS_WITH_AS(parser_helper("\"\x01\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0001 (SOH) must be escaped to \\u0001; last read: '\"<U+0001>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x02\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0002 (STX) must be escaped to \\u0002; last read: '\"<U+0002>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x03\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0003 (ETX) must be escaped to \\u0003; last read: '\"<U+0003>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x04\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0004 (EOT) must be escaped to \\u0004; last read: '\"<U+0004>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x05\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0005 (ENQ) must be escaped to \\u0005; last read: '\"<U+0005>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x06\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0006 (ACK) must be escaped to \\u0006; last read: '\"<U+0006>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x07\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0007 (BEL) must be escaped to \\u0007; last read: '\"<U+0007>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x08\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0008 (BS) must be escaped to \\u0008 or \\b; last read: '\"<U+0008>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x09\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0009 (HT) must be escaped to \\u0009 or \\t; last read: '\"<U+0009>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x0a\""), "[json.exception.parse_error.101] parse error at line 2, column 0: syntax error while parsing value - invalid string: control character U+000A (LF) must be escaped to \\u000A or \\n; last read: '\"<U+000A>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x0b\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+000B (VT) must be escaped to \\u000B; last read: '\"<U+000B>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x0c\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+000C (FF) must be escaped to \\u000C or \\f; last read: '\"<U+000C>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x0d\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+000D (CR) must be escaped to \\u000D or \\r; last read: '\"<U+000D>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x0e\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+000E (SO) must be escaped to \\u000E; last read: '\"<U+000E>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x0f\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+000F (SI) must be escaped to \\u000F; last read: '\"<U+000F>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x10\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0010 (DLE) must be escaped to \\u0010; last read: '\"<U+0010>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x11\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0011 (DC1) must be escaped to \\u0011; last read: '\"<U+0011>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x12\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0012 (DC2) must be escaped to \\u0012; last read: '\"<U+0012>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x13\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0013 (DC3) must be escaped to \\u0013; last read: '\"<U+0013>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x14\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0014 (DC4) must be escaped to \\u0014; last read: '\"<U+0014>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x15\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0015 (NAK) must be escaped to \\u0015; last read: '\"<U+0015>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x16\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0016 (SYN) must be escaped to \\u0016; last read: '\"<U+0016>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x17\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0017 (ETB) must be escaped to \\u0017; last read: '\"<U+0017>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x18\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0018 (CAN) must be escaped to \\u0018; last read: '\"<U+0018>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x19\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0019 (EM) must be escaped to \\u0019; last read: '\"<U+0019>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x1a\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+001A (SUB) must be escaped to \\u001A; last read: '\"<U+001A>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x1b\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+001B (ESC) must be escaped to \\u001B; last read: '\"<U+001B>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x1c\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+001C (FS) must be escaped to \\u001C; last read: '\"<U+001C>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x1d\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+001D (GS) must be escaped to \\u001D; last read: '\"<U+001D>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x1e\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+001E (RS) must be escaped to \\u001E; last read: '\"<U+001E>'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("\"\x1f\""), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+001F (US) must be escaped to \\u001F; last read: '\"<U+001F>'", json::parse_error&);

                SECTION("additional test for null byte")
                {
                    // The test above for the null byte is wrong, because passing
                    // a string to the parser only reads int until it encounters
                    // a null byte. This test inserts the null byte later on and
                    // uses an iterator range.
                    std::string s = "\"1\"";
                    s[1] = '\0';
                    json _;
                    CHECK_THROWS_WITH_AS(_ = json::parse(s.begin(), s.end()), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0000 (NUL) must be escaped to \\u0000; last read: '\"<U+0000>'", json::parse_error&);
                }
            }

            SECTION("escaped")
            {
                // quotation mark "\""
                auto r1 = R"("\"")"_json;
                CHECK(parser_helper("\"\\\"\"") == r1);
                // reverse solidus "\\"
                auto r2 = R"("\\")"_json;
                CHECK(parser_helper("\"\\\\\"") == r2);
                // solidus
                CHECK(parser_helper("\"\\/\"") == R"("/")"_json);
                // backspace
                CHECK(parser_helper("\"\\b\"") == json("\b"));
                // formfeed
                CHECK(parser_helper("\"\\f\"") == json("\f"));
                // newline
                CHECK(parser_helper("\"\\n\"") == json("\n"));
                // carriage return
                CHECK(parser_helper("\"\\r\"") == json("\r"));
                // horizontal tab
                CHECK(parser_helper("\"\\t\"") == json("\t"));

                CHECK(parser_helper("\"\\u0001\"").get<json::string_t>() == "\x01");
                CHECK(parser_helper("\"\\u000a\"").get<json::string_t>() == "\n");
                CHECK(parser_helper("\"\\u00b0\"").get<json::string_t>() == "°");
                CHECK(parser_helper("\"\\u0c00\"").get<json::string_t>() == "ఀ");
                CHECK(parser_helper("\"\\ud000\"").get<json::string_t>() == "퀀");
                CHECK(parser_helper("\"\\u000E\"").get<json::string_t>() == "\x0E");
                CHECK(parser_helper("\"\\u00F0\"").get<json::string_t>() == "ð");
                CHECK(parser_helper("\"\\u0100\"").get<json::string_t>() == "Ā");
                CHECK(parser_helper("\"\\u2000\"").get<json::string_t>() == " ");
                CHECK(parser_helper("\"\\uFFFF\"").get<json::string_t>() == "￿");
                CHECK(parser_helper("\"\\u20AC\"").get<json::string_t>() == "€");
                CHECK(parser_helper("\"€\"").get<json::string_t>() == "€");
                CHECK(parser_helper("\"🎈\"").get<json::string_t>() == "🎈");

                CHECK(parser_helper("\"\\ud80c\\udc60\"").get<json::string_t>() == "\xf0\x93\x81\xa0");
                CHECK(parser_helper("\"\\ud83c\\udf1e\"").get<json::string_t>() == "🌞");
            }
        }

        SECTION("NUL byte handling (issue #5530, JSON_STRICT_NUL_HANDLING)")
        {
            // by default, a NUL byte anywhere in the input (not inside a quoted
            // string, which is covered above) is silently treated the same as
            // real end of input; JSON_STRICT_NUL_HANDLING (off by default, see
            // docs/mkdocs/docs/api/macros/json_strict_nul_handling.md) makes a
            // NUL byte an error like any other unexpected byte instead.
            //
            // The two sections below are mutually exclusive: this whole test
            // binary is compiled once, with JSON_STRICT_NUL_HANDLING either
            // left at its default or forced to 1 (e.g. by the dedicated
            // ci_test_strict_nul_handling CI target), so only the section
            // matching the actual, compiled-in behavior can pass.
            SECTION("the macro is part of the ABI tag")
            {
                const std::string ns = JSON_TEST_STRINGIZE(NLOHMANN_JSON_NAMESPACE);
#if defined(JSON_TEST_STRICT_NUL_HANDLING_ENABLED)
                CHECK(ns.find("_snul") != std::string::npos);
#else
                CHECK(ns.find("_snul") == std::string::npos);
#endif
            }

#if !defined(JSON_TEST_STRICT_NUL_HANDLING_ENABLED)
            SECTION("default behavior (macro not enabled)")
            {
                // a NUL byte after a complete value silently truncates the input
                std::string s = "123";
                s.push_back('\0');
                s += "4";
                CHECK(json::parse(s) == json(123));
                CHECK(json::accept(s));

                // parsing from a string literal is unaffected either way
                CHECK(json::parse("123") == json(123));

                // a NUL byte that ends a // comment ends the input just
                // like a NUL byte anywhere else (issue #5659); before the
                // fix, the NUL was consumed as part of the comment, and
                // scanning continued with whatever followed it
                {
                    // same as "//c" alone (real end of input after the
                    // comment), rather than continuing with "[1]"
                    std::string s1 = "//c";
                    s1.push_back('\0');
                    s1 += "[1]";
                    json _; // NOLINT(readability-identifier-naming)
                    CHECK_THROWS_WITH_AS(_ = json::parse(s1, nullptr, true, true),
                                         "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - unexpected end of input; expected '[', '{', or a literal",
                                         json::parse_error&);
                    CHECK_FALSE(json::accept(s1, true, true));
                }

                {
                    // same as "[1, //c" alone, rather than continuing with " 2]"
                    std::string s2 = "[1, //c";
                    s2.push_back('\0');
                    s2 += " 2]";
                    json _; // NOLINT(readability-identifier-naming)
                    CHECK_THROWS_WITH_AS(_ = json::parse(s2, nullptr, true, true),
                                         "[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected end of input; expected '[', '{', or a literal",
                                         json::parse_error&);
                    CHECK_FALSE(json::accept(s2, true, true));
                }

                {
                    // same as "1 //c" alone: the comment (and the NUL that
                    // ends it) is ignored, and "x" is never reached
                    std::string s3 = "1 //c";
                    s3.push_back('\0');
                    s3 += "x";
                    CHECK(json::parse(s3, nullptr, true, true) == json(1));
                    CHECK(json::accept(s3, true, true));
                }
            }
#endif

#if defined(JSON_TEST_STRICT_NUL_HANDLING_ENABLED)
            SECTION("opt-in strict behavior (JSON_STRICT_NUL_HANDLING == 1)")
            {
                // a NUL byte after a complete value is now a parse error,
                // instead of silently truncating the input
                {
                    std::string s = "123";
                    s.push_back('\0');
                    json _; // NOLINT(readability-identifier-naming)
                    CHECK_THROWS_WITH_AS(_ = json::parse(s),
                                         "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid literal; last read: '123<U+0000>'; expected end of input",
                                         json::parse_error&);
                    CHECK_FALSE(json::accept(s));
                }

                // a NUL byte where a value is expected is now a parse error,
                // instead of being treated the same as an empty input
                {
                    const std::string s(1, '\0');
                    json _; // NOLINT(readability-identifier-naming)
                    CHECK_THROWS_WITH_AS(_ = json::parse(s),
                                         "[json.exception.parse_error.101] parse error at line 1, column 1: syntax error while parsing value - invalid literal; last read: '<U+0000>'",
                                         json::parse_error&);
                    CHECK_FALSE(json::accept(s));
                }

                // a NUL byte inside a // comment no longer stops the comment
                // scan early; scanning continues correctly past it
                {
                    std::string s = "1 // a";
                    s.push_back('\0');
                    s += "b\n";
                    CHECK(json::parse(s, nullptr, true, true) == json(1));
                    CHECK(json::accept(s, true, true));
                }

                // a NUL byte inside a /* */ comment no longer stops the
                // comment scan early either
                {
                    std::string s = "1 /* a";
                    s.push_back('\0');
                    s += "b */ ";
                    CHECK(json::parse(s, nullptr, true, true) == json(1));
                    CHECK(json::accept(s, true, true));
                }

                // regression guard: parsing from a string literal (which
                // carries a compiler-appended trailing '\0') still works,
                // even though a NUL byte is now rejected everywhere else
                CHECK(json::parse("123") == json(123));

                // regression test for issue #5658: the same holds for wide,
                // UTF-16, UTF-32, and (C++20) UTF-8 string literals, whose
                // compiler-appended trailing '\0' is not of type `char`
                CHECK(json::parse(L"[1]") == json({1}));
                CHECK(json::accept(L"[1]"));
                CHECK(json::parse(u"[1]") == json({1}));
                CHECK(json::accept(u"[1]"));
                CHECK(json::parse(U"[1]") == json({1}));
                CHECK(json::accept(U"[1]"));
#if defined(__cpp_char8_t)
                CHECK(json::parse(u8"[1]") == json({1}));
                CHECK(json::accept(u8"[1]"));
#endif

                // a NUL byte inside such a literal, as opposed to the single
                // compiler-appended trailing one, is still rejected
                {
                    json _; // NOLINT(readability-identifier-naming)
                    CHECK_THROWS_WITH_AS(_ = json::parse(L"[1\0]"),
                                         "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing array - invalid literal; last read: '1<U+0000>'; expected ']'",
                                         json::parse_error&);
                    CHECK_FALSE(json::accept(L"[1\0]"));
                }
            }
#endif
        }

        SECTION("number")
        {
            SECTION("integers")
            {
                SECTION("without exponent")
                {
                    CHECK(parser_helper("-128") == json(-128));
                    CHECK(parser_helper("-0") == json(-0));
                    CHECK(parser_helper("0") == json(0));
                    CHECK(parser_helper("128") == json(128));
                }

                SECTION("with exponent")
                {
                    CHECK(parser_helper("0e1") == json(0e1));
                    CHECK(parser_helper("0E1") == json(0e1));

                    CHECK(parser_helper("10000E-4") == json(10000e-4));
                    CHECK(parser_helper("10000E-3") == json(10000e-3));
                    CHECK(parser_helper("10000E-2") == json(10000e-2));
                    CHECK(parser_helper("10000E-1") == json(10000e-1));
                    CHECK(parser_helper("10000E0") == json(10000e0));
                    CHECK(parser_helper("10000E1") == json(10000e1));
                    CHECK(parser_helper("10000E2") == json(10000e2));
                    CHECK(parser_helper("10000E3") == json(10000e3));
                    CHECK(parser_helper("10000E4") == json(10000e4));

                    CHECK(parser_helper("10000e-4") == json(10000e-4));
                    CHECK(parser_helper("10000e-3") == json(10000e-3));
                    CHECK(parser_helper("10000e-2") == json(10000e-2));
                    CHECK(parser_helper("10000e-1") == json(10000e-1));
                    CHECK(parser_helper("10000e0") == json(10000e0));
                    CHECK(parser_helper("10000e1") == json(10000e1));
                    CHECK(parser_helper("10000e2") == json(10000e2));
                    CHECK(parser_helper("10000e3") == json(10000e3));
                    CHECK(parser_helper("10000e4") == json(10000e4));

                    CHECK(parser_helper("-0e1") == json(-0e1));
                    CHECK(parser_helper("-0E1") == json(-0e1));
                    CHECK(parser_helper("-0E123") == json(-0e123));

                    // numbers after exponent
                    CHECK(parser_helper("10E0") == json(10e0));
                    CHECK(parser_helper("10E1") == json(10e1));
                    CHECK(parser_helper("10E2") == json(10e2));
                    CHECK(parser_helper("10E3") == json(10e3));
                    CHECK(parser_helper("10E4") == json(10e4));
                    CHECK(parser_helper("10E5") == json(10e5));
                    CHECK(parser_helper("10E6") == json(10e6));
                    CHECK(parser_helper("10E7") == json(10e7));
                    CHECK(parser_helper("10E8") == json(10e8));
                    CHECK(parser_helper("10E9") == json(10e9));
                    CHECK(parser_helper("10E+0") == json(10e0));
                    CHECK(parser_helper("10E+1") == json(10e1));
                    CHECK(parser_helper("10E+2") == json(10e2));
                    CHECK(parser_helper("10E+3") == json(10e3));
                    CHECK(parser_helper("10E+4") == json(10e4));
                    CHECK(parser_helper("10E+5") == json(10e5));
                    CHECK(parser_helper("10E+6") == json(10e6));
                    CHECK(parser_helper("10E+7") == json(10e7));
                    CHECK(parser_helper("10E+8") == json(10e8));
                    CHECK(parser_helper("10E+9") == json(10e9));
                    CHECK(parser_helper("10E-1") == json(10e-1));
                    CHECK(parser_helper("10E-2") == json(10e-2));
                    CHECK(parser_helper("10E-3") == json(10e-3));
                    CHECK(parser_helper("10E-4") == json(10e-4));
                    CHECK(parser_helper("10E-5") == json(10e-5));
                    CHECK(parser_helper("10E-6") == json(10e-6));
                    CHECK(parser_helper("10E-7") == json(10e-7));
                    CHECK(parser_helper("10E-8") == json(10e-8));
                    CHECK(parser_helper("10E-9") == json(10e-9));
                }

                SECTION("edge cases")
                {
                    // From RFC8259, Section 6:
                    // Note that when such software is used, numbers that are
                    // integers and are in the range [-(2**53)+1, (2**53)-1]
                    // are interoperable in the sense that implementations will
                    // agree exactly on their numeric values.

                    // -(2**53)+1
                    CHECK(parser_helper("-9007199254740991").get<int64_t>() == -9007199254740991);
                    // (2**53)-1
                    CHECK(parser_helper("9007199254740991").get<int64_t>() == 9007199254740991);
                }

                SECTION("over the edge cases")  // issue #178 - Integer conversion to unsigned (incorrect handling of 64-bit integers)
                {
                    // While RFC8259, Section 6 specifies a preference for support
                    // for ranges in range of IEEE 754-2008 binary64 (double precision)
                    // this does not accommodate 64-bit integers without loss of accuracy.
                    // As 64-bit integers are now widely used in software, it is desirable
                    // to expand support to the full 64 bit (signed and unsigned) range
                    // i.e. -(2**63) -> (2**64)-1.

                    // -(2**63)    ** Note: compilers see negative literals as negated positive numbers (hence the -1))
                    CHECK(parser_helper("-9223372036854775808").get<int64_t>() == -9223372036854775807 - 1);
                    // (2**63)-1
                    CHECK(parser_helper("9223372036854775807").get<int64_t>() == 9223372036854775807);
                    // (2**64)-1
                    CHECK(parser_helper("18446744073709551615").get<uint64_t>() == 18446744073709551615u);
                }
            }

            SECTION("floating-point")
            {
                SECTION("without exponent")
                {
                    CHECK(parser_helper("-128.5") == json(-128.5));
                    CHECK(parser_helper("0.999") == json(0.999));
                    CHECK(parser_helper("128.5") == json(128.5));
                    CHECK(parser_helper("-0.0") == json(-0.0));
                }

                SECTION("with exponent")
                {
                    CHECK(parser_helper("-128.5E3") == json(-128.5E3));
                    CHECK(parser_helper("-128.5E-3") == json(-128.5E-3));
                    CHECK(parser_helper("-0.0e1") == json(-0.0e1));
                    CHECK(parser_helper("-0.0E1") == json(-0.0e1));
                }
            }

            SECTION("overflow")
            {
                // overflows during parsing yield an exception
                // empty() is nodiscard; the exception is thrown by parser_helper() itself, before empty() would run
                CHECK_THROWS_WITH_AS(utils::ignore_return_value(parser_helper("1.18973e+4932").empty()), "[json.exception.out_of_range.406] number overflow parsing '1.18973e+4932'", json::out_of_range&);
            }

            SECTION("invalid numbers")
            {
                // numbers must not begin with "+"
                CHECK_THROWS_AS(parser_helper("+1"), json::parse_error&);
                CHECK_THROWS_AS(parser_helper("+0"), json::parse_error&);

                CHECK_THROWS_WITH_AS(parser_helper("01"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - unexpected number literal; expected end of input", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("-01"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - unexpected number literal; expected end of input", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("--1"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid number; expected digit after '-'; last read: '--'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("1."),
                                     "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid number; expected digit after '.'; last read: '1.'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("1E"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid number; expected '+', '-', or digit after exponent; last read: '1E'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("1E-"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid number; expected digit after exponent sign; last read: '1E-'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("1.E1"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid number; expected digit after '.'; last read: '1.E'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("-1E"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid number; expected '+', '-', or digit after exponent; last read: '-1E'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("-0E#"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid number; expected '+', '-', or digit after exponent; last read: '-0E#'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("-0E-#"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 5: syntax error while parsing value - invalid number; expected digit after exponent sign; last read: '-0E-#'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("-0#"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid literal; last read: '-0#'; expected end of input", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("-0.0:"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 5: syntax error while parsing value - unexpected ':'; expected end of input", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("-0.0Z"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 5: syntax error while parsing value - invalid literal; last read: '-0.0Z'; expected end of input", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("-0E123:"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 7: syntax error while parsing value - unexpected ':'; expected end of input", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("-0e0-:"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 6: syntax error while parsing value - invalid number; expected digit after '-'; last read: '-:'; expected end of input", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("-0e-:"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 5: syntax error while parsing value - invalid number; expected digit after exponent sign; last read: '-0e-:'", json::parse_error&);
                CHECK_THROWS_WITH_AS(parser_helper("-0f"),
                                     "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid literal; last read: '-0f'; expected end of input", json::parse_error&);
            }
        }
    }

    SECTION("accept")
    {
        SECTION("null")
        {
            CHECK(accept_helper("null"));
        }

        SECTION("true")
        {
            CHECK(accept_helper("true"));
        }

        SECTION("false")
        {
            CHECK(accept_helper("false"));
        }

        SECTION("array")
        {
            SECTION("empty array")
            {
                CHECK(accept_helper("[]"));
                CHECK(accept_helper("[ ]"));
            }

            SECTION("nonempty array")
            {
                CHECK(accept_helper("[true, false, null]"));
            }
        }

        SECTION("object")
        {
            SECTION("empty object")
            {
                CHECK(accept_helper("{}"));
                CHECK(accept_helper("{ }"));
            }

            SECTION("nonempty object")
            {
                CHECK(accept_helper("{\"\": true, \"one\": 1, \"two\": null}"));
            }
        }

        SECTION("string")
        {
            // empty string
            CHECK(accept_helper("\"\""));

            SECTION("errors")
            {
                // error: tab in string
                CHECK(accept_helper("\"\t\"") == false);
                // error: newline in string
                CHECK(accept_helper("\"\n\"") == false);
                CHECK(accept_helper("\"\r\"") == false);
                // error: backspace in string
                CHECK(accept_helper("\"\b\"") == false);
                // improve code coverage
                CHECK(accept_helper("\uFF01") == false);
                CHECK(accept_helper("[-4:1,]") == false);
                // unescaped control characters
                CHECK(accept_helper("\"\x00\"") == false); // NOLINT(bugprone-string-literal-with-embedded-nul)
                CHECK(accept_helper("\"\x01\"") == false);
                CHECK(accept_helper("\"\x02\"") == false);
                CHECK(accept_helper("\"\x03\"") == false);
                CHECK(accept_helper("\"\x04\"") == false);
                CHECK(accept_helper("\"\x05\"") == false);
                CHECK(accept_helper("\"\x06\"") == false);
                CHECK(accept_helper("\"\x07\"") == false);
                CHECK(accept_helper("\"\x08\"") == false);
                CHECK(accept_helper("\"\x09\"") == false);
                CHECK(accept_helper("\"\x0a\"") == false);
                CHECK(accept_helper("\"\x0b\"") == false);
                CHECK(accept_helper("\"\x0c\"") == false);
                CHECK(accept_helper("\"\x0d\"") == false);
                CHECK(accept_helper("\"\x0e\"") == false);
                CHECK(accept_helper("\"\x0f\"") == false);
                CHECK(accept_helper("\"\x10\"") == false);
                CHECK(accept_helper("\"\x11\"") == false);
                CHECK(accept_helper("\"\x12\"") == false);
                CHECK(accept_helper("\"\x13\"") == false);
                CHECK(accept_helper("\"\x14\"") == false);
                CHECK(accept_helper("\"\x15\"") == false);
                CHECK(accept_helper("\"\x16\"") == false);
                CHECK(accept_helper("\"\x17\"") == false);
                CHECK(accept_helper("\"\x18\"") == false);
                CHECK(accept_helper("\"\x19\"") == false);
                CHECK(accept_helper("\"\x1a\"") == false);
                CHECK(accept_helper("\"\x1b\"") == false);
                CHECK(accept_helper("\"\x1c\"") == false);
                CHECK(accept_helper("\"\x1d\"") == false);
                CHECK(accept_helper("\"\x1e\"") == false);
                CHECK(accept_helper("\"\x1f\"") == false);
            }

            SECTION("escaped")
            {
                // quotation mark "\""
                auto r1 = R"("\"")"_json;
                CHECK(accept_helper("\"\\\"\""));
                // reverse solidus "\\"
                auto r2 = R"("\\")"_json;
                CHECK(accept_helper("\"\\\\\""));
                // solidus
                CHECK(accept_helper("\"\\/\""));
                // backspace
                CHECK(accept_helper("\"\\b\""));
                // formfeed
                CHECK(accept_helper("\"\\f\""));
                // newline
                CHECK(accept_helper("\"\\n\""));
                // carriage return
                CHECK(accept_helper("\"\\r\""));
                // horizontal tab
                CHECK(accept_helper("\"\\t\""));

                CHECK(accept_helper("\"\\u0001\""));
                CHECK(accept_helper("\"\\u000a\""));
                CHECK(accept_helper("\"\\u00b0\""));
                CHECK(accept_helper("\"\\u0c00\""));
                CHECK(accept_helper("\"\\ud000\""));
                CHECK(accept_helper("\"\\u000E\""));
                CHECK(accept_helper("\"\\u00F0\""));
                CHECK(accept_helper("\"\\u0100\""));
                CHECK(accept_helper("\"\\u2000\""));
                CHECK(accept_helper("\"\\uFFFF\""));
                CHECK(accept_helper("\"\\u20AC\""));
                CHECK(accept_helper("\"€\""));
                CHECK(accept_helper("\"🎈\""));

                CHECK(accept_helper("\"\\ud80c\\udc60\""));
                CHECK(accept_helper("\"\\ud83c\\udf1e\""));
            }
        }

        SECTION("number")
        {
            SECTION("integers")
            {
                SECTION("without exponent")
                {
                    CHECK(accept_helper("-128"));
                    CHECK(accept_helper("-0"));
                    CHECK(accept_helper("0"));
                    CHECK(accept_helper("128"));
                }

                SECTION("with exponent")
                {
                    CHECK(accept_helper("0e1"));
                    CHECK(accept_helper("0E1"));

                    CHECK(accept_helper("10000E-4"));
                    CHECK(accept_helper("10000E-3"));
                    CHECK(accept_helper("10000E-2"));
                    CHECK(accept_helper("10000E-1"));
                    CHECK(accept_helper("10000E0"));
                    CHECK(accept_helper("10000E1"));
                    CHECK(accept_helper("10000E2"));
                    CHECK(accept_helper("10000E3"));
                    CHECK(accept_helper("10000E4"));

                    CHECK(accept_helper("10000e-4"));
                    CHECK(accept_helper("10000e-3"));
                    CHECK(accept_helper("10000e-2"));
                    CHECK(accept_helper("10000e-1"));
                    CHECK(accept_helper("10000e0"));
                    CHECK(accept_helper("10000e1"));
                    CHECK(accept_helper("10000e2"));
                    CHECK(accept_helper("10000e3"));
                    CHECK(accept_helper("10000e4"));

                    CHECK(accept_helper("-0e1"));
                    CHECK(accept_helper("-0E1"));
                    CHECK(accept_helper("-0E123"));
                }

                SECTION("edge cases")
                {
                    // From RFC8259, Section 6:
                    // Note that when such software is used, numbers that are
                    // integers and are in the range [-(2**53)+1, (2**53)-1]
                    // are interoperable in the sense that implementations will
                    // agree exactly on their numeric values.

                    // -(2**53)+1
                    CHECK(accept_helper("-9007199254740991"));
                    // (2**53)-1
                    CHECK(accept_helper("9007199254740991"));
                }

                SECTION("over the edge cases")  // issue #178 - Integer conversion to unsigned (incorrect handling of 64-bit integers)
                {
                    // While RFC8259, Section 6 specifies a preference for support
                    // for ranges in range of IEEE 754-2008 binary64 (double precision)
                    // this does not accommodate 64 bit integers without loss of accuracy.
                    // As 64 bit integers are now widely used in software, it is desirable
                    // to expand support to the full 64 bit (signed and unsigned) range
                    // i.e. -(2**63) -> (2**64)-1.

                    // -(2**63)    ** Note: compilers see negative literals as negated positive numbers (hence the -1))
                    CHECK(accept_helper("-9223372036854775808"));
                    // (2**63)-1
                    CHECK(accept_helper("9223372036854775807"));
                    // (2**64)-1
                    CHECK(accept_helper("18446744073709551615"));
                }
            }

            SECTION("floating-point")
            {
                SECTION("without exponent")
                {
                    CHECK(accept_helper("-128.5"));
                    CHECK(accept_helper("0.999"));
                    CHECK(accept_helper("128.5"));
                    CHECK(accept_helper("-0.0"));
                }

                SECTION("with exponent")
                {
                    CHECK(accept_helper("-128.5E3"));
                    CHECK(accept_helper("-128.5E-3"));
                    CHECK(accept_helper("-0.0e1"));
                    CHECK(accept_helper("-0.0E1"));
                }
            }

            SECTION("overflow")
            {
                // overflows during parsing
                CHECK(!accept_helper("1.18973e+4932"));
            }

            SECTION("invalid numbers")
            {
                CHECK(accept_helper("01") == false);
                CHECK(accept_helper("--1") == false);
                CHECK(accept_helper("1.") == false);
                CHECK(accept_helper("1E") == false);
                CHECK(accept_helper("1E-") == false);
                CHECK(accept_helper("1.E1") == false);
                CHECK(accept_helper("-1E") == false);
                CHECK(accept_helper("-0E#") == false);
                CHECK(accept_helper("-0E-#") == false);
                CHECK(accept_helper("-0#") == false);
                CHECK(accept_helper("-0.0:") == false);
                CHECK(accept_helper("-0.0Z") == false);
                CHECK(accept_helper("-0E123:") == false);
                CHECK(accept_helper("-0e0-:") == false);
                CHECK(accept_helper("-0e-:") == false);
                CHECK(accept_helper("-0f") == false);

                // numbers must not begin with "+"
                CHECK(accept_helper("+1") == false);
                CHECK(accept_helper("+0") == false);
            }

            SECTION("issue #5411 - skip conversion when accept() does not need the numeric value")
            {
                // lexer::scan_number() may skip strtoull()/strtoll() for
                // value_unsigned/value_integer tokens when the caller (e.g.
                // json::accept()) does not need the converted value, as long
                // as the digit count alone guarantees no 64-bit overflow (see
                // the "safe_digit_count" fast path in scan_number()). This
                // differential test checks that json::accept() (which enables
                // the fast path) and json::parse() (which never does) always
                // agree, over a corpus that exercises both the fast path
                // (<=18 digits) and the untouched, exact fallback path (>=19
                // digits) -- including reclassification of huge digit-only
                // integers to a (possibly non-finite) floating-point value.
                const std::vector<std::pair<std::string, bool>> cases =
                {
                    // normal small/large integers, both signs
                    {"0", true}, {"1", true}, {"-1", true}, {"42", true}, {"-42", true},
                    {"123456789", true}, {"-123456789", true},

                    // digit-count boundary around the 18-digit safe cutoff (both signs)
                    {std::string(17, '9'), true},
                    {std::string(18, '9'), true},
                    {std::string(19, '9'), true},
                    {std::string(20, '9'), true},
                    {"-" + std::string(17, '9'), true},
                    {"-" + std::string(18, '9'), true},
                    {"-" + std::string(19, '9'), true},
                    {"-" + std::string(20, '9'), true},

                    // 64-bit boundaries
                    {"9223372036854775807", true},    // INT64_MAX
                    {"-9223372036854775808", true},   // INT64_MIN
                    {"18446744073709551615", true},   // UINT64_MAX
                    {"18446744073709551616", true},   // UINT64_MAX + 1 (overflows uint64_t, finite double)

                    // the 28-digit example from the issue: overflows uint64_t
                    // but is finite as a double, so the scanner reclassifies
                    // it to value_float and it is accepted
                    {"9999999999999999999999999999", true},

                    // huge digit-only integers that overflow even a double -> rejected
                    {std::string(309, '9'), false},
                    {std::string(400, '9'), false},
                    {"1" + std::string(400, '0'), false},

                    // 1e999 / 1e400 style overflow -> rejected
                    {"1e999", false},
                    {"1e400", false},
                    {"-1e999", false},
                    {"1E999", false},

                    // values straddling DBL_MAX
                    {"1.7976931348623157e308", true},   // <= DBL_MAX, finite
                    {"1.7976931348623159e308", false},  // > DBL_MAX, overflows to inf

                    // a mix of other valid/invalid numeric syntax
                    {"3.14159", true},
                    {"-0.0", true},
                    {"1.0e10", true},
                    {"01", false},
                    {"-", false},
                    {"1.", false},
                    {"1e", false},
                    {"+1", false},
                };

                for (const auto& c : cases)
                {
                    const std::string& number = c.first;
                    const bool expected = c.second;
                    CAPTURE(number)
                    CAPTURE(expected)

                    // accept() takes the fast path (skips conversion when possible)
                    CHECK(json::accept(number) == expected);

                    // parse() always performs the full conversion; it must agree
                    json j;
                    CHECK_NOTHROW(json::parser(nlohmann::detail::input_adapter(number), nullptr, false).parse(true, j));
                    CHECK(!j.is_discarded() == expected);

                    // wrap in an array so get_token() is exercised beyond the
                    // very first (constructor-time) scan as well
                    std::string wrapped = "[";
                    wrapped += number;
                    wrapped += ",";
                    wrapped += number;
                    wrapped += "]";
                    CHECK(json::accept(wrapped) == expected);
                }
            }
        }
    }

    SECTION("parse errors")
    {
        // unexpected end of number
        CHECK_THROWS_WITH_AS(parser_helper("0."),
                             "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid number; expected digit after '.'; last read: '0.'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("-"),
                             "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid number; expected digit after '-'; last read: '-'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("--"),
                             "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid number; expected digit after '-'; last read: '--'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("-0."),
                             "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid number; expected digit after '.'; last read: '-0.'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("-."),
                             "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid number; expected digit after '-'; last read: '-.'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("-:"),
                             "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid number; expected digit after '-'; last read: '-:'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("0.:"),
                             "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid number; expected digit after '.'; last read: '0.:'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("e."),
                             "[json.exception.parse_error.101] parse error at line 1, column 1: syntax error while parsing value - invalid literal; last read: 'e'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("1e."),
                             "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid number; expected '+', '-', or digit after exponent; last read: '1e.'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("1e/"),
                             "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid number; expected '+', '-', or digit after exponent; last read: '1e/'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("1e:"),
                             "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid number; expected '+', '-', or digit after exponent; last read: '1e:'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("1E."),
                             "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid number; expected '+', '-', or digit after exponent; last read: '1E.'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("1E/"),
                             "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid number; expected '+', '-', or digit after exponent; last read: '1E/'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("1E:"),
                             "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid number; expected '+', '-', or digit after exponent; last read: '1E:'", json::parse_error&);

        // unexpected end of null
        CHECK_THROWS_WITH_AS(parser_helper("n"),
                             "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid literal; last read: 'n'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("nu"),
                             "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid literal; last read: 'nu'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("nul"),
                             "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid literal; last read: 'nul'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("nulk"),
                             "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid literal; last read: 'nulk'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("nulm"),
                             "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid literal; last read: 'nulm'", json::parse_error&);

        // unexpected end of true
        CHECK_THROWS_WITH_AS(parser_helper("t"),
                             "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid literal; last read: 't'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("tr"),
                             "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid literal; last read: 'tr'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("tru"),
                             "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid literal; last read: 'tru'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("trud"),
                             "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid literal; last read: 'trud'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("truf"),
                             "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid literal; last read: 'truf'", json::parse_error&);

        // unexpected end of false
        CHECK_THROWS_WITH_AS(parser_helper("f"),
                             "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid literal; last read: 'f'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("fa"),
                             "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid literal; last read: 'fa'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("fal"),
                             "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid literal; last read: 'fal'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("fals"),
                             "[json.exception.parse_error.101] parse error at line 1, column 5: syntax error while parsing value - invalid literal; last read: 'fals'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("falsd"),
                             "[json.exception.parse_error.101] parse error at line 1, column 5: syntax error while parsing value - invalid literal; last read: 'falsd'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("falsf"),
                             "[json.exception.parse_error.101] parse error at line 1, column 5: syntax error while parsing value - invalid literal; last read: 'falsf'", json::parse_error&);

        // missing/unexpected end of array
        CHECK_THROWS_WITH_AS(parser_helper("["),
                             "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - unexpected end of input; expected '[', '{', or a literal", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("[1"),
                             "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing array - unexpected end of input; expected ']'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("[1,"),
                             "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - unexpected end of input; expected '[', '{', or a literal", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("[1,]"),
                             "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("]"),
                             "[json.exception.parse_error.101] parse error at line 1, column 1: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal", json::parse_error&);

        // missing/unexpected end of object
        CHECK_THROWS_WITH_AS(parser_helper("{"),
                             "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing object key - unexpected end of input; expected string literal", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("{\"foo\""),
                             "[json.exception.parse_error.101] parse error at line 1, column 7: syntax error while parsing object separator - unexpected end of input; expected ':'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("{\"foo\":"),
                             "[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected end of input; expected '[', '{', or a literal", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("{\"foo\":}"),
                             "[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected '}'; expected '[', '{', or a literal", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("{\"foo\":1,}"),
                             "[json.exception.parse_error.101] parse error at line 1, column 10: syntax error while parsing object key - unexpected '}'; expected string literal", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("}"),
                             "[json.exception.parse_error.101] parse error at line 1, column 1: syntax error while parsing value - unexpected '}'; expected '[', '{', or a literal", json::parse_error&);

        // missing/unexpected end of string
        CHECK_THROWS_WITH_AS(parser_helper("\""),
                             "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: missing closing quote; last read: '\"'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("\"\\\""),
                             "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid string: missing closing quote; last read: '\"\\\"'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("\"\\u\""),
                             "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid string: '\\u' must be followed by 4 hex digits; last read: '\"\\u\"'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("\"\\u0\""),
                             "[json.exception.parse_error.101] parse error at line 1, column 5: syntax error while parsing value - invalid string: '\\u' must be followed by 4 hex digits; last read: '\"\\u0\"'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("\"\\u01\""),
                             "[json.exception.parse_error.101] parse error at line 1, column 6: syntax error while parsing value - invalid string: '\\u' must be followed by 4 hex digits; last read: '\"\\u01\"'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("\"\\u012\""),
                             "[json.exception.parse_error.101] parse error at line 1, column 7: syntax error while parsing value - invalid string: '\\u' must be followed by 4 hex digits; last read: '\"\\u012\"'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("\"\\u"),
                             "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid string: '\\u' must be followed by 4 hex digits; last read: '\"\\u'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("\"\\u0"),
                             "[json.exception.parse_error.101] parse error at line 1, column 5: syntax error while parsing value - invalid string: '\\u' must be followed by 4 hex digits; last read: '\"\\u0'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("\"\\u01"),
                             "[json.exception.parse_error.101] parse error at line 1, column 6: syntax error while parsing value - invalid string: '\\u' must be followed by 4 hex digits; last read: '\"\\u01'", json::parse_error&);
        CHECK_THROWS_WITH_AS(parser_helper("\"\\u012"),
                             "[json.exception.parse_error.101] parse error at line 1, column 7: syntax error while parsing value - invalid string: '\\u' must be followed by 4 hex digits; last read: '\"\\u012'", json::parse_error&);

        // invalid escapes
        for (int c = 1; c < 128; ++c)
        {
            auto s = std::string("\"\\") + std::string(1, static_cast<char>(c)) + "\"";

            switch (c)
            {
                // valid escapes
                case ('"'):
                case ('\\'):
                case ('/'):
                case ('b'):
                case ('f'):
                case ('n'):
                case ('r'):
                case ('t'):
                {
                    CHECK_NOTHROW(parser_helper(s));
                    break;
                }

                // \u must be followed with four numbers, so we skip it here
                case ('u'):
                {
                    break;
                }

                // any other combination of backslash and character is invalid
                default:
                {
                    CHECK_THROWS_AS(parser_helper(s), json::parse_error&);
                    // only check error message if c is not a control character
                    if (c > 0x1f)
                    {
                        CHECK_THROWS_WITH_STD_STR(parser_helper(s),
                                                  "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid string: forbidden character after backslash; last read: '\"\\" + std::string(1, static_cast<char>(c)) + "'");
                    }
                    break;
                }
            }
        }

        // invalid \uxxxx escapes
        {
            // check whether character is a valid hex character
            const auto valid = [](int c)
            {
                switch (c)
                {
                    case ('0'):
                    case ('1'):
                    case ('2'):
                    case ('3'):
                    case ('4'):
                    case ('5'):
                    case ('6'):
                    case ('7'):
                    case ('8'):
                    case ('9'):
                    case ('a'):
                    case ('b'):
                    case ('c'):
                    case ('d'):
                    case ('e'):
                    case ('f'):
                    case ('A'):
                    case ('B'):
                    case ('C'):
                    case ('D'):
                    case ('E'):
                    case ('F'):
                    {
                        return true;
                    }

                    default:
                    {
                        return false;
                    }
                }
            };

            for (int c = 1; c < 128; ++c)
            {
                std::string const s = "\"\\u";

                // create a string with the iterated character at each position
                auto s1 = s + "000" + std::string(1, static_cast<char>(c)) + "\"";
                auto s2 = s + "00" + std::string(1, static_cast<char>(c)) + "0\"";
                auto s3 = s + "0" + std::string(1, static_cast<char>(c)) + "00\"";
                auto s4 = s + std::string(1, static_cast<char>(c)) + "000\"";

                if (valid(c))
                {
                    CAPTURE(s1)
                    CHECK_NOTHROW(parser_helper(s1));
                    CAPTURE(s2)
                    CHECK_NOTHROW(parser_helper(s2));
                    CAPTURE(s3)
                    CHECK_NOTHROW(parser_helper(s3));
                    CAPTURE(s4)
                    CHECK_NOTHROW(parser_helper(s4));
                }
                else
                {
                    CAPTURE(s1)
                    CHECK_THROWS_AS(parser_helper(s1), json::parse_error&);
                    // only check error message if c is not a control character
                    if (c > 0x1f)
                    {
                        CHECK_THROWS_WITH_STD_STR(parser_helper(s1),
                                                  "[json.exception.parse_error.101] parse error at line 1, column 7: syntax error while parsing value - invalid string: '\\u' must be followed by 4 hex digits; last read: '" + s1.substr(0, 7) + "'");
                    }

                    CAPTURE(s2)
                    CHECK_THROWS_AS(parser_helper(s2), json::parse_error&);
                    // only check error message if c is not a control character
                    if (c > 0x1f)
                    {
                        CHECK_THROWS_WITH_STD_STR(parser_helper(s2),
                                                  "[json.exception.parse_error.101] parse error at line 1, column 6: syntax error while parsing value - invalid string: '\\u' must be followed by 4 hex digits; last read: '" + s2.substr(0, 6) + "'");
                    }

                    CAPTURE(s3)
                    CHECK_THROWS_AS(parser_helper(s3), json::parse_error&);
                    // only check error message if c is not a control character
                    if (c > 0x1f)
                    {
                        CHECK_THROWS_WITH_STD_STR(parser_helper(s3),
                                                  "[json.exception.parse_error.101] parse error at line 1, column 5: syntax error while parsing value - invalid string: '\\u' must be followed by 4 hex digits; last read: '" + s3.substr(0, 5) + "'");
                    }

                    CAPTURE(s4)
                    CHECK_THROWS_AS(parser_helper(s4), json::parse_error&);
                    // only check error message if c is not a control character
                    if (c > 0x1f)
                    {
                        CHECK_THROWS_WITH_STD_STR(parser_helper(s4),
                                                  "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing value - invalid string: '\\u' must be followed by 4 hex digits; last read: '" + s4.substr(0, 4) + "'");
                    }
                }
            }
        }

        json _;

        // missing part of a surrogate pair
        CHECK_THROWS_WITH_AS(_ = json::parse("\"\\uD80C\""), "[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - invalid string: surrogate U+D800..U+DBFF must be followed by U+DC00..U+DFFF; last read: '\"\\uD80C\"'", json::parse_error&);
        // invalid surrogate pair
        CHECK_THROWS_WITH_AS(_ = json::parse("\"\\uD80C\\uD80C\""),
                             "[json.exception.parse_error.101] parse error at line 1, column 13: syntax error while parsing value - invalid string: surrogate U+D800..U+DBFF must be followed by U+DC00..U+DFFF; last read: '\"\\uD80C\\uD80C'", json::parse_error&);
        CHECK_THROWS_WITH_AS(_ = json::parse("\"\\uD80C\\u0000\""),
                             "[json.exception.parse_error.101] parse error at line 1, column 13: syntax error while parsing value - invalid string: surrogate U+D800..U+DBFF must be followed by U+DC00..U+DFFF; last read: '\"\\uD80C\\u0000'", json::parse_error&);
        CHECK_THROWS_WITH_AS(_ = json::parse("\"\\uD80C\\uFFFF\""),
                             "[json.exception.parse_error.101] parse error at line 1, column 13: syntax error while parsing value - invalid string: surrogate U+D800..U+DBFF must be followed by U+DC00..U+DFFF; last read: '\"\\uD80C\\uFFFF'", json::parse_error&);
    }

    SECTION("parse errors (accept)")
    {
        // unexpected end of number
        CHECK(accept_helper("0.") == false);
        CHECK(accept_helper("-") == false);
        CHECK(accept_helper("--") == false);
        CHECK(accept_helper("-0.") == false);
        CHECK(accept_helper("-.") == false);
        CHECK(accept_helper("-:") == false);
        CHECK(accept_helper("0.:") == false);
        CHECK(accept_helper("e.") == false);
        CHECK(accept_helper("1e.") == false);
        CHECK(accept_helper("1e/") == false);
        CHECK(accept_helper("1e:") == false);
        CHECK(accept_helper("1E.") == false);
        CHECK(accept_helper("1E/") == false);
        CHECK(accept_helper("1E:") == false);

        // unexpected end of null
        CHECK(accept_helper("n") == false);
        CHECK(accept_helper("nu") == false);
        CHECK(accept_helper("nul") == false);

        // unexpected end of true
        CHECK(accept_helper("t") == false);
        CHECK(accept_helper("tr") == false);
        CHECK(accept_helper("tru") == false);

        // unexpected end of false
        CHECK(accept_helper("f") == false);
        CHECK(accept_helper("fa") == false);
        CHECK(accept_helper("fal") == false);
        CHECK(accept_helper("fals") == false);

        // missing/unexpected end of array
        CHECK(accept_helper("[") == false);
        CHECK(accept_helper("[1") == false);
        CHECK(accept_helper("[1,") == false);
        CHECK(accept_helper("[1,]") == false);
        CHECK(accept_helper("]") == false);

        // missing/unexpected end of object
        CHECK(accept_helper("{") == false);
        CHECK(accept_helper("{\"foo\"") == false);
        CHECK(accept_helper("{\"foo\":") == false);
        CHECK(accept_helper("{\"foo\":}") == false);
        CHECK(accept_helper("{\"foo\":1,}") == false);
        CHECK(accept_helper("}") == false);

        // missing/unexpected end of string
        CHECK(accept_helper("\"") == false);
        CHECK(accept_helper("\"\\\"") == false);
        CHECK(accept_helper("\"\\u\"") == false);
        CHECK(accept_helper("\"\\u0\"") == false);
        CHECK(accept_helper("\"\\u01\"") == false);
        CHECK(accept_helper("\"\\u012\"") == false);
        CHECK(accept_helper("\"\\u") == false);
        CHECK(accept_helper("\"\\u0") == false);
        CHECK(accept_helper("\"\\u01") == false);
        CHECK(accept_helper("\"\\u012") == false);

        // unget of newline
        CHECK(parser_helper("\n123\n") == 123);

        // invalid escapes
        for (int c = 1; c < 128; ++c)
        {
            auto s = std::string("\"\\") + std::string(1, static_cast<char>(c)) + "\"";

            switch (c)
            {
                // valid escapes
                case ('"'):
                case ('\\'):
                case ('/'):
                case ('b'):
                case ('f'):
                case ('n'):
                case ('r'):
                case ('t'):
                {
                    CHECK(json::parser(nlohmann::detail::input_adapter(s)).accept());
                    break;
                }

                // \u must be followed with four numbers, so we skip it here
                case ('u'):
                {
                    break;
                }

                // any other combination of backslash and character is invalid
                default:
                {
                    CHECK(json::parser(nlohmann::detail::input_adapter(s)).accept() == false);
                    break;
                }
            }
        }

        // invalid \uxxxx escapes
        {
            // check whether character is a valid hex character
            const auto valid = [](int c)
            {
                switch (c)
                {
                    case ('0'):
                    case ('1'):
                    case ('2'):
                    case ('3'):
                    case ('4'):
                    case ('5'):
                    case ('6'):
                    case ('7'):
                    case ('8'):
                    case ('9'):
                    case ('a'):
                    case ('b'):
                    case ('c'):
                    case ('d'):
                    case ('e'):
                    case ('f'):
                    case ('A'):
                    case ('B'):
                    case ('C'):
                    case ('D'):
                    case ('E'):
                    case ('F'):
                    {
                        return true;
                    }

                    default:
                    {
                        return false;
                    }
                }
            };

            for (int c = 1; c < 128; ++c)
            {
                std::string const s = "\"\\u";

                // create a string with the iterated character at each position
                const auto s1 = s + "000" + std::string(1, static_cast<char>(c)) + "\"";
                const auto s2 = s + "00" + std::string(1, static_cast<char>(c)) + "0\"";
                const auto s3 = s + "0" + std::string(1, static_cast<char>(c)) + "00\"";
                const auto s4 = s + std::string(1, static_cast<char>(c)) + "000\"";

                if (valid(c))
                {
                    CAPTURE(s1)
                    CHECK(json::parser(nlohmann::detail::input_adapter(s1)).accept());
                    CAPTURE(s2)
                    CHECK(json::parser(nlohmann::detail::input_adapter(s2)).accept());
                    CAPTURE(s3)
                    CHECK(json::parser(nlohmann::detail::input_adapter(s3)).accept());
                    CAPTURE(s4)
                    CHECK(json::parser(nlohmann::detail::input_adapter(s4)).accept());
                }
                else
                {
                    CAPTURE(s1)
                    CHECK(json::parser(nlohmann::detail::input_adapter(s1)).accept() == false);

                    CAPTURE(s2)
                    CHECK(json::parser(nlohmann::detail::input_adapter(s2)).accept() == false);

                    CAPTURE(s3)
                    CHECK(json::parser(nlohmann::detail::input_adapter(s3)).accept() == false);

                    CAPTURE(s4)
                    CHECK(json::parser(nlohmann::detail::input_adapter(s4)).accept() == false);
                }
            }
        }

        // missing part of a surrogate pair
        CHECK(accept_helper("\"\\uD80C\"") == false);
        // invalid surrogate pair
        CHECK(accept_helper("\"\\uD80C\\uD80C\"") == false);
        CHECK(accept_helper("\"\\uD80C\\u0000\"") == false);
        CHECK(accept_helper("\"\\uD80C\\uFFFF\"") == false);
    }

#if !defined(JSON_NOEXCEPTION)
    SECTION("issue #5412 - whitespace skipping bookkeeping (compact vs. pretty-printed)")
    {
        // lexer::skip_whitespace() reads its first character with get() (to
        // honor a possibly pending unget() from the previous token) and every
        // further whitespace character with get_ignoring_pending_unget() (a
        // get() variant that skips the then-always-false next_unget check).
        // This must not change the reported byte offset, line, or column of
        // a syntax error, even when a long run of whitespace containing
        // multiple newlines is skipped beforehand (as with pretty-printed
        // input). The expected values below were captured from the
        // unmodified do-while(get()) loop, so any regression that miscounts
        // characters or newlines while skipping whitespace changes them.
        const auto check_error = [](const std::string & input, std::size_t expected_byte,
                                    const std::string & expected_what)
        {
            CAPTURE(input)
            try
            {
                json _ = json::parse(input);
                FAIL_CHECK("expected a parse_error, but parsing succeeded");
            }
            catch (const json::parse_error& e)
            {
                CHECK(e.byte == expected_byte);
                CHECK(std::string(e.what()) == expected_what);
            }
        };

        // a nested document, serialized both compactly and pretty-printed
        // (dump(4)), each truncated right before the final closing '}' so
        // that the parser hits EOF after skipping all of the (in the
        // pretty-printed case, substantial) indentation whitespace
        const json doc =
        {
            {"a", 1},
            {"b", json::array({true, false, nullptr, "x"})},
            {"c", json::object({{"d", 3.14}, {"e", json::array({1, 2, 3})}})}
        };

        const std::string compact = doc.dump();
        const std::string pretty = doc.dump(4);

        check_error(compact.substr(0, compact.size() - 1), 60,
                    "[json.exception.parse_error.101] parse error at line 1, column 60: syntax error while parsing object - unexpected end of input; expected '}'");
        check_error(pretty.substr(0, pretty.size() - 1), 193,
                    "[json.exception.parse_error.101] parse error at line 17, column 1: syntax error while parsing object - unexpected end of input; expected '}'");

        // an invalid token appearing after several indented, multi-line
        // whitespace runs vs. the same document without any of that
        // whitespace
        check_error(R"({
    "a": 1,
    "b": [
        true,
        false
    ],
    "c": @
})", 70,
                    "[json.exception.parse_error.101] parse error at line 7, column 10: syntax error while parsing value - invalid literal; last read: '\"c\": @'");
        check_error(R"({"a":1,"b":[true,false],"c":@})", 29,
                    "[json.exception.parse_error.101] parse error at line 1, column 29: syntax error while parsing value - invalid literal; last read: '\"c\":@'");
    }
#endif

    SECTION("tests found by mutate++")
    {
        // test case to make sure no comma precedes the first key
        CHECK_THROWS_WITH_AS(parser_helper("{,\"key\": false}"), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing object key - unexpected ','; expected string literal", json::parse_error&);
        // test case to make sure an object is properly closed
        CHECK_THROWS_WITH_AS(parser_helper("[{\"key\": false true]"), "[json.exception.parse_error.101] parse error at line 1, column 19: syntax error while parsing object - unexpected true literal; expected '}'", json::parse_error&);

        // test case to make sure the callback is properly evaluated after reading a key
        {
            json::parser_callback_t const cb = [](int /*unused*/, json::parse_event_t event, json& /*unused*/) noexcept
            {
                return event != json::parse_event_t::key;
            };

            const json x = json::parse("{\"key\": false}", cb);
            CHECK(x == json::object());
        }
    }

    SECTION("callback function")
    {
        const auto* s_object = R"(
            {
                "foo": 2,
                "bar": {
                    "baz": 1
                }
            }
        )";

        const auto* s_array = R"(
            [1,2,[3,4,5],4,5]
        )";

        const auto* structured_array = R"(
            [
                1,
                {
                     "foo": "bar"
                },
                {
                     "qux": "baz"
                }
            ]
        )";

        const auto* structured_object = R"(
            {
                "foo": [1, 2],
                "bar": 3
            }
        )";

        SECTION("filter nothing")
        {
            const json j_object = json::parse(s_object, [](int /*unused*/, json::parse_event_t /*unused*/, const json& /*unused*/) noexcept
            {
                return true;
            });

            CHECK (j_object == json({{"foo", 2}, {"bar", {{"baz", 1}}}}));

            const json j_array = json::parse(s_array, [](int /*unused*/, json::parse_event_t /*unused*/, const json& /*unused*/) noexcept
            {
                return true;
            });

            CHECK (j_array == json({1, 2, {3, 4, 5}, 4, 5}));
        }

        SECTION("filter everything")
        {
            json const j_object = json::parse(s_object, [](int /*unused*/, json::parse_event_t /*unused*/, const json& /*unused*/) noexcept
            {
                return false;
            });

            // the top-level object will be discarded, leaving a null
            CHECK (j_object.is_null());

            json const j_array = json::parse(s_array, [](int /*unused*/, json::parse_event_t /*unused*/, const json& /*unused*/) noexcept
            {
                return false;
            });

            // the top-level array will be discarded, leaving a null
            CHECK (j_array.is_null());
        }

        SECTION("filter specific element")
        {
            const json j_object = json::parse(s_object, [](int /*unused*/, json::parse_event_t event, const json & j) noexcept
            {
                // filter all number(2) elements
                return event != json::parse_event_t::value || j != json(2);
            });

            CHECK (j_object == json({{"bar", {{"baz", 1}}}}));

            const json j_array = json::parse(s_array, [](int /*unused*/, json::parse_event_t event, const json & j) noexcept
            {
                return event != json::parse_event_t::value || j != json(2);
            });

            CHECK (j_array == json({1, {3, 4, 5}, 4, 5}));
        }

        SECTION("filter object in array")
        {
            const json j_filtered1 = json::parse(structured_array, [](int /*unused*/, json::parse_event_t e, const json & parsed)
            {
                return !(e == json::parse_event_t::object_end && parsed.contains("foo"));
            });

            // the specified object will be discarded, and removed.
            CHECK (j_filtered1.size() == 2);
            CHECK (j_filtered1 == json({1, {{"qux", "baz"}}}));

            const json j_filtered2 = json::parse(structured_array, [](int /*unused*/, json::parse_event_t e, const json& /*parsed*/) noexcept
            {
                return e != json::parse_event_t::object_end;
            });

            // removed all objects in array.
            CHECK (j_filtered2.size() == 1);
            CHECK (j_filtered2 == json({1}));
        }

        SECTION("filter array in object")
        {
            // the array is discarded once it is already stored under its key
            const json j_filtered1 = json::parse(structured_object, [](int /*unused*/, json::parse_event_t e, const json& /*parsed*/) noexcept
            {
                return e != json::parse_event_t::array_end;
            });

            CHECK (j_filtered1 == json({{"bar", 3}}));

            // the array is discarded before it is stored, leaving the
            // placeholder the key event wrote
            const json j_filtered2 = json::parse(structured_object, [](int /*unused*/, json::parse_event_t e, const json& /*parsed*/) noexcept
            {
                return e != json::parse_event_t::array_start;
            });

            CHECK (j_filtered2 == json({{"bar", 3}}));
        }

        SECTION("filter value in object")
        {
            // the value is discarded after its key was kept, leaving the
            // placeholder the key event wrote
            const json j_filtered1 = json::parse(structured_object, [](int /*unused*/, json::parse_event_t e, const json & parsed) noexcept
            {
                return !(e == json::parse_event_t::value && parsed == json(3));
            });

            CHECK (j_filtered1 == json({{"foo", {1, 2}}}));

            // the same value is discarded together with its key, so no
            // placeholder was stored for it
            const json j_filtered2 = json::parse(structured_object, [](int /*unused*/, json::parse_event_t e, const json & parsed) noexcept
            {
                return !((e == json::parse_event_t::key && parsed == json("bar")) ||
                         (e == json::parse_event_t::value && parsed == json(3)));
            });

            CHECK (j_filtered2 == json({{"foo", {1, 2}}}));
        }

        SECTION("filter many members of one container")
        {
            // Rejecting a value makes the parser remove the placeholder its key
            // event stored. Locating that placeholder used to be a scan of the
            // whole parent, which made filtering a large container quadratic:
            // 128k members took ~25 s. These cases keep many members alive
            // while discarding many others, so the removal cost is the whole
            // point; they run in milliseconds when the placeholder is erased
            // directly.
            constexpr int count = 20000;

            std::string s = "{";
            for (int i = 0; i < count; ++i)
            {
                // "a<i>" is kept, "z<i>" is discarded
                s += "\"a" + std::to_string(i) + "\":" + std::to_string(i) + ",";
                s += "\"z" + std::to_string(i) + "\":-1,";
            }
            s.back() = '}';

            const json j_values = json::parse(s, [](int /*unused*/, json::parse_event_t e, const json & parsed) noexcept
            {
                return !(e == json::parse_event_t::value && parsed == json(-1));
            });

            CHECK(j_values.size() == count);
            CHECK(j_values.at("a0") == json(0));
            CHECK(j_values.at("a" + std::to_string(count - 1)) == json(count - 1));
            CHECK_FALSE(j_values.contains("z0"));
            CHECK_FALSE(j_values.contains("z" + std::to_string(count - 1)));

            // the same, but discarding whole containers rather than values,
            // which takes the end_object()/end_array() removal path
            std::string s_nested = "{";
            for (int i = 0; i < count; ++i)
            {
                s_nested += "\"a" + std::to_string(i) + "\":" + std::to_string(i) + ",";
                s_nested += "\"z" + std::to_string(i) + "\":[1,2],";
            }
            s_nested.back() = '}';

            const json j_arrays = json::parse(s_nested, [](int /*unused*/, json::parse_event_t e, const json& /*unused*/) noexcept
            {
                return e != json::parse_event_t::array_end;
            });

            CHECK(j_arrays.size() == count);
            CHECK(j_arrays.at("a0") == json(0));
            CHECK_FALSE(j_arrays.contains("z0"));
            CHECK_FALSE(j_arrays.contains("z" + std::to_string(count - 1)));
        }

        SECTION("filter specific events")
        {
            SECTION("first closing event")
            {
                {
                    const json j_object = json::parse(s_object, [](int /*unused*/, json::parse_event_t e, const json& /*unused*/) noexcept
                    {
                        static bool first = true;
                        if (e == json::parse_event_t::object_end && first)
                        {
                            first = false;
                            return false;
                        }

                        return true;
                    });

                    // the first completed object will be discarded
                    CHECK (j_object == json({{"foo", 2}}));
                }

                {
                    const json j_array = json::parse(s_array, [](int /*unused*/, json::parse_event_t e, const json& /*unused*/) noexcept
                    {
                        static bool first = true;
                        if (e == json::parse_event_t::array_end && first)
                        {
                            first = false;
                            return false;
                        }

                        return true;
                    });

                    // the first completed array will be discarded
                    CHECK (j_array == json({1, 2, 4, 5}));
                }
            }
        }

        SECTION("no callback for the content of a discarded container (#5643)")
        {
            // discarding a container at its start event must also hide
            // everything inside it from the callback: none of the nested
            // keys, values, or nested containers' own start/end events may
            // be reported
            std::vector<std::string> log;
            bool first = true;
            const json j = json::parse(R"({"skip": {"k1": 1, "k2": [2, {"k3": 3}]}, "keep": 1})",
                                       [&](int depth, json::parse_event_t event, json & parsed)
            {
                static const char* const names[] = {"object_start", "object_end", "array_start", "array_end", "key", "value"}; // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
                log.push_back(std::to_string(depth) + " " + names[static_cast<int>(event)] + " " + parsed.dump());

                if (depth == 1 && event == json::parse_event_t::object_start && first)
                {
                    // discard "skip" right at its object_start event
                    first = false;
                    return false;
                }
                return true;
            });

            CHECK(log == std::vector<std::string>
            {
                "0 object_start <discarded>",
                "1 key \"skip\"",
                "1 object_start <discarded>",
                "1 key \"keep\"",
                "1 value 1",
                "0 object_end {\"keep\":1}"
            });
            CHECK(j == json({{"keep", 1}}));
        }

        SECTION("callback still called inside a container whose key was rejected (#5643)")
        {
            // rejecting a key does not discard its value's container at the
            // container's own start event, so the callback is still called
            // for that container's content; only storing the container
            // under the rejected key is skipped
            // (documented for parser_callback_t: "the callback is still
            // called for the associated value, but its return value has no
            // further effect")
            const auto record = [](std::vector<std::string>& log, int depth, json::parse_event_t event, const json & parsed)
            {
                static const char* const names[] = {"object_start", "object_end", "array_start", "array_end", "key", "value"}; // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
                log.push_back(std::to_string(depth) + " " + names[static_cast<int>(event)] + " " + parsed.dump());
            };

            std::vector<std::string> log_object;
            const json j_object = json::parse(R"({"skip": {"k1": 1}, "keep": 2})",
                                              [&](int depth, json::parse_event_t event, json & parsed)
            {
                record(log_object, depth, event, parsed);
                return !(event == json::parse_event_t::key && parsed == json("skip"));
            });

            CHECK(log_object == std::vector<std::string>
            {
                "0 object_start <discarded>",
                "1 key \"skip\"",
                "1 object_start <discarded>",
                "2 key \"k1\"",
                "2 value 1",
                "1 key \"keep\"",
                "1 value 2",
                "0 object_end {\"keep\":2}"
            });
            CHECK(j_object == json({{"keep", 2}}));

            // same for a rejected key whose value is an array rather than an object
            std::vector<std::string> log_array;
            const json j_array = json::parse(R"({"skip": [1, {"k1": 2}], "keep": 2})",
                                             [&](int depth, json::parse_event_t event, json & parsed)
            {
                record(log_array, depth, event, parsed);
                return !(event == json::parse_event_t::key && parsed == json("skip"));
            });

            CHECK(log_array == std::vector<std::string>
            {
                "0 object_start <discarded>",
                "1 key \"skip\"",
                "1 array_start <discarded>",
                "2 value 1",
                "2 object_start <discarded>",
                "3 key \"k1\"",
                "3 value 2",
                "1 key \"keep\"",
                "1 value 2",
                "0 object_end {\"keep\":2}"
            });
            CHECK(j_array == json({{"keep", 2}}));
        }

        SECTION("special cases")
        {
            // the following test cases cover the situation in which an empty
            // object and array is discarded only after the closing character
            // has been read

            const json j_empty_object = json::parse("{}", [](int /*unused*/, json::parse_event_t e, const json& /*unused*/) noexcept
            {
                return e != json::parse_event_t::object_end;
            });
            CHECK(j_empty_object == json());

            const json j_empty_array = json::parse("[]", [](int /*unused*/, json::parse_event_t e, const json& /*unused*/) noexcept
            {
                return e != json::parse_event_t::array_end;
            });
            CHECK(j_empty_array == json());
        }
    }

    SECTION("constructing from contiguous containers")
    {
        SECTION("from std::vector")
        {
            std::vector<uint8_t> v = {'t', 'r', 'u', 'e'};
            json j;
            json::parser(nlohmann::detail::input_adapter(std::begin(v), std::end(v))).parse(true, j);
            CHECK(j == json(true));
        }

        SECTION("from std::array")
        {
            // NOTE: this array is sized to exactly the length of "true" (unlike
            // the trailing-NUL-tolerant default behavior elsewhere in this file,
            // see the "NUL byte handling" section above); a size of 5 here would
            // leave a value-initialized trailing 0x00 element that is only
            // silently accepted as end-of-input by default and would fail under
            // JSON_STRICT_NUL_HANDLING
            std::array<uint8_t, 4> v { {'t', 'r', 'u', 'e'} };
            json j;
            json::parser(nlohmann::detail::input_adapter(std::begin(v), std::end(v))).parse(true, j);
            CHECK(j == json(true));
        }

        SECTION("from array")
        {
            uint8_t v[] = {'t', 'r', 'u', 'e'}; // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
            json j;
            json::parser(nlohmann::detail::input_adapter(std::begin(v), std::end(v))).parse(true, j);
            CHECK(j == json(true));
        }

        SECTION("from char literal")
        {
            CHECK(parser_helper("true") == json(true));
        }

        SECTION("from std::string")
        {
            std::string v = {'t', 'r', 'u', 'e'};
            json j;
            json::parser(nlohmann::detail::input_adapter(std::begin(v), std::end(v))).parse(true, j);
            CHECK(j == json(true));
        }

        SECTION("from std::initializer_list")
        {
            std::initializer_list<uint8_t> const v = {'t', 'r', 'u', 'e'};
            json j;
            json::parser(nlohmann::detail::input_adapter(std::begin(v), std::end(v))).parse(true, j);
            CHECK(j == json(true));
        }

        SECTION("from std::valarray")
        {
            std::valarray<uint8_t> v = {'t', 'r', 'u', 'e'};
            json j;
            json::parser(nlohmann::detail::input_adapter(std::begin(v), std::end(v))).parse(true, j);
            CHECK(j == json(true));
        }
    }

    SECTION("improve test coverage")
    {
        SECTION("parser with callback")
        {
            json::parser_callback_t const cb = [](int /*unused*/, json::parse_event_t /*unused*/, json& /*unused*/) noexcept
            {
                return true;
            };

            CHECK(json::parse("{\"foo\": true:", cb, false).is_discarded());

            json _;
            CHECK_THROWS_WITH_AS(_ = json::parse("{\"foo\": true:", cb), "[json.exception.parse_error.101] parse error at line 1, column 13: syntax error while parsing object - unexpected ':'; expected '}'", json::parse_error&);

            CHECK_THROWS_WITH_AS(_ = json::parse("1.18973e+4932", cb), "[json.exception.out_of_range.406] number overflow parsing '1.18973e+4932'", json::out_of_range&);
        }

        SECTION("SAX parser")
        {
            SECTION("} without value")
            {
                SaxCountdown s(1);
                CHECK(json::sax_parse("{}", &s) == false);
            }

            SECTION("} with value")
            {
                SaxCountdown s(3);
                CHECK(json::sax_parse("{\"k1\": true}", &s) == false);
            }

            SECTION("second key")
            {
                SaxCountdown s(3);
                CHECK(json::sax_parse("{\"k1\": true, \"k2\": false}", &s) == false);
            }

            SECTION("] without value")
            {
                SaxCountdown s(1);
                CHECK(json::sax_parse("[]", &s) == false);
            }

            SECTION("] with value")
            {
                SaxCountdown s(2);
                CHECK(json::sax_parse("[1]", &s) == false);
            }

            SECTION("float")
            {
                SaxCountdown s(0);
                CHECK(json::sax_parse("3.14", &s) == false);
            }

            SECTION("false")
            {
                SaxCountdown s(0);
                CHECK(json::sax_parse("false", &s) == false);
            }

            SECTION("null")
            {
                SaxCountdown s(0);
                CHECK(json::sax_parse("null", &s) == false);
            }

            SECTION("true")
            {
                SaxCountdown s(0);
                CHECK(json::sax_parse("true", &s) == false);
            }

            SECTION("unsigned")
            {
                SaxCountdown s(0);
                CHECK(json::sax_parse("12", &s) == false);
            }

            SECTION("integer")
            {
                SaxCountdown s(0);
                CHECK(json::sax_parse("-12", &s) == false);
            }

            SECTION("string")
            {
                SaxCountdown s(0);
                CHECK(json::sax_parse("\"foo\"", &s) == false);
            }
        }
    }

    SECTION("error messages for comments")
    {
        json _;
        CHECK_THROWS_WITH_AS(_ = json::parse("/a", nullptr, true, true), "[json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid comment; expecting '/' or '*' after '/'; last read: '/a'", json::parse_error);
        // "/*" is a string literal, so it carries a compiler-appended trailing
        // '\0'; by default that NUL is read like any other byte and shows up
        // in "last read", but JSON_STRICT_NUL_HANDLING trims exactly that one
        // trailing byte from a char array (see
        // docs/mkdocs/docs/api/macros/json_strict_nul_handling.md), so it no
        // longer appears in the message in that state
#if defined(JSON_TEST_STRICT_NUL_HANDLING_ENABLED)
        CHECK_THROWS_WITH_AS(_ = json::parse("/*", nullptr, true, true), "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid comment; missing closing '*/'; last read: '/*'", json::parse_error);
#else
        CHECK_THROWS_WITH_AS(_ = json::parse("/*", nullptr, true, true), "[json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid comment; missing closing '*/'; last read: '/*<U+0000>'", json::parse_error);
#endif
    }

    SECTION("comments before separators")
    {
        // The parser first checks for the expected ':' or ',' and only then
        // falls back to the full token switch, which skips comments. A comment
        // directly before a separator takes that fallback.
        json _;

        SECTION("ignored")
        {
            const std::vector<std::pair<std::string, json>> inputs =
            {
                {"{\"a\" /* c */ : 1}", {{"a", 1}}},
                {"{\"a\" // c\n: 1}", {{"a", 1}}},
                {R"({"a": 1, "b" /* c */ : 2})", {{"a", 1}, {"b", 2}}},
                {R"({"a": 1 /* c */ , "b": 2})", {{"a", 1}, {"b", 2}}},
                {"{\"a\": 1 // c\n, \"b\": 2}", {{"a", 1}, {"b", 2}}},
                {"[1 /* c */ , 2]", {1, 2}},
                {"[1 // c\n, 2]", {1, 2}},
                {"{\"a\" /* c */ /* d */ : [1 // c\n , 2 /**/ ] /**/ , \"b\" : 3}", {{"a", {1, 2}}, {"b", 3}}}
            };
            for (const auto& input : inputs)
            {
                CAPTURE(input.first)
                CHECK(json::parse(input.first, nullptr, true, true) == input.second);
                CHECK(json::accept(input.first, true));
            }
        }

        SECTION("ignored, with trailing commas")
        {
            CHECK(json::parse(std::string("[1 /* c */ , ]"), nullptr, true, true, true) == json({1}));
            CHECK(json::parse(std::string("{\"a\": 1 /* c */ , }"), nullptr, true, true, true) == json({{"a", 1}}));
            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("[1 /* c */ , ]"), nullptr, true, true),
                                 "[json.exception.parse_error.101] parse error at line 1, column 14: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal", json::parse_error);
            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\": 1 /* c */ , }"), nullptr, true, true),
                                 "[json.exception.parse_error.101] parse error at line 1, column 19: syntax error while parsing object key - unexpected '}'; expected string literal", json::parse_error);
        }

        SECTION("not ignored")
        {
            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\" /* c */ : 1}")),
                                 "[json.exception.parse_error.101] parse error at line 1, column 6: syntax error while parsing object separator - invalid literal; last read: '\"a\" /'; expected ':'", json::parse_error);
            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\": 1, \"b\" /* c */ : 2}")),
                                 "[json.exception.parse_error.101] parse error at line 1, column 14: syntax error while parsing object separator - invalid literal; last read: '\"b\" /'; expected ':'", json::parse_error);
            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("{\"a\": 1 /* c */ , \"b\": 2}")),
                                 "[json.exception.parse_error.101] parse error at line 1, column 9: syntax error while parsing object - invalid literal; last read: '1 /'; expected '}'", json::parse_error);
            CHECK_THROWS_WITH_AS(_ = json::parse(std::string("[1 /* c */ , 2]")),
                                 "[json.exception.parse_error.101] parse error at line 1, column 4: syntax error while parsing array - invalid literal; last read: '1 /'; expected ']'", json::parse_error);
            CHECK(!json::accept(std::string("[1 /* c */ , 2]")));
        }
    }

#if JSON_DIAGNOSTIC_POSITIONS
    // Macro for all test cases for start_pos and end_pos
#define SETUP_TESTCASES() \
    SECTION("with callback") \
    { \
        SECTION("filter nothing") \
        { \
            json::parser_callback_t const cb = [](int /*unused*/, json::parse_event_t /*unused*/, json& /*unused*/) noexcept \
            { \
                return true; \
            }; \
            validate_start_end_pos_for_nested_obj_helper(nested_type_json_str, root_type_json_str, expected, cb); \
        } \
        SECTION("filter element") \
        { \
            json::parser_callback_t const cb = [](int /*unused*/, json::parse_event_t event, json& j) noexcept \
            { \
                return (event != json::parse_event_t::key && event != json::parse_event_t::value) || j != json("a"); \
            }; \
            validate_start_end_pos_for_nested_obj_helper(nested_type_json_str, root_type_json_str, filteredExpected, cb); \
        } \
    } \
    SECTION("without callback") \
    { \
        validate_start_end_pos_for_nested_obj_helper(nested_type_json_str, root_type_json_str, expected); \
    }

    SECTION("retrieve start position and end position")
    {
        SECTION("for object")
        {
            // Create an object with spaces to test the start and end positions. Spaces will not be included in the
            // JSON object, however, the start and end positions should include the spaces from the input JSON string.
            const std::string nested_type_json_str =  R"({    "a":       1,"b"      : "test1"})";
            const std::string root_type_json_str =  R"({    "nested": )" + nested_type_json_str + R"(, "anotherValue": "test2"})";
            auto expected = json({{"nested", {{"a", 1}, {"b", "test1"}}}, {"anotherValue", "test2"}});
            auto filteredExpected = expected;
            filteredExpected["nested"].erase("a");

            SETUP_TESTCASES()
        }

        SECTION("for array")
        {
            const std::string nested_type_json_str =  R"(["a", "test", 45])";
            const std::string root_type_json_str =  R"({   "nested": )" + nested_type_json_str + R"(, "anotherValue": "test" })";
            auto expected = json({{"nested", {"a", "test", 45}}, {"anotherValue", "test"}});
            auto filteredExpected = expected;
            filteredExpected["nested"] = json({"test", 45});
            SETUP_TESTCASES()
        }

        SECTION("for array with objects")
        {
            const std::string nested_type_json_str =  R"([{"a": 1, "b": "test"}, {"c": 2, "d": "test2"}])";
            const std::string root_type_json_str =  R"({   "nested": )" + nested_type_json_str + R"(, "anotherValue": "test" })";
            auto expected = json({{"nested", {{{"a", 1}, {"b", "test"}}, {{"c", 2}, {"d", "test2"}}}}, {"anotherValue", "test"}});
            auto filteredExpected = expected;
            filteredExpected["nested"][0].erase("a");
            SETUP_TESTCASES()

            auto j = json::parse(root_type_json_str);
            auto nested_array = j["nested"];
            const auto& nested_obj = nested_array[0];
            CHECK(nested_type_json_str.substr(1, 21) == root_type_json_str.substr(nested_obj.start_pos(), nested_obj.end_pos() - nested_obj.start_pos()));
            CHECK(nested_type_json_str.substr(24, 22) == root_type_json_str.substr(nested_array[1].start_pos(), nested_array[1].end_pos() - nested_array[1].start_pos()));
        }

        SECTION("for two levels of nesting objects")
        {
            const std::string nested_type_json_str =  R"({"nested2": {"b": "test"}})";
            const std::string root_type_json_str =  R"({   "a": 2, "nested": )" + nested_type_json_str + R"(, "anotherValue": "test" })";
            auto expected = json({{"a", 2}, {"nested", {{"nested2", {{"b", "test"}}}}}, {"anotherValue", "test"}});
            auto filteredExpected = expected;
            filteredExpected.erase("a");
            SETUP_TESTCASES()

            auto j = json::parse(root_type_json_str);
            auto nested_obj = j["nested"]["nested2"];
            CHECK(nested_type_json_str.substr(12, 13) == root_type_json_str.substr(nested_obj.start_pos(), nested_obj.end_pos() - nested_obj.start_pos()));
        }

        SECTION("for simple types")
        {
            SECTION("no nested")
            {
                SECTION("with callback")
                {
                    json::parser_callback_t const cb = [](int /*unused*/, json::parse_event_t /*unused*/, json& /*unused*/) noexcept
                    {
                        return true;
                    };

                    // 1. string type
                    std::string json_str =  R"("test")";
                    auto j = json::parse(json_str, cb);
                    validate_generated_json_and_start_end_pos_helper(json_str, j, "test");

                    // 2. number type
                    json_str =  R"(1)";
                    j = json::parse(json_str, cb);
                    validate_generated_json_and_start_end_pos_helper(json_str, j, 1);

                    // 3. boolean type
                    json_str =  R"(true)";
                    j = json::parse(json_str, cb);
                    validate_generated_json_and_start_end_pos_helper(json_str, j, true);

                    // 4. null type
                    json_str =  R"(null)";
                    j = json::parse(json_str, cb);
                    validate_generated_json_and_start_end_pos_helper(json_str, j, nullptr);
                }

                SECTION("without callback")
                {
                    // 1. string type
                    std::string json_str =  R"("test")";
                    auto j = json::parse(json_str);
                    validate_generated_json_and_start_end_pos_helper(json_str, j, "test");

                    // 2. number type
                    json_str =  R"(1)";
                    j = json::parse(json_str);
                    validate_generated_json_and_start_end_pos_helper(json_str, j, 1);

                    json_str = R"(1.001239923)";
                    j = json::parse(json_str);
                    validate_generated_json_and_start_end_pos_helper(json_str, j, 1.001239923);

                    json_str = R"(1.123812389000000)";
                    j = json::parse(json_str);
                    validate_generated_json_and_start_end_pos_helper(json_str, j, 1.123812389);

                    // 3. boolean type
                    json_str =  R"(true)";
                    j = json::parse(json_str);
                    validate_generated_json_and_start_end_pos_helper(json_str, j, true);

                    json_str =  R"(false)";
                    j = json::parse(json_str);
                    validate_generated_json_and_start_end_pos_helper(json_str, j, false);

                    // 4. null type
                    json_str =  R"(null)";
                    j = json::parse(json_str);
                    validate_generated_json_and_start_end_pos_helper(json_str, j, nullptr);
                }
            }

            SECTION("string type")
            {
                const std::string nested_type_json_str =  R"("test")";
                const std::string root_type_json_str =  R"({ "a": 1,   "nested": )" + nested_type_json_str + R"(, "anotherValue": "test" })";
                auto expected = json({{"nested", "test"}, {"anotherValue", "test"}, {"a", 1}});
                auto filteredExpected = expected;
                filteredExpected.erase("a");
                SETUP_TESTCASES()
            }

            SECTION("number type")
            {
                const std::string nested_type_json_str =  R"(2)";
                const std::string root_type_json_str =  R"({ "a": 1,   "nested": )" + nested_type_json_str + R"(, "anotherValue": "test" })";
                auto expected = json({{"nested", 2}, {"anotherValue", "test"}, {"a", 1}});
                auto filteredExpected = expected;
                filteredExpected.erase("a");
                SETUP_TESTCASES()
            }

            SECTION("boolean type")
            {
                const std::string nested_type_json_str =  R"(true)";
                const std::string root_type_json_str =  R"({ "a": 1,   "nested": )" + nested_type_json_str + R"(, "anotherValue": "test" })";
                auto expected = json({{"nested", true}, {"anotherValue", "test"}, {"a", 1}});
                auto filteredExpected = expected;
                filteredExpected.erase("a");
                SETUP_TESTCASES()
            }

            SECTION("null type")
            {
                const std::string nested_type_json_str =  R"(null)";
                const std::string root_type_json_str =  R"({ "a": 1,   "nested": )" + nested_type_json_str + R"(, "anotherValue": "test" })";
                auto expected = json({{"nested", nullptr}, {"anotherValue", "test"}, {"a", 1}});
                auto filteredExpected = expected;
                filteredExpected.erase("a");
                SETUP_TESTCASES()
            }
        }
        SECTION("with leading whitespace and newlines around root JSON")
        {
            const std::string initial_whitespace = R"(
                
            )";
            const std::string nested_type_json_str = R"({
                "a": 1,
                "nested": {
                    "b": "test"
                },
                "anotherValue": "test"
            })";
            const std::string end_whitespace = R"(
                
            )";
            const std::string root_type_json_str = initial_whitespace + nested_type_json_str + end_whitespace;

            auto expected = json({{"a", 1}, {"nested", {{"b", "test"}}}, {"anotherValue", "test"}});

            auto j = json::parse(root_type_json_str);

            // 2. Check if the generated JSON is as expected
            CHECK(j == expected);

            // 3. Check if the start and end positions do not include the surrounding whitespace
            CHECK(j.start_pos() == initial_whitespace.size());
            CHECK(j.end_pos() == root_type_json_str.size() - end_whitespace.size());
        }
    }
#undef SETUP_TESTCASES
#endif
}

// this test relies on parse errors being thrown, so it is skipped when
// exceptions are disabled (json::parse aborts instead of throwing there)
#if !defined(JSON_NOEXCEPTION)
namespace
{
// Return the exception message from parsing @a input, or a "<no error ...>"
// sentinel if the parse unexpectedly succeeds. json::parse is nodiscard, so the
// result is consumed (via size()) to keep -Wunused-result / -Werror happy.
template<typename InputType>
std::string parse_error_message(InputType&& input)
{
    try
    {
        const json j = json::parse(std::forward<InputType>(input));
        return "<no error, size " + std::to_string(j.size()) + ">";
    }
    catch (const json::exception& e)
    {
        return e.what();
    }
}

template<typename IteratorType>
std::string parse_error_message_range(IteratorType first, IteratorType last)
{
    try
    {
        const json j = json::parse(first, last);
        return "<no error, size " + std::to_string(j.size()) + ">";
    }
    catch (const json::exception& e)
    {
        return e.what();
    }
}
} // namespace

TEST_CASE("last-read diagnostics are identical across input adapters")
{
    // The lexer reconstructs the "last read" token lazily for seekable adapters
    // (contiguous byte input) and copies it eagerly for streaming adapters.
    // Both strategies must yield byte-for-byte identical error messages.

    // a selection of malformed inputs that exercise different token kinds,
    // whitespace/structural accumulation, number overflow, and control-char
    // escaping in the reconstructed "last read" token
    const std::vector<std::string> inputs =
    {
        "[1,2,x]",
        "  \n  @",
        "{\"a\": }",
        "1.18973e+4932",
        "\"\t\"",
        "tru",
        "[1 2]",
        "\xEF\xBB\xBF   nul",
    };

    for (const auto& s : inputs)
    {
        CAPTURE(s)

        // reference: contiguous std::string -> seekable (lazy) path
        const std::string reference = parse_error_message(s);
        // every input is malformed, so parsing must fail (error messages start
        // with '['; the success sentinel returned above starts with '<')
        CHECK(reference.front() == '[');

        // const char* -> also seekable
        CHECK(parse_error_message(s.c_str()) == reference);

        // std::vector<char> iterators -> seekable (random-access)
        {
            const std::vector<char> v(s.begin(), s.end());
            CHECK(parse_error_message_range(v.begin(), v.end()) == reference);
        }

        // std::list iterators -> non-seekable (bidirectional) eager path
        {
            const std::list<char> l(s.begin(), s.end());
            CHECK(parse_error_message_range(l.begin(), l.end()) == reference);
        }

        // std::istringstream -> non-seekable streaming eager path
        {
            std::istringstream ss(s);
            CHECK(parse_error_message(ss) == reference);
        }

        // wide strings -> wide_string_input_adapter eager path; only comparable
        // for ASCII input, as non-ASCII bytes are transcoded to different UTF-8
        const bool is_ascii = std::all_of(s.begin(), s.end(), [](char c)
        {
            return static_cast<unsigned char>(c) < 0x80;
        });
        if (is_ascii)
        {
            const std::u16string w16(s.begin(), s.end());
            CHECK(parse_error_message(w16) == reference);

            const std::u32string w32(s.begin(), s.end());
            CHECK(parse_error_message(w32) == reference);
        }
    }
}
#endif // !defined(JSON_NOEXCEPTION)

// this test characterizes the current (documented-by-example, not otherwise
// specified) behavior of JSON_DIAGNOSTIC_POSITIONS positions with respect to
// value lifetime (copy/move/swap/mutation), the various input adapters, and
// user-driven SAX usage. It is regression protection, not a behavior
// specification: if any of these checks fail after a change to json.hpp,
// that change deliberately altered observable behavior and the test (and
// this comment) should be updated accordingly, rather than "fixed" blindly.
#if JSON_DIAGNOSTIC_POSITIONS
TEST_CASE("diagnostic positions: value lifetime, input adapters, and SAX")
{
    SECTION("value lifetime")
    {
        SECTION("copy constructor copies positions, recursively")
        {
            // basic_json(const basic_json&) (json.hpp, around line 1192) copies
            // start_position/end_position for the value itself; nested values
            // are copied via their own copy constructor (through the copied
            // object/array container), so positions are preserved throughout
            // the whole tree.
            const std::string s = R"({"a":1,"b":[1,2,3]})";
            const json a = json::parse(s);
            const json b = a; // NOLINT(performance-unnecessary-copy-initialization)

            CHECK(b.start_pos() == a.start_pos());
            CHECK(b.end_pos() == a.end_pos());
            CHECK(b["b"].start_pos() == a["b"].start_pos());
            CHECK(b["b"].end_pos() == a["b"].end_pos());
            CHECK(b["b"][0].start_pos() == a["b"][0].start_pos());
            CHECK(b["b"][0].end_pos() == a["b"][0].end_pos());

            // sanity: the positions are meaningful (not all npos)
            CHECK(b.start_pos() == 0);
            CHECK(b.end_pos() == s.size());
        }

        SECTION("move constructor resets the moved-from value to npos")
        {
            // basic_json(basic_json&&) copies
            // other's start_position/end_position into *this and then resets
            // other's to npos. Only the top-level moved-from value is
            // affected; its (moved-away) children are gone along with it.
            const std::string s = R"({"a":1,"b":[1,2,3]})";
            json a = json::parse(s);
            const auto a_start = a.start_pos();
            const auto a_end = a.end_pos();
            const auto nested_start = a["b"].start_pos();
            const auto nested_end = a["b"].end_pos();

            const json b(std::move(a));

            // the destination retains the original positions, recursively
            CHECK(b.start_pos() == a_start);
            CHECK(b.end_pos() == a_end);
            CHECK(b["b"].start_pos() == nested_start);
            CHECK(b["b"].end_pos() == nested_end);

            // the moved-from value is reset to a null and reports npos
            CHECK(a.is_null()); // NOLINT(bugprone-use-after-move,hicpp-invalid-access-moved,clang-analyzer-cplusplus.Move)
            CHECK(a.start_pos() == std::string::npos); // NOLINT(bugprone-use-after-move,hicpp-invalid-access-moved,clang-analyzer-cplusplus.Move)
            CHECK(a.end_pos() == std::string::npos); // NOLINT(bugprone-use-after-move,hicpp-invalid-access-moved,clang-analyzer-cplusplus.Move)
        }

        SECTION("swap() exchanges positions along with values")
        {
            // basic_json::swap() (json.hpp, around line 3626, and the friend
            // swap() that forwards to it) swaps start_position/end_position
            // together with m_data.m_type and m_data.m_value, so after
            // swap(a, b) each variable's position describes its own new
            // content, consistent with copy-assignment's
            // operator=(basic_json) (json.hpp, around line 1291), which also
            // swaps positions as part of its copy-and-swap implementation.
            json a = json::parse(R"({"a":1})");
            json b = json::parse(R"([1,2,3,4,5])");
            const auto a_start = a.start_pos();
            const auto a_end = a.end_pos();
            const auto b_start = b.start_pos();
            const auto b_end = b.end_pos();
            // both start at 0 (root values start right away), but their
            // lengths (and thus end positions) differ, which is enough to
            // tell after the swap whether positions actually moved with
            // the values
            CHECK(a_end != b_end);

            using std::swap;
            swap(a, b);

            // values were exchanged as expected ...
            CHECK(a == json::parse(R"([1,2,3,4,5])"));
            CHECK(b == json::parse(R"({"a":1})"));

            // ... and so were positions: each variable now carries the
            // other's original position, describing its own new content
            CHECK(a.start_pos() == b_start);
            CHECK(a.end_pos() == b_end);
            CHECK(b.start_pos() == a_start);
            CHECK(b.end_pos() == a_end);

            // member swap() behaves the same as the free function
            json c = json::parse(R"({"a":1})");
            json d = json::parse(R"([1,2,3,4,5])");
            const auto c_start = c.start_pos();
            const auto c_end = c.end_pos();
            const auto d_start = d.start_pos();
            const auto d_end = d.end_pos();

            c.swap(d);

            CHECK(c.start_pos() == d_start);
            CHECK(c.end_pos() == d_end);
            CHECK(d.start_pos() == c_start);
            CHECK(d.end_pos() == c_end);
        }

        SECTION("mutating a parsed document leaves positions of unrelated values untouched")
        {
            // Positions are recorded once, during parsing, and are not
            // recomputed on mutation. As a consequence, after a mutation the
            // parent's own recorded span may no longer describe its current
            // (serialized) content -- it still describes what was originally
            // parsed. This is characterized here as current behavior, not
            // asserted to be desirable or specified.
            SECTION("operator[] adding a new object key")
            {
                const std::string s = R"({"a":1})";
                json j = json::parse(s);
                const auto root_start = j.start_pos();
                const auto root_end = j.end_pos();
                const auto a_start = j["a"].start_pos();
                const auto a_end = j["a"].end_pos();

                j["c"] = 42;

                // the newly-added value was never parsed, so it has no position
                CHECK(j["c"].start_pos() == std::string::npos);
                CHECK(j["c"].end_pos() == std::string::npos);

                // the existing sibling's position is unaffected
                CHECK(j["a"].start_pos() == a_start);
                CHECK(j["a"].end_pos() == a_end);

                // the parent's own recorded span is left as-is (now stale:
                // it still reflects the original, shorter `{"a":1}` string)
                CHECK(j.start_pos() == root_start);
                CHECK(j.end_pos() == root_end);
            }

            SECTION("push_back on a parsed array")
            {
                const std::string s = R"([1,2,3])";
                json j = json::parse(s);
                const auto root_start = j.start_pos();
                const auto root_end = j.end_pos();
                const auto first_start = j[0].start_pos();

                j.push_back(4);

                CHECK(j.back().start_pos() == std::string::npos);
                CHECK(j.back().end_pos() == std::string::npos);
                CHECK(j[0].start_pos() == first_start);
                CHECK(j.start_pos() == root_start);
                CHECK(j.end_pos() == root_end);
            }

            SECTION("erase on a parsed array shifts elements but keeps their own positions")
            {
                const std::string s = R"([1,2,3])";
                json j = json::parse(s);
                const auto second_start = j[1].start_pos();
                const auto third_start = j[2].start_pos();
                const auto root_start = j.start_pos();
                const auto root_end = j.end_pos();

                j.erase(0);

                // remaining elements moved down an index, but each one still
                // reports the position it had *before* the erase (i.e. its
                // position in the original source string, not a
                // recalculated one)
                CHECK(j[0].start_pos() == second_start);
                CHECK(j[1].start_pos() == third_start);

                // the parent's own recorded span is again left as-is
                CHECK(j.start_pos() == root_start);
                CHECK(j.end_pos() == root_end);
            }
        }
    }

    SECTION("input adapters")
    {
        SECTION("wide string input: positions count transcoded UTF-8 bytes, not wide characters")
        {
            // 'é' (U+00E9) is a single code unit in a wchar_t/UTF-16 string, but
            // transcodes to 2 bytes in UTF-8; the lexer only ever sees the
            // transcoded UTF-8 byte stream, so reported positions are byte
            // offsets into that UTF-8 stream, not indices into the original
            // std::wstring.
            // é (rather than a literal 'é' byte sequence in this source
            // file) so the wide-string literal's meaning does not depend on
            // the compiler's assumed source character set (MSVC, without
            // /utf-8, would otherwise decode the raw UTF-8 bytes using the
            // system code page instead of as UTF-8)
            const std::wstring ws = L"{\"a\":\"\u00e9\u00e9\"}";
            CHECK(ws.size() == 10); // 10 wide characters

            const json j = json::parse(ws);
            CHECK(j.start_pos() == 0);
            // the transcoded UTF-8 form is 2 bytes longer than the wide string,
            // because each of the two 'é' characters becomes 2 UTF-8 bytes
            CHECK(j.end_pos() == 12);
            CHECK(j.end_pos() != ws.size());

            const json& a = j["a"];
            CHECK(a.start_pos() == 5);
            CHECK(a.end_pos() == 11);
        }

        SECTION("BOM-prefixed input: start_pos() reflects the skipped 3-byte BOM")
        {
            const std::string s = "\xEF\xBB\xBF{\"a\":1}";
            const json j = json::parse(s);

            // the lexer silently skips the BOM before parsing the value, so
            // the root value's recorded span starts right after it
            CHECK(j.start_pos() == 3);
            CHECK(j.end_pos() == s.size());
        }

        SECTION("std::istringstream: positions are consistent, not npos")
        {
            const std::string s = R"({"a":1,"b":2})";
            std::istringstream ss(s);
            const json j = json::parse(ss);

            CHECK(j.start_pos() == 0);
            CHECK(j.end_pos() == s.size());
            CHECK(j["a"].start_pos() == 5);
        }

        SECTION("std::ifstream: positions are consistent, not npos")
        {
            const std::string s = R"({"a":1,"b":2})";
            {
                std::ofstream file("unit-class_parser_diagnostic_positions.tmp");
                file << s;
            }

            {
                std::ifstream f("unit-class_parser_diagnostic_positions.tmp");
                const json j = json::parse(f);

                CHECK(j.start_pos() == 0);
                CHECK(j.end_pos() == s.size());
                CHECK(j["a"].start_pos() == 5);
            }

            static_cast<void>(std::remove("unit-class_parser_diagnostic_positions.tmp"));
        }

        SECTION("iterator-pair input: positions are consistent, not npos")
        {
            const std::string s = R"({"a":1,"b":2})";
            const json j = json::parse(s.begin(), s.end());

            CHECK(j.start_pos() == 0);
            CHECK(j.end_pos() == s.size());
            CHECK(j["a"].start_pos() == 5);
        }

        SECTION("binary formats have no text positions")
        {
            // binary formats (BJData, BON8, BSON, CBOR, MessagePack, UBJSON) are
            // parsed via detail::binary_reader, which never sets
            // start_position/end_position on the values it produces (they
            // have no notion of a text offset), so every value's position
            // stays at its default of npos.
            const json src = json::parse(R"({"a":1,"b":[1,2]})");

            const json from_cbor = json::from_cbor(json::to_cbor(src));
            CHECK(from_cbor.start_pos() == std::string::npos);
            CHECK(from_cbor.end_pos() == std::string::npos);
            CHECK(from_cbor["a"].start_pos() == std::string::npos);
            CHECK(from_cbor["b"][0].start_pos() == std::string::npos);

            const json from_msgpack = json::from_msgpack(json::to_msgpack(src));
            CHECK(from_msgpack.start_pos() == std::string::npos);
            CHECK(from_msgpack.end_pos() == std::string::npos);

            const json from_bon8 = json::from_bon8(json::to_bon8(src));
            CHECK(from_bon8.start_pos() == std::string::npos);
            CHECK(from_bon8.end_pos() == std::string::npos);

            const json from_ubjson = json::from_ubjson(json::to_ubjson(src));
            CHECK(from_ubjson.start_pos() == std::string::npos);
            CHECK(from_ubjson.end_pos() == std::string::npos);

            const json from_bson_val = json::from_bson(json::to_bson(src));
            CHECK(from_bson_val.start_pos() == std::string::npos);
            CHECK(from_bson_val.end_pos() == std::string::npos);
        }
    }

    SECTION("user-driven SAX consumers with no lexer report npos")
    {
        // json::parse() internally wires up its json_sax_dom_parser with a
        // pointer to its own lexer (see parser.hpp), which is how positions
        // get set at all. A user who constructs a json_sax_dom_parser
        // directly (e.g. to drive it via json::sax_parse()) and does not
        // supply a lexer pointer gets a consumer with m_lexer_ref == nullptr;
        // every "if (m_lexer_ref)" guard in json_sax.hpp is then skipped, so
        // every value it produces keeps its default, unset position (npos).
        // This was previously true but silently unasserted (operator==
        // ignores positions), see #5420.
        json result;
        nlohmann::detail::json_sax_dom_parser<json, nlohmann::detail::string_input_adapter_type> sdp(result);
        const std::string s = R"({"a":1,"b":[1,2,3]})";
        CHECK(json::sax_parse(s, &sdp));

        CHECK(result.start_pos() == std::string::npos);
        CHECK(result.end_pos() == std::string::npos);
        CHECK(result["a"].start_pos() == std::string::npos);
        CHECK(result["a"].end_pos() == std::string::npos);
        CHECK(result["b"][0].start_pos() == std::string::npos);
        CHECK(result["b"][0].end_pos() == std::string::npos);
    }
}
#endif