Document why the digit-count fast path is safe regardless of number_unsigned_t/number_integer_t width

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

include/nlohmann/detail/input/lexer.hpp

@@ -149,10 +149,11 @@ class lexer : public lexer_base<BasicJsonType>
   public:
     using token_type = typename lexer_base<BasicJsonType>::token_type;
 
-    explicit lexer(InputAdapterType&& adapter, bool ignore_comments_ = false) noexcept
+    explicit lexer(InputAdapterType&& adapter, bool ignore_comments_ = false, bool discard_number_values_ = false) noexcept
         : ia(std::move(adapter))
         , ignore_comments(ignore_comments_)
         , decimal_point_char(static_cast<char_int_type>(get_decimal_point()))
+        , discard_number_values(discard_number_values_)
     {}
 
     // deleted because of pointer members
@@ -1279,6 +1280,58 @@ scan_number_done:
         // we are done scanning a number)
         unget();
 
+        // If the caller does not need the converted value (only whether the
+        // input is syntactically valid; see json_sax_acceptor/accept()), an
+        // unsigned/integer token can be reported without calling
+        // strtoull()/strtoll() at all, *provided* we can already tell from
+        // the digit count alone that the conversion cannot overflow 64 bits.
+        // Such tokens are always finite and are accepted unconditionally by
+        // the parser regardless of their actual value (parser::sax_parse_internal()
+        // never checks finiteness for value_unsigned/value_integer), so the
+        // classification below is all that is needed.
+        //
+        // A decimal number with up to 18 digits is always representable in
+        // both std::uint64_t and std::int64_t (18 nines is ~1e18, well below
+        // both UINT64_MAX ~1.8e19 and INT64_MAX ~9.2e18), so strtoull()/strtoll()
+        // could not have set errno to ERANGE for it. Numbers with more digits
+        // (rare in practice) fall through to the exact code below, unchanged,
+        // so their handling -- including reclassification to value_float when
+        // the value overflows 64 bits, and rejection when it is not even
+        // finite as a double -- is bit-for-bit identical to before this
+        // optimization.
+        //
+        // Note this reasons about std::uint64_t/std::int64_t, not about
+        // number_unsigned_t/number_integer_t (BasicJsonType's own, possibly
+        // narrower, template parameters -- e.g. std::uint32_t). That is fine
+        // *only* because discard_number_values is exclusively set by
+        // accept() (see json.hpp), and accept() always parses through the
+        // library's own json_sax_acceptor -- never a user-supplied SAX
+        // consumer -- whose number_unsigned()/number_integer()/number_float()
+        // callbacks unconditionally discard their argument and return true.
+        // So for every caller that can reach this branch, neither the token
+        // classification below nor the eventual (possibly narrowed, and on
+        // this fast path left stale/unset) value_unsigned/value_integer is
+        // ever consulted -- an unsigned/integer token is accepted outright,
+        // and even a >18-digit token that this fast path deliberately falls
+        // through for is, once reclassified to value_float, still finite
+        // (and thus accepted) for any digit count that fits in number_unsigned_t
+        // or number_integer_t regardless of that type's width. If this
+        // function is ever taught to run with discard_number_values true for
+        // a caller that *does* read the converted value, this reasoning (and
+        // the fast path below) would need to be revisited.
+        if (discard_number_values)
+        {
+            constexpr std::size_t safe_digit_count = 18;
+            if (number_type == token_type::value_unsigned && token_buffer.size() <= safe_digit_count)
+            {
+                return token_type::value_unsigned;
+            }
+            if (number_type == token_type::value_integer && token_buffer.size() - 1 <= safe_digit_count)
+            {
+                return token_type::value_integer;
+            }
+        }
+
         char* endptr = nullptr; // NOLINT(misc-const-correctness,cppcoreguidelines-pro-type-vararg,hicpp-vararg)
         errno = 0;
 
@@ -1754,6 +1807,13 @@ scan_number_done:
     const char_int_type decimal_point_char = '.';
     /// the position of the decimal point in the input
     std::size_t decimal_point_position = std::string::npos;
+
+    /// whether the caller (e.g. accept()/json_sax_acceptor) only needs the
+    /// token classification and never looks at the converted numeric value;
+    /// when set, scan_number() may skip strtoull()/strtoll() for
+    /// value_unsigned/value_integer tokens whose digit count guarantees they
+    /// fit into 64 bits (see scan_number())
+    const bool discard_number_values = false;
 };
 
 }  // namespace detail

include/nlohmann/detail/input/parser.hpp

@@ -72,9 +72,10 @@ class parser
                     parser_callback_t<BasicJsonType> cb = nullptr,
                     const bool allow_exceptions_ = true,
                     const bool ignore_comments = false,
-                    const bool ignore_trailing_commas_ = false)
+                    const bool ignore_trailing_commas_ = false,
+                    const bool discard_number_values_ = false)
         : callback(std::move(cb))
-        , m_lexer(std::move(adapter), ignore_comments)
+        , m_lexer(std::move(adapter), ignore_comments, discard_number_values_)
         , allow_exceptions(allow_exceptions_)
         , ignore_trailing_commas(ignore_trailing_commas_)
     {

include/nlohmann/json.hpp

@@ -164,11 +164,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         detail::parser_callback_t<basic_json>cb = nullptr,
         const bool allow_exceptions = true,
         const bool ignore_comments = false,
-        const bool ignore_trailing_commas = false
+        const bool ignore_trailing_commas = false,
+        const bool discard_number_values = false
     )
     {
         return ::nlohmann::detail::parser<basic_json, InputAdapterType>(std::move(adapter),
-               std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas);
+               std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas, discard_number_values);
     }
 
   private:
@@ -1335,7 +1336,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief serialization
     /// @sa https://json.nlohmann.me/api/basic_json/dump/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     string_t dump(const int indent = -1,
                   const char indent_char = ' ',
                   const bool ensure_ascii = false,
@@ -1358,7 +1358,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return the type of the JSON value (explicit)
     /// @sa https://json.nlohmann.me/api/basic_json/type/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr value_t type() const noexcept
     {
         return m_data.m_type;
@@ -1366,7 +1365,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether type is primitive
     /// @sa https://json.nlohmann.me/api/basic_json/is_primitive/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_primitive() const noexcept
     {
         return is_null() || is_string() || is_boolean() || is_number() || is_binary();
@@ -1374,7 +1372,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether type is structured
     /// @sa https://json.nlohmann.me/api/basic_json/is_structured/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_structured() const noexcept
     {
         return is_array() || is_object();
@@ -1382,7 +1379,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is null
     /// @sa https://json.nlohmann.me/api/basic_json/is_null/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_null() const noexcept
     {
         return m_data.m_type == value_t::null;
@@ -1390,7 +1386,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a boolean
     /// @sa https://json.nlohmann.me/api/basic_json/is_boolean/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_boolean() const noexcept
     {
         return m_data.m_type == value_t::boolean;
@@ -1398,7 +1393,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number() const noexcept
     {
         return is_number_integer() || is_number_float();
@@ -1406,7 +1400,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an integer number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number_integer/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number_integer() const noexcept
     {
         return m_data.m_type == value_t::number_integer || m_data.m_type == value_t::number_unsigned;
@@ -1414,7 +1407,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an unsigned integer number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number_unsigned/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number_unsigned() const noexcept
     {
         return m_data.m_type == value_t::number_unsigned;
@@ -1422,7 +1414,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a floating-point number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number_float/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number_float() const noexcept
     {
         return m_data.m_type == value_t::number_float;
@@ -1430,7 +1421,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an object
     /// @sa https://json.nlohmann.me/api/basic_json/is_object/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_object() const noexcept
     {
         return m_data.m_type == value_t::object;
@@ -1438,7 +1428,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an array
     /// @sa https://json.nlohmann.me/api/basic_json/is_array/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_array() const noexcept
     {
         return m_data.m_type == value_t::array;
@@ -1446,7 +1435,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a string
     /// @sa https://json.nlohmann.me/api/basic_json/is_string/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_string() const noexcept
     {
         return m_data.m_type == value_t::string;
@@ -1454,7 +1442,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a binary array
     /// @sa https://json.nlohmann.me/api/basic_json/is_binary/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_binary() const noexcept
     {
         return m_data.m_type == value_t::binary;
@@ -1462,7 +1449,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is discarded
     /// @sa https://json.nlohmann.me/api/basic_json/is_discarded/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_discarded() const noexcept
     {
         return m_data.m_type == value_t::discarded;
@@ -2794,7 +2780,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief returns the number of occurrences of a key in a JSON object
     /// @sa https://json.nlohmann.me/api/basic_json/count/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type count(const typename object_t::key_type& key) const
     {
         // return 0 for all nonobject types
@@ -2805,7 +2790,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/count/
     template<class KeyType, detail::enable_if_t<
                  detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type count(KeyType && key) const
     {
         // return 0 for all nonobject types
@@ -2814,7 +2798,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief check the existence of an element in a JSON object
     /// @sa https://json.nlohmann.me/api/basic_json/contains/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool contains(const typename object_t::key_type& key) const
     {
         return is_object() && m_data.m_value.object->find(key) != m_data.m_value.object->end();
@@ -2824,7 +2807,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/contains/
     template<class KeyType, detail::enable_if_t<
                  detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool contains(KeyType && key) const
     {
         return is_object() && m_data.m_value.object->find(std::forward<KeyType>(key)) != m_data.m_value.object->end();
@@ -2832,14 +2814,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief check the existence of an element in a JSON object given a JSON pointer
     /// @sa https://json.nlohmann.me/api/basic_json/contains/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool contains(const json_pointer& ptr) const
     {
         return ptr.contains(this);
     }
 
     template<typename BasicJsonType, detail::enable_if_t<detail::is_basic_json<BasicJsonType>::value, int> = 0>
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     JSON_HEDLEY_DEPRECATED_FOR(3.11.0, basic_json::json_pointer or nlohmann::json_pointer<basic_json::string_t>) // NOLINT(readability/alt_tokens)
     bool contains(const typename ::nlohmann::json_pointer<BasicJsonType>& ptr) const
     {
@@ -2995,7 +2975,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief checks whether the container is empty.
     /// @sa https://json.nlohmann.me/api/basic_json/empty/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool empty() const noexcept
     {
         switch (m_data.m_type)
@@ -3035,7 +3014,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief returns the number of elements
     /// @sa https://json.nlohmann.me/api/basic_json/size/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type size() const noexcept
     {
         switch (m_data.m_type)
@@ -3075,7 +3053,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief returns the maximum possible number of elements
     /// @sa https://json.nlohmann.me/api/basic_json/max_size/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type max_size() const noexcept
     {
         switch (m_data.m_type)
@@ -4153,24 +4130,22 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @brief check if the input is valid JSON
     /// @sa https://json.nlohmann.me/api/basic_json/accept/
     template<typename InputType>
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     static bool accept(InputType&& i,
                        const bool ignore_comments = false,
                        const bool ignore_trailing_commas = false)
     {
-        return parser(detail::input_adapter(std::forward<InputType>(i)), nullptr, false, ignore_comments, ignore_trailing_commas).accept(true);
+        return parser(detail::input_adapter(std::forward<InputType>(i)), nullptr, false, ignore_comments, ignore_trailing_commas, true).accept(true);
     }
 
     /// @brief check if the input is valid JSON (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/accept/
     template<typename IteratorType, typename SentinelType = IteratorType,
              detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     static bool accept(IteratorType first, SentinelType last,
                        const bool ignore_comments = false,
                        const bool ignore_trailing_commas = false)
     {
-        return parser(detail::input_adapter(std::move(first), std::move(last)), nullptr, false, ignore_comments, ignore_trailing_commas).accept(true);
+        return parser(detail::input_adapter(std::move(first), std::move(last)), nullptr, false, ignore_comments, ignore_trailing_commas, true).accept(true);
     }
 
     JSON_HEDLEY_WARN_UNUSED_RESULT
@@ -4179,7 +4154,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                        const bool ignore_comments = false,
                        const bool ignore_trailing_commas = false)
     {
-        return parser(i.get(), nullptr, false, ignore_comments, ignore_trailing_commas).accept(true);
+        return parser(i.get(), nullptr, false, ignore_comments, ignore_trailing_commas, true).accept(true);
     }
 
     /// @brief generate SAX events
@@ -4265,7 +4240,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return the type as string
     /// @sa https://json.nlohmann.me/api/basic_json/type_name/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     JSON_HEDLEY_RETURNS_NON_NULL
     const char* type_name() const noexcept
     {

single_include/nlohmann/json.hpp

@@ -7932,10 +7932,11 @@ class lexer : public lexer_base<BasicJsonType>
   public:
     using token_type = typename lexer_base<BasicJsonType>::token_type;
 
-    explicit lexer(InputAdapterType&& adapter, bool ignore_comments_ = false) noexcept
+    explicit lexer(InputAdapterType&& adapter, bool ignore_comments_ = false, bool discard_number_values_ = false) noexcept
         : ia(std::move(adapter))
         , ignore_comments(ignore_comments_)
         , decimal_point_char(static_cast<char_int_type>(get_decimal_point()))
+        , discard_number_values(discard_number_values_)
     {}
 
     // deleted because of pointer members
@@ -9062,6 +9063,58 @@ scan_number_done:
         // we are done scanning a number)
         unget();
 
+        // If the caller does not need the converted value (only whether the
+        // input is syntactically valid; see json_sax_acceptor/accept()), an
+        // unsigned/integer token can be reported without calling
+        // strtoull()/strtoll() at all, *provided* we can already tell from
+        // the digit count alone that the conversion cannot overflow 64 bits.
+        // Such tokens are always finite and are accepted unconditionally by
+        // the parser regardless of their actual value (parser::sax_parse_internal()
+        // never checks finiteness for value_unsigned/value_integer), so the
+        // classification below is all that is needed.
+        //
+        // A decimal number with up to 18 digits is always representable in
+        // both std::uint64_t and std::int64_t (18 nines is ~1e18, well below
+        // both UINT64_MAX ~1.8e19 and INT64_MAX ~9.2e18), so strtoull()/strtoll()
+        // could not have set errno to ERANGE for it. Numbers with more digits
+        // (rare in practice) fall through to the exact code below, unchanged,
+        // so their handling -- including reclassification to value_float when
+        // the value overflows 64 bits, and rejection when it is not even
+        // finite as a double -- is bit-for-bit identical to before this
+        // optimization.
+        //
+        // Note this reasons about std::uint64_t/std::int64_t, not about
+        // number_unsigned_t/number_integer_t (BasicJsonType's own, possibly
+        // narrower, template parameters -- e.g. std::uint32_t). That is fine
+        // *only* because discard_number_values is exclusively set by
+        // accept() (see json.hpp), and accept() always parses through the
+        // library's own json_sax_acceptor -- never a user-supplied SAX
+        // consumer -- whose number_unsigned()/number_integer()/number_float()
+        // callbacks unconditionally discard their argument and return true.
+        // So for every caller that can reach this branch, neither the token
+        // classification below nor the eventual (possibly narrowed, and on
+        // this fast path left stale/unset) value_unsigned/value_integer is
+        // ever consulted -- an unsigned/integer token is accepted outright,
+        // and even a >18-digit token that this fast path deliberately falls
+        // through for is, once reclassified to value_float, still finite
+        // (and thus accepted) for any digit count that fits in number_unsigned_t
+        // or number_integer_t regardless of that type's width. If this
+        // function is ever taught to run with discard_number_values true for
+        // a caller that *does* read the converted value, this reasoning (and
+        // the fast path below) would need to be revisited.
+        if (discard_number_values)
+        {
+            constexpr std::size_t safe_digit_count = 18;
+            if (number_type == token_type::value_unsigned && token_buffer.size() <= safe_digit_count)
+            {
+                return token_type::value_unsigned;
+            }
+            if (number_type == token_type::value_integer && token_buffer.size() - 1 <= safe_digit_count)
+            {
+                return token_type::value_integer;
+            }
+        }
+
         char* endptr = nullptr; // NOLINT(misc-const-correctness,cppcoreguidelines-pro-type-vararg,hicpp-vararg)
         errno = 0;
 
@@ -9537,6 +9590,13 @@ scan_number_done:
     const char_int_type decimal_point_char = '.';
     /// the position of the decimal point in the input
     std::size_t decimal_point_position = std::string::npos;
+
+    /// whether the caller (e.g. accept()/json_sax_acceptor) only needs the
+    /// token classification and never looks at the converted numeric value;
+    /// when set, scan_number() may skip strtoull()/strtoll() for
+    /// value_unsigned/value_integer tokens whose digit count guarantees they
+    /// fit into 64 bits (see scan_number())
+    const bool discard_number_values = false;
 };
 
 }  // namespace detail
@@ -14043,9 +14103,10 @@ class parser
                     parser_callback_t<BasicJsonType> cb = nullptr,
                     const bool allow_exceptions_ = true,
                     const bool ignore_comments = false,
-                    const bool ignore_trailing_commas_ = false)
+                    const bool ignore_trailing_commas_ = false,
+                    const bool discard_number_values_ = false)
         : callback(std::move(cb))
-        , m_lexer(std::move(adapter), ignore_comments)
+        , m_lexer(std::move(adapter), ignore_comments, discard_number_values_)
         , allow_exceptions(allow_exceptions_)
         , ignore_trailing_commas(ignore_trailing_commas_)
     {
@@ -21592,11 +21653,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         detail::parser_callback_t<basic_json>cb = nullptr,
         const bool allow_exceptions = true,
         const bool ignore_comments = false,
-        const bool ignore_trailing_commas = false
+        const bool ignore_trailing_commas = false,
+        const bool discard_number_values = false
                                  )
     {
         return ::nlohmann::detail::parser<basic_json, InputAdapterType>(std::move(adapter),
-            std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas);
+            std::move(cb), allow_exceptions, ignore_comments, ignore_trailing_commas, discard_number_values);
     }
 
   private:
@@ -22763,7 +22825,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief serialization
     /// @sa https://json.nlohmann.me/api/basic_json/dump/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     string_t dump(const int indent = -1,
                   const char indent_char = ' ',
                   const bool ensure_ascii = false,
@@ -22786,7 +22847,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return the type of the JSON value (explicit)
     /// @sa https://json.nlohmann.me/api/basic_json/type/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr value_t type() const noexcept
     {
         return m_data.m_type;
@@ -22794,7 +22854,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether type is primitive
     /// @sa https://json.nlohmann.me/api/basic_json/is_primitive/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_primitive() const noexcept
     {
         return is_null() || is_string() || is_boolean() || is_number() || is_binary();
@@ -22802,7 +22861,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether type is structured
     /// @sa https://json.nlohmann.me/api/basic_json/is_structured/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_structured() const noexcept
     {
         return is_array() || is_object();
@@ -22810,7 +22868,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is null
     /// @sa https://json.nlohmann.me/api/basic_json/is_null/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_null() const noexcept
     {
         return m_data.m_type == value_t::null;
@@ -22818,7 +22875,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a boolean
     /// @sa https://json.nlohmann.me/api/basic_json/is_boolean/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_boolean() const noexcept
     {
         return m_data.m_type == value_t::boolean;
@@ -22826,7 +22882,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number() const noexcept
     {
         return is_number_integer() || is_number_float();
@@ -22834,7 +22889,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an integer number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number_integer/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number_integer() const noexcept
     {
         return m_data.m_type == value_t::number_integer || m_data.m_type == value_t::number_unsigned;
@@ -22842,7 +22896,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an unsigned integer number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number_unsigned/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number_unsigned() const noexcept
     {
         return m_data.m_type == value_t::number_unsigned;
@@ -22850,7 +22903,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a floating-point number
     /// @sa https://json.nlohmann.me/api/basic_json/is_number_float/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_number_float() const noexcept
     {
         return m_data.m_type == value_t::number_float;
@@ -22858,7 +22910,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an object
     /// @sa https://json.nlohmann.me/api/basic_json/is_object/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_object() const noexcept
     {
         return m_data.m_type == value_t::object;
@@ -22866,7 +22917,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is an array
     /// @sa https://json.nlohmann.me/api/basic_json/is_array/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_array() const noexcept
     {
         return m_data.m_type == value_t::array;
@@ -22874,7 +22924,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a string
     /// @sa https://json.nlohmann.me/api/basic_json/is_string/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_string() const noexcept
     {
         return m_data.m_type == value_t::string;
@@ -22882,7 +22931,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is a binary array
     /// @sa https://json.nlohmann.me/api/basic_json/is_binary/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_binary() const noexcept
     {
         return m_data.m_type == value_t::binary;
@@ -22890,7 +22938,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return whether value is discarded
     /// @sa https://json.nlohmann.me/api/basic_json/is_discarded/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     constexpr bool is_discarded() const noexcept
     {
         return m_data.m_type == value_t::discarded;
@@ -24222,7 +24269,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief returns the number of occurrences of a key in a JSON object
     /// @sa https://json.nlohmann.me/api/basic_json/count/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type count(const typename object_t::key_type& key) const
     {
         // return 0 for all nonobject types
@@ -24233,7 +24279,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/count/
     template<class KeyType, detail::enable_if_t<
                  detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type count(KeyType && key) const
     {
         // return 0 for all nonobject types
@@ -24242,7 +24287,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief check the existence of an element in a JSON object
     /// @sa https://json.nlohmann.me/api/basic_json/contains/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool contains(const typename object_t::key_type& key) const
     {
         return is_object() && m_data.m_value.object->find(key) != m_data.m_value.object->end();
@@ -24252,7 +24296,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @sa https://json.nlohmann.me/api/basic_json/contains/
     template<class KeyType, detail::enable_if_t<
                  detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool contains(KeyType && key) const
     {
         return is_object() && m_data.m_value.object->find(std::forward<KeyType>(key)) != m_data.m_value.object->end();
@@ -24260,14 +24303,12 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief check the existence of an element in a JSON object given a JSON pointer
     /// @sa https://json.nlohmann.me/api/basic_json/contains/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool contains(const json_pointer& ptr) const
     {
         return ptr.contains(this);
     }
 
     template<typename BasicJsonType, detail::enable_if_t<detail::is_basic_json<BasicJsonType>::value, int> = 0>
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     JSON_HEDLEY_DEPRECATED_FOR(3.11.0, basic_json::json_pointer or nlohmann::json_pointer<basic_json::string_t>) // NOLINT(readability/alt_tokens)
     bool contains(const typename ::nlohmann::json_pointer<BasicJsonType>& ptr) const
     {
@@ -24423,7 +24464,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief checks whether the container is empty.
     /// @sa https://json.nlohmann.me/api/basic_json/empty/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     bool empty() const noexcept
     {
         switch (m_data.m_type)
@@ -24463,7 +24503,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief returns the number of elements
     /// @sa https://json.nlohmann.me/api/basic_json/size/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type size() const noexcept
     {
         switch (m_data.m_type)
@@ -24503,7 +24542,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief returns the maximum possible number of elements
     /// @sa https://json.nlohmann.me/api/basic_json/max_size/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     size_type max_size() const noexcept
     {
         switch (m_data.m_type)
@@ -25581,24 +25619,22 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @brief check if the input is valid JSON
     /// @sa https://json.nlohmann.me/api/basic_json/accept/
     template<typename InputType>
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     static bool accept(InputType&& i,
                        const bool ignore_comments = false,
                        const bool ignore_trailing_commas = false)
     {
-        return parser(detail::input_adapter(std::forward<InputType>(i)), nullptr, false, ignore_comments, ignore_trailing_commas).accept(true);
+        return parser(detail::input_adapter(std::forward<InputType>(i)), nullptr, false, ignore_comments, ignore_trailing_commas, true).accept(true);
     }
 
     /// @brief check if the input is valid JSON (iterator pair, or iterator+sentinel pair for C++20 ranges support)
     /// @sa https://json.nlohmann.me/api/basic_json/accept/
     template<typename IteratorType, typename SentinelType = IteratorType,
              detail::enable_if_t<detail::can_compare_ne<IteratorType, SentinelType>::value, int> = 0>
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     static bool accept(IteratorType first, SentinelType last,
                        const bool ignore_comments = false,
                        const bool ignore_trailing_commas = false)
     {
-        return parser(detail::input_adapter(std::move(first), std::move(last)), nullptr, false, ignore_comments, ignore_trailing_commas).accept(true);
+        return parser(detail::input_adapter(std::move(first), std::move(last)), nullptr, false, ignore_comments, ignore_trailing_commas, true).accept(true);
     }
 
     JSON_HEDLEY_WARN_UNUSED_RESULT
@@ -25607,7 +25643,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                        const bool ignore_comments = false,
                        const bool ignore_trailing_commas = false)
     {
-        return parser(i.get(), nullptr, false, ignore_comments, ignore_trailing_commas).accept(true);
+        return parser(i.get(), nullptr, false, ignore_comments, ignore_trailing_commas, true).accept(true);
     }
 
     /// @brief generate SAX events
@@ -25693,7 +25729,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
     /// @brief return the type as string
     /// @sa https://json.nlohmann.me/api/basic_json/type_name/
-    JSON_HEDLEY_WARN_UNUSED_RESULT
     JSON_HEDLEY_RETURNS_NON_NULL
     const char* type_name() const noexcept
     {

tests/src/unit-class_parser.cpp

@@ -930,6 +930,94 @@ TEST_CASE("parser class")
                 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
+                    const std::string wrapped = "[" + number + "," + number + "]";
+                    CHECK(json::accept(wrapped) == expected);
+                }
+            }
         }
     }
 

tests/src/unit-regression2.cpp

@@ -639,8 +639,7 @@ TEST_CASE("regression tests 2")
                 s += static_cast<char>(i);
             }
             dump_test["1"] = s;
-            // dump() is nodiscard; this only checks that dumping does not throw/crash
-            (void)dump_test.dump(-1, ' ', true, nlohmann::json::error_handler_t::replace);
+            dump_test.dump(-1, ' ', true, nlohmann::json::error_handler_t::replace);
         }
     }