Escape characters below 0x60 from a table straight into the write buffer

When the bulk scanner in dump_escaped_impl() stops at a control
character, a quote or a backslash, that byte used to go through the
UTF-8 decoder and a switch that wrote the escape byte by byte into
string_buffer, which was then copied into write_buffer at the next run.

A byte below 0x60 at a character boundary is a complete code point, so
it now skips the decoder: its escape sequence comes from a 96-entry
table of 8-byte rows (sequence padded with zeros, length in the last
byte), and all 8 bytes are copied straight into write_buffer with one
room check. The check sits inside the existing character-boundary
block, so it runs once per scanner stop and not once per byte; testing
every byte cost about 6% on twitter.json with ensure_ascii.

The switch in the decoder's accept branch lost its cases for
characters below 0x60, which can no longer reach it.

Output is byte-identical. The string-buffer boundary tests now use
\u-escaped non-ASCII characters, since simple escapes no longer reach
string_buffer, and a new test case checks every ASCII character at
every position against a reference escaper.

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

docs/mkdocs/docs/api/basic_json/is_number_unsigned.md

@@ -6,9 +6,6 @@ constexpr bool is_number_unsigned() const noexcept;
 
 This function returns `#!cpp true` if and only if the JSON value is an unsigned integer number. This excludes
 floating-point and signed integer values.
-
-If [`number_unsigned_t`](number_unsigned_t.md) is the same type as [`number_integer_t`](number_integer_t.md), every
-integer is stored as `number_integer_t`, and this function always returns `#!cpp false`.
     
 ## Return value
 

docs/mkdocs/docs/api/basic_json/number_unsigned_t.md

@@ -25,8 +25,7 @@ the type to use.
 `NumberUnsignedType`
 :   the type to store unsigned integers. It must be an **unsigned integral** type (`#!cpp std::is_integral`) with a
     `#!cpp std::numeric_limits` specialization, and it must be able to represent the absolute value of every
-    [`number_integer_t`](number_integer_t.md) value. Alternatively, it can be the same type as
-    [`number_integer_t`](number_integer_t.md); then every integer is stored as `number_integer_t`. See
+    [`number_integer_t`](number_integer_t.md) value. See
     [Template Parameter Requirements](../../features/types/template_parameters.md#numberintegertype-and-numberunsignedtype).
 
 ## Notes
@@ -90,4 +89,3 @@ Integer number values are stored directly inside a `basic_json` type.
 ## Version history
 
 - Added in version 2.0.0.
-- Can be the same type as `number_integer_t` since version 3.13.0.

docs/mkdocs/docs/api/basic_json/operator_eq.md

@@ -11,12 +11,10 @@ template<typename ScalarType>
 bool operator==(ScalarType lhs, const const_reference rhs) noexcept(/* see below */); // (2)
 
 // since C++20
-class basic_json {
-    bool operator==(const_reference rhs) const noexcept;              // (1)
+bool operator==(const_reference rhs) const noexcept;             // (1)
 
-    template<typename ScalarType>
-    bool operator==(ScalarType rhs) const noexcept(/* see below */); // (2)
-};
+template<typename ScalarType>
+bool operator==(ScalarType rhs) const noexcept(/* see below */); // (2)
 ```
 
 1. Compares two JSON values for equality according to the following rules:

docs/mkdocs/docs/api/basic_json/operator_spaceship.md

@@ -2,12 +2,10 @@
 
 ```cpp
 // since C++20
-class basic_json {
-    std::partial_ordering operator<=>(const_reference rhs) const noexcept;  // (1)
+std::partial_ordering operator<=>(const_reference rhs) const noexcept;                   // (1)
 
-    template<typename ScalarType>
-    std::partial_ordering operator<=>(const ScalarType rhs) const noexcept(/* see below */); // (2)
-};
+template<typename ScalarType>
+std::partial_ordering operator<=>(const ScalarType rhs) const noexcept(/* see below */); // (2)
 ```
 
 1. 3-way compares two JSON values producing a result of type `std::partial_ordering` according to the following rules:

docs/mkdocs/docs/api/json_pointer/operator_eq.md

@@ -18,13 +18,11 @@ bool operator==(
     const json_pointer<RefStringTypeRhs>& rhs);                     // (2)
 
 // since C++20
-class json_pointer {
-    template<typename RefStringTypeRhs>
-    bool operator==(
-        const json_pointer<RefStringTypeRhs>& rhs) const noexcept;  // (1)
+template<typename RefStringTypeRhs>
+bool operator==(
+    const json_pointer<RefStringTypeRhs>& rhs) const noexcept;      // (1)
 
-    bool operator==(const string_t& rhs) const;                     // (2)
-};
+bool operator==(const string_t& rhs) const;                         // (2)
 ```
 
 1. Compares two JSON pointers for equality by comparing their reference tokens.

docs/mkdocs/docs/api/json_pointer/operator_spaceship.md

@@ -2,10 +2,8 @@
 
 ```cpp
 // since C++20
-class json_pointer {
-    template<typename RefStringTypeRhs>
-    std::strong_ordering operator<=>(const json_pointer<RefStringTypeRhs>& rhs) const noexcept; // *NOPAD*
-};
+template<typename RefStringTypeRhs>
+std::strong_ordering operator<=>(const json_pointer<RefStringTypeRhs>& rhs) const noexcept; // *NOPAD*
 ```
 
 3-way compares two JSON pointers by lexicographically comparing their sequences of reference tokens: corresponding

docs/mkdocs/docs/features/types/number_handling.md

@@ -343,11 +343,8 @@ The number types can be changed with template parameters.
       in case of overflow.
     - The type for unsigned integers must be convertible from `#!c unsigned long long`.  The type for floating-point
       numbers is used in case of overflow.
-    - The type for unsigned integers can also be the same signed type as the type for signed integers (e.g.,
-      `#!c std::int64_t` for both). Then every integer is stored as a signed integer, and
-      [`is_number_unsigned()`](../../api/basic_json/is_number_unsigned.md) is always `#!c false`, see
-      [Template Parameter Requirements](template_parameters.md#same-type-for-both).
-    - The type for floating-point numbers must be distinct from the integer types.
+    - The types for signed and unsigned integers must be distinct, see
+      [#2573](https://github.com/nlohmann/json/issues/2573).
     - Only `#!c double`, `#!c float`, and `#!c long double` are supported for floating-point numbers.
 
 !!! example

docs/mkdocs/docs/features/types/template_parameters.md

@@ -40,19 +40,19 @@ Requirements are split into two groups:
 
 ## Overview
 
-| Template parameter                                                | Default                           | Notable substitutes                                                                              |
-|-------------------------------------------------------------------|-----------------------------------|--------------------------------------------------------------------------------------------------|
-| [`ObjectType`](#objecttype)                                       | `std::map`                        | [`nlohmann::ordered_map`](../../api/ordered_map.md), Abseil hash maps                            |
-| [`ArrayType`](#arraytype)                                         | `std::vector`                     | `#!cpp std::deque`                                                                               |
-| [`StringType`](#stringtype)                                       | `std::string`                     | `std::string`-like types over `char`                                                             |
-| [`BooleanType`](#booleantype)                                     | `bool`                            | none worth using                                                                                 |
-| [`NumberIntegerType`](#numberintegertype-and-numberunsignedtype)  | `std::int64_t`                    | any signed integer type                                                                          |
-| [`NumberUnsignedType`](#numberintegertype-and-numberunsignedtype) | `std::uint64_t`                   | any unsigned integer type at least as wide as `NumberIntegerType`, or `NumberIntegerType` itself |
-| [`NumberFloatType`](#numberfloattype)                             | `double`                          | `float` (`long double`: no binary formats)                                                       |
-| [`AllocatorType`](#allocatortype)                                 | `std::allocator`                  | stateless allocators                                                                             |
-| [`JSONSerializer`](#jsonserializer)                               | `adl_serializer`                  | serializers with the same interface                                                              |
-| [`BinaryType`](#binarytype)                                       | `#!cpp std::vector<std::uint8_t>` | `#!cpp std::vector<char>`                                                                        |
-| [`CustomBaseClass`](#custombaseclass)                             | `void`                            | any default-constructible class                                                                  |
+| Template parameter                                                | Default                           | Notable substitutes                                                   |
+|-------------------------------------------------------------------|-----------------------------------|-----------------------------------------------------------------------|
+| [`ObjectType`](#objecttype)                                       | `std::map`                        | [`nlohmann::ordered_map`](../../api/ordered_map.md), Abseil hash maps |
+| [`ArrayType`](#arraytype)                                         | `std::vector`                     | `#!cpp std::deque`                                                    |
+| [`StringType`](#stringtype)                                       | `std::string`                     | `std::string`-like types over `char`                                  |
+| [`BooleanType`](#booleantype)                                     | `bool`                            | none worth using                                                      |
+| [`NumberIntegerType`](#numberintegertype-and-numberunsignedtype)  | `std::int64_t`                    | any signed integer type                                               |
+| [`NumberUnsignedType`](#numberintegertype-and-numberunsignedtype) | `std::uint64_t`                   | any unsigned integer type at least as wide as `NumberIntegerType`     |
+| [`NumberFloatType`](#numberfloattype)                             | `double`                          | `float` (`long double`: no binary formats)                            |
+| [`AllocatorType`](#allocatortype)                                 | `std::allocator`                  | stateless allocators                                                  |
+| [`JSONSerializer`](#jsonserializer)                               | `adl_serializer`                  | serializers with the same interface                                   |
+| [`BinaryType`](#binarytype)                                       | `#!cpp std::vector<std::uint8_t>` | `#!cpp std::vector<char>`                                             |
+| [`CustomBaseClass`](#custombaseclass)                             | `void`                            | any default-constructible class                                       |
 
 !!! warning "Third-party containers and incomplete types"
 
@@ -499,14 +499,13 @@ Both types are stored **directly** inside `basic_json`'s union.
 ### Always required
 
 - `#!cpp std::is_integral` must be satisfied: `NumberIntegerType` must be a **signed** integer type,
-  `NumberUnsignedType` an **unsigned** integer type or the same type as `NumberIntegerType` (see
-  [below](#same-type-for-both)). Class types are not supported -- among others, the constructors taking integer values
-  are constrained on `#!cpp std::is_integral`.
+  `NumberUnsignedType` an **unsigned** integer type. Class types are not supported -- among others, the constructors
+  taking integer values are constrained on `#!cpp std::is_integral`.
 - Trivially default-constructible, trivially copyable, and trivially destructible (union member).
 - `#!cpp std::numeric_limits` must be specialized for both types.
-- An unsigned `NumberUnsignedType` must be able to represent the absolute value of every `NumberIntegerType` value;
-  serialization of negative numbers converts the value to `NumberUnsignedType`. A `#!cpp static_assert` requires it to
-  be at least as wide as `NumberIntegerType`, which is what that amounts to for the standard integer types.
+- `NumberUnsignedType` must be able to represent the absolute value of every `NumberIntegerType` value; serialization
+  of negative numbers converts the value to `NumberUnsignedType`. A `#!cpp static_assert` requires it to be at least as
+  wide as `NumberIntegerType`, which is what that amounts to for the standard integer types.
 - Both types must fit into the internal 64-character number buffer used by
   [`dump`](../../api/basic_json/dump.md), which is the case for all standard integer types.
 - [`std::hash<basic_json>`](../../api/basic_json/std_hash.md) additionally requires `#!cpp std::hash` specializations.
@@ -518,23 +517,6 @@ is stored as [`number_float_t`](../../api/basic_json/number_float_t.md) instead.
 therefore silently changes parse results rather than raising an error. See
 [Number Handling](number_handling.md) for details.
 
-#### Same type for both
-
-`NumberUnsignedType` may be the same type as `NumberIntegerType`, e.g.
-`#!cpp nlohmann::json::with_integers_t<std::int64_t, std::int64_t>` (see
-[`with_integers_t`](../../api/basic_json/with_t.md)), for code that only wants to handle one integer type. Such a `basic_json` stores every integer as
-[`number_integer_t`](../../api/basic_json/number_integer_t.md):
-
-- [`is_number_unsigned()`](../../api/basic_json/is_number_unsigned.md) is always `#!cpp false`, and
-  [`type()`](../../api/basic_json/type.md) never returns `value_t::number_unsigned`. Constructing a value from
-  `value_t::number_unsigned` gives a `value_t::number_integer` value.
-- Values of unsigned C++ types (e.g., `#!cpp 5u`) are converted to `NumberIntegerType`, so values above its maximum
-  wrap around, as values outside the range of a narrower `NumberIntegerType` already do.
-- The parsers store a non-negative integer above the maximum of `NumberIntegerType` (e.g., `18446744073709551615` for
-  `#!cpp std::int64_t`) as [`number_float_t`](../../api/basic_json/number_float_t.md), like any other integer that does
-  not fit.
-- Converting from a `basic_json` with distinct types turns its unsigned values into `number_integer_t` values.
-
 ### Compatible types
 
 | Type pair                                                                                    | Support                                                                                                                                                                                          |
@@ -543,7 +525,6 @@ therefore silently changes parse results rather than raising an error. See
 | `#!cpp std::int32_t` / `#!cpp std::uint32_t`, `#!cpp long long` / `#!cpp unsigned long long` | full; narrower types change which literals the parser can represent                                                                                                                              |
 | any other pair of standard signed/unsigned integer types                                     | full                                                                                                                                                                                             |
 | class types, enumerations                                                                    | not usable; `#!cpp std::is_integral` must hold                                                                                                                                                   |
-| the same signed integer type for both, e.g. `#!cpp std::int64_t` / `#!cpp std::int64_t`      | full; every integer is stored as `number_integer_t`, see [above](#same-type-for-both)                                                                                                            |
 | `#!cpp bool`, or a type already used for another member of the union                         | not usable; `#!cpp std::is_integral<bool>` is in fact `#!cpp true`, but the `get_impl_ptr` overloads for `boolean_t`, `number_integer_t`, `number_unsigned_t` and `number_float_t` would collide |
 
 ## `NumberFloatType`

include/nlohmann/detail/conversions/from_json.hpp

@@ -80,11 +80,7 @@ void get_arithmetic_value(const BasicJsonType& j, ArithmeticType& val)
     {
         case value_t::number_unsigned:
         {
-            // never null: a basic_json whose number_unsigned_t is number_integer_t
-            // stores no number_unsigned values (#2573)
-            const auto* ptr = j.template get_ptr<const typename BasicJsonType::number_unsigned_t*>();
-            JSON_ASSERT(ptr != nullptr);
-            val = static_cast<ArithmeticType>(*ptr);
+            val = static_cast<ArithmeticType>(*j.template get_ptr<const typename BasicJsonType::number_unsigned_t*>());
             break;
         }
         case value_t::number_integer:
@@ -154,7 +150,7 @@ inline void from_json(const BasicJsonType& j, typename BasicJsonType::number_flo
 }
 
 template<typename BasicJsonType>
-inline void from_json(const BasicJsonType& j, number_unsigned_overload_t<BasicJsonType>& val)
+inline void from_json(const BasicJsonType& j, typename BasicJsonType::number_unsigned_t& val)
 {
     get_arithmetic_value(j, val);
 }
@@ -410,11 +406,7 @@ inline void from_json(const BasicJsonType& j, ArithmeticType& val)
     {
         case value_t::number_unsigned:
         {
-            // never null: a basic_json whose number_unsigned_t is number_integer_t
-            // stores no number_unsigned values (#2573)
-            const auto* ptr = j.template get_ptr<const typename BasicJsonType::number_unsigned_t*>();
-            JSON_ASSERT(ptr != nullptr);
-            val = static_cast<ArithmeticType>(*ptr);
+            val = static_cast<ArithmeticType>(*j.template get_ptr<const typename BasicJsonType::number_unsigned_t*>());
             break;
         }
         case value_t::number_integer:

include/nlohmann/detail/conversions/to_json.hpp

@@ -303,14 +303,14 @@ inline void to_json(BasicJsonType& j, FloatType val) noexcept
 }
 
 template<typename BasicJsonType, typename CompatibleNumberUnsignedType,
-         enable_if_t<is_compatible_integer_type<number_unsigned_overload_t<BasicJsonType>, CompatibleNumberUnsignedType>::value, int> = 0>
+         enable_if_t<is_compatible_integer_type<typename BasicJsonType::number_unsigned_t, CompatibleNumberUnsignedType>::value, int> = 0>
 inline void to_json(BasicJsonType& j, CompatibleNumberUnsignedType val) noexcept
 {
     external_constructor<value_t::number_unsigned>::construct(j, static_cast<typename BasicJsonType::number_unsigned_t>(val));
 }
 
 template<typename BasicJsonType, typename CompatibleNumberIntegerType,
-         enable_if_t<is_compatible_number_integer_type<BasicJsonType, CompatibleNumberIntegerType>::value, int> = 0>
+         enable_if_t<is_compatible_integer_type<typename BasicJsonType::number_integer_t, CompatibleNumberIntegerType>::value, int> = 0>
 inline void to_json(BasicJsonType& j, CompatibleNumberIntegerType val) noexcept
 {
     external_constructor<value_t::number_integer>::construct(j, static_cast<typename BasicJsonType::number_integer_t>(val));

include/nlohmann/detail/input/number_parse.hpp

@@ -18,7 +18,6 @@
 #include <cstring> // memcpy
 #include <limits> // numeric_limits
 #include <string> // string
-#include <type_traits> // is_signed, true_type, false_type
 
 #include <nlohmann/detail/bit_ops.hpp>
 #include <nlohmann/detail/input/pow5_table.hpp>
@@ -45,20 +44,6 @@ NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
 
-// whether an integer of a signed type is negative; the overload for other types
-// avoids comparing an unsigned value with zero
-template<typename NumberType>
-constexpr bool is_negative_integer(NumberType x, std::true_type /*is_signed*/) noexcept
-{
-    return x < 0;
-}
-
-template<typename NumberType>
-constexpr bool is_negative_integer(NumberType /*unused*/, std::false_type /*is_signed*/) noexcept
-{
-    return false;
-}
-
 /*!
 @brief fast integer parser for an already-validated unsigned integer
 
@@ -92,10 +77,8 @@ bool parse_integer_unsigned(const char* first, const char* last, NumberUnsignedT
         x = (x * 10u) + digit;
     }
     value = static_cast<NumberUnsignedType>(x);
-    // reject values that do not round-trip into a narrower NumberUnsignedType,
-    // and values that wrap around to negative numbers if it is a signed type
-    // (number_unsigned_t may be the same type as number_integer_t, #2573)
-    return static_cast<std::uint64_t>(value) == x && !is_negative_integer(value, std::is_signed<NumberUnsignedType>());
+    // reject values that do not round-trip into a narrower NumberUnsignedType
+    return static_cast<std::uint64_t>(value) == x;
 }
 
 /*!

include/nlohmann/detail/meta/type_traits.hpp

@@ -683,22 +683,6 @@ struct is_compatible_integer_type
     : is_compatible_integer_type_impl<RealIntegerType,
       CompatibleNumberIntegerType> {};
 
-// whether to_json() stores an integer of type CompatibleNumberIntegerType as
-// number_integer
-template<typename BasicJsonType, typename CompatibleNumberIntegerType, typename = void>
-struct is_compatible_number_integer_type
-    : is_compatible_integer_type<typename BasicJsonType::number_integer_t, CompatibleNumberIntegerType> {};
-
-// if number_unsigned_t is the same type as number_integer_t, unsigned integers
-// are stored as number_integer as well (#2573)
-template<typename BasicJsonType, typename CompatibleNumberIntegerType>
-struct is_compatible_number_integer_type < BasicJsonType, CompatibleNumberIntegerType,
-           enable_if_t < std::is_same<typename BasicJsonType::number_integer_t, typename BasicJsonType::number_unsigned_t>::value&&
-           std::is_integral<CompatibleNumberIntegerType>::value&&
-           std::is_unsigned<CompatibleNumberIntegerType>::value&&
-           !std::is_same<bool, CompatibleNumberIntegerType>::value >>
-           : std::is_constructible<typename BasicJsonType::number_integer_t, CompatibleNumberIntegerType> {};
-
 template<typename BasicJsonType, typename CompatibleType, typename = void>
 struct is_compatible_type_impl: std::false_type {};
 
@@ -1036,27 +1020,6 @@ constexpr bool value_in_range_of(T val)
 template<bool Value>
 using bool_constant = std::integral_constant<bool, Value>;
 
-// Stands in for number_unsigned_t in overload sets if it is the same type as
-// number_integer_t, so that the unsigned overloads do not collide with the
-// integer ones. Never constructed. Such a basic_json stores every integer as
-// number_integer (#2573).
-struct same_as_number_integer_type {};
-
-// NumberUnsignedType, or same_as_number_integer_type if it is the same type as
-// NumberIntegerType; basic_json uses this one, as it cannot name its own member
-// types through an incomplete basic_json (MSVC reports a recursive alias, and
-// old GCC versions crash)
-template<typename NumberIntegerType, typename NumberUnsignedType>
-using number_unsigned_overload_type = typename std::conditional <
-                                      std::is_same<NumberIntegerType, NumberUnsignedType>::value,
-                                      same_as_number_integer_type,
-                                      NumberUnsignedType >::type;
-
-// number_unsigned_t of BasicJsonType, or same_as_number_integer_type if it is
-// the same type as number_integer_t
-template<typename BasicJsonType>
-using number_unsigned_overload_t = number_unsigned_overload_type<typename BasicJsonType::number_integer_t, typename BasicJsonType::number_unsigned_t>;
-
 ///////////////////////////////////////////////////////////////////////////////
 // is_c_string
 ///////////////////////////////////////////////////////////////////////////////

include/nlohmann/detail/output/binary_writer.hpp

@@ -221,7 +221,7 @@ class binary_writer
 
             case value_t::number_unsigned:
             {
-                write_cbor_head(0x00, conditional_static_cast<std::uint64_t>(j.m_data.m_value.number_unsigned));
+                write_cbor_head(0x00, j.m_data.m_value.number_unsigned);
                 break;
             }
 
@@ -1601,7 +1601,7 @@ class binary_writer
                 return calc_bson_integer_size(j.m_data.m_value.number_integer);
 
             case value_t::number_unsigned:
-                return calc_bson_unsigned_size(conditional_static_cast<std::uint64_t>(j.m_data.m_value.number_unsigned));
+                return calc_bson_unsigned_size(j.m_data.m_value.number_unsigned);
 
             case value_t::string:
                 return calc_bson_string_size(*j.m_data.m_value.string, j);
@@ -1643,7 +1643,7 @@ class binary_writer
                 return write_bson_integer(name, j.m_data.m_value.number_integer);
 
             case value_t::number_unsigned:
-                return write_bson_unsigned(name, conditional_static_cast<std::uint64_t>(j.m_data.m_value.number_unsigned));
+                return write_bson_unsigned(name, j.m_data.m_value.number_unsigned);
 
             case value_t::string:
                 return write_bson_string(name, *j.m_data.m_value.string);

include/nlohmann/detail/output/serializer.hpp

@@ -49,12 +49,6 @@ class serializer
     using number_float_t = typename BasicJsonType::number_float_t;
     using number_integer_t = typename BasicJsonType::number_integer_t;
     using number_unsigned_t = typename BasicJsonType::number_unsigned_t;
-    /// type of the absolute value of an integer: number_unsigned_t, or its
-    /// unsigned counterpart if it is signed (it may be number_integer_t, #2573)
-    using number_magnitude_t = typename std::conditional <
-                               std::is_integral<number_unsigned_t>::value && std::is_signed<number_unsigned_t>::value,
-                               std::make_unsigned<number_unsigned_t>,
-                               std::enable_if<true, number_unsigned_t >>::type::type;
     using binary_char_t = typename BasicJsonType::binary_t::value_type;
 
   public:
@@ -778,6 +772,33 @@ class serializer
                         break;
                     }
                 }
+
+                // A byte below 0x60 is a complete code point and needs no
+                // UTF-8 decoding. The scanner stops only at a control
+                // character, a quote, or a backslash in that range, so look up
+                // its escape sequence and write all 8 bytes of the table entry
+                // straight into the write buffer; only the length stored in the
+                // last byte of the entry is kept.
+                if (data[i] < 0x60)
+                {
+                    // emit any bytes still pending in string_buffer first to
+                    // preserve output order
+                    if (bytes != 0)
+                    {
+                        put_buffer(string_buffer, bytes);
+                        bytes = 0;
+                    }
+                    const auto& escape = ascii_escapes()[data[i]];
+                    if (JSON_HEDLEY_UNLIKELY(write_buffer.size() - write_buffer_pos < escape.size()))
+                    {
+                        flush();
+                    }
+                    std::memcpy(write_buffer.data() + write_buffer_pos, escape.data(), escape.size());
+                    write_buffer_pos += static_cast<std::size_t>(escape[7]);
+                    bytes_after_last_accept = 0;
+                    undumped_chars = 0;
+                    continue;
+                }
             }
 
             const auto byte = static_cast<std::uint8_t>(s[i]);
@@ -786,92 +807,39 @@ class serializer
             {
                 case UTF8_ACCEPT:  // decode found a new code point
                 {
-                    switch (codepoint)
+                    // the code point is at least 0x60 (smaller ones are handled
+                    // above) and is not printable ASCII (which the scanner
+                    // copies): it is DEL (0x7F) under EnsureAscii or a non-ASCII
+                    // character
+                    if (EnsureAscii)
                     {
-                        case 0x08: // backspace
+                        JSON_ASSERT(codepoint >= 0x7F);
+                        if (error_handler == error_handler_t::keep)
                         {
-                            string_buffer[bytes++] = '\\';
-                            string_buffer[bytes++] = 'b';
-                            break;
+                            // this character was buffered as raw bytes below
+                            // in case it turned out to be part of an
+                            // ill-formed sequence (which is kept as is); now
+                            // that it decoded to a well-formed code point,
+                            // undo that and \u-escape it like any other
+                            // character instead
+                            bytes = bytes_after_last_accept;
                         }
-
-                        case 0x09: // horizontal tab
+                        if (codepoint <= 0xFFFF)
                         {
-                            string_buffer[bytes++] = '\\';
-                            string_buffer[bytes++] = 't';
-                            break;
+                            write_u_escape(bytes, static_cast<std::uint16_t>(codepoint));
                         }
-
-                        case 0x0A: // newline
+                        else
                         {
-                            string_buffer[bytes++] = '\\';
-                            string_buffer[bytes++] = 'n';
-                            break;
-                        }
-
-                        case 0x0C: // formfeed
-                        {
-                            string_buffer[bytes++] = '\\';
-                            string_buffer[bytes++] = 'f';
-                            break;
-                        }
-
-                        case 0x0D: // carriage return
-                        {
-                            string_buffer[bytes++] = '\\';
-                            string_buffer[bytes++] = 'r';
-                            break;
-                        }
-
-                        case 0x22: // quotation mark
-                        {
-                            string_buffer[bytes++] = '\\';
-                            string_buffer[bytes++] = '"';
-                            break;
-                        }
-
-                        case 0x5C: // reverse solidus
-                        {
-                            string_buffer[bytes++] = '\\';
-                            string_buffer[bytes++] = '\\';
-                            break;
-                        }
-
-                        default:
-                        {
-                            // escape control characters (0x00..0x1F) or, if
-                            // EnsureAscii parameter is used, non-ASCII characters
-                            if ((codepoint <= 0x1F) || (EnsureAscii && (codepoint >= 0x7F)))
-                            {
-                                if (EnsureAscii && error_handler == error_handler_t::keep)
-                                {
-                                    // this character was buffered as raw bytes
-                                    // below in case it turned out to be part of
-                                    // an ill-formed sequence (which is kept as
-                                    // is); now that it decoded to a well-formed
-                                    // code point, undo that and \u-escape it
-                                    // like any other character instead
-                                    bytes = bytes_after_last_accept;
-                                }
-                                if (codepoint <= 0xFFFF)
-                                {
-                                    write_u_escape(bytes, static_cast<std::uint16_t>(codepoint));
-                                }
-                                else
-                                {
-                                    write_u_escape(bytes, static_cast<std::uint16_t>(0xD7C0u + (codepoint >> 10u)));
-                                    write_u_escape(bytes, static_cast<std::uint16_t>(0xDC00u + (codepoint & 0x3FFu)));
-                                }
-                            }
-                            else
-                            {
-                                // copy byte to buffer (all previous bytes
-                                // been copied have in default case above)
-                                string_buffer[bytes++] = s[i];
-                            }
-                            break;
+                            write_u_escape(bytes, static_cast<std::uint16_t>(0xD7C0u + (codepoint >> 10u)));
+                            write_u_escape(bytes, static_cast<std::uint16_t>(0xDC00u + (codepoint & 0x3FFu)));
                         }
                     }
+                    else
+                    {
+                        // copy byte to buffer (all previous bytes of the
+                        // sequence have been copied in the default case below)
+                        string_buffer[bytes++] = s[i];
+                    }
 
                     // write buffer and reset index; there must be 13 bytes
                     // left, as this is the maximal number of bytes to be
@@ -1233,7 +1201,7 @@ class serializer
     @param[in] x  unsigned integer number to count its digits
     @return    number of decimal digits
     */
-    unsigned int count_digits(number_magnitude_t x) noexcept
+    unsigned int count_digits(number_unsigned_t x) noexcept
     {
         unsigned int n_digits = 1;
         for (;;)
@@ -1259,6 +1227,121 @@ class serializer
         }
     }
 
+    /*!
+    @brief the JSON form of each character below 0x60
+
+    Entry c holds the escape sequence of character c - the short form for
+    \\b, \\t, \\n, \\f, \\r, \\" and \\\\, \\u00xx for the other control characters -
+    or c itself if it needs no escaping, padded with zeros to 7 bytes. The last
+    byte is the length of the sequence. The 8 bytes of an entry are copied at
+    once, so writing an escape needs neither a branch nor a byte-wise loop.
+    */
+    static const std::array<std::array<char, 8>, 0x60>& ascii_escapes() noexcept
+    {
+        static const std::array<std::array<char, 8>, 0x60> escapes =
+        {
+            {
+                {{'\\', 'u', '0', '0', '0', '0', 0, 6}}, // 0x00
+                {{'\\', 'u', '0', '0', '0', '1', 0, 6}}, // 0x01
+                {{'\\', 'u', '0', '0', '0', '2', 0, 6}}, // 0x02
+                {{'\\', 'u', '0', '0', '0', '3', 0, 6}}, // 0x03
+                {{'\\', 'u', '0', '0', '0', '4', 0, 6}}, // 0x04
+                {{'\\', 'u', '0', '0', '0', '5', 0, 6}}, // 0x05
+                {{'\\', 'u', '0', '0', '0', '6', 0, 6}}, // 0x06
+                {{'\\', 'u', '0', '0', '0', '7', 0, 6}}, // 0x07
+                {{'\\', 'b', 0, 0, 0, 0, 0, 2}}, // 0x08
+                {{'\\', 't', 0, 0, 0, 0, 0, 2}}, // 0x09
+                {{'\\', 'n', 0, 0, 0, 0, 0, 2}}, // 0x0A
+                {{'\\', 'u', '0', '0', '0', 'b', 0, 6}}, // 0x0B
+                {{'\\', 'f', 0, 0, 0, 0, 0, 2}}, // 0x0C
+                {{'\\', 'r', 0, 0, 0, 0, 0, 2}}, // 0x0D
+                {{'\\', 'u', '0', '0', '0', 'e', 0, 6}}, // 0x0E
+                {{'\\', 'u', '0', '0', '0', 'f', 0, 6}}, // 0x0F
+                {{'\\', 'u', '0', '0', '1', '0', 0, 6}}, // 0x10
+                {{'\\', 'u', '0', '0', '1', '1', 0, 6}}, // 0x11
+                {{'\\', 'u', '0', '0', '1', '2', 0, 6}}, // 0x12
+                {{'\\', 'u', '0', '0', '1', '3', 0, 6}}, // 0x13
+                {{'\\', 'u', '0', '0', '1', '4', 0, 6}}, // 0x14
+                {{'\\', 'u', '0', '0', '1', '5', 0, 6}}, // 0x15
+                {{'\\', 'u', '0', '0', '1', '6', 0, 6}}, // 0x16
+                {{'\\', 'u', '0', '0', '1', '7', 0, 6}}, // 0x17
+                {{'\\', 'u', '0', '0', '1', '8', 0, 6}}, // 0x18
+                {{'\\', 'u', '0', '0', '1', '9', 0, 6}}, // 0x19
+                {{'\\', 'u', '0', '0', '1', 'a', 0, 6}}, // 0x1A
+                {{'\\', 'u', '0', '0', '1', 'b', 0, 6}}, // 0x1B
+                {{'\\', 'u', '0', '0', '1', 'c', 0, 6}}, // 0x1C
+                {{'\\', 'u', '0', '0', '1', 'd', 0, 6}}, // 0x1D
+                {{'\\', 'u', '0', '0', '1', 'e', 0, 6}}, // 0x1E
+                {{'\\', 'u', '0', '0', '1', 'f', 0, 6}}, // 0x1F
+                {{' ', 0, 0, 0, 0, 0, 0, 1}}, // 0x20
+                {{'!', 0, 0, 0, 0, 0, 0, 1}}, // 0x21
+                {{'\\', '"', 0, 0, 0, 0, 0, 2}}, // 0x22
+                {{'#', 0, 0, 0, 0, 0, 0, 1}}, // 0x23
+                {{'$', 0, 0, 0, 0, 0, 0, 1}}, // 0x24
+                {{'%', 0, 0, 0, 0, 0, 0, 1}}, // 0x25
+                {{'&', 0, 0, 0, 0, 0, 0, 1}}, // 0x26
+                {{'\'', 0, 0, 0, 0, 0, 0, 1}}, // 0x27
+                {{'(', 0, 0, 0, 0, 0, 0, 1}}, // 0x28
+                {{')', 0, 0, 0, 0, 0, 0, 1}}, // 0x29
+                {{'*', 0, 0, 0, 0, 0, 0, 1}}, // 0x2A
+                {{'+', 0, 0, 0, 0, 0, 0, 1}}, // 0x2B
+                {{',', 0, 0, 0, 0, 0, 0, 1}}, // 0x2C
+                {{'-', 0, 0, 0, 0, 0, 0, 1}}, // 0x2D
+                {{'.', 0, 0, 0, 0, 0, 0, 1}}, // 0x2E
+                {{'/', 0, 0, 0, 0, 0, 0, 1}}, // 0x2F
+                {{'0', 0, 0, 0, 0, 0, 0, 1}}, // 0x30
+                {{'1', 0, 0, 0, 0, 0, 0, 1}}, // 0x31
+                {{'2', 0, 0, 0, 0, 0, 0, 1}}, // 0x32
+                {{'3', 0, 0, 0, 0, 0, 0, 1}}, // 0x33
+                {{'4', 0, 0, 0, 0, 0, 0, 1}}, // 0x34
+                {{'5', 0, 0, 0, 0, 0, 0, 1}}, // 0x35
+                {{'6', 0, 0, 0, 0, 0, 0, 1}}, // 0x36
+                {{'7', 0, 0, 0, 0, 0, 0, 1}}, // 0x37
+                {{'8', 0, 0, 0, 0, 0, 0, 1}}, // 0x38
+                {{'9', 0, 0, 0, 0, 0, 0, 1}}, // 0x39
+                {{':', 0, 0, 0, 0, 0, 0, 1}}, // 0x3A
+                {{';', 0, 0, 0, 0, 0, 0, 1}}, // 0x3B
+                {{'<', 0, 0, 0, 0, 0, 0, 1}}, // 0x3C
+                {{'=', 0, 0, 0, 0, 0, 0, 1}}, // 0x3D
+                {{'>', 0, 0, 0, 0, 0, 0, 1}}, // 0x3E
+                {{'?', 0, 0, 0, 0, 0, 0, 1}}, // 0x3F
+                {{'@', 0, 0, 0, 0, 0, 0, 1}}, // 0x40
+                {{'A', 0, 0, 0, 0, 0, 0, 1}}, // 0x41
+                {{'B', 0, 0, 0, 0, 0, 0, 1}}, // 0x42
+                {{'C', 0, 0, 0, 0, 0, 0, 1}}, // 0x43
+                {{'D', 0, 0, 0, 0, 0, 0, 1}}, // 0x44
+                {{'E', 0, 0, 0, 0, 0, 0, 1}}, // 0x45
+                {{'F', 0, 0, 0, 0, 0, 0, 1}}, // 0x46
+                {{'G', 0, 0, 0, 0, 0, 0, 1}}, // 0x47
+                {{'H', 0, 0, 0, 0, 0, 0, 1}}, // 0x48
+                {{'I', 0, 0, 0, 0, 0, 0, 1}}, // 0x49
+                {{'J', 0, 0, 0, 0, 0, 0, 1}}, // 0x4A
+                {{'K', 0, 0, 0, 0, 0, 0, 1}}, // 0x4B
+                {{'L', 0, 0, 0, 0, 0, 0, 1}}, // 0x4C
+                {{'M', 0, 0, 0, 0, 0, 0, 1}}, // 0x4D
+                {{'N', 0, 0, 0, 0, 0, 0, 1}}, // 0x4E
+                {{'O', 0, 0, 0, 0, 0, 0, 1}}, // 0x4F
+                {{'P', 0, 0, 0, 0, 0, 0, 1}}, // 0x50
+                {{'Q', 0, 0, 0, 0, 0, 0, 1}}, // 0x51
+                {{'R', 0, 0, 0, 0, 0, 0, 1}}, // 0x52
+                {{'S', 0, 0, 0, 0, 0, 0, 1}}, // 0x53
+                {{'T', 0, 0, 0, 0, 0, 0, 1}}, // 0x54
+                {{'U', 0, 0, 0, 0, 0, 0, 1}}, // 0x55
+                {{'V', 0, 0, 0, 0, 0, 0, 1}}, // 0x56
+                {{'W', 0, 0, 0, 0, 0, 0, 1}}, // 0x57
+                {{'X', 0, 0, 0, 0, 0, 0, 1}}, // 0x58
+                {{'Y', 0, 0, 0, 0, 0, 0, 1}}, // 0x59
+                {{'Z', 0, 0, 0, 0, 0, 0, 1}}, // 0x5A
+                {{'[', 0, 0, 0, 0, 0, 0, 1}}, // 0x5B
+                {{'\\', '\\', 0, 0, 0, 0, 0, 2}}, // 0x5C
+                {{']', 0, 0, 0, 0, 0, 0, 1}}, // 0x5D
+                {{'^', 0, 0, 0, 0, 0, 0, 1}}, // 0x5E
+                {{'_', 0, 0, 0, 0, 0, 0, 1}}, // 0x5F
+            }
+        };
+        return escapes;
+    }
+
     /*!
      * @brief write a lowercase "\\uXXXX" escape sequence into @a string_buffer
      *
@@ -1411,7 +1494,7 @@ class serializer
         // use a pointer to fill the buffer
         auto buffer_ptr = number_buffer.begin(); // NOLINT(llvm-qualified-auto,readability-qualified-auto)
 
-        number_magnitude_t abs_value;
+        number_unsigned_t abs_value;
 
         unsigned int n_chars{};
 
@@ -1425,7 +1508,7 @@ class serializer
         }
         else
         {
-            abs_value = static_cast<number_magnitude_t>(x);
+            abs_value = static_cast<number_unsigned_t>(x);
             n_chars = count_digits(abs_value);
         }
 
@@ -1568,6 +1651,17 @@ class serializer
         }
     }
 
+    /*
+     * Overload to make the compiler happy while it is instantiating
+     * dump_integer for number_unsigned_t.
+     * Must never be called.
+     */
+    number_unsigned_t remove_sign(number_unsigned_t x)
+    {
+        JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
+        return x; // LCOV_EXCL_LINE
+    }
+
     /*
      * Helper function for dump_integer
      *
@@ -1577,10 +1671,10 @@ class serializer
      * absolute values of INT_MIN and INT_MAX are usually not the same. See
      * #1708 for details.
      */
-    number_magnitude_t remove_sign(number_integer_t x) noexcept
+    number_unsigned_t remove_sign(number_integer_t x) noexcept
     {
         JSON_ASSERT(x < 0);
-        return static_cast<number_magnitude_t>(-(x + 1)) + 1;
+        return static_cast<number_unsigned_t>(-(x + 1)) + 1;
     }
 
   private:

include/nlohmann/json.hpp

@@ -500,22 +500,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                   "NumberUnsignedType must be at least as wide as NumberIntegerType, "
                   "because it has to hold the absolute value of every NumberIntegerType value");
 
-    // NumberUnsignedType may be the same type as NumberIntegerType (#2573), but
-    // a repeated NumberFloatType would give overloads that collide, so it gets
-    // a readable diagnostic instead.
-    static_assert(!std::is_same<NumberFloatType, NumberIntegerType>::value && !std::is_same<NumberFloatType, NumberUnsignedType>::value,
-                  "NumberFloatType must be a different type than NumberIntegerType and NumberUnsignedType");
-
   private:
 
-    /// the kind a value of kind @a t is stored as: if number_unsigned_t is the
-    /// same type as number_integer_t, every integer is stored as number_integer
-    /// (#2573); every other kind is stored as itself
-    static constexpr value_t stored_kind(const value_t t) noexcept
-    {
-        return (std::is_same<number_integer_t, number_unsigned_t>::value && t == value_t::number_unsigned) ? value_t::number_integer : t;
-    }
-
     /// helper for exception-safe object creation
     template<typename T, typename... Args>
     JSON_HEDLEY_RETURNS_NON_NULL
@@ -589,8 +575,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         json_value(boolean_t v) noexcept : boolean(v) {}
         /// constructor for numbers (integer)
         json_value(number_integer_t v) noexcept : number_integer(v) {}
-        /// constructor for numbers (unsigned); takes a placeholder if number_unsigned_t is number_integer_t
-        json_value(detail::number_unsigned_overload_type<NumberIntegerType, NumberUnsignedType> v) noexcept : number_unsigned(v) {}
+        /// constructor for numbers (unsigned)
+        json_value(number_unsigned_t v) noexcept : number_unsigned(v) {}
         /// constructor for numbers (floating-point)
         json_value(number_float_t v) noexcept : number_float(v) {}
         /// constructor for empty values of a given type
@@ -2195,7 +2181,7 @@ public:
     /// @brief create an empty value with a given type
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
     basic_json(const value_t v)
-        : m_data(stored_kind(v))
+        : m_data(v)
     {
         assert_invariant();
     }
@@ -2276,7 +2262,7 @@ public:
         {
             convert_leaf(val);
         }
-        JSON_ASSERT(m_data.m_type == stored_kind(val.type()));
+        JSON_ASSERT(m_data.m_type == val.type());
 
         set_parents();
         assert_invariant();
@@ -2861,16 +2847,14 @@ public:
         return m_data.m_type == value_t::number_integer ? &m_data.m_value.number_integer : nullptr;
     }
 
-    /// get a pointer to the value (unsigned number); takes a placeholder if
-    /// number_unsigned_t is number_integer_t (the integer overload handles it then)
-    number_unsigned_t* get_impl_ptr(detail::number_unsigned_overload_type<NumberIntegerType, NumberUnsignedType>* /*unused*/) noexcept
+    /// get a pointer to the value (unsigned number)
+    number_unsigned_t* get_impl_ptr(number_unsigned_t* /*unused*/) noexcept
     {
         return is_number_unsigned() ? &m_data.m_value.number_unsigned : nullptr;
     }
 
-    /// get a pointer to the value (unsigned number); takes a placeholder if
-    /// number_unsigned_t is number_integer_t (the integer overload handles it then)
-    constexpr const number_unsigned_t* get_impl_ptr(const detail::number_unsigned_overload_type<NumberIntegerType, NumberUnsignedType>* /*unused*/) const noexcept
+    /// get a pointer to the value (unsigned number)
+    constexpr const number_unsigned_t* get_impl_ptr(const number_unsigned_t* /*unused*/) const noexcept
     {
         return is_number_unsigned() ? &m_data.m_value.number_unsigned : nullptr;
     }

single_include/nlohmann/json.hpp

@@ -4687,22 +4687,6 @@ struct is_compatible_integer_type
     : is_compatible_integer_type_impl<RealIntegerType,
       CompatibleNumberIntegerType> {};
 
-// whether to_json() stores an integer of type CompatibleNumberIntegerType as
-// number_integer
-template<typename BasicJsonType, typename CompatibleNumberIntegerType, typename = void>
-struct is_compatible_number_integer_type
-    : is_compatible_integer_type<typename BasicJsonType::number_integer_t, CompatibleNumberIntegerType> {};
-
-// if number_unsigned_t is the same type as number_integer_t, unsigned integers
-// are stored as number_integer as well (#2573)
-template<typename BasicJsonType, typename CompatibleNumberIntegerType>
-struct is_compatible_number_integer_type < BasicJsonType, CompatibleNumberIntegerType,
-           enable_if_t < std::is_same<typename BasicJsonType::number_integer_t, typename BasicJsonType::number_unsigned_t>::value&&
-           std::is_integral<CompatibleNumberIntegerType>::value&&
-           std::is_unsigned<CompatibleNumberIntegerType>::value&&
-           !std::is_same<bool, CompatibleNumberIntegerType>::value >>
-           : std::is_constructible<typename BasicJsonType::number_integer_t, CompatibleNumberIntegerType> {};
-
 template<typename BasicJsonType, typename CompatibleType, typename = void>
 struct is_compatible_type_impl: std::false_type {};
 
@@ -5040,27 +5024,6 @@ constexpr bool value_in_range_of(T val)
 template<bool Value>
 using bool_constant = std::integral_constant<bool, Value>;
 
-// Stands in for number_unsigned_t in overload sets if it is the same type as
-// number_integer_t, so that the unsigned overloads do not collide with the
-// integer ones. Never constructed. Such a basic_json stores every integer as
-// number_integer (#2573).
-struct same_as_number_integer_type {};
-
-// NumberUnsignedType, or same_as_number_integer_type if it is the same type as
-// NumberIntegerType; basic_json uses this one, as it cannot name its own member
-// types through an incomplete basic_json (MSVC reports a recursive alias, and
-// old GCC versions crash)
-template<typename NumberIntegerType, typename NumberUnsignedType>
-using number_unsigned_overload_type = typename std::conditional <
-                                      std::is_same<NumberIntegerType, NumberUnsignedType>::value,
-                                      same_as_number_integer_type,
-                                      NumberUnsignedType >::type;
-
-// number_unsigned_t of BasicJsonType, or same_as_number_integer_type if it is
-// the same type as number_integer_t
-template<typename BasicJsonType>
-using number_unsigned_overload_t = number_unsigned_overload_type<typename BasicJsonType::number_integer_t, typename BasicJsonType::number_unsigned_t>;
-
 ///////////////////////////////////////////////////////////////////////////////
 // is_c_string
 ///////////////////////////////////////////////////////////////////////////////
@@ -5813,11 +5776,7 @@ void get_arithmetic_value(const BasicJsonType& j, ArithmeticType& val)
     {
         case value_t::number_unsigned:
         {
-            // never null: a basic_json whose number_unsigned_t is number_integer_t
-            // stores no number_unsigned values (#2573)
-            const auto* ptr = j.template get_ptr<const typename BasicJsonType::number_unsigned_t*>();
-            JSON_ASSERT(ptr != nullptr);
-            val = static_cast<ArithmeticType>(*ptr);
+            val = static_cast<ArithmeticType>(*j.template get_ptr<const typename BasicJsonType::number_unsigned_t*>());
             break;
         }
         case value_t::number_integer:
@@ -5887,7 +5846,7 @@ inline void from_json(const BasicJsonType& j, typename BasicJsonType::number_flo
 }
 
 template<typename BasicJsonType>
-inline void from_json(const BasicJsonType& j, number_unsigned_overload_t<BasicJsonType>& val)
+inline void from_json(const BasicJsonType& j, typename BasicJsonType::number_unsigned_t& val)
 {
     get_arithmetic_value(j, val);
 }
@@ -6143,11 +6102,7 @@ inline void from_json(const BasicJsonType& j, ArithmeticType& val)
     {
         case value_t::number_unsigned:
         {
-            // never null: a basic_json whose number_unsigned_t is number_integer_t
-            // stores no number_unsigned values (#2573)
-            const auto* ptr = j.template get_ptr<const typename BasicJsonType::number_unsigned_t*>();
-            JSON_ASSERT(ptr != nullptr);
-            val = static_cast<ArithmeticType>(*ptr);
+            val = static_cast<ArithmeticType>(*j.template get_ptr<const typename BasicJsonType::number_unsigned_t*>());
             break;
         }
         case value_t::number_integer:
@@ -7295,14 +7250,14 @@ inline void to_json(BasicJsonType& j, FloatType val) noexcept
 }
 
 template<typename BasicJsonType, typename CompatibleNumberUnsignedType,
-         enable_if_t<is_compatible_integer_type<number_unsigned_overload_t<BasicJsonType>, CompatibleNumberUnsignedType>::value, int> = 0>
+         enable_if_t<is_compatible_integer_type<typename BasicJsonType::number_unsigned_t, CompatibleNumberUnsignedType>::value, int> = 0>
 inline void to_json(BasicJsonType& j, CompatibleNumberUnsignedType val) noexcept
 {
     external_constructor<value_t::number_unsigned>::construct(j, static_cast<typename BasicJsonType::number_unsigned_t>(val));
 }
 
 template<typename BasicJsonType, typename CompatibleNumberIntegerType,
-         enable_if_t<is_compatible_number_integer_type<BasicJsonType, CompatibleNumberIntegerType>::value, int> = 0>
+         enable_if_t<is_compatible_integer_type<typename BasicJsonType::number_integer_t, CompatibleNumberIntegerType>::value, int> = 0>
 inline void to_json(BasicJsonType& j, CompatibleNumberIntegerType val) noexcept
 {
     external_constructor<value_t::number_integer>::construct(j, static_cast<typename BasicJsonType::number_integer_t>(val));
@@ -9110,7 +9065,6 @@ NLOHMANN_JSON_NAMESPACE_END
 #include <cstring> // memcpy
 #include <limits> // numeric_limits
 #include <string> // string
-#include <type_traits> // is_signed, true_type, false_type
 
 // #include <nlohmann/detail/bit_ops.hpp>
 
@@ -9512,20 +9466,6 @@ NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
 {
 
-// whether an integer of a signed type is negative; the overload for other types
-// avoids comparing an unsigned value with zero
-template<typename NumberType>
-constexpr bool is_negative_integer(NumberType x, std::true_type /*is_signed*/) noexcept
-{
-    return x < 0;
-}
-
-template<typename NumberType>
-constexpr bool is_negative_integer(NumberType /*unused*/, std::false_type /*is_signed*/) noexcept
-{
-    return false;
-}
-
 /*!
 @brief fast integer parser for an already-validated unsigned integer
 
@@ -9559,10 +9499,8 @@ bool parse_integer_unsigned(const char* first, const char* last, NumberUnsignedT
         x = (x * 10u) + digit;
     }
     value = static_cast<NumberUnsignedType>(x);
-    // reject values that do not round-trip into a narrower NumberUnsignedType,
-    // and values that wrap around to negative numbers if it is a signed type
-    // (number_unsigned_t may be the same type as number_integer_t, #2573)
-    return static_cast<std::uint64_t>(value) == x && !is_negative_integer(value, std::is_signed<NumberUnsignedType>());
+    // reject values that do not round-trip into a narrower NumberUnsignedType
+    return static_cast<std::uint64_t>(value) == x;
 }
 
 /*!
@@ -22184,7 +22122,7 @@ class binary_writer
 
             case value_t::number_unsigned:
             {
-                write_cbor_head(0x00, conditional_static_cast<std::uint64_t>(j.m_data.m_value.number_unsigned));
+                write_cbor_head(0x00, j.m_data.m_value.number_unsigned);
                 break;
             }
 
@@ -23564,7 +23502,7 @@ class binary_writer
                 return calc_bson_integer_size(j.m_data.m_value.number_integer);
 
             case value_t::number_unsigned:
-                return calc_bson_unsigned_size(conditional_static_cast<std::uint64_t>(j.m_data.m_value.number_unsigned));
+                return calc_bson_unsigned_size(j.m_data.m_value.number_unsigned);
 
             case value_t::string:
                 return calc_bson_string_size(*j.m_data.m_value.string, j);
@@ -23606,7 +23544,7 @@ class binary_writer
                 return write_bson_integer(name, j.m_data.m_value.number_integer);
 
             case value_t::number_unsigned:
-                return write_bson_unsigned(name, conditional_static_cast<std::uint64_t>(j.m_data.m_value.number_unsigned));
+                return write_bson_unsigned(name, j.m_data.m_value.number_unsigned);
 
             case value_t::string:
                 return write_bson_string(name, *j.m_data.m_value.string);
@@ -26426,12 +26364,6 @@ class serializer
     using number_float_t = typename BasicJsonType::number_float_t;
     using number_integer_t = typename BasicJsonType::number_integer_t;
     using number_unsigned_t = typename BasicJsonType::number_unsigned_t;
-    /// type of the absolute value of an integer: number_unsigned_t, or its
-    /// unsigned counterpart if it is signed (it may be number_integer_t, #2573)
-    using number_magnitude_t = typename std::conditional <
-                               std::is_integral<number_unsigned_t>::value && std::is_signed<number_unsigned_t>::value,
-                               std::make_unsigned<number_unsigned_t>,
-                               std::enable_if<true, number_unsigned_t >>::type::type;
     using binary_char_t = typename BasicJsonType::binary_t::value_type;
 
   public:
@@ -27155,6 +27087,33 @@ class serializer
                         break;
                     }
                 }
+
+                // A byte below 0x60 is a complete code point and needs no
+                // UTF-8 decoding. The scanner stops only at a control
+                // character, a quote, or a backslash in that range, so look up
+                // its escape sequence and write all 8 bytes of the table entry
+                // straight into the write buffer; only the length stored in the
+                // last byte of the entry is kept.
+                if (data[i] < 0x60)
+                {
+                    // emit any bytes still pending in string_buffer first to
+                    // preserve output order
+                    if (bytes != 0)
+                    {
+                        put_buffer(string_buffer, bytes);
+                        bytes = 0;
+                    }
+                    const auto& escape = ascii_escapes()[data[i]];
+                    if (JSON_HEDLEY_UNLIKELY(write_buffer.size() - write_buffer_pos < escape.size()))
+                    {
+                        flush();
+                    }
+                    std::memcpy(write_buffer.data() + write_buffer_pos, escape.data(), escape.size());
+                    write_buffer_pos += static_cast<std::size_t>(escape[7]);
+                    bytes_after_last_accept = 0;
+                    undumped_chars = 0;
+                    continue;
+                }
             }
 
             const auto byte = static_cast<std::uint8_t>(s[i]);
@@ -27163,92 +27122,39 @@ class serializer
             {
                 case UTF8_ACCEPT:  // decode found a new code point
                 {
-                    switch (codepoint)
+                    // the code point is at least 0x60 (smaller ones are handled
+                    // above) and is not printable ASCII (which the scanner
+                    // copies): it is DEL (0x7F) under EnsureAscii or a non-ASCII
+                    // character
+                    if (EnsureAscii)
                     {
-                        case 0x08: // backspace
+                        JSON_ASSERT(codepoint >= 0x7F);
+                        if (error_handler == error_handler_t::keep)
                         {
-                            string_buffer[bytes++] = '\\';
-                            string_buffer[bytes++] = 'b';
-                            break;
+                            // this character was buffered as raw bytes below
+                            // in case it turned out to be part of an
+                            // ill-formed sequence (which is kept as is); now
+                            // that it decoded to a well-formed code point,
+                            // undo that and \u-escape it like any other
+                            // character instead
+                            bytes = bytes_after_last_accept;
                         }
-
-                        case 0x09: // horizontal tab
+                        if (codepoint <= 0xFFFF)
                         {
-                            string_buffer[bytes++] = '\\';
-                            string_buffer[bytes++] = 't';
-                            break;
+                            write_u_escape(bytes, static_cast<std::uint16_t>(codepoint));
                         }
-
-                        case 0x0A: // newline
+                        else
                         {
-                            string_buffer[bytes++] = '\\';
-                            string_buffer[bytes++] = 'n';
-                            break;
-                        }
-
-                        case 0x0C: // formfeed
-                        {
-                            string_buffer[bytes++] = '\\';
-                            string_buffer[bytes++] = 'f';
-                            break;
-                        }
-
-                        case 0x0D: // carriage return
-                        {
-                            string_buffer[bytes++] = '\\';
-                            string_buffer[bytes++] = 'r';
-                            break;
-                        }
-
-                        case 0x22: // quotation mark
-                        {
-                            string_buffer[bytes++] = '\\';
-                            string_buffer[bytes++] = '"';
-                            break;
-                        }
-
-                        case 0x5C: // reverse solidus
-                        {
-                            string_buffer[bytes++] = '\\';
-                            string_buffer[bytes++] = '\\';
-                            break;
-                        }
-
-                        default:
-                        {
-                            // escape control characters (0x00..0x1F) or, if
-                            // EnsureAscii parameter is used, non-ASCII characters
-                            if ((codepoint <= 0x1F) || (EnsureAscii && (codepoint >= 0x7F)))
-                            {
-                                if (EnsureAscii && error_handler == error_handler_t::keep)
-                                {
-                                    // this character was buffered as raw bytes
-                                    // below in case it turned out to be part of
-                                    // an ill-formed sequence (which is kept as
-                                    // is); now that it decoded to a well-formed
-                                    // code point, undo that and \u-escape it
-                                    // like any other character instead
-                                    bytes = bytes_after_last_accept;
-                                }
-                                if (codepoint <= 0xFFFF)
-                                {
-                                    write_u_escape(bytes, static_cast<std::uint16_t>(codepoint));
-                                }
-                                else
-                                {
-                                    write_u_escape(bytes, static_cast<std::uint16_t>(0xD7C0u + (codepoint >> 10u)));
-                                    write_u_escape(bytes, static_cast<std::uint16_t>(0xDC00u + (codepoint & 0x3FFu)));
-                                }
-                            }
-                            else
-                            {
-                                // copy byte to buffer (all previous bytes
-                                // been copied have in default case above)
-                                string_buffer[bytes++] = s[i];
-                            }
-                            break;
+                            write_u_escape(bytes, static_cast<std::uint16_t>(0xD7C0u + (codepoint >> 10u)));
+                            write_u_escape(bytes, static_cast<std::uint16_t>(0xDC00u + (codepoint & 0x3FFu)));
                         }
                     }
+                    else
+                    {
+                        // copy byte to buffer (all previous bytes of the
+                        // sequence have been copied in the default case below)
+                        string_buffer[bytes++] = s[i];
+                    }
 
                     // write buffer and reset index; there must be 13 bytes
                     // left, as this is the maximal number of bytes to be
@@ -27610,7 +27516,7 @@ class serializer
     @param[in] x  unsigned integer number to count its digits
     @return    number of decimal digits
     */
-    unsigned int count_digits(number_magnitude_t x) noexcept
+    unsigned int count_digits(number_unsigned_t x) noexcept
     {
         unsigned int n_digits = 1;
         for (;;)
@@ -27636,6 +27542,121 @@ class serializer
         }
     }
 
+    /*!
+    @brief the JSON form of each character below 0x60
+
+    Entry c holds the escape sequence of character c - the short form for
+    \\b, \\t, \\n, \\f, \\r, \\" and \\\\, \\u00xx for the other control characters -
+    or c itself if it needs no escaping, padded with zeros to 7 bytes. The last
+    byte is the length of the sequence. The 8 bytes of an entry are copied at
+    once, so writing an escape needs neither a branch nor a byte-wise loop.
+    */
+    static const std::array<std::array<char, 8>, 0x60>& ascii_escapes() noexcept
+    {
+        static const std::array<std::array<char, 8>, 0x60> escapes =
+        {
+            {
+                {{'\\', 'u', '0', '0', '0', '0', 0, 6}}, // 0x00
+                {{'\\', 'u', '0', '0', '0', '1', 0, 6}}, // 0x01
+                {{'\\', 'u', '0', '0', '0', '2', 0, 6}}, // 0x02
+                {{'\\', 'u', '0', '0', '0', '3', 0, 6}}, // 0x03
+                {{'\\', 'u', '0', '0', '0', '4', 0, 6}}, // 0x04
+                {{'\\', 'u', '0', '0', '0', '5', 0, 6}}, // 0x05
+                {{'\\', 'u', '0', '0', '0', '6', 0, 6}}, // 0x06
+                {{'\\', 'u', '0', '0', '0', '7', 0, 6}}, // 0x07
+                {{'\\', 'b', 0, 0, 0, 0, 0, 2}}, // 0x08
+                {{'\\', 't', 0, 0, 0, 0, 0, 2}}, // 0x09
+                {{'\\', 'n', 0, 0, 0, 0, 0, 2}}, // 0x0A
+                {{'\\', 'u', '0', '0', '0', 'b', 0, 6}}, // 0x0B
+                {{'\\', 'f', 0, 0, 0, 0, 0, 2}}, // 0x0C
+                {{'\\', 'r', 0, 0, 0, 0, 0, 2}}, // 0x0D
+                {{'\\', 'u', '0', '0', '0', 'e', 0, 6}}, // 0x0E
+                {{'\\', 'u', '0', '0', '0', 'f', 0, 6}}, // 0x0F
+                {{'\\', 'u', '0', '0', '1', '0', 0, 6}}, // 0x10
+                {{'\\', 'u', '0', '0', '1', '1', 0, 6}}, // 0x11
+                {{'\\', 'u', '0', '0', '1', '2', 0, 6}}, // 0x12
+                {{'\\', 'u', '0', '0', '1', '3', 0, 6}}, // 0x13
+                {{'\\', 'u', '0', '0', '1', '4', 0, 6}}, // 0x14
+                {{'\\', 'u', '0', '0', '1', '5', 0, 6}}, // 0x15
+                {{'\\', 'u', '0', '0', '1', '6', 0, 6}}, // 0x16
+                {{'\\', 'u', '0', '0', '1', '7', 0, 6}}, // 0x17
+                {{'\\', 'u', '0', '0', '1', '8', 0, 6}}, // 0x18
+                {{'\\', 'u', '0', '0', '1', '9', 0, 6}}, // 0x19
+                {{'\\', 'u', '0', '0', '1', 'a', 0, 6}}, // 0x1A
+                {{'\\', 'u', '0', '0', '1', 'b', 0, 6}}, // 0x1B
+                {{'\\', 'u', '0', '0', '1', 'c', 0, 6}}, // 0x1C
+                {{'\\', 'u', '0', '0', '1', 'd', 0, 6}}, // 0x1D
+                {{'\\', 'u', '0', '0', '1', 'e', 0, 6}}, // 0x1E
+                {{'\\', 'u', '0', '0', '1', 'f', 0, 6}}, // 0x1F
+                {{' ', 0, 0, 0, 0, 0, 0, 1}}, // 0x20
+                {{'!', 0, 0, 0, 0, 0, 0, 1}}, // 0x21
+                {{'\\', '"', 0, 0, 0, 0, 0, 2}}, // 0x22
+                {{'#', 0, 0, 0, 0, 0, 0, 1}}, // 0x23
+                {{'$', 0, 0, 0, 0, 0, 0, 1}}, // 0x24
+                {{'%', 0, 0, 0, 0, 0, 0, 1}}, // 0x25
+                {{'&', 0, 0, 0, 0, 0, 0, 1}}, // 0x26
+                {{'\'', 0, 0, 0, 0, 0, 0, 1}}, // 0x27
+                {{'(', 0, 0, 0, 0, 0, 0, 1}}, // 0x28
+                {{')', 0, 0, 0, 0, 0, 0, 1}}, // 0x29
+                {{'*', 0, 0, 0, 0, 0, 0, 1}}, // 0x2A
+                {{'+', 0, 0, 0, 0, 0, 0, 1}}, // 0x2B
+                {{',', 0, 0, 0, 0, 0, 0, 1}}, // 0x2C
+                {{'-', 0, 0, 0, 0, 0, 0, 1}}, // 0x2D
+                {{'.', 0, 0, 0, 0, 0, 0, 1}}, // 0x2E
+                {{'/', 0, 0, 0, 0, 0, 0, 1}}, // 0x2F
+                {{'0', 0, 0, 0, 0, 0, 0, 1}}, // 0x30
+                {{'1', 0, 0, 0, 0, 0, 0, 1}}, // 0x31
+                {{'2', 0, 0, 0, 0, 0, 0, 1}}, // 0x32
+                {{'3', 0, 0, 0, 0, 0, 0, 1}}, // 0x33
+                {{'4', 0, 0, 0, 0, 0, 0, 1}}, // 0x34
+                {{'5', 0, 0, 0, 0, 0, 0, 1}}, // 0x35
+                {{'6', 0, 0, 0, 0, 0, 0, 1}}, // 0x36
+                {{'7', 0, 0, 0, 0, 0, 0, 1}}, // 0x37
+                {{'8', 0, 0, 0, 0, 0, 0, 1}}, // 0x38
+                {{'9', 0, 0, 0, 0, 0, 0, 1}}, // 0x39
+                {{':', 0, 0, 0, 0, 0, 0, 1}}, // 0x3A
+                {{';', 0, 0, 0, 0, 0, 0, 1}}, // 0x3B
+                {{'<', 0, 0, 0, 0, 0, 0, 1}}, // 0x3C
+                {{'=', 0, 0, 0, 0, 0, 0, 1}}, // 0x3D
+                {{'>', 0, 0, 0, 0, 0, 0, 1}}, // 0x3E
+                {{'?', 0, 0, 0, 0, 0, 0, 1}}, // 0x3F
+                {{'@', 0, 0, 0, 0, 0, 0, 1}}, // 0x40
+                {{'A', 0, 0, 0, 0, 0, 0, 1}}, // 0x41
+                {{'B', 0, 0, 0, 0, 0, 0, 1}}, // 0x42
+                {{'C', 0, 0, 0, 0, 0, 0, 1}}, // 0x43
+                {{'D', 0, 0, 0, 0, 0, 0, 1}}, // 0x44
+                {{'E', 0, 0, 0, 0, 0, 0, 1}}, // 0x45
+                {{'F', 0, 0, 0, 0, 0, 0, 1}}, // 0x46
+                {{'G', 0, 0, 0, 0, 0, 0, 1}}, // 0x47
+                {{'H', 0, 0, 0, 0, 0, 0, 1}}, // 0x48
+                {{'I', 0, 0, 0, 0, 0, 0, 1}}, // 0x49
+                {{'J', 0, 0, 0, 0, 0, 0, 1}}, // 0x4A
+                {{'K', 0, 0, 0, 0, 0, 0, 1}}, // 0x4B
+                {{'L', 0, 0, 0, 0, 0, 0, 1}}, // 0x4C
+                {{'M', 0, 0, 0, 0, 0, 0, 1}}, // 0x4D
+                {{'N', 0, 0, 0, 0, 0, 0, 1}}, // 0x4E
+                {{'O', 0, 0, 0, 0, 0, 0, 1}}, // 0x4F
+                {{'P', 0, 0, 0, 0, 0, 0, 1}}, // 0x50
+                {{'Q', 0, 0, 0, 0, 0, 0, 1}}, // 0x51
+                {{'R', 0, 0, 0, 0, 0, 0, 1}}, // 0x52
+                {{'S', 0, 0, 0, 0, 0, 0, 1}}, // 0x53
+                {{'T', 0, 0, 0, 0, 0, 0, 1}}, // 0x54
+                {{'U', 0, 0, 0, 0, 0, 0, 1}}, // 0x55
+                {{'V', 0, 0, 0, 0, 0, 0, 1}}, // 0x56
+                {{'W', 0, 0, 0, 0, 0, 0, 1}}, // 0x57
+                {{'X', 0, 0, 0, 0, 0, 0, 1}}, // 0x58
+                {{'Y', 0, 0, 0, 0, 0, 0, 1}}, // 0x59
+                {{'Z', 0, 0, 0, 0, 0, 0, 1}}, // 0x5A
+                {{'[', 0, 0, 0, 0, 0, 0, 1}}, // 0x5B
+                {{'\\', '\\', 0, 0, 0, 0, 0, 2}}, // 0x5C
+                {{']', 0, 0, 0, 0, 0, 0, 1}}, // 0x5D
+                {{'^', 0, 0, 0, 0, 0, 0, 1}}, // 0x5E
+                {{'_', 0, 0, 0, 0, 0, 0, 1}}, // 0x5F
+            }
+        };
+        return escapes;
+    }
+
     /*!
      * @brief write a lowercase "\\uXXXX" escape sequence into @a string_buffer
      *
@@ -27788,7 +27809,7 @@ class serializer
         // use a pointer to fill the buffer
         auto buffer_ptr = number_buffer.begin(); // NOLINT(llvm-qualified-auto,readability-qualified-auto)
 
-        number_magnitude_t abs_value;
+        number_unsigned_t abs_value;
 
         unsigned int n_chars{};
 
@@ -27802,7 +27823,7 @@ class serializer
         }
         else
         {
-            abs_value = static_cast<number_magnitude_t>(x);
+            abs_value = static_cast<number_unsigned_t>(x);
             n_chars = count_digits(abs_value);
         }
 
@@ -27945,6 +27966,17 @@ class serializer
         }
     }
 
+    /*
+     * Overload to make the compiler happy while it is instantiating
+     * dump_integer for number_unsigned_t.
+     * Must never be called.
+     */
+    number_unsigned_t remove_sign(number_unsigned_t x)
+    {
+        JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert) LCOV_EXCL_LINE
+        return x; // LCOV_EXCL_LINE
+    }
+
     /*
      * Helper function for dump_integer
      *
@@ -27954,10 +27986,10 @@ class serializer
      * absolute values of INT_MIN and INT_MAX are usually not the same. See
      * #1708 for details.
      */
-    number_magnitude_t remove_sign(number_integer_t x) noexcept
+    number_unsigned_t remove_sign(number_integer_t x) noexcept
     {
         JSON_ASSERT(x < 0);
-        return static_cast<number_magnitude_t>(-(x + 1)) + 1;
+        return static_cast<number_unsigned_t>(-(x + 1)) + 1;
     }
 
   private:
@@ -28841,22 +28873,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                   "NumberUnsignedType must be at least as wide as NumberIntegerType, "
                   "because it has to hold the absolute value of every NumberIntegerType value");
 
-    // NumberUnsignedType may be the same type as NumberIntegerType (#2573), but
-    // a repeated NumberFloatType would give overloads that collide, so it gets
-    // a readable diagnostic instead.
-    static_assert(!std::is_same<NumberFloatType, NumberIntegerType>::value && !std::is_same<NumberFloatType, NumberUnsignedType>::value,
-                  "NumberFloatType must be a different type than NumberIntegerType and NumberUnsignedType");
-
   private:
 
-    /// the kind a value of kind @a t is stored as: if number_unsigned_t is the
-    /// same type as number_integer_t, every integer is stored as number_integer
-    /// (#2573); every other kind is stored as itself
-    static constexpr value_t stored_kind(const value_t t) noexcept
-    {
-        return (std::is_same<number_integer_t, number_unsigned_t>::value && t == value_t::number_unsigned) ? value_t::number_integer : t;
-    }
-
     /// helper for exception-safe object creation
     template<typename T, typename... Args>
     JSON_HEDLEY_RETURNS_NON_NULL
@@ -28930,8 +28948,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         json_value(boolean_t v) noexcept : boolean(v) {}
         /// constructor for numbers (integer)
         json_value(number_integer_t v) noexcept : number_integer(v) {}
-        /// constructor for numbers (unsigned); takes a placeholder if number_unsigned_t is number_integer_t
-        json_value(detail::number_unsigned_overload_type<NumberIntegerType, NumberUnsignedType> v) noexcept : number_unsigned(v) {}
+        /// constructor for numbers (unsigned)
+        json_value(number_unsigned_t v) noexcept : number_unsigned(v) {}
         /// constructor for numbers (floating-point)
         json_value(number_float_t v) noexcept : number_float(v) {}
         /// constructor for empty values of a given type
@@ -30536,7 +30554,7 @@ public:
     /// @brief create an empty value with a given type
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
     basic_json(const value_t v)
-        : m_data(stored_kind(v))
+        : m_data(v)
     {
         assert_invariant();
     }
@@ -30617,7 +30635,7 @@ public:
         {
             convert_leaf(val);
         }
-        JSON_ASSERT(m_data.m_type == stored_kind(val.type()));
+        JSON_ASSERT(m_data.m_type == val.type());
 
         set_parents();
         assert_invariant();
@@ -31202,16 +31220,14 @@ public:
         return m_data.m_type == value_t::number_integer ? &m_data.m_value.number_integer : nullptr;
     }
 
-    /// get a pointer to the value (unsigned number); takes a placeholder if
-    /// number_unsigned_t is number_integer_t (the integer overload handles it then)
-    number_unsigned_t* get_impl_ptr(detail::number_unsigned_overload_type<NumberIntegerType, NumberUnsignedType>* /*unused*/) noexcept
+    /// get a pointer to the value (unsigned number)
+    number_unsigned_t* get_impl_ptr(number_unsigned_t* /*unused*/) noexcept
     {
         return is_number_unsigned() ? &m_data.m_value.number_unsigned : nullptr;
     }
 
-    /// get a pointer to the value (unsigned number); takes a placeholder if
-    /// number_unsigned_t is number_integer_t (the integer overload handles it then)
-    constexpr const number_unsigned_t* get_impl_ptr(const detail::number_unsigned_overload_type<NumberIntegerType, NumberUnsignedType>* /*unused*/) const noexcept
+    /// get a pointer to the value (unsigned number)
+    constexpr const number_unsigned_t* get_impl_ptr(const number_unsigned_t* /*unused*/) const noexcept
     {
         return is_number_unsigned() ? &m_data.m_value.number_unsigned : nullptr;
     }

tests/src/unit-same-number-types.cpp

@@ -1,201 +0,0 @@
-//     __ _____ _____ _____
-//  __|  |   __|     |   | |  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"
-
-#include <nlohmann/json.hpp>
-
-#include <cstdint>
-#include <functional>
-#include <limits>
-#include <string>
-#include <vector>
-
-// basic_json with the same type for number_integer_t and number_unsigned_t
-// (#2573): every integer is stored as number_integer
-
-namespace
-{
-
-template<typename IntegerType>
-using same_types_json = nlohmann::json::with_integers_t<IntegerType, IntegerType>;
-
-using json64 = same_types_json<std::int64_t>;
-using json32 = same_types_json<std::int32_t>;
-
-} // namespace
-
-TEST_CASE_TEMPLATE("same type for number_integer_t and number_unsigned_t", Json, json64, json32)
-{
-    using integer_t = typename Json::number_integer_t;
-    using limits = std::numeric_limits<integer_t>;
-
-    SECTION("parsing stores integers as number_integer")
-    {
-        const Json j = Json::parse("[0, 1, -1, 2147483647, -2147483648]");
-        for (const auto& element : j)
-        {
-            CHECK(element.is_number_integer());
-            CHECK(!element.is_number_unsigned());
-            CHECK(element.type() == nlohmann::detail::value_t::number_integer);
-        }
-        CHECK(j[1].template get<integer_t>() == 1);
-        CHECK(j[2].template get<integer_t>() == -1);
-        CHECK(j[3].template get<integer_t>() == 2147483647);
-        CHECK(j[4].template get<std::int64_t>() == -2147483648LL);
-    }
-
-    SECTION("integers above the maximum are parsed as floats")
-    {
-        // a positive literal is parsed as unsigned first; it must not wrap
-        // around to a negative number_integer_t
-        const auto above_max = std::to_string(static_cast<std::uint64_t>(limits::max()) + 1);
-        const Json j1 = Json::parse(above_max);
-        CHECK(j1.is_number_float());
-        CHECK(j1.template get<double>() == static_cast<double>(static_cast<std::uint64_t>(limits::max()) + 1));
-
-        const Json j2 = Json::parse("18446744073709551615");
-        CHECK(j2.is_number_float());
-        CHECK(j2.template get<double>() == 18446744073709551615.0);
-
-        const Json j3 = Json::parse(std::to_string(limits::max()));
-        CHECK(j3.is_number_integer());
-        CHECK(j3.template get<integer_t>() == limits::max());
-    }
-
-    SECTION("serialization of the extremes")
-    {
-        CHECK(Json(limits::max()).dump() == std::to_string(limits::max()));
-        // the absolute value of the minimum does not fit into number_unsigned_t
-        CHECK(Json(limits::min()).dump() == std::to_string(limits::min()));
-        CHECK(Json::parse(std::to_string(limits::min())) == Json(limits::min()));
-        CHECK(Json(0).dump() == "0");
-    }
-
-    SECTION("construction from unsigned types")
-    {
-        const Json j1 = 5u;
-        CHECK(j1.is_number_integer());
-        CHECK(!j1.is_number_unsigned());
-        CHECK(j1 == 5);
-
-        const Json j2 = static_cast<std::uint8_t>(200);
-        CHECK(j2.is_number_integer());
-        CHECK(j2.template get<integer_t>() == 200);
-
-        const Json j3 = std::vector<unsigned> {1, 2, 3};
-        CHECK(j3.dump() == "[1,2,3]");
-        CHECK(j3[0].is_number_integer());
-
-        const Json j4 = true;
-        CHECK(j4.is_boolean());
-    }
-
-    SECTION("value_t::number_unsigned creates a number_integer")
-    {
-        const Json j(nlohmann::detail::value_t::number_unsigned);
-        CHECK(j.is_number_integer());
-        CHECK(!j.is_number_unsigned());
-        CHECK(j == 0);
-        CHECK(j.dump() == "0");
-    }
-
-    SECTION("conversions")
-    {
-        Json j = 42;
-        CHECK(j.template get<unsigned>() == 42u);
-        CHECK(j.template get<std::uint64_t>() == 42u);
-        CHECK(j.template get<int>() == 42);
-        CHECK(j.template get<double>() == 42.0);
-        CHECK(j.template get_ptr<integer_t*>() != nullptr);
-        CHECK(*j.template get_ptr<integer_t*>() == 42);
-        CHECK(j.template get_ref<integer_t&>() == 42);
-        j.template get_ref<integer_t&>() = 43;
-        CHECK(j == 43);
-
-        integer_t i = 0;
-        j.get_to(i);
-        CHECK(i == 43);
-
-        const Json object = {{"a", 1}};
-        CHECK(object.value("a", 0u) == 1u);
-        CHECK(object.value("b", 7u) == 7u);
-    }
-
-    SECTION("conversion from a basic_json with distinct types")
-    {
-        const nlohmann::json j = {1, 2u, -3, 4.5};
-        CHECK(j[1].is_number_unsigned());
-        const Json converted = j;
-        CHECK(converted.dump() == "[1,2,-3,4.5]");
-        CHECK(converted[1].is_number_integer());
-        CHECK(!converted[1].is_number_unsigned());
-
-        const nlohmann::json back = converted;
-        CHECK(back == j);
-    }
-
-    SECTION("comparison and hashing")
-    {
-        const Json j1 = 1;
-        const Json j2 = 2u;
-        CHECK(j1 < j2);
-        CHECK(j1 != j2);
-        CHECK(j2 == 2);
-        CHECK(j2 == 2u);
-        CHECK(j1 < 1.5);
-        CHECK(Json(-1) < Json(0u));
-        CHECK(std::hash<Json> {}(Json(5u)) == std::hash<Json> {}(Json(5)));
-    }
-
-    SECTION("JSON Patch, diff, and flatten")
-    {
-        const Json source = Json::parse(R"({"a": [1, 2, 3], "b": 4})");
-        const Json target = Json::parse(R"({"a": [1, 5], "c": 6})");
-        CHECK(source.patch(Json::diff(source, target)) == target);
-        CHECK(source.flatten().unflatten() == source);
-    }
-
-    SECTION("binary formats")
-    {
-        const Json j = {{"min", limits::min()}, {"max", limits::max()}, {"zero", 0}, {"small", 100}, {"negative", -100}, {"float", 1.5}};
-
-        CHECK(Json::from_cbor(Json::to_cbor(j)) == j);
-        CHECK(Json::from_msgpack(Json::to_msgpack(j)) == j);
-        CHECK(Json::from_ubjson(Json::to_ubjson(j)) == j);
-        CHECK(Json::from_bjdata(Json::to_bjdata(j)) == j);
-        CHECK(Json::from_bson(Json::to_bson(j)) == j);
-        CHECK(Json::from_bon8(Json::to_bon8(j)) == j);
-
-        for (const auto& element : Json::from_cbor(Json::to_cbor(j)))
-        {
-            CHECK(!element.is_number_unsigned());
-        }
-
-        // CBOR unsigned integer 2^64-1 does not fit and is read as float
-        const std::vector<std::uint8_t> cbor_max = {0x1B, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF};
-        const Json from_cbor = Json::from_cbor(cbor_max);
-        CHECK(from_cbor.is_number_float());
-        CHECK(from_cbor.template get<double>() == 18446744073709551615.0);
-
-        // MessagePack uint 8 is read as number_integer
-        const std::vector<std::uint8_t> msgpack_uint8 = {0xCC, 0xC8};
-        const Json from_msgpack = Json::from_msgpack(msgpack_uint8);
-        CHECK(from_msgpack.is_number_integer());
-        CHECK(from_msgpack == 200);
-    }
-}
-
-TEST_CASE("same type for number_integer_t and number_unsigned_t: type traits")
-{
-    CHECK(std::is_same<nlohmann::detail::number_unsigned_overload_t<json64>, nlohmann::detail::same_as_number_integer_type>::value);
-    CHECK(std::is_same<nlohmann::detail::number_unsigned_overload_t<nlohmann::json>, std::uint64_t>::value);
-    CHECK(nlohmann::detail::is_compatible_number_integer_type<json64, unsigned>::value);
-    CHECK(!nlohmann::detail::is_compatible_number_integer_type<nlohmann::json, unsigned>::value);
-    CHECK(!nlohmann::detail::is_compatible_number_integer_type<json64, bool>::value);
-}

tests/src/unit-serialization.cpp

@@ -923,66 +923,68 @@ TEST_CASE("serialization boundary values for the string buffer")
     // mid-string once fewer than 13 bytes remain (`string_buffer.size() - bytes
     // < 13`), 13 being one more than the most a single code point can ever
     // write at once (a surrogate pair: two back-to-back "\uXXXX" escapes, 12
-    // bytes). Every write into string_buffer that this check protects happens
-    // in steps of 2 (a simple "\\x" escape) or 6 (one "\uXXXX" unit), so
-    // `bytes` only ever takes even values at the point the check runs - the
-    // tightest values actually reachable are therefore 498 (512 - 498 == 14,
-    // one simple escape away from the threshold) and 500 (512 - 500 == 12,
-    // where the flush fires immediately and resets bytes to 0).
+    // bytes). Characters below 0x60 (control characters, '"' and '\\') are
+    // escaped straight into the write buffer and never reach string_buffer, so
+    // under ensure_ascii it fills in steps of 6 (one "\uXXXX" unit for a
+    // character below U+10000) or 12 (a surrogate pair). With 2-byte characters
+    // such as U+00E4, `bytes` takes multiples of 6 at the point the check runs;
+    // the tightest values reachable are therefore 498 (512 - 498 == 14, after
+    // 83 characters) and 504 (512 - 504 == 8, after 84 characters, where the
+    // flush fires immediately and resets bytes to 0).
 
-    SECTION("a run of 2-byte escapes lands bytes on, and one step past, the flush threshold")
+    SECTION("a run of 6-byte escapes lands bytes on, and one step past, the flush threshold")
     {
         for (const int count :
                 {
-                    249, 250, 251
+                    83, 84, 85
                 })
         {
             CAPTURE(count)
-            const json j = std::string(static_cast<std::size_t>(count), '\n');
+            std::string umlauts;
             std::string expected = "\"";
             for (int i = 0; i < count; ++i)
             {
-                expected += "\\n";
+                umlauts += "\xC3\xA4";
+                expected += "\\u00e4";
             }
             expected += '"';
-            CHECK(j.dump() == expected);
+            CHECK(json(umlauts).dump(-1, ' ', true) == expected);
         }
     }
 
-    SECTION("an ASCII prefix leaves the tightest reachable margin before a 12-byte surrogate pair")
+    SECTION("a prefix leaves the tightest reachable margin before a 12-byte surrogate pair")
     {
         // U+1F600 (the "\xF0\x9F\x98\x80" UTF-8 bytes) is dumped under
         // ensure_ascii as the 12-byte surrogate pair "\ud83d\ude00"; that
         // write happens in a single step with no intermediate flush check, so
         // it is the write most exposed by an off-by-one in the "< 13" guard.
-        // A prefix of 249 newlines leaves exactly 14 bytes of headroom
+        // A prefix of 83 U+00E4 characters leaves exactly 14 bytes of headroom
         // (512 - 498), the smallest margin the guard ever actually allows
-        // into a new code point; 250 newlines instead trigger the guard's own
+        // into a new code point; 84 characters instead trigger the guard's own
         // flush first, so the emoji starts from a freshly emptied (512-byte)
-        // buffer, and 251 repeats that with one more escape already past the
+        // buffer, and 85 repeats that with one more escape already past the
         // reset. Together they cover the margin the guard allows landing on,
         // one step before, and one step after - all must still produce the
         // identical, correct escapes.
         for (const int prefix_count :
                 {
-                    249, 250, 251
+                    83, 84, 85
                 })
         {
             CAPTURE(prefix_count)
-            const std::string prefix(static_cast<std::size_t>(prefix_count), '\n');
-            const std::string emoji = "\xF0\x9F\x98\x80";
-            const json j = prefix + emoji;
-
+            std::string prefix;
             std::string expected_prefix;
             for (int i = 0; i < prefix_count; ++i)
             {
-                expected_prefix += "\\n";
+                prefix += "\xC3\xA4";
+                expected_prefix += "\\u00e4";
             }
+            const std::string emoji = "\xF0\x9F\x98\x80";
+            const json j = prefix + emoji;
 
-            // newline escaping does not depend on ensure_ascii: only the
-            // emoji differs (raw UTF-8 bytes vs. a \u-escaped surrogate pair)
+            // without ensure_ascii, the whole string is copied unchanged
             std::string expected_raw = "\"";
-            expected_raw += expected_prefix;
+            expected_raw += prefix;
             expected_raw += emoji;
             expected_raw += '"';
             std::string expected_ascii = "\"";
@@ -1027,3 +1029,132 @@ TEST_CASE("serialization boundary values for the string buffer")
         }
     }
 }
+
+namespace
+{
+// reference escaper for ASCII strings, written independently of the table in
+// serializer::ascii_escapes()
+std::string reference_escape_ascii(const std::string& s, const bool ensure_ascii)
+{
+    std::string result = "\"";
+    for (const char c : s)
+    {
+        const auto byte = static_cast<unsigned char>(c);
+        switch (byte)
+        {
+            case '\b':
+                result += "\\b";
+                break;
+            case '\t':
+                result += "\\t";
+                break;
+            case '\n':
+                result += "\\n";
+                break;
+            case '\f':
+                result += "\\f";
+                break;
+            case '\r':
+                result += "\\r";
+                break;
+            case '"':
+                result += "\\\"";
+                break;
+            case '\\':
+                result += "\\\\";
+                break;
+            default:
+                if (byte < 0x20 || (ensure_ascii && byte == 0x7F))
+                {
+                    // byte < 0x80, so the first two hex digits are 0
+                    const char* const hex = "0123456789abcdef";
+                    result += "\\u00";
+                    result += hex[byte >> 4U];
+                    result += hex[byte & 0x0FU];
+                }
+                else
+                {
+                    result += c;
+                }
+                break;
+        }
+    }
+    result += '"';
+    return result;
+}
+} // namespace
+
+TEST_CASE("characters below 0x60 are escaped from a table straight into the write buffer")
+{
+    SECTION("every ASCII character at every position")
+    {
+        // lengths up to 17 cover the 8-byte strides of the bulk scanner on
+        // both sides of the character, so the scanner stops at it from every
+        // offset
+        for (std::size_t length = 1; length <= 17; ++length)
+        {
+            for (std::size_t pos = 0; pos < length; ++pos)
+            {
+                for (unsigned int c = 0; c < 0x80; ++c)
+                {
+                    std::string s(length, 'a');
+                    s[pos] = static_cast<char>(c);
+                    CAPTURE(length)
+                    CAPTURE(pos)
+                    CAPTURE(c)
+                    const json j = s;
+                    CHECK(j.dump(-1, ' ', false) == reference_escape_ascii(s, false));
+                    CHECK(j.dump(-1, ' ', true) == reference_escape_ascii(s, true));
+                }
+            }
+        }
+    }
+
+    SECTION("runs of escaped characters")
+    {
+        std::string s;
+        for (unsigned int c = 0; c < 0x20; ++c)
+        {
+            s += static_cast<char>(c);
+        }
+        s += R"("\"\)";
+        CHECK(json(s).dump() == reference_escape_ascii(s, false));
+        CHECK(json(s).dump(-1, ' ', true) == reference_escape_ascii(s, true));
+    }
+
+    SECTION("escapes straddle the end of the write buffer")
+    {
+        // the 8-byte store of a table entry needs room for all 8 bytes, even
+        // when the escape itself is shorter; a prefix of k plain characters
+        // shifts the 6-byte escapes across every offset of the 1024-byte
+        // write buffer
+        for (std::size_t k = 0; k < 8; ++k)
+        {
+            CAPTURE(k)
+            const std::string s = std::string(k, 'a') + std::string(400, '\x01');
+            CHECK(json(s).dump() == reference_escape_ascii(s, false));
+            CHECK(json::array({s, s}).dump() == "[" + reference_escape_ascii(s, false) + "," + reference_escape_ascii(s, false) + "]");
+        }
+    }
+
+    SECTION("pending \\u escapes of non-ASCII characters are written first")
+    {
+        // under ensure_ascii, U+00E4 is \u-escaped into the string buffer;
+        // a following escape from the table must not overtake it
+        const json j = "\xC3\xA4\n\xC3\xA4\"\xF0\x9F\x98\x80\\";
+        CHECK(j.dump(-1, ' ', true) == R"("\u00e4\n\u00e4\"\ud83d\ude00\\")");
+        CHECK(j.dump(-1, ' ', false) == "\"\xC3\xA4\\n\xC3\xA4\\\"\xF0\x9F\x98\x80\\\\\"");
+    }
+
+    SECTION("escapes after invalid UTF-8 for every error handler")
+    {
+        const json j = "a\xFF\n\xC3\"";
+        CHECK_THROWS_WITH_AS(utils::ignore_return_value(j.dump()), "[json.exception.type_error.316] invalid UTF-8 byte at index 1: 0xFF", json::type_error&);
+        CHECK(j.dump(-1, ' ', false, json::error_handler_t::ignore) == R"("a\n\"")");
+        CHECK(j.dump(-1, ' ', true, json::error_handler_t::ignore) == R"("a\n\"")");
+        CHECK(j.dump(-1, ' ', false, json::error_handler_t::replace) == "\"a\xEF\xBF\xBD\\n\xEF\xBF\xBD\\\"\"");
+        CHECK(j.dump(-1, ' ', true, json::error_handler_t::replace) == R"("a\ufffd\n\ufffd\"")");
+        CHECK(j.dump(-1, ' ', false, json::error_handler_t::keep) == "\"a\xFF\\n\xC3\\\"\"");
+        CHECK(j.dump(-1, ' ', true, json::error_handler_t::keep) == "\"a\xFF\\n\xC3\\\"\"");
+    }
+}