Update documentation

doc/mkdocs/docs/css/custom.css

@@ -0,0 +1,4 @@
+/* disable ligatures in code and preformatted blocks */
+code, pre {
+    font-variant-ligatures: none;
+}

docs/mkdocs/docs/api/basic_json/index.md

@@ -233,9 +233,10 @@ Access to the JSON value
 - [**operator==**](operator_eq.md) - comparison: equal
 - [**operator!=**](operator_ne.md) - comparison: not equal
 - [**operator<**](operator_lt.md) - comparison: less than
-- [**operator<=**](operator_le.md) - comparison: less than or equal
 - [**operator>**](operator_gt.md) - comparison: greater than
+- [**operator<=**](operator_le.md) - comparison: less than or equal
 - [**operator>=**](operator_ge.md) - comparison: greater than or equal
+- [**operator<=>**](operator_spaceship.md) - comparison: 3-way
 
 ### Serialization / Dumping
 

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

@@ -1,21 +1,31 @@
 # <small>nlohmann::basic_json::</small>operator==
 
 ```cpp
-bool operator==(const_reference lhs, const_reference rhs) noexcept;
+// until C++20
+bool operator==(const_reference lhs, const_reference rhs) noexcept;   // (1)
 
 template<typename ScalarType>
-bool operator==(const_reference lhs, const ScalarType rhs) noexcept;
+bool operator==(const_reference lhs, const ScalarType rhs) noexcept;  // (2)
 
 template<typename ScalarType>
-bool operator==(ScalarType lhs, const const_reference rhs) noexcept;
+bool operator==(ScalarType lhs, const const_reference rhs) noexcept;  // (2)
+
+// since C++20
+class basic_json {
+    bool operator==(const_reference rhs) const noexcept;              // (1)
+
+    template<typename ScalarType>
+    bool operator==(ScalarType rhs) const noexcept;                   // (2)
+};
 ```
 
-Compares two JSON values for equality according to the following rules:
+1. Compares two JSON values for equality according to the following rules:
+    - Two JSON values are equal if (1) neither value is discarded, or (2) they are of the same
+      type and their stored values are the same according to their respective `operator==`.
+    - Integer and floating-point numbers are automatically converted before comparison.
 
-- Two JSON values are equal if (1) they are not discarded, (2) they are from the same type, and (3) their stored values
+2. Compares a JSON value and a scalar or a scalar and a JSON value for equality by converting the
-  are the same according to their respective `operator==`.
+   scalar to a JSON value and comparing both JSON values according to 1.
-- Integer and floating-point numbers are automatically converted before comparison. Note that two NaN values are always
-  treated as unequal.
 
 ## Template parameters
 
@@ -32,7 +42,7 @@ Compares two JSON values for equality according to the following rules:
 
 ## Return value
 
-whether the values `lhs` and `rhs` are equal
+whether the values `lhs`/`*this` and `rhs` are equal
 
 ## Exception safety
 
@@ -44,13 +54,17 @@ Linear.
 
 ## Notes
 
-!!! note
+!!! note "Comparison of special values"
 
-    - NaN values never compare equal to themselves or to other NaN values.
+    - `NaN` values are unordered within the domain of numbers.
+      The following comparisons all yield `#!cpp false`:
+        1. Comparing a `NaN` with itself.
+        2. Comparing a `NaN` with another `NaN`.
+        3. Comparing a `NaN` and any other number.
     - JSON `#!cpp null` values are all equal.
     - Discarded values never compare equal to themselves.
 
-!!! note
+!!! note "Floating-point comparison with epsilon"
 
     Floating-point numbers inside JSON values numbers are compared with `json::number_float_t::operator==` which is
     `double::operator==` by default. To compare floating-point while respecting an epsilon, an alternative
@@ -117,4 +131,5 @@ Linear.
 
 ## Version history
 
-- Added in version 1.0.0.
+1. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.
+2. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.

docs/mkdocs/docs/api/basic_json/operator_ge.md

@@ -1,17 +1,25 @@
 # <small>nlohmann::basic_json::</small>operator>=
 
 ```cpp
-bool operator>=(const_reference lhs, const_reference rhs) noexcept,
+// until C++20
+bool operator>=(const_reference lhs, const_reference rhs) noexcept;   // (1)
 
 template<typename ScalarType>
-bool operator>=(const_reference lhs, const ScalarType rhs) noexcept;
+bool operator>=(const_reference lhs, const ScalarType rhs) noexcept;  // (2)
 
 template<typename ScalarType>
-bool operator>=(ScalarType lhs, const const_reference rhs) noexcept;
+bool operator>=(ScalarType lhs, const const_reference rhs) noexcept;  // (2)
 ```
 
-Compares whether one JSON value `lhs` is greater than or equal to another JSON value `rhs` by calculating
+1. Compares whether one JSON value `lhs` is greater than or equal to another JSON value `rhs`
-`#!cpp !(lhs < rhs)`.
+   according to the following rules:
+    - The comparison always yields `#!cpp false` if (1) either operand is discarded, or (2) either
+      operand is `NaN` and the other operand is either `NaN` or any other number.
+    - Otherwise, returns the result of `#!cpp !(lhs < rhs)`.
+
+2. Compares wether a JSON value is greater than or equal to a scalar or a scalar is greater than or
+   equal to a JSON value by converting the scalar to a JSON value and comparing both JSON values
+   according to 1.
 
 ## Template parameters
 
@@ -38,6 +46,21 @@ No-throw guarantee: this function never throws exceptions.
 
 Linear.
 
+## Notes
+
+!!! note "Comparison of `NaN`"
+
+    `NaN` values are unordered within the domain of numbers.
+    The following comparisons all yield `#!cpp false`:
+      1. Comparing a `NaN` with itself.
+      2. Comparing a `NaN` with another `NaN`.
+      3. Comparing a `NaN` and any other number.
+
+!!! note "Operator overload resolution"
+
+    Since C++20 overload resolution will consider the _rewritten candidate_ generated from
+    [`operator<=>`](operator_spaceship.md).
+
 ## Examples
 
 ??? example
@@ -54,6 +77,11 @@ Linear.
     --8<-- "examples/operator__greaterequal.output"
     ```
 
+## See also
+
+- [**operator<=>**](operator_spaceship.md) comparison: 3-way
+
 ## Version history
 
-- Added in version 1.0.0.
+1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
+2. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.

docs/mkdocs/docs/api/basic_json/operator_gt.md

@@ -1,16 +1,24 @@
 # <small>nlohmann::basic_json::</small>operator>
 
 ```cpp
-bool operator>(const_reference lhs, const_reference rhs) noexcept,
+// until C++20
+bool operator>(const_reference lhs, const_reference rhs) noexcept;   // (1)
 
 template<typename ScalarType>
-bool operator>(const_reference lhs, const ScalarType rhs) noexcept;
+bool operator>(const_reference lhs, const ScalarType rhs) noexcept;  // (2)
 
 template<typename ScalarType>
-bool operator>(ScalarType lhs, const const_reference rhs) noexcept;
+bool operator>(ScalarType lhs, const const_reference rhs) noexcept;  // (2)
 ```
 
-Compares whether one JSON value `lhs` is greater than another JSON value `rhs` by calculating `#!cpp !(lhs <= rhs)`.
+1. Compares whether one JSON value `lhs` is greater than another JSON value `rhs` according to the
+  following rules:
+    - The comparison always yields `#!cpp false` if (1) either operand is discarded, or (2) either
+      operand is `NaN` and the other operand is either `NaN` or any other number.
+    - Otherwise, returns the result of `#!cpp !(lhs <= rhs)`.
+
+2. Compares wether a JSON value is greater than a scalar or a scalar is greater than a JSON value by
+   converting the scalar to a JSON value and comparing both JSON values according to 1.
 
 ## Template parameters
 
@@ -37,6 +45,21 @@ No-throw guarantee: this function never throws exceptions.
 
 Linear.
 
+## Notes
+
+!!! note "Comparison of `NaN`"
+
+    `NaN` values are unordered within the domain of numbers.
+    The following comparisons all yield `#!cpp false`:
+      1. Comparing a `NaN` with itself.
+      2. Comparing a `NaN` with another `NaN`.
+      3. Comparing a `NaN` and any other number.
+
+!!! note "Operator overload resolution"
+
+    Since C++20 overload resolution will consider the _rewritten candidate_ generated from
+    [`operator<=>`](operator_spaceship.md).
+
 ## Examples
 
 ??? example
@@ -53,6 +76,11 @@ Linear.
     --8<-- "examples/operator__greater.output"
     ```
 
+## See also
+
+- [**operator<=>**](operator_spaceship.md) comparison: 3-way
+
 ## Version history
 
-- Added in version 1.0.0.
+1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
+2. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.

docs/mkdocs/docs/api/basic_json/operator_le.md

@@ -1,17 +1,25 @@
 # <small>nlohmann::basic_json::</small>operator<=
 
 ```cpp
-bool operator<=(const_reference lhs, const_reference rhs) noexcept,
+// until C++20
+bool operator<=(const_reference lhs, const_reference rhs) noexcept;   // (1)
 
 template<typename ScalarType>
-bool operator<=(const_reference lhs, const ScalarType rhs) noexcept;
+bool operator<=(const_reference lhs, const ScalarType rhs) noexcept;  // (2)
 
 template<typename ScalarType>
-bool operator<=(ScalarType lhs, const const_reference rhs) noexcept;
+bool operator<=(ScalarType lhs, const const_reference rhs) noexcept;  // (2)
 ```
 
-Compares whether one JSON value `lhs` is less than or equal to another JSON value `rhs` by calculating
+1. Compares whether one JSON value `lhs` is less than or equal to another JSON value `rhs`
-`#cpp !(rhs < lhs)`.
+   according to the following rules:
+    - The comparison always yields `#!cpp false` if (1) either operand is discarded, or (2) either
+      operand is `NaN` and the other operand is either `NaN` or any other number.
+    - Otherwise, returns the result of `#!cpp !(rhs < lhs)`.
+
+1. Compares wether a JSON value is less than or equal to a scalar or a scalar is less than or equal
+   to a JSON value by converting the scalar to a JSON value and comparing both JSON values according
+   to 1.
 
 ## Template parameters
 
@@ -38,6 +46,21 @@ No-throw guarantee: this function never throws exceptions.
 
 Linear.
 
+## Notes
+
+!!! note "Comparison of `NaN`"
+
+    `NaN` values are unordered within the domain of numbers.
+    The following comparisons all yield `#!cpp false`:
+      1. Comparing a `NaN` with itself.
+      2. Comparing a `NaN` with another `NaN`.
+      3. Comparing a `NaN` and any other number.
+
+!!! note "Operator overload resolution"
+
+    Since C++20 overload resolution will consider the _rewritten candidate_ generated from
+    [`operator<=>`](operator_spaceship.md).
+
 ## Examples
 
 ??? example
@@ -54,6 +77,11 @@ Linear.
     --8<-- "examples/operator__lessequal.output"
     ```
 
+## See also
+
+- [**operator<=>**](operator_spaceship.md) comparison: 3-way
+
 ## Version history
 
-- Added in version 1.0.0.
+1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
+2. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.

docs/mkdocs/docs/api/basic_json/operator_lt.md

@@ -1,22 +1,23 @@
 # <small>nlohmann::basic_json::</small>operator<
 
 ```cpp
-bool operator<(const_reference lhs, const_reference rhs) noexcept;
+// until C++20
+bool operator<(const_reference lhs, const_reference rhs) noexcept;   // (1)
 
 template<typename ScalarType>
-bool operator<(const_reference lhs, const ScalarType rhs) noexcept;
+bool operator<(const_reference lhs, const ScalarType rhs) noexcept;  // (2)
 
 template<typename ScalarType>
-bool operator<(ScalarType lhs, const const_reference rhs) noexcept;
+bool operator<(ScalarType lhs, const const_reference rhs) noexcept;  // (2)
 ```
 
-Compares whether one JSON value `lhs` is less than another JSON value `rhs` according to the following rules:
+1. Compares whether one JSON value `lhs` is less than another JSON value `rhs` according to the
-
+  following rules:
-- If `lhs` and `rhs` have the same type, the values are compared using the default `<` operator.
+    - If either operand is discarded, the comparison yields `#!cpp false`.
-- Integer and floating-point numbers are automatically converted before comparison
+    - If both operands have the same type, the values are compared using their respective `operator<`.
-- Discarded values a
+    - Integer and floating-point numbers are automatically converted before comparison.
-- In case `lhs` and `rhs` have different types, the values are ignored and the order of the types is considered, which
+    - In case `lhs` and `rhs` have different types, the values are ignored and the order of the types
-  is:
+      is considered, which is:
         1. null
         2. boolean
         3. number (all types)
@@ -24,9 +25,11 @@ Compares whether one JSON value `lhs` is less than another JSON value `rhs` acco
         5. array
         6. string
         7. binary
-
       For instance, any boolean value is considered less than any string.
 
+2. Compares wether a JSON value is less than a scalar or a scalar is less than a JSON value by converting
+   the scalar to a JSON value and comparing both JSON values according to 1.
+
 ## Template parameters
 
 `ScalarType`
@@ -52,6 +55,21 @@ No-throw guarantee: this function never throws exceptions.
 
 Linear.
 
+## Notes
+
+!!! note "Comparison of `NaN`"
+
+    `NaN` values are unordered within the domain of numbers.
+    The following comparisons all yield `#!cpp false`:
+      1. Comparing a `NaN` with itself.
+      2. Comparing a `NaN` with another `NaN`.
+      3. Comparing a `NaN` and any other number.
+
+!!! note "Operator overload resolution"
+
+    Since C++20 overload resolution will consider the _rewritten candidate_ generated from
+    [`operator<=>`](operator_spaceship.md).
+
 ## Examples
 
 ??? example
@@ -68,6 +86,11 @@ Linear.
     --8<-- "examples/operator__less.output"
     ```
 
+## See also
+
+- [**operator<=>**](operator_spaceship.md) comparison: 3-way
+
 ## Version history
 
-- Added in version 1.0.0.
+1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
+2. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.

docs/mkdocs/docs/api/basic_json/operator_ne.md

@@ -1,16 +1,32 @@
 # <small>nlohmann::basic_json::</small>operator!=
 
 ```cpp
-bool operator!=(const_reference lhs, const_reference rhs) noexcept;
+// until C++20
+bool operator!=(const_reference lhs, const_reference rhs) noexcept;   // (1)
 
 template<typename ScalarType>
-bool operator!=(const_reference lhs, const ScalarType rhs) noexcept;
+bool operator!=(const_reference lhs, const ScalarType rhs) noexcept;  // (2)
 
 template<typename ScalarType>
-bool operator!=(ScalarType lhs, const const_reference rhs) noexcept;
+bool operator!=(ScalarType lhs, const const_reference rhs) noexcept;  // (2)
+
+// since C++20
+class basic_json {
+    bool operator!=(const_reference rhs) const noexcept;              // (1)
+
+    template<typename ScalarType>
+    bool operator!=(ScalarType rhs) const noexcept;                   // (2)
+};
 ```
 
-Compares two JSON values for inequality by calculating `#!cpp !(lhs == rhs)`.
+1. Compares two JSON values for inequality according to the following rules:
+    - The comparison always yields `#!cpp false` if (1) either operand is discarded, or (2) either
+      operand is `NaN` and the other operand is either `NaN` or any other number.
+    - Otherwise, returns the result of `#!cpp !(lhs == rhs)` (until C++20) or
+      `#!cpp !(*this == rhs)` (since C++20).
+
+2. Compares a JSON value and a scalar or a scalar and a JSON value for inequality by converting the
+   scalar to a JSON value and comparing both JSON values according to 1.
 
 ## Template parameters
 
@@ -27,7 +43,7 @@ Compares two JSON values for inequality by calculating `#!cpp !(lhs == rhs)`.
 
 ## Return value
 
-whether the values `lhs` and `rhs` are not equal
+whether the values `lhs`/`*this` and `rhs` are not equal
 
 ## Exception safety
 
@@ -37,6 +53,16 @@ No-throw guarantee: this function never throws exceptions.
 
 Linear.
 
+## Notes
+
+!!! note "Comparison of `NaN`"
+
+    `NaN` values are unordered within the domain of numbers.
+    The following comparisons all yield `#!cpp false`:
+      1. Comparing a `NaN` with itself.
+      2. Comparing a `NaN` with another `NaN`.
+      3. Comparing a `NaN` and any other number.
+
 ## Examples
 
 ??? example
@@ -69,4 +95,5 @@ Linear.
 
 ## Version history
 
-- Added in version 1.0.0.
+1. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.
+2. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.

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

@@ -1,5 +1,70 @@
 # <small>nlohmann::basic_json::</small>operator<=>
 
+```cpp
+// since C++20
+class basic_json {
+    std::partial_ordering operator<=>(const_reference rhs) const noexcept;  // (1)
+
+    template<typename ScalarType>
+    std::partial_ordering operator<=>(const ScalarType rhs) const noexcept; // (2)
+};
+```
+
+1. 3-way compares two JSON values producing a result of type `std::partial_ordering` according to the following rules:
+    - Two JSON values compare with a result of `std::partial_ordering::unordered` if either value is discarded.
+    - If both JSON values are of the same type, the result is produced by 3-way comparing their stored values using their
+      respective `operator<=>`.
+    - Integer and floating-point numbers are converted to their common type and then 3-way compared using their respective
+      `operator<=>`.
+      For instance, comparing an integer and a floating-point value will 3-way compare the first value convertered to
+      floating-point with the second value.
+    - Otherwise, yields a result by comparing the type (see [`value_t`](value_t.md)).
+  
+2. 3-way compares a JSON value and a scalar or a scalar and a JSON value by converting the scalar to a JSON value and 3-way
+   comparing both JSON values (see 1).
+
+## Template parameters
+
+`ScalarType`
+:   a scalar type according to `std::is_scalar<ScalarType>::value`
+
+## Parameters
+
+`rhs` (in)
+:   second value to consider 
+
+## Return value
+
+the `std::partial_ordering` of the 3-way comparison of `*this` and `rhs`
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Linear.
+
+## Notes
+
+!!! note
+
+    - `NaN` values are unordered within the domain of numbers.
+      The following comparisons all yield `std::partial_ordering::unordered`:
+        1. Comparing a `NaN` with itself.
+        2. Comparing a `NaN` with another `NaN`.
+        3. Comparing a `NaN` and any other number.
+
+## See also
+
+- [**operator==**](operator_eq.md) - comparison: equal
+- [**operator!=**](operator_ne.md) - comparison: not equal
+- [**operator<**](operator_lt.md) - comparison: less than
+- [**operator<=**](operator_le.md) - comparison: less than or equal
+- [**operator>**](operator_gt.md) - comparison: greater than
+- [**operator>=**](operator_ge.md) - comparison: greater than or equal
+
 ## Version history
 
-- Added in version 3.11.0.
+1. Added in version 3.11.0.
+2. Added in version 3.11.0.

docs/mkdocs/docs/api/basic_json/value_t.md

@@ -24,10 +24,41 @@ functions [`is_null`](is_null.md), [`is_object`](is_object.md), [`is_array`](is_
 
 ## Notes
 
-There are three enumeration entries (number_integer, number_unsigned, and number_float), because the library
+!!! note "Ordering"
-distinguishes these three types for numbers: [`number_unsigned_t`](number_unsigned_t.md) is used for unsigned integers,
+
-[`number_integer_t`](number_integer_t.md) is used for signed integers, and [`number_float_t`](number_float_t.md) is used
+    The order of types is as follows:
-for floating-point numbers or to approximate integers which do not fit in the limits of their respective type.
+
+    1. `null`
+    2. `boolean`
+    3. `number_integer`, `number_unsigned`, `number_float`
+    4. `object`
+    5. `array`
+    6. `string`
+    7. `binary`
+
+    `discarded` is unordered.
+
+!!! note "Types of numbers"
+
+    There are three enumerators for numbers (`number_integer`, `number_unsigned`, and `number_float`) to distinguish
+    between different types of numbers:
+
+      - [`number_unsigned_t`](number_unsigned_t.md) for unsigned integers
+      - [`number_integer_t`](number_integer_t.md) for signed integers
+      - [`number_float_t`](number_float_t.md) for floating-point numbers or to approximate integers which do not fit
+        into the limits of their respective type
+
+!!! warning "Comparison operators"
+
+    `operator<` and `operator<=>` (since C++20) are overloaded and compare according to the ordering described above.
+    Until C++20 all other relational and equality operators yield results according to the integer value of each
+    enumerator.
+    Since C++20 some compilers consider the _rewritten candidates_ generated from `operator<=>` during overload
+    resolution, while others do not.
+    For predictable and portable behavior use:
+
+      - `operator<` or `operator<=>` when wanting to compare according to the order described above
+      - `operator==` or `operator!=` when wanting to compare according to each enumerators integer value
 
 ## Version history
 

docs/mkdocs/docs/api/macros/index.md

@@ -17,6 +17,8 @@ header. See also the [macro overview page](../../features/macros.md).
 
 - [**JSON_HAS_CPP_11**<br>**JSON_HAS_CPP_14**<br>**JSON_HAS_CPP_17**<br>**JSON_HAS_CPP_20**](json_has_cpp_11.md) - set supported C++ standard
 - [**JSON_HAS_FILESYSTEM**<br>**JSON_HAS_EXPERIMENTAL_FILESYSTEM**](json_has_filesystem.md) - control `std::filesystem` support
+- [**JSON_HAS_RANGES**](json_has_ranges.md) - control `std::ranges` support
+- [**JSON_HAS_THREE_WAY_COMPARISON**](json_has_three_way_comparison.md) - control 3-way comparison support
 - [**JSON_NO_IO**](json_no_io.md) - switch off functions relying on certain C++ I/O headers
 - [**JSON_SKIP_UNSUPPORTED_COMPILER_CHECK**](json_skip_unsupported_compiler_check.md) - do not warn about unsupported compilers
 
@@ -29,6 +31,12 @@ header. See also the [macro overview page](../../features/macros.md).
 
 - [**JSON_USE_IMPLICIT_CONVERSIONS**](json_use_implicit_conversions.md) - control implicit conversions
 
+<!-- comment-->
+## Comparison behavior
+
+- [**JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON**](json_use_legacy_discarded_value_comparison.md) -
+  control comparison of discarded values
+
 ## Serialization/deserialization macros
 
 - [**NLOHMANN_DEFINE_TYPE_INTRUSIVE(type, member...)**<br>**NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(type, member...)**](nlohmann_define_type_intrusive.md) - serialization/deserialization of types _with_ access to private variables

docs/mkdocs/docs/api/macros/json_has_ranges.md

@@ -0,0 +1,18 @@
+# JSON_HAS_RANGES
+
+```cpp
+#define JSON_HAS_RANGES /* value */
+```
+
+This macro indicates whether the standard library has any support for ranges. Implies support for concepts.
+Possible values are `1` when supported or `0` when unsupported.
+
+## Default definition
+
+The default value is detected based on the preprocessor macro `#!cpp __cpp_lib_ranges`.
+
+When the macro is not defined, the library will define it to its default value.
+
+## Version history
+
+- Added in version 3.11.0.

docs/mkdocs/docs/api/macros/json_has_three_way_comparison.md

@@ -0,0 +1,19 @@
+# JSON_HAS_THREE_WAY_COMPARISON
+
+```cpp
+#define JSON_HAS_THREE_WAY_COMPARISON /* value */
+```
+
+This macro indicates whether the compiler and standard library support 3-way comparison.
+Possible values are `1` when supported or `0` when unsupported.
+
+## Default definition
+
+The default value is detected based on the preprocessor macros `#!cpp __cpp_impl_three_way_comparison`
+and `#!cpp __cpp_lib_three_way_comparison`.
+
+When the macro is not defined, the library will define it to its default value.
+
+## Version history
+
+- Added in version 3.11.0.

docs/mkdocs/docs/api/macros/json_use_legacy_discarded_value_comparison.md

@@ -0,0 +1,61 @@
+# JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
+
+```cpp
+#define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON /* value */
+```
+
+This macro enables the (incorrect) legacy comparison behavior of discarded JSON values.
+Possible values are `1` to enable or `0` to disable (default).
+
+When enabled, comparisons involving at least one discarded JSON value yield results as follows:
+
+| **Operator** | **Result**    |
+|--------------|---------------|
+| `==`         | `#!cpp false` |
+| `!=`         | `#!cpp true`  |
+| `<`          | `#!cpp false` |
+| `<=`         | `#!cpp true`  |
+| `>=`         | `#!cpp true`  |
+| `>`          | `#!cpp false` |
+
+Otherwise, comparisons involving at least one discarded JSON value always yield `#!cpp false`.
+
+## Default definition
+
+The default value is `0`.
+
+```cpp
+#define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON 0
+```
+
+When the macro is not defined, the library will define it to its default value.
+
+## Notes
+
+!!! warning "Inconsistent behavior in C++20 and beyond"
+
+    When targeting C++20 or above, enabling the legacy comparison behavior is _strongly_
+    discouraged.
+
+      - The 3-way comparison operator (`<=>`) will always give the correct result
+        (`#!cpp std::partial_ordering::unordered`) regardless of the value of
+        `JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON`.
+      - Overloads for the equality and relational operators emulate the legacy behavior.
+
+    Code outside your control may use either 3-way comparison or the equality and
+    relational operators, resulting in inconsistent and unpredictable behavior.
+
+    See [`operator<=>`](../basic_json/operator_spaceship.md) for more information on 3-way
+    comparison.
+
+!!! warning "Deprecation"
+
+    The legacy comparison behavior is deprecated and may be removed in a future major
+    version release.
+
+    New code should not depend on it and existing code should try to remove or rewrite
+    expressions relying on it.
+
+## Version history
+
+- Added in version 3.11.0.

docs/mkdocs/mkdocs.yml

@@ -152,16 +152,16 @@ nav:
         - 'operator value_t': api/basic_json/operator_value_t.md
         - 'operator[]': api/basic_json/operator[].md
         - 'operator=': api/basic_json/operator=.md
-        - 'operator<=>': api/basic_json/operator_spaceship.md
+        - 'operator+=': api/basic_json/operator+=.md
         - 'operator==': api/basic_json/operator_eq.md
         - 'operator!=': api/basic_json/operator_ne.md
         - 'operator<': api/basic_json/operator_lt.md
-        - 'operator<<': api/basic_json/operator_ltlt.md
-        - 'operator<=': api/basic_json/operator_le.md
         - 'operator>': api/basic_json/operator_gt.md
-        - 'operator>>': api/basic_json/operator_gtgt.md
+        - 'operator<=': api/basic_json/operator_le.md
         - 'operator>=': api/basic_json/operator_ge.md
-        - 'operator+=': api/basic_json/operator+=.md
+        - 'operator<=>': api/basic_json/operator_spaceship.md
+        - 'operator<<': api/basic_json/operator_ltlt.md
+        - 'operator>>': api/basic_json/operator_gtgt.md
         - 'operator""_json': api/basic_json/operator_literal_json.md
         - 'operator""_json_pointer': api/basic_json/operator_literal_json_pointer.md
         - 'out_of_range': api/basic_json/out_of_range.md
@@ -244,6 +244,8 @@ nav:
         - 'JSON_HAS_CPP_20': api/macros/json_has_cpp_11.md
         - 'JSON_HAS_EXPERIMENTAL_FILESYSTEM': api/macros/json_has_filesystem.md
         - 'JSON_HAS_FILESYSTEM': api/macros/json_has_filesystem.md
+        - 'JSON_HAS_RANGES': api/macros/json_has_ranges.md
+        - 'JSON_HAS_THREE_WAY_COMPARISON': api/macros/json_has_three_way_comparison.md
         - 'JSON_NOEXCEPTION': api/macros/json_noexception.md
         - 'JSON_NO_IO': api/macros/json_no_io.md
         - 'JSON_SKIP_LIBRARY_VERSION_CHECK': api/macros/json_skip_library_version_check.md
@@ -251,6 +253,7 @@ nav:
         - 'JSON_THROW_USER': api/macros/json_throw_user.md
         - 'JSON_TRY_USER': api/macros/json_throw_user.md
         - 'JSON_USE_IMPLICIT_CONVERSIONS': api/macros/json_use_implicit_conversions.md
+        - 'JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON': api/macros/json_use_legacy_discarded_value_comparison.md
         - 'NLOHMANN_DEFINE_TYPE_INTRUSIVE': api/macros/nlohmann_define_type_intrusive.md
         - 'NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT': api/macros/nlohmann_define_type_intrusive.md
         - 'NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE': api/macros/nlohmann_define_type_non_intrusive.md
@@ -317,5 +320,8 @@ plugins:
         minify_html: true
     - git-revision-date-localized
 
+extra_css:
+    - css/custom.css
+
 extra_javascript:
   - https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/MathJax.js?config=TeX-MML-AM_CHTML