Fix with_t nav entry and document chaining of the with_*_t aliases

Indent the with_t entry in mkdocs.yml so it is listed under basic_json,
explain that the aliases can be chained and work on ordered_json, and
test both, including json::with_object_t<ordered_map> == ordered_json.

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

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

@@ -109,6 +109,9 @@ The class satisfies the following concept requirements:
 - **initializer_list_t** - type for initializer lists of `basic_json` values
 - [**input_format_t**](input_format_t.md) - type to choose the format to parse
 - [**json_sax_t**](../json_sax/index.md) - type for SAX events
+- [**with_object_t, with_array_t, with_string_t, with_boolean_t, with_integers_t, with_float_t, with_allocator_t,
+  with_json_serializer_t, with_binary_t, with_base_class_t**](with_t.md) - types to create a `basic_json` type with
+  one (or two) replaced template parameters
 
 ### Exceptions
 

docs/mkdocs/docs/api/basic_json/to_bjdata.md

@@ -68,8 +68,6 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
 - Throws [type_error.316](../../home/exceptions.md#jsonexceptiontype_error316) if a string or object key in `j` is
   not valid UTF-8 and `error_handler` is `strict` (the default only if
   [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled)
-- Throws [type_error.321](../../home/exceptions.md#jsonexceptiontype_error321) if `j` or a value nested in it is
-  discarded; example: `"cannot serialize discarded value to BJData"`
 
 ## Complexity
 
@@ -121,6 +119,4 @@ Linear in the size of the JSON value `j`.
 - BJData version parameter (for draft3 binary encoding) added in version 3.12.0.
 - Added `error_handler` parameter in version 3.13.0. Its default, `keep`, writes the bytes of a string or object key
   that is not valid UTF-8 unchanged, as before; `strict` (the default if
-  [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled) throws `type_error.316`.
+  [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled) throws `type_error.316`.
-- Throws `type_error.321` for a discarded value since version 3.13.0; previously, a discarded value nested in an
-  array or object was silently skipped, producing invalid BJData.

docs/mkdocs/docs/api/basic_json/to_bson.md

@@ -58,9 +58,6 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
 - Throws [type_error.316](../../home/exceptions.md#jsonexceptiontype_error316) if a string or object key is
   not valid UTF-8 and `error_handler` is `strict` (the default only if
   [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled)
-- Throws [type_error.321](../../home/exceptions.md#jsonexceptiontype_error321) if a value nested in `j` is discarded
-  (the top-level value itself is covered by `type_error.317` above, since it must be an object); example:
-  `"cannot serialize discarded value to BSON"`
 
 ## Complexity
 
@@ -113,8 +110,6 @@ pass before anything is written.
 - Throws `out_of_range.412` and `out_of_range.415` since version 3.13.0.
 - Linear in the size of `j`, and no longer limited by the call stack for deeply nested values, since version 3.13.0.
 - `out_of_range.415` is now detected before anything is written, like the other exceptions above, since version 3.13.0.
-- Throws `type_error.321` for a discarded value nested in `j` since version 3.13.0; previously, it was silently
-  skipped, producing a document whose declared size did not match what was actually written.
 - Added `error_handler` parameter in version 3.13.0. Its default, `keep`, writes the bytes of a string or object key
   that is not valid UTF-8 unchanged, as before; `strict` (the default if
   [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled) throws `type_error.316` before anything

docs/mkdocs/docs/api/basic_json/to_cbor.md

@@ -49,8 +49,6 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
 - Throws [type_error.316](../../home/exceptions.md#jsonexceptiontype_error316) if a string or object key in `j` is
   not valid UTF-8 and `error_handler` is `strict` (the default only if
   [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled)
-- Throws [type_error.321](../../home/exceptions.md#jsonexceptiontype_error321) if `j` or a value nested in it is
-  discarded; example: `"cannot serialize discarded value to CBOR"`
 
 ## Complexity
 
@@ -88,5 +86,3 @@ Linear in the size of the JSON value `j`.
 - Added `error_handler` parameter in version 3.13.0. Its default, `keep`, writes the bytes of a string or object key
   that is not valid UTF-8 unchanged, as before; `strict` (the default if
   [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled) throws `type_error.316`.
-- Throws `type_error.321` for a discarded value since version 3.13.0; previously, a discarded value nested in an
-  array or object was silently skipped, producing invalid CBOR.

docs/mkdocs/docs/api/basic_json/to_msgpack.md

@@ -54,8 +54,6 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
   `"subtype 70000 is too large for the MessagePack ext type (max 255)"`
 - Throws [type_error.316](../../home/exceptions.md#jsonexceptiontype_error316) if a string or object key in `j` is
   not valid UTF-8 and `error_handler` is `strict`
-- Throws [type_error.321](../../home/exceptions.md#jsonexceptiontype_error321) if `j` or a value nested in it is
-  discarded; example: `"cannot serialize discarded value to MessagePack"`
 
 ## Complexity
 
@@ -110,5 +108,3 @@ Linear in the size of the JSON value `j`.
 - Fixed in version 3.13.0 to serialize `number_integer_t`/`number_unsigned_t` pairs of different width correctly;
   before, integers could be serialized with the wrong value if `number_integer_t` was narrower than
   `number_unsigned_t`.
-- Throws `type_error.321` for a discarded value since version 3.13.0; previously, a discarded value nested in an
-  array or object was silently skipped, producing invalid MessagePack.

docs/mkdocs/docs/api/basic_json/to_ubjson.md

@@ -61,8 +61,6 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
 - Throws [type_error.316](../../home/exceptions.md#jsonexceptiontype_error316) if a string or object key in `j` is
   not valid UTF-8 and `error_handler` is `strict` (the default only if
   [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled)
-- Throws [type_error.321](../../home/exceptions.md#jsonexceptiontype_error321) if `j` or a value nested in it is
-  discarded; example: `"cannot serialize discarded value to UBJSON"`
 
 ## Complexity
 
@@ -114,5 +112,3 @@ Linear in the size of the JSON value `j`.
 - Added `error_handler` parameter in version 3.13.0. Its default, `keep`, writes the bytes of a string or object key
   that is not valid UTF-8 unchanged, as before; `strict` (the default if
   [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled) throws `type_error.316`.
-- Throws `type_error.321` for a discarded value since version 3.13.0; previously, a discarded value nested in an
-  array or object was silently skipped, producing invalid UBJSON.

docs/mkdocs/docs/api/basic_json/with_t.md

@@ -0,0 +1,136 @@
+# <small>nlohmann::basic_json::</small>with_t
+
+Member alias templates `with_object_t`, `with_array_t`, `with_string_t`, `with_boolean_t`, `with_integers_t`,
+`with_float_t`, `with_allocator_t`, `with_json_serializer_t`, `with_binary_t`, and `with_base_class_t`.
+
+```cpp
+template<template<typename, typename, typename...> class ObjectType2>
+using with_object_t = basic_json<ObjectType2, ArrayType, StringType, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+template<template<typename, typename...> class ArrayType2>
+using with_array_t = basic_json<ObjectType, ArrayType2, StringType, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+template<class StringType2>
+using with_string_t = basic_json<ObjectType, ArrayType, StringType2, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+template<class BooleanType2>
+using with_boolean_t = basic_json<ObjectType, ArrayType, StringType, BooleanType2,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+template<class NumberIntegerType2, class NumberUnsignedType2>
+using with_integers_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,
+      NumberIntegerType2, NumberUnsignedType2, NumberFloatType,
+      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+template<class NumberFloatType2>
+using with_float_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType2,
+      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+template<template<typename> class AllocatorType2>
+using with_allocator_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType2, JSONSerializer, BinaryType, CustomBaseClass>;
+
+template<template<typename, typename = void> class JSONSerializer2>
+using with_json_serializer_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType, JSONSerializer2, BinaryType, CustomBaseClass>;
+
+template<class BinaryType2>
+using with_binary_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType, JSONSerializer, BinaryType2, CustomBaseClass>;
+
+template<class CustomBaseClass2>
+using with_base_class_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,
+      NumberIntegerType, NumberUnsignedType, NumberFloatType,
+      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass2>;
+```
+
+These member alias templates make it easier to create a `basic_json` type that is identical to the current type except
+for one (or, in the case of `with_integers_t`, two) of its [template parameters](index.md#template-parameters).
+Spelling out all 11 template parameters of `basic_json` just to change a single one is verbose and error-prone; these
+aliases only require the replacement type(s).
+
+with_object_t&lt;ObjectType2&gt;
+:   replaces `ObjectType`
+
+with_array_t&lt;ArrayType2&gt;
+:   replaces `ArrayType`
+
+with_string_t&lt;StringType2&gt;
+:   replaces `StringType`
+
+with_boolean_t&lt;BooleanType2&gt;
+:   replaces `BooleanType`
+
+with_integers_t&lt;NumberIntegerType2, NumberUnsignedType2&gt;
+:   replaces both `NumberIntegerType` and `NumberUnsignedType`; the two are combined into a single alias because they
+    are usually changed together (for instance, when switching to fixed-width integer types)
+
+with_float_t&lt;NumberFloatType2&gt;
+:   replaces `NumberFloatType`
+
+with_allocator_t&lt;AllocatorType2&gt;
+:   replaces `AllocatorType`
+
+with_json_serializer_t&lt;JSONSerializer2&gt;
+:   replaces `JSONSerializer`
+
+with_binary_t&lt;BinaryType2&gt;
+:   replaces `BinaryType`
+
+with_base_class_t&lt;CustomBaseClass2&gt;
+:   replaces `CustomBaseClass`; see also [`json_base_class_t`](json_base_class_t.md)
+
+## Notes
+
+All other template parameters are kept unchanged, so the resulting type still uses, for instance, the same
+`ObjectType` unless `with_object_t` itself is used.
+
+The aliases are members of every `basic_json` specialization, including [`ordered_json`](../ordered_json.md), and the
+type they produce is again a `basic_json` specialization. They can therefore be chained to replace several template
+parameters at once:
+
+```cpp
+using my_json = nlohmann::json::with_integers_t<int, unsigned int>::with_float_t<float>;
+using my_ordered_json = nlohmann::ordered_json::with_string_t<std::wstring>;
+```
+
+The result is the same type as spelling out all template parameters, so the order of the chained aliases does not
+matter. For instance, `nlohmann::json::with_object_t<nlohmann::ordered_map>` is `nlohmann::ordered_json`.
+
+## Examples
+
+??? example
+
+    The following code shows how `with_object_t` can be used to create a JSON type that stores object elements in a
+    `std::map` and therefore keeps them sorted by key, unlike the default type which preserves insertion order
+    only when `nlohmann::ordered_json` is used.
+
+    ```cpp
+    --8<-- "examples/with_t.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/with_t.output"
+    ```
+
+## See also
+
+- [basic_json](index.md#template-parameters) - the template parameters that can be replaced
+- [json_base_class_t](json_base_class_t.md) - the type used for `CustomBaseClass`
+
+## Version history
+
+- Added in version 3.13.0.

docs/mkdocs/docs/examples/json_base_class_t.cpp

@@ -13,19 +13,7 @@ class visitor_adaptor_with_metadata
     void do_visit(const Ptr& ptr, const Fnc& fnc) const;
 };
 
-using json = nlohmann::basic_json <
+using json = nlohmann::json::with_base_class_t<visitor_adaptor_with_metadata>;
-             std::map,
-             std::vector,
-             std::string,
-             bool,
-             std::int64_t,
-             std::uint64_t,
-             double,
-             std::allocator,
-             nlohmann::adl_serializer,
-             std::vector<std::uint8_t>,
-             visitor_adaptor_with_metadata
-             >;
 
 template <class Fnc>
 void visitor_adaptor_with_metadata::visit(const Fnc& fnc) const

docs/mkdocs/docs/examples/with_t.cpp

@@ -0,0 +1,18 @@
+#include <iostream>
+#include <map>
+#include <nlohmann/json.hpp>
+
+// a JSON type that stores objects in a std::map (which keeps keys sorted)
+// instead of the default ordered associative container
+using sorted_json = nlohmann::json::with_object_t<std::map>;
+
+int main()
+{
+    sorted_json j;
+    j["c"] = 1;
+    j["a"] = 2;
+    j["b"] = 3;
+
+    // keys are sorted, because std::map is used to store the object
+    std::cout << j.dump() << std::endl;
+}

docs/mkdocs/docs/examples/with_t.output

@@ -0,0 +1 @@
+{"a":2,"b":3,"c":1}

docs/mkdocs/docs/home/exceptions.md

@@ -804,20 +804,6 @@ does not list an enumerator and it is therefore converted like the first listed
     [json.exception.type_error.318] duplicate object key 'red'
     ```
 
-### json.exception.type_error.321
-
-A discarded value (one created by [`parse()`](../api/basic_json/parse.md) with a callback that returns `false` for the
-value, or by default-constructing a [`basic_json`](../api/basic_json/index.md) with
-[`value_t::discarded`](../api/basic_json/value_t.md)) was passed to a binary serialization function, either directly or
-nested in an array or object. There is no way to represent a discarded value in CBOR, MessagePack, UBJSON, BJData, or BSON.
-
-!!! failure "Example message"
-
-    Serializing `#!json [1, 2]` to CBOR, where the second element was discarded by a parser callback:
-    ```
-    [json.exception.type_error.321] cannot serialize discarded value to CBOR
-    ```
-
 ## Out of range
 
 This exception is thrown in case a library function is called on an input parameter that exceeds the expected range, for instance, in the case of array indices or nonexisting object keys.

docs/mkdocs/mkdocs.yml

@@ -232,6 +232,7 @@ nav:
           - 'update': api/basic_json/update.md
           - 'value': api/basic_json/value.md
           - 'value_t': api/basic_json/value_t.md
+          - 'with_t': api/basic_json/with_t.md
       - byte_container_with_subtype:
           - 'Overview': api/byte_container_with_subtype/index.md
           - '(constructor)': api/byte_container_with_subtype/byte_container_with_subtype.md

include/nlohmann/detail/output/binary_writer.hpp

@@ -127,7 +127,6 @@ class binary_writer
     @throw type_error.316 if a string value or an object key is not valid
            UTF-8
     @throw type_error.317 if @a j is not an object
-    @throw type_error.321 if a value nested in @a j is discarded
     */
     void write_bson(const BasicJsonType& j)
     {
@@ -159,7 +158,6 @@ class binary_writer
     @param[in] j  JSON value to serialize
     @throw type_error.316 if a string value or an object key is not valid
            UTF-8
-    @throw type_error.321 if @a j or a value nested in it is discarded
     */
     void write_cbor(const BasicJsonType& j)
     {
@@ -324,7 +322,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                throw_on_discarded(j, "CBOR");
+                break;
         }
     }
 
@@ -384,7 +382,6 @@ class binary_writer
 
     /*!
     @param[in] j  JSON value to serialize
-    @throw type_error.321 if @a j or a value nested in it is discarded
     */
     void write_msgpack(const BasicJsonType& j)
     {
@@ -658,7 +655,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                throw_on_discarded(j, "MessagePack");
+                break;
         }
     }
 
@@ -671,7 +668,6 @@ class binary_writer
     @param[in] bjdata_version  which BJData version to use, default is draft2
     @throw type_error.316 if a string value or an object key is not valid
            UTF-8
-    @throw type_error.321 if @a j or a value nested in it is discarded
     */
     void write_ubjson(const BasicJsonType& j, const bool use_count,
                       const bool use_type, const bool add_prefix = true,
@@ -905,7 +901,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                throw_on_discarded(j, use_bjdata ? "BJData" : "UBJSON");
+                break;
         }
     }
 
@@ -925,15 +921,6 @@ class binary_writer
     }
 
   private:
-    /*!
-    @brief throws because @a j is discarded and cannot be serialized
-    @throw type_error.321 always
-    */
-    JSON_HEDLEY_NO_RETURN static void throw_on_discarded(const BasicJsonType& j, const char* format_name)
-    {
-        JSON_THROW(type_error::create(321, concat("cannot serialize discarded value to ", format_name), &j));
-    }
-
     //////////
     // BSON //
     //////////
@@ -1185,7 +1172,6 @@ class binary_writer
            into a byte, before anything is written
     @throw type_error.316 if @a j is a string that is not valid UTF-8, before
            anything is written
-    @throw type_error.321 if @a j is discarded
     */
     std::size_t calc_bson_value_size(const BasicJsonType& j)
     {
@@ -1212,12 +1198,10 @@ class binary_writer
             case value_t::null:
                 return 0ul;
 
-            case value_t::discarded:
-                throw_on_discarded(j, "BSON");
-
             // LCOV_EXCL_START
             case value_t::object:
             case value_t::array:
+            case value_t::discarded:
             default:
                 JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert)
                 return 0ul;
@@ -1254,12 +1238,10 @@ class binary_writer
             case value_t::null:
                 return write_bson_null(name);
 
-            case value_t::discarded:
-                throw_on_discarded(j, "BSON");
-
             // LCOV_EXCL_START
             case value_t::object:
             case value_t::array:
+            case value_t::discarded:
             default:
                 JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert)
                 return;
@@ -1326,8 +1308,6 @@ class binary_writer
            byte, before anything is written
     @throw type_error.316 if a string value or a key is not valid UTF-8,
            before anything is written
-    @throw type_error.321 if a value nested in @a document is discarded,
-           before anything is written
     */
     std::size_t calc_bson_sizes(const BasicJsonType& document, std::vector<std::size_t>& nested_sizes)
     {

include/nlohmann/json.hpp

@@ -226,6 +226,70 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// SAX interface type, see @ref nlohmann::json_sax
     using json_sax_t = json_sax<basic_json>;
 
+    ////////////////////////////////////////////////////////////////////////////////
+    // utility templates to create a json type with different template parameters //
+    ////////////////////////////////////////////////////////////////////////////////
+
+    /// Json type using a different type for storing objects
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename, typename, typename...> class ObjectType2>
+    using with_object_t = basic_json<ObjectType2, ArrayType, StringType, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing arrays
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename, typename...> class ArrayType2>
+    using with_array_t =  basic_json<ObjectType, ArrayType2, StringType, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing strings
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class StringType2>
+    using with_string_t =  basic_json<ObjectType, ArrayType, StringType2, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing booleans
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class BooleanType2>
+    using with_boolean_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType2,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using different types for storing signed and unsigned integers
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class NumberIntegerType2, class NumberUnsignedType2>
+    using with_integers_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+          NumberIntegerType2, NumberUnsignedType2, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing floating point numbers
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class NumberFloatType2>
+    using with_float_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType2, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type as base allocator
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename> class AllocatorType2>
+    using with_allocator_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType2, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type as json serializer
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename, typename = void> class JSONSerializer2>
+    using with_json_serializer_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer2, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing binary data
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class BinaryType2>
+    using with_binary_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType2, CustomBaseClass>;
+
+    /// Json type using a different type as base class
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class CustomBaseClass2>
+    using with_base_class_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass2>;
+
     ////////////////
     // exceptions //
     ////////////////

single_include/nlohmann/json.hpp

@@ -21529,7 +21529,6 @@ class binary_writer
     @throw type_error.316 if a string value or an object key is not valid
            UTF-8
     @throw type_error.317 if @a j is not an object
-    @throw type_error.321 if a value nested in @a j is discarded
     */
     void write_bson(const BasicJsonType& j)
     {
@@ -21561,7 +21560,6 @@ class binary_writer
     @param[in] j  JSON value to serialize
     @throw type_error.316 if a string value or an object key is not valid
            UTF-8
-    @throw type_error.321 if @a j or a value nested in it is discarded
     */
     void write_cbor(const BasicJsonType& j)
     {
@@ -21726,7 +21724,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                throw_on_discarded(j, "CBOR");
+                break;
         }
     }
 
@@ -21786,7 +21784,6 @@ class binary_writer
 
     /*!
     @param[in] j  JSON value to serialize
-    @throw type_error.321 if @a j or a value nested in it is discarded
     */
     void write_msgpack(const BasicJsonType& j)
     {
@@ -22060,7 +22057,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                throw_on_discarded(j, "MessagePack");
+                break;
         }
     }
 
@@ -22073,7 +22070,6 @@ class binary_writer
     @param[in] bjdata_version  which BJData version to use, default is draft2
     @throw type_error.316 if a string value or an object key is not valid
            UTF-8
-    @throw type_error.321 if @a j or a value nested in it is discarded
     */
     void write_ubjson(const BasicJsonType& j, const bool use_count,
                       const bool use_type, const bool add_prefix = true,
@@ -22307,7 +22303,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                throw_on_discarded(j, use_bjdata ? "BJData" : "UBJSON");
+                break;
         }
     }
 
@@ -22327,15 +22323,6 @@ class binary_writer
     }
 
   private:
-    /*!
-    @brief throws because @a j is discarded and cannot be serialized
-    @throw type_error.321 always
-    */
-    JSON_HEDLEY_NO_RETURN static void throw_on_discarded(const BasicJsonType& j, const char* format_name)
-    {
-        JSON_THROW(type_error::create(321, concat("cannot serialize discarded value to ", format_name), &j));
-    }
-
     //////////
     // BSON //
     //////////
@@ -22587,7 +22574,6 @@ class binary_writer
            into a byte, before anything is written
     @throw type_error.316 if @a j is a string that is not valid UTF-8, before
            anything is written
-    @throw type_error.321 if @a j is discarded
     */
     std::size_t calc_bson_value_size(const BasicJsonType& j)
     {
@@ -22614,12 +22600,10 @@ class binary_writer
             case value_t::null:
                 return 0ul;
 
-            case value_t::discarded:
-                throw_on_discarded(j, "BSON");
-
             // LCOV_EXCL_START
             case value_t::object:
             case value_t::array:
+            case value_t::discarded:
             default:
                 JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert)
                 return 0ul;
@@ -22656,12 +22640,10 @@ class binary_writer
             case value_t::null:
                 return write_bson_null(name);
 
-            case value_t::discarded:
-                throw_on_discarded(j, "BSON");
-
             // LCOV_EXCL_START
             case value_t::object:
             case value_t::array:
+            case value_t::discarded:
             default:
                 JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert)
                 return;
@@ -22728,8 +22710,6 @@ class binary_writer
            byte, before anything is written
     @throw type_error.316 if a string value or a key is not valid UTF-8,
            before anything is written
-    @throw type_error.321 if a value nested in @a document is discarded,
-           before anything is written
     */
     std::size_t calc_bson_sizes(const BasicJsonType& document, std::vector<std::size_t>& nested_sizes)
     {
@@ -27368,6 +27348,70 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// SAX interface type, see @ref nlohmann::json_sax
     using json_sax_t = json_sax<basic_json>;
 
+    ////////////////////////////////////////////////////////////////////////////////
+    // utility templates to create a json type with different template parameters //
+    ////////////////////////////////////////////////////////////////////////////////
+
+    /// Json type using a different type for storing objects
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename, typename, typename...> class ObjectType2>
+    using with_object_t = basic_json<ObjectType2, ArrayType, StringType, BooleanType,
+                                     NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing arrays
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename, typename...> class ArrayType2>
+    using with_array_t =  basic_json<ObjectType, ArrayType2, StringType, BooleanType,
+                                     NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing strings
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class StringType2>
+    using with_string_t =  basic_json<ObjectType, ArrayType, StringType2, BooleanType,
+                                      NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing booleans
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class BooleanType2>
+    using with_boolean_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType2,
+                                       NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using different types for storing signed and unsigned integers
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class NumberIntegerType2, class NumberUnsignedType2>
+    using with_integers_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+                                        NumberIntegerType2, NumberUnsignedType2, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing floating point numbers
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class NumberFloatType2>
+    using with_float_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+                                     NumberIntegerType, NumberUnsignedType, NumberFloatType2, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type as base allocator
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename> class AllocatorType2>
+    using with_allocator_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+                                         NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType2, JSONSerializer, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type as json serializer
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<template<typename, typename = void> class JSONSerializer2>
+    using with_json_serializer_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+        NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer2, BinaryType, CustomBaseClass>;
+
+    /// Json type using a different type for storing binary data
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class BinaryType2>
+    using with_binary_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+                                      NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType2, CustomBaseClass>;
+
+    /// Json type using a different type as base class
+    /// @sa https://json.nlohmann.me/api/basic_json/with_t/
+    template<class CustomBaseClass2>
+    using with_base_class_t =  basic_json<ObjectType, ArrayType, StringType, BooleanType,
+                                          NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType, CustomBaseClass2>;
+
     ////////////////
     // exceptions //
     ////////////////

tests/src/unit-allocator.cpp

@@ -48,14 +48,7 @@ TEST_CASE("bad_alloc")
     SECTION("bad_alloc")
     {
         // create JSON type using the throwing allocator
-        using bad_json = nlohmann::basic_json<std::map,
+        using bad_json = nlohmann::json::with_allocator_t<bad_allocator>;
-              std::vector,
-              std::string,
-              bool,
-              std::int64_t,
-              std::uint64_t,
-              double,
-              bad_allocator>;
 
         // creating an object should throw
         CHECK_THROWS_AS(bad_json(bad_json::value_t::object), std::bad_alloc&);
@@ -129,14 +122,7 @@ void my_allocator_clean_up(T* p)
 TEST_CASE("controlled bad_alloc")
 {
     // create JSON type using the throwing allocator
-    using my_json = nlohmann::basic_json<std::map,
+    using my_json = nlohmann::json::with_allocator_t<my_allocator>;
-          std::vector,
-          std::string,
-          bool,
-          std::int64_t,
-          std::uint64_t,
-          double,
-          my_allocator>;
 
     SECTION("class json_value")
     {
@@ -588,14 +574,7 @@ TEST_CASE("bad my_allocator::construct")
 {
     SECTION("my_allocator::construct doesn't forward")
     {
-        using bad_alloc_json = nlohmann::basic_json<std::map,
+        using bad_alloc_json = nlohmann::json::with_allocator_t<allocator_no_forward>;
-              std::vector,
-              std::string,
-              bool,
-              std::int64_t,
-              std::uint64_t,
-              double,
-              allocator_no_forward>;
 
         bad_alloc_json j;
         j["test"] = bad_alloc_json::array_t();

tests/src/unit-alt-string.cpp

@@ -163,16 +163,7 @@ void int_to_string(alt_string& target, std::size_t value)
     target = std::to_string(value).c_str();
 }
 
-using alt_json = nlohmann::basic_json <
+using alt_json = nlohmann::json::with_string_t<alt_string>;
-                 std::map,
-                 std::vector,
-                 alt_string,
-                 bool,
-                 std::int64_t,
-                 std::uint64_t,
-                 double,
-                 std::allocator,
-                 nlohmann::adl_serializer >;
 
 bool operator<(const char* op1, const alt_string& op2) noexcept
 {

tests/src/unit-bjdata.cpp

@@ -106,59 +106,10 @@ TEST_CASE("BJData")
     {
         SECTION("discarded")
         {
-            // a discarded value cannot be serialized to BJData
+            // discarded values are not serialized
             json const j = json::value_t::discarded;
-            CHECK_THROWS_WITH_AS(json::to_bjdata(j), "[json.exception.type_error.321] cannot serialize discarded value to BJData", json::type_error&);
+            const auto result = json::to_bjdata(j);
-        }
+            CHECK(result.empty());
-
-        SECTION("discarded values nested in a container")
-        {
-            json const discarded = json::value_t::discarded;
-
-            SECTION("in an array")
-            {
-                json const j = {1, discarded, 2};
-#if JSON_DIAGNOSTICS
-                CHECK_THROWS_WITH_AS(json::to_bjdata(j), "[json.exception.type_error.321] (/1) cannot serialize discarded value to BJData", json::type_error&);
-#else
-                CHECK_THROWS_WITH_AS(json::to_bjdata(j), "[json.exception.type_error.321] cannot serialize discarded value to BJData", json::type_error&);
-#endif
-            }
-
-            SECTION("as an object value")
-            {
-                json j;
-                j["a"] = 1;
-                j["b"] = discarded;
-#if JSON_DIAGNOSTICS
-                CHECK_THROWS_WITH_AS(json::to_bjdata(j), "[json.exception.type_error.321] (/b) cannot serialize discarded value to BJData", json::type_error&);
-#else
-                CHECK_THROWS_WITH_AS(json::to_bjdata(j), "[json.exception.type_error.321] cannot serialize discarded value to BJData", json::type_error&);
-#endif
-            }
-
-            SECTION("nested deeper (array in object in array)")
-            {
-                json inner_array = {1, discarded};
-                json middle_object;
-                middle_object["x"] = inner_array;
-                json const j = {middle_object};
-#if JSON_DIAGNOSTICS
-                CHECK_THROWS_WITH_AS(json::to_bjdata(j), "[json.exception.type_error.321] (/0/x/1) cannot serialize discarded value to BJData", json::type_error&);
-#else
-                CHECK_THROWS_WITH_AS(json::to_bjdata(j), "[json.exception.type_error.321] cannot serialize discarded value to BJData", json::type_error&);
-#endif
-            }
-
-            SECTION("optimized array of all-discarded elements")
-            {
-                json const j = {discarded, discarded};
-#if JSON_DIAGNOSTICS
-                CHECK_THROWS_WITH_AS(json::to_bjdata(j, true, true), "[json.exception.type_error.321] (/0) cannot serialize discarded value to BJData", json::type_error&);
-#else
-                CHECK_THROWS_WITH_AS(json::to_bjdata(j, true, true), "[json.exception.type_error.321] cannot serialize discarded value to BJData", json::type_error&);
-#endif
-            }
         }
 
         SECTION("null")

tests/src/unit-bson.cpp

@@ -141,54 +141,6 @@ TEST_CASE("BSON")
             json const j = std::vector<int> {1, 2, 3, 4, 5, 6, 7};
             CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.317] to serialize to BSON, top-level type must be object, but is array", json::type_error&);
         }
-
-        SECTION("discarded")
-        {
-            json const j = json::value_t::discarded;
-            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.317] to serialize to BSON, top-level type must be object, but is discarded", json::type_error&);
-        }
-    }
-
-    SECTION("discarded values nested in a container cannot be serialized to BSON")
-    {
-        json const discarded = json::value_t::discarded;
-
-        SECTION("as an object value")
-        {
-            json j;
-            j["a"] = 1;
-            j["b"] = discarded;
-#if JSON_DIAGNOSTICS
-            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] (/b) cannot serialize discarded value to BSON", json::type_error&);
-#else
-            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] cannot serialize discarded value to BSON", json::type_error&);
-#endif
-        }
-
-        SECTION("in an array that is an object value")
-        {
-            json j;
-            j["a"] = json::array({1, discarded, 2});
-#if JSON_DIAGNOSTICS
-            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] (/a/1) cannot serialize discarded value to BSON", json::type_error&);
-#else
-            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] cannot serialize discarded value to BSON", json::type_error&);
-#endif
-        }
-
-        SECTION("nested deeper (array in object in object)")
-        {
-            json inner_array = {1, discarded};
-            json middle_object;
-            middle_object["x"] = inner_array;
-            json j;
-            j["outer"] = middle_object;
-#if JSON_DIAGNOSTICS
-            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] (/outer/x/1) cannot serialize discarded value to BSON", json::type_error&);
-#else
-            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] cannot serialize discarded value to BSON", json::type_error&);
-#endif
-        }
     }
 
     SECTION("keys containing code-point U+0000 cannot be serialized to BSON")

tests/src/unit-cbor.cpp

@@ -30,49 +30,10 @@ TEST_CASE("CBOR")
     {
         SECTION("discarded")
         {
-            // a discarded value cannot be serialized to CBOR
+            // discarded values are not serialized
             json const j = json::value_t::discarded;
-            CHECK_THROWS_WITH_AS(json::to_cbor(j), "[json.exception.type_error.321] cannot serialize discarded value to CBOR", json::type_error&);
+            const auto result = json::to_cbor(j);
-        }
+            CHECK(result.empty());
-
-        SECTION("discarded values nested in a container")
-        {
-            json const discarded = json::value_t::discarded;
-
-            SECTION("in an array")
-            {
-                json const j = {1, discarded, 2};
-#if JSON_DIAGNOSTICS
-                CHECK_THROWS_WITH_AS(json::to_cbor(j), "[json.exception.type_error.321] (/1) cannot serialize discarded value to CBOR", json::type_error&);
-#else
-                CHECK_THROWS_WITH_AS(json::to_cbor(j), "[json.exception.type_error.321] cannot serialize discarded value to CBOR", json::type_error&);
-#endif
-            }
-
-            SECTION("as an object value")
-            {
-                json j;
-                j["a"] = 1;
-                j["b"] = discarded;
-#if JSON_DIAGNOSTICS
-                CHECK_THROWS_WITH_AS(json::to_cbor(j), "[json.exception.type_error.321] (/b) cannot serialize discarded value to CBOR", json::type_error&);
-#else
-                CHECK_THROWS_WITH_AS(json::to_cbor(j), "[json.exception.type_error.321] cannot serialize discarded value to CBOR", json::type_error&);
-#endif
-            }
-
-            SECTION("nested deeper (array in object in array)")
-            {
-                json inner_array = {1, discarded};
-                json middle_object;
-                middle_object["x"] = inner_array;
-                json const j = {middle_object};
-#if JSON_DIAGNOSTICS
-                CHECK_THROWS_WITH_AS(json::to_cbor(j), "[json.exception.type_error.321] (/0/x/1) cannot serialize discarded value to CBOR", json::type_error&);
-#else
-                CHECK_THROWS_WITH_AS(json::to_cbor(j), "[json.exception.type_error.321] cannot serialize discarded value to CBOR", json::type_error&);
-#endif
-            }
         }
 
         SECTION("NaN")

tests/src/unit-custom-base-class.cpp

@@ -38,20 +38,7 @@ class json_metadata
 };
 
 template<class T>
-using json_with_metadata =
+using json_with_metadata = nlohmann::json::with_base_class_t<json_metadata<T>>;
-    nlohmann::basic_json <
-    std::map,
-    std::vector,
-    std::string,
-    bool,
-    std::int64_t,
-    std::uint64_t,
-    double,
-    std::allocator,
-    nlohmann::adl_serializer,
-    std::vector<std::uint8_t>,
-    json_metadata<T>
-    >;
 
 TEST_CASE("JSON Node Metadata")
 {
@@ -268,19 +255,7 @@ class visitor_adaptor
     void do_visit(const Ptr& ptr, const Fnc& fnc) const;
 };
 
-using json_with_visitor_t = nlohmann::basic_json <
+using json_with_visitor_t = nlohmann::json::with_base_class_t<visitor_adaptor>;
-                            std::map,
-                            std::vector,
-                            std::string,
-                            bool,
-                            std::int64_t,
-                            std::uint64_t,
-                            double,
-                            std::allocator,
-                            nlohmann::adl_serializer,
-                            std::vector<std::uint8_t>,
-                            visitor_adaptor
-                            >;
 
 template <class Fnc>
 void visitor_adaptor::visit(const Fnc& fnc) const

tests/src/unit-msgpack.cpp

@@ -33,49 +33,10 @@ TEST_CASE("MessagePack")
     {
         SECTION("discarded")
         {
-            // a discarded value cannot be serialized to MessagePack
+            // discarded values are not serialized
             json const j = json::value_t::discarded;
-            CHECK_THROWS_WITH_AS(json::to_msgpack(j), "[json.exception.type_error.321] cannot serialize discarded value to MessagePack", json::type_error&);
+            const auto result = json::to_msgpack(j);
-        }
+            CHECK(result.empty());
-
-        SECTION("discarded values nested in a container")
-        {
-            json const discarded = json::value_t::discarded;
-
-            SECTION("in an array")
-            {
-                json const j = {1, discarded, 2};
-#if JSON_DIAGNOSTICS
-                CHECK_THROWS_WITH_AS(json::to_msgpack(j), "[json.exception.type_error.321] (/1) cannot serialize discarded value to MessagePack", json::type_error&);
-#else
-                CHECK_THROWS_WITH_AS(json::to_msgpack(j), "[json.exception.type_error.321] cannot serialize discarded value to MessagePack", json::type_error&);
-#endif
-            }
-
-            SECTION("as an object value")
-            {
-                json j;
-                j["a"] = 1;
-                j["b"] = discarded;
-#if JSON_DIAGNOSTICS
-                CHECK_THROWS_WITH_AS(json::to_msgpack(j), "[json.exception.type_error.321] (/b) cannot serialize discarded value to MessagePack", json::type_error&);
-#else
-                CHECK_THROWS_WITH_AS(json::to_msgpack(j), "[json.exception.type_error.321] cannot serialize discarded value to MessagePack", json::type_error&);
-#endif
-            }
-
-            SECTION("nested deeper (array in object in array)")
-            {
-                json inner_array = {1, discarded};
-                json middle_object;
-                middle_object["x"] = inner_array;
-                json const j = {middle_object};
-#if JSON_DIAGNOSTICS
-                CHECK_THROWS_WITH_AS(json::to_msgpack(j), "[json.exception.type_error.321] (/0/x/1) cannot serialize discarded value to MessagePack", json::type_error&);
-#else
-                CHECK_THROWS_WITH_AS(json::to_msgpack(j), "[json.exception.type_error.321] cannot serialize discarded value to MessagePack", json::type_error&);
-#endif
-            }
         }
 
         SECTION("null")

tests/src/unit-ubjson.cpp

@@ -27,59 +27,10 @@ TEST_CASE("UBJSON")
     {
         SECTION("discarded")
         {
-            // a discarded value cannot be serialized to UBJSON
+            // discarded values are not serialized
             json const j = json::value_t::discarded;
-            CHECK_THROWS_WITH_AS(json::to_ubjson(j), "[json.exception.type_error.321] cannot serialize discarded value to UBJSON", json::type_error&);
+            const auto result = json::to_ubjson(j);
-        }
+            CHECK(result.empty());
-
-        SECTION("discarded values nested in a container")
-        {
-            json const discarded = json::value_t::discarded;
-
-            SECTION("in an array")
-            {
-                json const j = {1, discarded, 2};
-#if JSON_DIAGNOSTICS
-                CHECK_THROWS_WITH_AS(json::to_ubjson(j), "[json.exception.type_error.321] (/1) cannot serialize discarded value to UBJSON", json::type_error&);
-#else
-                CHECK_THROWS_WITH_AS(json::to_ubjson(j), "[json.exception.type_error.321] cannot serialize discarded value to UBJSON", json::type_error&);
-#endif
-            }
-
-            SECTION("as an object value")
-            {
-                json j;
-                j["a"] = 1;
-                j["b"] = discarded;
-#if JSON_DIAGNOSTICS
-                CHECK_THROWS_WITH_AS(json::to_ubjson(j), "[json.exception.type_error.321] (/b) cannot serialize discarded value to UBJSON", json::type_error&);
-#else
-                CHECK_THROWS_WITH_AS(json::to_ubjson(j), "[json.exception.type_error.321] cannot serialize discarded value to UBJSON", json::type_error&);
-#endif
-            }
-
-            SECTION("nested deeper (array in object in array)")
-            {
-                json inner_array = {1, discarded};
-                json middle_object;
-                middle_object["x"] = inner_array;
-                json const j = {middle_object};
-#if JSON_DIAGNOSTICS
-                CHECK_THROWS_WITH_AS(json::to_ubjson(j), "[json.exception.type_error.321] (/0/x/1) cannot serialize discarded value to UBJSON", json::type_error&);
-#else
-                CHECK_THROWS_WITH_AS(json::to_ubjson(j), "[json.exception.type_error.321] cannot serialize discarded value to UBJSON", json::type_error&);
-#endif
-            }
-
-            SECTION("optimized array of all-discarded elements")
-            {
-                json const j = {discarded, discarded};
-#if JSON_DIAGNOSTICS
-                CHECK_THROWS_WITH_AS(json::to_ubjson(j, true, true), "[json.exception.type_error.321] (/0) cannot serialize discarded value to UBJSON", json::type_error&);
-#else
-                CHECK_THROWS_WITH_AS(json::to_ubjson(j, true, true), "[json.exception.type_error.321] cannot serialize discarded value to UBJSON", json::type_error&);
-#endif
-            }
         }
 
         SECTION("null")
@@ -2140,14 +2091,9 @@ TEST_CASE("UBJSON")
 
         SECTION("discarded")
         {
-            // a discarded value cannot be serialized to UBJSON, even as part
-            // of an optimized array of a single (here: valueless) type
             json const j = {json::value_t::discarded, json::value_t::discarded};
-#if JSON_DIAGNOSTICS
+            std::vector<uint8_t> expected = {'[', '$', 'N', '#', 'i', 2};
-            CHECK_THROWS_WITH_AS(json::to_ubjson(j, true, true), "[json.exception.type_error.321] (/0) cannot serialize discarded value to UBJSON", json::type_error&);
+            CHECK(json::to_ubjson(j, true, true) == expected);
-#else
-            CHECK_THROWS_WITH_AS(json::to_ubjson(j, true, true), "[json.exception.type_error.321] cannot serialize discarded value to UBJSON", json::type_error&);
-#endif
         }
     }
 }

tests/src/unit-udt.cpp

@@ -23,6 +23,7 @@ using nlohmann::json;
     using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
 #endif
 
+#include <deque>
 #include <map>
 #include <memory>
 #include <string>
@@ -684,8 +685,7 @@ static std::ostream& operator<<(std::ostream& os, small_pod l)
 TEST_CASE("custom serializer for pods" * doctest::test_suite("udt"))
 {
     using custom_json =
-        nlohmann::basic_json<std::map, std::vector, std::string, bool,
+        nlohmann::json::with_json_serializer_t<pod_serializer>;
-        std::int64_t, std::uint64_t, double, std::allocator, pod_serializer>;
 
     auto p = udt::small_pod{42, '/', 42};
     custom_json const j = p;
@@ -703,7 +703,7 @@ TEST_CASE("custom serializer for pods" * doctest::test_suite("udt"))
 template <typename T, typename>
 struct another_adl_serializer;
 
-using custom_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, double, std::allocator, another_adl_serializer>;
+using custom_json = nlohmann::json::with_json_serializer_t<another_adl_serializer>;
 
 template <typename T, typename>
 struct another_adl_serializer
@@ -734,6 +734,80 @@ TEST_CASE("custom serializer that does adl by default" * doctest::test_suite("ud
     CHECK(me == cj.get<udt::person>());
 }
 
+TEST_CASE("with_*_t aliases" * doctest::test_suite("udt"))
+{
+    // a custom base class used to check with_base_class_t
+    struct custom_base_class {};
+
+    CHECK(std::is_same<json::with_object_t<std::deque>,
+          nlohmann::basic_json<std::deque, std::vector, std::string, bool,
+          std::int64_t, std::uint64_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_array_t<std::deque>,
+          nlohmann::basic_json<std::map, std::deque, std::string, bool,
+          std::int64_t, std::uint64_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_string_t<std::wstring>,
+          nlohmann::basic_json<std::map, std::vector, std::wstring, bool,
+          std::int64_t, std::uint64_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_boolean_t<int>,
+          nlohmann::basic_json<std::map, std::vector, std::string, int,
+          std::int64_t, std::uint64_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_integers_t<std::int32_t, std::uint32_t>,
+          nlohmann::basic_json<std::map, std::vector, std::string, bool,
+          std::int32_t, std::uint32_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_float_t<float>,
+          nlohmann::basic_json<std::map, std::vector, std::string, bool,
+          std::int64_t, std::uint64_t, float, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_allocator_t<std::allocator>,
+          nlohmann::basic_json<std::map, std::vector, std::string, bool,
+          std::int64_t, std::uint64_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_json_serializer_t<nlohmann::adl_serializer>,
+          nlohmann::basic_json<std::map, std::vector, std::string, bool,
+          std::int64_t, std::uint64_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    CHECK(std::is_same<json::with_binary_t<std::vector<char>>,
+          nlohmann::basic_json<std::map, std::vector, std::string, bool,
+          std::int64_t, std::uint64_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<char>>>::value);
+
+    CHECK(std::is_same<json::with_base_class_t<custom_base_class>,
+          nlohmann::basic_json<std::map, std::vector, std::string, bool,
+          std::int64_t, std::uint64_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>, custom_base_class>>::value);
+
+    // with_string_t on ordered_json must keep ordered_map as the object type
+    CHECK(std::is_same<nlohmann::ordered_json::with_string_t<std::wstring>,
+          nlohmann::basic_json<nlohmann::ordered_map, std::vector, std::wstring, bool,
+          std::int64_t, std::uint64_t, double, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+
+    // the aliases are members of the resulting type, so they can be chained
+    CHECK(std::is_same<json::with_integers_t<int, unsigned int>::with_float_t<float>,
+          nlohmann::basic_json<std::map, std::vector, std::string, bool,
+          int, unsigned int, float, std::allocator,
+          nlohmann::adl_serializer, std::vector<std::uint8_t>>>::value);
+    CHECK(std::is_same<json::with_float_t<float>::with_integers_t<int, unsigned int>,
+          json::with_integers_t<int, unsigned int>::with_float_t<float>>::value);
+
+    // replacing the object type of json with ordered_map yields ordered_json
+    CHECK(std::is_same<json::with_object_t<nlohmann::ordered_map>, nlohmann::ordered_json>::value);
+    CHECK(std::is_same<nlohmann::ordered_json::with_object_t<std::map>, json>::value);
+}
+
 TEST_CASE("different basic_json types conversions")
 {
     SECTION("null")