Document and test type_error.321 for discarded binary values

Add docs for the new exception (home/exceptions.md and the Exceptions
sections of to_cbor/to_msgpack/to_ubjson/to_bjdata/to_bson) and test
coverage for a discarded value nested in an array or object, nested
deeper, and (for UBJSON/BJData) inside an optimized same-type array,
for each of CBOR, MessagePack, UBJSON, BJData, and BSON. Adjust the
three pre-existing "discarded" tests that asserted the old silent
behavior (empty/short output) to expect type_error.321 instead.

Co-authored-by: ameliabarnabyhub <312084480+ameliabarnabyhub@users.noreply.github.com>
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

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

@@ -109,9 +109,6 @@ 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

@@ -68,6 +68,8 @@ 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
 
@@ -120,3 +122,5 @@ 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 BJData.

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

@@ -58,6 +58,9 @@ 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
 
@@ -110,6 +113,8 @@ 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

@@ -49,6 +49,8 @@ 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
 
@@ -86,3 +88,5 @@ 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

@@ -54,6 +54,8 @@ 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
 
@@ -108,3 +110,5 @@ 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

@@ -61,6 +61,8 @@ 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
 
@@ -112,3 +114,5 @@ 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

@@ -1,136 +0,0 @@
-# <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

@@ -13,7 +13,19 @@ class visitor_adaptor_with_metadata
     void do_visit(const Ptr& ptr, const Fnc& fnc) const;
 };
 
-using json = nlohmann::json::with_base_class_t<visitor_adaptor_with_metadata>;
+using json = 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>,
+             visitor_adaptor_with_metadata
+             >;
 
 template <class Fnc>
 void visitor_adaptor_with_metadata::visit(const Fnc& fnc) const

docs/mkdocs/docs/examples/with_t.cpp

@@ -1,18 +0,0 @@
-#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

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

docs/mkdocs/docs/home/exceptions.md

@@ -804,6 +804,20 @@ 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

@@ -232,7 +232,6 @@ 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

@@ -127,6 +127,7 @@ 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)
     {
@@ -158,6 +159,7 @@ 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)
     {
@@ -322,7 +324,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                break;
+                throw_on_discarded(j, "CBOR");
         }
     }
 
@@ -382,6 +384,7 @@ 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)
     {
@@ -655,7 +658,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                break;
+                throw_on_discarded(j, "MessagePack");
         }
     }
 
@@ -668,6 +671,7 @@ 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,
@@ -901,7 +905,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                break;
+                throw_on_discarded(j, use_bjdata ? "BJData" : "UBJSON");
         }
     }
 
@@ -921,6 +925,15 @@ 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 //
     //////////
@@ -1172,6 +1185,7 @@ 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)
     {
@@ -1198,10 +1212,12 @@ 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;
@@ -1238,10 +1254,12 @@ 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;
@@ -1308,6 +1326,8 @@ 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

@@ -226,70 +226,6 @@ 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

@@ -21529,6 +21529,7 @@ 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)
     {
@@ -21560,6 +21561,7 @@ 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)
     {
@@ -21724,7 +21726,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                break;
+                throw_on_discarded(j, "CBOR");
         }
     }
 
@@ -21784,6 +21786,7 @@ 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)
     {
@@ -22057,7 +22060,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                break;
+                throw_on_discarded(j, "MessagePack");
         }
     }
 
@@ -22070,6 +22073,7 @@ 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,
@@ -22303,7 +22307,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                break;
+                throw_on_discarded(j, use_bjdata ? "BJData" : "UBJSON");
         }
     }
 
@@ -22323,6 +22327,15 @@ 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 //
     //////////
@@ -22574,6 +22587,7 @@ 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)
     {
@@ -22600,10 +22614,12 @@ 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;
@@ -22640,10 +22656,12 @@ 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;
@@ -22710,6 +22728,8 @@ 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)
     {
@@ -27348,70 +27368,6 @@ 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

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

@@ -163,7 +163,16 @@ void int_to_string(alt_string& target, std::size_t value)
     target = std::to_string(value).c_str();
 }
 
-using alt_json = nlohmann::json::with_string_t<alt_string>;
+using alt_json = nlohmann::basic_json <
+                 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

@@ -106,10 +106,59 @@ TEST_CASE("BJData")
     {
         SECTION("discarded")
         {
-            // discarded values are not serialized
+            // a discarded value cannot be serialized to BJData
             json const j = json::value_t::discarded;
-            const auto result = json::to_bjdata(j);
+            CHECK_THROWS_WITH_AS(json::to_bjdata(j), "[json.exception.type_error.321] cannot serialize discarded value to BJData", json::type_error&);
-            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

@@ -141,6 +141,54 @@ 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

@@ -30,10 +30,49 @@ TEST_CASE("CBOR")
     {
         SECTION("discarded")
         {
-            // discarded values are not serialized
+            // a discarded value cannot be serialized to CBOR
             json const j = json::value_t::discarded;
-            const auto result = json::to_cbor(j);
+            CHECK_THROWS_WITH_AS(json::to_cbor(j), "[json.exception.type_error.321] cannot serialize discarded value to CBOR", json::type_error&);
-            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

@@ -38,7 +38,20 @@ class json_metadata
 };
 
 template<class T>
-using json_with_metadata = nlohmann::json::with_base_class_t<json_metadata<T>>;
+using json_with_metadata =
+    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")
 {
@@ -255,7 +268,19 @@ class visitor_adaptor
     void do_visit(const Ptr& ptr, const Fnc& fnc) const;
 };
 
-using json_with_visitor_t = nlohmann::json::with_base_class_t<visitor_adaptor>;
+using json_with_visitor_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>,
+                            visitor_adaptor
+                            >;
 
 template <class Fnc>
 void visitor_adaptor::visit(const Fnc& fnc) const

tests/src/unit-msgpack.cpp

@@ -33,10 +33,49 @@ TEST_CASE("MessagePack")
     {
         SECTION("discarded")
         {
-            // discarded values are not serialized
+            // a discarded value cannot be serialized to MessagePack
             json const j = json::value_t::discarded;
-            const auto result = json::to_msgpack(j);
+            CHECK_THROWS_WITH_AS(json::to_msgpack(j), "[json.exception.type_error.321] cannot serialize discarded value to MessagePack", json::type_error&);
-            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

@@ -27,10 +27,59 @@ TEST_CASE("UBJSON")
     {
         SECTION("discarded")
         {
-            // discarded values are not serialized
+            // a discarded value cannot be serialized to UBJSON
             json const j = json::value_t::discarded;
-            const auto result = json::to_ubjson(j);
+            CHECK_THROWS_WITH_AS(json::to_ubjson(j), "[json.exception.type_error.321] cannot serialize discarded value to UBJSON", json::type_error&);
-            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")
@@ -2091,9 +2140,14 @@ 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};
-            std::vector<uint8_t> expected = {'[', '$', 'N', '#', 'i', 2};
+#if JSON_DIAGNOSTICS
-            CHECK(json::to_ubjson(j, true, true) == expected);
+            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
         }
     }
 }

tests/src/unit-udt.cpp

@@ -23,7 +23,6 @@ using nlohmann::json;
     using namespace nlohmann::literals; // NOLINT(google-build-using-namespace)
 #endif
 
-#include <deque>
 #include <map>
 #include <memory>
 #include <string>
@@ -685,7 +684,8 @@ static std::ostream& operator<<(std::ostream& os, small_pod l)
 TEST_CASE("custom serializer for pods" * doctest::test_suite("udt"))
 {
     using custom_json =
-        nlohmann::json::with_json_serializer_t<pod_serializer>;
+        nlohmann::basic_json<std::map, std::vector, std::string, bool,
+        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::json::with_json_serializer_t<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>;
 
 template <typename T, typename>
 struct another_adl_serializer
@@ -734,80 +734,6 @@ 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")