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>

.github/workflows/macos.yml

@@ -17,11 +17,11 @@ permissions:
   contents: read
 
 jobs:
-  macos-15:
-    runs-on: macos-15 # https://github.com/actions/runner-images/blob/main/images/macos/macos-15-Readme.md
+  macos-14:
+    runs-on: macos-14 # https://github.com/actions/runner-images/blob/main/images/macos/macos-14-Readme.md
     strategy:
       matrix:
-        xcode: ['16.0', '16.1', '16.2', '16.3', '16.4', '26.0.1', '26.1.1', '26.2', '26.3']
+        xcode: ['15.0.1', '15.1', '15.2', '15.3', '15.4']
     env:
       DEVELOPER_DIR: /Applications/Xcode_${{ matrix.xcode }}.app/Contents/Developer
 
@@ -36,11 +36,11 @@ jobs:
       - name: Test
         run: cd build ; ctest -j 10 --output-on-failure
 
-  macos-26:
-    runs-on: macos-26 # https://github.com/actions/runner-images/blob/main/images/macos/macos-26-arm64-Readme.md
+  macos-15:
+    runs-on: macos-15 # https://github.com/actions/runner-images/blob/main/images/macos/macos-15-Readme.md
     strategy:
       matrix:
-        xcode: ['26.4.1', '26.5', '26.6']
+        xcode: ['16.0', '16.1', '16.2', '16.3', '16.4', '26.0.1']
     env:
       DEVELOPER_DIR: /Applications/Xcode_${{ matrix.xcode }}.app/Contents/Developer
 

.github/workflows/ubuntu.yml

@@ -209,7 +209,7 @@ jobs:
     strategy:
       matrix:
         # older GCC docker images (4, 5, 6) fail to check out code
-        compiler: ['7', '8', '9', '10', '11', '12', '13', '14', '15', '16', 'latest']
+        compiler: ['7', '8', '9', '10', '11', '12', '13', '14', '15', 'latest']
     container: gcc:${{ matrix.compiler }}
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1

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`.
-- 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.
+  [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled) throws `type_error.316`.

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/community/quality_assurance.md

@@ -21,18 +21,17 @@ Note: Some modern features (like C++20 ranges or filesystem support) may be disa
       
         | Compiler                                     | Architecture | Operating System                  | CI        |
         |----------------------------------------------|--------------|-----------------------------------|-----------|
+        | AppleClang 15.0.0.15000040; Xcode 15.0.1     | arm64        | macOS 14.7.2 (Sonoma)             | GitHub    |
+        | AppleClang 15.0.0.15000100; Xcode 15.1       | arm64        | macOS 14.7.2 (Sonoma)             | GitHub    |
+        | AppleClang 15.0.0.15000100; Xcode 15.2       | arm64        | macOS 14.7.2 (Sonoma)             | GitHub    |
+        | AppleClang 15.0.0.15000309; Xcode 15.3       | arm64        | macOS 14.7.2 (Sonoma)             | GitHub    |
+        | AppleClang 15.0.0.15000309; Xcode 15.4       | arm64        | macOS 14.7.2 (Sonoma)             | GitHub    |
         | AppleClang 16.0.0.16000026; Xcode 16         | arm64        | macOS 15.2 (Sequoia)              | GitHub    |
         | AppleClang 16.0.0.16000026; Xcode 16.1       | arm64        | macOS 15.2 (Sequoia)              | GitHub    |
         | AppleClang 16.0.0.16000026; Xcode 16.2       | arm64        | macOS 15.2 (Sequoia)              | GitHub    |
         | AppleClang 17.0.0.17000013; Xcode 16.3       | arm64        | macOS 15.5 (Sequoia)              | GitHub    |
         | AppleClang 17.0.0.17000013; Xcode 16.4       | arm64        | macOS 15.5 (Sequoia)              | GitHub    |
         | AppleClang 17.0.0.17000319; Xcode 26.0.1     | arm64        | macOS 15.5 (Sequoia)              | GitHub    |
-        | AppleClang 17.0.0.17000404; Xcode 26.1.1     | arm64        | macOS 15.7.9 (Sequoia)            | GitHub    |
-        | AppleClang 17.0.0.17000603; Xcode 26.2       | arm64        | macOS 15.7.9 (Sequoia)            | GitHub    |
-        | AppleClang 17.0.0.17000604; Xcode 26.3       | arm64        | macOS 15.7.9 (Sequoia)            | GitHub    |
-        | AppleClang 21.0.0.21000099; Xcode 26.4.1     | arm64        | macOS 26.6.2 (Tahoe)              | GitHub    |
-        | AppleClang 21.0.0.21000101; Xcode 26.5       | arm64        | macOS 26.6.2 (Tahoe)              | GitHub    |
-        | AppleClang 21.0.0.21000101; Xcode 26.6       | arm64        | macOS 26.6.2 (Tahoe)              | GitHub    |
         | Clang 3.4.2                                  | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
         | Clang 3.5.2                                  | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
         | Clang 3.6.2                                  | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
@@ -90,7 +89,7 @@ Note: Some modern features (like C++20 ranges or filesystem support) may be disa
         | GNU 13.3.0                                   | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
         | GNU 14.2.0                                   | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
         | GNU 15.1.0                                   | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
-        | GNU 16.2.0                                   | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
+        | GNU 16.1.0                                   | x86_64       | Ubuntu 22.04.1 LTS                | GitHub    |
         | GNU 16.1.0                                   | arm64        | Ubuntu 24.04                      | GitHub    |
         | icpc (ICC) 2021.10.0 20230609                | x86_64       | Ubuntu 22.04 LTS                  | GitHub    |
         | icpx (Intel oneAPI DPC++/C++) 2025.3.2       | x86_64       | Ubuntu 24.04 LTS                  | GitHub    |

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 <
-             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
-             >;
+using json = nlohmann::json::with_base_class_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/features/binary_formats/cbor.md

@@ -131,7 +131,6 @@ The library maps CBOR types to JSON value types as follows:
 | Byte string            | binary          | 0x59       |
 | Byte string            | binary          | 0x5A       |
 | Byte string            | binary          | 0x5B       |
-| Byte string            | binary          | 0x5F       |
 | UTF-8 string           | string          | 0x60..0x77 |
 | UTF-8 string           | string          | 0x78       |
 | UTF-8 string           | string          | 0x79       |
@@ -157,9 +156,6 @@ The library maps CBOR types to JSON value types as follows:
 | Single-Precision Float | number_float    | 0xFA       |
 | Double-Precision Float | number_float    | 0xFB       |
 
-Indefinite-length UTF-8 strings (0x7F) and byte strings (0x5F) are supported. Each chunk must be a definite-length
-string of the same major type, as required by [RFC 8949, Section 3.2.3](https://www.rfc-editor.org/rfc/rfc8949.html#section-3.2.3).
-
 !!! warning "Incomplete mapping"
 
     The mapping is **incomplete** in the sense that not all CBOR types can be converted to a JSON value. The following CBOR types are not supported and will yield parse errors:

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/input/binary_reader.hpp

@@ -1072,20 +1072,6 @@ class binary_reader
         }
     }
 
-    /*!
-    @brief reports a nested indefinite-length CBOR string or byte array
-    @param[in] type_name  name of the rejected string type
-    @param[in] context  parsing context for the error message
-    @return whether the SAX consumer accepts the parse error
-    */
-    bool cbor_indefinite_string_error(const char* type_name, const char* context)
-    {
-        auto last_token = get_token_string();
-        return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                exception_message(concat("indefinite-length ", type_name,
-                                        " is not allowed inside indefinite-length ", type_name, "; last byte: 0x", last_token), context), nullptr));
-    }
-
     /*!
     @brief reads a definite-length CBOR string
 
@@ -1095,13 +1081,12 @@ class binary_reader
     into the same string.
 
     @param[out] result  string the bytes are appended to
-    @param[in] inside_indefinite  whether the bytes belong to an indefinite-length string
 
     @return whether string creation completed
 
     @pre @a current is not EOF
     */
-    bool get_cbor_string_chunk(string_t& result, const bool inside_indefinite)
+    bool get_cbor_string_chunk(string_t& result)
     {
         switch (current)
         {
@@ -1162,7 +1147,7 @@ class binary_reader
             {
                 auto last_token = get_token_string();
                 return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                        exception_message(concat("expected length specification (0x60-0x7B)", inside_indefinite ? "" : " or indefinite string type (0x7F)", "; last byte: 0x", last_token), "string"), nullptr));
+                                        exception_message(concat("expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x", last_token), "string"), nullptr));
             }
         }
     }
@@ -1180,9 +1165,13 @@ class binary_reader
     */
     bool get_cbor_string(string_t& result, const char* context = "string")
     {
-        // read chunks iteratively, but reject a second indefinite-length
-        // level as required by RFC 8949, Section 3.2.3
-        bool indefinite = false;
+        // number of indefinite-length strings that have been opened and not
+        // closed yet. RFC 8949, Section 3.2.3 does not permit nesting them,
+        // but this reader has always accepted it, so the open levels are
+        // counted instead of recursed through, which overflowed the stack for
+        // an input of repeated 0x7F bytes (see #5104). Every chunk is appended
+        // to the same result, so no per-level state is needed.
+        std::size_t open = 0;
 
         while (true)
         {
@@ -1193,28 +1182,29 @@ class binary_reader
 
             if (current == 0x7F) // UTF-8 string (indefinite length)
             {
-                if (JSON_HEDLEY_UNLIKELY(indefinite))
-                {
-                    return cbor_indefinite_string_error("string", "string");
-                }
-                indefinite = true;
+                ++open;
                 get();
                 continue;
             }
 
-            // a break marker closes the indefinite-length string; outside
-            // of one it falls through to the error below
-            if (indefinite && current == 0xFF)
+            // a break marker closes the innermost indefinite-length string;
+            // outside of one it is not a string and falls through to the error
+            if (open != 0 && current == 0xFF)
             {
-                return check_string_utf8(result, context);
+                if (--open == 0)
+                {
+                    return check_string_utf8(result, context);
+                }
+                get();
+                continue;
             }
 
-            if (JSON_HEDLEY_UNLIKELY(!get_cbor_string_chunk(result, indefinite)))
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_string_chunk(result)))
             {
                 return false;
             }
 
-            if (!indefinite)
+            if (open == 0)
             {
                 return check_string_utf8(result, context);
             }
@@ -1306,13 +1296,12 @@ class binary_reader
     read into the same byte array.
 
     @param[out] result  byte array the bytes are appended to
-    @param[in] inside_indefinite  whether the bytes belong to an indefinite-length string
 
     @return whether byte array creation completed
 
     @pre @a current is not EOF
     */
-    bool get_cbor_binary_chunk(binary_t& result, const bool inside_indefinite)
+    bool get_cbor_binary_chunk(binary_t& result)
     {
         switch (current)
         {
@@ -1377,7 +1366,7 @@ class binary_reader
             {
                 auto last_token = get_token_string();
                 return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                        exception_message(concat("expected length specification (0x40-0x5B)", inside_indefinite ? "" : " or indefinite binary array type (0x5F)", "; last byte: 0x", last_token), "binary"), nullptr));
+                                        exception_message(concat("expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x", last_token), "binary"), nullptr));
             }
         }
     }
@@ -1395,9 +1384,9 @@ class binary_reader
     */
     bool get_cbor_binary(binary_t& result)
     {
-        // read chunks iteratively, but reject a second indefinite-length
-        // level as required by RFC 8949, Section 3.2.3
-        bool indefinite = false;
+        // the open indefinite-length byte arrays are counted rather than
+        // recursed through, for the reason given in @ref get_cbor_string
+        std::size_t open = 0;
 
         while (true)
         {
@@ -1408,28 +1397,29 @@ class binary_reader
 
             if (current == 0x5F) // Binary data (indefinite length)
             {
-                if (JSON_HEDLEY_UNLIKELY(indefinite))
-                {
-                    return cbor_indefinite_string_error("binary array", "binary");
-                }
-                indefinite = true;
+                ++open;
                 get();
                 continue;
             }
 
-            // a break marker closes the indefinite-length string; outside
-            // of one it falls through to the error below
-            if (indefinite && current == 0xFF)
+            // a break marker closes the innermost indefinite-length byte
+            // array; outside of one it falls through to the error below
+            if (open != 0 && current == 0xFF)
             {
-                return true;
+                if (--open == 0)
+                {
+                    return true;
+                }
+                get();
+                continue;
             }
 
-            if (JSON_HEDLEY_UNLIKELY(!get_cbor_binary_chunk(result, indefinite)))
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_binary_chunk(result)))
             {
                 return false;
             }
 
-            if (!indefinite)
+            if (open == 0)
             {
                 return true;
             }

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 //
     ////////////////
@@ -612,210 +676,100 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         /// constructor for rvalue binary arrays (internal type)
         json_value(binary_t&& value) : binary(create<binary_t>(std::move(value))) {}
 
-private:
-        // raw, allocation-free transfer of m_data from src to dst: no
-        // set_parents()/assert_invariant() (the former is O(#children) per
-        // call under JSON_DIAGNOSTICS, which would make the walk below
-        // quadratic); dst takes ownership, src is left as value_t::null.
-        static void take(basic_json& dst, basic_json& src) noexcept
-        {
-            dst.m_data.m_type = src.m_data.m_type;
-            dst.m_data.m_value = src.m_data.m_value;
-            src.m_data.m_type = value_t::null;
-        }
-
-        // true if v is not an array/object, or is an already-empty one
-        static bool has_no_children(const basic_json& v) noexcept
-        {
-            switch (v.m_data.m_type)
-            {
-                case value_t::array:
-                    return v.m_data.m_value.array->empty();
-                case value_t::object:
-                    return v.m_data.m_value.object->empty();
-                default:
-                    return true;
-            }
-        }
-
-        static basic_json& last_child(basic_json& v)
-        {
-            if (v.m_data.m_type == value_t::array)
-            {
-                return v.m_data.m_value.array->back();
-            }
-            JSON_ASSERT(v.m_data.m_type == value_t::object);
-            return v.m_data.m_value.object->rbegin()->second;
-        }
-
-        // removes the last child of a non-empty array/object v; this never
-        // allocates, and since it is only ever called when that child is a
-        // scalar or an already-empty array/object, destroying it never
-        // recurses more than one level deep (see destroy() below)
-        static void pop_last_child(basic_json& v)
-        {
-            if (v.m_data.m_type == value_t::array)
-            {
-                v.m_data.m_value.array->pop_back();
-            }
-            else
-            {
-                JSON_ASSERT(v.m_data.m_type == value_t::object);
-                // erase() needs a forward iterator, so std::prev(end()) is
-                // used here rather than rbegin() (see last_child() above)
-                v.m_data.m_value.object->erase(std::prev(v.m_data.m_value.object->end()));
-            }
-        }
-
-        // deallocates the (already empty) array/object held by v; this is
-        // the same allocator-based free the old recursive implementation
-        // used, just factored out so every level of the walk in destroy()
-        // can share it
-        static void free_container(basic_json& v) noexcept
-        {
-            if (v.m_data.m_type == value_t::array)
-            {
-                JSON_ASSERT(v.m_data.m_value.array->empty());
-                AllocatorType<array_t> alloc;
-                std::allocator_traits<decltype(alloc)>::destroy(alloc, v.m_data.m_value.array);
-                std::allocator_traits<decltype(alloc)>::deallocate(alloc, v.m_data.m_value.array, 1);
-            }
-            else
-            {
-                JSON_ASSERT(v.m_data.m_type == value_t::object);
-                JSON_ASSERT(v.m_data.m_value.object->empty());
-                AllocatorType<object_t> alloc;
-                std::allocator_traits<decltype(alloc)>::destroy(alloc, v.m_data.m_value.object);
-                std::allocator_traits<decltype(alloc)>::deallocate(alloc, v.m_data.m_value.object, 1);
-            }
-            v.m_data.m_type = value_t::null; // avoid a double free if v is later destructed
-        }
-
-public:
-        void destroy_string() noexcept
-        {
-            if (string == nullptr)
-            {
-                // not initialized (e.g., due to exception in the ctor)
-                return;
-            }
-            AllocatorType<string_t> alloc;
-            std::allocator_traits<decltype(alloc)>::destroy(alloc, string);
-            std::allocator_traits<decltype(alloc)>::deallocate(alloc, string, 1);
-        }
-
-        void destroy_binary() noexcept
-        {
-            if (binary == nullptr)
-            {
-                // not initialized (e.g., due to exception in the ctor)
-                return;
-            }
-            AllocatorType<binary_t> alloc;
-            std::allocator_traits<decltype(alloc)>::destroy(alloc, binary);
-            std::allocator_traits<decltype(alloc)>::deallocate(alloc, binary, 1);
-        }
-
-        // t must be value_t::array or value_t::object
-        void destroy_container(value_t t) noexcept
+        void destroy(value_t t)
         {
             if (
                 (t == value_t::object && object == nullptr) ||
-                (t == value_t::array && array == nullptr)
+                (t == value_t::array && array == nullptr) ||
+                (t == value_t::string && string == nullptr) ||
+                (t == value_t::binary && binary == nullptr)
             )
             {
                 // not initialized (e.g., due to exception in the ctor)
                 return;
             }
-
-            // Destroy the tree without recursing per nesting level and
-            // without any heap allocation: a heap-allocated flattening
-            // stack (the previous implementation) can itself throw
-            // bad_alloc, which would escape this noexcept destructor and
-            // terminate the program (#5135).
-            //
-            // Instead, walk down the "last child" chain, reversing links
-            // as we go: cur is the container currently being emptied,
-            // and prev is its parent (value_t::null when there is none).
-            // Each parent's last child slot doubles as storage for that
-            // parent's own parent link while we are below it, so no
-            // extra memory is needed. We only ever remove a child once
-            // it is a scalar or an empty array/object, which neither
-            // allocates nor recurses more than one level deep.
-            //
-            // This json_value is not itself a basic_json, so the
-            // top-level container is first moved into a local stand-in
-            // ("cur"); a default-constructed basic_json has a null
-            // pointer in its m_value (see data::m_value's initializer),
-            // so swapping it with *this leaves this union's own pointer
-            // null, and it is never looked at or freed a second time.
-            basic_json cur;
-            cur.m_data.m_type = t;
-            using std::swap;
-            swap(cur.m_data.m_value, *this);
-
-            basic_json prev; // value_t::null: no parent
-
-            while (true)
+            if (t == value_t::array || t == value_t::object)
             {
-                if (has_no_children(cur))
+                // flatten the current json_value to a heap-allocated stack
+                std::vector<basic_json> stack;
+
+                // move the top-level items to stack
+                if (t == value_t::array)
                 {
-                    if (prev.m_data.m_type == value_t::null)
+                    stack.reserve(array->size());
+                    std::move(array->begin(), array->end(), std::back_inserter(stack));
+                }
+                else
+                {
+                    stack.reserve(object->size());
+                    for (auto&& it : *object)
                     {
-                        free_container(cur);
-                        return; // back at the top with nothing left to do
+                        stack.push_back(std::move(it.second));
+                    }
+                }
+
+                while (!stack.empty())
+                {
+                    // move the last item to a local variable to be processed
+                    basic_json current_item(std::move(stack.back()));
+                    stack.pop_back();
+
+                    // if current_item is array/object, move
+                    // its children to the stack to be processed later
+                    if (current_item.is_array())
+                    {
+                        std::move(current_item.m_data.m_value.array->begin(), current_item.m_data.m_value.array->end(), std::back_inserter(stack));
+
+                        current_item.m_data.m_value.array->clear();
+                    }
+                    else if (current_item.is_object())
+                    {
+                        for (auto&& it : *current_item.m_data.m_value.object)
+                        {
+                            stack.push_back(std::move(it.second));
+                        }
+
+                        current_item.m_data.m_value.object->clear();
                     }
 
-                    // ascend: detach the grandparent link from prev's
-                    // last slot, drop that (now null) slot, free cur
-                    // (it is empty), then move up one level
-                    basic_json gp;
-                    take(gp, last_child(prev));
-                    pop_last_child(prev);
-
-                    free_container(cur);
-
-                    take(cur, prev);
-                    take(prev, gp);
-                    continue;
+                    // it's now safe that current_item gets destructed
+                    // since it doesn't have any children
                 }
-
-                basic_json& cur_last_ref = last_child(cur);
-
-                if (has_no_children(cur_last_ref))
-                {
-                    // scalar, or already-empty array/object
-                    pop_last_child(cur);
-                    continue;
-                }
-
-                // descend into the non-empty last child, reversing the
-                // link: its slot takes over prev, and the child becomes
-                // the new cur
-                basic_json tmp;
-                take(tmp, cur_last_ref);
-                take(cur_last_ref, prev);
-                take(prev, cur);
-                take(cur, tmp);
             }
-        }
 
-        void destroy(value_t t)
-        {
             switch (t)
             {
-                case value_t::string:
-                    destroy_string();
+                case value_t::object:
+                {
+                    AllocatorType<object_t> alloc;
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, object);
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, object, 1);
                     break;
+                }
+
+                case value_t::array:
+                {
+                    AllocatorType<array_t> alloc;
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, array);
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, array, 1);
+                    break;
+                }
+
+                case value_t::string:
+                {
+                    AllocatorType<string_t> alloc;
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, string);
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, string, 1);
+                    break;
+                }
 
                 case value_t::binary:
-                    destroy_binary();
-                    break;
-
-                case value_t::object:
-                case value_t::array:
-                    destroy_container(t);
+                {
+                    AllocatorType<binary_t> alloc;
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, binary);
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, binary, 1);
                     break;
+                }
 
                 case value_t::null:
                 case value_t::boolean:
@@ -824,7 +778,9 @@ public:
                 case value_t::number_float:
                 case value_t::discarded:
                 default:
+                {
                     break;
+                }
             }
         }
     };

single_include/nlohmann/json.hpp

@@ -14789,20 +14789,6 @@ class binary_reader
         }
     }
 
-    /*!
-    @brief reports a nested indefinite-length CBOR string or byte array
-    @param[in] type_name  name of the rejected string type
-    @param[in] context  parsing context for the error message
-    @return whether the SAX consumer accepts the parse error
-    */
-    bool cbor_indefinite_string_error(const char* type_name, const char* context)
-    {
-        auto last_token = get_token_string();
-        return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                exception_message(concat("indefinite-length ", type_name,
-                                        " is not allowed inside indefinite-length ", type_name, "; last byte: 0x", last_token), context), nullptr));
-    }
-
     /*!
     @brief reads a definite-length CBOR string
 
@@ -14812,13 +14798,12 @@ class binary_reader
     into the same string.
 
     @param[out] result  string the bytes are appended to
-    @param[in] inside_indefinite  whether the bytes belong to an indefinite-length string
 
     @return whether string creation completed
 
     @pre @a current is not EOF
     */
-    bool get_cbor_string_chunk(string_t& result, const bool inside_indefinite)
+    bool get_cbor_string_chunk(string_t& result)
     {
         switch (current)
         {
@@ -14879,7 +14864,7 @@ class binary_reader
             {
                 auto last_token = get_token_string();
                 return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                        exception_message(concat("expected length specification (0x60-0x7B)", inside_indefinite ? "" : " or indefinite string type (0x7F)", "; last byte: 0x", last_token), "string"), nullptr));
+                                        exception_message(concat("expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x", last_token), "string"), nullptr));
             }
         }
     }
@@ -14897,9 +14882,13 @@ class binary_reader
     */
     bool get_cbor_string(string_t& result, const char* context = "string")
     {
-        // read chunks iteratively, but reject a second indefinite-length
-        // level as required by RFC 8949, Section 3.2.3
-        bool indefinite = false;
+        // number of indefinite-length strings that have been opened and not
+        // closed yet. RFC 8949, Section 3.2.3 does not permit nesting them,
+        // but this reader has always accepted it, so the open levels are
+        // counted instead of recursed through, which overflowed the stack for
+        // an input of repeated 0x7F bytes (see #5104). Every chunk is appended
+        // to the same result, so no per-level state is needed.
+        std::size_t open = 0;
 
         while (true)
         {
@@ -14910,28 +14899,29 @@ class binary_reader
 
             if (current == 0x7F) // UTF-8 string (indefinite length)
             {
-                if (JSON_HEDLEY_UNLIKELY(indefinite))
-                {
-                    return cbor_indefinite_string_error("string", "string");
-                }
-                indefinite = true;
+                ++open;
                 get();
                 continue;
             }
 
-            // a break marker closes the indefinite-length string; outside
-            // of one it falls through to the error below
-            if (indefinite && current == 0xFF)
+            // a break marker closes the innermost indefinite-length string;
+            // outside of one it is not a string and falls through to the error
+            if (open != 0 && current == 0xFF)
             {
-                return check_string_utf8(result, context);
+                if (--open == 0)
+                {
+                    return check_string_utf8(result, context);
+                }
+                get();
+                continue;
             }
 
-            if (JSON_HEDLEY_UNLIKELY(!get_cbor_string_chunk(result, indefinite)))
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_string_chunk(result)))
             {
                 return false;
             }
 
-            if (!indefinite)
+            if (open == 0)
             {
                 return check_string_utf8(result, context);
             }
@@ -15023,13 +15013,12 @@ class binary_reader
     read into the same byte array.
 
     @param[out] result  byte array the bytes are appended to
-    @param[in] inside_indefinite  whether the bytes belong to an indefinite-length string
 
     @return whether byte array creation completed
 
     @pre @a current is not EOF
     */
-    bool get_cbor_binary_chunk(binary_t& result, const bool inside_indefinite)
+    bool get_cbor_binary_chunk(binary_t& result)
     {
         switch (current)
         {
@@ -15094,7 +15083,7 @@ class binary_reader
             {
                 auto last_token = get_token_string();
                 return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                        exception_message(concat("expected length specification (0x40-0x5B)", inside_indefinite ? "" : " or indefinite binary array type (0x5F)", "; last byte: 0x", last_token), "binary"), nullptr));
+                                        exception_message(concat("expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x", last_token), "binary"), nullptr));
             }
         }
     }
@@ -15112,9 +15101,9 @@ class binary_reader
     */
     bool get_cbor_binary(binary_t& result)
     {
-        // read chunks iteratively, but reject a second indefinite-length
-        // level as required by RFC 8949, Section 3.2.3
-        bool indefinite = false;
+        // the open indefinite-length byte arrays are counted rather than
+        // recursed through, for the reason given in @ref get_cbor_string
+        std::size_t open = 0;
 
         while (true)
         {
@@ -15125,28 +15114,29 @@ class binary_reader
 
             if (current == 0x5F) // Binary data (indefinite length)
             {
-                if (JSON_HEDLEY_UNLIKELY(indefinite))
-                {
-                    return cbor_indefinite_string_error("binary array", "binary");
-                }
-                indefinite = true;
+                ++open;
                 get();
                 continue;
             }
 
-            // a break marker closes the indefinite-length string; outside
-            // of one it falls through to the error below
-            if (indefinite && current == 0xFF)
+            // a break marker closes the innermost indefinite-length byte
+            // array; outside of one it falls through to the error below
+            if (open != 0 && current == 0xFF)
             {
-                return true;
+                if (--open == 0)
+                {
+                    return true;
+                }
+                get();
+                continue;
             }
 
-            if (JSON_HEDLEY_UNLIKELY(!get_cbor_binary_chunk(result, indefinite)))
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_binary_chunk(result)))
             {
                 return false;
             }
 
-            if (!indefinite)
+            if (open == 0)
             {
                 return true;
             }
@@ -21597,7 +21587,6 @@ 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)
     {
@@ -21629,7 +21618,6 @@ 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)
     {
@@ -21794,7 +21782,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                throw_on_discarded(j, "CBOR");
+                break;
         }
     }
 
@@ -21854,7 +21842,6 @@ 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)
     {
@@ -22128,7 +22115,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                throw_on_discarded(j, "MessagePack");
+                break;
         }
     }
 
@@ -22141,7 +22128,6 @@ 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,
@@ -22375,7 +22361,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                throw_on_discarded(j, use_bjdata ? "BJData" : "UBJSON");
+                break;
         }
     }
 
@@ -22395,15 +22381,6 @@ 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 //
     //////////
@@ -22655,7 +22632,6 @@ 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)
     {
@@ -22682,12 +22658,10 @@ 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;
@@ -22724,12 +22698,10 @@ 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;
@@ -22796,8 +22768,6 @@ 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)
     {
@@ -27436,6 +27406,70 @@ 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 //
     ////////////////
@@ -27822,210 +27856,100 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         /// constructor for rvalue binary arrays (internal type)
         json_value(binary_t&& value) : binary(create<binary_t>(std::move(value))) {}
 
-private:
-        // raw, allocation-free transfer of m_data from src to dst: no
-        // set_parents()/assert_invariant() (the former is O(#children) per
-        // call under JSON_DIAGNOSTICS, which would make the walk below
-        // quadratic); dst takes ownership, src is left as value_t::null.
-        static void take(basic_json& dst, basic_json& src) noexcept
-        {
-            dst.m_data.m_type = src.m_data.m_type;
-            dst.m_data.m_value = src.m_data.m_value;
-            src.m_data.m_type = value_t::null;
-        }
-
-        // true if v is not an array/object, or is an already-empty one
-        static bool has_no_children(const basic_json& v) noexcept
-        {
-            switch (v.m_data.m_type)
-            {
-                case value_t::array:
-                    return v.m_data.m_value.array->empty();
-                case value_t::object:
-                    return v.m_data.m_value.object->empty();
-                default:
-                    return true;
-            }
-        }
-
-        static basic_json& last_child(basic_json& v)
-        {
-            if (v.m_data.m_type == value_t::array)
-            {
-                return v.m_data.m_value.array->back();
-            }
-            JSON_ASSERT(v.m_data.m_type == value_t::object);
-            return v.m_data.m_value.object->rbegin()->second;
-        }
-
-        // removes the last child of a non-empty array/object v; this never
-        // allocates, and since it is only ever called when that child is a
-        // scalar or an already-empty array/object, destroying it never
-        // recurses more than one level deep (see destroy() below)
-        static void pop_last_child(basic_json& v)
-        {
-            if (v.m_data.m_type == value_t::array)
-            {
-                v.m_data.m_value.array->pop_back();
-            }
-            else
-            {
-                JSON_ASSERT(v.m_data.m_type == value_t::object);
-                // erase() needs a forward iterator, so std::prev(end()) is
-                // used here rather than rbegin() (see last_child() above)
-                v.m_data.m_value.object->erase(std::prev(v.m_data.m_value.object->end()));
-            }
-        }
-
-        // deallocates the (already empty) array/object held by v; this is
-        // the same allocator-based free the old recursive implementation
-        // used, just factored out so every level of the walk in destroy()
-        // can share it
-        static void free_container(basic_json& v) noexcept
-        {
-            if (v.m_data.m_type == value_t::array)
-            {
-                JSON_ASSERT(v.m_data.m_value.array->empty());
-                AllocatorType<array_t> alloc;
-                std::allocator_traits<decltype(alloc)>::destroy(alloc, v.m_data.m_value.array);
-                std::allocator_traits<decltype(alloc)>::deallocate(alloc, v.m_data.m_value.array, 1);
-            }
-            else
-            {
-                JSON_ASSERT(v.m_data.m_type == value_t::object);
-                JSON_ASSERT(v.m_data.m_value.object->empty());
-                AllocatorType<object_t> alloc;
-                std::allocator_traits<decltype(alloc)>::destroy(alloc, v.m_data.m_value.object);
-                std::allocator_traits<decltype(alloc)>::deallocate(alloc, v.m_data.m_value.object, 1);
-            }
-            v.m_data.m_type = value_t::null; // avoid a double free if v is later destructed
-        }
-
-public:
-        void destroy_string() noexcept
-        {
-            if (string == nullptr)
-            {
-                // not initialized (e.g., due to exception in the ctor)
-                return;
-            }
-            AllocatorType<string_t> alloc;
-            std::allocator_traits<decltype(alloc)>::destroy(alloc, string);
-            std::allocator_traits<decltype(alloc)>::deallocate(alloc, string, 1);
-        }
-
-        void destroy_binary() noexcept
-        {
-            if (binary == nullptr)
-            {
-                // not initialized (e.g., due to exception in the ctor)
-                return;
-            }
-            AllocatorType<binary_t> alloc;
-            std::allocator_traits<decltype(alloc)>::destroy(alloc, binary);
-            std::allocator_traits<decltype(alloc)>::deallocate(alloc, binary, 1);
-        }
-
-        // t must be value_t::array or value_t::object
-        void destroy_container(value_t t) noexcept
+        void destroy(value_t t)
         {
             if (
                 (t == value_t::object && object == nullptr) ||
-                (t == value_t::array && array == nullptr)
+                (t == value_t::array && array == nullptr) ||
+                (t == value_t::string && string == nullptr) ||
+                (t == value_t::binary && binary == nullptr)
             )
             {
                 // not initialized (e.g., due to exception in the ctor)
                 return;
             }
-
-            // Destroy the tree without recursing per nesting level and
-            // without any heap allocation: a heap-allocated flattening
-            // stack (the previous implementation) can itself throw
-            // bad_alloc, which would escape this noexcept destructor and
-            // terminate the program (#5135).
-            //
-            // Instead, walk down the "last child" chain, reversing links
-            // as we go: cur is the container currently being emptied,
-            // and prev is its parent (value_t::null when there is none).
-            // Each parent's last child slot doubles as storage for that
-            // parent's own parent link while we are below it, so no
-            // extra memory is needed. We only ever remove a child once
-            // it is a scalar or an empty array/object, which neither
-            // allocates nor recurses more than one level deep.
-            //
-            // This json_value is not itself a basic_json, so the
-            // top-level container is first moved into a local stand-in
-            // ("cur"); a default-constructed basic_json has a null
-            // pointer in its m_value (see data::m_value's initializer),
-            // so swapping it with *this leaves this union's own pointer
-            // null, and it is never looked at or freed a second time.
-            basic_json cur;
-            cur.m_data.m_type = t;
-            using std::swap;
-            swap(cur.m_data.m_value, *this);
-
-            basic_json prev; // value_t::null: no parent
-
-            while (true)
+            if (t == value_t::array || t == value_t::object)
             {
-                if (has_no_children(cur))
+                // flatten the current json_value to a heap-allocated stack
+                std::vector<basic_json> stack;
+
+                // move the top-level items to stack
+                if (t == value_t::array)
                 {
-                    if (prev.m_data.m_type == value_t::null)
+                    stack.reserve(array->size());
+                    std::move(array->begin(), array->end(), std::back_inserter(stack));
+                }
+                else
+                {
+                    stack.reserve(object->size());
+                    for (auto&& it : *object)
                     {
-                        free_container(cur);
-                        return; // back at the top with nothing left to do
+                        stack.push_back(std::move(it.second));
+                    }
+                }
+
+                while (!stack.empty())
+                {
+                    // move the last item to a local variable to be processed
+                    basic_json current_item(std::move(stack.back()));
+                    stack.pop_back();
+
+                    // if current_item is array/object, move
+                    // its children to the stack to be processed later
+                    if (current_item.is_array())
+                    {
+                        std::move(current_item.m_data.m_value.array->begin(), current_item.m_data.m_value.array->end(), std::back_inserter(stack));
+
+                        current_item.m_data.m_value.array->clear();
+                    }
+                    else if (current_item.is_object())
+                    {
+                        for (auto&& it : *current_item.m_data.m_value.object)
+                        {
+                            stack.push_back(std::move(it.second));
+                        }
+
+                        current_item.m_data.m_value.object->clear();
                     }
 
-                    // ascend: detach the grandparent link from prev's
-                    // last slot, drop that (now null) slot, free cur
-                    // (it is empty), then move up one level
-                    basic_json gp;
-                    take(gp, last_child(prev));
-                    pop_last_child(prev);
-
-                    free_container(cur);
-
-                    take(cur, prev);
-                    take(prev, gp);
-                    continue;
+                    // it's now safe that current_item gets destructed
+                    // since it doesn't have any children
                 }
-
-                basic_json& cur_last_ref = last_child(cur);
-
-                if (has_no_children(cur_last_ref))
-                {
-                    // scalar, or already-empty array/object
-                    pop_last_child(cur);
-                    continue;
-                }
-
-                // descend into the non-empty last child, reversing the
-                // link: its slot takes over prev, and the child becomes
-                // the new cur
-                basic_json tmp;
-                take(tmp, cur_last_ref);
-                take(cur_last_ref, prev);
-                take(prev, cur);
-                take(cur, tmp);
             }
-        }
 
-        void destroy(value_t t)
-        {
             switch (t)
             {
-                case value_t::string:
-                    destroy_string();
+                case value_t::object:
+                {
+                    AllocatorType<object_t> alloc;
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, object);
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, object, 1);
                     break;
+                }
+
+                case value_t::array:
+                {
+                    AllocatorType<array_t> alloc;
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, array);
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, array, 1);
+                    break;
+                }
+
+                case value_t::string:
+                {
+                    AllocatorType<string_t> alloc;
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, string);
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, string, 1);
+                    break;
+                }
 
                 case value_t::binary:
-                    destroy_binary();
-                    break;
-
-                case value_t::object:
-                case value_t::array:
-                    destroy_container(t);
+                {
+                    AllocatorType<binary_t> alloc;
+                    std::allocator_traits<decltype(alloc)>::destroy(alloc, binary);
+                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, binary, 1);
                     break;
+                }
 
                 case value_t::null:
                 case value_t::boolean:
@@ -28034,7 +27958,9 @@ public:
                 case value_t::number_float:
                 case value_t::discarded:
                 default:
+                {
                     break;
+                }
             }
         }
     };

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,
-              std::vector,
-              std::string,
-              bool,
-              std::int64_t,
-              std::uint64_t,
-              double,
-              bad_allocator>;
+        using bad_json = nlohmann::json::with_allocator_t<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,
-          std::vector,
-          std::string,
-          bool,
-          std::int64_t,
-          std::uint64_t,
-          double,
-          my_allocator>;
+    using my_json = nlohmann::json::with_allocator_t<my_allocator>;
 
     SECTION("class json_value")
     {
@@ -593,102 +579,10 @@ TEST_CASE("bad my_allocator::construct")
 {
     SECTION("my_allocator::construct doesn't 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>;
+        using bad_alloc_json = nlohmann::json::with_allocator_t<allocator_no_forward>;
 
         bad_alloc_json j;
         j["test"] = bad_alloc_json::array_t();
         j["test"].push_back("should not leak");
     }
 }
-
-namespace
-{
-std::size_t counting_allocator_allocations = 0;
-std::size_t counting_allocator_deallocations = 0;
-
-template<class T>
-struct counting_allocator : std::allocator<T>
-{
-    using std::allocator<T>::allocator;
-
-    T* allocate(std::size_t n)
-    {
-        ++counting_allocator_allocations;
-        return std::allocator<T>::allocate(n);
-    }
-
-    void deallocate(T* p, std::size_t n)
-    {
-        ++counting_allocator_deallocations;
-        std::allocator<T>::deallocate(p, n);
-    }
-
-    template <class U>
-    struct rebind
-    {
-        using other = counting_allocator<U>;
-    };
-};
-} // namespace
-
-TEST_CASE("destructor performs no allocation, only deallocation")
-{
-    // see https://github.com/nlohmann/json/issues/4842 and
-    // https://github.com/nlohmann/json/issues/5135: destroying nested
-    // arrays/objects used to allocate a temporary stack (first with
-    // std::allocator, later - after #4842 - with the provided allocator).
-    // Since that stack could itself throw bad_alloc from inside the
-    // noexcept destructor (#5135), destroy() no longer allocates anything:
-    // it only ever frees what is already there.
-    using counting_json = nlohmann::basic_json<std::map,
-          std::vector,
-          std::string,
-          bool,
-          std::int64_t,
-          std::uint64_t,
-          double,
-          counting_allocator>;
-
-    SECTION("array")
-    {
-        auto* j = new counting_json({1, {2, {3, 4}}, 5}); // NOLINT(cppcoreguidelines-owning-memory)
-        const auto allocations_before = counting_allocator_allocations;
-        const auto deallocations_before = counting_allocator_deallocations;
-        delete j; // NOLINT(cppcoreguidelines-owning-memory)
-        CHECK(counting_allocator_allocations == allocations_before);
-        CHECK(counting_allocator_deallocations > deallocations_before);
-    }
-
-    SECTION("object")
-    {
-        auto* j = new counting_json({{"a", {{"b", {1, 2}}}}, {"c", 3}}); // NOLINT(cppcoreguidelines-owning-memory)
-        const auto allocations_before = counting_allocator_allocations;
-        const auto deallocations_before = counting_allocator_deallocations;
-        delete j; // NOLINT(cppcoreguidelines-owning-memory)
-        CHECK(counting_allocator_allocations == allocations_before);
-        CHECK(counting_allocator_deallocations > deallocations_before);
-    }
-
-    SECTION("mixed tree of empty/non-empty arrays and objects")
-    {
-        auto* j = new counting_json( // NOLINT(cppcoreguidelines-owning-memory)
-        {
-            {"empty_obj", counting_json::object()},
-            {"empty_arr", counting_json::array()},
-            {"nested", {{"a", counting_json::array({1, 2, counting_json::object()})}, {"b", 3}}},
-            {"tail", counting_json::array({counting_json::array({1}), 2, counting_json::array({3})})}
-        });
-        const auto allocations_before = counting_allocator_allocations;
-        const auto deallocations_before = counting_allocator_deallocations;
-        delete j; // NOLINT(cppcoreguidelines-owning-memory)
-        CHECK(counting_allocator_allocations == allocations_before);
-        CHECK(counting_allocator_deallocations > deallocations_before);
-    }
-}

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 <
-                 std::map,
-                 std::vector,
-                 alt_string,
-                 bool,
-                 std::int64_t,
-                 std::uint64_t,
-                 double,
-                 std::allocator,
-                 nlohmann::adl_serializer >;
+using alt_json = nlohmann::json::with_string_t<alt_string>;
 
 bool operator<(const char* op1, const alt_string& op2) noexcept
 {

tests/src/unit-bjdata.cpp

@@ -114,59 +114,10 @@ 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&);
-        }
-
-        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
-            }
+            const auto result = json::to_bjdata(j);
+            CHECK(result.empty());
         }
 
         SECTION("null")

tests/src/unit-bson.cpp

@@ -149,54 +149,6 @@ 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

@@ -38,49 +38,10 @@ 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&);
-        }
-
-        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
-            }
+            const auto result = json::to_cbor(j);
+            CHECK(result.empty());
         }
 
         SECTION("NaN")
@@ -1699,7 +1660,7 @@ TEST_CASE("CBOR")
             CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xA1, 0x61, 0X61})), "[json.exception.parse_error.110] parse error at byte 4: syntax error while parsing CBOR value: unexpected end of input", json::parse_error&);
             CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xBF, 0x61, 0X61})), "[json.exception.parse_error.110] parse error at byte 4: syntax error while parsing CBOR value: unexpected end of input", json::parse_error&);
             CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F})), "[json.exception.parse_error.110] parse error at byte 2: syntax error while parsing CBOR binary: unexpected end of input", json::parse_error&);
-            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B); last byte: 0x00", json::parse_error&);
+            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x00", json::parse_error&);
             CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x41})), "[json.exception.parse_error.110] parse error at byte 2: syntax error while parsing CBOR binary: unexpected end of input", json::parse_error&);
 
             CHECK(json::from_cbor(std::vector<uint8_t>({0x18}), true, false).is_discarded());
@@ -2305,21 +2266,22 @@ TEST_CASE("CBOR indefinite-length strings do not recurse per chunk")
 {
     // Reading an indefinite-length string or byte array used to call itself
     // once per chunk, so a payload of repeated 0x7F (or 0x5F) bytes exhausted
-    // the call stack before any of the input was rejected. Nested indefinite
-    // chunks are now rejected at the second byte, without recursing.
+    // the call stack before any of the input was rejected. The open levels are
+    // counted now, and the levels below prove the reader still reads the same
+    // values and reports the same errors at the same byte offsets.
     json _;
 
-    SECTION("nested levels are rejected, not crashed on")
+    SECTION("many open levels are reported, not crashed on")
     {
         const std::vector<uint8_t> input(200000, 0x7F);
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: indefinite-length string is not allowed inside indefinite-length string; last byte: 0x7F", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.110] parse error at byte 200001: syntax error while parsing CBOR string: unexpected end of input", json::parse_error&);
         CHECK(json::from_cbor(input, true, false).is_discarded());
     }
 
-    SECTION("nested levels are rejected, not crashed on (binary)")
+    SECTION("many open levels are reported, not crashed on (binary)")
     {
         const std::vector<uint8_t> input(200000, 0x5F);
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: indefinite-length binary array is not allowed inside indefinite-length binary array; last byte: 0x5F", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.110] parse error at byte 200001: syntax error while parsing CBOR binary: unexpected end of input", json::parse_error&);
         CHECK(json::from_cbor(input, true, false).is_discarded());
     }
 
@@ -2327,22 +2289,22 @@ TEST_CASE("CBOR indefinite-length strings do not recurse per chunk")
     {
         CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0xFF})) == json(""));
         CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x61, 0x61, 0xFF})) == json("a"));
-        // empty and nonempty definite-length chunks concatenate in order
-        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x61, 'a', 0x60, 0x61, 'b', 0x61, 'c', 0xFF})) == json("abc"));
+        // nested indefinite-length strings are concatenated across levels
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x61, 0x61, 0xFF, 0x61, 0x62, 0xFF})) == json("ab"));
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x7F, 0x61, 0x7A, 0xFF, 0xFF, 0xFF})) == json("z"));
         CHECK(json::from_cbor(std::vector<uint8_t>({0xA1, 0x7F, 0x61, 0x61, 0xFF, 0x01})) == json({{"a", 1}}));
     }
 
     SECTION("chunks are still concatenated (binary)")
     {
         CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x41, 0x61, 0xFF})) == json::binary({0x61}));
-        CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0xFF})) == json::binary({}));
-        CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x41, 0x61, 0x40, 0x41, 0x62, 0x41, 0x63, 0xFF})) == json::binary({0x61, 0x62, 0x63}));
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x5F, 0x41, 0x61, 0xFF, 0x41, 0x62, 0xFF})) == json::binary({0x61, 0x62}));
     }
 
     SECTION("a chunk that is not a string is still rejected")
     {
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x7F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B); last byte: 0x00", json::parse_error&);
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B); last byte: 0x00", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x00})), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x00", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x00", json::parse_error&);
     }
 
     SECTION("a break marker outside an indefinite-length string is not a string")
@@ -2895,17 +2857,9 @@ TEST_CASE("examples from RFC 8949 Appendix A")
     {
         const auto packed = utils::read_binary_file(TEST_DATA_DIRECTORY "/binary_data/cbor_binary.cbor");
         json j;
-        // the fixture's tail contains nested indefinite-length byte strings.
-        CHECK_THROWS_WITH_AS(j = json::from_cbor(packed), "[json.exception.parse_error.113] parse error at byte 513: syntax error while parsing CBOR binary: indefinite-length binary array is not allowed inside indefinite-length binary array; last byte: 0x5F", json::parse_error&);
+        CHECK_NOTHROW(j = json::from_cbor(packed));
 
-        // keep the byte-for-byte decoding check for its valid prefix: the first
-        // 512 encoded bytes contain 468 payload bytes in definite-length chunks.
-        auto valid_prefix = packed;
-        valid_prefix.resize(512);
-        valid_prefix.push_back(0xFF);
-        auto expected = utils::read_binary_file(TEST_DATA_DIRECTORY "/binary_data/cbor_binary.out");
-        expected.resize(468);
-        CHECK_NOTHROW(j = json::from_cbor(valid_prefix));
+        const auto expected = utils::read_binary_file(TEST_DATA_DIRECTORY "/binary_data/cbor_binary.out");
         CHECK(j == json::binary(expected));
 
         // 0xd8

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

@@ -38,20 +38,7 @@ class json_metadata
 };
 
 template<class 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>
-    >;
+using json_with_metadata = nlohmann::json::with_base_class_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 <
-                            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
-                            >;
+using json_with_visitor_t = nlohmann::json::with_base_class_t<visitor_adaptor>;
 
 template <class Fnc>
 void visitor_adaptor::visit(const Fnc& fnc) const

tests/src/unit-msgpack.cpp

@@ -41,49 +41,10 @@ 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&);
-        }
-
-        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
-            }
+            const auto result = json::to_msgpack(j);
+            CHECK(result.empty());
         }
 
         SECTION("null")

tests/src/unit-regression2.cpp

@@ -40,9 +40,7 @@ using ordered_json = nlohmann::ordered_json;
 #endif
 
 #include <cstdio>
-#include <cstdlib>
 #include <list>
-#include <new>
 #include <tuple>
 #include <type_traits>
 #include <utility>
@@ -109,84 +107,6 @@ DOCTEST_CLANG_SUPPRESS_WARNING("-Wexit-time-destructors")
 
 using float_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, float>;
 
-#if (defined(__cpp_exceptions) || defined(__EXCEPTIONS) || defined(_CPPUNWIND)) && !defined(JSON_NOEXCEPTION)
-namespace
-{
-// An allocator whose allocate() can be told to fail on demand, so tests can
-// check that ~basic_json() tolerates - in fact, after #5135, never even
-// triggers - an allocation failure. This replaces an earlier version of
-// this test that overrode the process-wide ::operator new/::operator
-// delete, which affected every allocation in the whole unit-regression2
-// binary rather than just the values under test.
-std::size_t failing_allocator_allocations = 0;
-std::size_t failing_allocator_deallocations = 0;
-bool fail_next_allocation = false;
-
-template<class T>
-struct failing_allocator : std::allocator<T>
-{
-    using std::allocator<T>::allocator;
-
-    failing_allocator() noexcept = default;
-    template<class U>
-    failing_allocator(const failing_allocator<U>& /*unused*/) noexcept {} // NOLINT(google-explicit-constructor)
-
-    T* allocate(std::size_t n)
-    {
-        if (fail_next_allocation)
-        {
-            fail_next_allocation = false;
-            throw std::bad_alloc();
-        }
-        ++failing_allocator_allocations;
-        return std::allocator<T>::allocate(n);
-    }
-
-    void deallocate(T* p, std::size_t n)
-    {
-        ++failing_allocator_deallocations;
-        std::allocator<T>::deallocate(p, n);
-    }
-
-    template<class U>
-    struct rebind
-    {
-        using other = failing_allocator<U>;
-    };
-};
-
-using failing_json = nlohmann::basic_json<std::map, std::vector, std::string, bool,
-      std::int64_t, std::uint64_t, double, failing_allocator>;
-using failing_ordered_json = nlohmann::basic_json<nlohmann::ordered_map, std::vector, std::string, bool,
-      std::int64_t, std::uint64_t, double, failing_allocator>;
-
-// builds `depth` levels of nesting around a scalar, iteratively (never
-// recursing: each wrap only moves the previous, already-built value, which
-// is O(1)), each level an array or an object depending on `nest_objects`
-template<class BasicJsonType>
-BasicJsonType make_deep_nest(std::size_t depth, bool nest_objects)
-{
-    BasicJsonType v = 0;
-    for (std::size_t i = 0; i < depth; ++i)
-    {
-        if (nest_objects)
-        {
-            BasicJsonType wrapper = BasicJsonType::object();
-            wrapper["x"] = std::move(v);
-            v = std::move(wrapper);
-        }
-        else
-        {
-            BasicJsonType wrapper = BasicJsonType::array();
-            wrapper.push_back(std::move(v));
-            v = std::move(wrapper);
-        }
-    }
-    return v;
-}
-} // namespace
-#endif
-
 /////////////////////////////////////////////////////////////////////
 // for #1647
 /////////////////////////////////////////////////////////////////////
@@ -1020,211 +940,4 @@ TEST_CASE("regression test - excessive binary container size honors allow_except
     CHECK(json::from_cbor(std::vector<std::uint8_t> {0x9b, 0, 0, 0, 0, 0, 0, 0, 0x02}, true, false).is_discarded());
 }
 
-#if (defined(__cpp_exceptions) || defined(__EXCEPTIONS) || defined(_CPPUNWIND)) && !defined(JSON_NOEXCEPTION)
-TEST_CASE("regression test #5135 - destructor never allocates, even under memory pressure")
-{
-    // Before the fix, ~basic_json() flattened a nested array/object into a
-    // heap-allocated std::vector to avoid recursing; that allocation could
-    // itself throw bad_alloc, which escapes a noexcept destructor and
-    // terminates the program. destroy() no longer allocates anything, so
-    // none of the sections below ever observe fail_next_allocation being
-    // consumed: CHECK(fail_next_allocation) confirms it was never touched.
-
-    SECTION("the original report: a small, mixed array/object nest")
-    {
-        failing_allocator_allocations = 0;
-        failing_allocator_deallocations = 0;
-        {
-            failing_json j = failing_json::array(
-            {
-                failing_json::array({1, 2}),
-                failing_json::object({{"key", failing_json::array({3})}})
-            });
-            fail_next_allocation = true;
-        } // j is destroyed here, with every further allocation set to fail
-
-        CHECK(fail_next_allocation);
-        fail_next_allocation = false;
-        CHECK(failing_allocator_deallocations > 0);
-    }
-
-    SECTION("100000-deep nested array")
-    {
-        std::size_t allocations_before = 0;
-        {
-            failing_json j = make_deep_nest<failing_json>(100000, false);
-            allocations_before = failing_allocator_allocations;
-            fail_next_allocation = true;
-        }
-
-        CHECK(fail_next_allocation);
-        fail_next_allocation = false;
-        CHECK(failing_allocator_allocations == allocations_before);
-    }
-
-    SECTION("100000-deep nested object")
-    {
-        std::size_t allocations_before = 0;
-        {
-            failing_json j = make_deep_nest<failing_json>(100000, true);
-            allocations_before = failing_allocator_allocations;
-            fail_next_allocation = true;
-        }
-
-        CHECK(fail_next_allocation);
-        fail_next_allocation = false;
-        CHECK(failing_allocator_allocations == allocations_before);
-    }
-
-    SECTION("100000-deep nested ordered_json")
-    {
-        std::size_t allocations_before = 0;
-        {
-            failing_ordered_json j = make_deep_nest<failing_ordered_json>(100000, true);
-            allocations_before = failing_allocator_allocations;
-            fail_next_allocation = true;
-        }
-
-        CHECK(fail_next_allocation);
-        fail_next_allocation = false;
-        CHECK(failing_allocator_allocations == allocations_before);
-    }
-
-    SECTION("wide and deep: 1000 arrays of 1000 elements, each a small nested object")
-    {
-        std::size_t allocations_before = 0;
-        {
-            failing_json wide = failing_json::array();
-            for (std::size_t i = 0; i < 1000; ++i)
-            {
-                failing_json inner = failing_json::array();
-                for (std::size_t k = 0; k < 1000; ++k)
-                {
-                    inner.push_back(failing_json::object({{"a", 1}, {"b", failing_json::array({1, 2, 3})}}));
-                }
-                wide.push_back(std::move(inner));
-            }
-
-            allocations_before = failing_allocator_allocations;
-            fail_next_allocation = true;
-        }
-
-        CHECK(fail_next_allocation);
-        fail_next_allocation = false;
-        CHECK(failing_allocator_allocations == allocations_before);
-    }
-}
-#endif
-
-namespace
-{
-// a single-element chain of `depth` arrays, built iteratively (never
-// recursing: each wrap only moves the previous, already-built value)
-template<class BasicJsonType>
-BasicJsonType make_single_chain(std::size_t depth)
-{
-    BasicJsonType v = 1;
-    for (std::size_t i = 0; i < depth; ++i)
-    {
-        BasicJsonType wrapper = BasicJsonType::array();
-        wrapper.push_back(std::move(v));
-        v = std::move(wrapper);
-    }
-    return v;
-}
-
-// copies value first, to make sure nothing was corrupted by building it,
-// then lets both the copy and the original destruct via normal scope exit
-template<class BasicJsonType>
-void check_destroy_edge_case(const BasicJsonType& value)
-{
-    const BasicJsonType copy = value;
-    CHECK(copy == value);
-}
-} // namespace
-
-TEST_CASE_TEMPLATE("regression test #5135 - destroy() edge cases", BasicJsonType, json, ordered_json)
-{
-    using binary_t = typename BasicJsonType::binary_t;
-
-    SECTION("mix of empty objects, empty arrays, non-empty containers, and scalars")
-    {
-        BasicJsonType root = BasicJsonType::array();
-        root.push_back(BasicJsonType::object());
-        root.push_back(BasicJsonType::array());
-        root.push_back(BasicJsonType::object({{"k", 1}}));
-        root.push_back(BasicJsonType::array({1, 2, 3}));
-        root.push_back(nullptr);
-        root.push_back(true);
-        root.push_back(42);
-        root.push_back(3.14);
-        root.push_back("a string");
-        root.push_back(BasicJsonType(binary_t({1, 2, 3})));
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("container child in first position only")
-    {
-        BasicJsonType root = BasicJsonType::array({BasicJsonType::array({1, 2}), 3, 4, 5});
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("container child in last position only")
-    {
-        BasicJsonType root = BasicJsonType::array({1, 2, 3, BasicJsonType::array({4, 5})});
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("container children in first and last position")
-    {
-        BasicJsonType root = BasicJsonType::array({BasicJsonType::array({1}), 2, 3, BasicJsonType::array({4})});
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("single-element chain, 1000 levels deep")
-    {
-        BasicJsonType root = make_single_chain<BasicJsonType>(1000);
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("top-level empty array")
-    {
-        BasicJsonType root = BasicJsonType::array();
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("top-level empty object")
-    {
-        BasicJsonType root = BasicJsonType::object();
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("object whose last child is a non-empty array whose last child is an empty object")
-    {
-        BasicJsonType inner_array = BasicJsonType::array({1, 2, BasicJsonType::object()});
-        BasicJsonType root = BasicJsonType::object({{"a", 1}, {"b", inner_array}});
-        check_destroy_edge_case(root);
-    }
-
-    SECTION("destruction via erase() on a deeply nested child")
-    {
-        BasicJsonType root = BasicJsonType::array();
-        root.push_back(make_single_chain<BasicJsonType>(500));
-        root.push_back(BasicJsonType::object({{"k", BasicJsonType::array({1, 2, 3})}}));
-        // erase() must destroy the removed subtree without recursing or
-        // allocating beyond what erase() itself needs
-        root.erase(0);
-        CAPTURE(root.size())
-        CHECK(root.size() == 1);
-    }
-
-    SECTION("destruction via assignment on a deep tree")
-    {
-        BasicJsonType root = make_single_chain<BasicJsonType>(2000);
-        // assigning a new value destroys the old one in place
-        root = nullptr;
-        CHECK(root.is_null());
-    }
-}
-
 DOCTEST_CLANG_SUPPRESS_WARNING_POP

tests/src/unit-regression3.cpp

@@ -920,12 +920,4 @@ TEST_CASE("regression test #5476 - array type without reserve()")
     }
 }
 
-TEST_CASE("issue #5317 - nested indefinite-length CBOR string chunks are rejected")
-{
-    json _;
-    CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<std::uint8_t>({0x7F, 0x7F, 0x61, 0x61, 0xFF, 0xFF})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: indefinite-length string is not allowed inside indefinite-length string; last byte: 0x7F", json::parse_error&);
-    CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<std::uint8_t>({0x5F, 0x5F, 0x41, 0x61, 0xFF, 0xFF})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: indefinite-length binary array is not allowed inside indefinite-length binary array; last byte: 0x5F", json::parse_error&);
-    CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<std::uint8_t>({0xA1, 0x7F, 0x7F, 0xFF, 0xFF, 0x01})), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR string: indefinite-length string is not allowed inside indefinite-length string; last byte: 0x7F", json::parse_error&);
-}
-
 DOCTEST_CLANG_SUPPRESS_WARNING_POP

tests/src/unit-ubjson.cpp

@@ -35,59 +35,10 @@ 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&);
-        }
-
-        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
-            }
+            const auto result = json::to_ubjson(j);
+            CHECK(result.empty());
         }
 
         SECTION("null")
@@ -2148,14 +2099,9 @@ 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
-            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
+            std::vector<uint8_t> expected = {'[', '$', 'N', '#', 'i', 2};
+            CHECK(json::to_ubjson(j, true, true) == expected);
         }
     }
 }

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,
-        std::int64_t, std::uint64_t, double, std::allocator, pod_serializer>;
+        nlohmann::json::with_json_serializer_t<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")