Merge remote-tracking branch 'origin/develop' into claude/fix-issue-3989-db7e45

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

.github/CONTRIBUTING.md

@@ -111,6 +111,8 @@ context which existing file needs to be extended, and only very few cases requir
 When fixing a bug, edit `unit-regression3.cpp` and add a section referencing the fixed issue.
 `unit-regression2.cpp` holds the older tests; the two files exist because a single one grew large enough for the
 MinGW linker to fail relocating it, so please keep adding to the smaller file rather than growing the larger one.
+Tests that call `sax_parse` go into `unit-sax_parse.cpp` rather than into a large test file: every call instantiates
+the parser and the binary reader that recover from errors, which grows a test file considerably.
 
 #### Exceptions
 

.github/workflows/windows.yml

@@ -166,6 +166,13 @@ jobs:
       # link, but the binaries clang 11.0.1 and clang 18.1.8 then produce crash
       # before doctest prints its first line - 39 of 102 tests on clang 18.
       # Keep the objects small by splitting the test files instead.
+      # Objects with more than 65535 sections fail to link with "relocation
+      # truncated to fit: IMAGE_REL_AMD64_REL32 against `.rdata'": the MinGW
+      # linker stores the section an associative COMDAT section belongs to in
+      # 16 bits (x_associated in binutils' include/coff/internal.h), so it
+      # discards the jump tables of inline functions together with the wrong
+      # function. Each basic_json specialization a test file instantiates adds
+      # many sections, so test a second one in a file of its own.
       - name: Run CMake
         run: cmake -S . -B build ^
              -DCMAKE_CXX_COMPILER="C:/Program Files/LLVM/bin/clang++.exe" ^

README.md

@@ -494,7 +494,7 @@ bool key(string_t& val);
 bool parse_error(std::size_t position, const std::string& last_token, const detail::exception& ex);
 ```
 
-The return value of each function determines whether parsing should proceed.
+The return value of each function determines whether parsing should proceed. For `parse_error`, returning `true` [recovers from the error](https://json.nlohmann.me/features/parsing/error_recovery/): the parser repairs the input and continues.
 
 To implement your own SAX handler, proceed as follows:
 
@@ -502,7 +502,7 @@ To implement your own SAX handler, proceed as follows:
 2. Create an object of your SAX interface class, e.g. `my_sax`.
 3. Call `bool json::sax_parse(input, &my_sax)`; where the first parameter can be any input like a string or an input stream and the second parameter is a pointer to your SAX interface.
 
-Note the `sax_parse` function only returns a `bool` indicating the result of the last executed SAX event. It does not return a  `json` value - it is up to you to decide what to do with the SAX events. Furthermore, no exceptions are thrown in case of a parse error -- it is up to you what to do with the exception object passed to your `parse_error` implementation. Internally, the SAX interface is used for the DOM parser (class `json_sax_dom_parser`) as well as the acceptor (`json_sax_acceptor`), see file [`json_sax.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/input/json_sax.hpp).
+Note the `sax_parse` function only returns a `bool` indicating whether the input was parsed without errors and no SAX event returned `false`. It does not return a  `json` value - it is up to you to decide what to do with the SAX events. Furthermore, no exceptions are thrown in case of a parse error -- it is up to you what to do with the exception object passed to your `parse_error` implementation. Internally, the SAX interface is used for the DOM parser (class `json_sax_dom_parser`) as well as the acceptor (`json_sax_acceptor`), see file [`json_sax.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/input/json_sax.hpp).
 
 ### STL-like access
 

docs/mkdocs/docs/api/basic_json/binary_t.md

@@ -64,18 +64,7 @@ values of that type directly to a `basic_json` instance, and they will automatic
 rather than arrays:
 
 ```cpp
-using custom_json = nlohmann::basic_json<
-    nlohmann::ordered_map,  // ObjectType
-    std::vector,            // ArrayType
-    std::string,            // StringType
-    bool,                   // BooleanType
-    std::int64_t,           // NumberIntegerType
-    std::uint64_t,          // NumberUnsignedType
-    double,                 // NumberFloatType
-    std::allocator,         // AllocatorType
-    nlohmann::adl_serializer,
-    std::vector<std::byte>  // Custom BinaryType
->;
+using custom_json = nlohmann::ordered_json::with_binary_t<std::vector<std::byte>>;
 
 std::vector<std::byte> data{std::byte{1}, std::byte{2}, std::byte{3}};
 custom_json j = data;  // Creates a binary value, not an array

docs/mkdocs/docs/api/basic_json/object_t.md

@@ -26,7 +26,8 @@ To store objects in C++, a type is defined by the template parameters described
 
 `StringType`
 :   the type of the keys or names (e.g., `std::string`). The comparison function `std::less<StringType>` is used to
-    order elements inside the container.
+    order elements inside the container. `object_t::key_type` must be implicitly convertible to `string_t` (required by the
+    binary formats).
 
 `AllocatorType`
 :   the allocator to use for objects (e.g., `std::allocator`)

docs/mkdocs/docs/api/basic_json/sax_parse.md

@@ -96,7 +96,9 @@ The SAX event lister must follow the interface of [`json_sax`](../json_sax/index
 
 ## Return value
 
-return value of the last processed SAX event
+`#!cpp true` if the input was parsed without errors and no SAX event returned `#!cpp false`; `#!cpp false` otherwise.
+In particular, the result is `#!cpp false` for input with errors, even if the SAX parser recovered from all of them
+(see [error recovery](../../features/parsing/error_recovery.md)).
 
 ## Exception safety
 
@@ -145,6 +147,7 @@ A UTF-8 byte order mark is silently ignored.
 - Added `ignore_trailing_commas` in version 3.13.0.
 - Added `tag_handler` in version 3.13.0.
 - Extended container support (1) to include types with lvalue-only ADL `begin`/`end` (matching `std::begin`/`std::end` semantics) in version 3.13.0.
+- Recovering from parse errors (see [`parse_error`](../json_sax/parse_error.md)) added in version 3.13.0.
 - Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.
 - `JSON_PRECISE_STREAM_POSITION` added in version 3.13.0 to optionally leave a `#!cpp std::istream` positioned right
   after the parsed value when `strict` is `#!cpp false`.

docs/mkdocs/docs/api/json_sax/index.md

@@ -7,7 +7,8 @@ struct json_sax;
 
 This class describes the SAX interface used by [sax_parse](../basic_json/sax_parse.md). Each function is called in
 different situations while the input is parsed. The boolean return value informs the parser whether to continue
-processing the input.
+processing the input; for [`parse_error`](parse_error.md), it decides whether to
+[recover from the error](../../features/parsing/error_recovery.md).
 
 For instance, parsing the JSON text `{"a": [1, true]}` triggers the following callbacks, in order:
 

docs/mkdocs/docs/api/json_sax/parse_error.md

@@ -21,11 +21,18 @@ A parse error occurred.
 
 ## Return value
 
-Whether parsing should proceed (**must return `#!cpp false`**).
+Whether to recover from the error:
+
+- `#!cpp false` stops parsing.
+- `#!cpp true` recovers from the error: the error is repaired and parsing continues. If that is not possible, which
+  happens in the binary formats when the end of the item with the error is unknown, the value read so far is completed
+  and parsing stops. See [error recovery](../../features/parsing/error_recovery.md) for how errors are repaired.
+
+Either way, [`sax_parse`](../basic_json/sax_parse.md) returns `#!cpp false`.
 
 ## Examples
 
-??? example
+??? example "Example: (1) the SAX interface"
 
     The example below shows how the SAX interface is used.
 
@@ -39,12 +46,29 @@ Whether parsing should proceed (**must return `#!cpp false`**).
     --8<-- "examples/sax_parse.output"
     ```
 
+??? example "Example: (2) recovering from errors"
+
+    The example below shows how a SAX parser recovers from errors.
+
+    ```cpp
+    --8<-- "examples/sax_parse__error_recovery.cpp"
+    ```
+    
+    Output:
+    
+    ```
+    --8<-- "examples/sax_parse__error_recovery.output"
+    ```
+
 ## See also
 
 - [sax_parse](../basic_json/sax_parse.md) - SAX parser
 - [Parsing and Exceptions](../../features/parsing/parse_exceptions.md) - the article on handling parse errors without
   exceptions
+- [Error Recovery](../../features/parsing/error_recovery.md) - the article on recovering from parse errors
 
 ## Version history
 
 - Added in version 3.2.0.
+- Returning `#!cpp true` recovers from the error since version 3.13.0; before, parsing stopped, but the result of
+  [`sax_parse`](../basic_json/sax_parse.md) could be wrong.

docs/mkdocs/docs/api/macros/json_use_implicit_conversions.md

@@ -67,7 +67,7 @@ By default, implicit conversions are enabled.
     `JSON_USE_IMPLICIT_CONVERSIONS` is defined to `0`:
 
     ```cpp
-    using wjson = nlohmann::basic_json<std::map, std::vector, std::wstring>;
+    using wjson = nlohmann::json::with_string_t<std::wstring>;
 
     void load(const nlohmann::json& j);
 

docs/mkdocs/docs/examples/as_base_class.cpp

@@ -15,19 +15,7 @@ class base_class_with_hidden_members
     }
 };
 
-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>,
-             base_class_with_hidden_members
-             >;
+using json = nlohmann::json::with_base_class_t<base_class_with_hidden_members>;
 
 int main()
 {

docs/mkdocs/docs/examples/contains__json_pointer.cpp

@@ -19,25 +19,9 @@ int main()
               << j.contains("/array/1"_json_pointer) << '\n'
               << j.contains("/array/-"_json_pointer) << '\n'
               << j.contains("/array/4"_json_pointer) << '\n'
-              << j.contains("/baz"_json_pointer) << std::endl;
-
-    try
-    {
-        // try to use an array index with leading '0'
-        j.contains("/array/01"_json_pointer);
-    }
-    catch (const json::parse_error& e)
-    {
-        std::cout << e.what() << '\n';
-    }
-
-    try
-    {
-        // try to use an array index that is not a number
-        j.contains("/array/one"_json_pointer);
-    }
-    catch (const json::parse_error& e)
-    {
-        std::cout << e.what() << '\n';
-    }
+              << j.contains("/baz"_json_pointer) << '\n'
+              // an array index with a leading '0' is not found
+              << j.contains("/array/01"_json_pointer) << '\n'
+              // an array index that is not a number is not found
+              << j.contains("/array/one"_json_pointer) << std::endl;
 }

docs/mkdocs/docs/examples/contains__json_pointer.output

@@ -5,3 +5,5 @@ true
 false
 false
 false
+false
+false

docs/mkdocs/docs/examples/custom_array_type.cpp

@@ -1,11 +1,10 @@
 #include <iostream>
-#include <map>
 
 #include <nlohmann/json.hpp>
 
 #include "custom_array_type.hpp"
 
-using custom_json = nlohmann::basic_json<std::map, custom_array_type>;
+using custom_json = nlohmann::json::with_array_t<custom_array_type>;
 
 int main()
 {

docs/mkdocs/docs/examples/custom_binary_type.cpp

@@ -1,16 +1,10 @@
-#include <cstdint>
 #include <iostream>
-#include <map>
-#include <string>
-#include <vector>
 
 #include <nlohmann/json.hpp>
 
 #include "custom_binary_type.hpp"
 
-using custom_json = nlohmann::basic_json<std::map, std::vector, std::string, bool,
-      std::int64_t, std::uint64_t, double, std::allocator,
-      nlohmann::adl_serializer, custom_binary_type>;
+using custom_json = nlohmann::json::with_binary_t<custom_binary_type>;
 
 int main()
 {

docs/mkdocs/docs/examples/custom_object_type.cpp

@@ -1,12 +1,11 @@
 #include <iostream>
 #include <type_traits>
-#include <vector>
 
 #include <nlohmann/json.hpp>
 
 #include "custom_object_type.hpp"
 
-using custom_json = nlohmann::basic_json<custom_object_type, std::vector>;
+using custom_json = nlohmann::json::with_object_t<custom_object_type>;
 
 int main()
 {

docs/mkdocs/docs/examples/custom_string_type.cpp

@@ -1,12 +1,10 @@
 #include <iostream>
-#include <map>
-#include <vector>
 
 #include <nlohmann/json.hpp>
 
 #include "custom_string_type.hpp"
 
-using custom_json = nlohmann::basic_json<std::map, std::vector, custom_string_type>;
+using custom_json = nlohmann::json::with_string_t<custom_string_type>;
 
 int main()
 {

docs/mkdocs/docs/examples/parse_error.cpp

@@ -8,7 +8,7 @@ int main()
     try
     {
         // parsing input with a syntax error
-        json::parse("[1,2,3,]");
+        json j = json::parse("[1,2,3,]");
     }
     catch (const json::parse_error& e)
     {

docs/mkdocs/docs/examples/sax_parse__error_recovery.cpp

@@ -0,0 +1,43 @@
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+// a SAX parser that creates a JSON value like json::parse does, but that
+// recovers from parse errors instead of stopping at the first one
+class recovering_parser : public nlohmann::detail::json_sax_dom_parser<json>
+{
+  public:
+    explicit recovering_parser(json& result)
+        : nlohmann::detail::json_sax_dom_parser<json>(result, false)
+    {}
+
+    bool parse_error(std::size_t position,
+                     const std::string& /*last_token*/,
+                     const json::exception& ex)
+    {
+        std::cout << "byte " << position << ": " << ex.what() << '\n';
+
+        // repair the input and continue
+        return true;
+    }
+};
+
+int main()
+{
+    // JSON text with several mistakes that ends too early
+    const std::string text = R"({
+    "name": "Hello World",
+    "tags": ["a" "b",],
+    "valid": tru,
+    "size": 1.,
+    "nested": {"x": 1)";
+
+    json result;
+    recovering_parser sax(result);
+    const bool valid = json::sax_parse(text, &sax);
+
+    std::cout << "\nvalid JSON: " << std::boolalpha << valid << '\n'
+              << std::setw(4) << result << std::endl;
+}

docs/mkdocs/docs/examples/sax_parse__error_recovery.output

@@ -0,0 +1,19 @@
+byte 49: [json.exception.parse_error.101] parse error at line 3, column 20: syntax error while parsing array - unexpected string literal; expected ']'
+byte 51: [json.exception.parse_error.101] parse error at line 3, column 22: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal
+byte 70: [json.exception.parse_error.101] parse error at line 4, column 17: syntax error while parsing value - invalid literal; last read: '"valid": tru,'
+byte 86: [json.exception.parse_error.101] parse error at line 5, column 15: syntax error while parsing value - invalid number; expected digit after '.'; last read: '1.,'
+byte 109: [json.exception.parse_error.101] parse error at line 6, column 22: syntax error while parsing object - unexpected end of input; expected '}'
+
+valid JSON: false
+{
+    "name": "Hello World",
+    "nested": {
+        "x": 1
+    },
+    "size": 1,
+    "tags": [
+        "a",
+        "b"
+    ],
+    "valid": null
+}

docs/mkdocs/docs/features/parsing/error_recovery.md

@@ -0,0 +1,122 @@
+# Error Recovery
+
+By default, parsing stops at the first error. With the [SAX interface](sax_interface.md), you can instead ask the
+parser to *recover*: to repair the error and continue, so that you get as much as possible out of malformed input, for
+instance a file that was cut off, JSON edited by hand, or the output of a language model.
+
+## Recovering from errors
+
+The SAX parser's [`parse_error`](../../api/json_sax/parse_error.md) function is called for every error. Its return value
+decides what happens next:
+
+- `#!cpp false` stops parsing. This is what the SAX parsers of the library do, so [`parse`](../../api/basic_json/parse.md)
+  and [`accept`](../../api/basic_json/accept.md) never recover.
+- `#!cpp true` repairs the error and continues parsing.
+
+When recovering, the SAX parser still receives well-formed events: every `start_object` or `start_array` is followed by
+the matching `end_object` or `end_array`, and every `key` is followed by exactly one value. A SAX parser that creates a
+JSON value, such as the one in the example below, therefore gets a complete value. Parsing always ends, and
+[`sax_parse`](../../api/basic_json/sax_parse.md) returns `#!cpp false` for input that is not valid JSON, even if every
+error was repaired. Each token is reported at most once, and the SAX parser can stop at any error by returning
+`#!cpp false`.
+
+!!! example
+
+    The example below derives a SAX parser from the library's parser for `json` values (`json_sax_dom_parser`),
+    and recovers from all errors.
+
+    ```cpp
+    --8<-- "examples/sax_parse__error_recovery.cpp"
+    ```
+    
+    Output:
+    
+    ```
+    --8<-- "examples/sax_parse__error_recovery.output"
+    ```
+
+## How errors are repaired
+
+Each error is repaired with the smallest local edit: a missing separator is inserted, a stray token is removed, what can
+be read of a broken string or number is kept, and a value that cannot be read at all becomes `#!json null`.
+
+| Mistake                   | Repair                                                                         | Example                                  | Result                     |
+|---------------------------|--------------------------------------------------------------------------------|------------------------------------------|----------------------------|
+| missing `,` or `:`        | inserted                                                                       | `#!json [1 2]`, `#!json {"a" 1}`         | `[1,2]`, `{"a":1}`         |
+| missing value             | `#!json null` for an object key or between commas in an array                  | `#!json {"a":}`, `#!json [1,,2]`         | `{"a":null}`, `[1,null,2]` |
+| trailing comma            | removed                                                                        | `#!json [1,2,]`                          | `[1,2]`                    |
+| broken string             | invalid escapes and bytes are replaced (see below); a line break ends the string | `#!json ["a\qb"]`                        | `["aqb"]`                  |
+| broken number             | the longest valid beginning is kept                                            | `#!json [1., 2e+]`                       | `[1,2]`                    |
+| unreadable value          | `#!json null`                                                                  | `#!json [1, NaN, tru]`                   | `[1,null,null]`            |
+| number too large          | passed as infinity, together with its text                                     | `#!json [1e999]`                         | infinity (see below)       |
+| stray `:`                 | removed                                                                        | `#!json ["a":1]`                         | `["a",1]`                  |
+| member without a key      | skipped up to the next `,` or `}`                                              | `#!json {1:2, "b":3}`                    | `{"b":3}`                  |
+| wrong closing bracket     | closes the innermost array or object                                           | `#!json {"a":[1,2}, "b":3}`              | `{"a":[1,2],"b":3}`        |
+| input ends too early      | all open arrays and objects are closed                                         | `#!json {"a":[1,2`                       | `{"a":[1,2]}`              |
+| text before the value     | skipped                                                                        | `#!json )]}'{"a":1}`                     | `{"a":1}`                  |
+
+In a string, an unknown escape like `\q` stands for the escaped character (`q`), as in JavaScript. An invalid `\u`
+escape, a lone surrogate, and ill-formed UTF-8 are each replaced by U+FFFD (REPLACEMENT CHARACTER), and control
+characters are kept. A string without its closing quote ends at the next line break or at the end of the input.
+
+The input after the top-level value is not repaired: as without recovery, it is reported as an error, and parsing stops.
+
+## Binary formats
+
+The binary formats ([BJData](../binary_formats/bjdata.md), [BON8](../binary_formats/bon8.md),
+[BSON](../binary_formats/bson.md), [CBOR](../binary_formats/cbor.md), [MessagePack](../binary_formats/messagepack.md),
+and [UBJSON](../binary_formats/ubjson.md)) have no delimiters to find the next value by. So what can be repaired depends
+on whether the end of the item with the error is known, a distinction that
+[RFC 8949, Section 5.3](https://www.rfc-editor.org/rfc/rfc8949.html#section-5.3) makes for CBOR, too.
+
+If the item is complete, but cannot be passed on as it is, it is replaced, and parsing continues after it:
+
+| Mistake                                                             | Formats                 | Repair                                                                  |
+|---------------------------------------------------------------------|-------------------------|-------------------------------------------------------------------------|
+| tag, if `tag_handler` is `cbor_tag_handler_t::error` (the default)  | CBOR                    | ignored                                                                 |
+| simple value other than `false`, `true`, and `null`, like undefined | CBOR                    | `#!json null`                                                           |
+| number too large for a custom `number_float_t`, like `float`        | all                     | infinity                                                                |
+| character (`C`) that is not ASCII                                   | BJData, UBJSON          | U+FFFD                                                                  |
+| invalid high-precision number (`H`)                                 | BJData, UBJSON          | the longest valid beginning is kept, as for JSON text, or `#!json null` |
+| high-precision number too large                                     | BJData, UBJSON          | passed as infinity, together with its text                              |
+| object key that is not a string                                     | BON8, CBOR, MessagePack | the member is skipped                                                   |
+| element of a type the library does not read, like ObjectId or date  | BSON                    | `#!json null`                                                           |
+| string without its terminator                                       | BSON                    | kept                                                                    |
+| document whose size does not match its content                      | BSON                    | kept                                                                    |
+
+CBOR tags and simple values are repaired as [RFC 8949, Section 6.1](https://www.rfc-editor.org/rfc/rfc8949.html#section-6.1)
+suggests for converting CBOR to JSON. By default, [`sax_parse`](../../api/basic_json/sax_parse.md) reports every tag as
+an error; when recovering, tags are then ignored like with
+[`cbor_tag_handler_t::ignore`](../../api/basic_json/cbor_tag_handler_t.md). Strings that are not valid UTF-8 are no
+error: like [`from_cbor`](../../api/basic_json/from_cbor.md) and the other functions by default, `sax_parse` passes
+them on as they are.
+
+After any other error, the end of the item is unknown: the input ended, a byte is not a valid type marker, or a size
+cannot be right. Parsing then stops, and the value read so far is completed: a key that waits for its value gets
+`#!json null`, and all open arrays and objects are closed. This keeps everything before the error of an input that was
+cut off. The exception is BSON, which stores the size of every document: an element whose end is unknown gets
+`#!json null`, the rest of its document is skipped, and parsing continues after the document.
+
+## Limitations
+
+- A repair is a guess. For example, `#!json {"a" "b": 1}` could be meant as `#!json {"a": "b"}` or as
+  `#!json {"a": null, "b": 1}`; it is repaired to the former. Treat recovered values as a best effort, and check the
+  reported errors.
+- A closing bracket always closes the innermost array or object. If a bracket is missing rather than wrong, the
+  repair differs from the intention: `#!json {"a": {"b": [1, 2}, "c": 3}` is repaired to
+  `#!json {"a": {"b": [1, 2], "c": 3}}`, although `#!json {"a": {"b": [1, 2]}, "c": 3}` may have been meant.
+- Keys without quotes, and strings in single quotes, are not supported; such members are skipped.
+- In the binary formats, a member that is skipped because its key is not a string is lost, and so are the elements of a
+  BSON document after one whose end is unknown.
+- A number that is too large for `number_float_t` is passed as positive or negative infinity. The SAX parser's
+  `number_float` also gets the number's text, but a JSON value cannot store it, and
+  [`dump`](../../api/basic_json/dump.md) serializes infinity as `#!json null`.
+- When parsing is not strict (see [`sax_parse`](../../api/basic_json/sax_parse.md)), a repair may read parts of the
+  input after the value, for instance of the next value in a stream of concatenated values.
+
+## See also
+
+- [SAX interface](sax_interface.md) - implement a custom SAX handler
+- [`parse_error`](../../api/json_sax/parse_error.md) - the SAX event for parse errors
+- [`sax_parse`](../../api/basic_json/sax_parse.md) - generate SAX events
+- [parsing and exceptions](parse_exceptions.md) - control error handling

docs/mkdocs/docs/features/parsing/index.md

@@ -75,7 +75,7 @@ You can influence a DOM parse without switching to the SAX interface by passing
 When the input is not valid JSON, the `parse` function throws an exception by default. If exceptions are undesired or
 unavailable, the parser can instead return a discarded value, or [`accept`](../../api/basic_json/accept.md) can be used
 to only check whether an input is valid JSON. See [parsing and exceptions](parse_exceptions.md) for the available
-options.
+options. To get as much as possible out of malformed input, a SAX parser can [recover from errors](error_recovery.md).
 
 ## See also
 
@@ -86,4 +86,5 @@ options.
 - [parser callbacks](parser_callbacks.md) - influence the parsing by a callback function
 - [SAX interface](sax_interface.md) - implement a custom SAX handler
 - [parsing and exceptions](parse_exceptions.md) - control error handling
+- [error recovery](error_recovery.md) - get as much as possible out of malformed input
 - [parsing untrusted input](untrusted_input.md) - what to consider when parsing input from untrusted sources

docs/mkdocs/docs/features/parsing/parse_exceptions.md

@@ -64,7 +64,8 @@ bool parse_error(std::size_t position,
                  const json::exception& ex);
 ```
 
-The return value indicates whether the parsing should continue, so the function should usually return `#!cpp false`.
+The return value decides whether to stop parsing (`#!cpp false`) or to repair the error and continue
+(`#!cpp true`); see [error recovery](error_recovery.md) for the latter.
 
 ??? example "Example: report parse errors without exceptions"
 

docs/mkdocs/docs/features/parsing/sax_interface.md

@@ -60,7 +60,8 @@ bool key(string_t& val);
 bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex);
 ```
 
-The return value of each function determines whether parsing should proceed.
+The return value of each function determines whether parsing should proceed. For `parse_error`, returning
+`#!cpp true` [recovers from the error](error_recovery.md).
 
 To implement your own SAX handler, proceed as follows:
 
@@ -68,7 +69,7 @@ To implement your own SAX handler, proceed as follows:
 2. Create an object of your SAX interface class, e.g. `my_sax`.
 3. Call `#!cpp bool json::sax_parse(input, &my_sax);` where the first parameter can be any input like a string or an input stream and the second parameter is a pointer to your SAX interface.
 
-Note the `sax_parse` function only returns a `#!cpp bool` indicating the result of the last executed SAX event. It does not return `json` value - it is up to you to decide what to do with the SAX events. Furthermore, no exceptions are thrown in case of a parse error - it is up to you what to do with the exception object passed to your `parse_error` implementation. Internally, the SAX interface is used for the DOM parser (class `json_sax_dom_parser`) as well as the acceptor (`json_sax_acceptor`), see file `json_sax.hpp`.
+Note the `sax_parse` function only returns a `#!cpp bool` indicating whether the input was parsed without errors and no SAX event returned `#!cpp false`. It does not return `json` value - it is up to you to decide what to do with the SAX events. Furthermore, no exceptions are thrown in case of a parse error - it is up to you what to do with the exception object passed to your `parse_error` implementation. Internally, the SAX interface is used for the DOM parser (class `json_sax_dom_parser`) as well as the acceptor (`json_sax_acceptor`), see file `json_sax.hpp`.
 
 ## See also
 

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

@@ -351,9 +351,8 @@ The number types can be changed with template parameters.
 
     A `basic_json` type that uses `#!c long double` as floating-point type.
 
-    ```cpp hl_lines="2"
-    using json_ld = nlohmann::basic_json<std::map, std::vector, std::string, bool,
-                                         std::int64_t, std::uint64_t, long double>;
+    ```cpp hl_lines="1"
+    using json_ld = nlohmann::json::with_float_t<long double>;
     ```
 
     Note values should then be parsed with `json_ld::parse` rather than `json::parse` as the latter would parse

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

@@ -8,6 +8,10 @@ these requirements so they do not have to be discovered by trial and error. Each
 that are known to work for that parameter and the ones that do not, checked against Boost 1.83, Abseil 20250127.0,
 Folly, EASTL 3.21, `ankerl::unordered_dense`, `phmap`, `gtl`, `robin_hood`, `tsl::ordered_map`, and Qt 6.
 
+To change a single template parameter and keep the others, use the member alias templates
+[`with_*_t`](../../api/basic_json/with_t.md); for instance, `nlohmann::json::with_float_t<long double>` is `json` with
+`#!cpp long double` as [`number_float_t`](../../api/basic_json/number_float_t.md).
+
 ## How to read this page
 
 Requirements are split into two groups:
@@ -143,7 +147,7 @@ struct unordered_map_object
     using base_t::base_t;
 };
 
-using unordered_json = nlohmann::basic_json<unordered_map_object>;
+using unordered_json = nlohmann::json::with_object_t<unordered_map_object>;
 ```
 
 Whether `#!cpp std::unordered_map` can be instantiated at all depends on the standard library: `object_t` is formed
@@ -176,7 +180,7 @@ struct flat_hash_object
     using base_t::base_t;
 };
 
-using flat_hash_json = nlohmann::basic_json<flat_hash_object>;
+using flat_hash_json = nlohmann::json::with_object_t<flat_hash_object>;
 ```
 
 `absl::node_hash_map` keeps references to the mapped values valid across insertions; `absl::flat_hash_map` does not,

docs/mkdocs/docs/integration/migration_guide.md

@@ -116,7 +116,7 @@ function to use instead.
     === "Deprecated"
   
           ```cpp
-          using my_json = nlohmann::basic_json<std::map, std::vector, my_string_type>;
+          using my_json = nlohmann::json::with_string_t<my_string_type>;
           nlohmann::json_pointer<my_json> ptr("/foo/bar/1");
           ```
   

docs/mkdocs/mkdocs.yml

@@ -88,6 +88,7 @@ nav:
       - features/performance.md
       - Parsing:
           - features/parsing/index.md
+          - features/parsing/error_recovery.md
           - features/parsing/json_lines.md
           - features/parsing/parse_exceptions.md
           - features/parsing/parser_callbacks.md

include/nlohmann/detail/bit_ops.hpp

@@ -9,12 +9,15 @@
 #pragma once
 
 #include <cstdint> // uint64_t
+#if !defined(__SIZEOF_INT128__) && defined(_MSC_VER) && (defined(_M_X64) || defined(_M_ARM64))
+    #include <intrin0.h> // __umulh, _umul128
+#endif
 
 #include <nlohmann/detail/abi_macros.hpp>
 
 // Portable bit-level helpers for the number and string scanners. They use
-// compiler builtins where available and plain C++ otherwise, so they need no
-// platform headers and work regardless of byte order.
+// compiler builtins or platform-specific intrinsics where available and plain
+// C++ otherwise, so they work regardless of byte order.
 
 NLOHMANN_JSON_NAMESPACE_BEGIN
 namespace detail
@@ -52,6 +55,12 @@ inline uint128_parts full_multiplication(std::uint64_t a, std::uint64_t b) noexc
     __extension__ using uint128 = unsigned __int128;
     const uint128 r = static_cast<uint128>(a) * b;
     return {static_cast<std::uint64_t>(r), static_cast<std::uint64_t>(r >> 64u)};
+#elif defined(_MSC_VER) && defined(_M_X64)
+    std::uint64_t high = 0;
+    const std::uint64_t low = _umul128(a, b, &high);
+    return {low, high};
+#elif defined(_MSC_VER) && defined(_M_ARM64)
+    return {a * b, __umulh(a, b)};
 #else
     const std::uint64_t a_lo = a & 0xFFFFFFFFu;
     const std::uint64_t a_hi = a >> 32u;

include/nlohmann/detail/input/json_sax.hpp

@@ -132,7 +132,9 @@ struct json_sax
     @param[in] position    the position in the input where the error occurs
     @param[in] last_token  the last read token
     @param[in] ex          an exception object describing the error
-    @return whether parsing should proceed (must return false)
+    @return whether to recover from the error: false stops parsing; true
+            repairs the error and continues, or, if that is not possible,
+            stops after completing the value read so far
     */
     virtual bool parse_error(std::size_t position,
                              const std::string& last_token,
@@ -269,9 +271,12 @@ a pointer to the respective array or object for each recursion depth.
 After successful parsing, the value that is passed by reference to the
 constructor contains the parsed value.
 
-@tparam BasicJsonType  the JSON type
+@tparam BasicJsonType     the JSON type
+@tparam InputAdapterType  the input adapter of the lexer that can be passed to
+                          the constructor to record diagnostic positions; it
+                          does not matter if no lexer is passed
 */
-template<typename BasicJsonType, typename InputAdapterType>
+template<typename BasicJsonType, typename InputAdapterType = string_input_adapter_type>
 class json_sax_dom_parser
 {
   public:
@@ -518,7 +523,7 @@ class json_sax_dom_parser
     lexer_t* m_lexer_ref = nullptr;
 };
 
-template<typename BasicJsonType, typename InputAdapterType>
+template<typename BasicJsonType, typename InputAdapterType = string_input_adapter_type>
 class json_sax_dom_callback_parser
 {
   public:

include/nlohmann/detail/input/lexer.hpp

@@ -10,7 +10,7 @@
 
 #include <array> // array
 #include <cstddef> // size_t
-#include <cstdint> // uint32_t
+#include <cstdint> // uint8_t, uint32_t
 #include <cstdio> // snprintf
 #include <initializer_list> // initializer_list
 #include <string> // char_traits, string
@@ -439,8 +439,16 @@ class lexer : public lexer_base<BasicJsonType>
                             if (0xD800 <= codepoint1 && codepoint1 <= 0xDBFF)
                             {
                                 // expect next \uxxxx entry
-                                if (JSON_HEDLEY_LIKELY(get() == '\\' && get() == 'u'))
+                                if (JSON_HEDLEY_LIKELY(get() == '\\'))
                                 {
+                                    if (JSON_HEDLEY_UNLIKELY(get() != 'u'))
+                                    {
+                                        // current is the character escaped by the backslash
+                                        error_message = "invalid string: surrogate U+D800..U+DBFF must be followed by U+DC00..U+DFFF";
+                                        string_error_resume = resume_kind::escaped_character;
+                                        return token_type::parse_error;
+                                    }
+
                                     const int codepoint2 = get_codepoint();
 
                                     if (JSON_HEDLEY_UNLIKELY(codepoint2 == -1))
@@ -465,7 +473,11 @@ class lexer : public lexer_base<BasicJsonType>
                                     }
                                     else
                                     {
+                                        // the second escape was read completely and is a
+                                        // code point of its own
                                         error_message = "invalid string: surrogate U+D800..U+DBFF must be followed by U+DC00..U+DFFF";
+                                        string_error_resume = resume_kind::after_escape;
+                                        string_error_codepoint = codepoint2;
                                         return token_type::parse_error;
                                     }
                                 }
@@ -479,7 +491,9 @@ class lexer : public lexer_base<BasicJsonType>
                             {
                                 if (JSON_HEDLEY_UNLIKELY(0xDC00 <= codepoint1 && codepoint1 <= 0xDFFF))
                                 {
+                                    // the escape was read completely
                                     error_message = "invalid string: surrogate U+DC00..U+DFFF must follow U+D800..U+DBFF";
+                                    string_error_resume = resume_kind::after_escape;
                                     return token_type::parse_error;
                                 }
                             }
@@ -2133,6 +2147,574 @@ scan_number_done:
         }
     }
 
+  public:
+    /////////////////////
+    // error recovery
+    /////////////////////
+
+    /*!
+    @brief make the best of the token that scan() rejected
+
+    Called by the parser after scan() returned token_type::parse_error and the
+    SAX parser asked to recover from the error (see #3989). Keeps what can be
+    read of the token and skips the rest:
+
+    - A string keeps its characters. An unknown escape stands for the escaped
+      character itself (as in JavaScript), an invalid Unicode escape and ill-formed
+      UTF-8 become U+FFFD, and a control character is kept. A line break or the
+      end of the input ends a string that lacks its closing quote.
+    - A number keeps its longest valid prefix, e.g. `1` for `1.` or `1e+`.
+    - A block comment that is not closed runs to the end of the input.
+    - Anything else is skipped.
+
+    The rest of an invalid token is skipped up to the next delimiter
+    (whitespace, a structural character, or a quote). A delimiter that the
+    invalid token consumed is returned to the input, so that the next scan()
+    reads it.
+
+    @return token_type::value_string or a number token type if a string or a
+            number could be read, token_type::end_of_input for a block comment
+            that is not closed, token_type::uninitialized otherwise
+    */
+    token_type recover_token()
+    {
+        const resume_kind resume = string_error_resume;
+        const int codepoint = string_error_codepoint;
+        string_error_resume = resume_kind::character;
+        string_error_codepoint = -1;
+
+        if (error_message_starts_with("invalid string"))
+        {
+            return recover_string(resume, codepoint);
+        }
+
+        if (error_message_starts_with("invalid number"))
+        {
+            return recover_number();
+        }
+
+        if (error_message_starts_with("invalid comment; missing"))
+        {
+            // the comment runs to the end of the input
+            return token_type::end_of_input;
+        }
+
+        skip_to_delimiter();
+        return token_type::uninitialized;
+    }
+
+    /*!
+    @brief return the token that scan() read last to the input, so that the
+           next scan() reads it again
+
+    Called by the parser when recovering from an error. The token must be a
+    single character (',', ':', '[', ']', '{', or '}') or the end of the
+    input, and scan() must have read it last.
+    */
+    void unget_token()
+    {
+        JSON_ASSERT(!next_unget);
+        unget();
+    }
+
+    /*!
+    @brief let the token string for the next error begin at the current character
+
+    The token string of an error reaches back to the beginning of the last
+    string or number. After an error, the parser calls this function so that
+    the next error does not report (and, with many errors, copy) everything
+    read since then.
+    */
+    void restart_token_string()
+    {
+        restart_token_string_impl(std::integral_constant<bool, lazy_token_string> {});
+    }
+
+  private:
+    /// how recover_string() continues after the error scan_string() reported
+    enum class resume_kind : std::uint8_t
+    {
+        /// current is the next character of the string (or the end of input)
+        character,
+        /// current is the character escaped by the preceding backslash
+        escaped_character,
+        /// current is the last character of a complete escape
+        after_escape
+    };
+
+    /// whether error_message begins with @a prefix
+    bool error_message_starts_with(const char* prefix) const noexcept
+    {
+        const char* message = error_message;
+        while (*prefix != '\0')
+        {
+            if (*message++ != *prefix++)
+            {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    /// whether current ends an invalid token (see recover_token())
+    bool current_is_delimiter() const noexcept
+    {
+        switch (current)
+        {
+            case ' ':
+            case '\t':
+            case '\n':
+            case '\r':
+            case '[':
+            case ']':
+            case '{':
+            case '}':
+            case ',':
+            case ':':
+            case '\"':
+#if !JSON_STRICT_NUL_HANDLING
+            case '\0':
+#endif
+            case char_traits<char_type>::eof():
+                return true;
+
+            case '/':
+                return ignore_comments;
+
+            default:
+                return false;
+        }
+    }
+
+    /// skip the rest of an invalid token and return its delimiter to the input
+    void skip_to_delimiter()
+    {
+        while (!current_is_delimiter())
+        {
+            get();
+        }
+
+        if (current != char_traits<char_type>::eof())
+        {
+            unget();
+        }
+    }
+
+    /// append U+FFFD REPLACEMENT CHARACTER to token_buffer
+    void add_replacement_character()
+    {
+        add(0xEF);
+        add(0xBF);
+        add(0xBD);
+    }
+
+    /// append the UTF-8 encoding of @a codepoint (not a surrogate) to token_buffer
+    void add_codepoint(const int codepoint)
+    {
+        JSON_ASSERT(0x00 <= codepoint && codepoint <= 0x10FFFF);
+        const auto cp = static_cast<unsigned int>(codepoint);
+        if (cp < 0x80)
+        {
+            add(static_cast<char_int_type>(cp));
+        }
+        else if (cp <= 0x7FF)
+        {
+            add(static_cast<char_int_type>(0xC0u | (cp >> 6u)));
+            add(static_cast<char_int_type>(0x80u | (cp & 0x3Fu)));
+        }
+        else if (cp <= 0xFFFF)
+        {
+            add(static_cast<char_int_type>(0xE0u | (cp >> 12u)));
+            add(static_cast<char_int_type>(0x80u | ((cp >> 6u) & 0x3Fu)));
+            add(static_cast<char_int_type>(0x80u | (cp & 0x3Fu)));
+        }
+        else
+        {
+            add(static_cast<char_int_type>(0xF0u | (cp >> 18u)));
+            add(static_cast<char_int_type>(0x80u | ((cp >> 12u) & 0x3Fu)));
+            add(static_cast<char_int_type>(0x80u | ((cp >> 6u) & 0x3Fu)));
+            add(static_cast<char_int_type>(0x80u | (cp & 0x3Fu)));
+        }
+    }
+
+    /// append a code point read from a Unicode escape; a surrogate becomes U+FFFD
+    void add_escaped_codepoint(const int codepoint)
+    {
+        if (0xD800 <= codepoint && codepoint <= 0xDFFF)
+        {
+            add_replacement_character();
+        }
+        else
+        {
+            add_codepoint(codepoint);
+        }
+    }
+
+    /*!
+    @brief remove an incomplete UTF-8 sequence from the end of token_buffer
+
+    next_byte_in_range() adds the bytes of a sequence as it checks them, so
+    when it rejects a byte, the beginning of the sequence is already in
+    token_buffer, which otherwise holds only complete sequences.
+
+    @return whether an incomplete sequence was removed
+    */
+    bool remove_incomplete_utf8_sequence()
+    {
+        std::size_t lead = token_buffer.size();
+        std::size_t continuation_bytes = 0;
+        while (lead > 0 && continuation_bytes < 3
+                && (static_cast<unsigned char>(token_buffer[lead - 1]) & 0xC0u) == 0x80u)
+        {
+            --lead;
+            ++continuation_bytes;
+        }
+        if (lead == 0)
+        {
+            return false;
+        }
+
+        const auto lead_byte = static_cast<unsigned char>(token_buffer[lead - 1]);
+        std::size_t expected = 0;
+        if (lead_byte >= 0xF0)
+        {
+            expected = 3;
+        }
+        else if (lead_byte >= 0xE0)
+        {
+            expected = 2;
+        }
+        else if (lead_byte >= 0xC0)
+        {
+            expected = 1;
+        }
+        if (continuation_bytes >= expected)
+        {
+            return false;
+        }
+
+        token_buffer.resize(lead - 1);
+        return true;
+    }
+
+    /*!
+    @brief read the UTF-8 sequence that begins with current, which is not ASCII
+    @return whether the next character must be read; false if current still
+            needs to be handled, because it does not belong to the sequence
+    */
+    bool recover_utf8_sequence()
+    {
+        // the number of continuation bytes and the range of the first one;
+        // see the ranges in scan_string()
+        std::size_t count = 0;
+        char_int_type low = 0x80;
+        char_int_type high = 0xBF;
+        if (current >= 0xC2 && current <= 0xDF)
+        {
+            count = 1;
+        }
+        else if (current >= 0xE0 && current <= 0xEF)
+        {
+            count = 2;
+            low = (current == 0xE0) ? 0xA0 : 0x80;
+            high = (current == 0xED) ? 0x9F : 0xBF;
+        }
+        else if (current >= 0xF0 && current <= 0xF4)
+        {
+            count = 3;
+            low = (current == 0xF0) ? 0x90 : 0x80;
+            high = (current == 0xF4) ? 0x8F : 0xBF;
+        }
+        else
+        {
+            // an ill-formed byte
+            add_replacement_character();
+            return true;
+        }
+
+        const std::size_t start = token_buffer.size();
+        add(current);
+        for (std::size_t i = 0; i < count; ++i)
+        {
+            get();
+            if (current < low || current > high)
+            {
+                token_buffer.resize(start);
+                add_replacement_character();
+                return false;
+            }
+            add(current);
+            low = 0x80;
+            high = 0xBF;
+        }
+        return true;
+    }
+
+    /*!
+    @brief read the low surrogate that must follow the high surrogate @a high
+    @return whether the next character must be read; false if current still
+            needs to be handled
+    */
+    bool recover_low_surrogate(int high)
+    {
+        while (true)
+        {
+            if (get() != '\\')
+            {
+                add_replacement_character();
+                return false;
+            }
+            if (get() != 'u')
+            {
+                add_replacement_character();
+                // not 'u', so this does not come back here
+                return recover_escape();
+            }
+
+            const int low = get_codepoint();
+            if (low == -1)
+            {
+                add_replacement_character();
+                return false;
+            }
+            if (0xDC00 <= low && low <= 0xDFFF)
+            {
+                add_codepoint(static_cast<int>((static_cast<unsigned int>(high) << 10u)
+                                               + static_cast<unsigned int>(low) - 0x35FDC00u));
+                return true;
+            }
+
+            // high has no low surrogate
+            add_replacement_character();
+            if (low < 0xD800 || low > 0xDBFF)
+            {
+                add_codepoint(low);
+                return true;
+            }
+            // another high surrogate
+            high = low;
+        }
+    }
+
+    /*!
+    @brief read the escape whose backslash was read; current is the escaped character
+    @return whether the next character must be read; false if current still
+            needs to be handled
+    */
+    bool recover_escape()
+    {
+        switch (current)
+        {
+            case '\"':
+                add('\"');
+                return true;
+            case '\\':
+                add('\\');
+                return true;
+            case '/':
+                add('/');
+                return true;
+            case 'b':
+                add('\b');
+                return true;
+            case 'f':
+                add('\f');
+                return true;
+            case 'n':
+                add('\n');
+                return true;
+            case 'r':
+                add('\r');
+                return true;
+            case 't':
+                add('\t');
+                return true;
+
+            case 'u':
+            {
+                const int codepoint = get_codepoint();
+                if (codepoint == -1)
+                {
+                    add_replacement_character();
+                    return false;
+                }
+                if (0xD800 <= codepoint && codepoint <= 0xDBFF)
+                {
+                    return recover_low_surrogate(codepoint);
+                }
+                add_escaped_codepoint(codepoint);
+                return true;
+            }
+
+            // an unknown escape stands for the escaped character
+            default:
+                return false;
+        }
+    }
+
+    /*!
+    @brief read the rest of a string after scan_string() rejected it
+
+    token_buffer holds what scan_string() read before the error. See
+    recover_token() for how errors are repaired.
+
+    @param[in] resume     how to continue, see resume_kind
+    @param[in] codepoint  for a high surrogate followed by an escape of another
+                          code point: that code point; -1 otherwise
+    */
+    token_type recover_string(const resume_kind resume, const int codepoint)
+    {
+        // whether the next character must be read before it can be handled
+        bool fetch = false;
+
+        if (error_message_starts_with("invalid string: surrogate")
+                || error_message_starts_with("invalid string: '\\u'")
+                || (error_message_starts_with("invalid string: ill-formed UTF-8")
+                    && remove_incomplete_utf8_sequence()))
+        {
+            add_replacement_character();
+        }
+
+        switch (resume)
+        {
+            case resume_kind::escaped_character:
+                fetch = recover_escape();
+                break;
+            case resume_kind::after_escape:
+                if (0xD800 <= codepoint && codepoint <= 0xDBFF)
+                {
+                    fetch = recover_low_surrogate(codepoint);
+                }
+                else
+                {
+                    if (codepoint != -1)
+                    {
+                        add_escaped_codepoint(codepoint);
+                    }
+                    fetch = true;
+                }
+                break;
+            case resume_kind::character:
+            default:
+                break;
+        }
+
+        while (true)
+        {
+            if (fetch)
+            {
+                get();
+            }
+            fetch = true;
+
+            switch (current)
+            {
+                case '\"':
+                // a line break or the end of the input ends a string that
+                // lacks its closing quote
+                case '\n':
+                case '\r':
+                case char_traits<char_type>::eof():
+                    return token_type::value_string;
+
+#if !JSON_STRICT_NUL_HANDLING
+                case '\0':
+                    // the end of the input, see scan()
+                    unget();
+                    return token_type::value_string;
+#endif
+
+                case '\\':
+                    get();
+                    fetch = recover_escape();
+                    break;
+
+                default:
+                    if (current < 0x80)
+                    {
+                        // including control characters
+                        add(current);
+                    }
+                    else
+                    {
+                        fetch = recover_utf8_sequence();
+                    }
+                    break;
+            }
+        }
+    }
+
+    /*!
+    @brief keep the longest valid prefix of a number that scan_number() rejected
+
+    token_buffer holds the characters scan_number() accepted before the error,
+    so the prefix ends at its last digit.
+    */
+    token_type recover_number()
+    {
+        // only size(), operator[], and resize() are used, which every string
+        // type the library supports provides
+        std::size_t length = token_buffer.size();
+        while (length != 0 && (token_buffer[length - 1] < '0' || token_buffer[length - 1] > '9'))
+        {
+            --length;
+        }
+        token_buffer.resize(length);
+
+        if (length == 0)
+        {
+            skip_to_delimiter();
+            return token_type::uninitialized;
+        }
+
+        if (decimal_point_position >= length)
+        {
+            decimal_point_position = std::string::npos;
+        }
+
+        std::size_t exponent = std::string::npos;
+        for (std::size_t i = 0; i < length; ++i)
+        {
+            if (token_buffer[i] == 'e' || token_buffer[i] == 'E')
+            {
+                exponent = i;
+                break;
+            }
+        }
+        const std::size_t mantissa_end = (exponent == std::string::npos) ? length : exponent;
+        token_type number_type = token_type::value_unsigned;
+        if (decimal_point_position != std::string::npos || exponent != std::string::npos)
+        {
+            number_type = token_type::value_float;
+        }
+        else if (token_buffer[0] == '-')
+        {
+            number_type = token_type::value_integer;
+        }
+
+        const token_type result = convert_number(number_type, mantissa_end);
+        skip_to_delimiter();
+        return result;
+    }
+
+    /// seekable adapter: the token string begins at current, which was consumed
+    void restart_token_string_impl(std::true_type /*lazy*/) noexcept
+    {
+        const std::size_t consumed = ia.get_consumed_count();
+        token_string_start = (consumed > 0 && current != char_traits<char_type>::eof()) ? consumed - 1 : consumed;
+    }
+
+    /// streaming adapter: the token string begins at current; a character
+    /// that was put back is copied again when it is read again
+    void restart_token_string_impl(std::false_type /*lazy*/)
+    {
+        token_string.clear();
+        if (!next_unget && current != char_traits<char_type>::eof())
+        {
+            token_string.push_back(char_traits<char_type>::to_char_type(current));
+        }
+    }
+
     /// input adapter
     InputAdapterType ia;
 
@@ -2172,6 +2754,13 @@ scan_number_done:
     /// a description of occurred lexer errors
     const char* error_message = "";
 
+    /// how recover_token() continues a string that scan_string() rejected;
+    /// set only on the error paths that need more than error_message
+    resume_kind string_error_resume = resume_kind::character;
+    /// the code point of the second escape when a high surrogate is followed
+    /// by an escape that is not a low surrogate; -1 otherwise
+    int string_error_codepoint = -1;
+
     // number values
     number_integer_t value_integer = 0;
     number_unsigned_t value_unsigned = 0;

include/nlohmann/detail/input/parser.hpp

@@ -139,26 +139,59 @@ class parser
     bool accept(const bool strict = true)
     {
         json_sax_acceptor<BasicJsonType> sax_acceptor;
-        return sax_parse(&sax_acceptor, strict);
+        return sax_parse_impl<false>(&sax_acceptor, strict);
     }
 
+    /*!
+    @brief public SAX interface
+
+    If the SAX parser's parse_error() returns true, the parser recovers from
+    the error: it repairs the input and continues (see #3989).
+
+    @param[in] sax     the SAX parser
+    @param[in] strict  whether to expect the last token to be EOF
+    @return whether the input was parsed without errors and no SAX event
+            returned false
+    */
     template<typename SAX>
     JSON_HEDLEY_NON_NULL(2)
     bool sax_parse(SAX* sax, const bool strict = true)
+    {
+        return sax_parse_impl<true>(sax, strict);
+    }
+
+  private:
+    /// what sax_parse_internal() does after an object key was expected
+    enum class next_step : std::uint8_t
+    {
+        /// stop parsing
+        stop,
+        /// parse a value that begins with last_token
+        parse_value,
+        /// evaluate the state of the innermost container, which reads
+        /// last_token again
+        evaluate_state
+    };
+
+    template<bool AllowRecovery, typename SAX>
+    JSON_HEDLEY_NON_NULL(2)
+    bool sax_parse_impl(SAX* sax, const bool strict)
     {
         (void)detail::is_sax_static_asserts<SAX, BasicJsonType> {};
-        const bool result = sax_parse_internal(sax);
+        const bool result = sax_parse_internal<AllowRecovery>(sax);
 
         if (result)
         {
             if (strict)
             {
-                // strict mode: next byte must be EOF
-                if (get_token() != token_type::end_of_input)
+                // strict mode: next byte must be EOF; after recovering from an
+                // error, the end of the input may already have been read
+                if (last_token != token_type::end_of_input && get_token() != token_type::end_of_input)
                 {
-                    return sax->parse_error(m_lexer.get_position(),
-                                            m_lexer.get_token_string(),
-                                            parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_of_input, "value"), nullptr));
+                    // the value is complete, so there is nothing to recover
+                    static_cast<void>(report_error(sax, parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_of_input, "value"), nullptr),
+                                                   std::integral_constant<bool, AllowRecovery> {}));
+                    return false;
                 }
             }
             else
@@ -169,10 +202,9 @@ class parser
             }
         }
 
-        return result;
+        return result && !error_reported;
     }
 
-  private:
     /*!
     @brief run a DOM SAX parser to completion and position the lexer
 
@@ -190,7 +222,7 @@ class parser
     template<typename DomSax>
     bool parse_dom(DomSax& sdp, const bool strict)
     {
-        sax_parse_internal(&sdp);
+        sax_parse_internal<false>(&sdp);
 
         if (strict)
         {
@@ -213,10 +245,20 @@ class parser
         return !sdp.is_errored();
     }
 
-    template<typename SAX>
+    /*!
+    @brief parse a JSON value and pass it to a SAX parser
+
+    @tparam AllowRecovery  whether to recover from an error if the SAX parser's
+                           parse_error() returns true; false for the SAX parsers
+                           of parse() and accept(), which never do, so that no
+                           code for recovering is generated for them
+    */
+    template<bool AllowRecovery, typename SAX>
     JSON_HEDLEY_NON_NULL(2)
     bool sax_parse_internal(SAX* sax)
     {
+        const std::integral_constant<bool, AllowRecovery> allow_recovery{};
+
         // stack to remember the hierarchy of structured values we are parsing
         // true = array; false = object
         std::vector<bool> states;
@@ -247,12 +289,18 @@ class parser
                             break;
                         }
 
-                        // parse key
+                        // remember we are now inside an object
+                        states.push_back(false);
+
+                        // parse key (the steps of parse_key(), which are
+                        // repeated here and below for speed)
                         if (JSON_HEDLEY_UNLIKELY(last_token != token_type::value_string))
                         {
-                            return sax->parse_error(m_lexer.get_position(),
-                                                    m_lexer.get_token_string(),
-                                                    parse_error::create(101, m_lexer.get_position(), exception_message(token_type::value_string, "object key"), nullptr));
+                            if (!continue_after(key_error(sax, allow_recovery, false), skip_to_state_evaluation))
+                            {
+                                return false;
+                            }
+                            continue;
                         }
                         if (JSON_HEDLEY_UNLIKELY(!sax->key(m_lexer.get_string())))
                         {
@@ -262,14 +310,13 @@ class parser
                         // parse separator (:)
                         if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
                         {
-                            return sax->parse_error(m_lexer.get_position(),
-                                                    m_lexer.get_token_string(),
-                                                    parse_error::create(101, m_lexer.get_position(), exception_message(token_type::name_separator, "object separator"), nullptr));
+                            if (!continue_after(key_error(sax, allow_recovery, true), skip_to_state_evaluation))
+                            {
+                                return false;
+                            }
+                            continue;
                         }
 
-                        // remember we are now inside an object
-                        states.push_back(false);
-
                         // parse values
                         get_token();
                         continue;
@@ -305,9 +352,11 @@ class parser
 
                         if (JSON_HEDLEY_UNLIKELY(!std::isfinite(res)))
                         {
-                            return sax->parse_error(m_lexer.get_position(),
-                                                    m_lexer.get_token_string(),
-                                                    out_of_range::create(406, concat("number overflow parsing '", m_lexer.get_token_string(), '\''), nullptr));
+                            if (!overflow_error(sax, res, allow_recovery))
+                            {
+                                return false;
+                            }
+                            break;
                         }
 
                         if (JSON_HEDLEY_UNLIKELY(!sax->number_float(res, m_lexer.get_string())))
@@ -375,23 +424,63 @@ class parser
                     case token_type::parse_error:
                     {
                         // using "uninitialized" to avoid an "expected" message
-                        return sax->parse_error(m_lexer.get_position(),
-                                                m_lexer.get_token_string(),
-                                                parse_error::create(101, m_lexer.get_position(), exception_message(token_type::uninitialized, "value"), nullptr));
+                        if (!report_error(sax, parse_error::create(101, m_lexer.get_position(), exception_message(token_type::uninitialized, "value"), nullptr), allow_recovery))
+                        {
+                            return false;
+                        }
+
+                        // recover: keep what can be read of the token
+                        recover_token(allow_recovery);
+                        if (last_token != token_type::uninitialized)
+                        {
+                            // a string or a number
+                            continue;
+                        }
+                        if (states.empty())
+                        {
+                            // look for the value after the garbage
+                            if (!skip_to_value(allow_recovery))
+                            {
+                                return false;
+                            }
+                            continue;
+                        }
+                        // nothing could be read
+                        if (JSON_HEDLEY_UNLIKELY(!sax->null()))
+                        {
+                            return false;
+                        }
+                        break;
                     }
                     case token_type::end_of_input:
                     {
                         if (JSON_HEDLEY_UNLIKELY(m_lexer.get_position().chars_read_total == 1))
                         {
-                            return sax->parse_error(m_lexer.get_position(),
-                                                    m_lexer.get_token_string(),
-                                                    parse_error::create(101, m_lexer.get_position(),
-                                                            "attempting to parse an empty input; check that your input string or stream contains the expected JSON", nullptr));
+                            // there is nothing to recover
+                            static_cast<void>(report_error(sax, parse_error::create(101, m_lexer.get_position(),
+                                                           "attempting to parse an empty input; check that your input string or stream contains the expected JSON", nullptr), allow_recovery));
+                            return false;
                         }
 
-                        return sax->parse_error(m_lexer.get_position(),
-                                                m_lexer.get_token_string(),
-                                                parse_error::create(101, m_lexer.get_position(), exception_message(token_type::literal_or_value, "value"), nullptr));
+                        if (!report_error(sax, parse_error::create(101, m_lexer.get_position(), exception_message(token_type::literal_or_value, "value"), nullptr), allow_recovery))
+                        {
+                            return false;
+                        }
+
+                        // recover: the input ends where a value is missing
+                        if (states.empty())
+                        {
+                            // there is no value
+                            return false;
+                        }
+                        if (!recover_missing_value(sax, states, allow_recovery))
+                        {
+                            return false;
+                        }
+                        // the state evaluation reads the token again
+                        unget_token(allow_recovery);
+                        skip_to_state_evaluation = true;
+                        continue;
                     }
                     case token_type::uninitialized:
                     case token_type::end_array:
@@ -401,9 +490,35 @@ class parser
                     case token_type::literal_or_value:
                     default: // the last token was unexpected
                     {
-                        return sax->parse_error(m_lexer.get_position(),
-                                                m_lexer.get_token_string(),
-                                                parse_error::create(101, m_lexer.get_position(), exception_message(token_type::literal_or_value, "value"), nullptr));
+                        if (!report_error(sax, parse_error::create(101, m_lexer.get_position(), exception_message(token_type::literal_or_value, "value"), nullptr), allow_recovery))
+                        {
+                            return false;
+                        }
+
+                        // recover
+                        if (states.empty())
+                        {
+                            // look for the value after the garbage
+                            if (!skip_to_value(allow_recovery))
+                            {
+                                return false;
+                            }
+                            continue;
+                        }
+                        if (last_token == token_type::name_separator)
+                        {
+                            // a stray ':'; the value may follow
+                            get_token();
+                            continue;
+                        }
+                        if (!recover_missing_value(sax, states, allow_recovery))
+                        {
+                            return false;
+                        }
+                        // the state evaluation reads the token again
+                        unget_token(allow_recovery);
+                        skip_to_state_evaluation = true;
+                        continue;
                     }
                 }
             }
@@ -454,9 +569,30 @@ class parser
                     continue;
                 }
 
-                return sax->parse_error(m_lexer.get_position(),
-                                        m_lexer.get_token_string(),
-                                        parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_array, "array"), nullptr));
+                if (!report_error(sax, parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_array, "array"), nullptr), allow_recovery))
+                {
+                    return false;
+                }
+
+                // recover
+                if (last_token == token_type::end_of_input)
+                {
+                    // the input ends inside the array
+                    return close_containers(sax, states, allow_recovery);
+                }
+                if (last_token == token_type::end_object)
+                {
+                    // a wrong closing bracket closes the innermost container
+                    if (JSON_HEDLEY_UNLIKELY(!sax->end_array()))
+                    {
+                        return false;
+                    }
+                    states.pop_back();
+                    skip_to_state_evaluation = true;
+                }
+                // otherwise, a missing ',' (or a stray ':', which value
+                // parsing drops): the next value begins here
+                continue;
             }
 
             // states.back() is false -> object
@@ -473,11 +609,12 @@ class parser
                     // parse key
                     if (JSON_HEDLEY_UNLIKELY(last_token != token_type::value_string))
                     {
-                        return sax->parse_error(m_lexer.get_position(),
-                                                m_lexer.get_token_string(),
-                                                parse_error::create(101, m_lexer.get_position(), exception_message(token_type::value_string, "object key"), nullptr));
+                        if (!continue_after(key_error(sax, allow_recovery, false), skip_to_state_evaluation))
+                        {
+                            return false;
+                        }
+                        continue;
                     }
-
                     if (JSON_HEDLEY_UNLIKELY(!sax->key(m_lexer.get_string())))
                     {
                         return false;
@@ -486,9 +623,11 @@ class parser
                     // parse separator (:)
                     if (JSON_HEDLEY_UNLIKELY(!get_token_expecting(token_type::name_separator)))
                     {
-                        return sax->parse_error(m_lexer.get_position(),
-                                                m_lexer.get_token_string(),
-                                                parse_error::create(101, m_lexer.get_position(), exception_message(token_type::name_separator, "object separator"), nullptr));
+                        if (!continue_after(key_error(sax, allow_recovery, true), skip_to_state_evaluation))
+                        {
+                            return false;
+                        }
+                        continue;
                     }
 
                     // parse values
@@ -516,12 +655,546 @@ class parser
                 continue;
             }
 
-            return sax->parse_error(m_lexer.get_position(),
-                                    m_lexer.get_token_string(),
-                                    parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_object, "object"), nullptr));
+            if (!report_error(sax, parse_error::create(101, m_lexer.get_position(), exception_message(token_type::end_object, "object"), nullptr), allow_recovery))
+            {
+                return false;
+            }
+
+            // recover
+            if (last_token == token_type::end_of_input)
+            {
+                // the input ends inside the object
+                return close_containers(sax, states, allow_recovery);
+            }
+            if (last_token == token_type::end_array)
+            {
+                // a wrong closing bracket closes the innermost container
+                if (JSON_HEDLEY_UNLIKELY(!sax->end_object()))
+                {
+                    return false;
+                }
+                states.pop_back();
+                skip_to_state_evaluation = true;
+                continue;
+            }
+            if (!continue_after(recover_member(sax, allow_recovery), skip_to_state_evaluation))
+            {
+                return false;
+            }
         }
     }
 
+    /*!
+    @brief continue sax_parse_internal() after a recovery
+    @return whether to continue parsing
+    */
+    bool continue_after(const next_step step, bool& skip_to_state_evaluation)
+    {
+        if (step == next_step::evaluate_state)
+        {
+            // the state evaluation reads the token again
+            m_lexer.unget_token();
+            skip_to_state_evaluation = true;
+        }
+        return step != next_step::stop;
+    }
+
+    /// the parser for parse() and accept() never recovers: stop parsing
+    JSON_INTERNAL_ALWAYS_INLINE
+    static std::false_type continue_after(std::false_type /*step*/, bool& /*skip_to_state_evaluation*/) noexcept
+    {
+        return {};
+    }
+
+    /*!
+    @brief parse an object key and the name separator (:) after it
+
+    last_token is the token where the key is expected. sax_parse_internal()
+    repeats these steps rather than calling this function, which is used
+    when recovering from an error.
+
+    @return next_step::parse_value if the value follows, with last_token its
+            first token; next_step::evaluate_state if the object's state is
+            to be evaluated after recovering from an error; next_step::stop
+            to stop parsing
+    */
+    template<typename SAX>
+    next_step parse_key(SAX* sax)
+    {
+        const std::true_type allow_recovery{};
+
+        if (JSON_HEDLEY_UNLIKELY(last_token != token_type::value_string))
+        {
+            return key_error(sax, allow_recovery, false);
+        }
+
+        if (JSON_HEDLEY_UNLIKELY(!sax->key(m_lexer.get_string())))
+        {
+            return next_step::stop;
+        }
+
+        // parse separator (:)
+        if (JSON_HEDLEY_UNLIKELY(get_token() != token_type::name_separator))
+        {
+            return key_error(sax, allow_recovery, true);
+        }
+
+        // the value begins with the next token
+        get_token();
+        return next_step::parse_value;
+    }
+
+    /*!
+    @brief report a number that is too large for number_float_t, and recover
+           from the error by passing the value on; the SAX parser gets the
+           number's text as well
+
+    This is a separate function, as reading other numbers is measurably
+    slower if the error is handled where they are read.
+
+    @param[in] sax    the SAX parser
+    @param[in] value  the value that is not finite
+    @return whether to continue parsing
+    */
+    template<typename SAX>
+    bool overflow_error(SAX* sax, const number_float_t value, std::true_type allow_recovery)
+    {
+        if (!report_error(sax, out_of_range::create(406, concat("number overflow parsing '", m_lexer.get_token_string(), '\''), nullptr), allow_recovery))
+        {
+            return false;
+        }
+        return sax->number_float(value, m_lexer.get_string());
+    }
+
+    /// @copydoc overflow_error
+    template<typename SAX>
+    JSON_INTERNAL_ALWAYS_INLINE
+    std::false_type overflow_error(SAX* sax, const number_float_t /*value*/, std::false_type allow_recovery)
+    {
+        return report_error(sax, out_of_range::create(406, concat("number overflow parsing '", m_lexer.get_token_string(), '\''), nullptr), allow_recovery);
+    }
+
+    /*!
+    @brief report a missing key, or a missing name separator (:) after the
+           key; the parser for parse() and accept() never recovers
+
+    @param[in] key_read  whether the key was read, so that the name separator
+                         is missing
+    @return std::false_type, see report_error()
+    */
+    template<typename SAX>
+    JSON_INTERNAL_ALWAYS_INLINE
+    std::false_type key_error(SAX* sax, std::false_type allow_recovery, const bool key_read)
+    {
+        return report_error(sax, parse_error::create(101, m_lexer.get_position(), key_read
+                            ? exception_message(token_type::name_separator, "object separator")
+                            : exception_message(token_type::value_string, "object key"), nullptr), allow_recovery);
+    }
+
+    /*!
+    @brief report a missing key, or a missing name separator (:) after the
+           key, and recover from it
+
+    @param[in] key_read  whether the key was read, so that the name separator
+                         is missing
+    */
+    template<typename SAX>
+    next_step key_error(SAX* sax, std::true_type allow_recovery, const bool key_read)
+    {
+        if (!key_read)
+        {
+            if (!report_error(sax, parse_error::create(101, m_lexer.get_position(), exception_message(token_type::value_string, "object key"), nullptr), allow_recovery))
+            {
+                return next_step::stop;
+            }
+            return recover_key(sax);
+        }
+
+        if (!report_error(sax, parse_error::create(101, m_lexer.get_position(), exception_message(token_type::name_separator, "object separator"), nullptr), allow_recovery))
+        {
+            return next_step::stop;
+        }
+        return recover_name_separator(sax);
+    }
+
+    /////////////////////
+    // error recovery
+    /////////////////////
+
+    /*
+    The functions below repair an error after the SAX parser's parse_error()
+    returned true (see #3989). Each mistake is repaired by the smallest local
+    edit: a missing ',' or ':' is inserted, a stray token is removed, what can
+    be read of an invalid string or number is kept (see
+    lexer::recover_token()), a missing value becomes null, a wrong closing
+    bracket closes the innermost container, and the end of the input closes
+    all of them. The events stay balanced, and every key() is followed by
+    exactly one value.
+
+    A repair hands a token to the state evaluation, by returning it to the
+    lexer (lexer::unget_token()) so that the state evaluation reads it again,
+    only if it is ',', ']', '}', or the end of the input. The state evaluation
+    hands a token to value or key parsing only if it is none of them, so a
+    token is never handed back and forth. Every other step reads a token or
+    closes a container, so parsing always ends.
+    */
+
+    /*!
+    @brief report an error to the SAX parser; the parser for parse() and
+           accept() never recovers
+
+    @return std::false_type rather than false: its value is known where the
+            function is called even if the call is not inlined, so the code
+            for recovering is not generated
+    */
+    template<typename SAX, typename Exception>
+    JSON_INTERNAL_ALWAYS_INLINE
+    std::false_type report_error(SAX* sax, const Exception& ex, std::false_type /*allow_recovery*/)
+    {
+        static_cast<void>(sax); // MSVC 2015 does not count calling a static parse_error() as using it
+        error_reported = true;
+        static_cast<void>(sax->parse_error(m_lexer.get_position(), m_lexer.get_token_string(), ex));
+        return {};
+    }
+
+    /*!
+    @brief report an error to the SAX parser
+    @return whether to recover from the error
+    */
+    template<typename SAX, typename Exception>
+    bool report_error(SAX* sax, const Exception& ex, std::true_type /*allow_recovery*/)
+    {
+        static_cast<void>(sax); // MSVC 2015 does not count calling a static parse_error() as using it
+        const std::size_t position = m_lexer.get_position().chars_read_total;
+        if (error_reported && position == last_error_position && last_token == last_error_token)
+        {
+            // a repair handed on the token of the error it repaired; the
+            // token was reported already, and the SAX parser asked to recover
+            return true;
+        }
+
+        error_reported = true;
+        last_error_position = position;
+        last_error_token = last_token;
+
+        if (!sax->parse_error(m_lexer.get_position(), m_lexer.get_token_string(), ex))
+        {
+            return false;
+        }
+
+        // the token string of the next error begins here
+        m_lexer.restart_token_string();
+        return true;
+    }
+
+    /*!
+    @brief keep what can be read of the token that the lexer rejected
+
+    The error was reported for the rejected token, so it is not reported again
+    for the token it is repaired to (see lexer::recover_token()).
+    */
+    token_type recover_token()
+    {
+        last_token = m_lexer.recover_token();
+        last_error_position = m_lexer.get_position().chars_read_total;
+        last_error_token = last_token;
+        return last_token;
+    }
+
+    // The functions below are called from sax_parse_internal() after an error
+    // was reported. The parser for parse() and accept() stopped there, so for
+    // it they are stubs: the code for recovering is then not even referenced,
+    // and unoptimized builds do not emit it either.
+
+    /// @copydoc recover_token()
+    JSON_INTERNAL_ALWAYS_INLINE
+    token_type recover_token(std::true_type /*allow_recovery*/)
+    {
+        return recover_token();
+    }
+
+    /// @copydoc recover_token()
+    JSON_INTERNAL_ALWAYS_INLINE
+    static std::false_type recover_token(std::false_type /*allow_recovery*/) noexcept
+    {
+        return {};
+    }
+
+    /// return the token that was read last to the lexer (see lexer::unget_token())
+    JSON_INTERNAL_ALWAYS_INLINE
+    void unget_token(std::true_type /*allow_recovery*/)
+    {
+        m_lexer.unget_token();
+    }
+
+    /// @copydoc unget_token(std::true_type)
+    JSON_INTERNAL_ALWAYS_INLINE
+    static void unget_token(std::false_type /*allow_recovery*/) noexcept {}
+
+    /// @copydoc skip_to_value
+    JSON_INTERNAL_ALWAYS_INLINE
+    static std::false_type skip_to_value(std::false_type /*allow_recovery*/) noexcept
+    {
+        return {};
+    }
+
+    /// @copydoc recover_missing_value
+    template<typename SAX>
+    JSON_INTERNAL_ALWAYS_INLINE
+    static std::false_type recover_missing_value(SAX* /*sax*/, const std::vector<bool>& /*states*/, std::false_type /*allow_recovery*/) noexcept
+    {
+        return {};
+    }
+
+    /// pass the end events of all open containers
+    template<typename SAX>
+    bool close_containers(SAX* sax, std::vector<bool>& states, std::true_type /*allow_recovery*/)
+    {
+        while (!states.empty())
+        {
+            const bool is_array = states.back();
+            states.pop_back();
+            if (JSON_HEDLEY_UNLIKELY(is_array ? !sax->end_array() : !sax->end_object()))
+            {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    /// @copydoc close_containers
+    template<typename SAX>
+    JSON_INTERNAL_ALWAYS_INLINE
+    static std::false_type close_containers(SAX* /*sax*/, std::vector<bool>& /*states*/, std::false_type /*allow_recovery*/) noexcept
+    {
+        return {};
+    }
+
+    /*!
+    @brief read tokens until one begins a value, skipping everything before
+           the top-level value
+    @return whether a value begins with last_token
+    */
+    bool skip_to_value(std::true_type /*allow_recovery*/)
+    {
+        while (true)
+        {
+            switch (get_token())
+            {
+                case token_type::begin_array:
+                case token_type::begin_object:
+                case token_type::literal_false:
+                case token_type::literal_null:
+                case token_type::literal_true:
+                case token_type::value_float:
+                case token_type::value_integer:
+                case token_type::value_string:
+                case token_type::value_unsigned:
+                    return true;
+
+                case token_type::end_of_input:
+                    return false;
+
+                case token_type::parse_error:
+                    recover_token();
+                    if (last_token != token_type::uninitialized)
+                    {
+                        return true;
+                    }
+                    break;
+
+                case token_type::uninitialized:
+                case token_type::end_array:
+                case token_type::end_object:
+                case token_type::name_separator:
+                case token_type::value_separator:
+                case token_type::literal_or_value:
+                default:
+                    break;
+            }
+        }
+    }
+
+    /*!
+    @brief skip the rest of an object member that cannot be read
+
+    Reads tokens, beginning with last_token, until a ',', '}', or ']' that is
+    not inside a container that begins in the skipped tokens, or the end of
+    the input.
+    */
+    void skip_member()
+    {
+        std::size_t depth = 0;
+        while (true)
+        {
+            switch (last_token)
+            {
+                case token_type::begin_array:
+                case token_type::begin_object:
+                    ++depth;
+                    break;
+
+                case token_type::end_array:
+                case token_type::end_object:
+                    if (depth == 0)
+                    {
+                        return;
+                    }
+                    --depth;
+                    break;
+
+                case token_type::value_separator:
+                    if (depth == 0)
+                    {
+                        return;
+                    }
+                    break;
+
+                case token_type::end_of_input:
+                    return;
+
+                case token_type::parse_error:
+                    recover_token();
+                    break;
+
+                case token_type::uninitialized:
+                case token_type::literal_true:
+                case token_type::literal_false:
+                case token_type::literal_null:
+                case token_type::value_string:
+                case token_type::value_unsigned:
+                case token_type::value_integer:
+                case token_type::value_float:
+                case token_type::name_separator:
+                case token_type::literal_or_value:
+                default:
+                    break;
+            }
+            get_token();
+        }
+    }
+
+    /*!
+    @brief pass a value where it is missing
+
+    last_token is ',', ']', '}', or the end of the input, where a value was
+    expected. In an object, the key gets null; in an array, a ',' where a
+    value is missing stands for null (as in JavaScript), while an array that
+    ends there just ends.
+    */
+    template<typename SAX>
+    bool recover_missing_value(SAX* sax, const std::vector<bool>& states, std::true_type /*allow_recovery*/)
+    {
+        JSON_ASSERT(!states.empty());
+        if (!states.back() || last_token == token_type::value_separator)
+        {
+            return sax->null();
+        }
+        return true;
+    }
+
+    /// recover from a missing key; last_token is where it was expected
+    template<typename SAX>
+    next_step recover_key(SAX* sax)
+    {
+        switch (last_token)
+        {
+            case token_type::value_separator:
+            case token_type::end_object:
+            case token_type::end_array:
+            case token_type::end_of_input:
+                // no member: the object's state handles the token
+                return next_step::evaluate_state;
+
+            case token_type::parse_error:
+                recover_token();
+                if (last_token == token_type::value_string)
+                {
+                    // a key that could be repaired
+                    return parse_key(sax);
+                }
+                skip_member();
+                return next_step::evaluate_state;
+
+            case token_type::uninitialized:
+            case token_type::literal_true:
+            case token_type::literal_false:
+            case token_type::literal_null:
+            case token_type::value_string:
+            case token_type::value_unsigned:
+            case token_type::value_integer:
+            case token_type::value_float:
+            case token_type::begin_array:
+            case token_type::begin_object:
+            case token_type::name_separator:
+            case token_type::literal_or_value:
+            default:
+                // a member without a key
+                skip_member();
+                return next_step::evaluate_state;
+        }
+    }
+
+    /// recover from a missing name separator (:) after the key; last_token
+    /// is where it was expected
+    template<typename SAX>
+    next_step recover_name_separator(SAX* sax)
+    {
+        switch (last_token)
+        {
+            case token_type::value_separator:
+            case token_type::end_object:
+            case token_type::end_array:
+            case token_type::end_of_input:
+                // the value is missing as well
+                return sax->null() ? next_step::evaluate_state : next_step::stop;
+
+            case token_type::uninitialized:
+            case token_type::literal_true:
+            case token_type::literal_false:
+            case token_type::literal_null:
+            case token_type::value_string:
+            case token_type::value_unsigned:
+            case token_type::value_integer:
+            case token_type::value_float:
+            case token_type::begin_array:
+            case token_type::begin_object:
+            case token_type::name_separator:
+            case token_type::parse_error:
+            case token_type::literal_or_value:
+            default:
+                // a missing ':'; the value begins here
+                return next_step::parse_value;
+        }
+    }
+
+    /// recover from a token after an object member that is neither ',' nor
+    /// '}' (nor ']' or the end of the input, which the caller handles)
+    template<typename SAX>
+    next_step recover_member(SAX* sax, std::true_type /*allow_recovery*/)
+    {
+        if (last_token == token_type::parse_error)
+        {
+            recover_token();
+        }
+        if (last_token == token_type::value_string)
+        {
+            // a missing ','; the next key begins here
+            return parse_key(sax);
+        }
+        skip_member();
+        return next_step::evaluate_state;
+    }
+
+    /// the parser for parse() and accept() never recovers (and does not come
+    /// here, as report_error() returned false)
+    template<typename SAX>
+    JSON_INTERNAL_ALWAYS_INLINE
+    std::false_type recover_member(SAX* /*sax*/, std::false_type /*allow_recovery*/) const noexcept
+    {
+        return {};
+    }
+
     /// get next token from lexer
     token_type get_token()
     {
@@ -575,6 +1248,12 @@ class parser
     const bool allow_exceptions = true;
     /// whether trailing commas in objects and arrays should be ignored (true) or signaled as errors (false)
     const bool ignore_trailing_commas = false;
+    /// whether an error was reported to the SAX parser
+    bool error_reported = false;
+    /// the position of the last reported error
+    std::size_t last_error_position = 0;
+    /// the token of the last reported error
+    token_type last_error_token = token_type::uninitialized;
 };
 
 }  // namespace detail

include/nlohmann/detail/json_pointer.hpp

@@ -16,8 +16,8 @@
     #include <iosfwd> // ostream
 #endif  // JSON_NO_IO
 #include <limits> // max
+#include <map> // map
 #include <numeric> // accumulate
-#include <set> // set
 #include <string> // string
 #include <utility> // move
 #include <vector> // vector
@@ -359,33 +359,82 @@ class json_pointer
 
   private:
     /*!
-    @brief the reference token sequences that denote arrays
+    @brief the pointer prefixes of a flattened object, and which of them denote arrays
 
     @ref unflatten collects the pointer prefixes that have a reference token 0
     among their children; @ref get_and_create creates arrays exactly below
     those prefixes and objects everywhere else. Deciding this up front keeps
     the result independent of the order in which the flattened object is
     iterated, which is unspecified for some object types.
+
+    The prefixes form a tree and are numbered, so each of them is stored only
+    once (as a node) rather than as a copy of all of its reference tokens.
     */
-    using array_parents_t = std::set<std::vector<string_t>>;
+    struct prefix_tree
+    {
+        // children[id] maps a reference token to the number of the prefix
+        // extended by that token; number 0 is the empty prefix
+        std::vector<std::map<string_t, std::size_t>> children;
+        // is_array[id] is true iff some flattened key has the reference token
+        // 0 directly below the prefix with number id
+        std::vector<bool> is_array;
+
+        // start with the empty prefix only
+        prefix_tree()
+            : children(1)
+            , is_array(1, false)
+        {}
+
+        // return the number of the prefix with number id extended by
+        // reference_token, adding it if it is new
+        std::size_t add_child(std::size_t id, string_t&& reference_token)
+        {
+            if (reference_token == "0")
+            {
+                is_array[id] = true;
+            }
+
+            // read the number before the emplace_back below, which may
+            // reallocate children and invalidate the iterator
+            const std::size_t next = children.size();
+            const auto inserted = children[id].emplace(std::move(reference_token), next);
+            const std::size_t child = inserted.first->second;
+            if (inserted.second)
+            {
+                children.emplace_back();
+                is_array.push_back(false);
+            }
+            return child;
+        }
+
+        // return the number of the prefix with number id extended by
+        // reference_token, which must have been added before
+        std::size_t find_child(std::size_t id, const string_t& reference_token) const
+        {
+            const auto it = children[id].find(reference_token);
+            JSON_ASSERT(it != children[id].end());
+            return it->second;
+        }
+    };
 
     /*!
     @brief create and return a reference to the pointed to value
 
-    Complexity: Linear in the number of reference tokens.
+    Complexity: Linear in the number of reference tokens (times the logarithm
+    of the number of siblings for the prefix lookup).
 
     @throw parse_error.106 if an array index begins with '0'
     @throw parse_error.109 if array index is not a number
     @throw type_error.313 if value cannot be unflattened
     */
     template<typename BasicJsonType>
-    BasicJsonType& get_and_create(BasicJsonType& j, const array_parents_t& array_parents) const
+    BasicJsonType& get_and_create(BasicJsonType& j, const prefix_tree& tree) const
     {
         auto* result = &j;
 
-        // the reference tokens that have been consumed so far; used to look up
-        // whether the value to be created below is an array or an object
-        std::vector<string_t> prefix;
+        // the number of the prefix consumed so far; used to look up whether
+        // the value to be created below is an array or an object
+        std::size_t id = 0;
 
         // in case no reference tokens exist, return a reference to the JSON value
         // j which will be overwritten by a primitive value
@@ -395,7 +444,7 @@ class json_pointer
             {
                 case detail::value_t::null:
                 {
-                    if (array_parents.find(prefix) != array_parents.end())
+                    if (tree.is_array[id])
                     {
                         // some reference token below this position is 0, so the
                         // value is an array
@@ -440,7 +489,7 @@ class json_pointer
                     JSON_THROW(detail::type_error::create(313, "invalid value to unflatten", &j));
             }
 
-            prefix.push_back(reference_token);
+            id = tree.find_child(id, reference_token);
         }
 
         return *result;
@@ -878,64 +927,131 @@ class json_pointer
     @param[in,out] result        the result object to insert values to
 
     @note Empty objects or arrays are flattened to `null`.
+
+    The value is walked with an explicit stack rather than the call stack, so
+    arbitrarily deeply nested values can be flattened.
+
+    @sa https://github.com/nlohmann/json/issues/5393
     */
     template<typename BasicJsonType>
     static void flatten(const string_t& reference_string,
                         const BasicJsonType& value,
                         BasicJsonType& result)
     {
-        switch (value.type())
+        using object_const_iterator = typename BasicJsonType::object_t::const_iterator;
+
+        // an array or object being walked: the container, the array index or
+        // object iterator of the next child, and the length of the path of the
+        // container itself
+        struct frame
         {
-            case detail::value_t::array:
-            {
-                if (value.m_data.m_value.array->empty())
-                {
-                    // flatten empty array as null
-                    result[reference_string] = nullptr;
-                }
-                else
-                {
-                    // iterate array and use index as a reference string
-                    for (std::size_t i = 0; i < value.m_data.m_value.array->size(); ++i)
-                    {
-                        flatten(detail::concat<string_t>(reference_string, '/', std::to_string(i)),
-                                value.m_data.m_value.array->operator[](i), result);
-                    }
-                }
-                break;
-            }
+            frame(const BasicJsonType* container_, object_const_iterator member_, const std::size_t path_length_) noexcept
+                : container(container_), member(std::move(member_)), path_length(path_length_)
+            {}
 
-            case detail::value_t::object:
-            {
-                if (value.m_data.m_value.object->empty())
-                {
-                    // flatten empty object as null
-                    result[reference_string] = nullptr;
-                }
-                else
-                {
-                    // iterate object and use keys as reference string
-                    for (const auto& element : *value.m_data.m_value.object)
-                    {
-                        flatten(detail::concat<string_t>(reference_string, '/', detail::escape(element.first)), element.second, result);
-                    }
-                }
-                break;
-            }
+            const BasicJsonType* container;
+            std::size_t index = 0;
+            object_const_iterator member;
+            std::size_t path_length;
+        };
 
-            case detail::value_t::null:
-            case detail::value_t::string:
-            case detail::value_t::boolean:
-            case detail::value_t::number_integer:
-            case detail::value_t::number_unsigned:
-            case detail::value_t::number_float:
-            case detail::value_t::binary:
-            case detail::value_t::discarded:
-            default:
+        // The containers being flattened are kept on an explicit stack, and
+        // every child is flattened completely before the next one, so the
+        // entries come out in the same order as with a recursive walk. The
+        // path of the value being flattened is kept in one buffer that grows
+        // and shrinks with the stack, rather than in a new string per level.
+        std::vector<frame> stack;
+        string_t path = reference_string;
+
+        // flatten `v`, whose path is `path`: primitives and empty containers
+        // are added to the result right away; other containers get a frame
+        const auto enter = [&stack, &path, &result](const BasicJsonType & v)
+        {
+            switch (v.type())
             {
-                // add a primitive value with its reference string
-                result[reference_string] = value;
-                break;
+                case detail::value_t::array:
+                {
+                    if (v.m_data.m_value.array->empty())
+                    {
+                        // flatten empty array as null
+                        result[path] = nullptr;
+                    }
+                    else
+                    {
+                        stack.emplace_back(&v, object_const_iterator(), path.size());
+                    }
+                    return;
+                }
+
+                case detail::value_t::object:
+                {
+                    if (v.m_data.m_value.object->empty())
+                    {
+                        // flatten empty object as null
+                        result[path] = nullptr;
+                    }
+                    else
+                    {
+                        stack.emplace_back(&v, v.m_data.m_value.object->begin(), path.size());
+                    }
+                    return;
+                }
+
+                case detail::value_t::null:
+                case detail::value_t::string:
+                case detail::value_t::boolean:
+                case detail::value_t::number_integer:
+                case detail::value_t::number_unsigned:
+                case detail::value_t::number_float:
+                case detail::value_t::binary:
+                case detail::value_t::discarded:
+                default:
+                {
+                    // add a primitive value with its reference string
+                    result[path] = v;
+                    return;
+                }
+            }
+        };
+
+        enter(value);
+        while (!stack.empty())
+        {
+            // the frame is changed through stack.back(): enter() may push a
+            // frame, which would invalidate a reference to it
+            const BasicJsonType* const container = stack.back().container;
+
+            // drop the path of the previous child
+            path.resize(stack.back().path_length);
+
+            if (container->is_array())
+            {
+                const auto& array = *container->m_data.m_value.array;
+                const std::size_t i = stack.back().index;
+                if (i == array.size())
+                {
+                    stack.pop_back();
+                    continue;
+                }
+
+                // iterate array and use index as a reference string
+                ++stack.back().index;
+                detail::concat_into(path, '/', detail::to_string<string_t>(i));
+                enter(array[i]);
+            }
+            else
+            {
+                const object_const_iterator it = stack.back().member;
+                if (it == container->m_data.m_value.object->end())
+                {
+                    stack.pop_back();
+                    continue;
+                }
+
+                // iterate object and use keys as reference string
+                ++stack.back().member;
+                detail::concat_into(path, '/', detail::escape(it->first));
+                enter(it->second);
             }
         }
     }
@@ -963,19 +1079,15 @@ class json_pointer
 
         // collect the pointer prefixes that have a reference token 0 among
         // their children; the values below them are arrays, all others are
-        // objects (see array_parents_t)
-        array_parents_t array_parents;
+        // objects (see prefix_tree)
+        prefix_tree tree;
         for (const auto& element : *value.m_data.m_value.object)
         {
             json_pointer ptr(element.first);
-            std::vector<string_t> prefix;
+            std::size_t id = 0;
             for (auto& reference_token : ptr.reference_tokens)
             {
-                if (reference_token == "0")
-                {
-                    array_parents.insert(prefix);
-                }
-                prefix.push_back(std::move(reference_token));
+                id = tree.add_child(id, std::move(reference_token));
             }
         }
 
@@ -991,7 +1103,7 @@ class json_pointer
             // that if the JSON pointer is "" (i.e., points to the whole value),
             // function get_and_create returns a reference to the result itself.
             // An assignment will then create a primitive value.
-            json_pointer(element.first).get_and_create(result, array_parents) = element.second;
+            json_pointer(element.first).get_and_create(result, tree) = element.second;
         }
 
         return result;

include/nlohmann/detail/macro_scope.hpp

@@ -199,6 +199,17 @@
     #define JSON_NO_UNIQUE_ADDRESS
 #endif
 
+// Inlines small functions even in unoptimized builds, so that they are not
+// emitted. The parsers for parse(), accept(), and from_*() use it for the
+// functions that stand in for the code recovering from errors (see #3989).
+// MSVC is left to decide, as it warns (C4714) where it does not inline a
+// __forceinline function.
+#if defined(_MSC_VER) && !defined(__clang__)
+    #define JSON_INTERNAL_ALWAYS_INLINE
+#else
+    #define JSON_INTERNAL_ALWAYS_INLINE JSON_HEDLEY_ALWAYS_INLINE
+#endif
+
 // Clang targeting MinGW does not survive the thread_local storage the copy
 // constructor uses to bound its descent: every test that copies a value
 // segfaults with clang 11.0.1 and clang 18.1.8, while the same tests pass with

include/nlohmann/detail/macro_unscope.hpp

@@ -19,6 +19,7 @@
 #undef NLOHMANN_CAN_CALL_STD_FUNC_IMPL
 #undef JSON_INLINE_VARIABLE
 #undef JSON_NO_UNIQUE_ADDRESS
+#undef JSON_INTERNAL_ALWAYS_INLINE
 #undef JSON_DISABLE_ENUM_SERIALIZATION
 #undef JSON_DISABLE_TUPLE_REFERENCE_CONVERSION
 

include/nlohmann/detail/output/binary_writer.hpp

@@ -85,6 +85,12 @@ template<typename BasicJsonType, typename CharType, typename OutputSinkType = ou
 class binary_writer
 {
     using string_t = typename BasicJsonType::string_t;
+
+    /// an object key as string_t: a reference when object_t::key_type already is
+    /// string_t, otherwise a converted copy that outlives sanitize_utf8_for_write's result
+    using object_key_string_t = typename std::conditional <
+                                std::is_same<typename BasicJsonType::object_t::key_type, string_t>::value,
+                                const string_t&, string_t >::type;
     using binary_t = typename BasicJsonType::binary_t;
     using number_float_t = typename BasicJsonType::number_float_t;
 
@@ -244,16 +250,7 @@ class binary_writer
 
             case value_t::string:
             {
-                string_t storage;
-                const string_t& value = sanitize_utf8_for_write(*j.m_data.m_value.string, j, storage);
-
-                // step 1: write control byte and the string length
-                write_cbor_head(0x60, value.size());
-
-                // step 2: write the string
-                oa.write_characters(
-                      reinterpret_cast<const CharType*>(value.data()),
-                      value.size());
+                write_cbor_string(*j.m_data.m_value.string, j);
                 break;
             }
 
@@ -316,23 +313,20 @@ class binary_writer
 
             case value_t::object:
             {
+                static_assert(
+                    std::is_convertible <
+                    typename BasicJsonType::object_t::key_type,
+                    string_t >::value,
+                    "object_t::key_type must be implicitly convertible to string_t");
+
                 // step 1: write control byte and the object size
                 write_cbor_head(0xA0, j.m_data.m_value.object->size());
 
                 for (const auto& el : *j.m_data.m_value.object)
                 {
-                    // el.first is checked here, against the object as
-                    // diagnostics context, because write_cbor(el.first)
-                    // converts it to a temporary basic_json that would be
-                    // used as the context instead; for error_handler_t::keep
-                    // and ::replace/::ignore the recursive write_cbor(el.first)
-                    // call below handles the key like any other string, so no
-                    // separate check is needed here for those
-                    if (error_handler == error_handler_t::strict)
-                    {
-                        check_utf8(el.first, j);
-                    }
-                    write_cbor(el.first);
+                    // el.first is written directly (not via a temporary
+                    // basic_json), with the object as diagnostics context
+                    write_cbor_string(el.first, j);
                     write_cbor(el.second, depth + 1);
                 }
                 break;
@@ -491,39 +485,7 @@ class binary_writer
 
             case value_t::string:
             {
-                string_t storage;
-                const string_t& value = sanitize_utf8_for_write(*j.m_data.m_value.string, j, storage);
-
-                // step 1: write control byte and the string length
-                const auto N = to_msgpack_length(value.size(), j);
-                if (N <= 31)
-                {
-                    // fixstr
-                    write_number(static_cast<std::uint8_t>(0xA0 | N));
-                }
-                else if (N <= (std::numeric_limits<std::uint8_t>::max)())
-                {
-                    // str 8
-                    oa.write_character(to_char_type(0xD9));
-                    write_number(static_cast<std::uint8_t>(N));
-                }
-                else if (N <= (std::numeric_limits<std::uint16_t>::max)())
-                {
-                    // str 16
-                    oa.write_character(to_char_type(0xDA));
-                    write_number(static_cast<std::uint16_t>(N));
-                }
-                else
-                {
-                    // str 32
-                    oa.write_character(to_char_type(0xDB));
-                    write_number(static_cast<std::uint32_t>(N));
-                }
-
-                // step 2: write the string
-                oa.write_characters(
-                      reinterpret_cast<const CharType*>(value.data()),
-                      value.size());
+                write_msgpack_string(*j.m_data.m_value.string, j);
                 break;
             }
 
@@ -629,19 +591,20 @@ class binary_writer
 
             case value_t::object:
             {
+                static_assert(
+                    std::is_convertible <
+                    typename BasicJsonType::object_t::key_type,
+                    string_t >::value,
+                    "object_t::key_type must be implicitly convertible to string_t");
+
                 // step 1: write control byte and the object size
                 write_msgpack_object_prefix(j.m_data.m_value.object->size(), j);
 
                 for (const auto& el : *j.m_data.m_value.object)
                 {
-                    // as in write_cbor, el.first is checked here against the
-                    // object as diagnostics context; the recursive call below
-                    // handles keep/replace/ignore like any other string
-                    if (error_handler == error_handler_t::strict)
-                    {
-                        check_utf8(el.first, j);
-                    }
-                    write_msgpack(el.first);
+                    // as in write_cbor, el.first is written directly with the
+                    // object as diagnostics context
+                    write_msgpack_string(el.first, j);
                     write_msgpack(el.second, depth + 1);
                 }
                 break;
@@ -819,8 +782,10 @@ class binary_writer
 
                 for (const auto& el : *j.m_data.m_value.object)
                 {
+                    // a converted key must outlive the reference returned by sanitize_utf8_for_write
+                    const object_key_string_t key_string = el.first;
                     string_t storage;
-                    const string_t& key = sanitize_utf8_for_write(el.first, j, storage);
+                    const string_t& key = sanitize_utf8_for_write(key_string, j, storage);
                     write_number_with_ubjson_prefix(key.size(), true, use_bjdata);
                     oa.write_characters(
                           reinterpret_cast<const CharType*>(key.data()),
@@ -1025,13 +990,9 @@ class binary_writer
                     continue;
                 }
 
-                // el.first is checked here, against the object as diagnostics
-                // context, like the matching check in write_cbor's object case
-                if (error_handler == error_handler_t::strict)
-                {
-                    check_utf8(current.object_it->first, *current.value);
-                }
-                write_cbor(current.object_it->first);
+                // the key is written directly (not via a temporary basic_json),
+                // with the object as diagnostics context, as in write_cbor
+                write_cbor_string(current.object_it->first, *current.value);
                 const BasicJsonType* child = &(current.object_it->second);
                 ++stack.back().object_it;
                 write_cbor_value_or_push(*child, stack);
@@ -1106,11 +1067,9 @@ class binary_writer
                     continue;
                 }
 
-                if (error_handler == error_handler_t::strict)
-                {
-                    check_utf8(current.object_it->first, *current.value);
-                }
-                write_msgpack(current.object_it->first);
+                // as in write_cbor_iterative, the key is written directly with
+                // the object as diagnostics context
+                write_msgpack_string(current.object_it->first, *current.value);
                 const BasicJsonType* child = &(current.object_it->second);
                 ++stack.back().object_it;
                 write_msgpack_value_or_push(*child, stack);
@@ -1366,8 +1325,10 @@ class binary_writer
                     continue;
                 }
 
+                // a converted key must outlive the reference returned by sanitize_utf8_for_write
+                const object_key_string_t key_string = current.object_it->first;
                 string_t storage;
-                const string_t& key = sanitize_utf8_for_write(current.object_it->first, j, storage);
+                const string_t& key = sanitize_utf8_for_write(key_string, j, storage);
                 write_number_with_ubjson_prefix(key.size(), true, use_bjdata);
                 oa.write_characters(
                       reinterpret_cast<const CharType*>(key.data()),
@@ -1988,6 +1949,85 @@ class binary_writer
         }
     }
 
+    /*!
+    @brief write a CBOR text string
+
+    @a value is checked or sanitized according to @ref error_handler, with
+    @a context (the string value itself, or the object a key belongs to) used
+    as diagnostics context; this avoids converting object keys to a temporary
+    basic_json just to write them
+
+    @note When object_t::key_type is not string_t, @a value is a temporary
+          string_t converted from the key, which lives only until the end of
+          the caller's statement. The reference returned by
+          @ref sanitize_utf8_for_write may refer to it, so it must not escape
+          this function.
+    */
+    void write_cbor_string(const string_t& value, const BasicJsonType& context)
+    {
+        string_t storage;
+        const string_t& sanitized = sanitize_utf8_for_write(value, context, storage);
+
+        // step 1: write control byte and the string length
+        write_cbor_head(0x60, sanitized.size());
+
+        // step 2: write the string
+        oa.write_characters(
+              reinterpret_cast<const CharType*>(sanitized.data()),
+              sanitized.size());
+    }
+
+    /////////////
+    // MsgPack //
+    /////////////
+
+    /*!
+    @brief write a MessagePack str
+
+    @a value is checked or sanitized according to @ref error_handler, with
+    @a context used as diagnostics context, as in @ref write_cbor_string
+
+    @note As in @ref write_cbor_string, @a value may be a temporary string_t
+          converted from a key, so the reference returned by
+          @ref sanitize_utf8_for_write must not escape this function.
+    */
+    void write_msgpack_string(const string_t& value, const BasicJsonType& context)
+    {
+        string_t storage;
+        const string_t& sanitized = sanitize_utf8_for_write(value, context, storage);
+
+        // step 1: write control byte and the string length
+        const auto N = to_msgpack_length(sanitized.size(), context);
+        if (N <= 31)
+        {
+            // fixstr
+            write_number(static_cast<std::uint8_t>(0xA0 | N));
+        }
+        else if (N <= (std::numeric_limits<std::uint8_t>::max)())
+        {
+            // str 8
+            oa.write_character(to_char_type(0xD9));
+            write_number(static_cast<std::uint8_t>(N));
+        }
+        else if (N <= (std::numeric_limits<std::uint16_t>::max)())
+        {
+            // str 16
+            oa.write_character(to_char_type(0xDA));
+            write_number(static_cast<std::uint16_t>(N));
+        }
+        else
+        {
+            // str 32
+            oa.write_character(to_char_type(0xDB));
+            write_number(static_cast<std::uint32_t>(N));
+        }
+
+        // step 2: write the string
+        oa.write_characters(
+              reinterpret_cast<const CharType*>(sanitized.data()),
+              sanitized.size());
+    }
+
     ////////////
     // UBJSON //
     ////////////
@@ -2730,6 +2770,11 @@ class binary_writer
     itself in every case but a sanitized `replace`/`ignore` one, so @a
     storage must outlive the returned reference only then.
 
+    @a s must be an lvalue that outlives the returned reference. An object key
+    whose `key_type` is not @ref string_t must therefore first be converted
+    into a named string_t (see @ref object_key_string_t); the deleted overload
+    below enforces this at compile time.
+
     @param[in] s        the string (value or object key) to write
     @param[in] context  the value @a s belongs to (for diagnostics)
     @param[out] storage  backing storage for a sanitized copy
@@ -2759,6 +2804,10 @@ class binary_writer
         }
     }
 
+    /// deleted: anything but a string_t would bind a temporary that dies before the returned reference is used
+    template < typename T, enable_if_t < !std::is_same<T, string_t>::value, int > = 0 >
+    const string_t& sanitize_utf8_for_write(const T& /*s*/, const BasicJsonType& /*context*/, string_t& /*storage*/) const = delete; // NOLINT(hicpp-use-equals-delete,modernize-use-equals-delete): a private helper's guard, not part of the interface
+
     /*!
     @brief write an integer in the shortest encoding
 

include/nlohmann/detail/string_utils.hpp

@@ -13,6 +13,7 @@
 #include <cstddef> // size_t
 #include <cstdint> // uint8_t, uint32_t
 #include <string> // string, to_string
+#include <utility> // move
 
 #include <nlohmann/detail/abi_macros.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
@@ -108,8 +109,8 @@ void encode_utf8(std::uint32_t cp, const Out& out)
 ///////////////////
 
 // UTF-8 decoder states used by decode() below
-static constexpr std::uint8_t UTF8_ACCEPT = 0;
-static constexpr std::uint8_t UTF8_REJECT = 1;
+JSON_INLINE_VARIABLE constexpr std::uint8_t UTF8_ACCEPT = 0;
+JSON_INLINE_VARIABLE constexpr std::uint8_t UTF8_REJECT = 1;
 
 /*!
 @brief process a byte of a UTF-8 sequence

include/nlohmann/json.hpp

@@ -149,7 +149,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     friend class ::nlohmann::detail::iter_impl;
     template<typename BasicJsonType, typename CharType, typename OutputSinkType>
     friend class ::nlohmann::detail::binary_writer;
-    template<typename BasicJsonType, typename InputType, typename SAX>
+    template<typename BasicJsonType, typename InputType, typename SAX, bool AllowRecovery>
     friend class ::nlohmann::detail::binary_reader;
     template<typename BasicJsonType, typename InputAdapterType>
     friend class ::nlohmann::detail::json_sax_dom_parser;
@@ -5689,7 +5689,7 @@ public:
         auto ia = detail::input_adapter(std::forward<InputType>(i));
         return format == input_format_t::json
                ? parser(std::move(ia), nullptr, true, ignore_comments, ignore_trailing_commas).sax_parse(sax, strict)
-               : detail::binary_reader<basic_json, decltype(ia), SAX>(std::move(ia), format).sax_parse(sax, strict, tag_handler);
+               : detail::binary_reader<basic_json, decltype(ia), SAX, true>(std::move(ia), format).sax_parse(sax, strict, tag_handler);
     }
 
     /// @brief generate SAX events (iterator pair, or iterator+sentinel pair for C++20 ranges support)
@@ -5707,7 +5707,7 @@ public:
         auto ia = detail::input_adapter(std::move(first), std::move(last));
         return format == input_format_t::json
                ? parser(std::move(ia), nullptr, true, ignore_comments, ignore_trailing_commas).sax_parse(sax, strict)
-               : detail::binary_reader<basic_json, decltype(ia), SAX>(std::move(ia), format).sax_parse(sax, strict, tag_handler);
+               : detail::binary_reader<basic_json, decltype(ia), SAX, true>(std::move(ia), format).sax_parse(sax, strict, tag_handler);
     }
 
     /// @brief generate SAX events
@@ -5746,7 +5746,7 @@ public:
                // NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg)
                ? parser(std::move(ia), nullptr, true, ignore_comments, ignore_trailing_commas).sax_parse(sax, strict)
                // NOLINTNEXTLINE(hicpp-move-const-arg,performance-move-const-arg)
-               : detail::binary_reader<basic_json, decltype(ia), SAX>(std::move(ia), format).sax_parse(sax, strict, tag_handler);
+               : detail::binary_reader<basic_json, decltype(ia), SAX, true>(std::move(ia), format).sax_parse(sax, strict, tag_handler);
     }
 #endif
 #if defined(__clang__)

src/modules/json.cppm

@@ -47,6 +47,7 @@ inline namespace json_literals
 namespace detail
 {
     using NLOHMANN_JSON_NAMESPACE::detail::json_sax_dom_callback_parser;
+    using NLOHMANN_JSON_NAMESPACE::detail::json_sax_dom_parser;
     using NLOHMANN_JSON_NAMESPACE::detail::unknown_size;
 } // namespace detail
 

tests/CMakeLists.txt

@@ -138,7 +138,8 @@ json_test_set_test_options(test-disabled_exceptions
 # only the #972 regression test needs thirdparty/fifo_map on its include path
 json_test_set_test_options(test-regression1 LINK_LIBRARIES fifo_map_include)
 
-# GCC's false -Warray-bounds error with JSON_DIAGNOSTICS only shows up when optimizing (#5742).
+# Regression test for GCC's false -Warray-bounds error with JSON_DIAGNOSTICS (#5742, fixed in #5585). It only
+# showed up when optimizing, so build this test with -O3 and the warning as an error.
 # -O3 makes the optimizer-driven warnings of the ci_test_gcc flag set (-Winline,
 # -Wsuggest-attribute=...) fire on the library's inline functions; they are not
 # what this test checks, so turn them off for it.

tests/src/custom_object_key_type.hpp

@@ -0,0 +1,75 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <map>
+#include <memory>
+#include <string>
+#include <utility>
+#include <nlohmann/json.hpp>
+
+namespace custom_object_key_test
+{
+class key
+{
+  public:
+    key() = default;
+
+    key(const char* value)
+        : m_value(value)
+    {}
+
+    key(std::string value)
+        : m_value(std::move(value))
+    {}
+
+    operator std::string() const
+    {
+        return m_value;
+    }
+
+    // Required by JSON_DIAGNOSTICS, which reads object keys through data()
+    // when building the path of an exception.
+    const char* data() const noexcept
+    {
+        return m_value.data();
+    }
+
+    friend bool operator<(const key& lhs, const key& rhs)
+    {
+        return lhs.m_value < rhs.m_value;
+    }
+
+  private:
+    std::string m_value;
+};
+
+template<typename Key, typename Value, typename Compare, typename Allocator>
+class object
+    : public std::map <
+      key,
+      Value,
+      std::less<key>, // NOLINT(modernize-use-transparent-functors)
+      typename std::allocator_traits<Allocator>::template rebind_alloc <
+          std::pair<const key, Value >>>
+{
+  private:
+    using allocator_type =
+    typename std::allocator_traits<Allocator>::template rebind_alloc <
+        std::pair<const key, Value >>;
+
+    using base_type =
+        std::map<key, Value, std::less<key>, allocator_type>; // NOLINT(modernize-use-transparent-functors)
+
+  public:
+    using base_type::base_type;
+};
+
+using json = nlohmann::basic_json<object>;
+} // namespace custom_object_key_test

tests/src/fuzzer-parse_bjdata.cpp

@@ -47,6 +47,10 @@ dumps is stable under exactly the same values that break operator==.
 The unit tests run the same checks on a fixed corpus (see the "BJData round-trip
 invariants" test case), so keep both in sync.
 
+Furthermore, it reads data with a SAX parser that recovers from every error
+and checks that the events are balanced, that reading ends, and that it
+reports an error exactly when from_bjdata() fails (see #3989).
+
 The provided function `LLVMFuzzerTestOneInput` can be used in different fuzzer
 drivers.
 */
@@ -59,6 +63,8 @@ drivers.
     #error "the fuzzer drivers must be built without NDEBUG"
 #endif
 
+#include "fuzzer-recovering_checker.hpp"
+
 using json = nlohmann::json;
 
 // compares dumps rather than values, because NaN != NaN; keep writes strings
@@ -78,6 +84,9 @@ static bool is_value_stable(const json& lhs, const json& rhs)
 // see http://llvm.org/docs/LibFuzzer.html
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
 {
+    // recover from all errors, reading from memory and from a stream
+    const bool recovered_without_errors = check_recovering_parse(data, size, json::input_format_t::bjdata).errors == 0;
+
     std::vector<uint8_t> const vec1(data, data + size);
 
     // step 0: parse input without exceptions; a parse error must then be
@@ -110,6 +119,9 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
         // without exceptions, the same input must give the same value
         assert(!noexcept_threw && !j_noexcept.is_discarded() && same_value(j_noexcept, j1));
 
+        // the recovering parser must not have reported an error either
+        assert(recovered_without_errors);
+
         try
         {
             // step 2.1: round trip without adding size annotations to container types
@@ -143,6 +155,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
     {
         // parse errors are ok, because input may be random bytes
         assert(parsed || noexcept_threw || j_noexcept.is_discarded());
+        assert(parsed || !recovered_without_errors);
     }
     catch (const json::type_error&)
     {
@@ -153,6 +166,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
     {
         // out of range errors may happen if provided sizes are excessive
         assert(parsed || noexcept_threw || j_noexcept.is_discarded());
+        assert(parsed || !recovered_without_errors);
     }
 
     // return 0 - non-zero return values are reserved for future use

tests/src/fuzzer-parse_bon8.cpp

@@ -21,6 +21,10 @@ It also checks that reading the data from a stream, which reads strings byte by
 byte, gives the same value or error as reading it from contiguous memory, which
 copies strings in bulk.
 
+Furthermore, it reads data with a SAX parser that recovers from every error
+and checks that the events are balanced, that reading ends, and that it
+reports an error exactly when from_bon8() fails (see #3989).
+
 The provided function `LLVMFuzzerTestOneInput` can be used in different fuzzer
 drivers.
 */
@@ -34,6 +38,8 @@ drivers.
     #error "the fuzzer drivers must be built without NDEBUG"
 #endif
 
+#include "fuzzer-recovering_checker.hpp"
+
 using json = nlohmann::json;
 
 // compares dumps rather than values, because NaN != NaN; keep writes strings
@@ -64,6 +70,9 @@ std::string read_bon8(InputType&& input)
 // see http://llvm.org/docs/LibFuzzer.html
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
 {
+    // recover from all errors, reading from memory and from a stream
+    const bool recovered_without_errors = check_recovering_parse(data, size, json::input_format_t::bon8).errors == 0;
+
     // contiguous and stream input must be read alike
     {
         std::istringstream stream(std::string(reinterpret_cast<const char*>(data), size));
@@ -102,6 +111,9 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
         // without exceptions, the same input must give the same value
         assert(!noexcept_threw && !j_noexcept.is_discarded() && same_value(j_noexcept, j1));
 
+        // the recovering parser must not have reported an error either
+        assert(recovered_without_errors);
+
         try
         {
             // step 2: round trip
@@ -123,6 +135,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
     {
         // parse errors are ok, because input may be random bytes
         assert(parsed || noexcept_threw || j_noexcept.is_discarded());
+        assert(parsed || !recovered_without_errors);
     }
     catch (const json::type_error&)
     {
@@ -133,6 +146,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
     {
         // out of range errors may happen if provided sizes are excessive
         assert(parsed || noexcept_threw || j_noexcept.is_discarded());
+        assert(parsed || !recovered_without_errors);
     }
 
     // return 0 - non-zero return values are reserved for future use

tests/src/fuzzer-parse_bson.cpp

@@ -17,6 +17,10 @@ array data, it performs the following steps:
 - j2 = from_bson(vec)
 - assert(to_bson(j2) == vec)
 
+Furthermore, it reads data with a SAX parser that recovers from every error
+and checks that the events are balanced, that reading ends, and that it
+reports an error exactly when from_bson() fails (see #3989).
+
 The provided function `LLVMFuzzerTestOneInput` can be used in different fuzzer
 drivers.
 */
@@ -29,6 +33,8 @@ drivers.
     #error "the fuzzer drivers must be built without NDEBUG"
 #endif
 
+#include "fuzzer-recovering_checker.hpp"
+
 using json = nlohmann::json;
 
 // compares dumps rather than values, because NaN != NaN; keep writes strings
@@ -41,6 +47,9 @@ static bool same_value(const json& lhs, const json& rhs)
 // see http://llvm.org/docs/LibFuzzer.html
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
 {
+    // recover from all errors, reading from memory and from a stream
+    const bool recovered_without_errors = check_recovering_parse(data, size, json::input_format_t::bson).errors == 0;
+
     std::vector<uint8_t> const vec1(data, data + size);
 
     // step 0: parse input without exceptions; a parse error must then be
@@ -73,6 +82,9 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
         // without exceptions, the same input must give the same value
         assert(!noexcept_threw && !j_noexcept.is_discarded() && same_value(j_noexcept, j1));
 
+        // the recovering parser must not have reported an error either
+        assert(recovered_without_errors);
+
         try
         {
             // step 2: round trip
@@ -94,6 +106,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
     {
         // parse errors are ok, because input may be random bytes
         assert(parsed || noexcept_threw || j_noexcept.is_discarded());
+        assert(parsed || !recovered_without_errors);
     }
     catch (const json::type_error&)
     {
@@ -104,6 +117,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
     {
         // out of range errors can occur during parsing, too
         assert(parsed || noexcept_threw || j_noexcept.is_discarded());
+        assert(parsed || !recovered_without_errors);
     }
 
     // return 0 - non-zero return values are reserved for future use

tests/src/fuzzer-parse_cbor.cpp

@@ -17,6 +17,10 @@ array data, it performs the following steps:
 - j2 = from_cbor(vec)
 - assert(to_cbor(j2) == vec)
 
+Furthermore, it reads data with a SAX parser that recovers from every error
+and checks that the events are balanced, that reading ends, and that it
+reports an error exactly when from_cbor() fails (see #3989).
+
 The provided function `LLVMFuzzerTestOneInput` can be used in different fuzzer
 drivers.
 */
@@ -29,6 +33,8 @@ drivers.
     #error "the fuzzer drivers must be built without NDEBUG"
 #endif
 
+#include "fuzzer-recovering_checker.hpp"
+
 using json = nlohmann::json;
 
 // compares dumps rather than values, because NaN != NaN; keep writes strings
@@ -41,6 +47,9 @@ static bool same_value(const json& lhs, const json& rhs)
 // see http://llvm.org/docs/LibFuzzer.html
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
 {
+    // recover from all errors, reading from memory and from a stream
+    const bool recovered_without_errors = check_recovering_parse(data, size, json::input_format_t::cbor).errors == 0;
+
     std::vector<uint8_t> const vec1(data, data + size);
 
     // step 0: parse input without exceptions; a parse error must then be
@@ -73,6 +82,9 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
         // without exceptions, the same input must give the same value
         assert(!noexcept_threw && !j_noexcept.is_discarded() && same_value(j_noexcept, j1));
 
+        // the recovering parser must not have reported an error either
+        assert(recovered_without_errors);
+
         try
         {
             // step 2: round trip
@@ -94,6 +106,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
     {
         // parse errors are ok, because input may be random bytes
         assert(parsed || noexcept_threw || j_noexcept.is_discarded());
+        assert(parsed || !recovered_without_errors);
     }
     catch (const json::type_error&)
     {
@@ -104,6 +117,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
     {
         // out of range errors can occur during parsing, too
         assert(parsed || noexcept_threw || j_noexcept.is_discarded());
+        assert(parsed || !recovered_without_errors);
     }
 
     // return 0 - non-zero return values are reserved for future use

tests/src/fuzzer-parse_json.cpp

@@ -18,6 +18,10 @@ array data, it performs the following steps:
 - s2 = serialize(j2)
 - assert(s1 == s2)
 
+Furthermore, it parses data with a SAX parser that recovers from every error
+and checks that the events are balanced, that parsing ends, and that valid
+input is parsed without errors (see #3989).
+
 The provided function `LLVMFuzzerTestOneInput` can be used in different fuzzer
 drivers.
 */
@@ -30,6 +34,8 @@ drivers.
     #error "the fuzzer drivers must be built without NDEBUG"
 #endif
 
+#include "fuzzer-recovering_checker.hpp"
+
 using json = nlohmann::json;
 
 // compares dumps rather than values, because NaN != NaN; keep writes strings
@@ -42,6 +48,13 @@ static bool same_value(const json& lhs, const json& rhs)
 // see http://llvm.org/docs/LibFuzzer.html
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
 {
+    // recover from all errors, reading from memory and from a stream
+    {
+        const auto checker = check_recovering_parse(data, size, json::input_format_t::json);
+        assert(checker.events <= (4 * size) + 4);
+        assert((checker.errors == 0) == json::accept(data, data + size));
+    }
+
     // step 0: parse input without exceptions; a parse error must then be
     // reported as a discarded value, never thrown
     json j_noexcept;

tests/src/fuzzer-parse_msgpack.cpp

@@ -17,6 +17,10 @@ array data, it performs the following steps:
 - j2 = from_msgpack(vec)
 - assert(to_msgpack(j2) == vec)
 
+Furthermore, it reads data with a SAX parser that recovers from every error
+and checks that the events are balanced, that reading ends, and that it
+reports an error exactly when from_msgpack() fails (see #3989).
+
 The provided function `LLVMFuzzerTestOneInput` can be used in different fuzzer
 drivers.
 */
@@ -29,6 +33,8 @@ drivers.
     #error "the fuzzer drivers must be built without NDEBUG"
 #endif
 
+#include "fuzzer-recovering_checker.hpp"
+
 using json = nlohmann::json;
 
 // compares dumps rather than values, because NaN != NaN; keep writes strings
@@ -41,6 +47,9 @@ static bool same_value(const json& lhs, const json& rhs)
 // see http://llvm.org/docs/LibFuzzer.html
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
 {
+    // recover from all errors, reading from memory and from a stream
+    const bool recovered_without_errors = check_recovering_parse(data, size, json::input_format_t::msgpack).errors == 0;
+
     std::vector<uint8_t> const vec1(data, data + size);
 
     // step 0: parse input without exceptions; a parse error must then be
@@ -73,6 +82,9 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
         // without exceptions, the same input must give the same value
         assert(!noexcept_threw && !j_noexcept.is_discarded() && same_value(j_noexcept, j1));
 
+        // the recovering parser must not have reported an error either
+        assert(recovered_without_errors);
+
         try
         {
             // step 2: round trip
@@ -94,6 +106,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
     {
         // parse errors are ok, because input may be random bytes
         assert(parsed || noexcept_threw || j_noexcept.is_discarded());
+        assert(parsed || !recovered_without_errors);
     }
     catch (const json::type_error&)
     {
@@ -104,6 +117,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
     {
         // out of range errors may happen if provided sizes are excessive
         assert(parsed || noexcept_threw || j_noexcept.is_discarded());
+        assert(parsed || !recovered_without_errors);
     }
 
     // return 0 - non-zero return values are reserved for future use

tests/src/fuzzer-parse_ubjson.cpp

@@ -26,6 +26,10 @@ array data, it performs the following steps:
 The unit tests run the same checks on a fixed corpus (see the "UBJSON round-trip
 invariants" test case), so keep both in sync.
 
+Furthermore, it reads data with a SAX parser that recovers from every error
+and checks that the events are balanced, that reading ends, and that it
+reports an error exactly when from_ubjson() fails (see #3989).
+
 The provided function `LLVMFuzzerTestOneInput` can be used in different fuzzer
 drivers.
 */
@@ -38,6 +42,8 @@ drivers.
     #error "the fuzzer drivers must be built without NDEBUG"
 #endif
 
+#include "fuzzer-recovering_checker.hpp"
+
 using json = nlohmann::json;
 
 // compares dumps rather than values, because NaN != NaN; keep writes strings
@@ -50,6 +56,9 @@ static bool same_value(const json& lhs, const json& rhs)
 // see http://llvm.org/docs/LibFuzzer.html
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
 {
+    // recover from all errors, reading from memory and from a stream
+    const bool recovered_without_errors = check_recovering_parse(data, size, json::input_format_t::ubjson).errors == 0;
+
     std::vector<uint8_t> const vec1(data, data + size);
 
     // step 0: parse input without exceptions; a parse error must then be
@@ -82,6 +91,9 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
         // without exceptions, the same input must give the same value
         assert(!noexcept_threw && !j_noexcept.is_discarded() && same_value(j_noexcept, j1));
 
+        // the recovering parser must not have reported an error either
+        assert(recovered_without_errors);
+
         try
         {
             // step 2.1: round trip without adding size annotations to container types
@@ -113,6 +125,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
     {
         // parse errors are ok, because input may be random bytes
         assert(parsed || noexcept_threw || j_noexcept.is_discarded());
+        assert(parsed || !recovered_without_errors);
     }
     catch (const json::type_error&)
     {
@@ -123,6 +136,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
     {
         // out of range errors may happen if provided sizes are excessive
         assert(parsed || noexcept_threw || j_noexcept.is_discarded());
+        assert(parsed || !recovered_without_errors);
     }
 
     // return 0 - non-zero return values are reserved for future use

tests/src/fuzzer-recovering_checker.hpp

@@ -0,0 +1,154 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <sstream>
+#include <string>
+#include <vector>
+#include <nlohmann/json.hpp>
+
+namespace
+{
+// a SAX parser that recovers from every error and checks that the events are
+// balanced and that every key is followed by exactly one value
+class recovering_checker : public nlohmann::json_sax<nlohmann::json>
+{
+  public:
+    bool null() override
+    {
+        return value();
+    }
+
+    bool boolean(bool /*val*/) override
+    {
+        return value();
+    }
+
+    bool number_integer(number_integer_t /*val*/) override
+    {
+        return value();
+    }
+
+    bool number_unsigned(number_unsigned_t /*val*/) override
+    {
+        return value();
+    }
+
+    bool number_float(number_float_t /*val*/, const string_t& /*s*/) override
+    {
+        return value();
+    }
+
+    bool string(string_t& /*val*/) override
+    {
+        return value();
+    }
+
+    bool binary(binary_t& /*val*/) override
+    {
+        return value();
+    }
+
+    bool start_object(std::size_t /*elements*/) override
+    {
+        value();
+        stack.push_back('o');
+        return true;
+    }
+
+    bool key(string_t& /*val*/) override
+    {
+        ++events;
+        assert(!stack.empty() && stack.back() == 'o');
+        stack.back() = 'v';
+        return true;
+    }
+
+    bool end_object() override
+    {
+        ++events;
+        assert(!stack.empty() && stack.back() == 'o');
+        stack.pop_back();
+        return true;
+    }
+
+    bool start_array(std::size_t /*elements*/) override
+    {
+        value();
+        stack.push_back('a');
+        return true;
+    }
+
+    bool end_array() override
+    {
+        ++events;
+        assert(!stack.empty() && stack.back() == 'a');
+        stack.pop_back();
+        return true;
+    }
+
+    bool parse_error(std::size_t /*position*/, const std::string& /*last_token*/, const nlohmann::detail::exception& /*ex*/) override
+    {
+        ++errors;
+        return true;
+    }
+
+    bool complete() const
+    {
+        return stack.empty();
+    }
+
+    std::size_t events = 0;
+    std::size_t errors = 0;
+
+  private:
+    bool value()
+    {
+        ++events;
+        if (!stack.empty())
+        {
+            // an array element, or the value of a key
+            assert(stack.back() != 'o');
+            if (stack.back() == 'v')
+            {
+                stack.back() = 'o';
+            }
+        }
+        return true;
+    }
+
+    // 'a' for an array, 'o' for an object that expects a key, 'v' for an
+    // object that expects the value of a key
+    std::vector<char> stack {}; // NOLINT(readability-redundant-member-init)
+};
+
+/// parses @a data with a recovering_checker from memory and from a stream,
+/// checks that both see the same, that the events are balanced, and that the
+/// number of errors is bounded, and returns the checker (see #3989)
+inline recovering_checker check_recovering_parse(const std::uint8_t* data, const std::size_t size, const nlohmann::json::input_format_t format)
+{
+    recovering_checker checker;
+    const bool ok = nlohmann::json::sax_parse(data, data + size, &checker, format);
+    assert(checker.complete());
+    assert(checker.errors <= size + 1);
+    assert(ok == (checker.errors == 0));
+
+    std::istringstream stream(std::string(reinterpret_cast<const char*>(data), size));
+    recovering_checker stream_checker;
+    assert(nlohmann::json::sax_parse(stream, &stream_checker, format) == ok);
+    assert(stream_checker.complete());
+    assert(stream_checker.events == checker.events);
+    assert(stream_checker.errors == checker.errors);
+
+    return checker;
+}
+} // namespace

tests/src/unit-allocator.cpp

@@ -370,14 +370,7 @@ TEST_CASE("copy of a deeply nested value survives a failing allocation (#5640)")
 #if !(defined(_ITERATOR_DEBUG_LEVEL) && _ITERATOR_DEBUG_LEVEL > 0)
     SECTION("std::map-backed object_t")
     {
-        using bad_alloc_json = nlohmann::basic_json<std::map,
-              std::vector,
-              std::string,
-              bool,
-              std::int64_t,
-              std::uint64_t,
-              double,
-              nth_alloc_fails_allocator>;
+        using bad_alloc_json = nlohmann::json::with_allocator_t<nth_alloc_fails_allocator>;
 
         check_deep_copy_survives_failing_allocation<bad_alloc_json>(false);
         check_deep_copy_survives_failing_allocation<bad_alloc_json>(true);
@@ -385,14 +378,7 @@ TEST_CASE("copy of a deeply nested value survives a failing allocation (#5640)")
 
     SECTION("ordered_map-backed object_t")
     {
-        using bad_alloc_ordered_json = nlohmann::basic_json<nlohmann::ordered_map,
-              std::vector,
-              std::string,
-              bool,
-              std::int64_t,
-              std::uint64_t,
-              double,
-              nth_alloc_fails_allocator>;
+        using bad_alloc_ordered_json = nlohmann::ordered_json::with_allocator_t<nth_alloc_fails_allocator>;
 
         check_deep_copy_survives_failing_allocation<bad_alloc_ordered_json>(false);
         check_deep_copy_survives_failing_allocation<bad_alloc_ordered_json>(true);
@@ -450,14 +436,7 @@ struct scratch_counting_allocator : std::allocator<T>
 
 TEST_CASE("deep copy uses the provided allocator")
 {
-    using counting_json = nlohmann::basic_json<std::map,
-          std::vector,
-          std::string,
-          bool,
-          std::int64_t,
-          std::uint64_t,
-          double,
-          scratch_counting_allocator>;
+    using counting_json = nlohmann::json::with_allocator_t<scratch_counting_allocator>;
 
     // deeper than the 128 levels the copy constructor descends into, so the
     // innermost objects are copied by the iterative deep copy
@@ -516,14 +495,7 @@ TEST_CASE("converting a deeply nested value from another specialization fails cl
     // the allocator in noexcept constructors, so a failing construction crashes
     // the program there instead of throwing std::bad_alloc. Nothing to check.
 #if !(defined(_MSC_VER) && _MSC_VER < 1910 && defined(_ITERATOR_DEBUG_LEVEL) && _ITERATOR_DEBUG_LEVEL > 0)
-    using countdown_json = nlohmann::basic_json<std::map,
-          std::vector,
-          std::string,
-          bool,
-          std::int64_t,
-          std::uint64_t,
-          double,
-          countdown_allocator>;
+    using countdown_json = nlohmann::json::with_allocator_t<countdown_allocator>;
 
     // deeper than the 128 levels the converting constructor descends into, so
     // that failures land on both sides of the bound - or, built with
@@ -631,14 +603,7 @@ TEST_CASE("destructor performs no allocation, only deallocation")
     // 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>;
+    using counting_json = nlohmann::json::with_allocator_t<counting_allocator>;
 
     SECTION("array")
     {
@@ -683,14 +648,7 @@ TEST_CASE("destructor performs no allocation, only deallocation")
 TEST_CASE("a failed allocation leaves the value unchanged")
 {
     // 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>;
 
     // Each of these creates a string, array, object, or binary value. The
     // value must be created before the type is changed: otherwise, a failed

tests/src/unit-alt-string.cpp

@@ -415,6 +415,46 @@ TEST_CASE("alternative string type")
         CHECK(j2.dump() == R"({"/foo/0":"bar","/foo/1":"baz"})");
     }
 
+    SECTION("error recovery")
+    {
+        // a SAX parser that recovers from every error (see #3989)
+        struct recovering_parser : nlohmann::detail::json_sax_dom_parser<alt_json>
+        {
+            explicit recovering_parser(alt_json& j)
+                : nlohmann::detail::json_sax_dom_parser<alt_json>(j, false)
+            {}
+
+            // sax_parse() calls the SAX parser's own parse_error(), so hiding
+            // the one of the base class is what recovering takes
+            // NOLINTNEXTLINE(bugprone-derived-method-shadowing-base-method)
+            bool parse_error(std::size_t /*unused*/, const std::string& /*unused*/, const nlohmann::detail::exception& /*unused*/)
+            {
+                ++errors;
+                return true;
+            }
+
+            std::size_t errors = 0;
+        };
+
+        alt_json j;
+        recovering_parser sax(j);
+        // not inside CHECK(): MSVC reads the escape in a stringized raw string
+        const std::string input = R"([1., "a\qb", tru, {"k" 2}])";
+        CHECK(!alt_json::sax_parse(input, &sax));
+        CHECK(sax.errors == 4);
+        CHECK(j.dump() == R"([1,"aqb",null,{"k":2}])");
+
+        // a UBJSON high-precision number, a CBOR key that is not a string
+        alt_json u;
+        recovering_parser ubjson_sax(u);
+        CHECK(!alt_json::sax_parse(std::vector<std::uint8_t> {'[', 'H', 'i', 2, '1', '.', ']'}, &ubjson_sax, alt_json::input_format_t::ubjson));
+        CHECK(u.dump() == "[1]");
+        alt_json c;
+        recovering_parser cbor_sax(c);
+        CHECK(!alt_json::sax_parse(std::vector<std::uint8_t> {0xA2, 0x01, 0x02, 0x61, 'a', 0x03}, &cbor_sax, alt_json::input_format_t::cbor));
+        CHECK(c.dump() == R"({"a":3})");
+    }
+
     SECTION("conversion between basic_json specializations (#2649)")
     {
         // explicit conversions are always possible

tests/src/unit-binary_formats.cpp

@@ -237,7 +237,7 @@ namespace
 // the binary formats as function pointers for "Binary formats with narrow number types";
 // named functions rather than lambdas, because clang 3.5 cannot convert a lambda
 // to a function pointer in the braced initializer of the format table
-using narrow_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int32_t, std::uint32_t, float>;
+using narrow_json = nlohmann::json::with_integers_t<std::int32_t, std::uint32_t>::with_float_t<float>;
 using bytes = std::vector<std::uint8_t>;
 
 bytes encode_cbor(const json& j)

tests/src/unit-binary_utf8_error_handler.cpp

@@ -12,7 +12,11 @@
 #include <nlohmann/json.hpp>
 using nlohmann::json;
 
+#include <array>
+#include <map>
+#include <memory>
 #include <string>
+#include <utility>
 #include <vector>
 
 namespace
@@ -55,6 +59,53 @@ std::string dump_and_parse(const std::string& raw, eh error_handler)
     return json::parse(json(raw).dump(-1, ' ', false, error_handler)).get<std::string>();
 }
 
+// an object key type that is not string_t, but converts implicitly to it;
+// data() is only used when JSON_DIAGNOSTICS is enabled
+DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
+DOCTEST_CLANG_SUPPRESS_WARNING("-Wunused-member-function")
+class converting_key
+{
+  public:
+    converting_key(const char* s) : m_value(s) {} // NOLINT(google-explicit-constructor,hicpp-explicit-conversions)
+    converting_key(std::string s) : m_value(std::move(s)) {} // NOLINT(google-explicit-constructor,hicpp-explicit-conversions)
+
+    // the conversion yields a temporary string_t
+    operator std::string() const // NOLINT(google-explicit-constructor,hicpp-explicit-conversions)
+    {
+        return m_value;
+    }
+
+    // read by the exception messages when JSON_DIAGNOSTICS is enabled
+    const char* data() const noexcept
+    {
+        return m_value.data();
+    }
+
+    friend bool operator<(const converting_key& lhs, const converting_key& rhs)
+    {
+        return lhs.m_value < rhs.m_value;
+    }
+
+  private:
+    std::string m_value;
+};
+DOCTEST_CLANG_SUPPRESS_WARNING_POP
+
+// ObjectType using converting_key; the Key template argument is ignored
+template<typename Key, typename Value, typename Compare, typename Allocator>
+class converting_key_object : public std::map<converting_key, Value, std::less<converting_key>, // NOLINT(modernize-use-transparent-functors)
+    typename std::allocator_traits<Allocator>::template rebind_alloc<std::pair<const converting_key, Value>>>
+{
+    using base_type = std::map<converting_key, Value, std::less<converting_key>, // NOLINT(modernize-use-transparent-functors)
+                               typename std::allocator_traits<Allocator>::template rebind_alloc<std::pair<const converting_key, Value>>>;
+
+                             public:
+                               using base_type::base_type;
+                               using base_type::operator=;
+};
+
+using converting_key_json = nlohmann::basic_json<converting_key_object>;
+
 } // namespace
 
 TEST_CASE("UTF-8 error_handler for the binary readers and writers")
@@ -370,3 +421,109 @@ TEST_CASE("UTF-8 error_handler for the binary readers and writers")
         CHECK(json::from_bson(bson_bytes)["k"].get<std::string>() == ill_formed_cases()[0].bytes);
     }
 }
+
+// The UBJSON and BJData writers bind the (possibly sanitized) key to a const
+// string_t&. If key_type is not string_t but converts to it, the converted
+// temporary must outlive that reference; this was a use-after-scope found by
+// AddressSanitizer. Keys exceed the small string optimization on purpose.
+TEST_CASE("UBJSON and BJData writers with an object_t whose key_type is not string_t")
+{
+    const std::string long_prefix(70, 'k');
+
+    SECTION("well-formed keys, every error_handler")
+    {
+        const std::string key1 = long_prefix + "-first";
+        const std::string key2 = long_prefix + "-second";
+
+        converting_key_json::object_t o;
+        o.emplace(converting_key(key1), 1);
+        o.emplace(converting_key(key2), "value");
+        const converting_key_json v(std::move(o));
+
+        json expected;
+        expected[key1] = 1;
+        expected[key2] = "value";
+
+        const std::array<std::pair<bool, bool>, 3> combos = {{{false, false}, {true, false}, {true, true}}};
+        for (const auto h : all_handlers())
+        {
+            CAPTURE(static_cast<int>(h))
+            for (const auto& combo : combos)
+            {
+                const bool use_count = combo.first;
+                const bool use_type = combo.second;
+                CAPTURE(use_count)
+                CAPTURE(use_type)
+
+                CHECK(json::from_ubjson(converting_key_json::to_ubjson(v, use_count, use_type, h)) == expected);
+                CHECK(json::from_bjdata(converting_key_json::to_bjdata(v, use_count, use_type, json::bjdata_version_t::draft2, h)) == expected);
+                CHECK(json::from_bjdata(converting_key_json::to_bjdata(v, use_count, use_type, json::bjdata_version_t::draft3, h)) == expected);
+            }
+        }
+    }
+
+    SECTION("ill-formed keys")
+    {
+        for (const auto& c : ill_formed_cases())
+        {
+            CAPTURE(c.name)
+            const std::string key = long_prefix + c.bytes;
+
+            converting_key_json::object_t o;
+            o.emplace(converting_key(key), 1);
+            const converting_key_json v(std::move(o));
+
+            CHECK_THROWS_AS(converting_key_json::to_ubjson(v, false, false, eh::strict), converting_key_json::type_error&);
+            CHECK_THROWS_AS(converting_key_json::to_bjdata(v, false, false, json::bjdata_version_t::draft2, eh::strict), converting_key_json::type_error&);
+
+            for (const auto h :
+                    {
+                        eh::replace, eh::ignore
+                    })
+            {
+                CAPTURE(static_cast<int>(h))
+                const std::string expected = dump_and_parse(key, h);
+
+                CHECK(json::from_ubjson(converting_key_json::to_ubjson(v, false, false, h)).begin().key() == expected);
+                CHECK(json::from_bjdata(converting_key_json::to_bjdata(v, false, false, json::bjdata_version_t::draft2, h)).begin().key() == expected);
+            }
+
+            CHECK(json::from_ubjson(converting_key_json::to_ubjson(v, false, false, eh::keep)).begin().key() == key);
+            CHECK(json::from_bjdata(converting_key_json::to_bjdata(v, false, false, json::bjdata_version_t::draft2, eh::keep)).begin().key() == key);
+        }
+    }
+
+    SECTION("nested deeper than the recursion limit")
+    {
+        // wrap the previous value, innermost first
+        converting_key_json v = 42;
+        json expected = 42;
+        for (int i = 199; i >= 0; --i)
+        {
+            const std::string key = "level-" + std::to_string(i) + "-" + std::string(64, 'x');
+
+            converting_key_json::object_t o;
+            o.emplace(converting_key(key), std::move(v));
+            v = converting_key_json(std::move(o));
+
+            json e;
+            e[key] = std::move(expected);
+            expected = std::move(e);
+        }
+
+        for (const auto h : all_handlers())
+        {
+            CAPTURE(static_cast<int>(h))
+            for (const bool use_count :
+                    {
+                        false, true
+                    })
+            {
+                CAPTURE(use_count)
+
+                CHECK(json::from_ubjson(converting_key_json::to_ubjson(v, use_count, false, h)) == expected);
+                CHECK(json::from_bjdata(converting_key_json::to_bjdata(v, use_count, false, json::bjdata_version_t::draft2, h)) == expected);
+            }
+        }
+    }
+}

tests/src/unit-bson.cpp

@@ -46,9 +46,7 @@ class huge_binary_t : public std::vector<std::uint8_t>
     }
 };
 
-using huge_binary_json = nlohmann::basic_json <
-                         std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t,
-                         double, std::allocator, nlohmann::adl_serializer, huge_binary_t, void >;
+using huge_binary_json = nlohmann::json::with_binary_t<huge_binary_t>;
 
 // a string type that can be made to report a size beyond INT32_MAX without
 // allocating that much memory, so BSON length overflow can be tested for
@@ -96,9 +94,7 @@ class huge_string_t : public std::string
     bool pretend_huge = false;
 };
 
-using huge_string_json = nlohmann::basic_json <
-                         std::map, std::vector, huge_string_t, bool, std::int64_t, std::uint64_t,
-                         double, std::allocator, nlohmann::adl_serializer, std::vector<std::uint8_t>, void >;
+using huge_string_json = nlohmann::json::with_string_t<huge_string_t>;
 } // namespace
 
 TEST_CASE("BSON")

tests/src/unit-class_lexer.cpp

@@ -17,6 +17,7 @@ using nlohmann::json;
 #include <cstdint> // uint32_t, uint64_t
 #include <cstdlib> // strtod
 #include <cstring> // memcpy
+#include <limits> // numeric_limits
 #include <sstream> // stringstream
 #include <string> // string
 #include <utility> // pair
@@ -891,8 +892,39 @@ TEST_CASE("Eisel-Lemire float conversion")
 
     SECTION("128-bit products and leading zeros")
     {
+        const auto check_product = [](std::uint64_t a, std::uint64_t b)
+        {
+            const auto product = nlohmann::detail::full_multiplication(a, b);
+            CHECK(big_from(product.high, product.low) == big_mul(big_from(0, a), big_from(0, b)));
+        };
+
+        const std::uint64_t max = (std::numeric_limits<std::uint64_t>::max)();
+        const std::array<std::pair<std::uint64_t, std::uint64_t>, 13> edge_cases =
+        {
+            {
+                {0, 0},
+                {0, 1},
+                {1, 1},
+                {1, max},
+                {0xFFFFFFFFu, 0x100000000u},
+                {0x100000000u, 0x100000000u},
+                {0x100000001u, 0x100000001u},
+                {max, max},
+                {max, 2},
+                {0xFFFFFFFF00000000u, 0x100000001u},
+                {0x100000001u, 0xFFFFFFFF00000000u},
+                {max, 1},
+                {2, max},
+            }
+        };
+
+        for (const auto& test : edge_cases)
+        {
+            check_product(test.first, test.second);
+        }
+
         // whichever implementation the compiler gets (with or without a
-        // 128-bit integer type or a builtin)
+        // 128-bit integer type or a builtin / intrinsic)
         std::uint64_t state = 42;
         for (int i = 0; i < 10000; ++i)
         {
@@ -901,8 +933,7 @@ TEST_CASE("Eisel-Lemire float conversion")
             state ^= state << 17u;
             const std::uint64_t a = state;
             const std::uint64_t b = (state * 0x9E3779B97F4A7C15u) >> (i % 64);
-            const auto product = nlohmann::detail::full_multiplication(a, b);
-            CHECK(big_from(product.high, product.low) == big_mul(big_from(0, a), big_from(0, b)));
+            check_product(a, b);
 
             const int k = i % 64;
             const std::uint64_t x = (std::uint64_t{1} << k) | (a & ((std::uint64_t{1} << k) - 1));

tests/src/unit-class_parser.cpp

@@ -143,11 +143,13 @@ class SaxEventLogger
     {
         errored = true;
         events.push_back("parse_error(" + std::to_string(position) + ")");
-        return false;
+        return recover;
     }
 
     std::vector<std::string> events {}; // NOLINT(readability-redundant-member-init)
     bool errored = false;
+    /// whether parse_error() asks the parser to recover from the error (see #3989)
+    bool recover = false;
 };
 
 class SaxCountdown : public nlohmann::json::json_sax_t
@@ -3026,3 +3028,583 @@ TEST_CASE("diagnostic positions: value lifetime, input adapters, and SAX")
     }
 }
 #endif
+
+namespace
+{
+/// builds a value like json::parse(), but asks the parser to recover from
+/// errors (see #3989), and checks that the events it receives are balanced
+class RecoveringDomParser
+{
+  public:
+    explicit RecoveringDomParser(json& j, std::size_t max_errors_ = static_cast<std::size_t>(-1))
+        : dom(j, false)
+        , max_errors(max_errors_)
+    {}
+
+    bool null()
+    {
+        value();
+        return dom.null();
+    }
+
+    bool boolean(bool val)
+    {
+        value();
+        return dom.boolean(val);
+    }
+
+    bool number_integer(json::number_integer_t val)
+    {
+        value();
+        return dom.number_integer(val);
+    }
+
+    bool number_unsigned(json::number_unsigned_t val)
+    {
+        value();
+        return dom.number_unsigned(val);
+    }
+
+    bool number_float(json::number_float_t val, const std::string& s)
+    {
+        value();
+        return dom.number_float(val, s);
+    }
+
+    bool string(std::string& val)
+    {
+        value();
+        return dom.string(val);
+    }
+
+    bool binary(json::binary_t& val)
+    {
+        value();
+        return dom.binary(val);
+    }
+
+    bool start_object(std::size_t elements)
+    {
+        value();
+        stack.push_back('o');
+        return dom.start_object(elements);
+    }
+
+    bool key(std::string& val)
+    {
+        ++events;
+        if (stack.empty() || stack.back() != 'o')
+        {
+            well_formed = false;
+            return false;
+        }
+        stack.back() = 'v';
+        return dom.key(val);
+    }
+
+    bool end_object()
+    {
+        ++events;
+        if (stack.empty() || stack.back() != 'o')
+        {
+            well_formed = false;
+            return false;
+        }
+        stack.pop_back();
+        return dom.end_object();
+    }
+
+    bool start_array(std::size_t elements)
+    {
+        value();
+        stack.push_back('a');
+        return dom.start_array(elements);
+    }
+
+    bool end_array()
+    {
+        ++events;
+        if (stack.empty() || stack.back() != 'a')
+        {
+            well_formed = false;
+            return false;
+        }
+        stack.pop_back();
+        return dom.end_array();
+    }
+
+    bool parse_error(std::size_t /*unused*/, const std::string& /*unused*/, const json::exception& ex)
+    {
+        errors.emplace_back(ex.what());
+        return errors.size() < max_errors;
+    }
+
+    /// whether the events were balanced and every key was followed by a value
+    bool balanced() const
+    {
+        return well_formed && stack.empty();
+    }
+
+    /// builds the value
+    nlohmann::detail::json_sax_dom_parser<json> dom;
+    std::vector<std::string> errors {}; // NOLINT(readability-redundant-member-init)
+    std::size_t events = 0;
+    /// the open containers: 'a' for an array, 'o' for an object that expects
+    /// a key, 'v' for an object that expects the value of a key
+    std::vector<char> stack {}; // NOLINT(readability-redundant-member-init)
+    bool well_formed = true;
+    std::size_t max_errors;
+
+  private:
+    /// a value is passed: it is an array element, or the value of a key
+    void value()
+    {
+        ++events;
+        if (!stack.empty())
+        {
+            if (stack.back() == 'v')
+            {
+                stack.back() = 'o';
+            }
+            else if (stack.back() == 'o')
+            {
+                // a value without a key
+                well_formed = false;
+            }
+        }
+    }
+
+};
+
+struct RecoveryResult
+{
+    json value;
+    std::vector<std::string> errors;
+    std::size_t events;
+    bool ok;
+    bool balanced;
+};
+
+template<typename InputType>
+RecoveryResult parse_recovering(InputType&& input, const bool strict = true,
+                                const bool ignore_comments = false, const bool ignore_trailing_commas = false)
+{
+    json j;
+    RecoveringDomParser sax(j);
+    const bool ok = json::sax_parse(std::forward<InputType>(input), &sax, json::input_format_t::json,
+                                    strict, ignore_comments, ignore_trailing_commas);
+    return {j, sax.errors, sax.events, ok, sax.balanced()};
+}
+
+/// stops after a number of events, but recovers from errors
+class RecoveringCountdown : public SaxCountdown
+{
+  public:
+    using SaxCountdown::SaxCountdown;
+
+    bool parse_error(std::size_t /*position*/, const std::string& /*last_token*/, const json::exception& /*ex*/) override
+    {
+        return true;
+    }
+};
+
+/// a repaired input: the value it is repaired to, and the number of errors
+struct Repair
+{
+    const char* input;
+    const char* expected;
+    std::size_t errors;
+};
+} // namespace
+
+TEST_CASE("parser error recovery (#3989)")
+{
+    SECTION("repairs")
+    {
+        const std::vector<Repair> repairs =
+        {
+            // a missing separator is inserted
+            {"[1 2]", "[1,2]", 1},
+            {R"({"a":1 "b":2})", R"({"a":1,"b":2})", 1},
+            {R"({"a" 1})", R"({"a":1})", 1},
+            {"[1 tru 2]", "[1,null,2]", 2},
+            {R"({"a" "b": 1})", R"({"a":"b"})", 2},
+
+            // a missing value is null in an object; in an array, a ',' stands
+            // for null, while an array that ends there just ends
+            {R"({"a":})", R"({"a":null})", 1},
+            {R"({"a"})", R"({"a":null})", 1},
+            {R"({"a","b":1})", R"({"a":null,"b":1})", 1},
+            {"[1,,2]", "[1,null,2]", 1},
+            {"[,1]", "[null,1]", 1},
+            {"[1,]", "[1]", 1},
+            {"[1,2,3,]", "[1,2,3]", 1},
+            {R"({"a":1,})", R"({"a":1})", 1},
+
+            // a broken string keeps what can be read
+            {R"(["a\qb"])", R"(["aqb"])", 1},
+            {R"({"na\me":1})", R"({"name":1})", 1},
+            {"[\"\xFF\"]", R"(["\uFFFD"])", 1},
+            {"[\"a\xC3(\"]", R"(["a\uFFFD("])", 1},
+            {"[\"\xE2\x82\"]", R"(["\uFFFD"])", 1},
+            {"[\"\xC3\\\\\", 1]", R"(["\uFFFD\\",1])", 1},
+            {R"(["\u12"])", R"(["\uFFFD"])", 1},
+            {R"(["\u12G4"])", R"(["\uFFFDG4"])", 1},
+            {R"(["\uDC00x"])", R"(["\uFFFDx"])", 1},
+            {R"(["\uD800x"])", R"(["\uFFFDx"])", 1},
+            {R"(["\uD800\u0041"])", R"(["\uFFFDA"])", 1},
+            {R"(["\uD800\uD800\uDC00"])", R"(["\uFFFD\uD800\uDC00"])", 1},
+            {R"(["\uD800\uD800\uD800x"])", R"(["\uFFFD\uFFFD\uFFFDx"])", 1},
+            {
+                R"(["\uD800\"x", 1])", R"(["\uFFFD\"x",1])", 1
+            },
+            {R"(["\uD800\q"])", R"(["\uFFFDq"])", 1},
+            {"[\"a\tb\"]", R"(["a\tb"])", 1},
+            {R"(["a\qb\u0041\x"])", R"(["aqbAx"])", 1},
+
+            // a broken number keeps its longest valid prefix
+            {"[1.]", "[1]", 1},
+            {"[-2.]", "[-2]", 1},
+            {"[1.5e]", "[1.5]", 1},
+            {"[1e+]", "[1]", 1},
+            {"[1.x2, 3]", "[1,3]", 1},
+
+            // what cannot be read at all is null
+            {"[1,NaN,3]", "[1,null,3]", 1},
+            {"[tru]", "[null]", 1},
+            {"[-]", "[null]", 1},
+            {R"({"a":Infinity})", R"({"a":null})", 1},
+
+            // a stray token is dropped
+            {"[:1]", "[1]", 1},
+            {R"(["a":1])", R"(["a",1])", 1},
+            {R"({"a"::1})", R"({"a":1})", 1},
+
+            // a member that cannot be read is skipped
+            {R"({1:2,"b":3})", R"({"b":3})", 1},
+            {R"({"a":1 2})", R"({"a":1})", 1},
+            {R"({,"a":1})", R"({"a":1})", 1},
+            {R"({"a":1,,"b":2})", R"({"a":1,"b":2})", 1},
+            {"{a:1}", "{}", 1},
+            {R"({"a":1 [1,{"b":2}], "c":3})", R"({"a":1,"c":3})", 1},
+            {R"([{1}, "a"])", R"([{},"a"])", 1},
+
+            // a wrong closing bracket closes the innermost container
+            {R"({"a":[1,2}, "b":3})", R"({"a":[1,2],"b":3})", 1},
+            {R"([{"a":1], 2])", R"([{"a":1},2])", 1},
+            {"{]", "{}", 1},
+            {"[}", "[]", 1},
+
+            // the end of the input closes all containers
+            {R"({"a":[1,2)", R"({"a":[1,2]})", 1},
+            {"[", "[]", 1},
+            {"{", "{}", 1},
+            {R"({"a")", R"({"a":null})", 1},
+            {R"({"a":)", R"({"a":null})", 1},
+            {"[1,", "[1]", 1},
+            {"[[[1", "[[[1]]]", 1},
+            {
+                R"(["abc)", R"(["abc"])", 2
+            },
+            {"[1,tr", "[1,null]", 2},
+            {"\"abc", "\"abc\"", 1},
+            {"[\"ab\ncd\"]", R"(["ab",null,"]"])", 4},
+
+            // what comes before the top-level value is skipped
+            {")]}'\n{\"a\":1}", R"({"a":1})", 1},
+            {R"(data: {"a":1})", R"({"a":1})", 1},
+            {"\xEF\xBB[1]", "[1]", 1},
+
+            // what comes after it is an error that ends parsing
+            {R"({"a":1}})", R"({"a":1})", 1},
+            {"[1}]", "[1]", 2},
+            {"[1] [2]", "[1]", 1},
+        };
+
+        for (const auto& repair : repairs)
+        {
+            CAPTURE(repair.input)
+            const auto result = parse_recovering(std::string(repair.input));
+            CHECK(!result.ok);
+            CHECK(result.balanced);
+            CHECK(result.value == json::parse(repair.expected));
+            CHECK(result.errors.size() == repair.errors);
+        }
+    }
+
+    SECTION("number overflow")
+    {
+        const auto result = parse_recovering(std::string("[1e999,-1e999]"));
+        CHECK(!result.ok);
+        CHECK(result.balanced);
+        CHECK(result.errors.size() == 2);
+        CHECK(result.errors[0] == "[json.exception.out_of_range.406] number overflow parsing '1e999'");
+        REQUIRE(result.value.size() == 2);
+        CHECK(result.value[0].is_number_float());
+        CHECK(result.value[0].get<double>() == std::numeric_limits<double>::infinity());
+        CHECK(result.value[1].get<double>() == -std::numeric_limits<double>::infinity());
+
+        // the SAX parser gets the number's text
+        SaxEventLogger logger;
+        logger.recover = true;
+        CHECK(!json::sax_parse("1e999", &logger));
+        CHECK(logger.events == std::vector<std::string>({"parse_error(5)", "number_float(1e999)"}));
+    }
+
+    SECTION("nothing to recover")
+    {
+        for (const std::string s :
+                {
+                    "", "   ", "]", "tru", "NaN", ",:", "/* comment"
+                })
+        {
+            CAPTURE(s)
+            const auto result = parse_recovering(s, true, true);
+            CHECK(!result.ok);
+            CHECK(result.balanced);
+            CHECK(result.events == 0);
+            CHECK(result.value == nullptr);
+            CHECK(result.errors.size() == 1);
+        }
+    }
+
+    SECTION("error messages")
+    {
+        // the first error is reported as without recovery
+        for (const std::string s :
+                {
+                    "[1 2]", R"({"a":1 "b":2})", R"({"a" 1})", R"({"a":})", "[1,]", "[1.]",
+                    R"(["a\qb"])", "[1e999]", "{1:2}", R"({"a":[1,2}})", "[1,", "[1] [2]", "{a:1}"
+                })
+        {
+            CAPTURE(s)
+            const auto result = parse_recovering(s);
+            REQUIRE(!result.errors.empty());
+            json _;
+            CHECK_THROWS_WITH_STD_STR(_ = json::parse(s), result.errors.front());
+        }
+
+        // the token of an error begins where the previous error was
+        const auto result = parse_recovering(std::string("[tru, fals, nul]"));
+        CHECK(result.errors == std::vector<std::string>(
+        {
+            "[json.exception.parse_error.101] parse error at line 1, column 5: syntax error while parsing value - invalid literal; last read: '[tru,'",
+            "[json.exception.parse_error.101] parse error at line 1, column 11: syntax error while parsing value - invalid literal; last read: ', fals,'",
+            "[json.exception.parse_error.101] parse error at line 1, column 16: syntax error while parsing value - invalid literal; last read: ', nul]'"
+        }));
+        CHECK(result.value == json::parse("[null,null,null]"));
+    }
+
+    SECTION("events")
+    {
+        // see #4522
+        SaxEventLogger logger;
+        logger.recover = true;
+        CHECK(!json::sax_parse(R"([{1}, "a"])", &logger));
+        CHECK(logger.events == std::vector<std::string>(
+        {
+            "start_array()", "start_object()", "parse_error(3)", "end_object()", "string(a)", "end_array()"
+        }));
+    }
+
+    SECTION("options")
+    {
+        SECTION("strict")
+        {
+            const auto result = parse_recovering(std::string("[1 2] [3]"), false);
+            CHECK(!result.ok);
+            CHECK(result.value == json::parse("[1,2]"));
+            CHECK(result.errors.size() == 1);
+        }
+
+        SECTION("ignore_trailing_commas")
+        {
+            for (const std::string s :
+                    {
+                        "[1,]", R"({"a":1,})", "[[1,],]"
+                    })
+            {
+                CAPTURE(s)
+                const auto result = parse_recovering(s, true, false, true);
+                CHECK(result.ok);
+                CHECK(result.errors.empty());
+            }
+
+            auto result = parse_recovering(std::string("[1,,]"), true, false, true);
+            CHECK(result.value == json::parse("[1,null]"));
+            CHECK(result.errors.size() == 1);
+
+            result = parse_recovering(std::string(R"({"a":1,,})"), true, false, true);
+            CHECK(result.value == json::parse(R"({"a":1})"));
+            CHECK(result.errors.size() == 1);
+        }
+
+        SECTION("ignore_comments")
+        {
+            auto result = parse_recovering(std::string("[1 /* one */ 2]"), true, true);
+            CHECK(result.value == json::parse("[1,2]"));
+            CHECK(result.errors.size() == 1);
+
+            // a comment that is not closed runs to the end of the input, which
+            // is not reported again
+            result = parse_recovering(std::string("[1, 2 /* unterminated"), true, true);
+            CHECK(result.balanced);
+            CHECK(result.value == json::parse("[1,2]"));
+            CHECK(result.errors.size() == 1);
+
+            // a '/' that does not begin a comment is garbage
+            result = parse_recovering(std::string("[1, /x, 2]"), true, true);
+            CHECK(result.balanced);
+            CHECK(result.value == json::parse("[1,null,2]"));
+            CHECK(result.errors.size() == 1);
+        }
+    }
+
+    SECTION("null bytes")
+    {
+        // a null byte ends the input, unless JSON_STRICT_NUL_HANDLING is set
+        const auto result = parse_recovering(std::string("[1,\0x", 5));
+        CHECK(result.balanced);
+        CHECK(!result.ok);
+#ifdef JSON_TEST_STRICT_NUL_HANDLING_ENABLED
+        CHECK(result.value == json::parse("[1,null]"));
+#else
+        CHECK(result.value == json::parse("[1]"));
+        CHECK(result.errors.size() == 1);
+#endif
+
+        const auto in_string = parse_recovering(std::string("[\"a\0b\"]", 7));
+        CHECK(in_string.balanced);
+#ifdef JSON_TEST_STRICT_NUL_HANDLING_ENABLED
+        CHECK(in_string.value == json::array({std::string("a\0b", 3)}));
+#else
+        CHECK(in_string.value == json::parse(R"(["a"])"));
+#endif
+    }
+
+    SECTION("the SAX parser stops recovering")
+    {
+        json j;
+        RecoveringDomParser sax(j, 2);
+        CHECK(!json::sax_parse("[1 2 3 4 5]", &sax));
+        CHECK(sax.errors.size() == 2);
+
+        // an error at a delimiter that an invalid token consumed is reported
+        // to the SAX parser, too
+        json j2;
+        RecoveringDomParser sax2(j2, 2);
+        CHECK(!json::sax_parse("[tru}, 1]", &sax2));
+        CHECK(sax2.errors.size() == 2);
+    }
+
+    SECTION("an event stops parsing during a repair")
+    {
+        // start_object() and key() are passed, then null() for the missing
+        // value returns false
+        RecoveringCountdown countdown(2);
+        CHECK(!json::sax_parse(R"({"a":})", &countdown));
+
+        // the end of the input: end_array() for the second array returns false
+        RecoveringCountdown countdown2(4);
+        CHECK(!json::sax_parse("[[1", &countdown2));
+    }
+
+    SECTION("input adapters")
+    {
+        // the lexer reads contiguous and streaming input differently, and it
+        // puts back a character that ended an invalid token
+        for (const std::string s :
+                {
+                    "[1 2]", "[tru}, 1]", R"({"a" "b\q", "c":[1.x, 2}})", "[\"\xFF\xC3(\", -, 1e+]", "{a:1,\"b\":2", ")]}' [1]"
+                })
+        {
+            CAPTURE(s)
+            const auto reference = parse_recovering(s);
+            CHECK(reference.balanced);
+
+            const auto from_c_string = parse_recovering(s.c_str());
+            CHECK(from_c_string.value == reference.value);
+            CHECK(from_c_string.errors == reference.errors);
+
+            const std::list<char> l(s.begin(), s.end());
+            json j;
+            RecoveringDomParser sax(j);
+            CHECK(!json::sax_parse(l.begin(), l.end(), &sax));
+            CHECK(j == reference.value);
+            CHECK(sax.errors == reference.errors);
+
+            std::istringstream ss(s);
+            const auto from_stream = parse_recovering(ss);
+            CHECK(from_stream.value == reference.value);
+            CHECK(from_stream.errors == reference.errors);
+        }
+    }
+
+    SECTION("long runs of errors")
+    {
+        // no error may copy all the input read before it
+        const auto closing = parse_recovering("[" + std::string(100000, '}'));
+        CHECK(closing.balanced);
+        CHECK(closing.value == json::array());
+
+        const auto garbage = parse_recovering("[" + std::string(100000, 'x') + "]");
+        CHECK(garbage.balanced);
+        CHECK(garbage.errors.size() == 1);
+
+        const auto commas = parse_recovering("{" + std::string(100000, ',') + "}");
+        CHECK(commas.balanced);
+        CHECK(commas.value == json::object());
+    }
+
+    SECTION("mutations of valid input")
+    {
+        // whatever the input, the events are balanced, every error is reported
+        // at most once, and valid input is parsed as usual
+        const std::vector<std::string> documents =
+        {
+            R"({"name": "value", "list": [1, -2.5, true, null, {"x": [[]]}], "e": "\u00e9"})",
+            R"([{"a": [1, 2, {"b": "c"}]}, [], {}, "\ud83d\ude00", 1e10])",
+            "{\"\xC3\xA9\": \"\xF0\x9F\x98\x80\"}",
+            R"(  {"k" : [ "v" , 0 ] }  )",
+        };
+        // each character that can be inserted, including a null byte
+        const std::string insertions("[]{},:\"x\\\0\xFF", 11);
+
+        std::vector<std::string> inputs;
+        for (const auto& doc : documents)
+        {
+            for (std::size_t i = 0; i <= doc.size(); ++i)
+            {
+                inputs.push_back(doc.substr(0, i));
+                if (i < doc.size())
+                {
+                    inputs.push_back(doc.substr(0, i) + doc.substr(i + 1));
+                }
+                for (const char c : insertions)
+                {
+                    inputs.push_back(doc.substr(0, i) + c + doc.substr(i));
+                }
+            }
+        }
+
+        for (const auto& s : inputs)
+        {
+            CAPTURE(s)
+            const auto result = parse_recovering(s);
+            CHECK(result.balanced);
+            CHECK(result.errors.size() <= s.size() + 1);
+            CHECK(result.events <= (4 * s.size()) + 4);
+            if (json::accept(s))
+            {
+                CHECK(result.ok);
+                CHECK(result.errors.empty());
+                CHECK(result.value == json::parse(s));
+            }
+            else
+            {
+                CHECK(!result.ok);
+                CHECK(!result.errors.empty());
+            }
+        }
+    }
+}

tests/src/unit-comparison.cpp

@@ -824,7 +824,7 @@ struct unordered_object_t : std::map<Key, Value, directed_less<Key>, Allocator>
         return !(lhs == rhs);
     }
 };
-using unordered_json = nlohmann::basic_json<unordered_object_t>;
+using unordered_json = nlohmann::json::with_object_t<unordered_object_t>;
 
 // the entries "0" to "9", enumerated in ascending or in descending order
 unordered_json make_unordered_object(const bool descending)
@@ -875,7 +875,7 @@ struct key_case_less
 
 template<class Key, class Value, class /*Compare*/, class Allocator>
 using key_case_map = std::map<Key, Value, key_case_less, Allocator>;
-using key_case_json = nlohmann::basic_json<key_case_map>;
+using key_case_json = nlohmann::json::with_object_t<key_case_map>;
 
 // the innermost value of a chain of single-element arrays
 template<typename Json>
@@ -905,7 +905,7 @@ struct case_insensitive_less
 
 template<class Key, class Value, class /*Compare*/, class Allocator>
 using case_insensitive_map = std::map<Key, Value, case_insensitive_less, Allocator>;
-using ci_json = nlohmann::basic_json<case_insensitive_map>;
+using ci_json = nlohmann::json::with_object_t<case_insensitive_map>;
 } // namespace
 
 TEST_CASE("equality of objects whose entries have no fixed order")

tests/src/unit-custom-array-type.cpp

@@ -22,7 +22,7 @@ namespace
 
 // std::deque has no capacity() member function, which the library only needs
 // to detect a reallocation for JSON_DIAGNOSTICS
-using deque_json = nlohmann::basic_json<std::map, std::deque>;
+using deque_json = nlohmann::json::with_array_t<std::deque>;
 
 // a std::vector whose at() is hidden: the library performs its own bounds
 // check and must not fall back to the container's checked accessor
@@ -39,7 +39,7 @@ class vector_without_at : public std::vector<T, Allocator>
     void at() = delete;
 };
 
-using no_at_json = nlohmann::basic_json<std::map, vector_without_at>;
+using no_at_json = nlohmann::json::with_array_t<vector_without_at>;
 
 } // namespace
 

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

@@ -400,20 +400,7 @@ class base_class_with_hidden_members
     std::size_t m_size = 42;
 };
 
-using json_with_hidden_base_members =
-    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>,
-    base_class_with_hidden_members
-    >;
+using json_with_hidden_base_members = nlohmann::json::with_base_class_t<base_class_with_hidden_members>;
 
 TEST_CASE("JSON Node as_base_class")
 {
@@ -459,19 +446,7 @@ struct const_member_base
     const int id = 7; // NOLINT(misc-non-private-member-variables-in-classes)
 };
 
-using json_with_const_base = 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>,
-                             const_member_base
-                             >;
+using json_with_const_base = nlohmann::json::with_base_class_t<const_member_base>;
 
 // build an array nested @a depth levels deep, with the innermost value 1;
 // every level is constructed (never assigned), since const_member_base does

tests/src/unit-custom-binary-type.cpp

@@ -26,15 +26,11 @@ namespace
 
 // a BinaryType whose value type is signed: the elements must still be
 // processed as the numbers 0..255
-using char_binary_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<char>, void >;
+using char_binary_json = nlohmann::json::with_binary_t<std::vector<char>>;
 
 #ifdef JSON_HAS_CPP_17
     // a BinaryType whose value type is not an integer type at all
-    using byte_binary_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::byte>, void >;
+    using byte_binary_json = nlohmann::json::with_binary_t<std::vector<std::byte>>;
 #endif
 
 } // namespace

tests/src/unit-custom-object-key-type.cpp

@@ -0,0 +1,127 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+#include "doctest_compatibility.h"
+
+#include <nlohmann/json.hpp>
+
+#include <cstddef>
+#include <string>
+#include <utility>
+
+#include "custom_object_key_type.hpp"
+
+// These tests instantiate a second basic_json specialization. They live in
+// their own file rather than in unit-cbor.cpp and unit-msgpack.cpp to keep
+// those objects below 65535 sections: the MinGW linker stores the section a
+// COMDAT section is associated with in 16 bits, so it misplaces the jump
+// tables of larger objects (see the clang job in windows.yml).
+
+TEST_CASE("CBOR supports custom object key types")
+{
+    using custom_json = custom_object_key_test::json;
+    using custom_key = custom_object_key_test::key;
+
+    custom_json::object_t object;
+    object.emplace(custom_key{"short"}, 1);
+    object.emplace(
+              custom_key{"a key longer than twenty-three characters"},
+              2);
+
+    const custom_json value(std::move(object));
+    const auto encoded = custom_json::to_cbor(value);
+
+    CHECK(nlohmann::json::from_cbor(encoded) == nlohmann::json
+    {
+        {"short", 1},
+        {"a key longer than twenty-three characters", 2}
+    });
+}
+
+TEST_CASE("CBOR supports custom object key types nested deeper than the recursion depth limit")
+{
+    // below detail::recursion_depth_limit(), keys are written by
+    // write_cbor_iterative instead of write_cbor
+    using custom_json = custom_object_key_test::json;
+    using custom_key = custom_object_key_test::key;
+
+    const std::size_t depth = nlohmann::detail::recursion_depth_limit() + 10;
+
+    custom_json value = 1;
+    nlohmann::json expected = 1;
+    for (std::size_t i = 0; i < depth; ++i)
+    {
+        // alternate short keys with ones long enough to need a length byte
+        const std::string name = (i % 2 == 0) ? "k" + std::to_string(i)
+                                 : "a key longer than thirty-one characters " + std::to_string(i);
+
+        custom_json::object_t object;
+        object.emplace(custom_key{name}, std::move(value));
+        value = custom_json(std::move(object));
+
+        nlohmann::json::object_t expected_object;
+        expected_object.emplace(name, std::move(expected));
+        expected = nlohmann::json(std::move(expected_object));
+    }
+
+    const auto encoded = custom_json::to_cbor(value);
+    CHECK(encoded == nlohmann::json::to_cbor(expected));
+    CHECK(nlohmann::json::from_cbor(encoded) == expected);
+}
+
+TEST_CASE("MessagePack supports custom object key types")
+{
+    using custom_json = custom_object_key_test::json;
+    using custom_key = custom_object_key_test::key;
+
+    custom_json::object_t object;
+    object.emplace(custom_key{"short"}, 1);
+    object.emplace(
+              custom_key{"a key longer than thirty-one characters"},
+              2);
+
+    const custom_json value(std::move(object));
+    const auto encoded = custom_json::to_msgpack(value);
+
+    CHECK(nlohmann::json::from_msgpack(encoded) == nlohmann::json
+    {
+        {"short", 1},
+        {"a key longer than thirty-one characters", 2}
+    });
+}
+
+TEST_CASE("MessagePack supports custom object key types nested deeper than the recursion depth limit")
+{
+    // below detail::recursion_depth_limit(), keys are written by
+    // write_msgpack_iterative instead of write_msgpack
+    using custom_json = custom_object_key_test::json;
+    using custom_key = custom_object_key_test::key;
+
+    const std::size_t depth = nlohmann::detail::recursion_depth_limit() + 10;
+
+    custom_json value = 1;
+    nlohmann::json expected = 1;
+    for (std::size_t i = 0; i < depth; ++i)
+    {
+        // alternate short keys with ones long enough to need a length byte
+        const std::string name = (i % 2 == 0) ? "k" + std::to_string(i)
+                                 : "a key longer than thirty-one characters " + std::to_string(i);
+
+        custom_json::object_t object;
+        object.emplace(custom_key{name}, std::move(value));
+        value = custom_json(std::move(object));
+
+        nlohmann::json::object_t expected_object;
+        expected_object.emplace(name, std::move(expected));
+        expected = nlohmann::json(std::move(expected_object));
+    }
+
+    const auto encoded = custom_json::to_msgpack(value);
+    CHECK(encoded == nlohmann::json::to_msgpack(expected));
+    CHECK(nlohmann::json::from_msgpack(encoded) == expected);
+}

tests/src/unit-custom-object-type.cpp

@@ -179,7 +179,7 @@ class no_key_compare_map
     }
 };
 
-using no_key_compare_json = nlohmann::basic_json<no_key_compare_map>;
+using no_key_compare_json = nlohmann::json::with_object_t<no_key_compare_map>;
 
 // An ObjectType whose erase(iterator) returns void rather than the following
 // iterator, as for instance Abseil's hash maps do
@@ -196,7 +196,7 @@ struct void_erase_map : std::map<Key, T, Compare, Allocator>
     }
 };
 
-using void_erase_json = nlohmann::basic_json<void_erase_map>;
+using void_erase_json = nlohmann::json::with_object_t<void_erase_map>;
 
 // wraps an iterator, but only offers the LegacyForwardIterator operations,
 // like the iterators of std::unordered_map and other hash maps
@@ -388,7 +388,7 @@ class forward_only_map
     }
 };
 
-using forward_only_json = nlohmann::basic_json<forward_only_map>;
+using forward_only_json = nlohmann::json::with_object_t<forward_only_map>;
 
 } // namespace
 

tests/src/unit-hash.cpp

@@ -157,13 +157,13 @@ TEST_CASE("hash<nlohmann::json>")
     // the ends of the integer ranges, which equal floats exactly
     const auto int_min = (std::numeric_limits<json::number_integer_t>::min)();
     const auto int_max = (std::numeric_limits<json::number_integer_t>::max)();
-    const auto two_63 = json::number_unsigned_t(1) << 63U;
+    const auto two_63 = static_cast<json::number_unsigned_t>(1) << 63U;
     CHECK(json(int_min) == json(-9223372036854775808.0));
     CHECK(std::hash<json> {}(json(int_min)) == std::hash<json> {}(json(-9223372036854775808.0)));
     CHECK(json(two_63) == json(9223372036854775808.0));
     CHECK(std::hash<json> {}(json(two_63)) == std::hash<json> {}(json(9223372036854775808.0)));
-    CHECK(json(json::number_unsigned_t(int_max)) == json(int_max));
-    CHECK(std::hash<json> {}(json(json::number_unsigned_t(int_max))) == std::hash<json> {}(json(int_max)));
+    CHECK(json(static_cast<json::number_unsigned_t>(int_max)) == json(int_max));
+    CHECK(std::hash<json> {}(json(static_cast<json::number_unsigned_t>(int_max))) == std::hash<json> {}(json(int_max)));
 }
 
 TEST_CASE("hash<nlohmann::ordered_json>")

tests/src/unit-json_pointer.cpp

@@ -939,3 +939,114 @@ TEST_CASE("unescaping keeps a '~' that does not start an escape sequence")
     nlohmann::detail::unescape(s);
     CHECK(s == "~/~");
 }
+
+TEST_CASE("flatten of structured values")
+{
+    SECTION("values nested too deeply for the call stack (#5393)")
+    {
+        // flatten() used to recurse once per nesting level
+        const std::size_t depth = 100000;
+        for (const bool objects :
+                {
+                    false, true
+                })
+        {
+            CAPTURE(objects)
+            std::string text;
+            std::string path;
+            for (std::size_t i = 0; i < depth; ++i)
+            {
+                text += objects ? "{\"a\":" : "[";
+                path += objects ? "/a" : "/0";
+            }
+            text += "0";
+            text += std::string(depth, objects ? '}' : ']');
+            const auto value = json::parse(text);
+
+            const auto flat = value.flatten();
+            REQUIRE(flat.size() == 1);
+            REQUIRE(flat.begin().key().size() == path.size());
+            CHECK(flat.begin().key() == path);
+            CHECK(flat.begin().value() == 0);
+
+            // unflatten() is linear in the depth, so the value roundtrips
+            CHECK(flat.unflatten() == value);
+        }
+    }
+
+    SECTION("unflatten of a deeply nested pointer")
+    {
+        const std::size_t depth = 100000;
+        for (const bool objects :
+                {
+                    false, true
+                })
+        {
+            CAPTURE(objects)
+            std::string path;
+            for (std::size_t i = 0; i < depth; ++i)
+            {
+                path += objects ? "/a" : "/0";
+            }
+
+            json flat = json::object();
+            flat[path] = 1;
+            const json value = flat.unflatten();
+
+            // walk down iteratively
+            std::size_t levels = 0;
+            const json* current = &value;
+            while (objects ? current->is_object() : current->is_array())
+            {
+                REQUIRE(current->size() == 1);
+                current = objects ? &current->at("a") : &current->at(0);
+                ++levels;
+            }
+            CHECK(levels == depth);
+            CHECK(*current == 1);
+        }
+    }
+
+    SECTION("unflatten does not depend on the iteration order")
+    {
+        // the "0" key comes after its sibling in iteration order
+        const nlohmann::ordered_json flat_array = nlohmann::ordered_json::parse(R"({"/a/1": 2, "/a/0": 1})");
+        CHECK(flat_array.unflatten() == nlohmann::ordered_json::parse(R"({"a": [1, 2]})"));
+
+        const nlohmann::ordered_json flat_object = nlohmann::ordered_json::parse(R"({"/b/1": 2})");
+        CHECK(flat_object.unflatten() == nlohmann::ordered_json::parse(R"({"b": {"1": 2}})"));
+    }
+
+    SECTION("objects and arrays interleaved")
+    {
+        const json value =
+        {
+            {"a", {1, {{"b", json::array()}, {"c", json::object()}}, json::array({{{"x~/", {true, nullptr}}}})}},
+            {"a/b", {{"~", 1}}},
+            {"z", "s"}
+        };
+
+        const json expected =
+        {
+            {"/a/0", 1},
+            {"/a/1/b", nullptr},
+            {"/a/1/c", nullptr},
+            {"/a/2/0/x~0~1/0", true},
+            {"/a/2/0/x~0~1/1", nullptr},
+            {"/a~1b/~0", 1},
+            {"/z", "s"}
+        };
+
+        CHECK(value.flatten() == expected);
+    }
+
+    SECTION("order of the entries of an ordered_json")
+    {
+        const auto value = nlohmann::ordered_json::parse(
+                               R"({"z":"s","a/b":{"~":1,"k":[]},"a":[1,{"c":{},"b":[]},[{"x~/":[true,null],"w":2}]]})");
+
+        const auto flat = value.flatten();
+        CHECK(flat.dump() ==
+              R"({"/z":"s","/a~1b/~0":1,"/a~1b/k":null,"/a/0":1,"/a/1/c":null,"/a/1/b":null,"/a/2/0/x~0~1/0":true,"/a/2/0/x~0~1/1":null,"/a/2/0/w":2})");
+    }
+}

tests/src/unit-large_json.cpp

@@ -426,7 +426,7 @@ TEST_CASE("issue #5392 - binary writers on deeply nested values")
     {
         for (std::size_t depth = 120; depth <= 140; ++depth)
         {
-            CAPTURE(depth);
+            CAPTURE(depth)
 
             const json array = nested_array(depth, json(7));
             CHECK(json::from_cbor(json::to_cbor(array)) == array);
@@ -464,7 +464,7 @@ TEST_CASE("issue #5392 - binary writers on deeply nested values")
                     nlohmann::detail::recursion_depth_limit() + 1, nlohmann::detail::recursion_depth_limit() + 2
                 })
         {
-            CAPTURE(depth);
+            CAPTURE(depth)
             const json array = nested_array(depth, json(0));
 
             std::vector<std::uint8_t> expected_cbor(depth, 0x81);
@@ -505,10 +505,22 @@ TEST_CASE("issue #5392 - binary writers on deeply nested values")
         const json discarded_leaf(json::value_t::discarded);
         const json deep_discarded = nested_array(depth, discarded_leaf);
 
-        CHECK_THROWS_WITH_AS(json::to_cbor(deep_discarded), "[json.exception.type_error.321] cannot serialize discarded value to CBOR", json::type_error);
-        CHECK_THROWS_WITH_AS(json::to_msgpack(deep_discarded), "[json.exception.type_error.321] cannot serialize discarded value to MessagePack", json::type_error);
-        CHECK_THROWS_WITH_AS(json::to_ubjson(deep_discarded), "[json.exception.type_error.321] cannot serialize discarded value to UBJSON", json::type_error);
-        CHECK_THROWS_WITH_AS(json::to_bjdata(deep_discarded), "[json.exception.type_error.321] cannot serialize discarded value to BJData", json::type_error);
+        // with diagnostics, the message names the path to the discarded leaf
+#if JSON_DIAGNOSTICS
+        std::string path;
+        for (std::size_t i = 0; i < depth; ++i)
+        {
+            path += "/0";
+        }
+        const std::string prefix = "[json.exception.type_error.321] (" + path + ") ";
+#else
+        const std::string prefix = "[json.exception.type_error.321] ";
+#endif
+
+        CHECK_THROWS_WITH_AS(json::to_cbor(deep_discarded), (prefix + "cannot serialize discarded value to CBOR").c_str(), json::type_error);
+        CHECK_THROWS_WITH_AS(json::to_msgpack(deep_discarded), (prefix + "cannot serialize discarded value to MessagePack").c_str(), json::type_error);
+        CHECK_THROWS_WITH_AS(json::to_ubjson(deep_discarded), (prefix + "cannot serialize discarded value to UBJSON").c_str(), json::type_error);
+        CHECK_THROWS_WITH_AS(json::to_bjdata(deep_discarded), (prefix + "cannot serialize discarded value to BJData").c_str(), json::type_error);
     }
 
     SECTION("does not overflow the C++ stack")

tests/src/unit-locale-cpp.cpp

@@ -172,7 +172,7 @@ TEST_CASE("locale-dependent test (LC_NUMERIC=de_DE)")
             // a floating-point type that is not a float or a double is written
             // with snprintf, whose locale-specific decimal point and thousands
             // separator are undone afterwards
-            using long_double_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, long double>;
+            using long_double_json = nlohmann::json::with_float_t<long double>;
             CHECK(long_double_json(12345.5L).dump() == "12345.5");
             CHECK(long_double_json(1.0L).dump() == "1.0");
             CHECK(long_double_json(-0.25L).dump() == "-0.25");
@@ -272,7 +272,7 @@ TEST_CASE("locale changes between lexer construction and number conversion (#519
     }
     text += "]";
 
-    using long_double_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, long double>;
+    using long_double_json = nlohmann::json::with_float_t<long double>;
 
     // reference values, parsed without a locale switch
     REQUIRE(std::setlocale(LC_NUMERIC, "C") != nullptr);
@@ -432,7 +432,7 @@ TEST_CASE("locale changes during a single dump() (#5709 item 3)")
     // long double on 64-bit Arm, where it is IEEE-754 double) takes the
     // locale-independent to_chars() path instead, and this test is a no-op
     // there.
-    using long_double_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, long double>;
+    using long_double_json = nlohmann::json::with_float_t<long double>;
     using ld_limits = std::numeric_limits<long_double_json::number_float_t>;
     const bool is_ieee_single_or_double =
         (ld_limits::is_iec559 && ld_limits::digits == 24 && ld_limits::max_exponent == 128) ||

tests/src/unit-msgpack.cpp

@@ -31,8 +31,6 @@ using nlohmann::json;
 #include "make_test_data_available.hpp"
 #include "round_trip_corpus.hpp"
 #include "test_utils.hpp"
-#include "sax_countdown.hpp"
-using utils::SaxCountdown;
 
 
 TEST_CASE("MessagePack")
@@ -1645,30 +1643,6 @@ TEST_CASE("MessagePack")
             }
         }
     }
-
-    SECTION("SAX aborts")
-    {
-        SECTION("start_array(len)")
-        {
-            std::vector<uint8_t> const v = {0x93, 0x01, 0x02, 0x03};
-            SaxCountdown scp(0);
-            CHECK(!json::sax_parse(v, &scp, json::input_format_t::msgpack));
-        }
-
-        SECTION("start_object(len)")
-        {
-            std::vector<uint8_t> const v = {0x81, 0xa3, 0x66, 0x6F, 0x6F, 0xc2};
-            SaxCountdown scp(0);
-            CHECK(!json::sax_parse(v, &scp, json::input_format_t::msgpack));
-        }
-
-        SECTION("key()")
-        {
-            std::vector<uint8_t> const v = {0x81, 0xa3, 0x66, 0x6F, 0x6F, 0xc2};
-            SaxCountdown scp(1);
-            CHECK(!json::sax_parse(v, &scp, json::input_format_t::msgpack));
-        }
-    }
 }
 
 TEST_CASE("issue #5405 - array reserve for definite-length MessagePack arrays")
@@ -1739,21 +1713,6 @@ TEST_CASE("issue #5405 - array reserve for definite-length MessagePack arrays")
             CHECK(json::from_msgpack(packed) == j);
         }
     }
-
-    SECTION("a user-defined SAX consumer is unaffected by the internal DOM reserve optimization")
-    {
-        // the reserve() call is local to json_sax_dom_parser / json_sax_dom_callback_parser;
-        // a custom SAX consumer that does not touch a DOM array sees identical events
-        json j = json::array();
-        for (int i = 0; i < 100; ++i)
-        {
-            j.push_back(i);
-        }
-        const auto packed = json::to_msgpack(j);
-
-        SaxCountdown scp(1000000); // large enough to never trigger an abort
-        CHECK(json::sax_parse(packed, &scp, json::input_format_t::msgpack));
-    }
 }
 
 TEST_CASE("regression test - MessagePack ext type rejects a subtype that doesn't fit a single byte")
@@ -1792,15 +1751,6 @@ TEST_CASE("MessagePack nesting does not consume the call stack")
         CHECK(json::from_msgpack(input, true, false).is_discarded());
     }
 
-    SECTION("a well-formed deep value is read through the SAX interface")
-    {
-        std::vector<uint8_t> input(300000, 0x91);
-        input.push_back(0x01); // innermost value
-
-        SaxCountdown accept_all(600001);
-        CHECK(json::sax_parse(input, &accept_all, json::input_format_t::msgpack));
-    }
-
     SECTION("a well-formed deep value is read into a value")
     {
         const std::size_t depth = 10000;
@@ -1848,31 +1798,6 @@ TEST_CASE("MessagePack input that cannot be read is discarded by every overload"
 #endif
 }
 
-TEST_CASE("MessagePack SAX parsing stops at every event")
-{
-    // Containers are opened and closed by the loop that reads them; a SAX
-    // handler that rejects any event - including the end of a nested
-    // container - must stop the parse right there.
-    const auto count_events = [](const std::vector<std::uint8_t>& input)
-    {
-        int events = 0;
-        while (true)
-        {
-            SaxCountdown scp(events);
-            if (json::sax_parse(input, &scp, json::input_format_t::msgpack))
-            {
-                return events;
-            }
-            ++events;
-            REQUIRE(events < 1000);
-        }
-    };
-
-    // 20 events: every container kind closes inside another one
-    const json j = json::parse(R"({"a": [1, {"b": []}], "c": {"d": [[2]]}})");
-    CHECK(count_events(json::to_msgpack(j)) == 20);
-}
-
 TEST_CASE("single MessagePack roundtrip")
 {
     SECTION("sample.json")
@@ -2201,10 +2126,7 @@ struct huge_array : std::vector<T, A>
     }
 };
 
-using huge_array_json = nlohmann::basic_json <
-                        std::map, huge_array, std::string, bool, std::int64_t, std::uint64_t,
-                        double, std::allocator, nlohmann::adl_serializer,
-                        std::vector<std::uint8_t>, void >;
+using huge_array_json = nlohmann::json::with_array_t<huge_array>;
 
 TEST_CASE("MessagePack Size above uint32 for array")
 {
@@ -2249,18 +2171,7 @@ template<typename K, typename V,
     }
 };
 
-using huge_object_json = nlohmann::basic_json <
-                         huge_map,
-                         std::vector,
-                         std::string,
-                         bool,
-                         std::int64_t,
-                         std::uint64_t,
-                         double,
-                         std::allocator,
-                         nlohmann::adl_serializer,
-                         std::vector<std::uint8_t>,
-                         void >;
+using huge_object_json = nlohmann::json::with_object_t<huge_map>;
 
 TEST_CASE("MessagePack Size above uint32 for object")
 {
@@ -2295,18 +2206,7 @@ struct huge_string : std::string
     }
 };
 
-using huge_string_json = nlohmann::basic_json <
-                         std::map,
-                         std::vector,
-                         huge_string,
-                         bool,
-                         std::int64_t,
-                         std::uint64_t,
-                         double,
-                         std::allocator,
-                         nlohmann::adl_serializer,
-                         std::vector<std::uint8_t>,
-                         void >;
+using huge_string_json = nlohmann::json::with_string_t<huge_string>;
 
 TEST_CASE("MessagePack Size above uint32 for string")
 {
@@ -2329,18 +2229,7 @@ struct huge_binary : std::vector<std::uint8_t>
     }
 };
 
-using huge_binary_json = nlohmann::basic_json <
-                         std::map,
-                         std::vector,
-                         std::string,
-                         bool,
-                         std::int64_t,
-                         std::uint64_t,
-                         double,
-                         std::allocator,
-                         nlohmann::adl_serializer,
-                         huge_binary,
-                         void >;
+using huge_binary_json = nlohmann::json::with_binary_t<huge_binary>;
 
 TEST_CASE("MessagePack Size above uint32 for binary")
 {
@@ -2390,14 +2279,10 @@ class beyond_uint32_string_t : public std::string
     }
 };
 
-using beyond_uint32_string_json = nlohmann::basic_json <
-                                  std::map, std::vector, beyond_uint32_string_t, bool, std::int64_t, std::uint64_t,
-                                  double, std::allocator, nlohmann::adl_serializer, std::vector<std::uint8_t>, void >;
+using beyond_uint32_string_json = nlohmann::json::with_string_t<beyond_uint32_string_t>;
 #endif
 
-using beyond_uint32_binary_json = nlohmann::basic_json <
-                                  std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t,
-                                  double, std::allocator, nlohmann::adl_serializer, beyond_uint32_binary_t, void >;
+using beyond_uint32_binary_json = nlohmann::json::with_binary_t<beyond_uint32_binary_t>;
 } // namespace
 
 TEST_CASE("MessagePack lengths beyond UINT32_MAX cannot be serialized")
@@ -2432,8 +2317,8 @@ TEST_CASE("MessagePack numbers use the active union member (see #5644)")
     // used to read the union member that was not the active one, writing
     // wrong bytes for some values; std::int64_t/std::uint64_t (the default
     // types, where both members have the same width) were not affected
-    using int32_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int32_t, std::uint64_t, double>;
-    using int16_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int16_t, std::uint64_t, double>;
+    using int32_json = nlohmann::json::with_integers_t<std::int32_t, std::uint64_t>;
+    using int16_json = nlohmann::json::with_integers_t<std::int16_t, std::uint64_t>;
 
     SECTION("number_integer_t = std::int32_t")
     {

tests/src/unit-ordered_json2.cpp

@@ -217,16 +217,7 @@ 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-regression1.cpp

@@ -39,7 +39,7 @@ using nlohmann::json;
 
 template<class K, class V, class dummy_compare, class A>
 using my_workaround_fifo_map = nlohmann::fifo_map<K, V, nlohmann::fifo_map_compare<K>, A>;
-using my_json = nlohmann::basic_json<my_workaround_fifo_map>;
+using my_json = nlohmann::json::with_object_t<my_workaround_fifo_map>;
 
 /////////////////////////////////////////////////////////////////////
 // for #977
@@ -86,8 +86,7 @@ struct foo_serializer < T, typename std::enable_if < !std::is_same<foo, T>::valu
 };
 } // namespace ns
 
-using foo_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t,
-      std::uint64_t, double, std::allocator, ns::foo_serializer, std::vector<std::uint8_t>>;
+using foo_json = nlohmann::json::with_json_serializer_t<ns::foo_serializer>;
 
 /////////////////////////////////////////////////////////////////////
 // for #805
@@ -254,7 +253,7 @@ TEST_CASE("regression tests 1")
     {
         // create JSON class with nonstandard integer number type
         using custom_json =
-            nlohmann::basic_json<std::map, std::vector, std::string, bool, int32_t, uint32_t, float>;
+            nlohmann::json::with_integers_t<std::int32_t, std::uint32_t>::with_float_t<float>;
         custom_json j;
         j["int_1"] = 1;
         CHECK(j["int_1"] == 1);
@@ -470,18 +469,17 @@ TEST_CASE("regression tests 1")
         // create JSON class with nonstandard float number type
 
         // float
-        nlohmann::basic_json<std::map, std::vector, std::string, bool, int32_t, uint32_t, float> const j_float =
+        nlohmann::json::with_integers_t<std::int32_t, std::uint32_t>::with_float_t<float> const j_float =
             1.23e25f;
         CHECK(j_float.get<float>() == 1.23e25f);
 
         // double
-        nlohmann::basic_json<std::map, std::vector, std::string, bool, int64_t, uint64_t, double> const j_double =
+        nlohmann::json const j_double =
             1.23e35;
         CHECK(j_double.get<double>() == 1.23e35);
 
         // long double
-        nlohmann::basic_json<std::map, std::vector, std::string, bool, int64_t, uint64_t, long double>
-        const j_long_double = 1.23e45L;
+        nlohmann::json::with_float_t<long double> const j_long_double = 1.23e45L;
         CHECK(j_long_double.get<long double>() == 1.23e45L);
     }
 

tests/src/unit-regression2.cpp

@@ -64,18 +64,7 @@ using ordered_json = nlohmann::ordered_json;
     /////////////////////////////////////////////////////////////////////
     // for #4804
     /////////////////////////////////////////////////////////////////////
-    using json_4804 = nlohmann::basic_json<std::map,        // ObjectType
-    std::vector,     // ArrayType
-    std::string,     // StringType
-    bool,            // BooleanType
-    std::int64_t,    // NumberIntegerType
-    std::uint64_t,   // NumberUnsignedType
-    double,          // NumberFloatType
-    std::allocator,  // AllocatorType
-    nlohmann::adl_serializer,  // JSONSerializer
-    std::vector<std::byte>,    // BinaryType
-    void                       // CustomBaseClass
-    >;
+    using json_4804 = nlohmann::json::with_binary_t<std::vector<std::byte>>;
 #endif
 
 #ifdef JSON_HAS_CPP_20
@@ -107,7 +96,7 @@ DOCTEST_CLANG_SUPPRESS_WARNING("-Wexit-time-destructors")
 // for #1021
 /////////////////////////////////////////////////////////////////////
 
-using float_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, float>;
+using float_json = nlohmann::json::with_float_t<float>;
 
 #if (defined(__cpp_exceptions) || defined(__EXCEPTIONS) || defined(_CPPUNWIND)) && !defined(JSON_NOEXCEPTION)
 namespace
@@ -155,10 +144,8 @@ struct failing_allocator : std::allocator<T>
     };
 };
 
-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>;
+using failing_json = nlohmann::json::with_allocator_t<failing_allocator>;
+using failing_ordered_json = nlohmann::ordered_json::with_allocator_t<failing_allocator>;
 
 // builds `depth` levels of nesting around a scalar, iteratively (never
 // recursing: each wrap only moves the previous, already-built value, which

tests/src/unit-regression3.cpp

@@ -22,14 +22,6 @@
 // scoped enum, so get<std::byte>() (needed below to get<std::vector<std::byte>>()
 // from a plain JSON array, not just from an already-binary value) relies on
 // enum serialization being enabled
-// capture whether JSON_DELETE_DEPRECATED_FUNCTIONS was enabled on the command
-// line *before* including json.hpp, since the library #undefs it once the header
-// has been fully processed (see include/nlohmann/detail/macro_unscope.hpp); the
-// tests of deprecated functions are skipped if these functions are deleted
-#if defined(JSON_DELETE_DEPRECATED_FUNCTIONS) && (JSON_DELETE_DEPRECATED_FUNCTIONS == 1)
-    #define JSON_TEST_DEPRECATED_FUNCTIONS_DELETED
-#endif
-
 #if defined(JSON_DISABLE_ENUM_SERIALIZATION) && (JSON_DISABLE_ENUM_SERIALIZATION == 1)
     #define SKIP_TESTS_FOR_ENUM_SERIALIZATION
 #endif
@@ -62,18 +54,7 @@ using ordered_json = nlohmann::ordered_json;
     /////////////////////////////////////////////////////////////////////
     // for #4804
     /////////////////////////////////////////////////////////////////////
-    using json_4804 = nlohmann::basic_json<std::map,        // ObjectType
-    std::vector,     // ArrayType
-    std::string,     // StringType
-    bool,            // BooleanType
-    std::int64_t,    // NumberIntegerType
-    std::uint64_t,   // NumberUnsignedType
-    double,          // NumberFloatType
-    std::allocator,  // AllocatorType
-    nlohmann::adl_serializer,  // JSONSerializer
-    std::vector<std::byte>,    // BinaryType
-    void                       // CustomBaseClass
-    >;
+    using json_4804 = nlohmann::json::with_binary_t<std::vector<std::byte>>;
 #endif
 
 #ifdef JSON_HAS_CPP_20
@@ -870,50 +851,6 @@ TEST_CASE("issue #5338 - truncated CBOR tagged binary subtype is rejected")
     }
 }
 
-TEST_CASE("issue #5676 - SAX parsing of CBOR tags")
-{
-    const json expected = json::binary({1, 2, 3}, 42);
-    const auto cbor = json::to_cbor(expected);
-
-    nlohmann::detail::json_sax_acceptor<json> acceptor;
-    CHECK_FALSE(json::sax_parse(cbor, &acceptor, json::input_format_t::cbor));
-    CHECK_FALSE(json::sax_parse(cbor, &acceptor, json::input_format_t::cbor,
-                                true, false, false, json::cbor_tag_handler_t::error));
-
-    CHECK(json::sax_parse(cbor, &acceptor, json::input_format_t::cbor,
-                          true, false, false, json::cbor_tag_handler_t::ignore));
-
-    json parsed;
-    nlohmann::detail::json_sax_dom_parser<json, nlohmann::detail::string_input_adapter_type> sax(parsed);
-    CHECK(json::sax_parse(cbor, &sax, json::input_format_t::cbor,
-                          true, false, false, json::cbor_tag_handler_t::store));
-    CHECK(parsed == expected);
-
-    json iterator_parsed;
-    nlohmann::detail::json_sax_dom_parser<json, nlohmann::detail::string_input_adapter_type> iterator_sax(iterator_parsed);
-    CHECK(json::sax_parse(cbor.begin(), cbor.end(), &iterator_sax, json::input_format_t::cbor,
-                          true, false, false, json::cbor_tag_handler_t::store));
-    CHECK(iterator_parsed == expected);
-
-#ifndef JSON_TEST_DEPRECATED_FUNCTIONS_DELETED
-    json span_parsed;
-    nlohmann::detail::json_sax_dom_parser<json, nlohmann::detail::string_input_adapter_type> span_sax(span_parsed);
-    CHECK(json::sax_parse(nlohmann::detail::span_input_adapter(cbor.data(), cbor.size()), &span_sax,
-                          json::input_format_t::cbor, true, false, false, json::cbor_tag_handler_t::store));
-    CHECK(span_parsed == expected);
-#endif
-
-    const std::string text = "null";
-    CHECK(json::sax_parse(text, &acceptor, json::input_format_t::json,
-                          true, false, false, json::cbor_tag_handler_t::store));
-    CHECK(json::sax_parse(text.begin(), text.end(), &acceptor, json::input_format_t::json,
-                          true, false, false, json::cbor_tag_handler_t::store));
-#ifndef JSON_TEST_DEPRECATED_FUNCTIONS_DELETED
-    CHECK(json::sax_parse(nlohmann::detail::span_input_adapter(text.data(), text.size()), &acceptor,
-                          json::input_format_t::json, true, false, false, json::cbor_tag_handler_t::store));
-#endif
-}
-
 TEST_CASE("issue #5402 - update(merge_objects=true) overwrites a primitive with an object")
 {
     json t = {{"k", 1}};
@@ -930,7 +867,7 @@ TEST_CASE("regression test #5476 - array type without reserve()")
 {
     // the capacity reserved for definite-length arrays must not require the
     // array type to have a reserve() member function
-    using deque_json = nlohmann::basic_json<std::map, std::deque>;
+    using deque_json = nlohmann::json::with_array_t<std::deque>;
 
     SECTION("std::deque")
     {

tests/src/unit-sax_parse.cpp

@@ -0,0 +1,768 @@
+//     __ _____ _____ _____
+//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
+// |  |  |__   |  |  | | | |  version 3.12.0
+// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
+//
+// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
+// SPDX-License-Identifier: MIT
+
+/////////////////////////////////////////////////////////////////////
+// Tests that call basic_json::sax_parse have a file of their own: every
+// sax_parse call instantiates the parser and binary reader that recover from
+// errors (see #3989), and in unit-regression2.cpp, unit-regression3.cpp, and
+// unit-msgpack.cpp this made the objects too large for the MinGW linker to
+// relocate (see #5511).
+/////////////////////////////////////////////////////////////////////
+
+#include "doctest_compatibility.h"
+
+// capture whether JSON_DELETE_DEPRECATED_FUNCTIONS was enabled on the command
+// line *before* including json.hpp, since the library #undefs it once the header
+// has been fully processed (see include/nlohmann/detail/macro_unscope.hpp); the
+// tests of deprecated functions are skipped if these functions are deleted
+#if defined(JSON_DELETE_DEPRECATED_FUNCTIONS) && (JSON_DELETE_DEPRECATED_FUNCTIONS == 1)
+    #define JSON_TEST_DEPRECATED_FUNCTIONS_DELETED
+#endif
+
+#include <nlohmann/json.hpp>
+using json = nlohmann::json;
+
+#include <cmath>
+#include <cstddef>
+#include <cstdint>
+#include <initializer_list>
+#include <map>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "sax_countdown.hpp"
+using utils::SaxCountdown;
+
+// a narrow number_float_t, so that a double read from binary input can
+// overflow it
+using float_json = nlohmann::basic_json<std::map, std::vector, std::string, bool, std::int64_t, std::uint64_t, float>;
+
+DOCTEST_CLANG_SUPPRESS_WARNING_PUSH
+DOCTEST_CLANG_SUPPRESS_WARNING("-Wexit-time-destructors")
+
+namespace
+{
+/// builds a value from SAX events, asks the parser to recover from its first
+/// 100 errors, and checks that the events are balanced (see #3989)
+template<typename BasicJsonType>
+class BasicRecoveringParser
+{
+  public:
+    explicit BasicRecoveringParser(BasicJsonType& j)
+        : dom(j, false)
+    {}
+
+    bool null()
+    {
+        value();
+        return dom.null();
+    }
+
+    bool boolean(bool val)
+    {
+        value();
+        return dom.boolean(val);
+    }
+
+    bool number_integer(typename BasicJsonType::number_integer_t val)
+    {
+        value();
+        return dom.number_integer(val);
+    }
+
+    bool number_unsigned(typename BasicJsonType::number_unsigned_t val)
+    {
+        value();
+        return dom.number_unsigned(val);
+    }
+
+    bool number_float(typename BasicJsonType::number_float_t val, const std::string& s)
+    {
+        value();
+        return dom.number_float(val, s);
+    }
+
+    bool string(std::string& val)
+    {
+        value();
+        return dom.string(val);
+    }
+
+    bool binary(typename BasicJsonType::binary_t& val)
+    {
+        value();
+        return dom.binary(val);
+    }
+
+    bool start_object(std::size_t elements)
+    {
+        value();
+        stack.push_back('o');
+        return dom.start_object(elements);
+    }
+
+    bool key(std::string& val)
+    {
+        if (stack.empty() || stack.back() != 'o')
+        {
+            well_formed = false;
+            return false;
+        }
+        stack.back() = 'v';
+        return dom.key(val);
+    }
+
+    bool end_object()
+    {
+        if (stack.empty() || stack.back() != 'o')
+        {
+            well_formed = false;
+            return false;
+        }
+        stack.pop_back();
+        return dom.end_object();
+    }
+
+    bool start_array(std::size_t elements)
+    {
+        value();
+        stack.push_back('a');
+        return dom.start_array(elements);
+    }
+
+    bool end_array()
+    {
+        if (stack.empty() || stack.back() != 'a')
+        {
+            well_formed = false;
+            return false;
+        }
+        stack.pop_back();
+        return dom.end_array();
+    }
+
+    bool parse_error(std::size_t /*unused*/, const std::string& /*unused*/, const json::exception& ex)
+    {
+        messages.emplace_back(ex.what());
+        // a limit, so that a reader that does not stop fails the test
+        // instead of making it hang
+        return ++errors < 100;
+    }
+
+    /// whether the events were balanced and every key was followed by a value
+    bool balanced() const
+    {
+        return well_formed && stack.empty();
+    }
+
+    /// builds the value
+    nlohmann::detail::json_sax_dom_parser<BasicJsonType> dom;
+    std::size_t errors = 0;
+    std::vector<std::string> messages {}; // NOLINT(readability-redundant-member-init)
+    std::vector<char> stack {}; // NOLINT(readability-redundant-member-init)
+    bool well_formed = true;
+
+  private:
+    void value()
+    {
+        if (!stack.empty())
+        {
+            if (stack.back() == 'v')
+            {
+                stack.back() = 'o';
+            }
+            else if (stack.back() == 'o')
+            {
+                well_formed = false;
+            }
+        }
+    }
+
+};
+
+using RecoveringParser = BasicRecoveringParser<json>;
+
+struct BinaryParseResult
+{
+    json value;
+    std::size_t errors;
+    std::vector<std::string> messages;
+    bool ok;
+    bool balanced;
+};
+
+BinaryParseResult parse_binary_recovering(const std::vector<std::uint8_t>& input, const json::input_format_t format)
+{
+    json j;
+    RecoveringParser sax(j);
+    const bool ok = json::sax_parse(input, &sax, format);
+    return {j, sax.errors, sax.messages, ok, sax.balanced()};
+}
+
+#if !defined(JSON_NOEXCEPTION)
+/// the message of the exception that reading @a input into a JSON value
+/// throws, or an empty string if reading succeeds
+std::string binary_error_message(const std::vector<std::uint8_t>& input, const json::input_format_t format)
+{
+    try
+    {
+        json _;
+        switch (format)
+        {
+            case json::input_format_t::cbor:
+                _ = json::from_cbor(input);
+                break;
+            case json::input_format_t::msgpack:
+                _ = json::from_msgpack(input);
+                break;
+            case json::input_format_t::ubjson:
+                _ = json::from_ubjson(input);
+                break;
+            case json::input_format_t::bjdata:
+                _ = json::from_bjdata(input);
+                break;
+            case json::input_format_t::bson:
+                _ = json::from_bson(input);
+                break;
+            case json::input_format_t::bon8:
+                _ = json::from_bon8(input);
+                break;
+            case json::input_format_t::json:
+            default:
+                break;
+        }
+    }
+    catch (const json::exception& e)
+    {
+        return e.what();
+    }
+    return "";
+}
+#endif
+
+/// a BSON element: its type, its name, and its value
+std::vector<std::uint8_t> bson_element(const std::uint8_t type, const std::string& name, const std::vector<std::uint8_t>& value)
+{
+    std::vector<std::uint8_t> result = {type};
+    result.insert(result.end(), name.begin(), name.end());
+    result.push_back(0x00);
+    result.insert(result.end(), value.begin(), value.end());
+    return result;
+}
+
+/// a BSON document of the given elements; @a size_offset is added to the
+/// size it declares
+std::vector<std::uint8_t> bson_document(const std::vector<std::vector<std::uint8_t>>& elements, const int size_offset = 0)
+{
+    std::vector<std::uint8_t> body;
+    for (const auto& element : elements)
+    {
+        body.insert(body.end(), element.begin(), element.end());
+    }
+    const auto size = static_cast<std::uint32_t>(static_cast<int>(body.size()) + 5 + size_offset);
+    std::vector<std::uint8_t> result = {static_cast<std::uint8_t>(size & 0xFFu), static_cast<std::uint8_t>((size >> 8u) & 0xFFu),
+                                        static_cast<std::uint8_t>((size >> 16u) & 0xFFu), static_cast<std::uint8_t>((size >> 24u) & 0xFFu)
+                                       };
+    result.insert(result.end(), body.begin(), body.end());
+    result.push_back(0x00);
+    return result;
+}
+
+/// a BSON int32 value
+std::vector<std::uint8_t> bson_int32(const std::int32_t value)
+{
+    const auto u = static_cast<std::uint32_t>(value);
+    return {static_cast<std::uint8_t>(u & 0xFFu), static_cast<std::uint8_t>((u >> 8u) & 0xFFu),
+            static_cast<std::uint8_t>((u >> 16u) & 0xFFu), static_cast<std::uint8_t>((u >> 24u) & 0xFFu)};
+}
+
+/// a BSON string value, whose length is @a length_offset off
+std::vector<std::uint8_t> bson_string(const std::string& value, const std::int32_t length_offset = 0)
+{
+    auto result = bson_int32(static_cast<std::int32_t>(value.size() + 1) + length_offset);
+    result.insert(result.end(), value.begin(), value.end());
+    result.push_back(0x00);
+    return result;
+}
+
+/// @a count bytes of value 0xAB
+std::vector<std::uint8_t> bytes(const std::size_t count)
+{
+    return std::vector<std::uint8_t>(count, 0xAB);
+}
+
+/// the bytes of @a parts, one after the other
+std::vector<std::uint8_t> concatenated(std::initializer_list<std::vector<std::uint8_t>> parts)
+{
+    std::vector<std::uint8_t> result;
+    for (const auto& part : parts)
+    {
+        result.insert(result.end(), part.begin(), part.end());
+    }
+    return result;
+}
+
+/// U+FFFD REPLACEMENT CHARACTER
+std::string replacement_character()
+{
+    return "\xEF\xBF\xBD";
+}
+}  // namespace
+
+TEST_CASE("regression test - #3989 SAX parse_error() returning true")
+{
+    SECTION("binary formats complete what was read before the input ends")
+    {
+        const json j = {{"a", {1, -2, {{"b", "c"}}, json::array()}}, {"d", {{"e", nullptr}, {"f", true}}}, {"g", 1.5}, {"h", json::binary({1, 2, 3})}};
+
+        const std::vector<std::pair<json::input_format_t, std::vector<std::uint8_t>>> encodings =
+        {
+            {json::input_format_t::cbor, json::to_cbor(j)},
+            {json::input_format_t::msgpack, json::to_msgpack(j)},
+            {json::input_format_t::ubjson, json::to_ubjson(j)},
+            {json::input_format_t::ubjson, json::to_ubjson(j, true, true)},
+            {json::input_format_t::bjdata, json::to_bjdata(j)},
+            {json::input_format_t::bjdata, json::to_bjdata(j, true, true)},
+            {json::input_format_t::bson, json::to_bson(j)},
+            {json::input_format_t::bon8, json::to_bon8(j)},
+        };
+
+        for (const auto& encoding : encodings)
+        {
+            const auto format = encoding.first;
+            const auto& bytes = encoding.second;
+            CAPTURE(format)
+
+            // every prefix is truncated input
+            for (std::size_t length = 0; length < bytes.size(); ++length)
+            {
+                CAPTURE(length)
+                const auto result = parse_binary_recovering(std::vector<std::uint8_t>(bytes.begin(), bytes.begin() + static_cast<std::ptrdiff_t>(length)), format);
+                CHECK(!result.ok);
+                CHECK(result.errors == 1);
+                CHECK(result.balanced);
+            }
+
+            // the complete input is read as usual (binary values do not
+            // round-trip through every format, so compare with a plain parse)
+            json expected;
+            nlohmann::detail::json_sax_dom_parser<json> dom(expected);
+            CHECK(json::sax_parse(bytes, &dom, format));
+            const auto complete = parse_binary_recovering(bytes, format);
+            CHECK(complete.ok);
+            CHECK(complete.errors == 0);
+            CHECK(complete.value == expected);
+
+            // a byte after the value
+            auto trailing_bytes = bytes;
+            trailing_bytes.push_back(0x01);
+            const auto trailing = parse_binary_recovering(trailing_bytes, format);
+            CHECK(!trailing.ok);
+            CHECK(trailing.errors == 1);
+            CHECK(trailing.value == expected);
+        }
+    }
+
+    SECTION("containers without an end")
+    {
+        // these made the readers loop, or read on, after the error
+        const auto cbor_array = parse_binary_recovering({0x9F}, json::input_format_t::cbor);
+        CHECK(cbor_array.errors == 1);
+        CHECK(cbor_array.value == json::array());
+
+        const auto cbor_map = parse_binary_recovering({0xBF, 0x61, 'a'}, json::input_format_t::cbor);
+        CHECK(cbor_map.errors == 1);
+        CHECK(cbor_map.value == json({{"a", nullptr}}));
+
+        const auto msgpack_array = parse_binary_recovering({0xDD, 0xFF, 0xFF, 0xFF, 0xFF}, json::input_format_t::msgpack);
+        CHECK(msgpack_array.errors == 1);
+        CHECK(msgpack_array.value == json::array());
+
+        const auto msgpack_map = parse_binary_recovering({0x81, 0xA1, 'a', 0x92, 0x01}, json::input_format_t::msgpack);
+        CHECK(msgpack_map.errors == 1);
+        CHECK(msgpack_map.value == json({{"a", {1}}}));
+    }
+
+    SECTION("BJData ndarray")
+    {
+        // a 2x3 int8 array with two of its six elements; the annotated array
+        // format opens an object and two arrays of its own
+        const auto result = parse_binary_recovering({'[', '$', 'i', '#', '[', '$', 'i', '#', 'i', 2, 2, 3, 1, 2}, json::input_format_t::bjdata);
+        CHECK(result.errors == 1);
+        CHECK(result.balanced);
+        CHECK(result.value == json({{"_ArrayType_", "int8"}, {"_ArraySize_", {2, 3}}, {"_ArrayData_", {1, 2}}}));
+    }
+
+    SECTION("binary formats repair items whose end is known")
+    {
+        struct Repair
+        {
+            json::input_format_t format;
+            std::vector<std::uint8_t> input;
+            json expected;
+            std::size_t errors;
+        };
+
+        const std::vector<Repair> repairs =
+        {
+            // CBOR: tags are ignored (here tag 1 and the self-describe tag 55799)
+            {json::input_format_t::cbor, {0x82, 0xC1, 0x05, 0xD9, 0xD9, 0xF7, 0x06}, {5, 6}, 2},
+            // CBOR: undefined and other simple values become null
+            {json::input_format_t::cbor, {0x84, 0xF7, 0xE0, 0xF8, 0x20, 0x01}, {nullptr, nullptr, nullptr, 1}, 3},
+            // CBOR: members whose key is not a string are skipped, whatever their key and value
+            {json::input_format_t::cbor, {0xA4, 0x01, 0x02, 0x82, 0x01, 0x02, 0xA1, 0x61, 'x', 0x9F, 0xFF, 0xC1, 0x01, 0x5F, 0x41, 0x00, 0xFF, 0x61, 'a', 0x03}, {{"a", 3}}, 3},
+            {json::input_format_t::cbor, {0xBF, 0xF5, 0xBF, 0x61, 'x', 0x7F, 0x61, 'y', 0xFF, 0xFF, 0x61, 'a', 0x03, 0xFF}, {{"a", 3}}, 1},
+            // MessagePack: members whose key is not a string are skipped
+            {json::input_format_t::msgpack, {0x84, 0x01, 0x02, 0x81, 0xA1, 'x', 0x01, 0x92, 0x01, 0x02, 0xD4, 0x01, 0x02, 0xC0, 0xA1, 'a', 0x04}, {{"a", 4}}, 3},
+            // UBJSON: a char that is not ASCII becomes U+FFFD
+            {json::input_format_t::ubjson, {'[', 'C', 0x80, 'C', 'A', ']'}, {replacement_character(), "A"}, 1},
+            // UBJSON: the longest beginning of a high-precision number is kept
+            {json::input_format_t::ubjson, {'[', 'H', 'i', 5, '1', '2', 'a', 'b', 'c', 'H', 'i', 2, '1', '.', 'H', 'i', 3, 'a', 'b', 'c', 'H', 'i', 3, '4', '.', '5', ']'}, {12, 1, nullptr, 4.5}, 3},
+            // BJData, too
+            {json::input_format_t::bjdata, {'[', 'C', 0xFF, 'H', 'i', 2, '-', '1', 'H', 'i', 2, '-', 'x', ']'}, {replacement_character(), -1, nullptr}, 2},
+            // a NUL ends a high-precision number, as it ends JSON text
+            {json::input_format_t::ubjson, {'[', 'H', 'i', 4, '1', '2', 0, '9', 'H', 'i', 2, 0, '1', 'i', 3, ']'}, {12, nullptr, 3}, 2},
+            // BON8: members whose key is not a string are skipped
+            {json::input_format_t::bon8, {0x89, 0x91, 0x92, 0xC9, 0x40, 0x82, 0x91, 0x92, 0x61, 0x93}, {{"a", 3}}, 2},
+            {json::input_format_t::bon8, {0x8B, 0x91, 0x85, 0x91, 0xFE, 0xFA, 0x8B, 'x', 0x91, 0xFE, 0x61, 0x93, 0xFE}, {{"a", 3}}, 2},
+            // BSON: elements of types the library does not read become null
+            {
+                json::input_format_t::bson, bson_document(
+                {
+                    bson_element(0x07, "_id", bytes(12)),                         // ObjectId
+                    bson_element(0x09, "date", bytes(8)),                         // UTC datetime
+                    bson_element(0x13, "decimal", bytes(16)),                     // 128-bit decimal
+                    bson_element(0x0B, "regex", {'a', '+', 0, 'i', 0}),           // regular expression
+                    bson_element(0x0D, "code", bson_string("f()")),               // JavaScript code
+                    bson_element(0x0E, "symbol", bson_string("s")),               // symbol
+                    bson_element(0x0C, "pointer", concatenated({bson_string("c"), bytes(12)})), // DBPointer
+                    bson_element(0x0F, "scope", concatenated({bson_int32(15), bson_string("g"), bson_document({})})), // code with scope
+                    bson_element(0x06, "undefined", {}),                          // undefined
+                    bson_element(0xFF, "min", {}),                                // min key
+                    bson_element(0x7F, "max", {}),                                // max key
+                    bson_element(0x10, "z", bson_int32(7)),
+                }),
+                {{"_id", nullptr}, {"date", nullptr}, {"decimal", nullptr}, {"regex", nullptr}, {"code", nullptr}, {"symbol", nullptr}, {"pointer", nullptr}, {"scope", nullptr}, {"undefined", nullptr}, {"min", nullptr}, {"max", nullptr}, {"z", 7}},
+                11
+            },
+            // BSON: an element of an unknown type becomes null, and the rest of its document is skipped
+            {
+                json::input_format_t::bson, bson_document(
+                {
+                    bson_element(0x03, "inner", bson_document({bson_element(0x10, "a", bson_int32(1)), bson_element(0x42, "x", bytes(3)), bson_element(0x10, "b", bson_int32(2))})),
+                    bson_element(0x04, "array", bson_document({bson_element(0x10, "0", bson_int32(1)), bson_element(0x42, "1", bytes(3))})),
+                    bson_element(0x10, "after", bson_int32(3)),
+                }),
+                {{"inner", {{"a", 1}, {"x", nullptr}}}, {"array", {1, nullptr}}, {"after", 3}},
+                2
+            },
+            // BSON: so does a string or byte array whose length cannot be right
+            {
+                json::input_format_t::bson, bson_document(
+                {
+                    bson_element(0x03, "inner", bson_document({bson_element(0x02, "s", bson_string("abc", -10)), bson_element(0x10, "b", bson_int32(2))})),
+                    bson_element(0x03, "bin", bson_document({bson_element(0x05, "b", concatenated({bson_int32(-1), bytes(1)})), bson_element(0x10, "b", bson_int32(2))})),
+                    bson_element(0x10, "after", bson_int32(3)),
+                }),
+                {{"inner", {{"s", nullptr}}}, {"bin", {{"b", nullptr}}}, {"after", 3}},
+                2
+            },
+            // BSON: a string without its terminator, and a document whose size does not match, are kept
+            {
+                json::input_format_t::bson, bson_document(
+                {
+                    bson_element(0x02, "s", {2, 0, 0, 0, 'a', 'X'}),
+                    bson_element(0x03, "inner", bson_document({bson_element(0x10, "a", bson_int32(1))}, 1)),
+                }),
+                {{"s", "a"}, {"inner", {{"a", 1}}}},
+                2
+            },
+        };
+
+        for (const auto& repair : repairs)
+        {
+            CAPTURE(repair.format)
+            CAPTURE(repair.input)
+            const auto result = parse_binary_recovering(repair.input, repair.format);
+            CHECK(!result.ok);
+            CHECK(result.balanced);
+            CHECK(result.errors == repair.errors);
+            CHECK(result.value == repair.expected);
+            REQUIRE(!result.messages.empty());
+#if !defined(JSON_NOEXCEPTION)
+            // the first error is the one reported without recovering; under
+            // JSON_NOEXCEPTION, reading without recovering aborts instead of
+            // throwing, so there is no message to compare with
+            CHECK(result.messages.front() == binary_error_message(repair.input, repair.format));
+#endif
+        }
+    }
+
+    SECTION("binary formats repair numbers that are out of range")
+    {
+        // CBOR: a double too large for a float number_float_t
+        float_json cbor;
+        BasicRecoveringParser<float_json> sax(cbor);
+        const std::vector<std::uint8_t> cbor_input = {0x82, 0xFB, 0x7E, 0x37, 0xE4, 0x3C, 0x88, 0x00, 0x75, 0x9C, 0x01}; // [1e300, 1]
+        CHECK(!float_json::sax_parse(cbor_input, &sax, float_json::input_format_t::cbor));
+        CHECK(sax.errors == 1);
+        CHECK(sax.messages.front() == "[json.exception.out_of_range.406] syntax error while parsing CBOR value: number overflow");
+        REQUIRE(cbor.size() == 2);
+        CHECK(std::isinf(cbor[0].get<float>()));
+        CHECK(cbor[1] == 1);
+
+        // UBJSON: a high-precision number too large for number_float_t
+        const auto ubjson = parse_binary_recovering({'H', 'i', 5, '1', 'e', '9', '9', '9'}, json::input_format_t::ubjson);
+        CHECK(ubjson.errors == 1);
+        CHECK(ubjson.value.is_number_float());
+        CHECK(std::isinf(ubjson.value.get<double>()));
+    }
+
+    SECTION("binary formats stop where the end of an item is not known")
+    {
+        // a byte that begins no item
+        const auto cbor = parse_binary_recovering({0x82, 0x01, 0x1C, 0x02}, json::input_format_t::cbor);
+        CHECK(cbor.errors == 1);
+        CHECK(cbor.value == json({1}));
+
+        // a key that is no item: the unused MessagePack byte, a CBOR break
+        // in a map of known size, and the end of a BON8 container
+        const auto msgpack = parse_binary_recovering({0x82, 0xA1, 'a', 0x01, 0xC1, 0x02}, json::input_format_t::msgpack);
+        CHECK(msgpack.errors == 1);
+        CHECK(msgpack.value == json({{"a", 1}}));
+        const auto cbor_break = parse_binary_recovering({0xA2, 0x61, 'a', 0x01, 0xFF, 0x02}, json::input_format_t::cbor);
+        CHECK(cbor_break.errors == 1);
+        CHECK(cbor_break.value == json({{"a", 1}}));
+        const auto bon8 = parse_binary_recovering({0x88, 0x61, 0x91, 0xFE}, json::input_format_t::bon8);
+        CHECK(bon8.errors == 1);
+        CHECK(bon8.value == json({{"a", 1}}));
+
+        // an indefinite-length string inside an indefinite-length string
+        const auto nested = parse_binary_recovering({0x82, 0x01, 0x7F, 0x7F, 0x61, 'a', 0xFF, 0xFF}, json::input_format_t::cbor);
+        CHECK(nested.errors == 1);
+        CHECK(nested.value == json({1}));
+
+        // a BJData ndarray whose element type has no name: the object that
+        // holds the ndarray was already begun
+        const auto ndarray = parse_binary_recovering({'[', '$', 0x01, '#', '[', '$', 'i', '#', 'i', 2, 2, 3}, json::input_format_t::bjdata);
+        CHECK(ndarray.errors == 1);
+        CHECK(ndarray.balanced);
+        CHECK(ndarray.value == json::object());
+
+        // a skipped member that the input ends in
+        const auto truncated = parse_binary_recovering({0xA2, 0x01, 0x82, 0x01}, json::input_format_t::cbor);
+        CHECK(truncated.errors == 2);
+        CHECK(truncated.balanced);
+        CHECK(truncated.value == json::object());
+
+        // a BSON element of an unknown type in a document whose size cannot be right
+        const auto bson = parse_binary_recovering(bson_document({bson_element(0x10, "a", bson_int32(1)), bson_element(0x42, "x", bytes(3))}, -10), json::input_format_t::bson);
+        CHECK(bson.errors == 1);
+        CHECK(bson.value == json({{"a", 1}, {"x", nullptr}}));
+    }
+
+    SECTION("changed bytes in binary input")
+    {
+        const json j = {{"a", {1, -2, {{"b", "c"}}, json::array()}}, {"d", {{"e", nullptr}, {"f", true}}}, {"g", 1.5}, {"h", json::binary({1, 2, 3})}, {"i", "\xC3\xA4"}};
+
+        const std::vector<std::pair<json::input_format_t, std::vector<std::uint8_t>>> encodings =
+        {
+            {json::input_format_t::cbor, json::to_cbor(j)},
+            {json::input_format_t::msgpack, json::to_msgpack(j)},
+            {json::input_format_t::ubjson, json::to_ubjson(j)},
+            {json::input_format_t::ubjson, json::to_ubjson(j, true, true)},
+            {json::input_format_t::bjdata, json::to_bjdata(j)},
+            {json::input_format_t::bjdata, json::to_bjdata(j, true, true)},
+            {json::input_format_t::bson, json::to_bson(j)},
+            {json::input_format_t::bon8, json::to_bon8(j)},
+        };
+        const std::vector<std::uint8_t> replacements = {0x00, 0x01, 0x7F, 0x80, 0xC1, 0xD9, 0xE0, 0xF7, 0xFE, 0xFF};
+
+        for (const auto& encoding : encodings)
+        {
+            const auto format = encoding.first;
+            const auto& original = encoding.second;
+            CAPTURE(format)
+
+            std::vector<std::vector<std::uint8_t>> inputs;
+            for (std::size_t position = 0; position < original.size(); ++position)
+            {
+                for (const auto replacement : replacements)
+                {
+                    auto changed = original;
+                    changed[position] = replacement;
+                    inputs.push_back(changed);
+                }
+                auto removed = original;
+                removed.erase(removed.begin() + static_cast<std::ptrdiff_t>(position));
+                inputs.push_back(removed);
+            }
+
+            for (const auto& input : inputs)
+            {
+                CAPTURE(input)
+                const auto result = parse_binary_recovering(input, format);
+                CHECK(result.balanced);
+                CHECK(result.errors <= input.size() + 1);
+#if !defined(JSON_NOEXCEPTION)
+                // an error is reported exactly if reading into a JSON value
+                // fails, and the first one is the same (under JSON_NOEXCEPTION,
+                // that reading aborts instead of throwing)
+                const auto message = binary_error_message(input, format);
+                CHECK(result.ok == message.empty());
+                if (!result.ok && result.errors < 100)
+                {
+                    CHECK(result.messages.front() == message);
+                }
+#endif
+            }
+        }
+    }
+
+    SECTION("JSON text")
+    {
+        // the parser stopped, but reported success
+        json j;
+        RecoveringParser sax(j);
+        CHECK(!json::sax_parse("[1,2,3,]", &sax));
+        CHECK(sax.errors == 1);
+        CHECK(j == json({1, 2, 3}));
+    }
+
+    SECTION("the SAX parsers of the library stop")
+    {
+        json _;
+        CHECK(json::from_cbor(std::vector<std::uint8_t> {0x9F}, true, false).is_discarded());
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<std::uint8_t> {0x9F}), "[json.exception.parse_error.110] parse error at byte 2: syntax error while parsing CBOR value: unexpected end of input", json::parse_error&);
+        CHECK(json::parse("[1,2,3,]", nullptr, false).is_discarded());
+        CHECK(!json::accept("[1,2,3,]"));
+    }
+}
+
+
+TEST_CASE("issue #5676 - SAX parsing of CBOR tags")
+{
+    const json expected = json::binary({1, 2, 3}, 42);
+    const auto cbor = json::to_cbor(expected);
+
+    nlohmann::detail::json_sax_acceptor<json> acceptor;
+    CHECK_FALSE(json::sax_parse(cbor, &acceptor, json::input_format_t::cbor));
+    CHECK_FALSE(json::sax_parse(cbor, &acceptor, json::input_format_t::cbor,
+                                true, false, false, json::cbor_tag_handler_t::error));
+
+    CHECK(json::sax_parse(cbor, &acceptor, json::input_format_t::cbor,
+                          true, false, false, json::cbor_tag_handler_t::ignore));
+
+    json parsed;
+    nlohmann::detail::json_sax_dom_parser<json, nlohmann::detail::string_input_adapter_type> sax(parsed);
+    CHECK(json::sax_parse(cbor, &sax, json::input_format_t::cbor,
+                          true, false, false, json::cbor_tag_handler_t::store));
+    CHECK(parsed == expected);
+
+    json iterator_parsed;
+    nlohmann::detail::json_sax_dom_parser<json, nlohmann::detail::string_input_adapter_type> iterator_sax(iterator_parsed);
+    CHECK(json::sax_parse(cbor.begin(), cbor.end(), &iterator_sax, json::input_format_t::cbor,
+                          true, false, false, json::cbor_tag_handler_t::store));
+    CHECK(iterator_parsed == expected);
+
+#ifndef JSON_TEST_DEPRECATED_FUNCTIONS_DELETED
+    json span_parsed;
+    nlohmann::detail::json_sax_dom_parser<json, nlohmann::detail::string_input_adapter_type> span_sax(span_parsed);
+    CHECK(json::sax_parse(nlohmann::detail::span_input_adapter(cbor.data(), cbor.size()), &span_sax,
+                          json::input_format_t::cbor, true, false, false, json::cbor_tag_handler_t::store));
+    CHECK(span_parsed == expected);
+#endif
+
+    const std::string text = "null";
+    CHECK(json::sax_parse(text, &acceptor, json::input_format_t::json,
+                          true, false, false, json::cbor_tag_handler_t::store));
+    CHECK(json::sax_parse(text.begin(), text.end(), &acceptor, json::input_format_t::json,
+                          true, false, false, json::cbor_tag_handler_t::store));
+#ifndef JSON_TEST_DEPRECATED_FUNCTIONS_DELETED
+    CHECK(json::sax_parse(nlohmann::detail::span_input_adapter(text.data(), text.size()), &acceptor,
+                          json::input_format_t::json, true, false, false, json::cbor_tag_handler_t::store));
+#endif
+}
+
+
+TEST_CASE("MessagePack SAX aborts")
+{
+    SECTION("start_array(len)")
+    {
+        std::vector<uint8_t> const v = {0x93, 0x01, 0x02, 0x03};
+        SaxCountdown scp(0);
+        CHECK(!json::sax_parse(v, &scp, json::input_format_t::msgpack));
+    }
+
+    SECTION("start_object(len)")
+    {
+        std::vector<uint8_t> const v = {0x81, 0xa3, 0x66, 0x6F, 0x6F, 0xc2};
+        SaxCountdown scp(0);
+        CHECK(!json::sax_parse(v, &scp, json::input_format_t::msgpack));
+    }
+
+    SECTION("key()")
+    {
+        std::vector<uint8_t> const v = {0x81, 0xa3, 0x66, 0x6F, 0x6F, 0xc2};
+        SaxCountdown scp(1);
+        CHECK(!json::sax_parse(v, &scp, json::input_format_t::msgpack));
+    }
+}
+
+TEST_CASE("issue #5405 - a user-defined SAX consumer is unaffected by the internal DOM reserve optimization")
+{
+    // the reserve() call is local to json_sax_dom_parser / json_sax_dom_callback_parser;
+    // a custom SAX consumer that does not touch a DOM array sees identical events
+    json j = json::array();
+    for (int i = 0; i < 100; ++i)
+    {
+        j.push_back(i);
+    }
+    const auto packed = json::to_msgpack(j);
+
+    SaxCountdown scp(1000000); // large enough to never trigger an abort
+    CHECK(json::sax_parse(packed, &scp, json::input_format_t::msgpack));
+}
+
+TEST_CASE("MessagePack nesting does not consume the call stack - SAX interface")
+{
+    // see the test case of the same name in unit-msgpack.cpp (#5104)
+    std::vector<uint8_t> input(300000, 0x91);
+    input.push_back(0x01); // innermost value
+
+    SaxCountdown accept_all(600001);
+    CHECK(json::sax_parse(input, &accept_all, json::input_format_t::msgpack));
+}
+
+TEST_CASE("MessagePack SAX parsing stops at every event")
+{
+    // Containers are opened and closed by the loop that reads them; a SAX
+    // handler that rejects any event - including the end of a nested
+    // container - must stop the parse right there.
+    const auto count_events = [](const std::vector<std::uint8_t>& input)
+    {
+        int events = 0;
+        while (true)
+        {
+            SaxCountdown scp(events);
+            if (json::sax_parse(input, &scp, json::input_format_t::msgpack))
+            {
+                return events;
+            }
+            ++events;
+            REQUIRE(events < 1000);
+        }
+    };
+
+    // 20 events: every container kind closes inside another one
+    const json j = json::parse(R"({"a": [1, {"b": []}], "c": {"d": [[2]]}})");
+    CHECK(count_events(json::to_msgpack(j)) == 20);
+}
+
+DOCTEST_CLANG_SUPPRESS_WARNING_POP

tests/src/unit-serialization.cpp

@@ -367,8 +367,7 @@ TEST_CASE("dump for basic_json with long double number_float_t")
     // serializer::dump_float(x, std::false_type). That branch must use the
     // "%.*Lg" format specifier; using "%.*g" with a long double argument is
     // undefined behavior and corrupts the output.
-    using long_double_json = nlohmann::basic_json<std::map, std::vector, std::string,
-          bool, std::int64_t, std::uint64_t, long double>;
+    using long_double_json = nlohmann::json::with_float_t<long double>;
 
     SECTION("round-trip dump/parse")
     {