Address review comments on nested indefinite-length CBOR strings

Rename is_chunk to inside_indefinite, update the stale test section
names, and use lowercase comments like the surrounding code.

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

.github/workflows/macos.yml

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

.github/workflows/ubuntu.yml

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

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

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

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

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

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

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

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

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

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

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

docs/mkdocs/docs/community/quality_assurance.md

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

docs/mkdocs/docs/features/binary_formats/cbor.md

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

docs/mkdocs/docs/home/exceptions.md

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

include/nlohmann/detail/input/binary_reader.hpp

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

include/nlohmann/detail/output/binary_writer.hpp

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

include/nlohmann/json.hpp

@@ -612,100 +612,210 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         /// constructor for rvalue binary arrays (internal type)
         json_value(binary_t&& value) : binary(create<binary_t>(std::move(value))) {}
 
-        void destroy(value_t t)
+private:
+        // raw, allocation-free transfer of m_data from src to dst: no
+        // set_parents()/assert_invariant() (the former is O(#children) per
+        // call under JSON_DIAGNOSTICS, which would make the walk below
+        // quadratic); dst takes ownership, src is left as value_t::null.
+        static void take(basic_json& dst, basic_json& src) noexcept
+        {
+            dst.m_data.m_type = src.m_data.m_type;
+            dst.m_data.m_value = src.m_data.m_value;
+            src.m_data.m_type = value_t::null;
+        }
+
+        // true if v is not an array/object, or is an already-empty one
+        static bool has_no_children(const basic_json& v) noexcept
+        {
+            switch (v.m_data.m_type)
+            {
+                case value_t::array:
+                    return v.m_data.m_value.array->empty();
+                case value_t::object:
+                    return v.m_data.m_value.object->empty();
+                default:
+                    return true;
+            }
+        }
+
+        static basic_json& last_child(basic_json& v)
+        {
+            if (v.m_data.m_type == value_t::array)
+            {
+                return v.m_data.m_value.array->back();
+            }
+            JSON_ASSERT(v.m_data.m_type == value_t::object);
+            return v.m_data.m_value.object->rbegin()->second;
+        }
+
+        // removes the last child of a non-empty array/object v; this never
+        // allocates, and since it is only ever called when that child is a
+        // scalar or an already-empty array/object, destroying it never
+        // recurses more than one level deep (see destroy() below)
+        static void pop_last_child(basic_json& v)
+        {
+            if (v.m_data.m_type == value_t::array)
+            {
+                v.m_data.m_value.array->pop_back();
+            }
+            else
+            {
+                JSON_ASSERT(v.m_data.m_type == value_t::object);
+                // erase() needs a forward iterator, so std::prev(end()) is
+                // used here rather than rbegin() (see last_child() above)
+                v.m_data.m_value.object->erase(std::prev(v.m_data.m_value.object->end()));
+            }
+        }
+
+        // deallocates the (already empty) array/object held by v; this is
+        // the same allocator-based free the old recursive implementation
+        // used, just factored out so every level of the walk in destroy()
+        // can share it
+        static void free_container(basic_json& v) noexcept
+        {
+            if (v.m_data.m_type == value_t::array)
+            {
+                JSON_ASSERT(v.m_data.m_value.array->empty());
+                AllocatorType<array_t> alloc;
+                std::allocator_traits<decltype(alloc)>::destroy(alloc, v.m_data.m_value.array);
+                std::allocator_traits<decltype(alloc)>::deallocate(alloc, v.m_data.m_value.array, 1);
+            }
+            else
+            {
+                JSON_ASSERT(v.m_data.m_type == value_t::object);
+                JSON_ASSERT(v.m_data.m_value.object->empty());
+                AllocatorType<object_t> alloc;
+                std::allocator_traits<decltype(alloc)>::destroy(alloc, v.m_data.m_value.object);
+                std::allocator_traits<decltype(alloc)>::deallocate(alloc, v.m_data.m_value.object, 1);
+            }
+            v.m_data.m_type = value_t::null; // avoid a double free if v is later destructed
+        }
+
+public:
+        void destroy_string() noexcept
+        {
+            if (string == nullptr)
+            {
+                // not initialized (e.g., due to exception in the ctor)
+                return;
+            }
+            AllocatorType<string_t> alloc;
+            std::allocator_traits<decltype(alloc)>::destroy(alloc, string);
+            std::allocator_traits<decltype(alloc)>::deallocate(alloc, string, 1);
+        }
+
+        void destroy_binary() noexcept
+        {
+            if (binary == nullptr)
+            {
+                // not initialized (e.g., due to exception in the ctor)
+                return;
+            }
+            AllocatorType<binary_t> alloc;
+            std::allocator_traits<decltype(alloc)>::destroy(alloc, binary);
+            std::allocator_traits<decltype(alloc)>::deallocate(alloc, binary, 1);
+        }
+
+        // t must be value_t::array or value_t::object
+        void destroy_container(value_t t) noexcept
         {
             if (
                 (t == value_t::object && object == nullptr) ||
-                (t == value_t::array && array == nullptr) ||
-                (t == value_t::string && string == nullptr) ||
-                (t == value_t::binary && binary == nullptr)
+                (t == value_t::array && array == nullptr)
             )
             {
                 // not initialized (e.g., due to exception in the ctor)
                 return;
             }
-            if (t == value_t::array || t == value_t::object)
+
+            // Destroy the tree without recursing per nesting level and
+            // without any heap allocation: a heap-allocated flattening
+            // stack (the previous implementation) can itself throw
+            // bad_alloc, which would escape this noexcept destructor and
+            // terminate the program (#5135).
+            //
+            // Instead, walk down the "last child" chain, reversing links
+            // as we go: cur is the container currently being emptied,
+            // and prev is its parent (value_t::null when there is none).
+            // Each parent's last child slot doubles as storage for that
+            // parent's own parent link while we are below it, so no
+            // extra memory is needed. We only ever remove a child once
+            // it is a scalar or an empty array/object, which neither
+            // allocates nor recurses more than one level deep.
+            //
+            // This json_value is not itself a basic_json, so the
+            // top-level container is first moved into a local stand-in
+            // ("cur"); a default-constructed basic_json has a null
+            // pointer in its m_value (see data::m_value's initializer),
+            // so swapping it with *this leaves this union's own pointer
+            // null, and it is never looked at or freed a second time.
+            basic_json cur;
+            cur.m_data.m_type = t;
+            using std::swap;
+            swap(cur.m_data.m_value, *this);
+
+            basic_json prev; // value_t::null: no parent
+
+            while (true)
             {
-                // flatten the current json_value to a heap-allocated stack
-                std::vector<basic_json> stack;
-
-                // move the top-level items to stack
-                if (t == value_t::array)
+                if (has_no_children(cur))
                 {
-                    stack.reserve(array->size());
-                    std::move(array->begin(), array->end(), std::back_inserter(stack));
-                }
-                else
-                {
-                    stack.reserve(object->size());
-                    for (auto&& it : *object)
+                    if (prev.m_data.m_type == value_t::null)
                     {
-                        stack.push_back(std::move(it.second));
-                    }
-                }
-
-                while (!stack.empty())
-                {
-                    // move the last item to a local variable to be processed
-                    basic_json current_item(std::move(stack.back()));
-                    stack.pop_back();
-
-                    // if current_item is array/object, move
-                    // its children to the stack to be processed later
-                    if (current_item.is_array())
-                    {
-                        std::move(current_item.m_data.m_value.array->begin(), current_item.m_data.m_value.array->end(), std::back_inserter(stack));
-
-                        current_item.m_data.m_value.array->clear();
-                    }
-                    else if (current_item.is_object())
-                    {
-                        for (auto&& it : *current_item.m_data.m_value.object)
-                        {
-                            stack.push_back(std::move(it.second));
-                        }
-
-                        current_item.m_data.m_value.object->clear();
+                        free_container(cur);
+                        return; // back at the top with nothing left to do
                     }
 
-                    // it's now safe that current_item gets destructed
-                    // since it doesn't have any children
+                    // ascend: detach the grandparent link from prev's
+                    // last slot, drop that (now null) slot, free cur
+                    // (it is empty), then move up one level
+                    basic_json gp;
+                    take(gp, last_child(prev));
+                    pop_last_child(prev);
+
+                    free_container(cur);
+
+                    take(cur, prev);
+                    take(prev, gp);
+                    continue;
                 }
+
+                basic_json& cur_last_ref = last_child(cur);
+
+                if (has_no_children(cur_last_ref))
+                {
+                    // scalar, or already-empty array/object
+                    pop_last_child(cur);
+                    continue;
+                }
+
+                // descend into the non-empty last child, reversing the
+                // link: its slot takes over prev, and the child becomes
+                // the new cur
+                basic_json tmp;
+                take(tmp, cur_last_ref);
+                take(cur_last_ref, prev);
+                take(prev, cur);
+                take(cur, tmp);
             }
+        }
 
+        void destroy(value_t t)
+        {
             switch (t)
             {
-                case value_t::object:
-                {
-                    AllocatorType<object_t> alloc;
-                    std::allocator_traits<decltype(alloc)>::destroy(alloc, object);
-                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, object, 1);
-                    break;
-                }
-
-                case value_t::array:
-                {
-                    AllocatorType<array_t> alloc;
-                    std::allocator_traits<decltype(alloc)>::destroy(alloc, array);
-                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, array, 1);
-                    break;
-                }
-
                 case value_t::string:
-                {
-                    AllocatorType<string_t> alloc;
-                    std::allocator_traits<decltype(alloc)>::destroy(alloc, string);
-                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, string, 1);
+                    destroy_string();
                     break;
-                }
 
                 case value_t::binary:
-                {
-                    AllocatorType<binary_t> alloc;
-                    std::allocator_traits<decltype(alloc)>::destroy(alloc, binary);
-                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, binary, 1);
+                    destroy_binary();
+                    break;
+
+                case value_t::object:
+                case value_t::array:
+                    destroy_container(t);
                     break;
-                }
 
                 case value_t::null:
                 case value_t::boolean:
@@ -714,9 +824,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                 case value_t::number_float:
                 case value_t::discarded:
                 default:
-                {
                     break;
-                }
             }
         }
     };

single_include/nlohmann/json.hpp

@@ -14789,6 +14789,20 @@ class binary_reader
         }
     }
 
+    /*!
+    @brief reports a nested indefinite-length CBOR string or byte array
+    @param[in] type_name  name of the rejected string type
+    @param[in] context  parsing context for the error message
+    @return whether the SAX consumer accepts the parse error
+    */
+    bool cbor_indefinite_string_error(const char* type_name, const char* context)
+    {
+        auto last_token = get_token_string();
+        return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
+                                exception_message(concat("indefinite-length ", type_name,
+                                        " is not allowed inside indefinite-length ", type_name, "; last byte: 0x", last_token), context), nullptr));
+    }
+
     /*!
     @brief reads a definite-length CBOR string
 
@@ -14798,12 +14812,13 @@ class binary_reader
     into the same string.
 
     @param[out] result  string the bytes are appended to
+    @param[in] inside_indefinite  whether the bytes belong to an indefinite-length string
 
     @return whether string creation completed
 
     @pre @a current is not EOF
     */
-    bool get_cbor_string_chunk(string_t& result)
+    bool get_cbor_string_chunk(string_t& result, const bool inside_indefinite)
     {
         switch (current)
         {
@@ -14864,7 +14879,7 @@ class binary_reader
             {
                 auto last_token = get_token_string();
                 return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                        exception_message(concat("expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x", last_token), "string"), nullptr));
+                                        exception_message(concat("expected length specification (0x60-0x7B)", inside_indefinite ? "" : " or indefinite string type (0x7F)", "; last byte: 0x", last_token), "string"), nullptr));
             }
         }
     }
@@ -14882,13 +14897,9 @@ class binary_reader
     */
     bool get_cbor_string(string_t& result, const char* context = "string")
     {
-        // number of indefinite-length strings that have been opened and not
-        // closed yet. RFC 8949, Section 3.2.3 does not permit nesting them,
-        // but this reader has always accepted it, so the open levels are
-        // counted instead of recursed through, which overflowed the stack for
-        // an input of repeated 0x7F bytes (see #5104). Every chunk is appended
-        // to the same result, so no per-level state is needed.
-        std::size_t open = 0;
+        // read chunks iteratively, but reject a second indefinite-length
+        // level as required by RFC 8949, Section 3.2.3
+        bool indefinite = false;
 
         while (true)
         {
@@ -14899,29 +14910,28 @@ class binary_reader
 
             if (current == 0x7F) // UTF-8 string (indefinite length)
             {
-                ++open;
-                get();
-                continue;
-            }
-
-            // a break marker closes the innermost indefinite-length string;
-            // outside of one it is not a string and falls through to the error
-            if (open != 0 && current == 0xFF)
-            {
-                if (--open == 0)
+                if (JSON_HEDLEY_UNLIKELY(indefinite))
                 {
-                    return check_string_utf8(result, context);
+                    return cbor_indefinite_string_error("string", "string");
                 }
+                indefinite = true;
                 get();
                 continue;
             }
 
-            if (JSON_HEDLEY_UNLIKELY(!get_cbor_string_chunk(result)))
+            // a break marker closes the indefinite-length string; outside
+            // of one it falls through to the error below
+            if (indefinite && current == 0xFF)
+            {
+                return check_string_utf8(result, context);
+            }
+
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_string_chunk(result, indefinite)))
             {
                 return false;
             }
 
-            if (open == 0)
+            if (!indefinite)
             {
                 return check_string_utf8(result, context);
             }
@@ -15013,12 +15023,13 @@ class binary_reader
     read into the same byte array.
 
     @param[out] result  byte array the bytes are appended to
+    @param[in] inside_indefinite  whether the bytes belong to an indefinite-length string
 
     @return whether byte array creation completed
 
     @pre @a current is not EOF
     */
-    bool get_cbor_binary_chunk(binary_t& result)
+    bool get_cbor_binary_chunk(binary_t& result, const bool inside_indefinite)
     {
         switch (current)
         {
@@ -15083,7 +15094,7 @@ class binary_reader
             {
                 auto last_token = get_token_string();
                 return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                        exception_message(concat("expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x", last_token), "binary"), nullptr));
+                                        exception_message(concat("expected length specification (0x40-0x5B)", inside_indefinite ? "" : " or indefinite binary array type (0x5F)", "; last byte: 0x", last_token), "binary"), nullptr));
             }
         }
     }
@@ -15101,9 +15112,9 @@ class binary_reader
     */
     bool get_cbor_binary(binary_t& result)
     {
-        // the open indefinite-length byte arrays are counted rather than
-        // recursed through, for the reason given in @ref get_cbor_string
-        std::size_t open = 0;
+        // read chunks iteratively, but reject a second indefinite-length
+        // level as required by RFC 8949, Section 3.2.3
+        bool indefinite = false;
 
         while (true)
         {
@@ -15114,29 +15125,28 @@ class binary_reader
 
             if (current == 0x5F) // Binary data (indefinite length)
             {
-                ++open;
-                get();
-                continue;
-            }
-
-            // a break marker closes the innermost indefinite-length byte
-            // array; outside of one it falls through to the error below
-            if (open != 0 && current == 0xFF)
-            {
-                if (--open == 0)
+                if (JSON_HEDLEY_UNLIKELY(indefinite))
                 {
-                    return true;
+                    return cbor_indefinite_string_error("binary array", "binary");
                 }
+                indefinite = true;
                 get();
                 continue;
             }
 
-            if (JSON_HEDLEY_UNLIKELY(!get_cbor_binary_chunk(result)))
+            // a break marker closes the indefinite-length string; outside
+            // of one it falls through to the error below
+            if (indefinite && current == 0xFF)
+            {
+                return true;
+            }
+
+            if (JSON_HEDLEY_UNLIKELY(!get_cbor_binary_chunk(result, indefinite)))
             {
                 return false;
             }
 
-            if (open == 0)
+            if (!indefinite)
             {
                 return true;
             }
@@ -21587,6 +21597,7 @@ class binary_writer
     @throw type_error.316 if a string value or an object key is not valid
            UTF-8
     @throw type_error.317 if @a j is not an object
+    @throw type_error.321 if a value nested in @a j is discarded
     */
     void write_bson(const BasicJsonType& j)
     {
@@ -21618,6 +21629,7 @@ class binary_writer
     @param[in] j  JSON value to serialize
     @throw type_error.316 if a string value or an object key is not valid
            UTF-8
+    @throw type_error.321 if @a j or a value nested in it is discarded
     */
     void write_cbor(const BasicJsonType& j)
     {
@@ -21782,7 +21794,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                break;
+                throw_on_discarded(j, "CBOR");
         }
     }
 
@@ -21842,6 +21854,7 @@ class binary_writer
 
     /*!
     @param[in] j  JSON value to serialize
+    @throw type_error.321 if @a j or a value nested in it is discarded
     */
     void write_msgpack(const BasicJsonType& j)
     {
@@ -22115,7 +22128,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                break;
+                throw_on_discarded(j, "MessagePack");
         }
     }
 
@@ -22128,6 +22141,7 @@ class binary_writer
     @param[in] bjdata_version  which BJData version to use, default is draft2
     @throw type_error.316 if a string value or an object key is not valid
            UTF-8
+    @throw type_error.321 if @a j or a value nested in it is discarded
     */
     void write_ubjson(const BasicJsonType& j, const bool use_count,
                       const bool use_type, const bool add_prefix = true,
@@ -22361,7 +22375,7 @@ class binary_writer
 
             case value_t::discarded:
             default:
-                break;
+                throw_on_discarded(j, use_bjdata ? "BJData" : "UBJSON");
         }
     }
 
@@ -22381,6 +22395,15 @@ class binary_writer
     }
 
   private:
+    /*!
+    @brief throws because @a j is discarded and cannot be serialized
+    @throw type_error.321 always
+    */
+    JSON_HEDLEY_NO_RETURN static void throw_on_discarded(const BasicJsonType& j, const char* format_name)
+    {
+        JSON_THROW(type_error::create(321, concat("cannot serialize discarded value to ", format_name), &j));
+    }
+
     //////////
     // BSON //
     //////////
@@ -22632,6 +22655,7 @@ class binary_writer
            into a byte, before anything is written
     @throw type_error.316 if @a j is a string that is not valid UTF-8, before
            anything is written
+    @throw type_error.321 if @a j is discarded
     */
     std::size_t calc_bson_value_size(const BasicJsonType& j)
     {
@@ -22658,10 +22682,12 @@ class binary_writer
             case value_t::null:
                 return 0ul;
 
+            case value_t::discarded:
+                throw_on_discarded(j, "BSON");
+
             // LCOV_EXCL_START
             case value_t::object:
             case value_t::array:
-            case value_t::discarded:
             default:
                 JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert)
                 return 0ul;
@@ -22698,10 +22724,12 @@ class binary_writer
             case value_t::null:
                 return write_bson_null(name);
 
+            case value_t::discarded:
+                throw_on_discarded(j, "BSON");
+
             // LCOV_EXCL_START
             case value_t::object:
             case value_t::array:
-            case value_t::discarded:
             default:
                 JSON_ASSERT(false); // NOLINT(cert-dcl03-c,hicpp-static-assert,misc-static-assert)
                 return;
@@ -22768,6 +22796,8 @@ class binary_writer
            byte, before anything is written
     @throw type_error.316 if a string value or a key is not valid UTF-8,
            before anything is written
+    @throw type_error.321 if a value nested in @a document is discarded,
+           before anything is written
     */
     std::size_t calc_bson_sizes(const BasicJsonType& document, std::vector<std::size_t>& nested_sizes)
     {
@@ -27792,100 +27822,210 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         /// constructor for rvalue binary arrays (internal type)
         json_value(binary_t&& value) : binary(create<binary_t>(std::move(value))) {}
 
-        void destroy(value_t t)
+private:
+        // raw, allocation-free transfer of m_data from src to dst: no
+        // set_parents()/assert_invariant() (the former is O(#children) per
+        // call under JSON_DIAGNOSTICS, which would make the walk below
+        // quadratic); dst takes ownership, src is left as value_t::null.
+        static void take(basic_json& dst, basic_json& src) noexcept
+        {
+            dst.m_data.m_type = src.m_data.m_type;
+            dst.m_data.m_value = src.m_data.m_value;
+            src.m_data.m_type = value_t::null;
+        }
+
+        // true if v is not an array/object, or is an already-empty one
+        static bool has_no_children(const basic_json& v) noexcept
+        {
+            switch (v.m_data.m_type)
+            {
+                case value_t::array:
+                    return v.m_data.m_value.array->empty();
+                case value_t::object:
+                    return v.m_data.m_value.object->empty();
+                default:
+                    return true;
+            }
+        }
+
+        static basic_json& last_child(basic_json& v)
+        {
+            if (v.m_data.m_type == value_t::array)
+            {
+                return v.m_data.m_value.array->back();
+            }
+            JSON_ASSERT(v.m_data.m_type == value_t::object);
+            return v.m_data.m_value.object->rbegin()->second;
+        }
+
+        // removes the last child of a non-empty array/object v; this never
+        // allocates, and since it is only ever called when that child is a
+        // scalar or an already-empty array/object, destroying it never
+        // recurses more than one level deep (see destroy() below)
+        static void pop_last_child(basic_json& v)
+        {
+            if (v.m_data.m_type == value_t::array)
+            {
+                v.m_data.m_value.array->pop_back();
+            }
+            else
+            {
+                JSON_ASSERT(v.m_data.m_type == value_t::object);
+                // erase() needs a forward iterator, so std::prev(end()) is
+                // used here rather than rbegin() (see last_child() above)
+                v.m_data.m_value.object->erase(std::prev(v.m_data.m_value.object->end()));
+            }
+        }
+
+        // deallocates the (already empty) array/object held by v; this is
+        // the same allocator-based free the old recursive implementation
+        // used, just factored out so every level of the walk in destroy()
+        // can share it
+        static void free_container(basic_json& v) noexcept
+        {
+            if (v.m_data.m_type == value_t::array)
+            {
+                JSON_ASSERT(v.m_data.m_value.array->empty());
+                AllocatorType<array_t> alloc;
+                std::allocator_traits<decltype(alloc)>::destroy(alloc, v.m_data.m_value.array);
+                std::allocator_traits<decltype(alloc)>::deallocate(alloc, v.m_data.m_value.array, 1);
+            }
+            else
+            {
+                JSON_ASSERT(v.m_data.m_type == value_t::object);
+                JSON_ASSERT(v.m_data.m_value.object->empty());
+                AllocatorType<object_t> alloc;
+                std::allocator_traits<decltype(alloc)>::destroy(alloc, v.m_data.m_value.object);
+                std::allocator_traits<decltype(alloc)>::deallocate(alloc, v.m_data.m_value.object, 1);
+            }
+            v.m_data.m_type = value_t::null; // avoid a double free if v is later destructed
+        }
+
+public:
+        void destroy_string() noexcept
+        {
+            if (string == nullptr)
+            {
+                // not initialized (e.g., due to exception in the ctor)
+                return;
+            }
+            AllocatorType<string_t> alloc;
+            std::allocator_traits<decltype(alloc)>::destroy(alloc, string);
+            std::allocator_traits<decltype(alloc)>::deallocate(alloc, string, 1);
+        }
+
+        void destroy_binary() noexcept
+        {
+            if (binary == nullptr)
+            {
+                // not initialized (e.g., due to exception in the ctor)
+                return;
+            }
+            AllocatorType<binary_t> alloc;
+            std::allocator_traits<decltype(alloc)>::destroy(alloc, binary);
+            std::allocator_traits<decltype(alloc)>::deallocate(alloc, binary, 1);
+        }
+
+        // t must be value_t::array or value_t::object
+        void destroy_container(value_t t) noexcept
         {
             if (
                 (t == value_t::object && object == nullptr) ||
-                (t == value_t::array && array == nullptr) ||
-                (t == value_t::string && string == nullptr) ||
-                (t == value_t::binary && binary == nullptr)
+                (t == value_t::array && array == nullptr)
             )
             {
                 // not initialized (e.g., due to exception in the ctor)
                 return;
             }
-            if (t == value_t::array || t == value_t::object)
+
+            // Destroy the tree without recursing per nesting level and
+            // without any heap allocation: a heap-allocated flattening
+            // stack (the previous implementation) can itself throw
+            // bad_alloc, which would escape this noexcept destructor and
+            // terminate the program (#5135).
+            //
+            // Instead, walk down the "last child" chain, reversing links
+            // as we go: cur is the container currently being emptied,
+            // and prev is its parent (value_t::null when there is none).
+            // Each parent's last child slot doubles as storage for that
+            // parent's own parent link while we are below it, so no
+            // extra memory is needed. We only ever remove a child once
+            // it is a scalar or an empty array/object, which neither
+            // allocates nor recurses more than one level deep.
+            //
+            // This json_value is not itself a basic_json, so the
+            // top-level container is first moved into a local stand-in
+            // ("cur"); a default-constructed basic_json has a null
+            // pointer in its m_value (see data::m_value's initializer),
+            // so swapping it with *this leaves this union's own pointer
+            // null, and it is never looked at or freed a second time.
+            basic_json cur;
+            cur.m_data.m_type = t;
+            using std::swap;
+            swap(cur.m_data.m_value, *this);
+
+            basic_json prev; // value_t::null: no parent
+
+            while (true)
             {
-                // flatten the current json_value to a heap-allocated stack
-                std::vector<basic_json> stack;
-
-                // move the top-level items to stack
-                if (t == value_t::array)
+                if (has_no_children(cur))
                 {
-                    stack.reserve(array->size());
-                    std::move(array->begin(), array->end(), std::back_inserter(stack));
-                }
-                else
-                {
-                    stack.reserve(object->size());
-                    for (auto&& it : *object)
+                    if (prev.m_data.m_type == value_t::null)
                     {
-                        stack.push_back(std::move(it.second));
-                    }
-                }
-
-                while (!stack.empty())
-                {
-                    // move the last item to a local variable to be processed
-                    basic_json current_item(std::move(stack.back()));
-                    stack.pop_back();
-
-                    // if current_item is array/object, move
-                    // its children to the stack to be processed later
-                    if (current_item.is_array())
-                    {
-                        std::move(current_item.m_data.m_value.array->begin(), current_item.m_data.m_value.array->end(), std::back_inserter(stack));
-
-                        current_item.m_data.m_value.array->clear();
-                    }
-                    else if (current_item.is_object())
-                    {
-                        for (auto&& it : *current_item.m_data.m_value.object)
-                        {
-                            stack.push_back(std::move(it.second));
-                        }
-
-                        current_item.m_data.m_value.object->clear();
+                        free_container(cur);
+                        return; // back at the top with nothing left to do
                     }
 
-                    // it's now safe that current_item gets destructed
-                    // since it doesn't have any children
+                    // ascend: detach the grandparent link from prev's
+                    // last slot, drop that (now null) slot, free cur
+                    // (it is empty), then move up one level
+                    basic_json gp;
+                    take(gp, last_child(prev));
+                    pop_last_child(prev);
+
+                    free_container(cur);
+
+                    take(cur, prev);
+                    take(prev, gp);
+                    continue;
                 }
+
+                basic_json& cur_last_ref = last_child(cur);
+
+                if (has_no_children(cur_last_ref))
+                {
+                    // scalar, or already-empty array/object
+                    pop_last_child(cur);
+                    continue;
+                }
+
+                // descend into the non-empty last child, reversing the
+                // link: its slot takes over prev, and the child becomes
+                // the new cur
+                basic_json tmp;
+                take(tmp, cur_last_ref);
+                take(cur_last_ref, prev);
+                take(prev, cur);
+                take(cur, tmp);
             }
+        }
 
+        void destroy(value_t t)
+        {
             switch (t)
             {
-                case value_t::object:
-                {
-                    AllocatorType<object_t> alloc;
-                    std::allocator_traits<decltype(alloc)>::destroy(alloc, object);
-                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, object, 1);
-                    break;
-                }
-
-                case value_t::array:
-                {
-                    AllocatorType<array_t> alloc;
-                    std::allocator_traits<decltype(alloc)>::destroy(alloc, array);
-                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, array, 1);
-                    break;
-                }
-
                 case value_t::string:
-                {
-                    AllocatorType<string_t> alloc;
-                    std::allocator_traits<decltype(alloc)>::destroy(alloc, string);
-                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, string, 1);
+                    destroy_string();
                     break;
-                }
 
                 case value_t::binary:
-                {
-                    AllocatorType<binary_t> alloc;
-                    std::allocator_traits<decltype(alloc)>::destroy(alloc, binary);
-                    std::allocator_traits<decltype(alloc)>::deallocate(alloc, binary, 1);
+                    destroy_binary();
+                    break;
+
+                case value_t::object:
+                case value_t::array:
+                    destroy_container(t);
                     break;
-                }
 
                 case value_t::null:
                 case value_t::boolean:
@@ -27894,9 +28034,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                 case value_t::number_float:
                 case value_t::discarded:
                 default:
-                {
                     break;
-                }
             }
         }
     };

tests/src/unit-allocator.cpp

@@ -607,3 +607,88 @@ TEST_CASE("bad my_allocator::construct")
         j["test"].push_back("should not leak");
     }
 }
+
+namespace
+{
+std::size_t counting_allocator_allocations = 0;
+std::size_t counting_allocator_deallocations = 0;
+
+template<class T>
+struct counting_allocator : std::allocator<T>
+{
+    using std::allocator<T>::allocator;
+
+    T* allocate(std::size_t n)
+    {
+        ++counting_allocator_allocations;
+        return std::allocator<T>::allocate(n);
+    }
+
+    void deallocate(T* p, std::size_t n)
+    {
+        ++counting_allocator_deallocations;
+        std::allocator<T>::deallocate(p, n);
+    }
+
+    template <class U>
+    struct rebind
+    {
+        using other = counting_allocator<U>;
+    };
+};
+} // namespace
+
+TEST_CASE("destructor performs no allocation, only deallocation")
+{
+    // see https://github.com/nlohmann/json/issues/4842 and
+    // https://github.com/nlohmann/json/issues/5135: destroying nested
+    // arrays/objects used to allocate a temporary stack (first with
+    // std::allocator, later - after #4842 - with the provided allocator).
+    // Since that stack could itself throw bad_alloc from inside the
+    // noexcept destructor (#5135), destroy() no longer allocates anything:
+    // it only ever frees what is already there.
+    using counting_json = nlohmann::basic_json<std::map,
+          std::vector,
+          std::string,
+          bool,
+          std::int64_t,
+          std::uint64_t,
+          double,
+          counting_allocator>;
+
+    SECTION("array")
+    {
+        auto* j = new counting_json({1, {2, {3, 4}}, 5}); // NOLINT(cppcoreguidelines-owning-memory)
+        const auto allocations_before = counting_allocator_allocations;
+        const auto deallocations_before = counting_allocator_deallocations;
+        delete j; // NOLINT(cppcoreguidelines-owning-memory)
+        CHECK(counting_allocator_allocations == allocations_before);
+        CHECK(counting_allocator_deallocations > deallocations_before);
+    }
+
+    SECTION("object")
+    {
+        auto* j = new counting_json({{"a", {{"b", {1, 2}}}}, {"c", 3}}); // NOLINT(cppcoreguidelines-owning-memory)
+        const auto allocations_before = counting_allocator_allocations;
+        const auto deallocations_before = counting_allocator_deallocations;
+        delete j; // NOLINT(cppcoreguidelines-owning-memory)
+        CHECK(counting_allocator_allocations == allocations_before);
+        CHECK(counting_allocator_deallocations > deallocations_before);
+    }
+
+    SECTION("mixed tree of empty/non-empty arrays and objects")
+    {
+        auto* j = new counting_json( // NOLINT(cppcoreguidelines-owning-memory)
+        {
+            {"empty_obj", counting_json::object()},
+            {"empty_arr", counting_json::array()},
+            {"nested", {{"a", counting_json::array({1, 2, counting_json::object()})}, {"b", 3}}},
+            {"tail", counting_json::array({counting_json::array({1}), 2, counting_json::array({3})})}
+        });
+        const auto allocations_before = counting_allocator_allocations;
+        const auto deallocations_before = counting_allocator_deallocations;
+        delete j; // NOLINT(cppcoreguidelines-owning-memory)
+        CHECK(counting_allocator_allocations == allocations_before);
+        CHECK(counting_allocator_deallocations > deallocations_before);
+    }
+}

tests/src/unit-bjdata.cpp

@@ -114,10 +114,59 @@ TEST_CASE("BJData")
     {
         SECTION("discarded")
         {
-            // discarded values are not serialized
+            // a discarded value cannot be serialized to BJData
             json const j = json::value_t::discarded;
-            const auto result = json::to_bjdata(j);
-            CHECK(result.empty());
+            CHECK_THROWS_WITH_AS(json::to_bjdata(j), "[json.exception.type_error.321] cannot serialize discarded value to BJData", json::type_error&);
+        }
+
+        SECTION("discarded values nested in a container")
+        {
+            json const discarded = json::value_t::discarded;
+
+            SECTION("in an array")
+            {
+                json const j = {1, discarded, 2};
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_bjdata(j), "[json.exception.type_error.321] (/1) cannot serialize discarded value to BJData", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_bjdata(j), "[json.exception.type_error.321] cannot serialize discarded value to BJData", json::type_error&);
+#endif
+            }
+
+            SECTION("as an object value")
+            {
+                json j;
+                j["a"] = 1;
+                j["b"] = discarded;
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_bjdata(j), "[json.exception.type_error.321] (/b) cannot serialize discarded value to BJData", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_bjdata(j), "[json.exception.type_error.321] cannot serialize discarded value to BJData", json::type_error&);
+#endif
+            }
+
+            SECTION("nested deeper (array in object in array)")
+            {
+                json inner_array = {1, discarded};
+                json middle_object;
+                middle_object["x"] = inner_array;
+                json const j = {middle_object};
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_bjdata(j), "[json.exception.type_error.321] (/0/x/1) cannot serialize discarded value to BJData", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_bjdata(j), "[json.exception.type_error.321] cannot serialize discarded value to BJData", json::type_error&);
+#endif
+            }
+
+            SECTION("optimized array of all-discarded elements")
+            {
+                json const j = {discarded, discarded};
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_bjdata(j, true, true), "[json.exception.type_error.321] (/0) cannot serialize discarded value to BJData", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_bjdata(j, true, true), "[json.exception.type_error.321] cannot serialize discarded value to BJData", json::type_error&);
+#endif
+            }
         }
 
         SECTION("null")

tests/src/unit-bson.cpp

@@ -149,6 +149,54 @@ TEST_CASE("BSON")
             json const j = std::vector<int> {1, 2, 3, 4, 5, 6, 7};
             CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.317] to serialize to BSON, top-level type must be object, but is array", json::type_error&);
         }
+
+        SECTION("discarded")
+        {
+            json const j = json::value_t::discarded;
+            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.317] to serialize to BSON, top-level type must be object, but is discarded", json::type_error&);
+        }
+    }
+
+    SECTION("discarded values nested in a container cannot be serialized to BSON")
+    {
+        json const discarded = json::value_t::discarded;
+
+        SECTION("as an object value")
+        {
+            json j;
+            j["a"] = 1;
+            j["b"] = discarded;
+#if JSON_DIAGNOSTICS
+            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] (/b) cannot serialize discarded value to BSON", json::type_error&);
+#else
+            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] cannot serialize discarded value to BSON", json::type_error&);
+#endif
+        }
+
+        SECTION("in an array that is an object value")
+        {
+            json j;
+            j["a"] = json::array({1, discarded, 2});
+#if JSON_DIAGNOSTICS
+            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] (/a/1) cannot serialize discarded value to BSON", json::type_error&);
+#else
+            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] cannot serialize discarded value to BSON", json::type_error&);
+#endif
+        }
+
+        SECTION("nested deeper (array in object in object)")
+        {
+            json inner_array = {1, discarded};
+            json middle_object;
+            middle_object["x"] = inner_array;
+            json j;
+            j["outer"] = middle_object;
+#if JSON_DIAGNOSTICS
+            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] (/outer/x/1) cannot serialize discarded value to BSON", json::type_error&);
+#else
+            CHECK_THROWS_WITH_AS(json::to_bson(j), "[json.exception.type_error.321] cannot serialize discarded value to BSON", json::type_error&);
+#endif
+        }
     }
 
     SECTION("keys containing code-point U+0000 cannot be serialized to BSON")

tests/src/unit-cbor.cpp

@@ -38,10 +38,49 @@ TEST_CASE("CBOR")
     {
         SECTION("discarded")
         {
-            // discarded values are not serialized
+            // a discarded value cannot be serialized to CBOR
             json const j = json::value_t::discarded;
-            const auto result = json::to_cbor(j);
-            CHECK(result.empty());
+            CHECK_THROWS_WITH_AS(json::to_cbor(j), "[json.exception.type_error.321] cannot serialize discarded value to CBOR", json::type_error&);
+        }
+
+        SECTION("discarded values nested in a container")
+        {
+            json const discarded = json::value_t::discarded;
+
+            SECTION("in an array")
+            {
+                json const j = {1, discarded, 2};
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_cbor(j), "[json.exception.type_error.321] (/1) cannot serialize discarded value to CBOR", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_cbor(j), "[json.exception.type_error.321] cannot serialize discarded value to CBOR", json::type_error&);
+#endif
+            }
+
+            SECTION("as an object value")
+            {
+                json j;
+                j["a"] = 1;
+                j["b"] = discarded;
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_cbor(j), "[json.exception.type_error.321] (/b) cannot serialize discarded value to CBOR", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_cbor(j), "[json.exception.type_error.321] cannot serialize discarded value to CBOR", json::type_error&);
+#endif
+            }
+
+            SECTION("nested deeper (array in object in array)")
+            {
+                json inner_array = {1, discarded};
+                json middle_object;
+                middle_object["x"] = inner_array;
+                json const j = {middle_object};
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_cbor(j), "[json.exception.type_error.321] (/0/x/1) cannot serialize discarded value to CBOR", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_cbor(j), "[json.exception.type_error.321] cannot serialize discarded value to CBOR", json::type_error&);
+#endif
+            }
         }
 
         SECTION("NaN")
@@ -1660,7 +1699,7 @@ TEST_CASE("CBOR")
             CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xA1, 0x61, 0X61})), "[json.exception.parse_error.110] parse error at byte 4: syntax error while parsing CBOR value: unexpected end of input", json::parse_error&);
             CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xBF, 0x61, 0X61})), "[json.exception.parse_error.110] parse error at byte 4: syntax error while parsing CBOR value: unexpected end of input", json::parse_error&);
             CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F})), "[json.exception.parse_error.110] parse error at byte 2: syntax error while parsing CBOR binary: unexpected end of input", json::parse_error&);
-            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x00", json::parse_error&);
+            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B); last byte: 0x00", json::parse_error&);
             CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x41})), "[json.exception.parse_error.110] parse error at byte 2: syntax error while parsing CBOR binary: unexpected end of input", json::parse_error&);
 
             CHECK(json::from_cbor(std::vector<uint8_t>({0x18}), true, false).is_discarded());
@@ -2266,22 +2305,21 @@ TEST_CASE("CBOR indefinite-length strings do not recurse per chunk")
 {
     // Reading an indefinite-length string or byte array used to call itself
     // once per chunk, so a payload of repeated 0x7F (or 0x5F) bytes exhausted
-    // the call stack before any of the input was rejected. The open levels are
-    // counted now, and the levels below prove the reader still reads the same
-    // values and reports the same errors at the same byte offsets.
+    // the call stack before any of the input was rejected. Nested indefinite
+    // chunks are now rejected at the second byte, without recursing.
     json _;
 
-    SECTION("many open levels are reported, not crashed on")
+    SECTION("nested levels are rejected, not crashed on")
     {
         const std::vector<uint8_t> input(200000, 0x7F);
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.110] parse error at byte 200001: syntax error while parsing CBOR string: unexpected end of input", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: indefinite-length string is not allowed inside indefinite-length string; last byte: 0x7F", json::parse_error&);
         CHECK(json::from_cbor(input, true, false).is_discarded());
     }
 
-    SECTION("many open levels are reported, not crashed on (binary)")
+    SECTION("nested levels are rejected, not crashed on (binary)")
     {
         const std::vector<uint8_t> input(200000, 0x5F);
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.110] parse error at byte 200001: syntax error while parsing CBOR binary: unexpected end of input", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(input), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: indefinite-length binary array is not allowed inside indefinite-length binary array; last byte: 0x5F", json::parse_error&);
         CHECK(json::from_cbor(input, true, false).is_discarded());
     }
 
@@ -2289,22 +2327,22 @@ TEST_CASE("CBOR indefinite-length strings do not recurse per chunk")
     {
         CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0xFF})) == json(""));
         CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x61, 0x61, 0xFF})) == json("a"));
-        // nested indefinite-length strings are concatenated across levels
-        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x61, 0x61, 0xFF, 0x61, 0x62, 0xFF})) == json("ab"));
-        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x7F, 0x61, 0x7A, 0xFF, 0xFF, 0xFF})) == json("z"));
+        // empty and nonempty definite-length chunks concatenate in order
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x7F, 0x61, 'a', 0x60, 0x61, 'b', 0x61, 'c', 0xFF})) == json("abc"));
         CHECK(json::from_cbor(std::vector<uint8_t>({0xA1, 0x7F, 0x61, 0x61, 0xFF, 0x01})) == json({{"a", 1}}));
     }
 
     SECTION("chunks are still concatenated (binary)")
     {
         CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x41, 0x61, 0xFF})) == json::binary({0x61}));
-        CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x5F, 0x41, 0x61, 0xFF, 0x41, 0x62, 0xFF})) == json::binary({0x61, 0x62}));
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0xFF})) == json::binary({}));
+        CHECK(json::from_cbor(std::vector<uint8_t>({0x5F, 0x41, 0x61, 0x40, 0x41, 0x62, 0x41, 0x63, 0xFF})) == json::binary({0x61, 0x62, 0x63}));
     }
 
     SECTION("a chunk that is not a string is still rejected")
     {
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x7F, 0x7F, 0x00})), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x00", json::parse_error&);
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B) or indefinite binary array type (0x5F); last byte: 0x00", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x7F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B); last byte: 0x00", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0x5F, 0x00})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR binary: expected length specification (0x40-0x5B); last byte: 0x00", json::parse_error&);
     }
 
     SECTION("a break marker outside an indefinite-length string is not a string")
@@ -2857,9 +2895,17 @@ TEST_CASE("examples from RFC 8949 Appendix A")
     {
         const auto packed = utils::read_binary_file(TEST_DATA_DIRECTORY "/binary_data/cbor_binary.cbor");
         json j;
-        CHECK_NOTHROW(j = json::from_cbor(packed));
+        // the fixture's tail contains nested indefinite-length byte strings.
+        CHECK_THROWS_WITH_AS(j = json::from_cbor(packed), "[json.exception.parse_error.113] parse error at byte 513: syntax error while parsing CBOR binary: indefinite-length binary array is not allowed inside indefinite-length binary array; last byte: 0x5F", json::parse_error&);
 
-        const auto expected = utils::read_binary_file(TEST_DATA_DIRECTORY "/binary_data/cbor_binary.out");
+        // keep the byte-for-byte decoding check for its valid prefix: the first
+        // 512 encoded bytes contain 468 payload bytes in definite-length chunks.
+        auto valid_prefix = packed;
+        valid_prefix.resize(512);
+        valid_prefix.push_back(0xFF);
+        auto expected = utils::read_binary_file(TEST_DATA_DIRECTORY "/binary_data/cbor_binary.out");
+        expected.resize(468);
+        CHECK_NOTHROW(j = json::from_cbor(valid_prefix));
         CHECK(j == json::binary(expected));
 
         // 0xd8

tests/src/unit-msgpack.cpp

@@ -41,10 +41,49 @@ TEST_CASE("MessagePack")
     {
         SECTION("discarded")
         {
-            // discarded values are not serialized
+            // a discarded value cannot be serialized to MessagePack
             json const j = json::value_t::discarded;
-            const auto result = json::to_msgpack(j);
-            CHECK(result.empty());
+            CHECK_THROWS_WITH_AS(json::to_msgpack(j), "[json.exception.type_error.321] cannot serialize discarded value to MessagePack", json::type_error&);
+        }
+
+        SECTION("discarded values nested in a container")
+        {
+            json const discarded = json::value_t::discarded;
+
+            SECTION("in an array")
+            {
+                json const j = {1, discarded, 2};
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_msgpack(j), "[json.exception.type_error.321] (/1) cannot serialize discarded value to MessagePack", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_msgpack(j), "[json.exception.type_error.321] cannot serialize discarded value to MessagePack", json::type_error&);
+#endif
+            }
+
+            SECTION("as an object value")
+            {
+                json j;
+                j["a"] = 1;
+                j["b"] = discarded;
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_msgpack(j), "[json.exception.type_error.321] (/b) cannot serialize discarded value to MessagePack", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_msgpack(j), "[json.exception.type_error.321] cannot serialize discarded value to MessagePack", json::type_error&);
+#endif
+            }
+
+            SECTION("nested deeper (array in object in array)")
+            {
+                json inner_array = {1, discarded};
+                json middle_object;
+                middle_object["x"] = inner_array;
+                json const j = {middle_object};
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_msgpack(j), "[json.exception.type_error.321] (/0/x/1) cannot serialize discarded value to MessagePack", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_msgpack(j), "[json.exception.type_error.321] cannot serialize discarded value to MessagePack", json::type_error&);
+#endif
+            }
         }
 
         SECTION("null")

tests/src/unit-regression2.cpp

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

tests/src/unit-regression3.cpp

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

tests/src/unit-ubjson.cpp

@@ -35,10 +35,59 @@ TEST_CASE("UBJSON")
     {
         SECTION("discarded")
         {
-            // discarded values are not serialized
+            // a discarded value cannot be serialized to UBJSON
             json const j = json::value_t::discarded;
-            const auto result = json::to_ubjson(j);
-            CHECK(result.empty());
+            CHECK_THROWS_WITH_AS(json::to_ubjson(j), "[json.exception.type_error.321] cannot serialize discarded value to UBJSON", json::type_error&);
+        }
+
+        SECTION("discarded values nested in a container")
+        {
+            json const discarded = json::value_t::discarded;
+
+            SECTION("in an array")
+            {
+                json const j = {1, discarded, 2};
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_ubjson(j), "[json.exception.type_error.321] (/1) cannot serialize discarded value to UBJSON", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_ubjson(j), "[json.exception.type_error.321] cannot serialize discarded value to UBJSON", json::type_error&);
+#endif
+            }
+
+            SECTION("as an object value")
+            {
+                json j;
+                j["a"] = 1;
+                j["b"] = discarded;
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_ubjson(j), "[json.exception.type_error.321] (/b) cannot serialize discarded value to UBJSON", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_ubjson(j), "[json.exception.type_error.321] cannot serialize discarded value to UBJSON", json::type_error&);
+#endif
+            }
+
+            SECTION("nested deeper (array in object in array)")
+            {
+                json inner_array = {1, discarded};
+                json middle_object;
+                middle_object["x"] = inner_array;
+                json const j = {middle_object};
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_ubjson(j), "[json.exception.type_error.321] (/0/x/1) cannot serialize discarded value to UBJSON", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_ubjson(j), "[json.exception.type_error.321] cannot serialize discarded value to UBJSON", json::type_error&);
+#endif
+            }
+
+            SECTION("optimized array of all-discarded elements")
+            {
+                json const j = {discarded, discarded};
+#if JSON_DIAGNOSTICS
+                CHECK_THROWS_WITH_AS(json::to_ubjson(j, true, true), "[json.exception.type_error.321] (/0) cannot serialize discarded value to UBJSON", json::type_error&);
+#else
+                CHECK_THROWS_WITH_AS(json::to_ubjson(j, true, true), "[json.exception.type_error.321] cannot serialize discarded value to UBJSON", json::type_error&);
+#endif
+            }
         }
 
         SECTION("null")
@@ -2099,9 +2148,14 @@ TEST_CASE("UBJSON")
 
         SECTION("discarded")
         {
+            // a discarded value cannot be serialized to UBJSON, even as part
+            // of an optimized array of a single (here: valueless) type
             json const j = {json::value_t::discarded, json::value_t::discarded};
-            std::vector<uint8_t> expected = {'[', '$', 'N', '#', 'i', 2};
-            CHECK(json::to_ubjson(j, true, true) == expected);
+#if JSON_DIAGNOSTICS
+            CHECK_THROWS_WITH_AS(json::to_ubjson(j, true, true), "[json.exception.type_error.321] (/0) cannot serialize discarded value to UBJSON", json::type_error&);
+#else
+            CHECK_THROWS_WITH_AS(json::to_ubjson(j, true, true), "[json.exception.type_error.321] cannot serialize discarded value to UBJSON", json::type_error&);
+#endif
         }
     }
 }