Merge branch 'develop' into claude/fix-issue-2649-738aba

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

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

@@ -272,6 +272,15 @@ basic_json(basic_json&& other) noexcept;
         When used without parentheses around an empty initializer list, `basic_json()` is called instead of this
         function, yielding the JSON `#!json null` value.
 
+- Overload 4:
+
+    !!! info "Implicit conversion"
+
+        The conversion is implicit unless [`JSON_USE_IMPLICIT_CONVERSIONS`](../macros/json_use_implicit_conversions.md)
+        is defined to `0` and `BasicJsonType::string_t` differs from `string_t`. In that case, the constructor is
+        `explicit`, so a JSON value with a different string type is no longer silently converted, for example when it is
+        passed to a function taking `#!cpp const json&`. Write `#!cpp json(other)` or `#!cpp other.get<json>()` instead.
+
 - Overload 7:
 
     !!! info "Preconditions"
@@ -420,7 +429,8 @@ basic_json(basic_json&& other) noexcept;
 1. Since version 1.0.0.
 2. Since version 1.0.0.
 3. Since version 2.1.0.
-4. Since version 3.2.0.
+4. Since version 3.2.0. Explicit for different string types if `JSON_USE_IMPLICIT_CONVERSIONS` is `0` since
+   version 3.13.0.
 5. Since version 1.0.0.
 6. Since version 1.0.0.
 7. Since version 1.0.0.

docs/mkdocs/docs/api/basic_json/from_cbor.md

@@ -80,8 +80,8 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
   the end of the file was not reached when `strict` was set to true
 - Throws [parse_error.112](../../home/exceptions.md#jsonexceptionparse_error112) if unsupported features from CBOR were
   used in the given input or if the input is not valid CBOR
-- Throws [parse_error.113](../../home/exceptions.md#jsonexceptionparse_error113) if a map key is not a string (keys of other
-  types are not supported, as JSON object keys are always strings) or a string is malformed
+- Throws [parse_error.113](../../home/exceptions.md#jsonexceptionparse_error113) if a string was expected as a map key,
+  but not found
 
 ## Complexity
 

docs/mkdocs/docs/api/basic_json/from_msgpack.md

@@ -73,8 +73,8 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va
   the end of the file was not reached when `strict` was set to true
 - Throws [parse_error.112](../../home/exceptions.md#jsonexceptionparse_error112) if unsupported features from
   MessagePack were used in the given input or if the input is not valid MessagePack
-- Throws [parse_error.113](../../home/exceptions.md#jsonexceptionparse_error113) if a map key is not a string (keys of other
-  types are not supported, as JSON object keys are always strings) or a string is malformed
+- Throws [parse_error.113](../../home/exceptions.md#jsonexceptionparse_error113) if a string was expected as a map key,
+  but not found
 
 ## Complexity
 

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

@@ -5,7 +5,9 @@
 ```
 
 When defined to `0`, implicit conversions are switched off. By default, implicit conversions are switched on. The
-value directly affects [`operator ValueType`](../basic_json/operator_ValueType.md).
+value directly affects [`operator ValueType`](../basic_json/operator_ValueType.md) and the
+[converting constructor](../basic_json/basic_json.md) from a `basic_json` specialization with a different string
+type (overload 4).
 
 ## Default definition
 
@@ -57,6 +59,25 @@ By default, implicit conversions are enabled.
     auto s = j.get<std::string>();
     ```
 
+??? example "Conversion between `basic_json` specializations"
+
+    A `basic_json` specialization with a different string type is also no longer converted implicitly when
+    `JSON_USE_IMPLICIT_CONVERSIONS` is defined to `0`:
+
+    ```cpp
+    using wjson = nlohmann::basic_json<std::map, std::vector, std::wstring>;
+
+    void load(const nlohmann::json& j);
+
+    wjson wj = /* ... */;
+    load(wj);                            // error: no implicit conversion
+    load(nlohmann::json(wj));            // OK: explicit conversion
+    load(wj.get<nlohmann::json>());      // OK: explicit conversion
+    ```
+
+    Specializations that share the same string type, such as `json` and `ordered_json`, remain implicitly
+    convertible.
+
 ## See also
 
 - [**operator ValueType**](../basic_json/operator_ValueType.md) - get a value (implicit)
@@ -66,3 +87,4 @@ By default, implicit conversions are enabled.
 ## Version history
 
 - Added in version 3.9.0.
+- Also affects the conversion between `basic_json` specializations with different string types since version 3.13.0.

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

@@ -174,20 +174,7 @@ The library maps CBOR types to JSON value types as follows:
 
 !!! warning "Object keys"
 
-    CBOR allows map keys of any type, whereas JSON only allows strings as keys in object values. Therefore, CBOR maps
-    with keys other than text strings (major type 3) are rejected with a
-    [`parse_error.113`](../../home/exceptions.md#jsonexceptionparse_error113) exception (or, with `allow_exceptions` set
-    to `false`, a discarded value) naming the type of the key that was found, for instance:
-
-    ```
-    [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR object key: only string keys are supported, but found an unsigned integer; last byte: 0x01
-    ```
-
-    This applies to the [SAX interface](../parsing/sax_interface.md) as well, as the key is read before it is passed
-    on. This is a deliberate restriction of the library's JSON value model, not an oversight: formats built on CBOR
-    maps with integer keys, such as COSE ([RFC 9052](https://www.rfc-editor.org/rfc/rfc9052.html)) or CWT
-    ([RFC 8392](https://www.rfc-editor.org/rfc/rfc8392.html)), cannot be read with this library and need a
-    general-purpose CBOR library instead.
+    CBOR allows map keys of any type, whereas JSON only allows strings as keys in object values. Therefore, CBOR maps with keys other than UTF-8 strings are rejected.
 
 !!! warning "UTF-8 validation of text strings"
 

docs/mkdocs/docs/features/binary_formats/messagepack.md

@@ -138,21 +138,6 @@ The library maps MessagePack types to JSON value types as follows:
 
     Any MessagePack output created by `to_msgpack` can be successfully parsed by `from_msgpack`.
 
-!!! warning "Object keys"
-
-    MessagePack allows map keys of any type, whereas JSON only allows strings as keys in object values. Like the
-    JSON-compatible [profile](https://github.com/msgpack/msgpack/blob/master/spec.md#profile) sketched in the
-    MessagePack specification, this library restricts map keys to `str` values. Maps with keys of any other type are
-    rejected with a [`parse_error.113`](../../home/exceptions.md#jsonexceptionparse_error113) exception (or, with
-    `allow_exceptions` set to `false`, a discarded value) naming the type of the key that was found, for instance:
-
-    ```
-    [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack object key: only string keys are supported, but found nil; last byte: 0xC0
-    ```
-
-    This applies to the [SAX interface](../parsing/sax_interface.md) as well, as the key is read before it is passed
-    on. Such input needs a general-purpose MessagePack library instead.
-
 !!! warning "UTF-8 validation of string values"
 
     The MessagePack specification requires `str` values (`fixstr`, `str 8`, `str 16`, `str 32`) to be valid UTF-8.

docs/mkdocs/docs/home/exceptions.md

@@ -343,20 +343,13 @@ A string could not be read from a [binary format](../features/binary_formats/ind
 string was read where one was required (for instance as a map key), the string's length specification is invalid, or
 the string's bytes are not valid UTF-8.
 
-CBOR and MessagePack allow map keys of any type, but JSON object keys are always strings. Maps with keys of any other
-type (for instance integers or `null`) are therefore not supported; see the notes on
-[CBOR](../features/binary_formats/cbor.md) and [MessagePack](../features/binary_formats/messagepack.md).
-
 !!! failure "Example messages"
 
     ```
-    [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR object key: only string keys are supported, but found an unsigned integer; last byte: 0x01
+    [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0xFF
     ```
     ```
-    [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack object key: only string keys are supported, but found nil; last byte: 0xC0
-    ```
-    ```
-    [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x7C
+    [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack string: expected length specification (0xA0-0xBF, 0xD9-0xDB); last byte: 0xFF
     ```
     ```
     [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing UBJSON char: byte after 'C' must be in range 0x00..0x7F; last byte: 0x82

include/nlohmann/detail/conversions/to_json.hpp

@@ -291,7 +291,9 @@ void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept
 {
     if (opt.has_value())
     {
-        j = *opt;
+        // explicit construction, as the conversion from a basic_json with a different
+        // string type is explicit if JSON_USE_IMPLICIT_CONVERSIONS is 0 (#2649)
+        j = BasicJsonType(*opt);
     }
     else
     {

include/nlohmann/detail/input/binary_reader.hpp

@@ -1324,80 +1324,6 @@ class binary_reader
         }
     }
 
-    /*!
-    @brief reads a CBOR object key
-
-    RFC 8949 allows any data item as a map key, but only strings have a
-    counterpart in JSON. A key of any other type is rejected with a message
-    naming that type, rather than the one @ref get_cbor_string gives for a
-    malformed string.
-
-    @param[out] result  created key
-
-    @return whether key creation completed
-    */
-    bool get_cbor_object_key(string_t& result)
-    {
-        // EOF and major type 3 (text string) are left to get_cbor_string
-        if (current == char_traits<char_type>::eof() || (static_cast<unsigned int>(current) & 0xE0u) == 0x60u)
-        {
-            return get_cbor_string(result);
-        }
-
-        const char* found = nullptr;
-        switch (static_cast<unsigned int>(current) >> 5u)
-        {
-            case 0:
-                found = "an unsigned integer";
-                break;
-            case 1:
-                found = "a negative integer";
-                break;
-            case 2:
-                found = "a byte string";
-                break;
-            case 4:
-                found = "an array";
-                break;
-            case 5:
-                found = "a map";
-                break;
-            case 6:
-                found = "a tag";
-                break;
-            default: // major type 7
-                switch (current)
-                {
-                    case 0xF4:
-                    case 0xF5:
-                        found = "a boolean";
-                        break;
-                    case 0xF6:
-                        found = "null";
-                        break;
-                    case 0xF7:
-                        found = "undefined";
-                        break;
-                    case 0xF9:
-                    case 0xFA:
-                    case 0xFB:
-                        found = "a floating-point number";
-                        break;
-                    case 0xFF:
-                        found = "a break stop code";
-                        break;
-                    default:
-                        found = "a simple value";
-                        break;
-                }
-                break;
-        }
-
-        auto last_token = get_token_string();
-        return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                exception_message(input_format_t::cbor, concat("only string keys are supported, but found ", found, "; last byte: 0x", last_token), "object key"), nullptr));
-    }
-
     /*!
     @brief reads a definite-length CBOR byte array
 
@@ -1642,7 +1568,7 @@ class binary_reader
                 if (top.is_object)
                 {
                     key.clear();
-                    if (JSON_HEDLEY_UNLIKELY(!get_cbor_object_key(key) || !sax->key(key)))
+                    if (JSON_HEDLEY_UNLIKELY(!get_cbor_string(key) || !sax->key(key)))
                     {
                         return false;
                     }
@@ -2143,98 +2069,6 @@ class binary_reader
         }
     }
 
-    /*!
-    @brief reads a MessagePack object key
-
-    The MessagePack specification allows any type as a map key, but only
-    strings have a counterpart in JSON. A key of any other type is rejected
-    with a message naming that type, rather than the one @ref
-    get_msgpack_string gives for a malformed string.
-
-    @param[out] result  created key
-
-    @return whether key creation completed
-    */
-    bool get_msgpack_object_key(string_t& result)
-    {
-        const char* found = nullptr;
-        switch (current)
-        {
-            case 0xC0:
-                found = "nil";
-                break;
-            case 0xC2:
-            case 0xC3:
-                found = "a boolean";
-                break;
-            case 0xCA:
-            case 0xCB:
-                found = "a float";
-                break;
-            case 0xC4:
-            case 0xC5:
-            case 0xC6:
-                found = "a bin";
-                break;
-            case 0xC7:
-            case 0xC8:
-            case 0xC9:
-            case 0xD4:
-            case 0xD5:
-            case 0xD6:
-            case 0xD7:
-            case 0xD8:
-                found = "an ext";
-                break;
-            case 0xCC:
-            case 0xCD:
-            case 0xCE:
-            case 0xCF:
-            case 0xD0:
-            case 0xD1:
-            case 0xD2:
-            case 0xD3:
-                found = "an integer";
-                break;
-            case 0xDC:
-            case 0xDD:
-                found = "an array";
-                break;
-            case 0xDE:
-            case 0xDF:
-                found = "a map";
-                break;
-            default:
-                // fixint, fixmap, and fixarray; strings, EOF, and the unused
-                // byte 0xC1 are left to get_msgpack_string
-                if (current == char_traits<char_type>::eof())
-                {
-                    return get_msgpack_string(result);
-                }
-                if (current <= 0x7F || current >= 0xE0)
-                {
-                    found = "an integer";
-                }
-                else if (current <= 0x8F)
-                {
-                    found = "a map";
-                }
-                else if (current <= 0x9F)
-                {
-                    found = "an array";
-                }
-                else
-                {
-                    return get_msgpack_string(result);
-                }
-                break;
-        }
-
-        auto last_token = get_token_string();
-        return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                exception_message(input_format_t::msgpack, concat("only string keys are supported, but found ", found, "; last byte: 0x", last_token), "object key"), nullptr));
-    }
-
     /*!
     @brief reads a MessagePack byte array
 
@@ -2397,7 +2231,7 @@ class binary_reader
                 {
                     get();
                     key.clear();
-                    if (JSON_HEDLEY_UNLIKELY(!get_msgpack_object_key(key) || !sax->key(key)))
+                    if (JSON_HEDLEY_UNLIKELY(!get_msgpack_string(key) || !sax->key(key)))
                     {
                         return false;
                     }

include/nlohmann/json.hpp

@@ -1605,12 +1605,42 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         assert_invariant();
     }
 
+  private:
+    /// whether a basic_json specialization can be converted implicitly into this one;
+    /// with JSON_USE_IMPLICIT_CONVERSIONS set to 0, this is only the case if both share
+    /// the same string type (see https://github.com/nlohmann/json/issues/2649)
+    template<typename BasicJsonType>
+    using is_implicitly_convertible_basic_json = std::integral_constant < bool,
+          (JSON_USE_IMPLICIT_CONVERSIONS != 0)
+          || std::is_same<typename BasicJsonType::string_t, string_t>::value >;
+
+    /// tag to select the constructor that performs the conversion from another basic_json specialization
+    struct convert_basic_json_tag {};
+
+  public:
     /// @brief create a JSON value from an existing one
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
     template < typename BasicJsonType,
                detail::enable_if_t <
-                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value, int > = 0 >
+                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value
+                   && is_implicitly_convertible_basic_json<BasicJsonType>::value, int > = 0 >
     basic_json(const BasicJsonType& val)
+        : basic_json(val, convert_basic_json_tag{})
+    {}
+
+    /// @brief create a JSON value from an existing one
+    /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
+    template < typename BasicJsonType,
+               detail::enable_if_t <
+                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value
+                   && !is_implicitly_convertible_basic_json<BasicJsonType>::value, int > = 0 >
+    explicit basic_json(const BasicJsonType& val)
+        : basic_json(val, convert_basic_json_tag{})
+    {}
+
+  private:
+    template<typename BasicJsonType>
+    basic_json(const BasicJsonType& val, convert_basic_json_tag /*unused*/)
 #if JSON_DIAGNOSTIC_POSITIONS
         : start_position(val.start_pos()),
           end_position(val.end_pos())
@@ -1666,6 +1696,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         assert_invariant();
     }
 
+  public:
     /// @brief create a container (array or object) from an initializer list
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
     basic_json(initializer_list_t init,
@@ -2432,7 +2463,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                    int > = 0 >
     BasicJsonType get_impl(detail::priority_tag<2> /*unused*/) const
     {
-        return *this;
+        return BasicJsonType(*this);
     }
 
     /*!
@@ -2571,7 +2602,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                  int> = 0>
     ValueType & get_to(ValueType& v) const
     {
-        v = *this;
+        v = ValueType(*this);
         return v;
     }
 

single_include/nlohmann/json.hpp

@@ -6820,7 +6820,9 @@ void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept
 {
     if (opt.has_value())
     {
-        j = *opt;
+        // explicit construction, as the conversion from a basic_json with a different
+        // string type is explicit if JSON_USE_IMPLICIT_CONVERSIONS is 0 (#2649)
+        j = BasicJsonType(*opt);
     }
     else
     {
@@ -14059,80 +14061,6 @@ class binary_reader
         }
     }
 
-    /*!
-    @brief reads a CBOR object key
-
-    RFC 8949 allows any data item as a map key, but only strings have a
-    counterpart in JSON. A key of any other type is rejected with a message
-    naming that type, rather than the one @ref get_cbor_string gives for a
-    malformed string.
-
-    @param[out] result  created key
-
-    @return whether key creation completed
-    */
-    bool get_cbor_object_key(string_t& result)
-    {
-        // EOF and major type 3 (text string) are left to get_cbor_string
-        if (current == char_traits<char_type>::eof() || (static_cast<unsigned int>(current) & 0xE0u) == 0x60u)
-        {
-            return get_cbor_string(result);
-        }
-
-        const char* found = nullptr;
-        switch (static_cast<unsigned int>(current) >> 5u)
-        {
-            case 0:
-                found = "an unsigned integer";
-                break;
-            case 1:
-                found = "a negative integer";
-                break;
-            case 2:
-                found = "a byte string";
-                break;
-            case 4:
-                found = "an array";
-                break;
-            case 5:
-                found = "a map";
-                break;
-            case 6:
-                found = "a tag";
-                break;
-            default: // major type 7
-                switch (current)
-                {
-                    case 0xF4:
-                    case 0xF5:
-                        found = "a boolean";
-                        break;
-                    case 0xF6:
-                        found = "null";
-                        break;
-                    case 0xF7:
-                        found = "undefined";
-                        break;
-                    case 0xF9:
-                    case 0xFA:
-                    case 0xFB:
-                        found = "a floating-point number";
-                        break;
-                    case 0xFF:
-                        found = "a break stop code";
-                        break;
-                    default:
-                        found = "a simple value";
-                        break;
-                }
-                break;
-        }
-
-        auto last_token = get_token_string();
-        return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                exception_message(input_format_t::cbor, concat("only string keys are supported, but found ", found, "; last byte: 0x", last_token), "object key"), nullptr));
-    }
-
     /*!
     @brief reads a definite-length CBOR byte array
 
@@ -14377,7 +14305,7 @@ class binary_reader
                 if (top.is_object)
                 {
                     key.clear();
-                    if (JSON_HEDLEY_UNLIKELY(!get_cbor_object_key(key) || !sax->key(key)))
+                    if (JSON_HEDLEY_UNLIKELY(!get_cbor_string(key) || !sax->key(key)))
                     {
                         return false;
                     }
@@ -14878,98 +14806,6 @@ class binary_reader
         }
     }
 
-    /*!
-    @brief reads a MessagePack object key
-
-    The MessagePack specification allows any type as a map key, but only
-    strings have a counterpart in JSON. A key of any other type is rejected
-    with a message naming that type, rather than the one @ref
-    get_msgpack_string gives for a malformed string.
-
-    @param[out] result  created key
-
-    @return whether key creation completed
-    */
-    bool get_msgpack_object_key(string_t& result)
-    {
-        const char* found = nullptr;
-        switch (current)
-        {
-            case 0xC0:
-                found = "nil";
-                break;
-            case 0xC2:
-            case 0xC3:
-                found = "a boolean";
-                break;
-            case 0xCA:
-            case 0xCB:
-                found = "a float";
-                break;
-            case 0xC4:
-            case 0xC5:
-            case 0xC6:
-                found = "a bin";
-                break;
-            case 0xC7:
-            case 0xC8:
-            case 0xC9:
-            case 0xD4:
-            case 0xD5:
-            case 0xD6:
-            case 0xD7:
-            case 0xD8:
-                found = "an ext";
-                break;
-            case 0xCC:
-            case 0xCD:
-            case 0xCE:
-            case 0xCF:
-            case 0xD0:
-            case 0xD1:
-            case 0xD2:
-            case 0xD3:
-                found = "an integer";
-                break;
-            case 0xDC:
-            case 0xDD:
-                found = "an array";
-                break;
-            case 0xDE:
-            case 0xDF:
-                found = "a map";
-                break;
-            default:
-                // fixint, fixmap, and fixarray; strings, EOF, and the unused
-                // byte 0xC1 are left to get_msgpack_string
-                if (current == char_traits<char_type>::eof())
-                {
-                    return get_msgpack_string(result);
-                }
-                if (current <= 0x7F || current >= 0xE0)
-                {
-                    found = "an integer";
-                }
-                else if (current <= 0x8F)
-                {
-                    found = "a map";
-                }
-                else if (current <= 0x9F)
-                {
-                    found = "an array";
-                }
-                else
-                {
-                    return get_msgpack_string(result);
-                }
-                break;
-        }
-
-        auto last_token = get_token_string();
-        return sax->parse_error(chars_read, last_token, parse_error::create(113, chars_read,
-                                exception_message(input_format_t::msgpack, concat("only string keys are supported, but found ", found, "; last byte: 0x", last_token), "object key"), nullptr));
-    }
-
     /*!
     @brief reads a MessagePack byte array
 
@@ -15132,7 +14968,7 @@ class binary_reader
                 {
                     get();
                     key.clear();
-                    if (JSON_HEDLEY_UNLIKELY(!get_msgpack_object_key(key) || !sax->key(key)))
+                    if (JSON_HEDLEY_UNLIKELY(!get_msgpack_string(key) || !sax->key(key)))
                     {
                         return false;
                     }
@@ -27654,12 +27490,42 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         assert_invariant();
     }
 
+  private:
+    /// whether a basic_json specialization can be converted implicitly into this one;
+    /// with JSON_USE_IMPLICIT_CONVERSIONS set to 0, this is only the case if both share
+    /// the same string type (see https://github.com/nlohmann/json/issues/2649)
+    template<typename BasicJsonType>
+    using is_implicitly_convertible_basic_json = std::integral_constant < bool,
+        (JSON_USE_IMPLICIT_CONVERSIONS != 0)
+        || std::is_same<typename BasicJsonType::string_t, string_t>::value >;
+
+    /// tag to select the constructor that performs the conversion from another basic_json specialization
+    struct convert_basic_json_tag {};
+
+  public:
     /// @brief create a JSON value from an existing one
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
     template < typename BasicJsonType,
                detail::enable_if_t <
-                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value, int > = 0 >
+                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value
+                   && is_implicitly_convertible_basic_json<BasicJsonType>::value, int > = 0 >
     basic_json(const BasicJsonType& val)
+        : basic_json(val, convert_basic_json_tag{})
+    {}
+
+    /// @brief create a JSON value from an existing one
+    /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
+    template < typename BasicJsonType,
+               detail::enable_if_t <
+                   detail::is_basic_json<BasicJsonType>::value&& !std::is_same<basic_json, BasicJsonType>::value
+                   && !is_implicitly_convertible_basic_json<BasicJsonType>::value, int > = 0 >
+    explicit basic_json(const BasicJsonType& val)
+        : basic_json(val, convert_basic_json_tag{})
+    {}
+
+  private:
+    template<typename BasicJsonType>
+    basic_json(const BasicJsonType& val, convert_basic_json_tag /*unused*/)
 #if JSON_DIAGNOSTIC_POSITIONS
         : start_position(val.start_pos()),
           end_position(val.end_pos())
@@ -27715,6 +27581,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         assert_invariant();
     }
 
+  public:
     /// @brief create a container (array or object) from an initializer list
     /// @sa https://json.nlohmann.me/api/basic_json/basic_json/
     basic_json(initializer_list_t init,
@@ -28481,7 +28348,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                    int > = 0 >
     BasicJsonType get_impl(detail::priority_tag<2> /*unused*/) const
     {
-        return *this;
+        return BasicJsonType(*this);
     }
 
     /*!
@@ -28620,7 +28487,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                  int> = 0>
     ValueType & get_to(ValueType& v) const
     {
-        v = *this;
+        v = ValueType(*this);
         return v;
     }
 

tests/src/unit-alt-string.cpp

@@ -13,6 +13,7 @@
 
 #include <cstdint>
 #include <string>
+#include <type_traits>
 #include <utility>
 #include <vector>
 
@@ -374,4 +375,39 @@ TEST_CASE("alternative string type")
         const auto j2 = j.flatten();
         CHECK(j2.dump() == R"({"/foo/0":"bar","/foo/1":"baz"})");
     }
+
+    SECTION("conversion between basic_json specializations (#2649)")
+    {
+        // explicit conversions are always possible
+        CHECK(std::is_constructible<nlohmann::json, alt_json>::value);
+        CHECK(std::is_constructible<alt_json, nlohmann::json>::value);
+        CHECK(std::is_constructible<nlohmann::json, nlohmann::ordered_json>::value);
+        CHECK(std::is_constructible<nlohmann::ordered_json, nlohmann::json>::value);
+
+        // specializations with the same string type are implicitly convertible
+        CHECK(std::is_convertible<nlohmann::ordered_json, nlohmann::json>::value);
+        CHECK(std::is_convertible<nlohmann::json, nlohmann::ordered_json>::value);
+
+        // specializations with different string types are only implicitly convertible
+        // if implicit conversions are enabled
+#if JSON_USE_IMPLICIT_CONVERSIONS
+        CHECK(std::is_convertible<alt_json, nlohmann::json>::value);
+        CHECK(std::is_convertible<nlohmann::json, alt_json>::value);
+#else
+        CHECK_FALSE(std::is_convertible<alt_json, nlohmann::json>::value);
+        CHECK_FALSE(std::is_convertible<nlohmann::json, alt_json>::value);
+#endif
+
+        // get<BasicJsonType>() works in either case
+        const nlohmann::json j = {{"foo", 1}, {"bar", true}};
+        CHECK(j.get<nlohmann::ordered_json>() == nlohmann::ordered_json(j));
+        // (only a number is converted here, as objects and strings are affected by #3425)
+        CHECK(nlohmann::json(42).get<alt_json>() == 42);
+        CHECK(alt_json(nlohmann::json(42)) == 42);
+
+        // get_to() also works in either case
+        alt_json a;
+        nlohmann::json(42).get_to(a);
+        CHECK(a == 42);
+    }
 }

tests/src/unit-cbor.cpp

@@ -1830,51 +1830,10 @@ TEST_CASE("CBOR")
         SECTION("invalid string in map")
         {
             json _;
-            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xa1, 0xff, 0x01})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR object key: only string keys are supported, but found a break stop code; last byte: 0xFF", json::parse_error&);
+            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xa1, 0xff, 0x01})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0xFF", json::parse_error&);
             CHECK(json::from_cbor(std::vector<uint8_t>({0xa1, 0xff, 0x01}), true, false).is_discarded());
         }
 
-        SECTION("non-string key (see #2766 and #3381)")
-        {
-            // only text strings map to JSON object keys; any other key is
-            // rejected with a message naming its type
-            const std::vector<std::pair<std::vector<std::uint8_t>, std::string>> cases =
-            {
-                {{0xA1, 0x01, 0x01}, "an unsigned integer; last byte: 0x01"},
-                {{0xA1, 0x20, 0x01}, "a negative integer; last byte: 0x20"},
-                {{0xA1, 0x41, 0x61, 0x01}, "a byte string; last byte: 0x41"},
-                {{0xA1, 0x80, 0x01}, "an array; last byte: 0x80"},
-                {{0xA1, 0xA0, 0x01}, "a map; last byte: 0xA0"},
-                {{0xA1, 0xC0, 0x61, 0x61, 0x01}, "a tag; last byte: 0xC0"},
-                {{0xA1, 0xF4, 0x01}, "a boolean; last byte: 0xF4"},
-                {{0xA1, 0xF5, 0x01}, "a boolean; last byte: 0xF5"},
-                {{0xA1, 0xF6, 0x01}, "null; last byte: 0xF6"},
-                {{0xA1, 0xF7, 0x01}, "undefined; last byte: 0xF7"},
-                {{0xA1, 0xF9, 0x3C, 0x00, 0x01}, "a floating-point number; last byte: 0xF9"},
-                {{0xA1, 0xFA, 0x3F, 0x80, 0x00, 0x00, 0x01}, "a floating-point number; last byte: 0xFA"},
-                {{0xA1, 0xFB, 0x3F, 0xF0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01}, "a floating-point number; last byte: 0xFB"},
-                {{0xA1, 0xE0, 0x01}, "a simple value; last byte: 0xE0"},
-                {{0xA1, 0xF8, 0x20, 0x01}, "a simple value; last byte: 0xF8"},
-                // indefinite-length map
-                {{0xBF, 0x01, 0x01, 0xFF}, "an unsigned integer; last byte: 0x01"},
-            };
-
-            for (const auto& c : cases)
-            {
-                CAPTURE(c.first)
-                const std::string expected = "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR object key: only string keys are supported, but found " + c.second;
-                json _;
-                CHECK_THROWS_WITH_AS(_ = json::from_cbor(c.first), expected.c_str(), json::parse_error&);
-                CHECK(json::from_cbor(c.first, true, false).is_discarded());
-            }
-
-            // a key of major type 3 with a reserved length is still reported as
-            // a malformed string, and a missing key as the end of input
-            json _;
-            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xA1})), "[json.exception.parse_error.110] parse error at byte 2: syntax error while parsing CBOR string: unexpected end of input", json::parse_error&);
-            CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xA1, 0x7C, 0x01})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x7C", json::parse_error&);
-        }
-
         SECTION("invalid UTF-8 in string (see #5529)")
         {
             // a two-character text string (major type 3) whose bytes are not
@@ -2325,7 +2284,7 @@ TEST_CASE("CBOR indefinite-length strings do not recurse per chunk")
     SECTION("a break marker outside an indefinite-length string is not a string")
     {
         // 0xFF only closes a string that was opened; on its own it is not one
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xA1, 0xFF, 0x01})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR object key: only string keys are supported, but found a break stop code; last byte: 0xFF", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(std::vector<uint8_t>({0xA1, 0xFF, 0x01})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0xFF", json::parse_error&);
     }
 }
 

tests/src/unit-msgpack.cpp

@@ -1551,69 +1551,10 @@ TEST_CASE("MessagePack")
         SECTION("invalid string in map")
         {
             json _;
-            CHECK_THROWS_WITH_AS(_ = json::from_msgpack(std::vector<uint8_t>({0x81, 0xff, 0x01})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack object key: only string keys are supported, but found an integer; last byte: 0xFF", json::parse_error&);
+            CHECK_THROWS_WITH_AS(_ = json::from_msgpack(std::vector<uint8_t>({0x81, 0xff, 0x01})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack string: expected length specification (0xA0-0xBF, 0xD9-0xDB); last byte: 0xFF", json::parse_error&);
             CHECK(json::from_msgpack(std::vector<uint8_t>({0x81, 0xff, 0x01}), true, false).is_discarded());
         }
 
-        SECTION("non-string key (see #3381)")
-        {
-            // only strings map to JSON object keys; any other key is rejected
-            // with a message naming its type
-            const std::vector<std::pair<std::vector<std::uint8_t>, std::string>> cases =
-            {
-                {{0x81, 0xC0, 0x01}, "nil; last byte: 0xC0"},
-                {{0x81, 0xC2, 0x01}, "a boolean; last byte: 0xC2"},
-                {{0x81, 0xC3, 0x01}, "a boolean; last byte: 0xC3"},
-                {{0x81, 0xCA, 0x3F, 0x80, 0x00, 0x00, 0x01}, "a float; last byte: 0xCA"},
-                {{0x81, 0xCB, 0x3F, 0xF0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01}, "a float; last byte: 0xCB"},
-                {{0x81, 0xC4, 0x00, 0x01}, "a bin; last byte: 0xC4"},
-                {{0x81, 0xC5, 0x00, 0x00, 0x01}, "a bin; last byte: 0xC5"},
-                {{0x81, 0xC6, 0x00, 0x00, 0x00, 0x00, 0x01}, "a bin; last byte: 0xC6"},
-                {{0x81, 0xC7, 0x00, 0x01, 0x01}, "an ext; last byte: 0xC7"},
-                {{0x81, 0xC8, 0x00, 0x00, 0x01, 0x01}, "an ext; last byte: 0xC8"},
-                {{0x81, 0xC9, 0x00, 0x00, 0x00, 0x00, 0x01, 0x01}, "an ext; last byte: 0xC9"},
-                {{0x81, 0xD4, 0x01, 0x00, 0x01}, "an ext; last byte: 0xD4"},
-                {{0x81, 0xD5, 0x01, 0x00, 0x00, 0x01}, "an ext; last byte: 0xD5"},
-                {{0x81, 0xD6, 0x01, 0x00, 0x00, 0x00, 0x00, 0x01}, "an ext; last byte: 0xD6"},
-                {{0x81, 0xD7, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01}, "an ext; last byte: 0xD7"},
-                {{0x81, 0xD8, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01}, "an ext; last byte: 0xD8"},
-                {{0x81, 0xCC, 0x01, 0x01}, "an integer; last byte: 0xCC"},
-                {{0x81, 0xCD, 0x00, 0x01, 0x01}, "an integer; last byte: 0xCD"},
-                {{0x81, 0xCE, 0x00, 0x00, 0x00, 0x01, 0x01}, "an integer; last byte: 0xCE"},
-                {{0x81, 0xCF, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x01}, "an integer; last byte: 0xCF"},
-                {{0x81, 0xD0, 0x01, 0x01}, "an integer; last byte: 0xD0"},
-                {{0x81, 0xD1, 0x00, 0x01, 0x01}, "an integer; last byte: 0xD1"},
-                {{0x81, 0xD2, 0x00, 0x00, 0x00, 0x01, 0x01}, "an integer; last byte: 0xD2"},
-                {{0x81, 0xD3, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x01}, "an integer; last byte: 0xD3"},
-                {{0x81, 0x00, 0x01}, "an integer; last byte: 0x00"},
-                {{0x81, 0x7F, 0x01}, "an integer; last byte: 0x7F"},
-                {{0x81, 0xE0, 0x01}, "an integer; last byte: 0xE0"},
-                {{0x81, 0x80, 0x01}, "a map; last byte: 0x80"},
-                {{0x81, 0x8F, 0x01}, "a map; last byte: 0x8F"},
-                {{0x81, 0xDE, 0x00, 0x00, 0x01}, "a map; last byte: 0xDE"},
-                {{0x81, 0xDF, 0x00, 0x00, 0x00, 0x00, 0x01}, "a map; last byte: 0xDF"},
-                {{0x81, 0x90, 0x01}, "an array; last byte: 0x90"},
-                {{0x81, 0x9F, 0x01}, "an array; last byte: 0x9F"},
-                {{0x81, 0xDC, 0x00, 0x00, 0x01}, "an array; last byte: 0xDC"},
-                {{0x81, 0xDD, 0x00, 0x00, 0x00, 0x00, 0x01}, "an array; last byte: 0xDD"},
-            };
-
-            for (const auto& c : cases)
-            {
-                CAPTURE(c.first)
-                const std::string expected = "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack object key: only string keys are supported, but found " + c.second;
-                json _;
-                CHECK_THROWS_WITH_AS(_ = json::from_msgpack(c.first), expected.c_str(), json::parse_error&);
-                CHECK(json::from_msgpack(c.first, true, false).is_discarded());
-            }
-
-            json _;
-            // the unused byte 0xC1 is still reported as a malformed string
-            CHECK_THROWS_WITH_AS(_ = json::from_msgpack(std::vector<uint8_t>({0x81, 0xC1, 0x01})), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack string: expected length specification (0xA0-0xBF, 0xD9-0xDB); last byte: 0xC1", json::parse_error&);
-            // a missing key is still reported as the end of input
-            CHECK_THROWS_WITH_AS(_ = json::from_msgpack(std::vector<uint8_t>({0x81})), "[json.exception.parse_error.110] parse error at byte 2: syntax error while parsing MessagePack string: unexpected end of input", json::parse_error&);
-        }
-
         SECTION("invalid UTF-8 in string (see #5529)")
         {
             // a fixstr of length 2 (0xA0 | 2) whose bytes are not valid UTF-8

tests/src/unit-regression1.cpp

@@ -1018,7 +1018,7 @@ TEST_CASE("regression tests 1")
         };
 
         json _;
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(vec), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR object key: only string keys are supported, but found an array; last byte: 0x98", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(vec), "[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x98", json::parse_error&);
 
         // related test case: nonempty UTF-8 string (indefinite length)
         std::vector<uint8_t> const vec1 {0x7f, 0x61, 0x61};
@@ -1065,7 +1065,7 @@ TEST_CASE("regression tests 1")
         };
 
         json _;
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(vec1), "[json.exception.parse_error.113] parse error at byte 13: syntax error while parsing CBOR object key: only string keys are supported, but found a map; last byte: 0xB4", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(vec1), "[json.exception.parse_error.113] parse error at byte 13: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0xB4", json::parse_error&);
 
         // related test case: double-precision
         std::vector<uint8_t> const vec2
@@ -1077,7 +1077,7 @@ TEST_CASE("regression tests 1")
             0x96, 0x96, 0xb4, 0xb4, 0xfa, 0x94, 0x94, 0x61,
             0x61, 0x61, 0x61, 0x61, 0x61, 0x61, 0x61, 0xfb
         };
-        CHECK_THROWS_WITH_AS(_ = json::from_cbor(vec2), "[json.exception.parse_error.113] parse error at byte 13: syntax error while parsing CBOR object key: only string keys are supported, but found a map; last byte: 0xB4", json::parse_error&);
+        CHECK_THROWS_WITH_AS(_ = json::from_cbor(vec2), "[json.exception.parse_error.113] parse error at byte 13: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0xB4", json::parse_error&);
     }
 
     SECTION("issue #452 - Heap-buffer-overflow (OSS-Fuzz issue 585)")