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

Conflicts:
- tests/src/unit-alt-string.cpp: kept both new sections (this PR's
  "conversion between basic_json specializations (#2649)" and develop's
  "strict enum" regression test for #5667)

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

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

@@ -224,7 +224,5 @@ Strong exception safety: if an exception occurs, the original value stays intact
 
 1. Added in version 1.0.0.
 2. Added in version 1.0.0.
-3. Added in version 3.11.0. Fixed in version 3.13.0 to consistently accept `std::string_view`-convertible keys, as
-   already supported by [`operator[]`](operator[].md), [`value`](value.md), [`find`](find.md), and other lookup
-   functions.
+3. Added in version 3.11.0.
 4. Added in version 2.0.0.

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. Fixed in version 3.13.0 to also check the iterator range for binary values; before, a range

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

@@ -119,9 +119,7 @@ Logarithmic in the size of the JSON object.
 ## Version history
 
 1. Added in version 3.11.0.
-2. Added in version 3.6.0. Extended template `KeyType` to support comparable types in version 3.11.0. Fixed in
-   version 3.13.0 to consistently accept `std::string_view`-convertible keys, as already supported by
-   [`operator[]`](operator[].md), [`at`](at.md), [`value`](value.md), and other lookup functions.
+2. Added in version 3.6.0. Extended template `KeyType` to support comparable types in version 3.11.0.
 3. Added in version 3.7.0.
 4. Deleted overloads for integral key types added in version 3.13.0 to reject such calls at compile time instead of
    causing undefined behavior at runtime.

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

@@ -84,8 +84,6 @@ Logarithmic in the size of the JSON object.
 ## Version history
 
 1. Added in version 3.11.0.
-2. Added in version 1.0.0. Changed parameter `key` type to `KeyType&&` in version 3.11.0. Fixed in version 3.13.0 to
-   consistently accept `std::string_view`-convertible keys, as already supported by [`operator[]`](operator[].md),
-   [`at`](at.md), [`value`](value.md), and other lookup functions.
+2. Added in version 1.0.0. Changed parameter `key` type to `KeyType&&` in version 3.11.0.
 3. Deleted overload for integral key types added in version 3.13.0 to reject such calls at compile time instead of
    causing undefined behavior at runtime.

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

@@ -213,7 +213,5 @@ Strong exception safety: if an exception occurs, the original value stays intact
 1. Added in version 1.0.0. Added support for binary types in version 3.8.0.
 2. Added in version 1.0.0. Added support for binary types in version 3.8.0.
 3. Added in version 1.0.0.
-4. Added in version 3.11.0. Fixed in version 3.13.0 to consistently accept `std::string_view`-convertible keys, as
-   already supported by [`operator[]`](operator[].md), [`at`](at.md), [`value`](value.md), and other lookup
-   functions.
+4. Added in version 3.11.0.
 5. Added in version 1.0.0.

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

@@ -88,8 +88,6 @@ Logarithmic in the size of the JSON object.
 ## Version history
 
 1. Added in version 3.11.0.
-2. Added in version 1.0.0. Changed to support comparable types in version 3.11.0. Fixed in version 3.13.0 to
-   consistently accept `std::string_view`-convertible keys, as already supported by [`operator[]`](operator[].md),
-   [`at`](at.md), [`value`](value.md), and other lookup functions.
+2. Added in version 1.0.0. Changed to support comparable types in version 3.11.0.
 3. Deleted overloads for integral key types added in version 3.13.0 to reject such calls at compile time instead of
    causing undefined behavior at runtime.

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

@@ -195,9 +195,7 @@ changes to any JSON value.
 1. Added in version 1.0.0. Changed parameter `default_value` type from `const ValueType&` to `ValueType&&` in version
    3.11.0. Deleted overload for integral key types added in version 3.13.0 to reject such calls at compile time
    instead of causing undefined behavior at runtime.
-2. Added in version 3.11.0. Made `ValueType` the first template parameter in version 3.11.2. Fixed in version 3.13.0
-   to consistently accept `std::string_view`-convertible keys, as already supported by
-   [`operator[]`](operator[].md), [`at`](at.md), [`find`](find.md), and other lookup functions.
+2. Added in version 3.11.0. Made `ValueType` the first template parameter in version 3.11.2.
 3. Added in version 2.0.2. Extended to work with arrays in version 3.13.0, including fixing an issue where resolving
    `ptr` through an array unexpectedly threw `out_of_range` instead of returning the resolved element (or
    `default_value`, as documented).

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.

include/nlohmann/detail/conversions/to_json.hpp

@@ -291,7 +291,9 @@ void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept(std::is_not
 {
     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/meta/type_traits.hpp

@@ -748,30 +748,6 @@ using is_usable_as_key_type = typename std::conditional <
                               std::true_type,
                               std::false_type >::type;
 
-#ifdef JSON_HAS_CPP_17
-// type trait to check if KeyType can only be used as an object key after
-// converting it to std::string_view: it is convertible to std::string_view, the
-// object's comparator cannot compare it with object_t::key_type directly, but
-// can compare a std::string_view. JSON pointers and JSON iterators are ruled out
-// first, so that the conversion checks are never instantiated for them (a JSON
-// pointer's deprecated conversion to string_t would be named otherwise).
-template < typename BasicJsonType, typename KeyTypeCVRef, typename KeyType = uncvref_t<KeyTypeCVRef>,
-           bool = is_json_pointer<KeyType>::value || is_json_iterator_of<BasicJsonType, KeyType>::value >
-struct is_string_view_convertible_key_type : std::false_type {};
-
-template<typename BasicJsonType, typename KeyTypeCVRef, typename KeyType>
-struct is_string_view_convertible_key_type<BasicJsonType, KeyTypeCVRef, KeyType, false>
-    : std::integral_constant < bool,
-      std::is_convertible<KeyTypeCVRef, std::string_view>::value
-      && !is_usable_as_key_type<typename BasicJsonType::object_comparator_t,
-      typename BasicJsonType::object_t::key_type, KeyTypeCVRef, true, false>::value
-      && is_usable_as_key_type<typename BasicJsonType::object_comparator_t,
-      typename BasicJsonType::object_t::key_type, std::string_view, true, false>::value > {};
-#else
-template<typename BasicJsonType, typename KeyTypeCVRef>
-struct is_string_view_convertible_key_type : std::false_type {};
-#endif
-
 // type trait to check if KeyType can be used as an object key
 // true if:
 //   - KeyType is comparable with BasicJsonType::object_t::key_type
@@ -785,7 +761,9 @@ using is_usable_as_basic_json_key_type = typename std::conditional <
      typename BasicJsonType::object_t::key_type, KeyTypeCVRef,
      RequireTransparentComparator, ExcludeObjectKeyType>::value
      && !is_json_iterator_of<BasicJsonType, KeyType>::value)
-    || is_string_view_convertible_key_type<BasicJsonType, KeyTypeCVRef>::value
+#ifdef JSON_HAS_CPP_17
+    || std::is_convertible<KeyType, std::string_view>::value
+#endif
     , std::true_type,
     std::false_type >::type;
 

include/nlohmann/json.hpp

@@ -807,24 +807,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return it;
     }
 
-    /// @brief the key to look up an object member with: the key itself, or its
-    ///        std::string_view if the object can only be searched with that
-    template < typename KeyType, detail::enable_if_t <
-                   !detail::is_string_view_convertible_key_type<basic_json_t, KeyType>::value, int > = 0 >
-    static KeyType && lookup_key(KeyType && key) noexcept
-    {
-        return std::forward<KeyType>(key);
-    }
-
-#ifdef JSON_HAS_CPP_17
-    template < typename KeyType, detail::enable_if_t <
-                   detail::is_string_view_convertible_key_type<basic_json_t, KeyType>::value, int > = 0 >
-    static std::string_view lookup_key(KeyType && key)
-    {
-        return std::forward<KeyType>(key);
-    }
-#endif
-
     /// @brief erase an element from the object and return the following one
     /// Not every map returns an iterator from erase(iterator): some containers
     /// (e.g., Abseil's hash maps) return void to avoid computing a successor
@@ -1624,12 +1606,42 @@ 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())
@@ -1685,6 +1697,7 @@ 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,
@@ -2451,7 +2464,7 @@ 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);
     }
 
     /*!
@@ -2590,7 +2603,7 @@ 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;
     }
 
@@ -2785,7 +2798,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             JSON_THROW(type_error::create(304, detail::concat("cannot use at() with ", type_name()), this));
         }
 
-        auto it = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
+        auto it = m_data.m_value.object->find(std::forward<KeyType>(key));
         if (it == m_data.m_value.object->end())
         {
             JSON_THROW(out_of_range::create(403, detail::concat("key '", string_t(std::forward<KeyType>(key)), "' not found"), this));
@@ -2823,7 +2836,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             JSON_THROW(type_error::create(304, detail::concat("cannot use at() with ", type_name()), this));
         }
 
-        auto it = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
+        auto it = m_data.m_value.object->find(std::forward<KeyType>(key));
         if (it == m_data.m_value.object->end())
         {
             JSON_THROW(out_of_range::create(403, detail::concat("key '", string_t(std::forward<KeyType>(key)), "' not found"), this));
@@ -2966,7 +2979,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         // operator[] only works for objects
         if (JSON_HEDLEY_LIKELY(is_object()))
         {
-            auto result = m_data.m_value.object->emplace(lookup_key(std::forward<KeyType>(key)), nullptr);
+            auto result = m_data.m_value.object->emplace(std::forward<KeyType>(key), nullptr);
             return set_parent(result.first->second);
         }
 
@@ -2982,7 +2995,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         // const operator[] only works for objects
         if (JSON_HEDLEY_LIKELY(is_object()))
         {
-            auto it = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
+            auto it = m_data.m_value.object->find(std::forward<KeyType>(key));
             JSON_ASSERT(it != m_data.m_value.object->end());
             return it->second;
         }
@@ -2992,10 +3005,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
   private:
     template<typename KeyType>
-    using is_comparable_with_object_key = std::integral_constant < bool,
-        detail::is_comparable <
-            object_comparator_t, const typename object_t::key_type&, KeyType >::value
-        || detail::is_string_view_convertible_key_type<basic_json_t, KeyType>::value >;
+    using is_comparable_with_object_key = detail::is_comparable <
+        object_comparator_t, const typename object_t::key_type&, KeyType >;
 
     template<typename ValueType>
     using value_return_type = std::conditional <
@@ -3382,7 +3393,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             JSON_THROW(type_error::create(307, detail::concat("cannot use erase() with ", type_name()), this));
         }
 
-        const auto it = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
+        const auto it = m_data.m_value.object->find(std::forward<KeyType>(key));
         if (it != m_data.m_value.object->end())
         {
             m_data.m_value.object->erase(it);
@@ -3409,7 +3420,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                  detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
     size_type erase(KeyType && key)
     {
-        return erase_internal(lookup_key(std::forward<KeyType>(key)));
+        return erase_internal(std::forward<KeyType>(key));
     }
 
     /// @brief remove element from a JSON array given an index
@@ -3489,7 +3500,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
         if (is_object())
         {
-            result.m_it.object_iterator = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
+            result.m_it.object_iterator = m_data.m_value.object->find(std::forward<KeyType>(key));
         }
 
         return result;
@@ -3505,7 +3516,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
         if (is_object())
         {
-            result.m_it.object_iterator = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
+            result.m_it.object_iterator = m_data.m_value.object->find(std::forward<KeyType>(key));
         }
 
         return result;
@@ -3528,7 +3539,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     size_type count(KeyType && key) const
     {
         // return 0 for all nonobject types
-        return is_object() ? m_data.m_value.object->count(lookup_key(std::forward<KeyType>(key))) : 0;
+        return is_object() ? m_data.m_value.object->count(std::forward<KeyType>(key)) : 0;
     }
 
     /// @brief check the existence of an element in a JSON object
@@ -3546,7 +3557,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     JSON_HEDLEY_WARN_UNUSED_RESULT
     bool contains(KeyType && key) const
     {
-        return is_object() && m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key))) != m_data.m_value.object->end();
+        return is_object() && m_data.m_value.object->find(std::forward<KeyType>(key)) != m_data.m_value.object->end();
     }
 
     /// @brief check the existence of an element in a JSON object given a JSON pointer

single_include/nlohmann/json.hpp

@@ -4756,30 +4756,6 @@ using is_usable_as_key_type = typename std::conditional <
                               std::true_type,
                               std::false_type >::type;
 
-#ifdef JSON_HAS_CPP_17
-// type trait to check if KeyType can only be used as an object key after
-// converting it to std::string_view: it is convertible to std::string_view, the
-// object's comparator cannot compare it with object_t::key_type directly, but
-// can compare a std::string_view. JSON pointers and JSON iterators are ruled out
-// first, so that the conversion checks are never instantiated for them (a JSON
-// pointer's deprecated conversion to string_t would be named otherwise).
-template < typename BasicJsonType, typename KeyTypeCVRef, typename KeyType = uncvref_t<KeyTypeCVRef>,
-           bool = is_json_pointer<KeyType>::value || is_json_iterator_of<BasicJsonType, KeyType>::value >
-struct is_string_view_convertible_key_type : std::false_type {};
-
-template<typename BasicJsonType, typename KeyTypeCVRef, typename KeyType>
-struct is_string_view_convertible_key_type<BasicJsonType, KeyTypeCVRef, KeyType, false>
-    : std::integral_constant < bool,
-      std::is_convertible<KeyTypeCVRef, std::string_view>::value
-      && !is_usable_as_key_type<typename BasicJsonType::object_comparator_t,
-      typename BasicJsonType::object_t::key_type, KeyTypeCVRef, true, false>::value
-      && is_usable_as_key_type<typename BasicJsonType::object_comparator_t,
-      typename BasicJsonType::object_t::key_type, std::string_view, true, false>::value > {};
-#else
-template<typename BasicJsonType, typename KeyTypeCVRef>
-struct is_string_view_convertible_key_type : std::false_type {};
-#endif
-
 // type trait to check if KeyType can be used as an object key
 // true if:
 //   - KeyType is comparable with BasicJsonType::object_t::key_type
@@ -4793,7 +4769,9 @@ using is_usable_as_basic_json_key_type = typename std::conditional <
      typename BasicJsonType::object_t::key_type, KeyTypeCVRef,
      RequireTransparentComparator, ExcludeObjectKeyType>::value
      && !is_json_iterator_of<BasicJsonType, KeyType>::value)
-    || is_string_view_convertible_key_type<BasicJsonType, KeyTypeCVRef>::value
+#ifdef JSON_HAS_CPP_17
+    || std::is_convertible<KeyType, std::string_view>::value
+#endif
     , std::true_type,
     std::false_type >::type;
 
@@ -6844,7 +6822,9 @@ void to_json(BasicJsonType& j, const std::optional<T>& opt) noexcept(std::is_not
 {
     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
     {
@@ -27756,24 +27736,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return it;
     }
 
-    /// @brief the key to look up an object member with: the key itself, or its
-    ///        std::string_view if the object can only be searched with that
-    template < typename KeyType, detail::enable_if_t <
-                   !detail::is_string_view_convertible_key_type<basic_json_t, KeyType>::value, int > = 0 >
-    static KeyType && lookup_key(KeyType && key) noexcept
-    {
-        return std::forward<KeyType>(key);
-    }
-
-#ifdef JSON_HAS_CPP_17
-    template < typename KeyType, detail::enable_if_t <
-                   detail::is_string_view_convertible_key_type<basic_json_t, KeyType>::value, int > = 0 >
-    static std::string_view lookup_key(KeyType && key)
-    {
-        return std::forward<KeyType>(key);
-    }
-#endif
-
     /// @brief erase an element from the object and return the following one
     /// Not every map returns an iterator from erase(iterator): some containers
     /// (e.g., Abseil's hash maps) return void to avoid computing a successor
@@ -28573,12 +28535,42 @@ 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())
@@ -28634,6 +28626,7 @@ 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,
@@ -29400,7 +29393,7 @@ 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);
     }
 
     /*!
@@ -29539,7 +29532,7 @@ 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;
     }
 
@@ -29734,7 +29727,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             JSON_THROW(type_error::create(304, detail::concat("cannot use at() with ", type_name()), this));
         }
 
-        auto it = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
+        auto it = m_data.m_value.object->find(std::forward<KeyType>(key));
         if (it == m_data.m_value.object->end())
         {
             JSON_THROW(out_of_range::create(403, detail::concat("key '", string_t(std::forward<KeyType>(key)), "' not found"), this));
@@ -29772,7 +29765,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             JSON_THROW(type_error::create(304, detail::concat("cannot use at() with ", type_name()), this));
         }
 
-        auto it = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
+        auto it = m_data.m_value.object->find(std::forward<KeyType>(key));
         if (it == m_data.m_value.object->end())
         {
             JSON_THROW(out_of_range::create(403, detail::concat("key '", string_t(std::forward<KeyType>(key)), "' not found"), this));
@@ -29915,7 +29908,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         // operator[] only works for objects
         if (JSON_HEDLEY_LIKELY(is_object()))
         {
-            auto result = m_data.m_value.object->emplace(lookup_key(std::forward<KeyType>(key)), nullptr);
+            auto result = m_data.m_value.object->emplace(std::forward<KeyType>(key), nullptr);
             return set_parent(result.first->second);
         }
 
@@ -29931,7 +29924,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         // const operator[] only works for objects
         if (JSON_HEDLEY_LIKELY(is_object()))
         {
-            auto it = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
+            auto it = m_data.m_value.object->find(std::forward<KeyType>(key));
             JSON_ASSERT(it != m_data.m_value.object->end());
             return it->second;
         }
@@ -29941,10 +29934,8 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
   private:
     template<typename KeyType>
-    using is_comparable_with_object_key = std::integral_constant < bool,
-        detail::is_comparable <
-            object_comparator_t, const typename object_t::key_type&, KeyType >::value
-        || detail::is_string_view_convertible_key_type<basic_json_t, KeyType>::value >;
+    using is_comparable_with_object_key = detail::is_comparable <
+        object_comparator_t, const typename object_t::key_type&, KeyType >;
 
     template<typename ValueType>
     using value_return_type = std::conditional <
@@ -30331,7 +30322,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
             JSON_THROW(type_error::create(307, detail::concat("cannot use erase() with ", type_name()), this));
         }
 
-        const auto it = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
+        const auto it = m_data.m_value.object->find(std::forward<KeyType>(key));
         if (it != m_data.m_value.object->end())
         {
             m_data.m_value.object->erase(it);
@@ -30358,7 +30349,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
                  detail::is_usable_as_basic_json_key_type<basic_json_t, KeyType>::value, int> = 0>
     size_type erase(KeyType && key)
     {
-        return erase_internal(lookup_key(std::forward<KeyType>(key)));
+        return erase_internal(std::forward<KeyType>(key));
     }
 
     /// @brief remove element from a JSON array given an index
@@ -30438,7 +30429,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
         if (is_object())
         {
-            result.m_it.object_iterator = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
+            result.m_it.object_iterator = m_data.m_value.object->find(std::forward<KeyType>(key));
         }
 
         return result;
@@ -30454,7 +30445,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
         if (is_object())
         {
-            result.m_it.object_iterator = m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key)));
+            result.m_it.object_iterator = m_data.m_value.object->find(std::forward<KeyType>(key));
         }
 
         return result;
@@ -30477,7 +30468,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     size_type count(KeyType && key) const
     {
         // return 0 for all nonobject types
-        return is_object() ? m_data.m_value.object->count(lookup_key(std::forward<KeyType>(key))) : 0;
+        return is_object() ? m_data.m_value.object->count(std::forward<KeyType>(key)) : 0;
     }
 
     /// @brief check the existence of an element in a JSON object
@@ -30495,7 +30486,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     JSON_HEDLEY_WARN_UNUSED_RESULT
     bool contains(KeyType && key) const
     {
-        return is_object() && m_data.m_value.object->find(lookup_key(std::forward<KeyType>(key))) != m_data.m_value.object->end();
+        return is_object() && m_data.m_value.object->find(std::forward<KeyType>(key)) != m_data.m_value.object->end();
     }
 
     /// @brief check the existence of an element in a JSON object given a JSON pointer

tests/src/unit-alt-string.cpp

@@ -13,6 +13,7 @@
 
 #include <cstdint>
 #include <string>
+#include <type_traits>
 #include <utility>
 #include <vector>
 
@@ -419,6 +420,41 @@ TEST_CASE("alternative string type")
         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);
+    }
+
     SECTION("strict enum")
     {
         // regression test for #5667: NLOHMANN_JSON_SERIALIZE_ENUM_STRICT's from_json

tests/src/unit-element_access2.cpp

@@ -1973,115 +1973,4 @@ TEST_CASE("operator[] with user-defined std::string_view-convertible types")
         }
     }
 }
-
-TEST_CASE("keys convertible to std::string_view work with all lookup functions (regression test for #5663)")
-{
-    // a key type convertible only to std::string_view: the case #4958 added
-    // support for, but only the non-const operator[] compiled with it
-    struct ViewKey
-    {
-        operator std::string_view() const
-        {
-            return "a";
-        }
-    };
-
-    // a key type convertible to both std::string and std::string_view: with
-    // 3.12.0, such a key worked with at, the const operator[], find, count and
-    // contains via the conversion to std::string; #4958 made the KeyType&&
-    // templates win overload resolution for it instead, and those then failed
-    struct DualKey
-    {
-        operator std::string() const
-        {
-            return "a";
-        }
-        operator std::string_view() const
-        {
-            return "a";
-        }
-    };
-
-    SECTION("nlohmann::json")
-    {
-        using json = nlohmann::json;
-
-        SECTION("ViewKey")
-        {
-            json j = {{"a", 1}};
-            const json& cj = j;
-
-            CHECK(j[ViewKey{}] == 1);
-            CHECK(cj[ViewKey{}] == 1);
-            CHECK(j.at(ViewKey{}) == 1);
-            CHECK(cj.at(ViewKey{}) == 1);
-            CHECK(j.find(ViewKey{}) != j.end());
-            CHECK(cj.find(ViewKey{}) != cj.end());
-            CHECK(j.count(ViewKey{}) == 1);
-            CHECK(j.contains(ViewKey{}));
-            CHECK(j.value(ViewKey{}, 0) == 1);
-            CHECK(j.erase(ViewKey{}) == 1);
-            CHECK(!j.contains("a"));
-        }
-
-        SECTION("DualKey")
-        {
-            json j = {{"a", 1}};
-            const json& cj = j;
-
-            CHECK(j[DualKey{}] == 1);
-            CHECK(cj[DualKey{}] == 1);
-            CHECK(j.at(DualKey{}) == 1);
-            CHECK(cj.at(DualKey{}) == 1);
-            CHECK(j.find(DualKey{}) != j.end());
-            CHECK(cj.find(DualKey{}) != cj.end());
-            CHECK(j.count(DualKey{}) == 1);
-            CHECK(j.contains(DualKey{}));
-            CHECK(j.value(DualKey{}, 0) == 1);
-            CHECK(j.erase(DualKey{}) == 1);
-            CHECK(!j.contains("a"));
-        }
-    }
-
-    SECTION("nlohmann::ordered_json")
-    {
-        using ordered_json = nlohmann::ordered_json;
-
-        SECTION("ViewKey")
-        {
-            ordered_json j = {{"a", 1}};
-            const ordered_json& cj = j;
-
-            CHECK(j[ViewKey{}] == 1);
-            CHECK(cj[ViewKey{}] == 1);
-            CHECK(j.at(ViewKey{}) == 1);
-            CHECK(cj.at(ViewKey{}) == 1);
-            CHECK(j.find(ViewKey{}) != j.end());
-            CHECK(cj.find(ViewKey{}) != cj.end());
-            CHECK(j.count(ViewKey{}) == 1);
-            CHECK(j.contains(ViewKey{}));
-            CHECK(j.value(ViewKey{}, 0) == 1);
-            CHECK(j.erase(ViewKey{}) == 1);
-            CHECK(!j.contains("a"));
-        }
-
-        SECTION("DualKey")
-        {
-            ordered_json j = {{"a", 1}};
-            const ordered_json& cj = j;
-
-            CHECK(j[DualKey{}] == 1);
-            CHECK(cj[DualKey{}] == 1);
-            CHECK(j.at(DualKey{}) == 1);
-            CHECK(cj.at(DualKey{}) == 1);
-            CHECK(j.find(DualKey{}) != j.end());
-            CHECK(cj.find(DualKey{}) != cj.end());
-            CHECK(j.count(DualKey{}) == 1);
-            CHECK(j.contains(DualKey{}));
-            CHECK(j.value(DualKey{}, 0) == 1);
-            CHECK(j.erase(DualKey{}) == 1);
-            CHECK(!j.contains("a"));
-        }
-    }
-}
 #endif