Merge branch 'develop' into claude/copy-without-assignable-base-5674

Conflicts:
- include/nlohmann/json.hpp: copy_array_level() keeps develop's #5721
  "set the array type only once the container exists" and then builds the
  elements with the PR's copy_construct_tag emplace_back loop
- single_include/nlohmann/json.hpp: regenerated with make amalgamate

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/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).

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
@@ -1022,19 +1004,39 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     using copy_scratch_value_t = std::pair<typename object_t::key_type, basic_json>;
     using copy_scratch_t = std::vector<copy_scratch_value_t, AllocatorType<copy_scratch_value_t>>;
 
-    /// @brief copy everything of @a src into @a dst but its type and value
-    static void copy_metadata(const basic_json& src, basic_json& dst)
-    {
-        // a custom base class is only required to be copy-constructible and
-        // move-assignable, so the copy has to go through a temporary
-        static_cast<json_base_class_t&>(dst) = json_base_class_t(static_cast<const json_base_class_t&>(src));
+    /// @brief tag selecting the constructor below; used only to build the
+    /// elements of a deep copy (@ref copy_array_level, @ref copy_object_level)
+    struct copy_construct_tag {};
 
+  public:
+    /*!
+    @brief construct a null value whose base class - and, with @ref
+           JSON_DIAGNOSTIC_POSITIONS, positions - are copied from @a src
+
+    Copy-constructing @ref json_base_class_t here, rather than default-
+    constructing the element and assigning its base class afterwards, means
+    that copying a @ref basic_json only ever requires a copy-constructible
+    base class, and never a move-assignable one as well.
+
+    @note this constructor has to be public: @ref copy_array_level and
+          @ref copy_object_level reach it through @ref array_t's or @ref
+          object_t's own emplace_back(), which constructs the element from
+          outside @ref basic_json and so cannot call a private constructor.
+          @ref copy_construct_tag is private, though, and nothing in the
+          public interface hands out a value of it, so outside code can still
+          never name it to call this constructor itself.
+    */
+    basic_json(copy_construct_tag /*unused*/, const basic_json& src)
+        : json_base_class_t(src)
 #if JSON_DIAGNOSTIC_POSITIONS
-        dst.start_position = src.start_position;
-        dst.end_position = src.end_position;
+        , start_position(src.start_position)
+        , end_position(src.end_position)
 #endif
+    {
     }
 
+  private:
+
     /*!
     @brief copy the value of @a src into @a dst, which must not be structured
 
@@ -1097,8 +1099,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     }
 
     /*!
-    @brief copy everything of @a src into the null value @a dst but the children
+    @brief finish the copy @a dst of @a src that a @ref copy_construct_tag
+           constructor started, other than the children of an object or array
 
+    @a dst already has @a src's base class and, with @ref
+    JSON_DIAGNOSTIC_POSITIONS, positions; only its value is still missing.
     Objects and arrays are not copied here; they are appended to @a worklist to
     be created later by @ref copy_iteratively. Until that happens, @a dst remains
     a null value, so that a partially built copy can be destroyed at any point
@@ -1106,8 +1111,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     */
     static void copy_shallow(const basic_json& src, basic_json& dst, copy_worklist_t& worklist)
     {
-        copy_metadata(src, dst);
-
         if (src.m_data.m_type == value_t::object || src.m_data.m_type == value_t::array)
         {
             // defer: dst stays a null value until its container exists
@@ -1128,15 +1131,19 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     {
         const array_t& src_array = *src.m_data.m_value.array;
 
-        // create all elements up front: growing the array afterwards could
-        // invalidate the pointers that are handed to the worklist; resize()
-        // rather than the fill constructor, because not every array type
-        // provides the latter (e.g., ones without a matching allocator-aware
-        // fill constructor)
         dst.m_data.m_value.array = create<array_t>();
         // only now that the array exists may dst stop being a null value
         dst.m_data.m_type = value_t::array;
-        dst.m_data.m_value.array->resize(src_array.size());
+
+        // create every element - its base class already copy-constructed from
+        // its counterpart in src, via the copy_construct_tag constructor -
+        // before any of their addresses are handed to worklist below: growing
+        // the array while that is going on could reallocate it and invalidate
+        // addresses taken from an earlier iteration
+        for (const auto& src_element : src_array)
+        {
+            dst.m_data.m_value.array->emplace_back(copy_construct_tag{}, src_element);
+        }
 
         auto dst_it = dst.m_data.m_value.array->begin();
         for (auto src_it = src_array.cbegin(); src_it != src_array.cend(); ++src_it, ++dst_it)
@@ -1154,12 +1161,14 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
         // build the complete key skeleton and hand it to the object's range
         // constructor: adding the keys one by one would be quadratic for object
-        // types that are backed by a vector, such as nlohmann::ordered_map
+        // types that are backed by a vector, such as nlohmann::ordered_map; each
+        // value's base class is already copy-constructed from its counterpart
+        // in src, via the copy_construct_tag constructor
         scratch.clear();
         scratch.reserve(src_object.size());
         for (const auto& element : src_object)
         {
-            scratch.emplace_back(element.first, basic_json());
+            scratch.emplace_back(element.first, basic_json(copy_construct_tag{}, element.second));
         }
 
         dst.m_data.m_value.object = create<object_t>(std::make_move_iterator(scratch.begin()),
@@ -2785,7 +2794,7 @@ 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 +2832,7 @@ 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 +2975,7 @@ 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 +2991,7 @@ 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 +3001,8 @@ 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 +3389,7 @@ 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 +3416,7 @@ 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 +3496,7 @@ 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 +3512,7 @@ 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 +3535,7 @@ 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 +3553,7 @@ 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;
 
@@ -27756,24 +27734,6 @@ 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
@@ -27971,19 +27931,39 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     using copy_scratch_value_t = std::pair<typename object_t::key_type, basic_json>;
     using copy_scratch_t = std::vector<copy_scratch_value_t, AllocatorType<copy_scratch_value_t>>;
 
-    /// @brief copy everything of @a src into @a dst but its type and value
-    static void copy_metadata(const basic_json& src, basic_json& dst)
-    {
-        // a custom base class is only required to be copy-constructible and
-        // move-assignable, so the copy has to go through a temporary
-        static_cast<json_base_class_t&>(dst) = json_base_class_t(static_cast<const json_base_class_t&>(src));
+    /// @brief tag selecting the constructor below; used only to build the
+    /// elements of a deep copy (@ref copy_array_level, @ref copy_object_level)
+    struct copy_construct_tag {};
 
+  public:
+    /*!
+    @brief construct a null value whose base class - and, with @ref
+           JSON_DIAGNOSTIC_POSITIONS, positions - are copied from @a src
+
+    Copy-constructing @ref json_base_class_t here, rather than default-
+    constructing the element and assigning its base class afterwards, means
+    that copying a @ref basic_json only ever requires a copy-constructible
+    base class, and never a move-assignable one as well.
+
+    @note this constructor has to be public: @ref copy_array_level and
+          @ref copy_object_level reach it through @ref array_t's or @ref
+          object_t's own emplace_back(), which constructs the element from
+          outside @ref basic_json and so cannot call a private constructor.
+          @ref copy_construct_tag is private, though, and nothing in the
+          public interface hands out a value of it, so outside code can still
+          never name it to call this constructor itself.
+    */
+    basic_json(copy_construct_tag /*unused*/, const basic_json& src)
+        : json_base_class_t(src)
 #if JSON_DIAGNOSTIC_POSITIONS
-        dst.start_position = src.start_position;
-        dst.end_position = src.end_position;
+        , start_position(src.start_position)
+        , end_position(src.end_position)
 #endif
+    {
     }
 
+  private:
+
     /*!
     @brief copy the value of @a src into @a dst, which must not be structured
 
@@ -28046,8 +28026,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     }
 
     /*!
-    @brief copy everything of @a src into the null value @a dst but the children
+    @brief finish the copy @a dst of @a src that a @ref copy_construct_tag
+           constructor started, other than the children of an object or array
 
+    @a dst already has @a src's base class and, with @ref
+    JSON_DIAGNOSTIC_POSITIONS, positions; only its value is still missing.
     Objects and arrays are not copied here; they are appended to @a worklist to
     be created later by @ref copy_iteratively. Until that happens, @a dst remains
     a null value, so that a partially built copy can be destroyed at any point
@@ -28055,8 +28038,6 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     */
     static void copy_shallow(const basic_json& src, basic_json& dst, copy_worklist_t& worklist)
     {
-        copy_metadata(src, dst);
-
         if (src.m_data.m_type == value_t::object || src.m_data.m_type == value_t::array)
         {
             // defer: dst stays a null value until its container exists
@@ -28077,15 +28058,19 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     {
         const array_t& src_array = *src.m_data.m_value.array;
 
-        // create all elements up front: growing the array afterwards could
-        // invalidate the pointers that are handed to the worklist; resize()
-        // rather than the fill constructor, because not every array type
-        // provides the latter (e.g., ones without a matching allocator-aware
-        // fill constructor)
         dst.m_data.m_value.array = create<array_t>();
         // only now that the array exists may dst stop being a null value
         dst.m_data.m_type = value_t::array;
-        dst.m_data.m_value.array->resize(src_array.size());
+
+        // create every element - its base class already copy-constructed from
+        // its counterpart in src, via the copy_construct_tag constructor -
+        // before any of their addresses are handed to worklist below: growing
+        // the array while that is going on could reallocate it and invalidate
+        // addresses taken from an earlier iteration
+        for (const auto& src_element : src_array)
+        {
+            dst.m_data.m_value.array->emplace_back(copy_construct_tag{}, src_element);
+        }
 
         auto dst_it = dst.m_data.m_value.array->begin();
         for (auto src_it = src_array.cbegin(); src_it != src_array.cend(); ++src_it, ++dst_it)
@@ -28103,12 +28088,14 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 
         // build the complete key skeleton and hand it to the object's range
         // constructor: adding the keys one by one would be quadratic for object
-        // types that are backed by a vector, such as nlohmann::ordered_map
+        // types that are backed by a vector, such as nlohmann::ordered_map; each
+        // value's base class is already copy-constructed from its counterpart
+        // in src, via the copy_construct_tag constructor
         scratch.clear();
         scratch.reserve(src_object.size());
         for (const auto& element : src_object)
         {
-            scratch.emplace_back(element.first, basic_json());
+            scratch.emplace_back(element.first, basic_json(copy_construct_tag{}, element.second));
         }
 
         dst.m_data.m_value.object = create<object_t>(std::make_move_iterator(scratch.begin()),
@@ -29734,7 +29721,7 @@ 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 +29759,7 @@ 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 +29902,7 @@ 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 +29918,7 @@ 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 +29928,8 @@ 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 +30316,7 @@ 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 +30343,7 @@ 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 +30423,7 @@ 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 +30439,7 @@ 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 +30462,7 @@ 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 +30480,7 @@ 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-custom-base-class.cpp

@@ -405,3 +405,73 @@ TEST_CASE("JSON Visit Node")
         );
     CHECK(expected.empty());
 }
+
+// A custom base class with a const member: copy-constructible (initializing a
+// const member works fine), but not copy-/move-assignable (assigning one does
+// not). Used to check that copy construction never requires more than that.
+struct const_member_base
+{
+    const int id = 7; // NOLINT(misc-non-private-member-variables-in-classes)
+};
+
+using json_with_const_base = nlohmann::basic_json <
+                             std::map,
+                             std::vector,
+                             std::string,
+                             bool,
+                             std::int64_t,
+                             std::uint64_t,
+                             double,
+                             std::allocator,
+                             nlohmann::adl_serializer,
+                             std::vector<std::uint8_t>,
+                             const_member_base
+                             >;
+
+// build an array nested @a depth levels deep, with the innermost value 1;
+// every level is constructed (never assigned), since const_member_base does
+// not support assignment
+static json_with_const_base make_nested_array(std::size_t depth)
+{
+    if (depth == 0)
+    {
+        return json_with_const_base(1);
+    }
+    return json_with_const_base::array({make_nested_array(depth - 1)});
+}
+
+TEST_CASE("Regression test for issue #5674 - copy construction must not require an assignable base class")
+{
+    SECTION("depth 0")
+    {
+        // as in the original bug report: copy construction only, no assignment
+        const json_with_const_base j = {1, 2};
+        const json_with_const_base copy = j; // NOLINT(performance-unnecessary-copy-initialization)
+
+        CHECK(copy.size() == 2);
+        CHECK(copy.id == 7);
+    }
+
+    SECTION("nested deeper than the copy constructor's descent bound")
+    {
+        // beyond nesting_depth_limit() (128) levels, the copy constructor
+        // copies without the call stack (copy_iteratively / copy_array_level),
+        // which used to assign the base class of every element it created
+        const std::size_t depth = 300;
+
+        const json_with_const_base j = make_nested_array(depth);
+        const json_with_const_base copy = j; // NOLINT(performance-unnecessary-copy-initialization)
+
+        const json_with_const_base* c = &copy;
+        for (std::size_t level = 0; level <= depth; ++level)
+        {
+            CAPTURE(level)
+            REQUIRE(c->id == 7);
+            if (level < depth)
+            {
+                c = &c->at(0);
+            }
+        }
+        CHECK(*c == 1);
+    }
+}

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