add new is_constructible_* traits used in from_json

is_compatible_* traits were used in from_json, but it made no sense
whatsoever.

It used to work because of non-SFINAE correctness + json_ref
unconstrained variadic template constructor.

SFINAE checks are becoming quite complex, we need a specification of
some sort describing:

* which concepts the library uses
* how the conversion to/from json works in detail

Having such a specification would really help simplifying the current
code (as well as having meaningful checks).

Fixes !1299

include/nlohmann/detail/conversions/from_json.hpp

@@ -84,13 +84,13 @@ void from_json(const BasicJsonType& j, typename BasicJsonType::string_t& s)
 }
 
 template <
-    typename BasicJsonType, typename CompatibleStringType,
+    typename BasicJsonType, typename ConstructibleStringType,
     enable_if_t <
-        is_compatible_string_type<BasicJsonType, CompatibleStringType>::value and
+        is_constructible_string_type<BasicJsonType, ConstructibleStringType>::value and
         not std::is_same<typename BasicJsonType::string_t,
-                         CompatibleStringType>::value,
+                         ConstructibleStringType>::value,
         int > = 0 >
-void from_json(const BasicJsonType& j, CompatibleStringType& s)
+void from_json(const BasicJsonType& j, ConstructibleStringType& s)
 {
     if (JSON_UNLIKELY(not j.is_string()))
     {
@@ -173,11 +173,11 @@ auto from_json_array_impl(const BasicJsonType& j, std::array<T, N>& arr,
     }
 }
 
-template<typename BasicJsonType, typename CompatibleArrayType>
+template<typename BasicJsonType, typename ConstructibleArrayType>
-auto from_json_array_impl(const BasicJsonType& j, CompatibleArrayType& arr, priority_tag<1> /*unused*/)
+auto from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr, priority_tag<1> /*unused*/)
 -> decltype(
-    arr.reserve(std::declval<typename CompatibleArrayType::size_type>()),
+    arr.reserve(std::declval<typename ConstructibleArrayType::size_type>()),
-    j.template get<typename CompatibleArrayType::value_type>(),
+    j.template get<typename ConstructibleArrayType::value_type>(),
     void())
 {
     using std::end;
@@ -188,12 +188,12 @@ auto from_json_array_impl(const BasicJsonType& j, CompatibleArrayType& arr, prio
     {
         // get<BasicJsonType>() returns *this, this won't call a from_json
         // method when value_type is BasicJsonType
-        return i.template get<typename CompatibleArrayType::value_type>();
+        return i.template get<typename ConstructibleArrayType::value_type>();
     });
 }
 
-template <typename BasicJsonType, typename CompatibleArrayType>
+template <typename BasicJsonType, typename ConstructibleArrayType>
-void from_json_array_impl(const BasicJsonType& j, CompatibleArrayType& arr,
+void from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr,
                           priority_tag<0> /*unused*/)
 {
     using std::end;
@@ -204,21 +204,21 @@ void from_json_array_impl(const BasicJsonType& j, CompatibleArrayType& arr,
     {
         // get<BasicJsonType>() returns *this, this won't call a from_json
         // method when value_type is BasicJsonType
-        return i.template get<typename CompatibleArrayType::value_type>();
+        return i.template get<typename ConstructibleArrayType::value_type>();
     });
 }
 
-template <typename BasicJsonType, typename CompatibleArrayType,
+template <typename BasicJsonType, typename ConstructibleArrayType,
           enable_if_t <
-              is_compatible_array_type<BasicJsonType, CompatibleArrayType>::value and
+              is_constructible_array_type<BasicJsonType, ConstructibleArrayType>::value and
-              not is_compatible_object_type<BasicJsonType, CompatibleArrayType>::value and
+              not is_constructible_object_type<BasicJsonType, ConstructibleArrayType>::value and
-              not is_compatible_string_type<BasicJsonType, CompatibleArrayType>::value and
+              not is_constructible_string_type<BasicJsonType, ConstructibleArrayType>::value and
-              not is_basic_json<CompatibleArrayType>::value,
+              not is_basic_json<ConstructibleArrayType>::value,
               int > = 0 >
 
-auto from_json(const BasicJsonType& j, CompatibleArrayType& arr)
+auto from_json(const BasicJsonType& j, ConstructibleArrayType& arr)
 -> decltype(from_json_array_impl(j, arr, priority_tag<3> {}),
-j.template get<typename CompatibleArrayType::value_type>(),
+j.template get<typename ConstructibleArrayType::value_type>(),
 void())
 {
     if (JSON_UNLIKELY(not j.is_array()))
@@ -230,9 +230,9 @@ void())
     from_json_array_impl(j, arr, priority_tag<3> {});
 }
 
-template<typename BasicJsonType, typename CompatibleObjectType,
+template<typename BasicJsonType, typename ConstructibleObjectType,
-         enable_if_t<is_compatible_object_type<BasicJsonType, CompatibleObjectType>::value, int> = 0>
+         enable_if_t<is_constructible_object_type<BasicJsonType, ConstructibleObjectType>::value, int> = 0>
-void from_json(const BasicJsonType& j, CompatibleObjectType& obj)
+void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
 {
     if (JSON_UNLIKELY(not j.is_object()))
     {
@@ -240,13 +240,13 @@ void from_json(const BasicJsonType& j, CompatibleObjectType& obj)
     }
 
     auto inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
-    using value_type = typename CompatibleObjectType::value_type;
+    using value_type = typename ConstructibleObjectType::value_type;
     std::transform(
         inner_object->begin(), inner_object->end(),
         std::inserter(obj, obj.begin()),
         [](typename BasicJsonType::object_t::value_type const & p)
     {
-        return value_type(p.first, p.second.template get<typename CompatibleObjectType::mapped_type>());
+        return value_type(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
     });
 }
 

include/nlohmann/detail/meta/type_traits.hpp

@@ -68,6 +68,52 @@ using from_json_function = decltype(T::from_json(std::declval<Args>()...));
 template <typename T, typename U>
 using get_template_function = decltype(std::declval<T>().template get<U>());
 
+// trait checking if JSONSerializer<T>::from_json(json const&, udt&) exists
+template <typename BasicJsonType, typename T, typename = void>
+struct has_from_json : std::false_type {};
+
+template <typename BasicJsonType, typename T>
+struct has_from_json<BasicJsonType, T,
+           enable_if_t<not is_basic_json<T>::value>>
+{
+    using serializer = typename BasicJsonType::template json_serializer<T, void>;
+
+    static constexpr bool value =
+        is_detected_exact<void, from_json_function, serializer,
+        const BasicJsonType&, T&>::value;
+};
+
+// This trait checks if JSONSerializer<T>::from_json(json const&) exists
+// this overload is used for non-default-constructible user-defined-types
+template <typename BasicJsonType, typename T, typename = void>
+struct has_non_default_from_json : std::false_type {};
+
+template<typename BasicJsonType, typename T>
+struct has_non_default_from_json<BasicJsonType, T, enable_if_t<not is_basic_json<T>::value>>
+{
+    using serializer = typename BasicJsonType::template json_serializer<T, void>;
+
+    static constexpr bool value =
+        is_detected_exact<T, from_json_function, serializer,
+        const BasicJsonType&>::value;
+};
+
+// This trait checks if BasicJsonType::json_serializer<T>::to_json exists
+// Do not evaluate the trait when T is a basic_json type, to avoid template instantiation infinite recursion.
+template <typename BasicJsonType, typename T, typename = void>
+struct has_to_json : std::false_type {};
+
+template <typename BasicJsonType, typename T>
+struct has_to_json<BasicJsonType, T, enable_if_t<not is_basic_json<T>::value>>
+{
+    using serializer = typename BasicJsonType::template json_serializer<T, void>;
+
+    static constexpr bool value =
+        is_detected_exact<void, to_json_function, serializer, BasicJsonType&,
+        T>::value;
+};
+
+
 ///////////////////
 // is_ functions //
 ///////////////////
@@ -123,6 +169,35 @@ template <typename BasicJsonType, typename CompatibleObjectType>
 struct is_compatible_object_type
     : is_compatible_object_type_impl<BasicJsonType, CompatibleObjectType> {};
 
+template <typename BasicJsonType, typename ConstructibleObjectType,
+          typename = void>
+struct is_constructible_object_type_impl : std::false_type {};
+
+template <typename BasicJsonType, typename ConstructibleObjectType>
+struct is_constructible_object_type_impl <
+    BasicJsonType, ConstructibleObjectType,
+    enable_if_t<is_detected<mapped_type_t, ConstructibleObjectType>::value and
+    is_detected<key_type_t, ConstructibleObjectType>::value >>
+{
+    using object_t = typename BasicJsonType::object_t;
+
+    static constexpr bool value =
+        std::is_constructible<typename ConstructibleObjectType::key_type,
+        typename object_t::key_type>::value and
+        std::is_same<typename object_t::mapped_type,
+        typename ConstructibleObjectType::mapped_type>::value or
+        (has_from_json<BasicJsonType,
+         typename ConstructibleObjectType::mapped_type>::value or
+         has_non_default_from_json <
+         BasicJsonType,
+         typename ConstructibleObjectType::mapped_type >::value);
+};
+
+template <typename BasicJsonType, typename ConstructibleObjectType>
+struct is_constructible_object_type
+    : is_constructible_object_type_impl<BasicJsonType,
+      ConstructibleObjectType> {};
+
 template <typename BasicJsonType, typename CompatibleStringType,
           typename = void>
 struct is_compatible_string_type_impl : std::false_type {};
@@ -137,9 +212,28 @@ struct is_compatible_string_type_impl <
         std::is_constructible<typename BasicJsonType::string_t, CompatibleStringType>::value;
 };
 
-template <typename BasicJsonType, typename CompatibleStringType>
+template <typename BasicJsonType, typename ConstrutibleStringType>
 struct is_compatible_string_type
-    : is_compatible_string_type_impl<BasicJsonType, CompatibleStringType> {};
+    : is_compatible_string_type_impl<BasicJsonType, ConstrutibleStringType> {};
+
+template <typename BasicJsonType, typename ConstrutibleStringType,
+          typename = void>
+struct is_constructible_string_type_impl : std::false_type {};
+
+template <typename BasicJsonType, typename ConstrutibleStringType>
+struct is_constructible_string_type_impl <
+    BasicJsonType, ConstrutibleStringType,
+    enable_if_t<is_detected_exact<typename BasicJsonType::string_t::value_type,
+    value_type_t, ConstrutibleStringType>::value >>
+{
+    static constexpr auto value =
+        std::is_constructible<ConstrutibleStringType,
+        typename BasicJsonType::string_t>::value;
+};
+
+template <typename BasicJsonType, typename ConstrutibleStringType>
+struct is_constructible_string_type
+    : is_constructible_string_type_impl<BasicJsonType, ConstrutibleStringType> {};
 
 template <typename BasicJsonType, typename CompatibleArrayType, typename = void>
 struct is_compatible_array_type_impl : std::false_type {};
@@ -148,18 +242,61 @@ template <typename BasicJsonType, typename CompatibleArrayType>
 struct is_compatible_array_type_impl <
     BasicJsonType, CompatibleArrayType,
     enable_if_t<is_detected<value_type_t, CompatibleArrayType>::value and
-    is_detected<iterator_t, CompatibleArrayType>::value >>
+    is_detected<iterator_t, CompatibleArrayType>::value and
-{
 // This is needed because json_reverse_iterator has a ::iterator type...
 // Therefore it is detected as a CompatibleArrayType.
 // The real fix would be to have an Iterable concept.
-    static constexpr bool value = not is_iterator_traits<std::iterator_traits<CompatibleArrayType>>::value;
+    not is_iterator_traits<
+    std::iterator_traits<CompatibleArrayType>>::value >>
+{
+    static constexpr bool value =
+        std::is_constructible<BasicJsonType,
+        typename CompatibleArrayType::value_type>::value;
 };
 
 template <typename BasicJsonType, typename CompatibleArrayType>
 struct is_compatible_array_type
     : is_compatible_array_type_impl<BasicJsonType, CompatibleArrayType> {};
 
+template <typename BasicJsonType, typename ConstructibleArrayType, typename = void>
+struct is_constructible_array_type_impl : std::false_type {};
+
+template <typename BasicJsonType, typename ConstructibleArrayType>
+struct is_constructible_array_type_impl <
+    BasicJsonType, ConstructibleArrayType,
+    enable_if_t<std::is_same<ConstructibleArrayType,
+    typename BasicJsonType::value_type>::value >>
+            : std::true_type {};
+
+template <typename BasicJsonType, typename ConstructibleArrayType>
+struct is_constructible_array_type_impl <
+    BasicJsonType, ConstructibleArrayType,
+    enable_if_t<not std::is_same<ConstructibleArrayType,
+    typename BasicJsonType::value_type>::value and
+    is_detected<value_type_t, ConstructibleArrayType>::value and
+    is_detected<iterator_t, ConstructibleArrayType>::value and
+    is_complete_type<
+    detected_t<value_type_t, ConstructibleArrayType>>::value >>
+{
+    static constexpr bool value =
+        // This is needed because json_reverse_iterator has a ::iterator type,
+        // furthermore, std::back_insert_iterator (and other iterators) have a base class `iterator`...
+        // Therefore it is detected as a ConstructibleArrayType.
+        // The real fix would be to have an Iterable concept.
+        not is_iterator_traits <
+        std::iterator_traits<ConstructibleArrayType >>::value and
+
+        (std::is_same<typename ConstructibleArrayType::value_type, typename BasicJsonType::array_t::value_type>::value or
+         has_from_json<BasicJsonType,
+         typename ConstructibleArrayType::value_type>::value or
+         has_non_default_from_json <
+         BasicJsonType, typename ConstructibleArrayType::value_type >::value);
+};
+
+template <typename BasicJsonType, typename ConstructibleArrayType>
+struct is_constructible_array_type
+    : is_constructible_array_type_impl<BasicJsonType, ConstructibleArrayType> {};
+
 template <typename RealIntegerType, typename CompatibleNumberIntegerType,
           typename = void>
 struct is_compatible_integer_type_impl : std::false_type {};
@@ -187,51 +324,6 @@ struct is_compatible_integer_type
     : is_compatible_integer_type_impl<RealIntegerType,
       CompatibleNumberIntegerType> {};
 
-// trait checking if JSONSerializer<T>::from_json(json const&, udt&) exists
-template <typename BasicJsonType, typename T, typename = void>
-struct has_from_json : std::false_type {};
-
-template <typename BasicJsonType, typename T>
-struct has_from_json<BasicJsonType, T,
-           enable_if_t<not is_basic_json<T>::value>>
-{
-    using serializer = typename BasicJsonType::template json_serializer<T, void>;
-
-    static constexpr bool value =
-        is_detected_exact<void, from_json_function, serializer,
-        const BasicJsonType&, T&>::value;
-};
-
-// This trait checks if JSONSerializer<T>::from_json(json const&) exists
-// this overload is used for non-default-constructible user-defined-types
-template <typename BasicJsonType, typename T, typename = void>
-struct has_non_default_from_json : std::false_type {};
-
-template<typename BasicJsonType, typename T>
-struct has_non_default_from_json<BasicJsonType, T, enable_if_t<not is_basic_json<T>::value>>
-{
-    using serializer = typename BasicJsonType::template json_serializer<T, void>;
-
-    static constexpr bool value =
-        is_detected_exact<T, from_json_function, serializer,
-        const BasicJsonType&>::value;
-};
-
-// This trait checks if BasicJsonType::json_serializer<T>::to_json exists
-// Do not evaluate the trait when T is a basic_json type, to avoid template instantiation infinite recursion.
-template <typename BasicJsonType, typename T, typename = void>
-struct has_to_json : std::false_type {};
-
-template <typename BasicJsonType, typename T>
-struct has_to_json<BasicJsonType, T, enable_if_t<not is_basic_json<T>::value>>
-{
-    using serializer = typename BasicJsonType::template json_serializer<T, void>;
-
-    static constexpr bool value =
-        is_detected_exact<void, to_json_function, serializer, BasicJsonType&,
-        T>::value;
-};
-
 template <typename BasicJsonType, typename CompatibleType, typename = void>
 struct is_compatible_type_impl: std::false_type {};
 

single_include/nlohmann/json.hpp

@@ -428,6 +428,52 @@ using from_json_function = decltype(T::from_json(std::declval<Args>()...));
 template <typename T, typename U>
 using get_template_function = decltype(std::declval<T>().template get<U>());
 
+// trait checking if JSONSerializer<T>::from_json(json const&, udt&) exists
+template <typename BasicJsonType, typename T, typename = void>
+struct has_from_json : std::false_type {};
+
+template <typename BasicJsonType, typename T>
+struct has_from_json<BasicJsonType, T,
+           enable_if_t<not is_basic_json<T>::value>>
+{
+    using serializer = typename BasicJsonType::template json_serializer<T, void>;
+
+    static constexpr bool value =
+        is_detected_exact<void, from_json_function, serializer,
+        const BasicJsonType&, T&>::value;
+};
+
+// This trait checks if JSONSerializer<T>::from_json(json const&) exists
+// this overload is used for non-default-constructible user-defined-types
+template <typename BasicJsonType, typename T, typename = void>
+struct has_non_default_from_json : std::false_type {};
+
+template<typename BasicJsonType, typename T>
+struct has_non_default_from_json<BasicJsonType, T, enable_if_t<not is_basic_json<T>::value>>
+{
+    using serializer = typename BasicJsonType::template json_serializer<T, void>;
+
+    static constexpr bool value =
+        is_detected_exact<T, from_json_function, serializer,
+        const BasicJsonType&>::value;
+};
+
+// This trait checks if BasicJsonType::json_serializer<T>::to_json exists
+// Do not evaluate the trait when T is a basic_json type, to avoid template instantiation infinite recursion.
+template <typename BasicJsonType, typename T, typename = void>
+struct has_to_json : std::false_type {};
+
+template <typename BasicJsonType, typename T>
+struct has_to_json<BasicJsonType, T, enable_if_t<not is_basic_json<T>::value>>
+{
+    using serializer = typename BasicJsonType::template json_serializer<T, void>;
+
+    static constexpr bool value =
+        is_detected_exact<void, to_json_function, serializer, BasicJsonType&,
+        T>::value;
+};
+
+
 ///////////////////
 // is_ functions //
 ///////////////////
@@ -483,6 +529,35 @@ template <typename BasicJsonType, typename CompatibleObjectType>
 struct is_compatible_object_type
     : is_compatible_object_type_impl<BasicJsonType, CompatibleObjectType> {};
 
+template <typename BasicJsonType, typename ConstructibleObjectType,
+          typename = void>
+struct is_constructible_object_type_impl : std::false_type {};
+
+template <typename BasicJsonType, typename ConstructibleObjectType>
+struct is_constructible_object_type_impl <
+    BasicJsonType, ConstructibleObjectType,
+    enable_if_t<is_detected<mapped_type_t, ConstructibleObjectType>::value and
+    is_detected<key_type_t, ConstructibleObjectType>::value >>
+{
+    using object_t = typename BasicJsonType::object_t;
+
+    static constexpr bool value =
+        std::is_constructible<typename ConstructibleObjectType::key_type,
+        typename object_t::key_type>::value and
+        std::is_same<typename object_t::mapped_type,
+        typename ConstructibleObjectType::mapped_type>::value or
+        (has_from_json<BasicJsonType,
+         typename ConstructibleObjectType::mapped_type>::value or
+         has_non_default_from_json <
+         BasicJsonType,
+         typename ConstructibleObjectType::mapped_type >::value);
+};
+
+template <typename BasicJsonType, typename ConstructibleObjectType>
+struct is_constructible_object_type
+    : is_constructible_object_type_impl<BasicJsonType,
+      ConstructibleObjectType> {};
+
 template <typename BasicJsonType, typename CompatibleStringType,
           typename = void>
 struct is_compatible_string_type_impl : std::false_type {};
@@ -497,9 +572,28 @@ struct is_compatible_string_type_impl <
         std::is_constructible<typename BasicJsonType::string_t, CompatibleStringType>::value;
 };
 
-template <typename BasicJsonType, typename CompatibleStringType>
+template <typename BasicJsonType, typename ConstrutibleStringType>
 struct is_compatible_string_type
-    : is_compatible_string_type_impl<BasicJsonType, CompatibleStringType> {};
+    : is_compatible_string_type_impl<BasicJsonType, ConstrutibleStringType> {};
+
+template <typename BasicJsonType, typename ConstrutibleStringType,
+          typename = void>
+struct is_constructible_string_type_impl : std::false_type {};
+
+template <typename BasicJsonType, typename ConstrutibleStringType>
+struct is_constructible_string_type_impl <
+    BasicJsonType, ConstrutibleStringType,
+    enable_if_t<is_detected_exact<typename BasicJsonType::string_t::value_type,
+    value_type_t, ConstrutibleStringType>::value >>
+{
+    static constexpr auto value =
+        std::is_constructible<ConstrutibleStringType,
+        typename BasicJsonType::string_t>::value;
+};
+
+template <typename BasicJsonType, typename ConstrutibleStringType>
+struct is_constructible_string_type
+    : is_constructible_string_type_impl<BasicJsonType, ConstrutibleStringType> {};
 
 template <typename BasicJsonType, typename CompatibleArrayType, typename = void>
 struct is_compatible_array_type_impl : std::false_type {};
@@ -508,18 +602,61 @@ template <typename BasicJsonType, typename CompatibleArrayType>
 struct is_compatible_array_type_impl <
     BasicJsonType, CompatibleArrayType,
     enable_if_t<is_detected<value_type_t, CompatibleArrayType>::value and
-    is_detected<iterator_t, CompatibleArrayType>::value >>
+    is_detected<iterator_t, CompatibleArrayType>::value and
-{
 // This is needed because json_reverse_iterator has a ::iterator type...
 // Therefore it is detected as a CompatibleArrayType.
 // The real fix would be to have an Iterable concept.
-    static constexpr bool value = not is_iterator_traits<std::iterator_traits<CompatibleArrayType>>::value;
+    not is_iterator_traits<
+    std::iterator_traits<CompatibleArrayType>>::value >>
+{
+    static constexpr bool value =
+        std::is_constructible<BasicJsonType,
+        typename CompatibleArrayType::value_type>::value;
 };
 
 template <typename BasicJsonType, typename CompatibleArrayType>
 struct is_compatible_array_type
     : is_compatible_array_type_impl<BasicJsonType, CompatibleArrayType> {};
 
+template <typename BasicJsonType, typename ConstructibleArrayType, typename = void>
+struct is_constructible_array_type_impl : std::false_type {};
+
+template <typename BasicJsonType, typename ConstructibleArrayType>
+struct is_constructible_array_type_impl <
+    BasicJsonType, ConstructibleArrayType,
+    enable_if_t<std::is_same<ConstructibleArrayType,
+    typename BasicJsonType::value_type>::value >>
+            : std::true_type {};
+
+template <typename BasicJsonType, typename ConstructibleArrayType>
+struct is_constructible_array_type_impl <
+    BasicJsonType, ConstructibleArrayType,
+    enable_if_t<not std::is_same<ConstructibleArrayType,
+    typename BasicJsonType::value_type>::value and
+    is_detected<value_type_t, ConstructibleArrayType>::value and
+    is_detected<iterator_t, ConstructibleArrayType>::value and
+    is_complete_type<
+    detected_t<value_type_t, ConstructibleArrayType>>::value >>
+{
+    static constexpr bool value =
+        // This is needed because json_reverse_iterator has a ::iterator type,
+        // furthermore, std::back_insert_iterator (and other iterators) have a base class `iterator`...
+        // Therefore it is detected as a ConstructibleArrayType.
+        // The real fix would be to have an Iterable concept.
+        not is_iterator_traits <
+        std::iterator_traits<ConstructibleArrayType >>::value and
+
+        (std::is_same<typename ConstructibleArrayType::value_type, typename BasicJsonType::array_t::value_type>::value or
+         has_from_json<BasicJsonType,
+         typename ConstructibleArrayType::value_type>::value or
+         has_non_default_from_json <
+         BasicJsonType, typename ConstructibleArrayType::value_type >::value);
+};
+
+template <typename BasicJsonType, typename ConstructibleArrayType>
+struct is_constructible_array_type
+    : is_constructible_array_type_impl<BasicJsonType, ConstructibleArrayType> {};
+
 template <typename RealIntegerType, typename CompatibleNumberIntegerType,
           typename = void>
 struct is_compatible_integer_type_impl : std::false_type {};
@@ -547,51 +684,6 @@ struct is_compatible_integer_type
     : is_compatible_integer_type_impl<RealIntegerType,
       CompatibleNumberIntegerType> {};
 
-// trait checking if JSONSerializer<T>::from_json(json const&, udt&) exists
-template <typename BasicJsonType, typename T, typename = void>
-struct has_from_json : std::false_type {};
-
-template <typename BasicJsonType, typename T>
-struct has_from_json<BasicJsonType, T,
-           enable_if_t<not is_basic_json<T>::value>>
-{
-    using serializer = typename BasicJsonType::template json_serializer<T, void>;
-
-    static constexpr bool value =
-        is_detected_exact<void, from_json_function, serializer,
-        const BasicJsonType&, T&>::value;
-};
-
-// This trait checks if JSONSerializer<T>::from_json(json const&) exists
-// this overload is used for non-default-constructible user-defined-types
-template <typename BasicJsonType, typename T, typename = void>
-struct has_non_default_from_json : std::false_type {};
-
-template<typename BasicJsonType, typename T>
-struct has_non_default_from_json<BasicJsonType, T, enable_if_t<not is_basic_json<T>::value>>
-{
-    using serializer = typename BasicJsonType::template json_serializer<T, void>;
-
-    static constexpr bool value =
-        is_detected_exact<T, from_json_function, serializer,
-        const BasicJsonType&>::value;
-};
-
-// This trait checks if BasicJsonType::json_serializer<T>::to_json exists
-// Do not evaluate the trait when T is a basic_json type, to avoid template instantiation infinite recursion.
-template <typename BasicJsonType, typename T, typename = void>
-struct has_to_json : std::false_type {};
-
-template <typename BasicJsonType, typename T>
-struct has_to_json<BasicJsonType, T, enable_if_t<not is_basic_json<T>::value>>
-{
-    using serializer = typename BasicJsonType::template json_serializer<T, void>;
-
-    static constexpr bool value =
-        is_detected_exact<void, to_json_function, serializer, BasicJsonType&,
-        T>::value;
-};
-
 template <typename BasicJsonType, typename CompatibleType, typename = void>
 struct is_compatible_type_impl: std::false_type {};
 
@@ -1156,13 +1248,13 @@ void from_json(const BasicJsonType& j, typename BasicJsonType::string_t& s)
 }
 
 template <
-    typename BasicJsonType, typename CompatibleStringType,
+    typename BasicJsonType, typename ConstructibleStringType,
     enable_if_t <
-        is_compatible_string_type<BasicJsonType, CompatibleStringType>::value and
+        is_constructible_string_type<BasicJsonType, ConstructibleStringType>::value and
         not std::is_same<typename BasicJsonType::string_t,
-                         CompatibleStringType>::value,
+                         ConstructibleStringType>::value,
         int > = 0 >
-void from_json(const BasicJsonType& j, CompatibleStringType& s)
+void from_json(const BasicJsonType& j, ConstructibleStringType& s)
 {
     if (JSON_UNLIKELY(not j.is_string()))
     {
@@ -1245,11 +1337,11 @@ auto from_json_array_impl(const BasicJsonType& j, std::array<T, N>& arr,
     }
 }
 
-template<typename BasicJsonType, typename CompatibleArrayType>
+template<typename BasicJsonType, typename ConstructibleArrayType>
-auto from_json_array_impl(const BasicJsonType& j, CompatibleArrayType& arr, priority_tag<1> /*unused*/)
+auto from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr, priority_tag<1> /*unused*/)
 -> decltype(
-    arr.reserve(std::declval<typename CompatibleArrayType::size_type>()),
+    arr.reserve(std::declval<typename ConstructibleArrayType::size_type>()),
-    j.template get<typename CompatibleArrayType::value_type>(),
+    j.template get<typename ConstructibleArrayType::value_type>(),
     void())
 {
     using std::end;
@@ -1260,12 +1352,12 @@ auto from_json_array_impl(const BasicJsonType& j, CompatibleArrayType& arr, prio
     {
         // get<BasicJsonType>() returns *this, this won't call a from_json
         // method when value_type is BasicJsonType
-        return i.template get<typename CompatibleArrayType::value_type>();
+        return i.template get<typename ConstructibleArrayType::value_type>();
     });
 }
 
-template <typename BasicJsonType, typename CompatibleArrayType>
+template <typename BasicJsonType, typename ConstructibleArrayType>
-void from_json_array_impl(const BasicJsonType& j, CompatibleArrayType& arr,
+void from_json_array_impl(const BasicJsonType& j, ConstructibleArrayType& arr,
                           priority_tag<0> /*unused*/)
 {
     using std::end;
@@ -1276,21 +1368,21 @@ void from_json_array_impl(const BasicJsonType& j, CompatibleArrayType& arr,
     {
         // get<BasicJsonType>() returns *this, this won't call a from_json
         // method when value_type is BasicJsonType
-        return i.template get<typename CompatibleArrayType::value_type>();
+        return i.template get<typename ConstructibleArrayType::value_type>();
     });
 }
 
-template <typename BasicJsonType, typename CompatibleArrayType,
+template <typename BasicJsonType, typename ConstructibleArrayType,
           enable_if_t <
-              is_compatible_array_type<BasicJsonType, CompatibleArrayType>::value and
+              is_constructible_array_type<BasicJsonType, ConstructibleArrayType>::value and
-              not is_compatible_object_type<BasicJsonType, CompatibleArrayType>::value and
+              not is_constructible_object_type<BasicJsonType, ConstructibleArrayType>::value and
-              not is_compatible_string_type<BasicJsonType, CompatibleArrayType>::value and
+              not is_constructible_string_type<BasicJsonType, ConstructibleArrayType>::value and
-              not is_basic_json<CompatibleArrayType>::value,
+              not is_basic_json<ConstructibleArrayType>::value,
               int > = 0 >
 
-auto from_json(const BasicJsonType& j, CompatibleArrayType& arr)
+auto from_json(const BasicJsonType& j, ConstructibleArrayType& arr)
 -> decltype(from_json_array_impl(j, arr, priority_tag<3> {}),
-j.template get<typename CompatibleArrayType::value_type>(),
+j.template get<typename ConstructibleArrayType::value_type>(),
 void())
 {
     if (JSON_UNLIKELY(not j.is_array()))
@@ -1302,9 +1394,9 @@ void())
     from_json_array_impl(j, arr, priority_tag<3> {});
 }
 
-template<typename BasicJsonType, typename CompatibleObjectType,
+template<typename BasicJsonType, typename ConstructibleObjectType,
-         enable_if_t<is_compatible_object_type<BasicJsonType, CompatibleObjectType>::value, int> = 0>
+         enable_if_t<is_constructible_object_type<BasicJsonType, ConstructibleObjectType>::value, int> = 0>
-void from_json(const BasicJsonType& j, CompatibleObjectType& obj)
+void from_json(const BasicJsonType& j, ConstructibleObjectType& obj)
 {
     if (JSON_UNLIKELY(not j.is_object()))
     {
@@ -1312,13 +1404,13 @@ void from_json(const BasicJsonType& j, CompatibleObjectType& obj)
     }
 
     auto inner_object = j.template get_ptr<const typename BasicJsonType::object_t*>();
-    using value_type = typename CompatibleObjectType::value_type;
+    using value_type = typename ConstructibleObjectType::value_type;
     std::transform(
         inner_object->begin(), inner_object->end(),
         std::inserter(obj, obj.begin()),
         [](typename BasicJsonType::object_t::value_type const & p)
     {
-        return value_type(p.first, p.second.template get<typename CompatibleObjectType::mapped_type>());
+        return value_type(p.first, p.second.template get<typename ConstructibleObjectType::mapped_type>());
     });
 }
 

test/src/unit-regression.cpp

@@ -121,6 +121,28 @@ struct nocopy
         j = {{"val", n.val}};
     }
 };
+
+struct Data
+{
+    std::string a;
+    std::string b;
+};
+
+void from_json(const json& j, Data& data)
+{
+    j["a"].get_to(data.a);
+    j["b"].get_to(data.b);
+}
+
+bool operator==(Data const& lhs, Data const& rhs)
+{
+    return lhs.a == rhs.a && lhs.b == rhs.b;
+}
+
+bool operator!=(Data const& lhs, Data const& rhs)
+{
+    return !(lhs == rhs);
+}
 }
 
 /////////////////////////////////////////////////////////////////////
@@ -1665,4 +1687,24 @@ TEST_CASE("regression tests")
             not std::is_constructible<json, std::variant<int, float>>::value, "");
     }
 #endif
+
+    SECTION("issue #1299 - compile error in from_json converting to container "
+            "with std::pair")
+    {
+        json j =
+        {
+            {"1", {{"a", "testa_1"}, {"b", "testb_1"}}},
+            {"2", {{"a", "testa_2"}, {"b", "testb_2"}}},
+            {"3", {{"a", "testa_3"}, {"b", "testb_3"}}},
+        };
+
+        std::map<std::string, Data> expected
+        {
+            {"1", {"testa_1", "testb_1" }},
+            {"2", {"testa_2", "testb_2"}},
+            {"3", {"testa_3", "testb_3"}},
+        };
+        const auto data = j.get<decltype(expected)>();
+        CHECK(expected == data);
+    }
 }