use detected instead of has_* traits

include/nlohmann/detail/macro_scope.hpp

@@ -102,22 +102,3 @@
     basic_json<ObjectType, ArrayType, StringType, BooleanType,             \
     NumberIntegerType, NumberUnsignedType, NumberFloatType,                \
     AllocatorType, JSONSerializer>
-
-/*!
-@brief Helper to determine whether there's a key_type for T.
-
-This helper is used to tell associative containers apart from other containers
-such as sequence containers. For instance, `std::map` passes the test as it
-contains a `mapped_type`, whereas `std::vector` fails the test.
-
-@sa http://stackoverflow.com/a/7728728/266378
-@since version 1.0.0, overworked in version 2.0.6
-*/
-#define NLOHMANN_JSON_HAS_HELPER(type)                                        \
-    template<typename T> struct has_##type {                                  \
-        template<typename U, typename = typename U::type>                     \
-        static int detect(U &&);                                              \
-        static void detect(...);                                              \
-        static constexpr bool value =                                         \
-                std::is_integral<decltype(detect(std::declval<T>()))>::value; \
-    }

include/nlohmann/detail/macro_unscope.hpp

@@ -20,4 +20,3 @@
 #undef JSON_HAS_CPP_17
 #undef NLOHMANN_BASIC_JSON_TPL_DECLARATION
 #undef NLOHMANN_BASIC_JSON_TPL
-#undef NLOHMANN_JSON_HAS_HELPER

include/nlohmann/detail/meta/type_traits.hpp

@@ -7,6 +7,7 @@
 
 #include <nlohmann/json_fwd.hpp>
 #include <nlohmann/detail/meta/cpp_future.hpp>
+#include <nlohmann/detail/meta/detected.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 
 namespace nlohmann
@@ -30,9 +31,31 @@ template<typename> struct is_basic_json : std::false_type {};
 NLOHMANN_BASIC_JSON_TPL_DECLARATION
 struct is_basic_json<NLOHMANN_BASIC_JSON_TPL> : std::true_type {};
 
-////////////////////////
-// has_/is_ functions //
-////////////////////////
+//////////////////////////
+// aliases for detected //
+//////////////////////////
+
+template <typename T>
+using mapped_type_t = typename T::mapped_type;
+
+template <typename T>
+using key_type_t = typename T::key_type;
+
+template <typename T>
+using value_type_t = typename T::value_type;
+
+template <typename T>
+using iterator_t = typename T::iterator;
+
+template <typename T, typename... Args>
+using to_json_function = decltype(T::to_json(std::declval<Args>()...));
+
+template <typename T, typename... Args>
+using from_json_function = decltype(T::from_json(std::declval<Args>()...));
+
+///////////////////
+// is_ functions //
+///////////////////
 
 // source: https://stackoverflow.com/a/37193089/4116453
 
@@ -42,11 +65,6 @@ struct is_complete_type : std::false_type {};
 template <typename T>
 struct is_complete_type<T, decltype(void(sizeof(T)))> : std::true_type {};
 
-NLOHMANN_JSON_HAS_HELPER(mapped_type);
-NLOHMANN_JSON_HAS_HELPER(key_type);
-NLOHMANN_JSON_HAS_HELPER(value_type);
-NLOHMANN_JSON_HAS_HELPER(iterator);
-
 template<bool B, class RealType, class CompatibleObjectType>
 struct is_compatible_object_type_impl : std::false_type {};
 
@@ -74,8 +92,8 @@ struct is_compatible_object_type
 {
     static auto constexpr value = is_compatible_object_type_impl <
                                   conjunction<negation<std::is_same<void, CompatibleObjectType>>,
-                                  has_mapped_type<CompatibleObjectType>,
-                                  has_key_type<CompatibleObjectType>>::value,
+                                  is_detected<mapped_type_t, CompatibleObjectType>,
+                                  is_detected<key_type_t, CompatibleObjectType>>::value,
                                   typename BasicJsonType::object_t, CompatibleObjectType >::value;
 };
 
@@ -84,7 +102,7 @@ struct is_compatible_string_type
 {
     static auto constexpr value = is_compatible_string_type_impl <
                                   conjunction<negation<std::is_same<void, CompatibleStringType>>,
-                                  has_value_type<CompatibleStringType>>::value,
+                                  is_detected<value_type_t, CompatibleStringType>>::value,
                                   typename BasicJsonType::string_t, CompatibleStringType >::value;
 };
 
@@ -107,8 +125,8 @@ struct is_compatible_array_type
         negation<std::is_constructible<typename BasicJsonType::string_t,
         CompatibleArrayType>>,
         negation<is_basic_json_nested_type<BasicJsonType, CompatibleArrayType>>,
-        has_value_type<CompatibleArrayType>,
-        has_iterator<CompatibleArrayType>>::value;
+        is_detected<value_type_t, CompatibleArrayType>,
+        is_detected<iterator_t, CompatibleArrayType>>::value;
 };
 
 template<bool, typename, typename>
@@ -141,14 +159,11 @@ struct is_compatible_integer_type
 template<typename BasicJsonType, typename T>
 struct has_from_json
 {
-    // also check the return type of from_json
-    template<typename U, typename = enable_if_t<std::is_same<void, decltype(uncvref_t<U>::from_json(
-                 std::declval<BasicJsonType>(), std::declval<T&>()))>::value>>
-    static int detect(U&&);
-    static void detect(...);
-
-    static constexpr bool value = std::is_integral<decltype(
-                                      detect(std::declval<typename BasicJsonType::template json_serializer<T, void>>()))>::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
@@ -156,28 +171,22 @@ struct has_from_json
 template<typename BasicJsonType, typename T>
 struct has_non_default_from_json
 {
-    template <
-        typename U,
-        typename = enable_if_t<std::is_same<
-                                   T, decltype(uncvref_t<U>::from_json(std::declval<BasicJsonType>()))>::value >>
-    static int detect(U&&);
-    static void detect(...);
-
-    static constexpr bool value = std::is_integral<decltype(detect(
-                                      std::declval<typename BasicJsonType::template json_serializer<T, void>>()))>::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
 template<typename BasicJsonType, typename T>
 struct has_to_json
 {
-    template<typename U, typename = decltype(uncvref_t<U>::to_json(
-                 std::declval<BasicJsonType&>(), std::declval<T>()))>
-    static int detect(U&&);
-    static void detect(...);
+    using serializer = typename BasicJsonType::template json_serializer<T, void>;
 
-    static constexpr bool value = std::is_integral<decltype(detect(
-                                      std::declval<typename BasicJsonType::template json_serializer<T, void>>()))>::value;
+    static constexpr bool value =
+        is_detected_exact<void, to_json_function, serializer, BasicJsonType&,
+        T>::value;
 };
 
 template <typename BasicJsonType, typename CompatibleCompleteType>