Add internal version of is_default_constructible to work around LWG 2367.

include/nlohmann/detail/meta/type_traits.hpp

@@ -149,6 +149,52 @@ struct has_to_json < BasicJsonType, T, enable_if_t < !is_basic_json<T>::value >>
 ///////////////////
 // is_ functions //
 ///////////////////
+// https://en.cppreference.com/w/cpp/types/conjunction
+template<class...> struct conjunction : std::true_type { };
+template<class B1> struct conjunction<B1> : B1 { };
+template<class B1, class... Bn>
+struct conjunction<B1, Bn...>
+: std::conditional<bool(B1::value), conjunction<Bn...>, B1>::type {};
+
+// Reimplementation of is_constructible and is_default_constructible, due to them being broken for
+// std::pair and std::tuple until LWG 2367 fix (see https://cplusplus.github.io/LWG/lwg-defects.html#2367).
+// This causes compile errors in e.g. clang 3.5 or gcc 4.9.
+// Based on commit fixing this in gcc: https://github.com/gcc-mirror/gcc/commit/d3c64041b32b6962ad6b2d879231537a477631fb
+template <typename T>
+struct is_default_constructible : std::is_default_constructible<T> {};
+
+template <typename T1, typename T2>
+struct is_default_constructible<std::pair<T1, T2>>
+            : conjunction<is_default_constructible<T1>, is_default_constructible<T2>> {};
+
+template <typename T1, typename T2>
+struct is_default_constructible<const std::pair<T1, T2>>
+            : conjunction<is_default_constructible<T1>, is_default_constructible<T2>> {};
+
+template <typename... Ts>
+struct is_default_constructible<std::tuple<Ts...>>
+            : conjunction<is_default_constructible<Ts>...> {};
+
+template <typename... Ts>
+struct is_default_constructible<const std::tuple<Ts...>>
+            : conjunction<is_default_constructible<Ts>...> {};
+
+
+template <typename T, typename... Args>
+struct is_constructible : std::is_constructible<T, Args...> {};
+
+template <typename T1, typename T2>
+struct is_constructible<std::pair<T1, T2>> : is_default_constructible<std::pair<T1, T2>> {};
+
+template <typename T1, typename T2>
+struct is_constructible<const std::pair<T1, T2>> : is_default_constructible<const std::pair<T1, T2>> {};
+
+template <typename... Ts>
+struct is_constructible<std::tuple<Ts...>> : is_default_constructible<std::tuple<Ts...>> {};
+
+template <typename... Ts>
+struct is_constructible<const std::tuple<Ts...>> : is_default_constructible<const std::tuple<Ts...>> {};
+
 
 template<typename T, typename = void>
 struct is_iterator_traits : std::false_type {};
@@ -192,9 +238,9 @@ struct is_compatible_object_type_impl <
 
     // macOS's is_constructible does not play well with nonesuch...
     static constexpr bool value =
-        std::is_constructible<typename object_t::key_type,
+        is_constructible<typename object_t::key_type,
         typename CompatibleObjectType::key_type>::value &&
-        std::is_constructible<typename object_t::mapped_type,
+        is_constructible<typename object_t::mapped_type,
         typename CompatibleObjectType::mapped_type>::value;
 };
 
@@ -215,10 +261,10 @@ struct is_constructible_object_type_impl <
     using object_t = typename BasicJsonType::object_t;
 
     static constexpr bool value =
-        (std::is_default_constructible<ConstructibleObjectType>::value &&
+        (is_default_constructible<ConstructibleObjectType>::value &&
          (std::is_move_assignable<ConstructibleObjectType>::value ||
           std::is_copy_assignable<ConstructibleObjectType>::value) &&
-         (std::is_constructible<typename ConstructibleObjectType::key_type,
+         (is_constructible<typename ConstructibleObjectType::key_type,
           typename object_t::key_type>::value &&
           std::is_same <
           typename object_t::mapped_type,
@@ -246,7 +292,7 @@ struct is_compatible_string_type_impl <
     value_type_t, CompatibleStringType>::value >>
 {
     static constexpr auto value =
-        std::is_constructible<typename BasicJsonType::string_t, CompatibleStringType>::value;
+        is_constructible<typename BasicJsonType::string_t, CompatibleStringType>::value;
 };
 
 template<typename BasicJsonType, typename ConstructibleStringType>
@@ -264,7 +310,7 @@ struct is_constructible_string_type_impl <
     value_type_t, ConstructibleStringType>::value >>
 {
     static constexpr auto value =
-        std::is_constructible<ConstructibleStringType,
+        is_constructible<ConstructibleStringType,
         typename BasicJsonType::string_t>::value;
 };
 
@@ -287,7 +333,7 @@ struct is_compatible_array_type_impl <
     iterator_traits<CompatibleArrayType >>::value >>
 {
     static constexpr bool value =
-        std::is_constructible<BasicJsonType,
+        is_constructible<BasicJsonType,
         typename CompatibleArrayType::value_type>::value;
 };
 
@@ -310,7 +356,7 @@ struct is_constructible_array_type_impl <
     BasicJsonType, ConstructibleArrayType,
     enable_if_t < !std::is_same<ConstructibleArrayType,
     typename BasicJsonType::value_type>::value&&
-    std::is_default_constructible<ConstructibleArrayType>::value&&
+    is_default_constructible<ConstructibleArrayType>::value&&
 (std::is_move_assignable<ConstructibleArrayType>::value ||
  std::is_copy_assignable<ConstructibleArrayType>::value)&&
 is_detected<value_type_t, ConstructibleArrayType>::value&&
@@ -354,7 +400,7 @@ struct is_compatible_integer_type_impl <
     using CompatibleLimits = std::numeric_limits<CompatibleNumberIntegerType>;
 
     static constexpr auto value =
-        std::is_constructible<RealIntegerType,
+        is_constructible<RealIntegerType,
         CompatibleNumberIntegerType>::value &&
         CompatibleLimits::is_integer &&
         RealLimits::is_signed == CompatibleLimits::is_signed;
@@ -381,17 +427,10 @@ template<typename BasicJsonType, typename CompatibleType>
 struct is_compatible_type
     : is_compatible_type_impl<BasicJsonType, CompatibleType> {};
 
-// https://en.cppreference.com/w/cpp/types/conjunction
-template<class...> struct conjunction : std::true_type { };
-template<class B1> struct conjunction<B1> : B1 { };
-template<class B1, class... Bn>
-struct conjunction<B1, Bn...>
-: std::conditional<bool(B1::value), conjunction<Bn...>, B1>::type {};
-
 template<typename T1, typename T2>
 struct is_constructible_tuple : std::false_type {};
 
 template<typename T1, typename... Args>
-struct is_constructible_tuple<T1, std::tuple<Args...>> : conjunction<std::is_constructible<T1, Args>...> {};
+struct is_constructible_tuple<T1, std::tuple<Args...>> : conjunction<is_constructible<T1, Args>...> {};
 }  // namespace detail
 }  // namespace nlohmann