Merge pull request #2533 from nlohmann/c++_future

Do not unconditionally redefine C++14 constructs

include/nlohmann/detail/meta/cpp_future.hpp

@@ -2,19 +2,32 @@
 
 #include <cstddef> // size_t
 #include <type_traits> // conditional, enable_if, false_type, integral_constant, is_constructible, is_integral, is_same, remove_cv, remove_reference, true_type
+#include <utility> // index_sequence, make_index_sequence, index_sequence_for
+
+#include <nlohmann/detail/macro_scope.hpp>
 
 namespace nlohmann
 {
 namespace detail
 {
-// alias templates to reduce boilerplate
-template<bool B, typename T = void>
-using enable_if_t = typename std::enable_if<B, T>::type;
 
 template<typename T>
 using uncvref_t = typename std::remove_cv<typename std::remove_reference<T>::type>::type;
 
-// implementation of C++14 index_sequence and affiliates
+#ifdef JSON_HAS_CPP_14
+
+// the following utilities are natively available in C++14
+using std::enable_if_t;
+using std::index_sequence;
+using std::make_index_sequence;
+using std::index_sequence_for;
+
+#else
+
+// alias templates to reduce boilerplate
+template<bool B, typename T = void>
+using enable_if_t = typename std::enable_if<B, T>::type;
+
 // source: https://stackoverflow.com/a/32223343
 template<std::size_t... Ints>
 struct index_sequence
@@ -45,6 +58,8 @@ template<> struct make_index_sequence<1> : index_sequence<0> {};
 template<typename... Ts>
 using index_sequence_for = make_index_sequence<sizeof...(Ts)>;
 
+#endif
+
 // dispatch utility (taken from ranges-v3)
 template<unsigned N> struct priority_tag : priority_tag < N - 1 > {};
 template<> struct priority_tag<0> {};
@@ -58,5 +73,6 @@ struct static_const
 
 template<typename T>
 constexpr T static_const<T>::value;
+
 }  // namespace detail
 }  // namespace nlohmann

single_include/nlohmann/json.hpp

@@ -2736,19 +2736,33 @@ class other_error : public exception
 
 #include <cstddef> // size_t
 #include <type_traits> // conditional, enable_if, false_type, integral_constant, is_constructible, is_integral, is_same, remove_cv, remove_reference, true_type
+#include <utility> // index_sequence, make_index_sequence, index_sequence_for
+
+// #include <nlohmann/detail/macro_scope.hpp>
+
 
 namespace nlohmann
 {
 namespace detail
 {
-// alias templates to reduce boilerplate
-template<bool B, typename T = void>
-using enable_if_t = typename std::enable_if<B, T>::type;
 
 template<typename T>
 using uncvref_t = typename std::remove_cv<typename std::remove_reference<T>::type>::type;
 
-// implementation of C++14 index_sequence and affiliates
+#ifdef JSON_HAS_CPP_14
+
+// the following utilities are natively available in C++14
+using std::enable_if_t;
+using std::index_sequence;
+using std::make_index_sequence;
+using std::index_sequence_for;
+
+#else
+
+// alias templates to reduce boilerplate
+template<bool B, typename T = void>
+using enable_if_t = typename std::enable_if<B, T>::type;
+
 // source: https://stackoverflow.com/a/32223343
 template<std::size_t... Ints>
 struct index_sequence
@@ -2779,6 +2793,8 @@ template<> struct make_index_sequence<1> : index_sequence<0> {};
 template<typename... Ts>
 using index_sequence_for = make_index_sequence<sizeof...(Ts)>;
 
+#endif
+
 // dispatch utility (taken from ranges-v3)
 template<unsigned N> struct priority_tag : priority_tag < N - 1 > {};
 template<> struct priority_tag<0> {};
@@ -2792,6 +2808,7 @@ struct static_const
 
 template<typename T>
 constexpr T static_const<T>::value;
+
 }  // namespace detail
 }  // namespace nlohmann