Add restriction for tuple specialization of to_json

include/nlohmann/detail/conversions/to_json.hpp

@@ -322,10 +322,10 @@ void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<Idx...>
     j = { std::get<Idx>(t)... };
 }
 
-template<typename BasicJsonType, typename... Args>
-void to_json(BasicJsonType& j, const std::tuple<Args...>& t)
+template<typename BasicJsonType, typename T, enable_if_t<is_constructible_tuple<BasicJsonType, T>::value, int > = 0>
+void to_json(BasicJsonType& j, const T& t)
 {
-    to_json_tuple_impl(j, t, index_sequence_for<Args...> {});
+    to_json_tuple_impl(j, t, make_index_sequence<std::tuple_size<T>::value> {});
 }
 
 struct to_json_fn

include/nlohmann/detail/meta/type_traits.hpp

@@ -357,5 +357,18 @@ struct is_compatible_type_impl <
 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>...> {};
 }  // namespace detail
 }  // namespace nlohmann

single_include/nlohmann/json.hpp

@@ -2794,6 +2794,19 @@ struct is_compatible_type_impl <
 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>...> {};
 }  // namespace detail
 }  // namespace nlohmann
 
@@ -3753,10 +3766,10 @@ void to_json_tuple_impl(BasicJsonType& j, const Tuple& t, index_sequence<Idx...>
     j = { std::get<Idx>(t)... };
 }
 
-template<typename BasicJsonType, typename... Args>
-void to_json(BasicJsonType& j, const std::tuple<Args...>& t)
+template<typename BasicJsonType, typename T, enable_if_t<is_constructible_tuple<BasicJsonType, T>::value, int > = 0>
+void to_json(BasicJsonType& j, const T& t)
 {
-    to_json_tuple_impl(j, t, index_sequence_for<Args...> {});
+    to_json_tuple_impl(j, t, make_index_sequence<std::tuple_size<T>::value> {});
 }
 
 struct to_json_fn

test/src/unit-regression.cpp

@@ -1842,6 +1842,12 @@ TEST_CASE("regression tests")
         static_assert(!std::is_constructible<json, std::pair<NotSerializableData, std::string>>::value, "");
         static_assert(std::is_constructible<json, std::pair<int, std::string>>::value, "");
     }
+    SECTION("issue #1825 - A tuple<Args..> is json constructible only if all T in Args are json constructible")
+    {
+        static_assert(!std::is_constructible<json, std::tuple<std::string, NotSerializableData>>::value, "");
+        static_assert(!std::is_constructible<json, std::tuple<NotSerializableData, std::string>>::value, "");
+        static_assert(std::is_constructible<json, std::tuple<int, std::string>>::value, "");
+    }
 }
 
 #if not defined(JSON_NOEXCEPTION)