make to_json SFINAE-correct

include/nlohmann/adl_serializer.hpp

@@ -35,9 +35,11 @@ struct adl_serializer
     @param[in,out] j  JSON value to write to
     @param[in] val     value to read from
     */
-    template<typename BasicJsonType, typename ValueType>
+    template <typename BasicJsonType, typename ValueType>
-    static void to_json(BasicJsonType& j, ValueType&& val) noexcept(
+    static auto to_json(BasicJsonType& j, ValueType&& val) noexcept(
         noexcept(::nlohmann::to_json(j, std::forward<ValueType>(val))))
+    -> decltype(::nlohmann::to_json(j, std::forward<ValueType>(val)),
+                void())
     {
         ::nlohmann::to_json(j, std::forward<ValueType>(val));
     }

include/nlohmann/detail/conversions/to_json.hpp

@@ -286,9 +286,12 @@ void to_json(BasicJsonType& j, typename BasicJsonType::object_t&& obj)
     external_constructor<value_t::object>::construct(j, std::move(obj));
 }
 
-template<typename BasicJsonType, typename T, std::size_t N,
+template <
-         enable_if_t<not std::is_constructible<typename BasicJsonType::string_t, T (&)[N]>::value, int> = 0>
+    typename BasicJsonType, typename T, std::size_t N,
-void to_json(BasicJsonType& j, T (&arr)[N])
+    enable_if_t<not std::is_constructible<typename BasicJsonType::string_t,
+                const T (&)[N]>::value,
+                int> = 0 >
+void to_json(BasicJsonType& j, const T (&arr)[N])
 {
     external_constructor<value_t::array>::construct(j, arr);
 }
@@ -321,35 +324,12 @@ void to_json(BasicJsonType& j, const std::tuple<Args...>& t)
 
 struct to_json_fn
 {
-  private:
     template<typename BasicJsonType, typename T>
-    auto call(BasicJsonType& j, T&& val, priority_tag<1> /*unused*/) const noexcept(noexcept(to_json(j, std::forward<T>(val))))
+    auto operator()(BasicJsonType& j, T&& val) const noexcept(noexcept(to_json(j, std::forward<T>(val))))
     -> decltype(to_json(j, std::forward<T>(val)), void())
     {
         return to_json(j, std::forward<T>(val));
     }
-
-    template<typename BasicJsonType, typename T>
-    void call(BasicJsonType& /*unused*/, T&& /*unused*/, priority_tag<0> /*unused*/) const noexcept
-    {
-        static_assert(sizeof(BasicJsonType) == 0,
-                      "could not find to_json() method in T's namespace");
-
-#ifdef _MSC_VER
-        // MSVC does not show a stacktrace for the above assert
-        using decayed = uncvref_t<T>;
-        static_assert(sizeof(typename decayed::force_msvc_stacktrace) == 0,
-                      "forcing MSVC stacktrace to show which T we're talking about.");
-#endif
-    }
-
-  public:
-    template<typename BasicJsonType, typename T>
-    void operator()(BasicJsonType& j, T&& val) const
-    noexcept(noexcept(std::declval<to_json_fn>().call(j, std::forward<T>(val), priority_tag<1> {})))
-    {
-        return call(j, std::forward<T>(val), priority_tag<1> {});
-    }
 };
 }
 

single_include/nlohmann/json.hpp

@@ -1807,9 +1807,12 @@ void to_json(BasicJsonType& j, typename BasicJsonType::object_t&& obj)
     external_constructor<value_t::object>::construct(j, std::move(obj));
 }
 
-template<typename BasicJsonType, typename T, std::size_t N,
+template <
-         enable_if_t<not std::is_constructible<typename BasicJsonType::string_t, T (&)[N]>::value, int> = 0>
+    typename BasicJsonType, typename T, std::size_t N,
-void to_json(BasicJsonType& j, T (&arr)[N])
+    enable_if_t<not std::is_constructible<typename BasicJsonType::string_t,
+                const T (&)[N]>::value,
+                int> = 0 >
+void to_json(BasicJsonType& j, const T (&arr)[N])
 {
     external_constructor<value_t::array>::construct(j, arr);
 }
@@ -1842,35 +1845,12 @@ void to_json(BasicJsonType& j, const std::tuple<Args...>& t)
 
 struct to_json_fn
 {
-  private:
     template<typename BasicJsonType, typename T>
-    auto call(BasicJsonType& j, T&& val, priority_tag<1> /*unused*/) const noexcept(noexcept(to_json(j, std::forward<T>(val))))
+    auto operator()(BasicJsonType& j, T&& val) const noexcept(noexcept(to_json(j, std::forward<T>(val))))
     -> decltype(to_json(j, std::forward<T>(val)), void())
     {
         return to_json(j, std::forward<T>(val));
     }
-
-    template<typename BasicJsonType, typename T>
-    void call(BasicJsonType& /*unused*/, T&& /*unused*/, priority_tag<0> /*unused*/) const noexcept
-    {
-        static_assert(sizeof(BasicJsonType) == 0,
-                      "could not find to_json() method in T's namespace");
-
-#ifdef _MSC_VER
-        // MSVC does not show a stacktrace for the above assert
-        using decayed = uncvref_t<T>;
-        static_assert(sizeof(typename decayed::force_msvc_stacktrace) == 0,
-                      "forcing MSVC stacktrace to show which T we're talking about.");
-#endif
-    }
-
-  public:
-    template<typename BasicJsonType, typename T>
-    void operator()(BasicJsonType& j, T&& val) const
-    noexcept(noexcept(std::declval<to_json_fn>().call(j, std::forward<T>(val), priority_tag<1> {})))
-    {
-        return call(j, std::forward<T>(val), priority_tag<1> {});
-    }
 };
 }
 
@@ -11147,9 +11127,11 @@ struct adl_serializer
     @param[in,out] j  JSON value to write to
     @param[in] val     value to read from
     */
-    template<typename BasicJsonType, typename ValueType>
+    template <typename BasicJsonType, typename ValueType>
-    static void to_json(BasicJsonType& j, ValueType&& val) noexcept(
+    static auto to_json(BasicJsonType& j, ValueType&& val) noexcept(
         noexcept(::nlohmann::to_json(j, std::forward<ValueType>(val))))
+    -> decltype(::nlohmann::to_json(j, std::forward<ValueType>(val)),
+                void())
     {
         ::nlohmann::to_json(j, std::forward<ValueType>(val));
     }