make from_json SFINAE-correct

include/nlohmann/adl_serializer.hpp

@@ -20,8 +20,10 @@ struct adl_serializer
     @param[in,out] val  value to write to
     */
     template<typename BasicJsonType, typename ValueType>
-    static void from_json(BasicJsonType&& j, ValueType& val) noexcept(
+    static auto from_json(BasicJsonType&& j, ValueType& val) noexcept(
-        noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), val)))
+        noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), val))) -> decltype(
+            ::nlohmann::from_json(std::forward<BasicJsonType>(j), val), void()
+        )
     {
         ::nlohmann::from_json(std::forward<BasicJsonType>(j), val);
     }

include/nlohmann/detail/conversions/from_json.hpp

@@ -218,8 +218,8 @@ template <typename BasicJsonType, typename CompatibleArrayType,
 
 auto from_json(const BasicJsonType& j, CompatibleArrayType& arr)
 -> decltype(from_json_array_impl(j, arr, priority_tag<3> {}),
-j.template get<typename CompatibleArrayType::value_type>(),
+            j.template get<typename CompatibleArrayType::value_type>(),
-void())
+            void())
 {
     if (JSON_UNLIKELY(not j.is_array()))
     {
@@ -350,35 +350,13 @@ void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyE
 
 struct from_json_fn
 {
-  private:
     template<typename BasicJsonType, typename T>
-    auto call(const BasicJsonType& j, T& val, priority_tag<1> /*unused*/) const
+    auto operator()(const BasicJsonType& j, T& val) const
     noexcept(noexcept(from_json(j, val)))
     -> decltype(from_json(j, val), void())
     {
         return from_json(j, val);
     }
-
-    template<typename BasicJsonType, typename T>
-    void call(const BasicJsonType& /*unused*/, T& /*unused*/, priority_tag<0> /*unused*/) const noexcept
-    {
-        static_assert(sizeof(BasicJsonType) == 0,
-                      "could not find from_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()(const BasicJsonType& j, T& val) const
-    noexcept(noexcept(std::declval<from_json_fn>().call(j, val, priority_tag<1> {})))
-    {
-        return call(j, val, priority_tag<1> {});
-    }
 };
 }
 

single_include/nlohmann/json.hpp

@@ -1212,8 +1212,8 @@ template <typename BasicJsonType, typename CompatibleArrayType,
 
 auto from_json(const BasicJsonType& j, CompatibleArrayType& arr)
 -> decltype(from_json_array_impl(j, arr, priority_tag<3> {}),
-j.template get<typename CompatibleArrayType::value_type>(),
+            j.template get<typename CompatibleArrayType::value_type>(),
-void())
+            void())
 {
     if (JSON_UNLIKELY(not j.is_array()))
     {
@@ -1344,35 +1344,13 @@ void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyE
 
 struct from_json_fn
 {
-  private:
     template<typename BasicJsonType, typename T>
-    auto call(const BasicJsonType& j, T& val, priority_tag<1> /*unused*/) const
+    auto operator()(const BasicJsonType& j, T& val) const
     noexcept(noexcept(from_json(j, val)))
     -> decltype(from_json(j, val), void())
     {
         return from_json(j, val);
     }
-
-    template<typename BasicJsonType, typename T>
-    void call(const BasicJsonType& /*unused*/, T& /*unused*/, priority_tag<0> /*unused*/) const noexcept
-    {
-        static_assert(sizeof(BasicJsonType) == 0,
-                      "could not find from_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()(const BasicJsonType& j, T& val) const
-    noexcept(noexcept(std::declval<from_json_fn>().call(j, val, priority_tag<1> {})))
-    {
-        return call(j, val, priority_tag<1> {});
-    }
 };
 }
 
@@ -11112,8 +11090,10 @@ struct adl_serializer
     @param[in,out] val  value to write to
     */
     template<typename BasicJsonType, typename ValueType>
-    static void from_json(BasicJsonType&& j, ValueType& val) noexcept(
+    static auto from_json(BasicJsonType&& j, ValueType& val) noexcept(
-        noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), val)))
+        noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), val))) -> decltype(
+            ::nlohmann::from_json(std::forward<BasicJsonType>(j), val), void()
+        )
     {
         ::nlohmann::from_json(std::forward<BasicJsonType>(j), val);
     }