Fix issue #1237

* Make the conversion operator SFINAE correct. * Workaround a GCC bug with some traits in type_traits.hpp
nlohmann · Sep 12, 2018 · 697dc23 · 697dc23
1 parent 186c747
commit 697dc23
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Showing 4 changed files with 48 additions and 10 deletions.
diff --git a/include/nlohmann/detail/meta/type_traits.hpp b/include/nlohmann/detail/meta/type_traits.hpp
@@ -65,6 +65,9 @@ using to_json_function = decltype(T::to_json(std::declval<Args>()...));
 template <typename T, typename... Args>
 using from_json_function = decltype(T::from_json(std::declval<Args>()...));
 
+template <typename T, typename U>
+using get_template_function = decltype(std::declval<T>().template get<U>());
+
 ///////////////////
 // is_ functions //
 ///////////////////
@@ -185,8 +188,12 @@ struct is_compatible_integer_type
       CompatibleNumberIntegerType> {};
 
 // trait checking if JSONSerializer<T>::from_json(json const&, udt&) exists
-template<typename BasicJsonType, typename T>
-struct has_from_json
+template <typename BasicJsonType, typename T, typename = void>
+struct has_from_json : std::false_type {};
+
+template <typename BasicJsonType, typename T>
+struct has_from_json<BasicJsonType, T,
+           enable_if_t<not is_basic_json<T>::value>>
 {
     using serializer = typename BasicJsonType::template json_serializer<T, void>;
 
@@ -197,8 +204,11 @@ struct has_from_json
 
 // This trait checks if JSONSerializer<T>::from_json(json const&) exists
 // this overload is used for non-default-constructible user-defined-types
+template <typename BasicJsonType, typename T, typename = void>
+struct has_non_default_from_json : std::false_type {};
+
 template<typename BasicJsonType, typename T>
-struct has_non_default_from_json
+struct has_non_default_from_json<BasicJsonType, T, enable_if_t<not is_basic_json<T>::value>>
 {
     using serializer = typename BasicJsonType::template json_serializer<T, void>;
 
@@ -208,8 +218,12 @@ struct has_non_default_from_json
 };
 
 // This trait checks if BasicJsonType::json_serializer<T>::to_json exists
-template<typename BasicJsonType, typename T>
-struct has_to_json
+// Do not evaluate the trait when T is a basic_json type, to avoid template instantiation infinite recursion.
+template <typename BasicJsonType, typename T, typename = void>
+struct has_to_json : std::false_type {};
+
+template <typename BasicJsonType, typename T>
+struct has_to_json<BasicJsonType, T, enable_if_t<not is_basic_json<T>::value>>
 {
     using serializer = typename BasicJsonType::template json_serializer<T, void>;
 

diff --git a/include/nlohmann/json.hpp b/include/nlohmann/json.hpp
@@ -2828,12 +2828,14 @@ class basic_json
                    not std::is_same<ValueType, detail::json_ref<basic_json>>::value and
                    not std::is_same<ValueType, typename string_t::value_type>::value and
                    not detail::is_basic_json<ValueType>::value
+
 #ifndef _MSC_VER  // fix for issue #167 operator<< ambiguity under VS2015
                    and not std::is_same<ValueType, std::initializer_list<typename string_t::value_type>>::value
 #if defined(JSON_HAS_CPP_17) && defined(_MSC_VER) and _MSC_VER <= 1914
                    and not std::is_same<ValueType, typename std::string_view>::value
 #endif
 #endif
+                   and detail::is_detected<detail::get_template_function, const basic_json_t&, ValueType>::value
                    , int >::type = 0 >
     operator ValueType() const
     {

diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
@@ -425,6 +425,9 @@ using to_json_function = decltype(T::to_json(std::declval<Args>()...));
 template <typename T, typename... Args>
 using from_json_function = decltype(T::from_json(std::declval<Args>()...));
 
+template <typename T, typename U>
+using get_template_function = decltype(std::declval<T>().template get<U>());
+
 ///////////////////
 // is_ functions //
 ///////////////////
@@ -545,8 +548,12 @@ struct is_compatible_integer_type
       CompatibleNumberIntegerType> {};
 
 // trait checking if JSONSerializer<T>::from_json(json const&, udt&) exists
-template<typename BasicJsonType, typename T>
-struct has_from_json
+template <typename BasicJsonType, typename T, typename = void>
+struct has_from_json : std::false_type {};
+
+template <typename BasicJsonType, typename T>
+struct has_from_json<BasicJsonType, T,
+           enable_if_t<not is_basic_json<T>::value>>
 {
     using serializer = typename BasicJsonType::template json_serializer<T, void>;
 
@@ -557,8 +564,11 @@ struct has_from_json
 
 // This trait checks if JSONSerializer<T>::from_json(json const&) exists
 // this overload is used for non-default-constructible user-defined-types
+template <typename BasicJsonType, typename T, typename = void>
+struct has_non_default_from_json : std::false_type {};
+
 template<typename BasicJsonType, typename T>
-struct has_non_default_from_json
+struct has_non_default_from_json<BasicJsonType, T, enable_if_t<not is_basic_json<T>::value>>
 {
     using serializer = typename BasicJsonType::template json_serializer<T, void>;
 
@@ -568,8 +578,12 @@ struct has_non_default_from_json
 };
 
 // This trait checks if BasicJsonType::json_serializer<T>::to_json exists
-template<typename BasicJsonType, typename T>
-struct has_to_json
+// Do not evaluate the trait when T is a basic_json type, to avoid template instantiation infinite recursion.
+template <typename BasicJsonType, typename T, typename = void>
+struct has_to_json : std::false_type {};
+
+template <typename BasicJsonType, typename T>
+struct has_to_json<BasicJsonType, T, enable_if_t<not is_basic_json<T>::value>>
 {
     using serializer = typename BasicJsonType::template json_serializer<T, void>;
 
@@ -13901,12 +13915,14 @@ class basic_json
                    not std::is_same<ValueType, detail::json_ref<basic_json>>::value and
                    not std::is_same<ValueType, typename string_t::value_type>::value and
                    not detail::is_basic_json<ValueType>::value
+
 #ifndef _MSC_VER  // fix for issue #167 operator<< ambiguity under VS2015
                    and not std::is_same<ValueType, std::initializer_list<typename string_t::value_type>>::value
 #if defined(JSON_HAS_CPP_17) && defined(_MSC_VER) and _MSC_VER <= 1914
                    and not std::is_same<ValueType, typename std::string_view>::value
 #endif
 #endif
+                   and detail::is_detected<detail::get_template_function, const basic_json_t&, ValueType>::value
                    , int >::type = 0 >
     operator ValueType() const
     {

diff --git a/test/src/unit-udt.cpp b/test/src/unit-udt.cpp
@@ -811,3 +811,9 @@ TEST_CASE("Issue #924")
     CHECK_NOTHROW(j.get<Evil>());
     CHECK_NOTHROW(j.get<std::vector<Evil>>());
 }
+
+TEST_CASE("Issue #1237")
+{
+    struct non_convertible_type {};
+    static_assert(not std::is_convertible<json, non_convertible_type>::value, "");
+}