Merge pull request #1238 from theodelrieu/fix/1237

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>;
 

include/nlohmann/json.hpp

@@ -2670,17 +2670,18 @@ class basic_json
 
 
     /*!
-    @brief get a pointer value (explicit)
+    @brief get a pointer value (implicit)
 
-    Explicit pointer access to the internally stored JSON value. No copies are
+    Implicit pointer access to the internally stored JSON value. No copies are
     made.
 
-    @warning The pointer becomes invalid if the underlying JSON object
-    changes.
+    @warning Writing data to the pointee of the result yields an undefined
+    state.
 
     @tparam PointerType pointer type; must be a pointer to @ref array_t, @ref
     object_t, @ref string_t, @ref boolean_t, @ref number_integer_t,
-    @ref number_unsigned_t, or @ref number_float_t.
+    @ref number_unsigned_t, or @ref number_float_t. Enforced by a static
+    assertion.
 
     @return pointer to the internally stored JSON value if the requested
     pointer type @a PointerType fits to the JSON value; `nullptr` otherwise
@@ -2690,45 +2691,43 @@ class basic_json
     @liveexample{The example below shows how pointers to internal values of a
     JSON value can be requested. Note that no type conversions are made and a
     `nullptr` is returned if the value and the requested pointer type does not
-    match.,get__PointerType}
-
-    @sa @ref get_ptr() for explicit pointer-member access
+    match.,get_ptr}
 
     @since version 1.0.0
     */
     template<typename PointerType, typename std::enable_if<
                  std::is_pointer<PointerType>::value, int>::type = 0>
-    PointerType get() noexcept
+    auto get_ptr() noexcept -> decltype(std::declval<basic_json_t&>().get_impl_ptr(std::declval<PointerType>()))
     {
-        // delegate the call to get_ptr
-        return get_ptr<PointerType>();
+        // delegate the call to get_impl_ptr<>()
+        return get_impl_ptr(static_cast<PointerType>(nullptr));
     }
 
     /*!
-    @brief get a pointer value (explicit)
-    @copydoc get()
+    @brief get a pointer value (implicit)
+    @copydoc get_ptr()
     */
     template<typename PointerType, typename std::enable_if<
-                 std::is_pointer<PointerType>::value, int>::type = 0>
-    constexpr const PointerType get() const noexcept
+                 std::is_pointer<PointerType>::value and
+                 std::is_const<typename std::remove_pointer<PointerType>::type>::value, int>::type = 0>
+    constexpr auto get_ptr() const noexcept -> decltype(std::declval<const basic_json_t&>().get_impl_ptr(std::declval<PointerType>()))
     {
-        // delegate the call to get_ptr
-        return get_ptr<PointerType>();
+        // delegate the call to get_impl_ptr<>() const
+        return get_impl_ptr(static_cast<PointerType>(nullptr));
     }
 
     /*!
-    @brief get a pointer value (implicit)
+    @brief get a pointer value (explicit)
 
-    Implicit pointer access to the internally stored JSON value. No copies are
+    Explicit pointer access to the internally stored JSON value. No copies are
     made.
 
-    @warning Writing data to the pointee of the result yields an undefined
-    state.
+    @warning The pointer becomes invalid if the underlying JSON object
+    changes.
 
     @tparam PointerType pointer type; must be a pointer to @ref array_t, @ref
     object_t, @ref string_t, @ref boolean_t, @ref number_integer_t,
-    @ref number_unsigned_t, or @ref number_float_t. Enforced by a static
-    assertion.
+    @ref number_unsigned_t, or @ref number_float_t.
 
     @return pointer to the internally stored JSON value if the requested
     pointer type @a PointerType fits to the JSON value; `nullptr` otherwise
@@ -2738,59 +2737,30 @@ class basic_json
     @liveexample{The example below shows how pointers to internal values of a
     JSON value can be requested. Note that no type conversions are made and a
     `nullptr` is returned if the value and the requested pointer type does not
-    match.,get_ptr}
+    match.,get__PointerType}
+
+    @sa @ref get_ptr() for explicit pointer-member access
 
     @since version 1.0.0
     */
     template<typename PointerType, typename std::enable_if<
                  std::is_pointer<PointerType>::value, int>::type = 0>
-    PointerType get_ptr() noexcept
-    {
-        // get the type of the PointerType (remove pointer and const)
-        using pointee_t = typename std::remove_const<typename
-                          std::remove_pointer<typename
-                          std::remove_const<PointerType>::type>::type>::type;
-        // make sure the type matches the allowed types
-        static_assert(
-            std::is_same<object_t, pointee_t>::value
-            or std::is_same<array_t, pointee_t>::value
-            or std::is_same<string_t, pointee_t>::value
-            or std::is_same<boolean_t, pointee_t>::value
-            or std::is_same<number_integer_t, pointee_t>::value
-            or std::is_same<number_unsigned_t, pointee_t>::value
-            or std::is_same<number_float_t, pointee_t>::value
-            , "incompatible pointer type");
-
-        // delegate the call to get_impl_ptr<>()
-        return get_impl_ptr(static_cast<PointerType>(nullptr));
+    auto get() noexcept -> decltype(std::declval<basic_json_t&>().template get_ptr<PointerType>())
+    {
+        // delegate the call to get_ptr
+        return get_ptr<PointerType>();
     }
 
     /*!
-    @brief get a pointer value (implicit)
-    @copydoc get_ptr()
+    @brief get a pointer value (explicit)
+    @copydoc get()
     */
     template<typename PointerType, typename std::enable_if<
-                 std::is_pointer<PointerType>::value and
-                 std::is_const<typename std::remove_pointer<PointerType>::type>::value, int>::type = 0>
-    constexpr const PointerType get_ptr() const noexcept
-    {
-        // get the type of the PointerType (remove pointer and const)
-        using pointee_t = typename std::remove_const<typename
-                          std::remove_pointer<typename
-                          std::remove_const<PointerType>::type>::type>::type;
-        // make sure the type matches the allowed types
-        static_assert(
-            std::is_same<object_t, pointee_t>::value
-            or std::is_same<array_t, pointee_t>::value
-            or std::is_same<string_t, pointee_t>::value
-            or std::is_same<boolean_t, pointee_t>::value
-            or std::is_same<number_integer_t, pointee_t>::value
-            or std::is_same<number_unsigned_t, pointee_t>::value
-            or std::is_same<number_float_t, pointee_t>::value
-            , "incompatible pointer type");
-
-        // delegate the call to get_impl_ptr<>() const
-        return get_impl_ptr(static_cast<PointerType>(nullptr));
+                 std::is_pointer<PointerType>::value, int>::type = 0>
+    constexpr auto get() const noexcept -> decltype(std::declval<const basic_json_t&>().template get_ptr<PointerType>())
+    {
+        // delegate the call to get_ptr
+        return get_ptr<PointerType>();
     }
 
     /*!
@@ -2874,12 +2844,14 @@ 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
     {

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>;
 
@@ -13743,17 +13757,18 @@ class basic_json
 
 
     /*!
-    @brief get a pointer value (explicit)
+    @brief get a pointer value (implicit)
 
-    Explicit pointer access to the internally stored JSON value. No copies are
+    Implicit pointer access to the internally stored JSON value. No copies are
     made.
 
-    @warning The pointer becomes invalid if the underlying JSON object
-    changes.
+    @warning Writing data to the pointee of the result yields an undefined
+    state.
 
     @tparam PointerType pointer type; must be a pointer to @ref array_t, @ref
     object_t, @ref string_t, @ref boolean_t, @ref number_integer_t,
-    @ref number_unsigned_t, or @ref number_float_t.
+    @ref number_unsigned_t, or @ref number_float_t. Enforced by a static
+    assertion.
 
     @return pointer to the internally stored JSON value if the requested
     pointer type @a PointerType fits to the JSON value; `nullptr` otherwise
@@ -13763,45 +13778,43 @@ class basic_json
     @liveexample{The example below shows how pointers to internal values of a
     JSON value can be requested. Note that no type conversions are made and a
     `nullptr` is returned if the value and the requested pointer type does not
-    match.,get__PointerType}
-
-    @sa @ref get_ptr() for explicit pointer-member access
+    match.,get_ptr}
 
     @since version 1.0.0
     */
     template<typename PointerType, typename std::enable_if<
                  std::is_pointer<PointerType>::value, int>::type = 0>
-    PointerType get() noexcept
+    auto get_ptr() noexcept -> decltype(std::declval<basic_json_t&>().get_impl_ptr(std::declval<PointerType>()))
     {
-        // delegate the call to get_ptr
-        return get_ptr<PointerType>();
+        // delegate the call to get_impl_ptr<>()
+        return get_impl_ptr(static_cast<PointerType>(nullptr));
     }
 
     /*!
-    @brief get a pointer value (explicit)
-    @copydoc get()
+    @brief get a pointer value (implicit)
+    @copydoc get_ptr()
     */
     template<typename PointerType, typename std::enable_if<
-                 std::is_pointer<PointerType>::value, int>::type = 0>
-    constexpr const PointerType get() const noexcept
+                 std::is_pointer<PointerType>::value and
+                 std::is_const<typename std::remove_pointer<PointerType>::type>::value, int>::type = 0>
+    constexpr auto get_ptr() const noexcept -> decltype(std::declval<const basic_json_t&>().get_impl_ptr(std::declval<PointerType>()))
     {
-        // delegate the call to get_ptr
-        return get_ptr<PointerType>();
+        // delegate the call to get_impl_ptr<>() const
+        return get_impl_ptr(static_cast<PointerType>(nullptr));
     }
 
     /*!
-    @brief get a pointer value (implicit)
+    @brief get a pointer value (explicit)
 
-    Implicit pointer access to the internally stored JSON value. No copies are
+    Explicit pointer access to the internally stored JSON value. No copies are
     made.
 
-    @warning Writing data to the pointee of the result yields an undefined
-    state.
+    @warning The pointer becomes invalid if the underlying JSON object
+    changes.
 
     @tparam PointerType pointer type; must be a pointer to @ref array_t, @ref
     object_t, @ref string_t, @ref boolean_t, @ref number_integer_t,
-    @ref number_unsigned_t, or @ref number_float_t. Enforced by a static
-    assertion.
+    @ref number_unsigned_t, or @ref number_float_t.
 
     @return pointer to the internally stored JSON value if the requested
     pointer type @a PointerType fits to the JSON value; `nullptr` otherwise
@@ -13811,59 +13824,30 @@ class basic_json
     @liveexample{The example below shows how pointers to internal values of a
     JSON value can be requested. Note that no type conversions are made and a
     `nullptr` is returned if the value and the requested pointer type does not
-    match.,get_ptr}
+    match.,get__PointerType}
+
+    @sa @ref get_ptr() for explicit pointer-member access
 
     @since version 1.0.0
     */
     template<typename PointerType, typename std::enable_if<
                  std::is_pointer<PointerType>::value, int>::type = 0>
-    PointerType get_ptr() noexcept
+    auto get() noexcept -> decltype(std::declval<basic_json_t&>().template get_ptr<PointerType>())
     {
-        // get the type of the PointerType (remove pointer and const)
-        using pointee_t = typename std::remove_const<typename
-                          std::remove_pointer<typename
-                          std::remove_const<PointerType>::type>::type>::type;
-        // make sure the type matches the allowed types
-        static_assert(
-            std::is_same<object_t, pointee_t>::value
-            or std::is_same<array_t, pointee_t>::value
-            or std::is_same<string_t, pointee_t>::value
-            or std::is_same<boolean_t, pointee_t>::value
-            or std::is_same<number_integer_t, pointee_t>::value
-            or std::is_same<number_unsigned_t, pointee_t>::value
-            or std::is_same<number_float_t, pointee_t>::value
-            , "incompatible pointer type");
-
-        // delegate the call to get_impl_ptr<>()
-        return get_impl_ptr(static_cast<PointerType>(nullptr));
+        // delegate the call to get_ptr
+        return get_ptr<PointerType>();
     }
 
     /*!
-    @brief get a pointer value (implicit)
-    @copydoc get_ptr()
+    @brief get a pointer value (explicit)
+    @copydoc get()
     */
     template<typename PointerType, typename std::enable_if<
-                 std::is_pointer<PointerType>::value and
-                 std::is_const<typename std::remove_pointer<PointerType>::type>::value, int>::type = 0>
-    constexpr const PointerType get_ptr() const noexcept
+                 std::is_pointer<PointerType>::value, int>::type = 0>
+    constexpr auto get() const noexcept -> decltype(std::declval<const basic_json_t&>().template get_ptr<PointerType>())
     {
-        // get the type of the PointerType (remove pointer and const)
-        using pointee_t = typename std::remove_const<typename
-                          std::remove_pointer<typename
-                          std::remove_const<PointerType>::type>::type>::type;
-        // make sure the type matches the allowed types
-        static_assert(
-            std::is_same<object_t, pointee_t>::value
-            or std::is_same<array_t, pointee_t>::value
-            or std::is_same<string_t, pointee_t>::value
-            or std::is_same<boolean_t, pointee_t>::value
-            or std::is_same<number_integer_t, pointee_t>::value
-            or std::is_same<number_unsigned_t, pointee_t>::value
-            or std::is_same<number_float_t, pointee_t>::value
-            , "incompatible pointer type");
-
-        // delegate the call to get_impl_ptr<>() const
-        return get_impl_ptr(static_cast<PointerType>(nullptr));
+        // delegate the call to get_ptr
+        return get_ptr<PointerType>();
     }
 
     /*!
@@ -13947,12 +13931,14 @@ 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
     {

test/src/unit-udt.cpp

@@ -824,3 +824,9 @@ 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, "");
+}