fix std::filesystem::path regression (#3073)

* meta: rework is_compatible/is_constructible_string_type

These type traits performed an incorrect and insufficient check.

Converting to a std::filesystem::path used to work by accident thanks to
these brittle constraints, but the clean-up performed in #3020 broke them.

* support std::filesystem::path

Fixes #3070

include/nlohmann/detail/conversions/from_json.hpp

@@ -19,6 +19,10 @@
 #include <nlohmann/detail/meta/type_traits.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
+#ifdef JSON_HAS_CPP_17
+    #include <filesystem>
+#endif
+
 namespace nlohmann
 {
 namespace detail
@@ -169,7 +173,7 @@ void from_json(const BasicJsonType& j, std::valarray<T>& l)
 }
 
 template<typename BasicJsonType, typename T, std::size_t N>
-auto from_json(const BasicJsonType& j, T (&arr)[N]) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
+auto from_json(const BasicJsonType& j, T (&arr)[N])  // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
 -> decltype(j.template get<T>(), void())
 {
     for (std::size_t i = 0; i < N; ++i)
@@ -444,6 +448,18 @@ void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyE
     }
 }
 
+#ifdef JSON_HAS_CPP_17
+template<typename BasicJsonType>
+void from_json(const BasicJsonType& j, std::filesystem::path& p)
+{
+    if (JSON_HEDLEY_UNLIKELY(!j.is_string()))
+    {
+        JSON_THROW(type_error::create(302, "type must be string, but is " + std::string(j.type_name()), j));
+    }
+    p = *j.template get_ptr<const typename BasicJsonType::string_t*>();
+}
+#endif
+
 struct from_json_fn
 {
     template<typename BasicJsonType, typename T>

include/nlohmann/detail/conversions/to_json.hpp

@@ -9,11 +9,16 @@
 #include <valarray> // valarray
 #include <vector> // vector
 
+#include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/iterators/iteration_proxy.hpp>
 #include <nlohmann/detail/meta/cpp_future.hpp>
 #include <nlohmann/detail/meta/type_traits.hpp>
 #include <nlohmann/detail/value_t.hpp>
 
+#ifdef JSON_HAS_CPP_17
+    #include <filesystem>
+#endif
+
 namespace nlohmann
 {
 namespace detail
@@ -386,6 +391,14 @@ void to_json(BasicJsonType& j, const T& t)
     to_json_tuple_impl(j, t, make_index_sequence<std::tuple_size<T>::value> {});
 }
 
+#ifdef JSON_HAS_CPP_17
+template<typename BasicJsonType>
+void to_json(BasicJsonType& j, const std::filesystem::path& p)
+{
+    j = p.string();
+}
+#endif
+
 struct to_json_fn
 {
     template<typename BasicJsonType, typename T>

include/nlohmann/detail/meta/type_traits.hpp

@@ -309,44 +309,21 @@ struct is_constructible_object_type
     : is_constructible_object_type_impl<BasicJsonType,
       ConstructibleObjectType> {};
 
-template<typename BasicJsonType, typename CompatibleStringType,
-         typename = void>
-struct is_compatible_string_type_impl : std::false_type {};
-
 template<typename BasicJsonType, typename CompatibleStringType>
-struct is_compatible_string_type_impl <
-    BasicJsonType, CompatibleStringType,
-    enable_if_t<is_detected_convertible<typename BasicJsonType::string_t::value_type,
-    range_value_t,
-    CompatibleStringType>::value >>
+struct is_compatible_string_type
 {
     static constexpr auto value =
         is_constructible<typename BasicJsonType::string_t, CompatibleStringType>::value;
 };
 
 template<typename BasicJsonType, typename ConstructibleStringType>
-struct is_compatible_string_type
-    : is_compatible_string_type_impl<BasicJsonType, ConstructibleStringType> {};
-
-template<typename BasicJsonType, typename ConstructibleStringType,
-         typename = void>
-struct is_constructible_string_type_impl : std::false_type {};
-
-template<typename BasicJsonType, typename ConstructibleStringType>
-struct is_constructible_string_type_impl <
-    BasicJsonType, ConstructibleStringType,
-    enable_if_t<is_detected_exact<typename BasicJsonType::string_t::value_type,
-    value_type_t, ConstructibleStringType>::value >>
+struct is_constructible_string_type
 {
     static constexpr auto value =
         is_constructible<ConstructibleStringType,
         typename BasicJsonType::string_t>::value;
 };
 
-template<typename BasicJsonType, typename ConstructibleStringType>
-struct is_constructible_string_type
-    : is_constructible_string_type_impl<BasicJsonType, ConstructibleStringType> {};
-
 template<typename BasicJsonType, typename CompatibleArrayType, typename = void>
 struct is_compatible_array_type_impl : std::false_type {};
 
@@ -355,7 +332,10 @@ struct is_compatible_array_type_impl <
     BasicJsonType, CompatibleArrayType,
     enable_if_t <
     is_detected<iterator_t, CompatibleArrayType>::value&&
-    is_iterator_traits<iterator_traits<detected_t<iterator_t, CompatibleArrayType>>>::value >>
+    is_iterator_traits<iterator_traits<detected_t<iterator_t, CompatibleArrayType>>>::value&&
+// special case for types like std::filesystem::path whose iterator's value_type are themselves
+// c.f. https://github.com/nlohmann/json/pull/3073
+    !std::is_same<CompatibleArrayType, detected_t<range_value_t, CompatibleArrayType>>::value >>
 {
     static constexpr bool value =
         is_constructible<BasicJsonType,
@@ -388,8 +368,11 @@ struct is_constructible_array_type_impl <
 is_detected<iterator_t, ConstructibleArrayType>::value&&
 is_iterator_traits<iterator_traits<detected_t<iterator_t, ConstructibleArrayType>>>::value&&
 is_detected<range_value_t, ConstructibleArrayType>::value&&
-is_complete_type <
-detected_t<range_value_t, ConstructibleArrayType >>::value >>
+// special case for types like std::filesystem::path whose iterator's value_type are themselves
+// c.f. https://github.com/nlohmann/json/pull/3073
+!std::is_same<ConstructibleArrayType, detected_t<range_value_t, ConstructibleArrayType>>::value&&
+        is_complete_type <
+        detected_t<range_value_t, ConstructibleArrayType >>::value >>
 {
     using value_type = range_value_t<ConstructibleArrayType>;
 

single_include/nlohmann/json.hpp

@@ -3783,44 +3783,21 @@ struct is_constructible_object_type
     : is_constructible_object_type_impl<BasicJsonType,
       ConstructibleObjectType> {};
 
-template<typename BasicJsonType, typename CompatibleStringType,
-         typename = void>
-struct is_compatible_string_type_impl : std::false_type {};
-
 template<typename BasicJsonType, typename CompatibleStringType>
-struct is_compatible_string_type_impl <
-    BasicJsonType, CompatibleStringType,
-    enable_if_t<is_detected_convertible<typename BasicJsonType::string_t::value_type,
-    range_value_t,
-    CompatibleStringType>::value >>
+struct is_compatible_string_type
 {
     static constexpr auto value =
         is_constructible<typename BasicJsonType::string_t, CompatibleStringType>::value;
 };
 
 template<typename BasicJsonType, typename ConstructibleStringType>
-struct is_compatible_string_type
-    : is_compatible_string_type_impl<BasicJsonType, ConstructibleStringType> {};
-
-template<typename BasicJsonType, typename ConstructibleStringType,
-         typename = void>
-struct is_constructible_string_type_impl : std::false_type {};
-
-template<typename BasicJsonType, typename ConstructibleStringType>
-struct is_constructible_string_type_impl <
-    BasicJsonType, ConstructibleStringType,
-    enable_if_t<is_detected_exact<typename BasicJsonType::string_t::value_type,
-    value_type_t, ConstructibleStringType>::value >>
+struct is_constructible_string_type
 {
     static constexpr auto value =
         is_constructible<ConstructibleStringType,
         typename BasicJsonType::string_t>::value;
 };
 
-template<typename BasicJsonType, typename ConstructibleStringType>
-struct is_constructible_string_type
-    : is_constructible_string_type_impl<BasicJsonType, ConstructibleStringType> {};
-
 template<typename BasicJsonType, typename CompatibleArrayType, typename = void>
 struct is_compatible_array_type_impl : std::false_type {};
 
@@ -3829,7 +3806,10 @@ struct is_compatible_array_type_impl <
     BasicJsonType, CompatibleArrayType,
     enable_if_t <
     is_detected<iterator_t, CompatibleArrayType>::value&&
-    is_iterator_traits<iterator_traits<detected_t<iterator_t, CompatibleArrayType>>>::value >>
+    is_iterator_traits<iterator_traits<detected_t<iterator_t, CompatibleArrayType>>>::value&&
+// special case for types like std::filesystem::path whose iterator's value_type are themselves
+// c.f. https://github.com/nlohmann/json/pull/3073
+    !std::is_same<CompatibleArrayType, detected_t<range_value_t, CompatibleArrayType>>::value >>
 {
     static constexpr bool value =
         is_constructible<BasicJsonType,
@@ -3862,8 +3842,11 @@ struct is_constructible_array_type_impl <
 is_detected<iterator_t, ConstructibleArrayType>::value&&
 is_iterator_traits<iterator_traits<detected_t<iterator_t, ConstructibleArrayType>>>::value&&
 is_detected<range_value_t, ConstructibleArrayType>::value&&
-is_complete_type <
-detected_t<range_value_t, ConstructibleArrayType >>::value >>
+// special case for types like std::filesystem::path whose iterator's value_type are themselves
+// c.f. https://github.com/nlohmann/json/pull/3073
+!std::is_same<ConstructibleArrayType, detected_t<range_value_t, ConstructibleArrayType>>::value&&
+        is_complete_type <
+        detected_t<range_value_t, ConstructibleArrayType >>::value >>
 {
     using value_type = range_value_t<ConstructibleArrayType>;
 
@@ -3967,6 +3950,10 @@ T conditional_static_cast(U value)
 // #include <nlohmann/detail/value_t.hpp>
 
 
+#ifdef JSON_HAS_CPP_17
+    #include <filesystem>
+#endif
+
 namespace nlohmann
 {
 namespace detail
@@ -4117,7 +4104,7 @@ void from_json(const BasicJsonType& j, std::valarray<T>& l)
 }
 
 template<typename BasicJsonType, typename T, std::size_t N>
-auto from_json(const BasicJsonType& j, T (&arr)[N]) // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
+auto from_json(const BasicJsonType& j, T (&arr)[N])  // NOLINT(cppcoreguidelines-avoid-c-arrays,hicpp-avoid-c-arrays,modernize-avoid-c-arrays)
 -> decltype(j.template get<T>(), void())
 {
     for (std::size_t i = 0; i < N; ++i)
@@ -4392,6 +4379,18 @@ void from_json(const BasicJsonType& j, std::unordered_map<Key, Value, Hash, KeyE
     }
 }
 
+#ifdef JSON_HAS_CPP_17
+template<typename BasicJsonType>
+void from_json(const BasicJsonType& j, std::filesystem::path& p)
+{
+    if (JSON_HEDLEY_UNLIKELY(!j.is_string()))
+    {
+        JSON_THROW(type_error::create(302, "type must be string, but is " + std::string(j.type_name()), j));
+    }
+    p = *j.template get_ptr<const typename BasicJsonType::string_t*>();
+}
+#endif
+
 struct from_json_fn
 {
     template<typename BasicJsonType, typename T>
@@ -4425,6 +4424,8 @@ constexpr const auto& from_json = detail::static_const<detail::from_json_fn>::va
 #include <valarray> // valarray
 #include <vector> // vector
 
+// #include <nlohmann/detail/macro_scope.hpp>
+
 // #include <nlohmann/detail/iterators/iteration_proxy.hpp>
 
 
@@ -4625,6 +4626,10 @@ class tuple_element<N, ::nlohmann::detail::iteration_proxy_value<IteratorType >>
 // #include <nlohmann/detail/value_t.hpp>
 
 
+#ifdef JSON_HAS_CPP_17
+    #include <filesystem>
+#endif
+
 namespace nlohmann
 {
 namespace detail
@@ -4997,6 +5002,14 @@ void to_json(BasicJsonType& j, const T& t)
     to_json_tuple_impl(j, t, make_index_sequence<std::tuple_size<T>::value> {});
 }
 
+#ifdef JSON_HAS_CPP_17
+template<typename BasicJsonType>
+void to_json(BasicJsonType& j, const std::filesystem::path& p)
+{
+    j = p.string();
+}
+#endif
+
 struct to_json_fn
 {
     template<typename BasicJsonType, typename T>