fix std::filesystem::path regression

Antiquated type traits performed an incorrect and insufficient check.

std::filesystem::path used to work by "chance" thanks to brittle
constraints, but the clean-up performed in nlohmann#3020 broke these.

Fixes nlohmann#3070

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 {};
 

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 {};
 

test/src/unit-regression2.cpp

@@ -47,6 +47,7 @@ using ordered_json = nlohmann::ordered_json;
 #endif
 
 #ifdef JSON_HAS_CPP_17
+    #include <filesystem>
     #include <variant>
 #endif
 
@@ -695,6 +696,17 @@ TEST_CASE("regression tests 2")
         json k = json::from_cbor(my_vector);
         CHECK(j == k);
     }
+
+#ifdef JSON_HAS_CPP_17
+    SECTION("issue #3070 - Version 3.10.3 breaks backward-compatibility with 3.10.2 ")
+    {
+        std::filesystem::path text_path("/tmp/text.txt");
+        json j(text_path);
+
+        const auto j_path = j.get<std::filesystem::path>();
+        CHECK(j_path == text_path);
+    }
+#endif
 }
 
 DOCTEST_CLANG_SUPPRESS_WARNING_POP