🐛 fix GCC16 issue

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json.hpp

@@ -3570,9 +3570,6 @@ NLOHMANN_JSON_NAMESPACE_END
 #include <tuple> // tuple
 #include <type_traits> // false_type, is_constructible, is_integral, is_same, true_type
 #include <utility> // declval
-#if defined(__cpp_lib_optional) && __cpp_lib_optional >= 201606L
-    #include <optional> // optional
-#endif
 #if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603L
     #include <cstddef> // byte
 #endif
@@ -3820,15 +3817,6 @@ struct is_json_ref : std::false_type {};
 template<typename T>
 struct is_json_ref<json_ref<T>> : std::true_type {};
 
-// trait to detect std::optional<T> specializations
-template<typename>
-struct is_std_optional : std::false_type {};
-
-#if defined(__cpp_lib_optional) && __cpp_lib_optional >= 201606L
-template<typename T>
-struct is_std_optional<std::optional<T>> : std::true_type {};
-#endif
-
 //////////////////////////
 // aliases for detected //
 //////////////////////////
@@ -20607,6 +20595,21 @@ NLOHMANN_JSON_NAMESPACE_END
 */
 NLOHMANN_JSON_NAMESPACE_BEGIN
 
+namespace detail
+{
+// Trait to detect std::optional<T> specializations. It is defined here rather
+// than in type_traits.hpp so that adding the <optional> include does not change
+// the include order of the C++20 module's global module fragment (the include
+// is already pulled in by the conversion headers above); see src/modules/json.cppm.
+template<typename>
+struct is_std_optional : std::false_type {};
+
+#ifdef JSON_HAS_CPP_17
+template<typename T>
+struct is_std_optional<std::optional<T>> : std::true_type {};
+#endif
+}  // namespace detail
+
 /*!
 @brief a class to store JSON values