🐛 fix GCC16 issue

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

include/nlohmann/json.hpp

@@ -86,6 +86,21 @@
 */
 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