From ca10ccfbc968e9ab0b1c853a469b9f84a7caca5f Mon Sep 17 00:00:00 2001
From: wuyangfan <1102042793@qq.com>
Date: Sun, 17 May 2026 22:33:42 +0800
Subject: [PATCH] fix: support zero-member types in NLOHMANN_DEFINE_TYPE_*
 macros

Add NLOHMANN_JSON_PASTE0 and NLOHMANN_JSON_DOUBLE_PASTE0 so empty
member lists expand to no-op serialization code.

Fixes #4041.
---
 include/nlohmann/detail/macro_scope.hpp |  8 ++++--
 tests/src/unit-udt_macro.cpp            | 33 +++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/include/nlohmann/detail/macro_scope.hpp b/include/nlohmann/detail/macro_scope.hpp
index 8a7608ec36..7e5178bc7d 100644
--- a/include/nlohmann/detail/macro_scope.hpp
+++ b/include/nlohmann/detail/macro_scope.hpp
@@ -339,7 +339,9 @@
         NLOHMANN_JSON_PASTE4, \
         NLOHMANN_JSON_PASTE3, \
         NLOHMANN_JSON_PASTE2, \
-        NLOHMANN_JSON_PASTE1)(__VA_ARGS__))
+        NLOHMANN_JSON_PASTE1, \
+        NLOHMANN_JSON_PASTE0)(__VA_ARGS__))
+#define NLOHMANN_JSON_PASTE0(func)
 #define NLOHMANN_JSON_PASTE2(func, v1) func(v1)
 #define NLOHMANN_JSON_PASTE3(func, v1, v2) NLOHMANN_JSON_PASTE2(func, v1) NLOHMANN_JSON_PASTE2(func, v2)
 #define NLOHMANN_JSON_PASTE4(func, v1, v2, v3) NLOHMANN_JSON_PASTE2(func, v1) NLOHMANN_JSON_PASTE3(func, v2, v3)
@@ -467,7 +469,9 @@
         NLOHMANN_JSON_DOUBLE_PASTE3, \
         NLOHMANN_JSON_DOUBLE_PASTE3, \
         NLOHMANN_JSON_DOUBLE_PASTE1, \
-        NLOHMANN_JSON_DOUBLE_PASTE1)(__VA_ARGS__))
+        NLOHMANN_JSON_DOUBLE_PASTE1, \
+        NLOHMANN_JSON_DOUBLE_PASTE0)(__VA_ARGS__))
+#define NLOHMANN_JSON_DOUBLE_PASTE0(func)
 #define NLOHMANN_JSON_DOUBLE_PASTE3(func, v1, v2) func(v1, v2)
 #define NLOHMANN_JSON_DOUBLE_PASTE5(func, v1, v2, v3, v4) NLOHMANN_JSON_DOUBLE_PASTE3(func, v1, v2) NLOHMANN_JSON_DOUBLE_PASTE3(func, v3, v4)
 #define NLOHMANN_JSON_DOUBLE_PASTE7(func, v1, v2, v3, v4, v5, v6) NLOHMANN_JSON_DOUBLE_PASTE3(func, v1, v2) NLOHMANN_JSON_DOUBLE_PASTE5(func, v3, v4, v5, v6)
diff --git a/tests/src/unit-udt_macro.cpp b/tests/src/unit-udt_macro.cpp
index 7fab809212..14dab29221 100644
--- a/tests/src/unit-udt_macro.cpp
+++ b/tests/src/unit-udt_macro.cpp
@@ -778,8 +778,41 @@ class derived_person_only_serialize_private_3 : person_without_default_construct
     NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES(derived_person_only_serialize_private_3, person_without_default_constructor_3, "json_hair_color", hair_color)
 };
 
+class empty_type
+{
+  public:
+    NLOHMANN_DEFINE_TYPE_INTRUSIVE(empty_type)
+
+    bool operator==(const empty_type& /*unused*/) const
+    {
+        return true;
+    }
+};
+
+class empty_type_non_intrusive
+{};
+
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(empty_type_non_intrusive)
+
 } // namespace persons
 
+TEST_CASE("NLOHMANN_DEFINE_TYPE_* with zero members")
+{
+    SECTION("intrusive")
+    {
+        persons::empty_type v;
+        CHECK(json(v).dump() == "{}");
+        CHECK(json::parse("{}").get<persons::empty_type>() == v);
+    }
+
+    SECTION("non-intrusive")
+    {
+        persons::empty_type_non_intrusive v;
+        CHECK(json(v).dump() == "{}");
+        CHECK(json::parse("{}").get<persons::empty_type_non_intrusive>() == v);
+    }
+}
+
 TEST_CASE_TEMPLATE("Serialization/deserialization via NLOHMANN_DEFINE_TYPE_INTRUSIVE and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE", Pair, // NOLINT(readability-math-missing-parentheses, bugprone-throwing-static-initialization)
                    std::pair<nlohmann::json, persons::person_with_private_data>,
                    std::pair<nlohmann::json, persons::person_without_private_data_1>,