Fix extra comma when unknown writer is empty (#1435)

Author: stephenberry

include/glaze/json/write.hpp

@@ -1363,14 +1363,26 @@ namespace glz
 
                using WriterType = meta_unknown_write_t<ValueType>;
                if constexpr (std::is_member_object_pointer_v<WriterType>) {
-                  // TODO: This intermediate is added to get GCC 14 to build
-                  decltype(auto) merged = glz::merge{value, value.*writer};
-                  write<JSON>::op<disable_write_unknown_on<Options>()>(std::move(merged), ctx, b, ix);
+                  decltype(auto) unknown_writer = value.*writer;
+                  if (unknown_writer.size() > 0) {
+                     // TODO: This intermediate is added to get GCC 14 to build
+                     decltype(auto) merged = glz::merge{value, unknown_writer};
+                     write<JSON>::op<disable_write_unknown_on<Options>()>(std::move(merged), ctx, b, ix);
+                  }
+                  else {
+                     write<JSON>::op<disable_write_unknown_on<Options>()>(value, ctx, b, ix);
+                  }
                }
                else if constexpr (std::is_member_function_pointer_v<WriterType>) {
-                  // TODO: This intermediate is added to get GCC 14 to build
-                  decltype(auto) merged = glz::merge{value, (value.*writer)()};
-                  write<JSON>::op<disable_write_unknown_on<Options>()>(std::move(merged), ctx, b, ix);
+                  decltype(auto) unknown_writer = (value.*writer)();
+                  if (unknown_writer.size() > 0) {
+                     // TODO: This intermediate is added to get GCC 14 to build
+                     decltype(auto) merged = glz::merge{value, unknown_writer};
+                     write<JSON>::op<disable_write_unknown_on<Options>()>(std::move(merged), ctx, b, ix);
+                  }
+                  else {
+                     write<JSON>::op<disable_write_unknown_on<Options>()>(value, ctx, b, ix);
+                  }
                }
                else {
                   static_assert(false_v<T>, "unknown_write type not handled");

tests/json_test/json_test.cpp

@@ -7193,6 +7193,26 @@ struct glz::meta<unknown_fields_2>
    static constexpr auto unknown_read{&T::extra};
 };
 
+struct my_unknown_struct
+{
+    int i = 287;
+    std::unordered_map<std::string, std::string> unknown;
+
+};
+
+template <>
+struct glz::meta<my_unknown_struct> {
+    using T = my_unknown_struct;
+    static constexpr auto value = glz::object(
+        "i", &T::i
+    );
+
+    static constexpr auto unknown_write{ &T::unknown };
+    //static constexpr auto unknown_read{
+    //    &T::unknown
+    //};
+};
+
 suite unknown_fields_member_test = [] {
    "decode_unknown"_test = [] {
       unknown_fields_member obj{};
@@ -7229,6 +7249,18 @@ suite unknown_fields_member_test = [] {
       expect(not glz::write_json(obj, out));
       expect(out == R"({"unk":"zzz","unk2":{"sub":3,"sub2":[{"a":"b"}]},"unk3":[]})") << out;
    };
+   
+   "my_unknown_struct"_test = [] {
+      my_unknown_struct obj;
+      std::string buffer;
+      expect(not glz::write < glz::opts{ .prettify = true } > (obj, buffer));
+      expect(buffer == R"({
+   "i": 287
+})") << buffer;
+      
+      expect(not glz::write < glz::opts{ } > (obj, buffer));
+      expect(buffer == R"({"i":287})") << buffer;
+   };
 };
 
 struct unknown_fields_method