lua: metatable for empty dict value

src/nvim/lua/converter.c

@@ -156,6 +156,13 @@ static LuaTableProps nlua_traverse_table(lua_State *const lstate)
             && other_keys_num == 0
             && ret.string_keys_num == 0)) {
       ret.type = kObjectTypeArray;
+      if (tsize == 0 && lua_getmetatable(lstate, -1)) {
+        nlua_pushref(lstate, nlua_empty_dict_ref);
+        if (lua_rawequal(lstate, -2, -1)) {
+          ret.type = kObjectTypeDictionary;
+        }
+        lua_pop(lstate, 2);
+      }
     } else if (ret.string_keys_num == tsize) {
       ret.type = kObjectTypeDictionary;
     } else {
@@ -465,6 +472,8 @@ static bool typval_conv_special = false;
         nlua_create_typed_table(lstate, 0, 0, kObjectTypeDictionary); \
       } else { \
         lua_createtable(lstate, 0, 0); \
+        nlua_pushref(lstate, nlua_empty_dict_ref); \
+        lua_setmetatable(lstate, -2); \
       } \
     } while (0)
 
@@ -695,6 +704,10 @@ void nlua_push_Dictionary(lua_State *lstate, const Dictionary dict,
     nlua_create_typed_table(lstate, 0, 0, kObjectTypeDictionary);
   } else {
     lua_createtable(lstate, 0, (int)dict.size);
+    if (dict.size == 0 && !special) {
+      nlua_pushref(lstate, nlua_empty_dict_ref);
+      lua_setmetatable(lstate, -2);
+    }
   }
   for (size_t i = 0; i < dict.size; i++) {
     nlua_push_String(lstate, dict.items[i].key, special);

src/nvim/lua/executor.c

@@ -324,6 +324,13 @@ static int nlua_state_init(lua_State *const lstate) FUNC_ATTR_NONNULL_ALL
   nlua_nil_ref = nlua_ref(lstate, -1);
   lua_setfield(lstate, -2, "NIL");
 
+  // vim._empty_dict_mt
+  lua_createtable(lstate, 0, 0);
+  lua_pushcfunction(lstate, &nlua_empty_dict_tostring);
+  lua_setfield(lstate, -2, "__tostring");
+  nlua_empty_dict_ref = nlua_ref(lstate, -1);
+  lua_setfield(lstate, -2, "_empty_dict_mt");
+
   // internal vim._treesitter... API
   nlua_add_treesitter(lstate);
 
@@ -665,6 +672,12 @@ static int nlua_nil_tostring(lua_State *lstate)
   return 1;
 }
 
+static int nlua_empty_dict_tostring(lua_State *lstate)
+{
+  lua_pushstring(lstate, "vim.empty_dict()");
+  return 1;
+}
+
 
 #ifdef WIN32
 /// os.getenv: override os.getenv to maintain coherency. #9681

src/nvim/lua/executor.h

@@ -13,6 +13,7 @@
 void nlua_add_api_functions(lua_State *lstate) REAL_FATTR_NONNULL_ALL;
 
 EXTERN LuaRef nlua_nil_ref INIT(= LUA_NOREF);
+EXTERN LuaRef nlua_empty_dict_ref INIT(= LUA_NOREF);
 
 #define set_api_error(s, err) \
     do { \

src/nvim/lua/vim.lua

@@ -243,6 +243,10 @@ function vim.schedule_wrap(cb)
   end)
 end
 
+function vim.empty_dict()
+  return setmetatable({}, vim._empty_dict_mt)
+end
+
 -- vim.fn.{func}(...)
 vim.fn = setmetatable({}, {
   __index = function(t, key)