Fix #1266

include/sol/stack.hpp

@@ -34,6 +34,7 @@
 #include <sol/stack_pop.hpp>
 #include <sol/stack_field.hpp>
 #include <sol/stack_probe.hpp>
+#include <sol/assert.hpp>
 
 #include <cstring>
 #include <array>
@@ -200,11 +201,21 @@ namespace sol {
 					     argument_types_, argument_indices_, L_, start_index_, handler, tracking, std::forward<Fx>(fx_), std::forward<FxArgs>(args_)...);
 				}
 			}
+
+			template <typename T>
+			void raw_table_set(lua_State* L, T&& arg, int tableindex = -2) {
+				int push_count = push(L, std::forward<T>(arg));
+				sol_c_assert(push_count == 1);
+				std::size_t unique_index = static_cast<std::size_t>(luaL_len(L, tableindex) + 1u);
+				lua_rawseti(L, tableindex, unique_index);
+			}
+
 		} // namespace stack_detail
 
 		template <typename T>
 		int set_ref(lua_State* L, T&& arg, int tableindex = -2) {
-			push(L, std::forward<T>(arg));
+			int push_count = push(L, std::forward<T>(arg));
+			sol_c_assert(push_count == 1);
 			return luaL_ref(L, tableindex);
 		}
 

include/sol/table_core.hpp

@@ -644,7 +644,7 @@ namespace sol {
 			auto pp = stack::push_pop(*this);
 			int table_index = pp.index_of(*this);
 			lua_State* L = base_t::lua_state();
-			(void)detail::swallow { 0, (stack::set_ref(L, std::forward<Args>(args), table_index), 0)... };
+			(void)detail::swallow { 0, (stack::stack_detail::raw_table_set(L, std::forward<Args>(args), table_index), 0)... };
 			return *this;
 		}
 

tests/regression_tests/simple/source/1266 - add method in Lua 5.4 showing freelist.cpp

@@ -0,0 +1,65 @@
+// sol2
+
+// The MIT License (MIT)
+
+// Copyright (c) 2013-2021 Rapptz, ThePhD and contributors
+
+// Permission is hereby granted, free of charge, to any person obtaining a copy of
+// this software and associated documentation files (the "Software"), to deal in
+// the Software without restriction, including without limitation the rights to
+// use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+// the Software, and to permit persons to whom the Software is furnished to do so,
+// subject to the following conditions:
+
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+// FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+// COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+// IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+// CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+#include <catch2/catch_all.hpp>
+
+#include <sol/sol.hpp>
+
+#include <string>
+
+TEST_CASE("Test for Issue #1266 - add method in Lua 5.4 exposes freelist in table, unfortunately", "[sol2][regression][Issue-1266]") {
+	sol::state lua;
+	lua.open_libraries(sol::lib::base);
+
+	const auto& code = R"(
+for k, v in pairs(table) do
+	assert(k == 1) assert(v == 'item')
+end)";
+
+	SECTION("sol2 API") {
+		// Create and add using sol.
+		sol::table table = lua.create_table();
+		table.add("item");
+		lua["table"] = table;
+
+		auto result = lua.safe_script(code, sol::script_pass_on_error);
+		REQUIRE(result.valid());
+	}
+	SECTION("sol2 + C API") {
+		// Create using C API and add using sol.
+		lua_createtable(lua.lua_state(), 0, 0);
+		lua_setglobal(lua.lua_state(), "table");
+		sol::table table = lua["table"];
+		table.add("item");
+
+		auto result = lua.safe_script(code, sol::script_pass_on_error);
+		REQUIRE(result.valid());
+	}
+
+	sol::table table = lua["table"];
+	REQUIRE(table.size() == 1);
+	std::string table_value = table[1];
+	std::string table_value_at_size = table[table.size()];
+	REQUIRE(table_value == "item");
+	REQUIRE(table_value_at_size == "item");
+}