3 files changed, 63 insertions, 18 deletions

diff --git a/indra/llcommon/lua_function.cpp b/indra/llcommon/lua_function.cpp
index 255385b8c4..2d08de68c5 100644
--- a/indra/llcommon/lua_function.cpp
+++ b/indra/llcommon/lua_function.cpp
@@ -38,6 +38,8 @@ const S32 INTERRUPTS_SUSPEND_LIMIT = 100;
 #define lua_register(L, n, f) (lua_pushcfunction(L, (f), n), lua_setglobal(L, (n)))
 #define lua_rawlen lua_objlen
 
+int DistinctInt::mValues{0};
+
 /*****************************************************************************
 *   luau namespace
 *****************************************************************************/
diff --git a/indra/llcommon/lua_function.h b/indra/llcommon/lua_function.h
index 284f911fa8..c32a586d79 100644
--- a/indra/llcommon/lua_function.h
+++ b/indra/llcommon/lua_function.h
@@ -195,26 +195,34 @@ int name##_luasub::call(lua_State* L)
 /*****************************************************************************
 *   lua_emplace<T>(), lua_toclass<T>()
 *****************************************************************************/
-namespace {
+// Every instance of DistinctInt has a different int value, barring int
+// wraparound.
+class DistinctInt
+{
+public:
+    DistinctInt(): mValue(++mValues) {}
+    int get() const { return mValue; }
+    operator int() const { return mValue; }
+private:
+    static int mValues;
+    int mValue;
+};
 
-// If we start engaging lua_emplace<T>() from more than one thread, type_tags
-// will need locking.
-std::unordered_map<std::type_index, int> type_tags;
+namespace {
 
-// find or create a new Luau userdata "tag" for type T
 template <typename T>
-int type_tag()
+struct TypeTag
 {
-    // The first time we encounter a given type T, assign a new distinct tag
-    // value based on the number of previously-created tags. But avoid tag 0,
-    // which is evidently the default for userdata objects created without
-    // explicit tags. Don't try to destroy a nonexistent T object in a random
-    // userdata object!
-    auto [entry, created] = type_tags.emplace(std::type_index(typeid(T)), int(type_tags.size()+1));
-    // Luau only permits up to LUA_UTAG_LIMIT distinct userdata tags (ca. 128)
-    llassert(entry->second < LUA_UTAG_LIMIT);
-    return entry->second;
-}
+    // For   (std::is_same<T, U>), &TypeTag<T>::value == &TypeTag<U>::value.
+    // For (! std::is_same<T, U>), &TypeTag<T>::value != &TypeTag<U>::value.
+    // And every distinct instance of DistinctInt has a distinct value.
+    // Therefore, TypeTag<T>::value is an int uniquely associated with each
+    // distinct T.
+    static DistinctInt value;
+};
+
+template <typename T>
+DistinctInt TypeTag<T>::value;
 
 } // anonymous namespace
 
@@ -233,7 +241,7 @@ template <class T, typename... ARGS>
 void lua_emplace(lua_State* L, ARGS&&... args)
 {
     luaL_checkstack(L, 1, nullptr);
-    int tag{ type_tag<T>() };
+    int tag{ TypeTag<T>::value };
     if (! lua_getuserdatadtor(L, tag))
     {
         // We haven't yet told THIS lua_State the destructor to use for this tag.
@@ -267,7 +275,7 @@ template <class T>
 T* lua_toclass(lua_State* L, int index)
 {
     // get void* pointer to userdata (if that's what it is)
-    void* ptr{ lua_touserdatatagged(L, index, type_tag<T>()) };
+    void* ptr{ lua_touserdatatagged(L, index, TypeTag<T>::value) };
     // Derive the T* from ptr. If in future lua_emplace() must manually
     // align our T* within the Lua-provided void*, adjust accordingly.
     return static_cast<T*>(ptr);
diff --git a/indra/newview/tests/llluamanager_test.cpp b/indra/newview/tests/llluamanager_test.cpp
index 2d525f7913..d3fc70dfd5 100644
--- a/indra/newview/tests/llluamanager_test.cpp
+++ b/indra/newview/tests/llluamanager_test.cpp
@@ -465,4 +465,39 @@ namespace tut
         ensure_equals(desc + " count: " + result.asString(), count, -1);
         ensure_contains(desc + " result", result.asString(), "terminated");
     }
+
+    template <typename T>
+    struct Visible
+    {
+        Visible(T name): name(name)
+        {
+            LL_INFOS() << "Visible<" << LLError::Log::classname<T>() << ">('" << name << "')" << LL_ENDL;
+        }
+        Visible(const Visible&) = delete;
+        Visible& operator=(const Visible&) = delete;
+        ~Visible()
+        {
+            LL_INFOS() << "~Visible<" << LLError::Log::classname<T>() << ">('" << name << "')" << LL_ENDL;
+        }
+        T name;
+    };
+
+    template<> template<>
+    void object::test<9>()
+    {
+        set_test_name("track distinct lua_emplace<T>() types");
+        LuaState L;
+        lua_emplace<Visible<std::string>>(L, "std::string 0");
+        int st0tag = lua_userdatatag(L, -1);
+        lua_emplace<Visible<const char*>>(L, "const char* 0");
+        int cp0tag = lua_userdatatag(L, -1);
+        lua_emplace<Visible<std::string>>(L, "std::string 1");
+        int st1tag = lua_userdatatag(L, -1);
+        lua_emplace<Visible<const char*>>(L, "const char* 1");
+        int cp1tag = lua_userdatatag(L, -1);
+        lua_settop(L, 0);
+        ensure_equals("lua_emplace<std::string>() tags diverge", st0tag, st1tag);
+        ensure_equals("lua_emplace<const char*>() tags diverge", cp0tag, cp1tag);
+        ensure_not_equals("lua_emplace<>() tags collide", st0tag, cp0tag);
+    }
 } // namespace tut