diff --git a/luprex/cpp/core/luastack.hpp b/luprex/cpp/core/luastack.hpp
index 1cb3ab44..c64ac676 100644
--- a/luprex/cpp/core/luastack.hpp
+++ b/luprex/cpp/core/luastack.hpp
@@ -206,6 +206,41 @@ public:
     friend class LuaDefStack;
 };
 
+struct LuaArgCounts {
+    int nret;
+    int narg;
+    int nvar;
+    int nextra;
+    constexpr LuaArgCounts(int nr, int na, int nv, int ne) : nret(nr), narg(na), nvar(nv), nextra(ne) {}
+    constexpr LuaArgCounts operator +(LuaArgCounts b) const {
+        return LuaArgCounts(nret + b.nret, narg + b.narg, nvar + b.nvar, nextra + b.nextra);
+    }
+};
+
+
+template<class... Ts>
+struct LuaCountArgs;
+template<>
+struct LuaCountArgs<> {
+    static constexpr LuaArgCounts value = LuaArgCounts(0,0,0,0);
+};
+template<class... Ts>
+struct LuaCountArgs<LuaRet, Ts...> {
+    static constexpr LuaArgCounts value = LuaArgCounts(1, 0, 0, 0) + LuaCountArgs<Ts...>::value;
+};
+template<class... Ts>
+struct LuaCountArgs<LuaArg, Ts...> {
+    static constexpr LuaArgCounts value = LuaArgCounts(0, 1, 0, 0) + LuaCountArgs<Ts...>::value;
+};
+template<class... Ts>
+struct LuaCountArgs<LuaVar, Ts...> {
+    static constexpr LuaArgCounts value = LuaArgCounts(0, 0, 1, 0) + LuaCountArgs<Ts...>::value;
+};
+template<class... Ts>
+struct LuaCountArgs<LuaExtraArgs, Ts...> {
+    static constexpr LuaArgCounts value = LuaArgCounts(0, 0, 0, 1) + LuaCountArgs<Ts...>::value;
+};
+
 
 class LuaNilMarker {};
 extern LuaNilMarker LuaNil;
@@ -291,51 +326,8 @@ protected:
     lua_State *L_;
 
 protected:
-    struct Counts {
-        int nret;
-        int narg;
-        int nvar;
-        int nextra;
-        constexpr Counts(int nr, int na, int nv, int ne) : nret(nr), narg(na), nvar(nv), nextra(ne) {}
-		constexpr Counts(Counts a, Counts b)
-			: nret(a.nret+b.nret), narg(a.narg+b.narg), nvar(a.nvar+b.nvar), nextra(a.nextra+b.nextra) {}
-    };
 
-    template<class... SS>
-    constexpr static Counts count_slots(LuaRet &v, SS && ... stackslots)
-    {
-        return Counts(Counts(1, 0, 0, 0), count_slots(stackslots...));
-    }
-    template<class... SS>
-    constexpr static Counts count_slots(LuaArg &v, SS && ... stackslots)
-    {
-        return Counts(Counts(0, 1, 0, 0), count_slots(stackslots...));
-    }
-    template<class... SS>
-    constexpr static Counts count_slots(LuaVar &v, SS && ... stackslots)
-    {
-        return Counts(Counts(0, 0, 1, 0), count_slots(stackslots...));
-    }
-    template<class... SS>
-    constexpr static Counts count_slots(LuaExtraArgs &v, SS && ... stackslots)
-    {
-        return Counts(Counts(0, 0, 0, 1), count_slots(stackslots...));
-    }
-    constexpr static Counts count_slots() {
-        return Counts(0, 0, 0, 0);
-    }
-
-
-    template<class... SS>
-    constexpr static int count_vars(LuaVar &v, SS && ... stackslots)
-    {
-        return 1 + count_vars(stackslots...);
-    }
-    constexpr static int count_vars() {
-        return 0;
-    }
-
-    private:
+private:
     // Push any value on the stack, by type.
     void push_any_value(LuaNewTableMarker s) const { lua_newtable(L_); }
     void push_any_value(LuaNilMarker s) const { lua_pushnil(L_); }
@@ -599,7 +591,7 @@ private:
 public:
     template<class... SS>
     inline LuaDefStack(lua_State *L, SS & ... stackslots) : LuaCoreStack(L) {
-        Counts counts = count_slots(stackslots...);
+        constexpr LuaArgCounts counts = LuaCountArgs<SS...>::value;
         int nargs = lua_gettop(L);
         if (counts.nextra == 0) {
             if (nargs != counts.narg) {
@@ -657,10 +649,13 @@ private:
 public:
     template<class... SS>
     LuaExtStack(lua_State *L, SS & ... stackslots) : LuaCoreStack(L) {
-        int nvars = count_vars(stackslots...);
-        lua_checkstack(L_, nvars + 20);
+        constexpr LuaArgCounts counts = LuaCountArgs<SS...>::value;
+        static_assert(counts.narg == 0, "LuaExtStack only allows LuaVar, not LuaArg");
+        static_assert(counts.nret == 0, "LuaExtStack only allows LuaVar, not LuaRet");
+        static_assert(counts.nextra == 0, "LuaExtStack only allows LuaVar, not LuaExtraArgs");
+        lua_checkstack(L_, counts.nvar + 20);
         oldtop_ = lua_gettop(L_);
-        for (int i = 0; i < nvars; i++) {
+        for (int i = 0; i < counts.nvar; i++) {
             lua_pushnil(L_);
         }
         assign_slots(oldtop_ + 1, stackslots...);