'luaD_tryfuncTM' checks stack space by itself

ldo.c

@@ -387,15 +387,17 @@ static void rethook (lua_State *L, CallInfo *ci, int nres) {
 ** stack, below original 'func', so that 'luaD_precall' can call it. Raise
 ** an error if there is no '__call' metafield.
 */
-void luaD_tryfuncTM (lua_State *L, StkId func) {
+StkId luaD_tryfuncTM (lua_State *L, StkId func) {
   const TValue *tm = luaT_gettmbyobj(L, s2v(func), TM_CALL);
   StkId p;
+  checkstackGCp(L, 1, func);  /* space for metamethod */
   if (l_unlikely(ttisnil(tm)))
     luaG_callerror(L, s2v(func));  /* nothing to call */
   for (p = L->top; p > func; p--)  /* open space for metamethod */
     setobjs2s(L, p, p-1);
   L->top++;  /* stack space pre-allocated by the caller */
   setobj2s(L, func, tm);  /* metamethod is the new function to be called */
+  return func;
 }
 
 
@@ -558,8 +560,7 @@ CallInfo *luaD_precall (lua_State *L, StkId func, int nresults) {
       return ci;
     }
     default: {  /* not a function */
-      checkstackGCp(L, 1, func);  /* space for metamethod */
-      luaD_tryfuncTM(L, func);  /* try to get '__call' metamethod */
+      func = luaD_tryfuncTM(L, func);  /* try to get '__call' metamethod */
       goto retry;  /* try again with metamethod */
     }
   }

ldo.h

@@ -62,7 +62,7 @@ LUAI_FUNC void luaD_pretailcall (lua_State *L, CallInfo *ci, StkId func, int n);
 LUAI_FUNC CallInfo *luaD_precall (lua_State *L, StkId func, int nResults);
 LUAI_FUNC void luaD_call (lua_State *L, StkId func, int nResults);
 LUAI_FUNC void luaD_callnoyield (lua_State *L, StkId func, int nResults);
-LUAI_FUNC void luaD_tryfuncTM (lua_State *L, StkId func);
+LUAI_FUNC StkId luaD_tryfuncTM (lua_State *L, StkId func);
 LUAI_FUNC int luaD_closeprotected (lua_State *L, ptrdiff_t level, int status);
 LUAI_FUNC int luaD_pcall (lua_State *L, Pfunc func, void *u,
                                         ptrdiff_t oldtop, ptrdiff_t ef);

lvm.c

@@ -1657,9 +1657,8 @@ void luaV_execute (lua_State *L, CallInfo *ci) {
           lua_assert(base == ci->func + 1);
         }
         while (!ttisfunction(s2v(ra))) {  /* not a function? */
-          luaD_tryfuncTM(L, ra);  /* try '__call' metamethod */
+          ra = luaD_tryfuncTM(L, ra);  /* try '__call' metamethod */
           b++;  /* there is now one extra argument */
-          checkstackGCp(L, 1, ra);
         }
         if (!ttisLclosure(s2v(ra))) {  /* C function? */
           luaD_precall(L, ra, LUA_MULTRET);  /* call it */
@@ -1670,9 +1669,11 @@ void luaV_execute (lua_State *L, CallInfo *ci) {
           updatetrap(ci);  /* 'luaD_poscall' can change hooks */
           goto ret;  /* caller returns after the tail call */
         }
-        ci->func -= delta;  /* restore 'func' (if vararg) */
-        luaD_pretailcall(L, ci, ra, b);  /* prepare call frame */
-        goto startfunc;  /* execute the callee */
+        else {  /* Lua function */
+          ci->func -= delta;  /* restore 'func' (if vararg) */
+          luaD_pretailcall(L, ci, ra, b);  /* prepare call frame */
+          goto startfunc;  /* execute the callee */
+        }
       }
       vmcase(OP_RETURN) {
         int n = GETARG_B(i) - 1;  /* number of results */