Function lua_upvaluejoin

lua/src/lapi.c:1456–1464

LUA_API void lua_upvaluejoin (lua_State *L, int fidx1, int n1,
                                            int fidx2, int n2) {
  LClosure *f1;
  UpVal **up1 = getupvalref(L, fidx1, n1, &f1);
  UpVal **up2 = getupvalref(L, fidx2, n2, NULL);
  api_check(L, *up1 != NULL && *up2 != NULL, "invalid upvalue index");
  *up1 = *up2;
  luaC_objbarrier(L, f1, *up1);
}