Function checkupval

extlibs/lua/src/ldblib.c:284–290

** Check whether a given upvalue from a given closure exists and ** returns its index */

** returns its index
*/
static int checkupval (lua_State *L, int argf, int argnup) {
  int nup = (int)luaL_checkinteger(L, argnup);  /* upvalue index */
  luaL_checktype(L, argf, LUA_TFUNCTION);  /* closure */
  luaL_argcheck(L, (lua_getupvalue(L, argf, nup) != NULL), argnup,
                   "invalid upvalue index");
  return nup;
}


static int db_upvalueid (lua_State *L) {