The standard explicitly permits documented behaviour, even though it doesn't have to, because "undefined" covers that option, along with anything else you care

The standard explicitly permits documented behaviour, even though it doesn't have to, because "undefined" covers that option, along with anything else you care to imagine: http://port70.net/~nsz/c/c11/n1570.html#3.4.3 http://port70.net/~nsz/c/c11/n1570.html#3.4.3