Two assignments to the same variable in the same statement are undefined for the same reason i++ + ++i is undefined. That said, even if it were not, you did no

Two assignments to the same variable in the same statement are undefined for the same reason i++ + ++i is undefined. That said, even if it were not, you did not define x and y. Reading them is also undefined behavior, which is what I initially though you meant until I read what followed them.