from C11 standards N1570 draft
I believe this is UB per the above: (it is obviously contrived)
J.2 Undefined behavior
— An lvalue designating an object of automatic storage duration that could have been declared with the register storage class is used in a context that requires the value of the designated object, but the object is uninitialized. (18.104.22.168).
If that is not the correct reference in the C11 standard, then please show me the correct one. Thanks.