Does const allow for (theoretical) optimization here?

The standard says in [dcl.type.cv]:

Except that any class member declared mutable […] can be modified, any attempt to modify […] a const object […] during its lifetime […] results in undefined behavior.

It is also not possible to make this defined by ending the lifetime of the object prematurely, according to [basic.life]:

Creating a new object within the storage that a const complete object with […] automatic storage duration occupies, or within the storage that such a const object used to occupy before its lifetime ended, results in undefined behavior.

This means that the optimization of x - y to zero is valid because any attempt to modify x in foo would result in undefined behavior.

The interesting question is if there is a reason for not performing this optimization in existing compilers. Considering that the const object definition is local to test2 and the fact is used within the same function, usual exceptions such as support for symbol interposition do not apply here.