Is taking the address of a local variable a constant expression in C++11?

Question

The following C++11 program:

int x = 42;

void f()
{
        int y = 43;

        static_assert(&x < &y, "foo");
}

int main()
{
        f();         


        

Answer 1

Taking the address of something is not the culprit here, but rather the pointer comparison using operator< on unrelated objects.

Relational operators on pointers is only specified for pointers to objects within the same class or within an array (5.9 Relational operators [expr.rel], points 3 and 4.) Relational comparison for pointers to unrelated objects is unspecified.

Comparing the address for equality rather than ordering does work:

int x = 42;

void f()
{
        int y = 43;

        static_assert(&x != &y, "foo");
                         ^^ <--- "<" on unrelated objects is unspecified
}

int main()
{
        f();
}

Live example

Just to show that this has nothing to do with const-expressions per se,

void f()
{
        int y[2] = { 42, 43 };

        static_assert(&y[0] < &y[1], "foo");
                            ^ <--- "<" on objects within an array is specified
}

int main()
{
        f();
}

Another live example.