Applying the lvalue-to-rvalue conversion to variable `x` does not yield a constant expression in the code below. Why is that?

The snippet below was obtained from DR 2083 by Hubert Tong.

extern int globx;
int main() {
    const int &x = globx;
    struct A {
        const int *foo() { return &x; }
    } a;
    return *a.foo();
}

Then the author writes:

x satisfies the requirements for appearing in a constant expression, but applying the lvalue-to-rvalue converstion to x does not yield a constant expression.

Why applying the lvalue-to-rvalue conversion to x does not yield a constant-expression?

Answer

Applying the lvalue-to-rvalue conversion to x reads the value of the mutable global variable globx, which makes it not a constant expression as the value of globx is subject to change (and, even if it were const, there would be the issue of its value not being known at compile time). Of course, this is not surprising: no one would expect int{x} to be a constant expression. The DR points out that this fact should, nonetheless, not cause the odr-use of x and thus the code should be well-formed: since x has constant initialization, the compiler can just translate &x into &globx.