I'm not even sure what that would mean since the machinery to compute the value won't exist in the binary.

You can get this behavior by initializing a variable with the value of another variable whose value is a constexpr. This is a case in which the order of initialization of global variables is not ambiguous!

    constexpr int foo_init_value = 2 + 4;
    int foo = foo_init_value;

Actually not a bad idiom.

But like you I do wish you could apply constexpr-like behavior to expressions rather than just statements:

What you're If you could really apply really applied to expressions you could write something like

    int foo = _compute_at_compile_time(2 + 4);

Of course the compiler can already do this degenerate example for you.