Initialization of a Struct with a static Member

Question

If I have a struct:

struct Vec2{
    double _x;
    double _y;
};

I can initialize that like this:

Vec2 foo = {13.0, 42.0};

But what if I have this struct?

struct Vec2{
    static const double INVALID;

    double _x;
    double _y;
};

Q. How do I initialize that struct?