How does this line work? `static bool const unused = (WillOnlyRunOnce(), true)`

I stumbled upon a piece of code similar to this one and I can't grasp it.

```
void WillOnlyRunOnce() {
    std::cout << "WillOnlyRunOnce" << std::endl;
}
void Init() {
    static bool const unused = (WillOnlyRunOnce(), true);
}
void main()
{
    Init();
    Init();
    Init();
}

```