Logical !! versus nothing

Question

I just found this in Microsoft's guiddef.h header file:

__inline bool operator==(REFGUID guidOne, REFGUID guidOther)
{
    return !!IsEqualGUID(guidOne,guidOther);
}

Is there any point to the !!, or was some dev just feeling cute that day?

Answer 1

It turns off Visual C++ silly performance warning for the conversion to boolean.

Answer 2

In this particular case, Alf is probably right.

Otherwise it is common idiom to standardize integer values to either 1 (if they start nonzero) or 0 (if they start as zero) for logical operations.