C++ :: Setting All Bits Within Signed Int With Exception To MSB

I tried to write a little bit of code to set all bits within a signed int with exception to the MSB, yielding the greatest max positive value. The odd part is that it works for shorts ints, and longs, which are 2, 4 and 4 bytes respectively, however long longs, with a size of 8 bytes, simply yields -1, which would indicate that it failed to clear the MSB. Heres the little segment in question:

template<typename T>
T getMax() {
return my::numerics<T>::is_signed ?
~0 ^ (1 << ((sizeof(T)*8)-1) ):
~0;
}

my::numerics is just an exercise- its thoroughly tested and I'm certain thats not the issue.

For shorts, ints, longs, this yields the maximum value. However, when I use it on long longs, the output is 0xFFFFFFFFFFFFFFFF, i.e. ~0. Obviously this means the maximum value for unsigned long longs, but -1 for signed long longs.