What is correct syntax to declare an unsigned 8 byte integer

In the documentation unsigned 8 byte integers are described as

unsigned 8-byte int

however the compiler will not accept

foo is unsigned 8-byte int

however it will accept

foo is unsigned int on 8 bytes

I forget where I found the syntax above, but it is accepted by the compiler. Since it is not documented, is there a documented way to declare an 8 byte integer that I am missing?