Function that calculates in compile time when it can

[Minas Mina]

I want to write a fibonacci(n) function that calculates the 
result.
a) if n is known at compile time, use a template
b) if not, use a normal function
I know how to write a template version:
template fib(ulong n)
{
        static if( n < 2 )
                const fib = n;
        else
                const fib = fib!(n-1) + fib!(n-2);
}

But how can I 'know' if n is known at compile time to make it use 
the other version? (which I won't post 'cause it is fairly easy).