I personally prefer to make the error state part of the objects: Streams can be in an error state, floats can be NaN and integers should be low(int) if they are invalid (low(int) is a pointless value anyway as it has no positive equivalent).
It's fine to pick sentinel values for errors in context, but describing 0x80000000 as "pointless" in general with such a weak justification doesn't inspire confidence. proc fib[T: Fibable](a: T): T =
if a <= 2:
result = 1
else:
result = fib(a-1) + fib(a-2)
Integer is the only possible type for T in this implementation, so what was the point of defining Fibable?