I have a question simplified as below:

let rec fib = fun x -> fib (x - 1);;

val fib : int -> 'a = <fun>

# let rec fib = (Obj.magic (fun x -> fib (x - 1)) : int -> 'a);;

Characters 15-47:

let rec fib = (Obj.magic (fun x -> fib (x - 1)) : int -> 'a);;

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Error: This kind of expression is not allowed as right-hand side of `let rec'

I know `fib` is a legal expression, is there any way to work around it?

Thanks -- Hongbo