n + k patterns

GHCi infers for

> fac 0                         =  1
> fac (n + 1)                   =  (n + 1) * fac n

the following type

  / _ \ /\  /\/ __(_)
 / /_\// /_/ / /  | |      GHC Interactive, version 5.04.2, for Haskell 98.
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.

Loading package base ... linking ... done.
Loading package haskell98 ... linking ... done.
Compiling Main             ( Fac.lhs, interpreted )
Ok, modules loaded: Main.
*Main> :t fac
forall a. (Num a, Ord a) => a -> a

The Report, however, states that

        3.17.2  Informal Semantics of Pattern Matching
        <...>
        An n+k pattern can only be matched against a value in the class 
Integral. 

Cheers, Ralf