Note about semantics of (!!) is not specific enough, if it's correct

The definition of (!!) in GHC.List, copied below, has a vague note about semantics for the Prelude and HBC versions being different. I can't see the difference myself. If there really is a difference, the details should be in the comment.

-- | List index (subscript) operator, starting from 0.
-- It is an instance of the more general 'Data.List.genericIndex',
-- which takes an index of any integral type.
(!!)                    :: [a] -> Int -> a
#ifdef USE_REPORT_PRELUDE
xs     !! n | n < 0 =  error "Prelude.!!: negative index"
[]     !! _         =  error "Prelude.!!: index too large"
(x:_)  !! 0         =  x
(_:xs) !! n         =  xs !! (n-1)
#else
-- HBC version (stolen), then unboxified
-- The semantics is not quite the same for error conditions
-- in the more efficient version.
--
xs !! (I# n0) | isTrue# (n0 <# 0#) =  error "Prelude.(!!): negative index\n"
              | otherwise          =  sub xs n0
                         where
                            sub :: [a] -> Int# -> a
                            sub []     _ = error "Prelude.(!!): index too large\n"
                            sub (y:ys) n = if isTrue# (n ==# 0#)
                                           then y
                                           else sub ys (n -# 1#)
#endif

Trac metadata

Trac field	Value
Version	7.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	ekmett, hvr
Operating system
Architecture