Sometimes GHC produces join points with unused parameters. In the example attached, we get join points like the following (when compiled with -O2):

...
    $j1_XHI
      :: GHC.Prim.Int#
         -> GHC.Types.Int
         -> (# Unboxed.FingerTree
                 Unboxed.Size (Unboxed.Node Unboxed.Size b_ahY),
               Unboxed.Node Unboxed.Size b_ahY,
               Unboxed.FingerTree
                 Unboxed.Size (Unboxed.Node Unboxed.Size b_ahY) #)
    [LclId, Arity=2, Str=DmdType LL]
    $j1_XHI =
      \ (x2_XE8 :: GHC.Prim.Int#) _ ->
...

which is always called as follows:

...
    $j1_XHI x2_XE2 (GHC.Types.I# x2_XE2)
...

i.e. where the second argument is a boxed version of the first. GHC should remove the dead parameter from the join point, to avoid unnecessary boxing.

I get this Core with 7.0.3 and with 7.1.20110629.

I've attached a self-contained example, as small as I can make it. (Making it smaller lets GHC do more unfolding and the problem disappears.) These join points occur inside the 'Deep' case of '$wsplitTree'.