With dependent types, error reported in seemingly unrelated function

In doing some routine dependently typed hackery, I wrote the following:

{-# LANGUAGE TypeFamilies, DataKinds, TypeOperators, GADTs #-}

data Nat = Zero | Succ Nat

data SNat :: Nat -> * where
  SZero :: SNat Zero
  SSucc :: SNat n -> SNat (Succ n)

data HList :: [*] -> * where
  HNil :: HList '[]
  HCons :: h -> HList t -> HList (h ': t)

data Index :: Nat -> [*] -> * where
  IZero :: Index Zero (h ': t)
  ISucc :: Index n l -> Index (Succ n) (h ': l)

type family Lookup (n :: Nat) (l :: [*]) :: *
type instance Lookup Zero (h ': t) = h
type instance Lookup (Succ n) (h ': t) = Lookup n t

hLookup :: Index n l -> HList l -> Lookup n l
hLookup IZero (HCons h _) = h
hLookup (ISucc n) (HCons _ t) = hLookup n t
hLookup _ _ = undefined

So far, so good. But, I wanted a way to convert SNats to Indexes. When I add the (wrong) code below

sNatToIndex :: SNat n -> HList l -> Index n l
sNatToIndex SZero HNil = IZero

I get an error in the second clause of hLookup, '''even though I haven't touched hLookup'''. (I also get an error in sNatToIndex, but that one's OK.)

The error is this:

/Users/rae/temp/Bug.hs:23:33:
    Couldn't match type ‛t1’ with ‛Lookup n l’
      ‛t1’ is untouchable
        inside the constraints (l ~ (':) * h1 t)
        bound by a pattern with constructor
                   HCons :: forall h (t :: [*]). h -> HList t -> HList ((':) * h t),
                 in an equation for ‛hLookup’
        at /Users/rae/temp/Bug.hs:23:20-28
    Expected type: Lookup n l
      Actual type: Lookup n1 l1
    Relevant bindings include
      hLookup :: Index n l -> HList l -> Lookup n l
        (bound at /Users/rae/temp/Bug.hs:22:1)
    In the expression: hLookup n t
    In an equation for ‛hLookup’:
        hLookup (ISucc n) (HCons _ t) = hLookup n t

This was tested with 7.7.20130528.

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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