type families: poor error massages when lacking instances

The following code:

{-# LANGUAGE TypeFamilies #-}

type family Elem a

class Collection a where
 cons :: Elem a -> a -> a

-- type instance Elem [a] = a

instance Collection [a] where
 cons = (:)

clearly should not compile, as I have commented out the instance declaration for 'Elem [a]'.

The error message provided is less than helpful:

GHCi, version 6.8.0.20071012: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
[1 of 1] Compiling Main             ( Test.hs, interpreted )

Test.hs:10:0:
    Occurs check: cannot construct the infinite type: a = Elem [a]
    When trying to generalise the type inferred for `cons'
      Signature type:     forall a. Elem [a] -> [a] -> [a]
      Type to generalise: Elem [a] -> [a] -> [a]
    In the instance declaration for `Collection [a]'
Failed, modules loaded: none.

Trac metadata

Trac field	Value
Version	6.8
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	aslatter@gmail.com
Operating system	Unknown
Architecture	Unknown