Bad occurs-check error message

Consider this:

{-# LANGUAGE  TypeFamilies, ScopedTypeVariables #-}
module Map where

import Prelude hiding ( lookup )

class Key k where
  type Map k :: * -> *
  lookup :: k -> Map k elt -> Maybe elt

instance (Key a, Key b) => Key (a,b) where
  type Map (a,b) = MP a b
  lookup (a,b) (m :: Map (a,b) elt) 
     = case lookup a m :: Maybe (Map b elt) of
	  Just (m2 :: Map b elt) -> lookup b m2 :: Maybe elt

data MP a b elt = MP (Map a (Map b elt))

This ought to typecheck, even in the absence of all those type signatures. But alas:

Map.hs:13:12:
    Occurs check: cannot construct the infinite type: elt = t elt
    In the expression: lookup a m :: Maybe (Map b elt)
    In the expression:
        case lookup a m :: Maybe (Map b elt) of {
          Just (m2 :: Map b elt) -> lookup b m2 :: Maybe elt }

Simon

Trac metadata

Trac field	Value
Version	6.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture