newtype vs data TC problem

The following  module stops with TC problems, but if
you replace the newtype with data, things work fine.
This problem is in the HEAD, but not in 5.00

Andy 

module State2 (StateM, runStateM, genUniqM) where

import ST

newtype StateM a = StateM (forall s . STRef s Int -> 
ST s a)

runStateM :: StateM a -> Int -> (a,Int)
runStateM (StateM fn) u = 
   runST (do { uRef <- newSTRef u
             ; res <- fn uRef
             ; u <- readSTRef uRef
	     ; return (res,u)
             })

unStateM :: StateM a -> STRef b Int -> ST b a
unStateM (StateM fn) u = fn u

genUniqM :: StateM Int
genUniqM = StateM (\ uRef -> do { u <- readSTRef uRef
				; writeSTRef uRef (u+1)
				; return u
				})

instance Monad StateM where
  return a = StateM (\ uRef -> return a)
  m >>= k  = StateM (\ uRef -> do { r <- unStateM m 
uRef
                                  ; unStateM (k r) uRef
				  })

Trac metadata

Trac field	Value
Version	5.0
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	ResolvedFixed
Component	Compiler (Type checker)
Test case
Differential revisions
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CC
Operating system
Architecture