Bad code with type families

{-# LANGUAGE TypeFamilies, ScopedTypeVariables #-}
module Foo (foo) where

import Control.Monad.ST
import Data.STRef

class V v where
  type M v :: * -> *

  rd :: v -> Int -> M v Int

data Vec s = Vec (STRef s Int)

instance V (Vec s) where
  type M (Vec s) = ST s

  rd (Vec r) n = do
                   m <- readSTRef r
                   return (m+n)

foo :: forall s. Vec s -> Int -> ST s Int
foo v n = go n
  where
    go 0 = return 0
    go n = do
             m <- rd v n -- :: ST s Int
             go m

ghc -O2 generates rather bad code for this (look at $wfoo). If I uncomment the type signature in the second to last line, the code becomes much better. It would be nice if this worked without the signature since I can't add it in the real code.

Trac metadata

Trac field	Value
Version	6.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Operating system	Unknown
Architecture	Unknown