fixST is a bit wrong

Lazy blackholing lets some ST calculations complete that shouldn't.

import Control.Monad.ST.Strict
import Control.Monad.Fix
import Data.STRef

foo :: ST s Int
foo = do
  ref <- newSTRef True
  mfix $ \res -> do
    x <- readSTRef ref
    if x
      then do
        writeSTRef ref False
        return $! (res + 5)
      else return 10

main = print $ runST foo

When this is compiled with -O -feager-blackholing, it produces a <<loop>> exception as expected. When it's compiled with -O0 or with -fno-eager-blackholing, it prints 15.

Should we reimplement fixST to do something like what fixIO does? Or should we consider this sometimes-lost bottom tolerable?

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Core Libraries
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture