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?