A code that exhibits "Impossible case alternative"

IntegerToInt.hs

{-# LANGUAGE MagicHash #-}
module IntegerToInt where
import GHC.Integer.GMP.Internals (Integer (S#))
-- import GHC.Num.Integer (Integer (IS))
import GHC.Exts (Int (I#))

-- Like Data.Bits.toIntegralSized, but optimized for Integer and Int
integerToIntMaybe :: Integer -> Maybe Int
integerToIntMaybe (S# x) = Just (I# x)
-- integerToIntMaybe (IS x) = Just (I# x)
integerToIntMaybe _      = Nothing -- relies on Integer's invariant
{-# INLINE [0] integerToIntMaybe #-} -- --> Impossible case alternative
-- {-# NOINLINE integerToIntMaybe #-} -- --> Segmentation fault

minBoundIntAsInteger :: Integer
minBoundIntAsInteger = fromIntegral (minBound :: Int)
{-# INLINE minBoundIntAsInteger #-}

maxBoundIntAsInteger :: Integer
maxBoundIntAsInteger = fromIntegral (maxBound :: Int)
{-# INLINE maxBoundIntAsInteger #-}

-- Helper function
integerToIntMaybe2 :: Bool -> Integer -> Maybe Int
integerToIntMaybe2 _ x = integerToIntMaybe x
-- No error with
--   integerToIntMaybe2 _ = integerToIntMaybe
{-# INLINE [0] integerToIntMaybe2 #-}

{-# RULES
"integerToIntMaybe" [~0] forall x.
  integerToIntMaybe x = integerToIntMaybe2 (minBoundIntAsInteger <= x && x <= maxBoundIntAsInteger) x
"integerToIntMaybe2/small" forall x.
  integerToIntMaybe2 True x = Just (fromIntegral x)
"integerToIntMaybe2/large" forall x.
  integerToIntMaybe2 False x = Nothing
  #-}

Main.hs

import IntegerToInt

noinline :: a -> a
noinline x = x
{-# NOINLINE noinline #-}

main = do
  print (noinline integerToIntMaybe 3141) -- Just 3141 (at runtime)
  print (integerToIntMaybe (2^20)) -- Just 1048576 (at runtime)
  print (integerToIntMaybe 314159265398979323846264338327950) -- Nothing (at compile time)
  print (integerToIntMaybe 3141) -- Just 3141 (at compile time)