[GHC] #5441: unsafeCoerce'ing function to GHC.Prim.Any causes segfault

[GHC]

#5441: unsafeCoerce'ing function to GHC.Prim.Any causes segfault
---------------------------------+------------------------------------------
    Reporter:  ulfn              |       Owner:              
        Type:  bug               |      Status:  new         
    Priority:  normal            |   Component:  Compiler    
     Version:  7.2.1             |    Keywords:              
    Testcase:                    |   Blockedby:              
          Os:  Unknown/Multiple  |    Blocking:              
Architecture:  x86               |     Failure:  None/Unknown
---------------------------------+------------------------------------------
 The following module is the smallest one I've been able to find exhibiting
 the bug. Any simplification (that I've tried) results in the problem going
 away.

 module Any where

 import Unsafe.Coerce (unsafeCoerce)
 import GHC.Prim (Any)

 listmap :: (a -> b) -> [a] -> [b]
 listmap f []       = []
 listmap f (x : xs) = f x : listmap f xs

 data Nat = Z | S Nat

 {-# NOINLINE inject #-}
 inject :: Nat -> Nat -> Nat
 inject m i = i

 {-# NOINLINE look #-}
 look :: Nat -> String -> Char
 look Z _ = '0'

 showDigit :: Nat -> () -> Nat -> Char
 showDigit base prf d = look (inject base d) ""

 toDigits :: Nat -> Nat -> [Nat]
 toDigits Z Z = [Z]

 coe1 :: (Nat -> String) -> Any
 coe1 = unsafeCoerce

 coe2 :: Any -> (Nat -> String)
 coe2 = unsafeCoerce

 showInBase :: Nat -> Any
 showInBase base
   = coe1 (\n -> listmap
                 (showDigit base ())
                 (toDigits base n))

 showNat :: Nat -> String
 showNat = coe2 (showInBase Z)

 Now given this main module (it needs to be a different module)

 import Any
 main = putStrLn (showNat Z)

 Compiling with optimisations:

 $ ghc -O --make Main.hs
 ...
 $ ./Main
 Bus error: 10

 Without optimizations it prints 0 as expected.

 This has been reproduced on 7.0.3 (Mac OS X) and 7.2.1 (some Linux).

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5441>
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