#8001: Coercion causes "impossible" error -------------------------------+-------------------------------------------- Reporter: gridaphobe | Owner: Type: bug | Status: new Priority: normal | Component: Compiler Version: 7.6.3 | Keywords: Os: Linux | Architecture: x86_64 (amd64) Failure: Compile-time crash | Blockedby: Blocking: | Related: -------------------------------+-------------------------------------------- The following, heavily-redacted version of a function in `text` is causing GHC to die after reporting that the impossible happened. {{{ module Encoding where import Data.ByteString as B import Data.ByteString.Internal as B import Data.Text.Internal (Text(..), safe, textP) import Data.Word (Word8) import Foreign.C.Types (CSize) import Foreign.ForeignPtr (withForeignPtr, ForeignPtr) import Foreign.Ptr (Ptr, minusPtr, plusPtr) -- | Encode text using UTF-8 encoding. encodeUtf8 :: Text -> ByteString encodeUtf8 (Text arr off len) = inlinePerformIO $ do let size0 = max len 4 mallocByteString size0 >>= start size0 off 0 where -- start :: Int -> Int -> Int -> ForeignPtr Word8 -> IO ByteString start size n0 m0 fp = withForeignPtr fp $ go n0 m0 where offLen = off + len go n m ptr | n == offLen = return (PS fp 0 m) | otherwise = do let ensure k act | size-m >= k = act | otherwise = do let newSize = size fp' <- mallocByteString newSize start newSize n m fp' {-# INLINE ensure #-} ensure 4 $ return $ PS fp 0 m }}} {{{ $ ghc -fext-core Encoding.hs [1 of 1] Compiling Encoding ( Encoding.hs, Encoding.o ) ghc: panic! (the 'impossible' happened) (GHC version 7.6.3 for x86_64-unknown-linux): make_exp (App _ (Coercion _)) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} Note that it only happens when I try to make GHC output the Core, it will happily compile otherwise. The crash can also be prevented by uncommenting the type-annotation or removing the inline-annotation. -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/8001 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler