Coercion causes "impossible" error

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.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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