GHC panic with UnliftedFFITypes+CApiFFI

The following code fails to compile with at least GHC 7.6, 7.8 and HEAD:

import GHC.Prim
import Foreign.C

type GmpLimb = CULong
type GmpSize = CLong

-- mp_limb_t mpn_add_1 (mp_limb_t *rp, const mp_limb_t *s1p, mp_size_t n, mp_limb_t s2limb)

foreign import capi unsafe "gmp.h mpn_add_1"
  c_mpn_add_1 :: MutableByteArray# s -> ByteArray# -> GmpSize -> GmpLimb -> IO GmpLimb

with the following compile error message:

ghc: panic! (the 'impossible' happened)
  (GHC version 7.9.20140704 for x86_64-unknown-linux):
	toCType MutableByteArray# s

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Otoh, replacing capi with ccall makes the code compile just fine

Trac metadata

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