Untouchable type, pattern synonyms

GHC panicked while experimenting

{-# LANGUAGE PatternSynonyms, ExistentialQuantification, GADTSyntax #-}

module Test where

data Expr a where
  Fun :: String -> (a -> b) -> (Expr a -> Expr b)

pattern IntFun :: () => (a ~ Int) => String -> (a -> b) -> (Expr a -> Expr b)
pattern IntFun str f x = Fun str f x

-- Alternative syntax for pattern synonyms:
--   pattern
--     Suc :: () => (a ~ Int) => Expr a -> Expr Int
--     Suc n <- IntFun _     _     n where
--     Suc n =  IntFun "suc" (+ 1) n
pattern Suc :: () => (a ~ Int) => Expr a -> Expr Int
pattern Suc n <- IntFun _     _     n where
         Suc n =  IntFun "suc" (+ 1) n

% ghc Test.hs
[1 of 1] Compiling Test             ( Test.hs, Test.o )

Test.hs:12:18:
    Couldn't match expected type ‘Int’ with actual type ‘a1’
      ‘a1’ is untouchable
        inside the constraints (a ~ Int)
        bound by the type signature for Suc :: Expr a -> Expr Int
        at Test.hs:12:9-11ghc: panic! (the 'impossible' happened)
  (GHC version 7.10.2 for i386-unknown-linux):
	No skolem info: a1_anA[ssk]

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