Yet another crash from typed holes

{-# OPTIONS_GHC -fdefer-typed-holes #-}

hylo a b = h where h = b . fmap h . a

foo a b = hylo a' b'
  where a' x = _
        b' = _

bar :: [Int]
bar = []

main = print bar

Loading this up in ghci and attempting to run main prints:

GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
unknown option: 'c'
[1 of 1] Compiling Main             ( bad.hs, interpreted )

bad.hs:6:16: Warning:
    Found hole ‘_’ with type: t4
    Where: ‘t4’ is a rigid type variable bound by
                the inferred type of a' :: t3 -> t4 at bad.hs:6:9
    Relevant bindings include
      x :: t3 (bound at bad.hs:6:12)
      a' :: t3 -> t4 (bound at bad.hs:6:9)
      b' :: forall t. t (bound at bad.hs:7:9)
      b :: t1 (bound at bad.hs:5:7)
      a :: t (bound at bad.hs:5:5)
      foo :: t -> t1 -> t2 -> b (bound at bad.hs:5:1)
    In the expression: _
    In an equation for ‘a'’: a' x = _
    In an equation for ‘foo’:
        foo a b
          = hylo a' b'
          where
              a' x = _
              b' = _

bad.hs:7:14: Warning:
    Found hole ‘_’ with type: t3
    Where: ‘t3’ is a rigid type variable bound by
                the inferred type of b' :: t3 at bad.hs:7:9
    Relevant bindings include
      b' :: t3 (bound at bad.hs:7:9)
      b :: t1 (bound at bad.hs:5:7)
      a :: t (bound at bad.hs:5:5)
      foo :: t -> t1 -> t2 -> b (bound at bad.hs:5:1)
    In the expression: _
    In an equation for ‘b'’: b' = _
    In an equation for ‘foo’:
        foo a b
          = hylo a' b'
          where
              a' x = _
              b' = _
Ok, modules loaded: Main.
*Main> main
ghc: panic! (the 'impossible' happened)
  (GHC version 7.10.2 for x86_64-unknown-linux):
	nameModule $dFunctor_aKj

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

Compiling with ghc and then running the resulting binary works.

Attempting to replicate with a recent version of GHC HEAD gives

GHCi, version 7.11.20150812: http://www.haskell.org/ghc/  :? for help
unknown option: 'c'
[1 of 1] Compiling Main             ( bad.hs, interpreted )

bad.hs:5:11: error:
    No instance for (Functor f0) arising from a use of ‘hylo’
    The type variable ‘f0’ is ambiguous
    Note: there are several potential instances:
      instance Functor Maybe -- Defined in ‘GHC.Base’
      instance Functor IO -- Defined in ‘GHC.Base’
      instance Functor ((->) r) -- Defined in ‘GHC.Base’
      ...plus two others
    In the expression: hylo a' b'
    In an equation for ‘foo’:
        foo a b
          = hylo a' b'
          where
              a' x = _
              b' = _
Failed, modules loaded: none.

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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