Too-eager variable capture in forall types

h's type is ineffable: GHCi infers Capture a, but listing that type in the souce code fails the typecheck. Listing no type allows the following:

{-# OPTIONS_GHC -fglasgow-exts #-}

data Capture a = Base a
               | Capture (Capture (forall x . Divide x -> Divide a))

g :: Capture (forall a . Divide a -> Divide a)
g = Base id

h = Capture g

l :: Capture a
l = h

m :: Capture Int
m = h

data Divide a where
    Pint :: Int -> Divide Int
    Pbool :: Bool -> Divide Bool

foo :: Capture a -> Divide a
foo (Capture (Base f)) = f (Pbool True)

unfoo :: Divide a -> a
unfoo (Pint x) = x
unfoo (Pbool x) = x

oops = unfoo (foo h) -- Illegal instruction
oopsAgain = unfoo (foo l) -- Illegal instruction
oopsOnceMore = unfoo (foo m)
{-
<interactive>: internal error: interpretBCO: hit a CASEFAIL
    (GHC version 6.6 for i386_unknown_linux)
    Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
Aborted
-}

Trac metadata

Trac field	Value
Version	6.6
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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