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{-# LANGUAGE ExistentialQuantification #-}
-- Without a type sig this is slightly tricky.
-- See Trac #1430
-- Reason: we get an implication constraint (forall a. Typeable a => Typeable b),
-- when generalising unExTypeable. We want to infer a context for the
-- whole thing of (Typeable b).
-- See Note [Inference and implication constraints] in TcSimplify
module Foo where
import Data.Typeable
data ExTypeable = forall a. Typeable a => ExTypeable a
-- unExTypeable :: Typeable h => ExTypeable -> Maybe h
unExTypeable (ExTypeable a) = cast a
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