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{-# LANGUAGE FlexibleInstances, FlexibleContexts, UndecidableInstances #-}
module SilentParametersOverlapping where
class C a where
c :: a -> ()
class C a => B a where
b :: a -> ()
instance C [a] where
c x = ()
instance {- silent: C [(a,b)] => -} B [(a,b)] where
b x = c [(undefined,undefined)]
-- We get wanted: C [(gamma, delta)],
-- and gamma,delta are unconstrained
-- We can apply the C [a] instance without difficulty, but
-- that fails due to silent dfun parameters
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