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{-#LANGUAGE TypeApplications#-}
{-#LANGUAGE MultiParamTypeClasses #-}
{-#LANGUAGE AllowAmbiguousTypes #-}
{-#LANGUAGE FlexibleInstances #-}
{-#LANGUAGE ScopedTypeVariables #-}
{-#LANGUAGE DefaultSignatures #-}
module T12220 where
-- | Type a is only used for
-- type application.
class ToUse a where
toUse :: Int -> Int
-- | The type used for
-- type application
data Default
-- | The instance using Default as type application.
-- To call use:
-- > toUse @Default
instance ToUse Default where
toUse a = 3*a
-- | Typeclass whose methods work
-- only with type application.
class Uses a b where
uses :: b -> [b]
-- | Default Signature, which generates the problem.
-- It is the same as the normal one
-- Comment it to 'fix' the bug.
default uses :: b -> [b]
uses v = [v]
-- | But this one doesn't.
-- Unless you comment the default signature.
instance (Uses t a, Uses t b, Uses t c) => Uses t (a,b,c)
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