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{-# LANGUAGE Haskell2010 #-}
{-# LANGUAGE MultiParamTypeClasses #-}
module ShouldFail where
data T a = MkT
class C a b where
op :: T a -> T b -> Bool
-- Repeated type variable in an instance constraint
-- should require UndecidableInstances
instance C a a => Eq (T a) where
(==) = op
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