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{-# LANGUAGE TypeFamilies, FunctionalDependencies, RankNTypes, MultiParamTypeClasses #-}
module T4254b where
class FD a b | a -> b where
op :: a -> b;
op = undefined
instance FD Int Bool
fails :: forall a b. (a~Int,FD a b) => a -> Bool
fails = op
-- Could fail: no proof that b~Bool
-- But can also succeed; it's not a *wanted* constraint
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