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{-# LANGUAGE TypeFamilies #-}
module ShouldCompile where
class Cls a where
type Fam a :: *
type Fam a = Maybe a
instance Cls Int where
-- Gets type family from default
inc :: (Fam a ~ Maybe Int) => a -> Fam a -> Fam a
inc _proxy (Just x) = Just (x + 1)
inc _proxy Nothing = Just 0
foo :: Maybe Int -> Maybe Int
foo = inc (undefined :: Int)
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