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{-# LANGUAGE ScopedTypeVariables #-}
module T12949 where
class Foo a where
foo :: Maybe a
data Result a b = Neither | This a | That b | Both a b
q :: forall a b . (Foo a, Foo b) => Result a b
q = case foo :: Maybe a of
Nothing -> case foo :: Maybe b of
Nothing -> Neither
Just c -> That c
Just i -> case foo :: Maybe b of
Nothing -> This i
Just c -> Both i c
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