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{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
module T17267 where
class a ~ b => Thing a b
instance a ~ b => Thing a b
unsafeCoerce :: forall a b. a -> b
unsafeCoerce a = oops a where
oops :: (a ~ b => Thing a b) => (Thing a b => r) -> r
oops r = r
{-
-- Now rejected
class C a b where
op :: a -> b
uc :: a -> b
uc = oops where
oops :: (C a b => C a b) => a -> b
oops x = op x
-}
{-
-- Now rejected
uc :: a -> b
uc = oops where
oops :: (a ~ b => a ~ b) => a -> b
oops x = x
-}
{-
-- Now rejected
class C a b where
op :: a -> b
class C a b => Thing a b
instance C a b => Thing a b
unsafeCoerce :: forall a b. a -> b
unsafeCoerce a = oops (op a :: Thing a b => b)
where
oops :: (C a b => Thing a b) => (Thing a b => x) -> x
oops r = r
-}
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