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{-# LANGUAGE DataKinds #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE AllowAmbiguousTypes #-} -- 'c' is ambiguous
module T8566 where
import Data.Kind (Type)
data U (s :: Type) = forall v. AA v [U s]
-- AA :: forall (s:*) (v:*). v -> [U s] -> U s
data I (u :: U Type) (r :: [Type]) :: Type where
A :: I (AA t as) r -- Existential k
-- A :: forall (u:U *) (r:[*]) Universal
-- (k:BOX) (t:k) (as:[U *]). Existential
-- (u ~ AA * k t as) =>
-- I u r
-- fs unused, but needs to be present for the bug
class C (u :: U Type) (r :: [Type]) (fs :: [Type]) where
c :: I u r -> I u r
-- c :: forall (u :: U *) (r :: [*]) (fs :: [*]). C u r fs => I u r -> I u r
instance (C (AA (t (I a ps)) as) ps fs) => C (AA t (a ': as)) ps fs where
-- instance C (AA t (a ': as)) ps fs where
c A = c undefined
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