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{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
{-# OPTIONS_GHC -fdefer-type-errors #-}
module T13487 where
import Data.Kind (Constraint)
import GHC.TypeLits
data Foo a b where
K :: Error a b => a -> b -> Foo a b
type family Error a b :: Constraint where
Error Int Int = ()
Error _ _ = TypeError ('Text "GHC panic in 3... 2... 1...")
foo = K 'a' 'b'
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