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{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE UndecidableInstances #-}
module Bug where
import GHC.TypeLits
newtype Foo = Foo Int
class Bar a where
bar :: a
instance (TypeError (Text "Boo")) => Bar Foo where
bar = undefined
newtype Baz = Baz Foo
deriving Bar
-- Apparently we derive
-- instance TypeError (Text "Boo") => Bar Baz
--
-- Is that really what we want? It defers the type
-- error... surely we should use standalone deriving
-- if that is what we want?
-- See GHC.Tc.Validity.validDerivPred and #22696.
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