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{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TypeFamilies #-}

data Test ext a where
  Foo :: a -> Test ext a
  Extend :: (ExtensionType ext a) -> Test ext a

type family ExtensionType ext a
data ListExtension
type instance ExtensionType ListExtension a = [a]

deriving instance Functor (Test ListExtension)

{-

a.hs:15:1: error:
    • Can't make a derived instance of ‘Functor (Test ListExtension)’:
        Constructor ‘Extend’ must use the type variable only as the last argument of a data type
    • In the stand-alone deriving instance for
        ‘Functor (Test ListExtension)’
   |
15 | deriving instance Functor (Test ListExtension)
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Failed, modules loaded: none.
-}

Trac metadata

Trac field	Value
Version	8.2.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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