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Module M:

{-# LANGUAGE PolyKinds #-}
module M where

data A a

Module N:

{-# LANGUAGE StandaloneDeriving, DeriveDataTypeable #-}
module N where

import M
import Data.Typeable

deriving instance Typeable A

We get a confusing error message:

N.hs:7:1:
    Derived typeable instance must be of form (Typeable A)
    In the stand-alone deriving instance for ‛Typeable A’

The error disappears when N is compiled with -XPolyKinds. The reason is: without -XPolyKinds, A defaults to A *. I think it's simplest to suggest the flag in the error message.

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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