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In file A.hs:

module A where

data Empty

In file B.hs:

{-# LANGUAGE StandaloneDeriving #-}

module B where

import A

deriving instance Show Empty

GHC 7.5.20120421 reports:

    Can't make a derived instance of `Show Empty':
      The data constructors of `Empty' are not all in scope
        so you cannot derive an instance for it
    In the stand-alone deriving instance for `Show Empty'

But Empty has no constructors, and the problem does not arise if Empty is declared in B.

I guess the problem is in this part of the code of TcDeriv:

      	   ; let hidden_data_cons = not (isWiredInName (tyConName rep_tc)) &&
                                    (isAbstractTyCon rep_tc || 
                                     any not_in_scope (tyConDataCons rep_tc))
      	         not_in_scope dc  = null (lookupGRE_Name rdr_env (dataConName dc))
      	   ; unless (isNothing mtheta || not hidden_data_cons)
      	   	    (bale_out (derivingHiddenErr tycon))

Trac metadata

Trac field	Value
Version	7.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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