id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
6031,Cannot derive instances for empty datatypes defined in other modules,dreixel,,"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))
}}}
",bug,closed,normal,,Compiler,7.5,fixed,,,Unknown/Multiple,Unknown/Multiple,None/Unknown,Unknown,deriving/should_compile/T6031,,,