#6031: Cannot derive instances for empty datatypes defined in other modules
------------------------------+---------------------------------------------
Reporter: dreixel | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.5 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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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))
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6031>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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