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The bug is triggered by the following minimal example. Types A.A and B.B share a constructor with the same name. Module B is imported qualified, so that the constructors do not conflict.

A.hs:

module A where

data A = C1 | C2 | C

B.hs:

module B where

data B = D1 | D2 | C

C.hs:

{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE StandaloneDeriving #-}

module C where

import Data.Data
import Data.Typeable

import A
import qualified B as B

deriving instance Typeable A
deriving instance Typeable B.B

deriving instance Data A
deriving instance Data B.B

main :: IO ()
main = return ()

When deriving a Data instance for the types, I get a name conflict for some generated code:

[au@lift ghc-bug]$ ~/Software/ghc-head/bin/ghc --make C.hs 
[1 of 3] Compiling B                ( B.hs, B.o )
[2 of 3] Compiling A                ( A.hs, A.o )
[3 of 3] Compiling C                ( C.hs, C.o )

C.hs:12:1:
    Duplicate type signatures for ‛$cC’
    at C.hs:11:1-24
       C.hs:12:1-26

Note that:

1. The problem occurs when deriving Data. Deriving Just the Typeable instance does not produce an error.
2. Moving the derivation of Typeable and Data to modules A and B respectively works fine.

The problem occurs both in ghc 7.6.2 as well as in ghc head (rev. cfb9bee7).

Trac metadata

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