[GHC] #13759: Strange error message for deriving Data

27 May 2017

      #13759: Strange error message for deriving Data
-------------------------------------+-------------------------------------
           Reporter:  alanz          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following code

 {{{#!hs
 {-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE FlexibleInstances #-}

 import Data.Data

 data Pass = Parsed | Renamed | Typechecked
          deriving (Data)

 data GHC (c :: Pass)

 deriving instance Data (GHC 'Parsed)
 deriving instance Data (GHC 'Renamed)
 deriving instance Data (GHC 'Typechecked)

 -- Type synonyms as a shorthand for tagging
 type GhcPs  = GHC 'Parsed
 type GhcRn  = GHC 'Renamed
 type GhcTc  = GHC 'Typechecked

 }}}

 loads fine in `ghci-8.2.0.20170507`.

 In `ghc-8.0.2` and current master (as at
 https://github.com/ghc/ghc/tree/wip/new-tree-one-param-2) it gives

 {{{#!hs
 alanz@alanz-laptop:~/tmp/ghc-bug-data-deriving$ ghci-8.0.2 Main.hs
 GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/alanz/.ghci
 [1 of 1] Compiling Main             ( Main.hs, interpreted )

 Main.hs:16:1: error:
     Multiple declarations of ‘$tFl4AgeEJ0Vk5ONrnm56S3r’
     Declared at: Main.hs:16:1
                  Main.hs:17:1

 Main.hs:16:1: error:
     Multiple declarations of ‘$tFl4AgeEJ0Vk5ONrnm56S3r’
     Declared at: Main.hs:15:1
                  Main.hs:17:1

 Main.hs:16:1: error:
     Duplicate type signatures for ‘$tFl4AgeEJ0Vk5ONrnm56S3r’
     at Main.hs:15:1-36
        Main.hs:16:1-37
        Main.hs:17:1-41
 Failed, modules loaded: none.
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13759
GHC http://www.haskell.org/ghc/
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