[GHC] #12127: ghc-8.0.1: panic! (the 'impossible' happened) filterImports/combine (double import)

28 May 2016

      #12127: ghc-8.0.1: panic! (the 'impossible' happened) filterImports/combine (double
import)
-------------------------------------+-------------------------------------
           Reporter:  slyfox         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 It's a distilled example or repa-devil-0.3.2.6 build failure:

 {{{#!hs
 -- Cursored.hs:
 {-# LANGUAGE TypeFamilies #-}
 module Cursored where

 class Source r where
     data Array r

 instance Source () where
     data Array () = F { f :: Int }
 }}}

 {{{#!hs
 -- Bug.hs:
 module DevIL () where

 import Cursored (Array(..), Source(..))
 }}}

 {{{
 [1 of 2] Compiling Cursored         ( Cursored.hs, Cursored.o )
 [2 of 2] Compiling DevIL            ( Bug.hs, Bug.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-unknown-linux):
         filterImports/combine
   (Array, Array{Array, F, f}, Nothing)
   (Array, Source{Source, Array}, Nothing)

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 The error is the same as #11959 but involved entities are slightly
 different.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12127
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