[GHC] #12604: panic when building repa-fftw

[GHC]

#12604: panic when building repa-fftw
-------------------------------------+-------------------------------------
        Reporter:  koshmar           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by mpickering):

 Here is a minimal reproduction. It is two files.

 1. `M.hs`
 {{{
 {-# LANGUAGE TypeFamilies #-}
 module M
         ( -- * Abstract array representation
          Array (..)
         , Source(..)

         )
 where

 class Source r where
  data Array r

 data C

 instance Source C where
  data Array C
         = ACursored { cursoredExtent :: Int }
 }}}

 2. `Test.hs`
 {{{
 module Test where

 import M (Array)
 }}}

 Reproduce with `ghc Test.hs`.

 Error being

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.1 for x86_64-apple-darwin):
         filterImports/combine
   (Array, Array{Array, ACursored, cursoredExtent}, Nothing)
   (Array, Source{Source, Array}, Nothing)

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

 Thanks for the report koshmar! Hopefully Simon will know a workaround.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12604#comment:5>
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