[GHC] #9160: Panic: Template variable unbound in rewrite rule

[GHC]

#9160: Panic: Template variable unbound in rewrite rule
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        Reporter:  mietek            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:  7.8.3
       Component:  Compiler          |                 Version:  7.8.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  crash                              |               Test Case:
      Blocked By:                    |  indexed_types/should_fail/T9160
 Related Tickets:  #4524             |                Blocking:
                                     |  Differential Revisions:
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Comment (by slyfox):

 Replying to [comment:13 George]:
 > I am also seeing this on ghc 7.10.1, not sure if this is another
 instance of this bug:
 >
 > cabal install bitmap
 > [ 3 of 10] Compiling Data.Bitmap.IO   ( Data/Bitmap/IO.hs,
 dist/build/Data/Bitmap/IO.o )

 I've distilled it a bit to a selfcontained test:
 {{{#!hs
 -- sf 9160 # cat B.hs
 {-# LANGUAGE NoImplicitPrelude #-}
 {-# OPTIONS_GHC -O2 #-}
 module B (bug) where

 data D = D
 data E = E

 class Storable a where
     poke2 :: a -> E
 instance Storable D where
     poke2 = poke2 -- undefined

 class Foo a where
 instance Foo D where

 class (Foo t, Storable t) => FooStorable t where

 instance FooStorable D where
     {-# SPECIALIZE instance FooStorable D #-}

 {-# SPECIALIZE bug :: D -> E #-}

 bug
   :: FooStorable t
   => t
   -> E
 bug = poke2
 {-
 sf 9160 # ghc -c -fforce-recomp -Wall B.hs

 B.hs:5:10: Warning: Defined but not used: data constructor ‘D’

 B.hs:6:10: Warning: Defined but not used: data constructor ‘E’
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.1 for x86_64-unknown-linux):
         Template variable unbound in rewrite rule
   $fFooStorableD_XU
   [$fFooStorableD_XU]
   [$fFooStorableD_XU]
   []
   []

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

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9160#comment:15>
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