[GHC] #8892: Ghc panics (variable not found)

[GHC]

#8892: Ghc panics (variable not found)
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        Reporter:  jwlato              |            Owner:
            Type:  bug                 |           Status:  new
        Priority:  normal              |        Milestone:
       Component:  Compiler            |          Version:  7.8.1-rc2
      Resolution:                      |         Keywords:
Operating System:  Unknown/Multiple    |     Architecture:
 Type of failure:  Compile-time crash  |  Unknown/Multiple
       Test Case:                      |       Difficulty:  Unknown
        Blocking:                      |       Blocked By:
                                       |  Related Tickets:
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Comment (by jwlato):

 I've made a test case that I hope is small enough you can work with.  This
 has dependencies on the `vector` and `vector-space` packages.  There are
 two files, `Piecewise.hs` and `Broken.hs`.  To trigger the bug, you need
 to first compile `Piecewise.hs` and install it to ghc's package database,
 then attempt to compile `Broken.hs` via `ghc -O Broken.hs`.  Trying to
 compile `Broken.hs` when ghc can find `Piecewise.hs` works properly for
 me.  I've put a repo on github with a minimal cabal file also,
 https://github.com/JohnLato/ghc-8892

 I can trigger the bug with any optimization level except `-O0`.

 `Piecewise.hs` defines `instance (AdditiveGroup (poly a), Ord a) =>
 AdditiveGroup (Piecewise poly a)`, with `{-# SPECIALISE instance
 AdditiveGroup (Piecewise Scale Double) #-}`.  That SPECIALIZE instance
 pragma, and the `INLINE negateV` pragma, both seem required to trigger
 this.  I haven't attempted to simplify the definition of `^+^` at all,
 I'll continue with that tomorrow.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8892#comment:4>
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