[GHC] #15586: Compilation panic! (the 'impossible' happened)

[GHC]

#15586: Compilation panic! (the 'impossible' happened)
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        Reporter:  subaruru          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5118
       Wiki Page:                    |
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Changes (by monoidal):

 * differential:   => Phab:D5118


Comment:

 Small testcase:
 {{{
 {-# LANGUAGE GADTs #-}

 module STree where

 data STree a where
     STreeIM :: {
         l :: v a ,
         stree :: a
     } -> STree a

 insert :: STree a -> STree a
 insert s = s { stree = undefined }
 }}}

 It crashes 8.4 but works on 8.0, 8.2 and master; I didn't check 8.6 but I
 presume it's fixed there. I couldn't find a similar program looking for
 `piResultTy` so I added a testcase in Phab:D5118.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15586#comment:3>
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