[GHC] #13708: Panic! (the "impossible" happened) bug in GHC 8.2.1 rc2

[GHC]

#13708: Panic! (the "impossible" happened) bug in GHC 8.2.1 rc2
-------------------------------------+-------------------------------------
        Reporter:  deech             |                Owner:  (none)
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.2.1-rc2
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  simplCore/should_compile/T13708
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by bgamari):

 * status:  closed => merge
 * milestone:   => 8.2.1


@@ -2,1 +2,1 @@
- {{{
+ {{{#!hs

New description:

 The following causes a panic in GHC 8.2.1 rc2:
 {{{#!hs
 module Main where

 indexOr :: a -> Int -> [a] -> a
 indexOr fallback idx xs =
   if (idx < length xs)
   then xs !! idx
   else fallback

 main :: IO ()
 main = undefined
 }}}

 When compiling I get:
 {{{
     [1 of 1] Compiling Main             ( src/Examples/table-sort.hs,
 .stack-work/dist/x86_64-linux/Cabal-2.0.0.0/build/fltkhs-table-sort
 /fltkhs-table-sort-tmp/Main.o )
     ghc: panic! (the 'impossible' happened)
       (GHC version 8.2.0.20170507 for x86_64-unknown-linux):
         idInfo
       a_a2ws
       Call stack:
           CallStack (from HasCallStack):
             prettyCurrentCallStack, called at
 compiler/utils/Outputable.hs:1134:58 in ghc:Outputable
             callStackDoc, called at compiler/utils/Outputable.hs:1138:37
 in ghc:Outputable
             pprPanic, called at compiler/basicTypes/Var.hs:526:34 in
 ghc:Var

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

--

Comment:

 I suspect we should merge this.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13708#comment:12>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler