[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:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by deech:

@@ -1,2 +1,3 @@
- The bug can be reproduced but it does take some setup. Here's the error
- report in case it's familiar:
+ The following causes a panic in GHC 8.2.1 rc2:
+ {{{
+ module Main where
@@ -4,18 +5,5 @@
- {{{
- ...
- [1 of 2] Compiling Valuators        ( .stack-
- work/dist/x86_64-linux/Cabal-2.0.0.0/build/fltkhs-fluid-valuators/fltkhs-
- fluid-valuators-tmp/Valuators.hs, .stack-
- work/dist/x86_64-linux/Cabal-2.0.0.0/build/fltkhs-fluid-valuators/fltkhs-
- fluid-valuators-tmp/Valuators.o )
- ghc: panic! (the 'impossible' happened)
-   (GHC version 8.2.0.20170507 for x86_64-unknown-linux):
-         idInfo
-   a_ad4D
-   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
+ indexOr :: a -> Int -> [a] -> a
+ indexOr fallback idx xs =
+   if (idx < length xs)
+   then xs !! idx
+   else fallback
@@ -23,1 +11,2 @@
- Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
+ main :: IO ()
+ main = undefined
@@ -26,1 +15,17 @@
- The bug is in a demo for a GUI binding. To reproduce:
+ 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
@@ -28,21 +33,2 @@
- {{{
- # Install the FLTK GUI toolkit from source.
- > wget http://fltk.org/pub/fltk/1.3.4/fltk-1.3.4-1-source.tar.gz
- > tar -zxf fltk-1.3.4-1-source.tar.gz
- > cd fltk-1.3.4-1
- > ./configure --enable-gl --enable-shared --enable-localjpeg --enable-
- localzlib --enable-localpng
- > make
- > sudo make install
- > fltk-config --version
-   1.3.4-1
-
- # Install the bindings from source
- > git clone -b 8.2.1-rc2 https://github.com/deech/fltkhs
- > cd fltkhs
- > cabal install
-
- # Install the package causing the bug from source
- > git clone -b 8.2.1-rc2 https://github.com/deech/fltkhs-fluid-demos
- > cd fltkhs-fluid-demos
- > cabal install
+     Please report this as a GHC bug:
+ http://www.haskell.org/ghc/reportabug

New description:

 The following causes a panic in GHC 8.2.1 rc2:
 {{{
 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
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13708#comment:2>
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