[GHC] #11291: DfltProb1(optasm): panic CoreToStg.myCollectArgs

[GHC]

#11291: DfltProb1(optasm): panic CoreToStg.myCollectArgs
-------------------------------------+-------------------------------------
        Reporter:  thomie            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.11
  (CodeGen)                          |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Compile-time      |            Test Case:
  crash                              |  typecheck/should_compile/DfltProb1
                                     |  (optasm)
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by bgamari):

 * failure:  None/Unknown => Compile-time crash
 * component:  Compiler => Compiler (CodeGen)


Old description:

> The program looks like this:
> {{{
> module DfltProb1 where
>
> import Control.Monad.ST
> import Prelude hiding (traverse)
>
> traverse :: a -> ST s [a]
> traverse = undefined
>
> -- WORKS with signature test :: Num a => [a]
> test = runST (traverse 1)
>
> main = print test
> }}}
>
> This is the failure:
> {{{
> $ ghc-7.11.20151225 -O DfltProb1.hs
> ghc-stage2: panic! (the 'impossible' happened)
>   (GHC version 7.11.20151224 for x86_64-unknown-linux):
>         CoreToStg.myCollectArgs
>   (case traverse of _ [Occ=Dead] { }) realWorld#
> }}}

New description:

 The program looks like this:
 {{{#!hs
 module DfltProb1 where

 import Control.Monad.ST
 import Prelude hiding (traverse)

 traverse :: a -> ST s [a]
 traverse = undefined

 -- WORKS with signature test :: Num a => [a]
 test = runST (traverse 1)

 main = print test
 }}}

 This is the failure:
 {{{
 $ ghc-7.11.20151225 -O DfltProb1.hs
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 7.11.20151224 for x86_64-unknown-linux):
         CoreToStg.myCollectArgs
   (case traverse of _ [Occ=Dead] { }) realWorld#
 }}}

--

Comment:

 I believe this is fixed on `master`. I have a suspicion that this is due
 to d990354473239943d83ee90f8906f3737b53fe65.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11291#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler