[GHC] #12571: GHC panic

[GHC]

#12571: GHC panic
-------------------------------------+-------------------------------------
           Reporter:  prsteele       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Linux
       Architecture:  x86_64         |   Type of failure:  Compile-time
  (amd64)                            |  crash
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I received the following error message from GHC:

 {{{
 <no location info>:
     ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.3 for x86_64-unknown-linux):
         Simplifier ticks exhausted
   When trying UnfoldingDone $fStorableWord21
   To increase the limit, use -fsimpl-tick-factor=N (default 100)
   If you need to do this, let GHC HQ know, and what factor you needed
   To see detailed counts use -ddump-simpl-stats
   Total ticks: 8040

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

 When I followed the suggestion of adding `-fsimpl-tick-factor=200` the
 compiler proceeds normally (finding a syntax error in a file). Removing
 this option, strangely, also causes the compiler to proceed normally. I
 can no longer reproduce this bug, although several compilations in a row
 (before adding the option) reproduced the bug.

 Although I cannot reproduce this bug, the syntax error that is present
 came from an error of the form:
 {{{#!hs
 f :: A -> M B
 f a =
   do
     x <- g a
   let
     c = h a
   in do
     ...
 }}}
 The actual file is attached.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12571>
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