[GHC] #13142: GHC emits enormous core starting with 'Arity decrease'

[GHC]

#13142: GHC emits enormous core starting with 'Arity decrease'
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           Reporter:  jeiea             |             Owner:
               Type:  bug               |            Status:  new
           Priority:  normal            |         Milestone:
          Component:  Compiler          |           Version:  8.0.2
           Keywords:                    |  Operating System:  Windows
       Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
          Test Case:                    |        Blocked By:
           Blocking:                    |   Related Tickets:
Differential Rev(s):                    |         Wiki Page:
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 I discovered this when building
 [https://github.com/commercialhaskell/stack/commit/f117ffb034658e3e1c3fea2a88972c685b4558e1
 stack] by stack on windows 10.

 I want to find problem area, but I can't even catch a clue.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13142>
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