[GHC] #11163: New exhaustiveness checker breaks T5642

[GHC]

#11163: New exhaustiveness checker breaks T5642
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  7.10.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The new exhaustiveness checker drastically increases compile time of the
 `T5642` testcase. From the profile it appears that a great deal of time is
 being spent evaluating `Check.mkPmId.occname`,

 {{{
 COST CENTRE                 MODULE       %time %alloc

 mkPmId.occname              Check         73.7   16.9
 mkOneConFull                Check          3.4   10.2
 deSugar                     HscMain        2.8   14.8
 mkOneConFull.arguments      Check          2.5    5.5
 pmTraverse                  Check          1.6    0.8
 mkOneConFull.subst1         Check          1.5    5.9
 wrapK.go                    Check          1.5    5.9
 cMatcher                    Check          1.2    2.7
 canEvVar                    TcCanonical    1.0    3.4
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11163>
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