[GHC] #4801: Typechecker performance regression 6.12 -> 7.0.1

[GHC]

#4801: Typechecker performance regression 6.12 -> 7.0.1
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    Reporter:  simonmar                 |        Owner:  simonpj     
        Type:  bug                      |       Status:  new         
    Priority:  high                     |    Milestone:  7.0.2       
   Component:  Compiler (Type checker)  |      Version:  7.0.1       
    Keywords:                           |     Testcase:              
   Blockedby:                           |   Difficulty:              
          Os:  Unknown/Multiple         |     Blocking:              
Architecture:  Unknown/Multiple         |      Failure:  None/Unknown
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Comment(by chak):

 Unfortunately, there is a similarly serious performance regressions in the
 type checker still present:  #4856

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4801#comment:4>
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