[GHC] #8654: Exponential-long compilation of code with Implicit params

[GHC]

#8654: Exponential-long compilation of code with Implicit params
-------------------------------------+-------------------------------------
       Reporter:  akamaus            |             Owner:
           Type:  bug                |            Status:  new
       Priority:  normal             |         Milestone:
      Component:  Compiler           |           Version:  7.6.2
       Keywords:  ImplicitParams     |  Operating System:  Unknown/Multiple
  slowness                           |   Type of failure:  Compile-time
   Architecture:  Unknown/Multiple   |  performance bug
     Difficulty:  Unknown            |         Test Case:
     Blocked By:                     |          Blocking:
Related Tickets:                     |
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 Some time ago I stumbled upon GHC hanging on my code. After some
 experiments I managed to minify it to:

 {-# LANGUAGE ImplicitParams #-}

 data D = D Int deriving Show

 slow_to_compile :: IO ()
 slow_to_compile = do
   tst1 <- return 1

   let ?tst1 = tst1
   let ?tst2 = tst1
   let ?tst3 = tst1
   let ?tst4 = tst1
   let ?tst5 = tst1
   let ?tst6 = tst1
   let ?tst7 = tst1

   print $ D ?tst1

 It compiles, but takes a while. Every additional binding doubles
 compilation time.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8654>
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