[GHC] #14766: Hole-y partial type signatures greatly slow down compile times

[GHC]

#14766: Hole-y partial type signatures greatly slow down compile times
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           Reporter:  harpocrates    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           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:
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 This time, I actually mean it. :)

 Originally reported [https://github.com/simonmar/happy/issues/109 here], I
 distilled the example from
 [https://github.com/simonmar/happy/issues/109#issuecomment-362957245 this
 comment] into a one file test case. `Sigs.hs` is exactly like `NoSigs.hs`,
 except for the fact that it has a bunch of extra type signatures that have
 a lot of holes. On my machine, this is what compilation times are (I gave
 up timing after 15 minutes):

 || GHC version || 8.0.2  || 8.2.1  || 8.4.1
 (445554b6d9a2263f969e25bb9f532dd0c3a9dc8c) ||
 || `NoSigs.hs` || 24.13s || 22.93s || 34.05s ||
 || `Sigs.hs`   || >15m   || >15m   || >15m   ||

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14766>
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