[GHC] #11371: Bogus in-scope set in substitutions

[GHC]

#11371: Bogus in-scope set in substitutions
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        Reporter:  simonpj           |                Owner:  niteria
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11360            |  Differential Rev(s):  phab:D1792,
       Wiki Page:                    |  phab:D1801, phab:D1802
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Comment (by niteria):

 `promoteCoercion`
 [https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/types/Unify.hs;4c6e95e4f92516a925fd2a1bce0c0f8b5b9cbd17$1073
 here] contributes half of the increase in allocations. `promoteCoercion`
 is called 86576 times in total for T5030.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11371#comment:42>
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