[GHC] #15019: Fix performance regressions from #14737

[GHC]

#15019: Fix performance regressions from #14737
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        Reporter:  tdammers          |                Owner:  tdammers
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14737            |  Differential Rev(s):  phab:D4568
       Wiki Page:                    |
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Comment (by simonpj):

 Ah, much easier to understand.  Now my next question is this: who built
 that coercion?  It is actually reflexive, and it's trivial to see that;
 but it is not `ReflCo`. Usually we build coercions with smart constructors
 that rapidly smash to `ReflCo`.

 Can you describe how to reproduce this exact example?  I can't get it to
 trip on `T9020` and `T12707` seems much larger.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15019#comment:7>
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