[GHC] #14899: Significant compilation time regression between 8.4 and HEAD due to coverage checking

[GHC]

#14899: Significant compilation time regression between 8.4 and HEAD due to
coverage checking
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 It seems to me like comment:4 (which I updated to fix a few mistakes) is
 only part of the story. In particular, the constraint `Δ'` proposed in
 that comment is not currently admitted under the constraint syntax given
 in the paper. In particular, the right hand size of a term equality (e.g.
 the `≈` production) is expected to be an expression. However, there is no
 way to lift `(u_0 |> sym co)` into an expression. More generally, it
 doesn't make sense to me to lift a value abstraction like `u_0` into an
 expression. Perhaps a new constraint variety is needed?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14899#comment:6>
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