[GHC] #14933: DeriveAnyClass can cause "No skolem info" GHC panic

[GHC]

#14933: DeriveAnyClass can cause "No skolem info" GHC panic
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14932            |  Differential Rev(s):  Phab:D4507
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * status:  new => patch
 * differential:   => Phab:D4507
 * related:   => #14932


Comment:

 Ack, it turns out that we are re-using the same unification variables
 across multiple iterations of `simplifyDeriv`, which results in utter
 disaster. (Honestly, I'm not sure how anything was working before.)

 Phab:D4507 fixes this issue (and #14932) by generating new unification
 variables across //each// iteration of `simplifyDeriv`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14933#comment:2>
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