[GHC] #14932: DeriveAnyClass produces unjustifiably untouchable unification variables

[GHC]

#14932: DeriveAnyClass produces unjustifiably untouchable unification variables
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.4.1
  checker)                           |
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13272            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Well, I do have a patch. But I also recently noticed the existence of
 #14933, which shares many similar characteristics with this issue, and now
 I'm wondering if perhaps I should clean up GHC's approach to
 `DeriveAnyClass` further to nail both bugs at once. Let me see how far I
 get...

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14932#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler