[GHC] #13272: DeriveAnyClass regression involving a rigid type variable

[GHC]

#13272: DeriveAnyClass regression involving a rigid type variable
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:  Generics
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"fd841f877ab7a991f667a50b401404927f6f599c/ghc"
 fd841f87/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="fd841f877ab7a991f667a50b401404927f6f599c"
 Fix DeriveAnyClass (again)

 This patch fixes Trac #13272.  The general approach was fine, but
 we were simply not generating the correct implication constraint
 (in particular generating fresh unification variables).  I added
 a lot more commentary to Note [Gathering and simplifying
 constraints for DeriveAnyClass]

 I'm still not very happy with the overall architecture.  It feels
 more complicate than it should.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13272#comment:9>
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