[GHC] #11837: GHC fails to unify kinds when deriving polykinded typeclass instance for polykinded newtype

[GHC]

#11837: GHC fails to unify kinds when deriving polykinded typeclass instance for
polykinded newtype
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        Reporter:  RyanGlScott       |                Owner:  RyanGlScott
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8865, #11833     |  Differential Rev(s):  Phab:D2117
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"e53f2180e89652c72e51ffa614c56294ba67cf37/ghc"
 e53f2180/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="e53f2180e89652c72e51ffa614c56294ba67cf37"
 Fix deriveTyData's kind unification when two kind variables are unified

 When `deriveTyData` attempts to unify two kind variables (which can
 happen if both the typeclass and the datatype are poly-kinded), it
 mistakenly adds an extra mapping to its substitution which causes the
 unification to fail when applying the substitution. This can be
 prevented by checking both the domain and the range of the original
 substitution to see which kind variables shouldn't be put into the
 domain of the substitution. A more in-depth explanation is included in
 `Note [Unification of two kind variables in deriving]`.

 Fixes #11837.

 Test Plan: ./validate

 Reviewers: simonpj, hvr, goldfire, niteria, austin, bgamari

 Reviewed By: bgamari

 Subscribers: niteria, thomie

 Differential Revision: https://phabricator.haskell.org/D2117

 GHC Trac Issues: #11837
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11837#comment:2>
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