[GHC] #13885: Template Haskell doesn't freshen GADT type variables properly

[GHC]

#13885: Template Haskell doesn't freshen GADT type variables properly
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3867
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"79b259ae6a0a0c17568d7d03d82e378ad4c4001a/ghc"
 79b259ae/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="79b259ae6a0a0c17568d7d03d82e378ad4c4001a"
 Fix #13885 by freshening reified GADT constructors' universal tyvars

 Summary:
 When reifying GADTs with Template Haskell, the universally quantified
 type variables were being reused across both the data type head and the
 constructors' type signatures. This had the annoying effect of causing
 sets
 of differently scoped variables to have the same uniques. To avoid this,
 we
 now freshen the universal tyvars before reifying the constructors so as to
 ensure they have distinct uniques.

 Test Plan: make test TEST=T13885

 Reviewers: goldfire, austin, bgamari, simonpj

 Reviewed By: simonpj

 Subscribers: rwbarton, thomie

 GHC Trac Issues: #13885

 Differential Revision: https://phabricator.haskell.org/D3867
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13885#comment:6>
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