[GHC] #15318: Core Lint error involving newtype family instances with wrappers

[GHC]

#15318: Core Lint error involving newtype family instances with wrappers
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.4.3
  checker)                           |
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4902
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"927518668111584a06f12bd9eb1b0910a38acf4f/ghc" 9275186/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="927518668111584a06f12bd9eb1b0910a38acf4f"
 Fix newtype instance GADTs

 Summary: This was taken from Richard's branch, which in turn was
 submitted to Phab by Matthew, which in turn was commandeered by Ryan.

 This fixes an issue with newtype instances in which too many
 coercions were being applied in the worker. This fixes the issue by
 removing the data family instance axiom from the worker and moving
 to the wrapper. Moreover, we now require all newtype instances
 to have wrappers, for symmetry with data instances.

 Reviewers: goldfire, bgamari, simonpj, mpickering

 Reviewed By: mpickering

 Subscribers: simonpj, rwbarton, thomie, carter

 GHC Trac Issues: #15318

 Differential Revision: https://phabricator.haskell.org/D4902
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15318#comment:3>
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