[GHC] #15852: Bad axiom produced for polykinded data family

[GHC]

#15852: Bad axiom produced for polykinded data family
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.7
      Resolution:                    |             Keywords:  TypeFamilies,
                                     |  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5328
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"014d6c1f08808c4dab6cba80efdc634527d91086/ghc" 014d6c1/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="014d6c1f08808c4dab6cba80efdc634527d91086"
 Fix #15852 by eta expanding data family instance RHSes, too

 When I defined `etaExpandFamInstLHS`, I blatantly forgot
 to eta expand the RHSes of data family instances. (Actually, I
 claimed that they didn't //need// to be eta expanded. I'm not sure
 what I was thinking.)

 This fixes the issue by changing `etaExpandFamInstLHS` to
 `etaExpandFamInst` and, well, making it actually eta expand the RHS.

 Test Plan: make test TEST=T15852

 Reviewers: goldfire, bgamari

 Reviewed By: goldfire

 Subscribers: rwbarton, carter

 GHC Trac Issues: #15852

 Differential Revision: https://phabricator.haskell.org/D5328
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15852#comment:7>
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