[GHC] #14817: GHC 8.4.1 pretty-prints data family instances with redundant kind signatures using -ddump-splices

[GHC]

#14817: GHC 8.4.1 pretty-prints data family instances with redundant kind
signatures using -ddump-splices
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.4.1-alpha3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4418
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"aef2b429072e3d3bbdbcb9e4082a0d86ba329d9e/ghc"
 aef2b429/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="aef2b429072e3d3bbdbcb9e4082a0d86ba329d9e"
 Fix #14817 by not double-printing data family instance kind signatures

 Within `pprDataFamInstDecl`, we were invoking `pprFamInstLHS` to
 pretty-print a data family instance header, and we were passing `Just` a
 kind signature to `pprFamInstLHS` to make it pretty-print the kind
 signature alongside it (this is a consequence of commit
 d1ef223cfebd23c25489a4b0c67fbaa2f91c1ec6). But this is silly, because
 then invoke `pp_data_defn`, which //also// pretty-prints the kind
 signature, resulting in the kind signature being printed twice by
 mistake.

 This fix is simple—pass `Nothing` to `pprFamInstLHS` instead.

 Test Plan: make test TEST=T14817

 Reviewers: alanz, bgamari, mpickering

 Reviewed By: mpickering

 Subscribers: mpickering, rwbarton, thomie, carter

 GHC Trac Issues: #14817

 Differential Revision: https://phabricator.haskell.org/D4418
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14817#comment:2>
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