[GHC] #14059: COMPLETE sets don't work at all with data family instances

[GHC]

#14059: COMPLETE sets don't work at all with data family instances
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms,
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4752
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"4d8004483387c087f5132736863d895ae4869163/ghc" 4d80044/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="4d8004483387c087f5132736863d895ae4869163"
 Fix a bad interaction between GADTs and COMPLETE sets

 As observed in #14059 (starting at comment 5), the error
 messages surrounding a program involving GADTs and a `COMPLETE` set
 became worse between 8.2 and 8.4. The culprit was a new validity
 check in 8.4 which filters out `COMPLETE` set candidates if a return
 type of any conlike in the set doesn't match the type of the
 scrutinee. However, this check was too conservative, since it removed
 perfectly valid `COMPLETE` sets that contained GADT constructors,
 which quite often have return types that don't match the type of a
 scrutinee.

 To fix this, I adopted the most straightforward possible solution of
 only performing this validity check on //pattern synonym//
 constructors, not //data// constructors.

 Note that this does not fix #14059 entirely, but instead simply fixes
 a particular buglet that was discovered in that ticket.

 Test Plan: make test TEST=T14059

 Reviewers: bgamari, mpickering

 Reviewed By: mpickering

 Subscribers: rwbarton, thomie, carter

 GHC Trac Issues: #14059

 Differential Revision: https://phabricator.haskell.org/D4752
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14059#comment:9>
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