[GHC] #16008: GHC HEAD type family regression involving invisible arguments

[GHC]

#16008: GHC HEAD type family regression involving invisible arguments
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  highest           |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.7
  checker)                           |             Keywords:  TypeFamilies,
      Resolution:                    |  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5435
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"3899966e4613ec18f365c28d64e9acc163cc1165/ghc"
 3899966e/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="3899966e4613ec18f365c28d64e9acc163cc1165"
 Fix #16008 with a pinch of addConsistencyConstraints

 Summary:
 #16008 happened because we forgot to typecheck nullary
 associated type family instances in a way that's consistent with the
 type variables bound by the parent class. Oops. Easily fixed with a
 use of `checkConsistencyConstraints`.

 Test Plan: make test TEST=T16008

 Reviewers: simonpj, goldfire, bgamari

 Reviewed By: goldfire

 Subscribers: rwbarton, carter

 GHC Trac Issues: #16008

 Differential Revision: https://phabricator.haskell.org/D5435
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16008#comment:6>
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