[GHC] #15872: Odd pretty printing of equality constraint in kind ('GHC.Types.Eq# <>)

[GHC]

#15872: Odd pretty printing of equality constraint in kind ('GHC.Types.Eq# <>)
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.2
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Poor/confusing    |            Test Case:
  error message                      |  typecheck/should_fail/T12102
      Blocked By:                    |             Blocking:
 Related Tickets:  #12102, #13933    |  Differential Rev(s):  Phab:D5397
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * status:  closed => new
 * resolution:  fixed =>
 * milestone:  8.8.1 =>


Comment:

 Commit
 [https://gitlab.haskell.org/ghc/ghc/commit/6cce36f83aec33d33545e0ef2135894d22dff5ca
 6cce36f83aec33d33545e0ef2135894d22dff5ca] (`Add AnonArgFlag to FunTy`)
 added back the ability to have equality constraints in kinds.
 Unfortunately, the issues in the original description persist:

 {{{
 GHCi, version 8.9.20190224: https://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )
 Ok, one module loaded.
 λ> :i MkFun
 type role Fun nominal representational representational
 data Fun (a1 :: a ~ 'OP) b c where
   MkFun :: b -> c -> Fun 'GHC.Types.Eq# <> b c
         -- Defined at Bug.hs:11:3
 λ> :k Fun
 Fun :: (a ~ 'OP) -> * -> * -> *
 }}}

 In fact, the situation is arguably //worse// now, since `:k Fun` reports
 the entirely bogus kind `Fun :: (a ~ 'OP) -> * -> * -> *`. (See also
 #12102.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15872#comment:8>
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