[GHC] #14887: Explicitly quantifying a kind variable causes a type family to fail to typecheck

[GHC]

#14887: Explicitly quantifying a kind variable causes a type family to fail to
typecheck
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        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.2
  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):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 The #12919 patch has landed. Interestingly, the program in comment:1 now
 gives a different error:

 {{{
 $ ~/Software/ghc/inplace/bin/ghc-stage2 --interactive Bug.hs
 GHCi, version 8.5.20180403: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:10:43: error:
     • Expected kind ‘k’, but ‘a’ has kind ‘k0’
     • In the first argument of ‘Proxy’, namely ‘(a :: k)’
       In the kind ‘Proxy (a :: k)’
    |
 10 | type family Foo2 (k :: Type) (e :: Proxy (a :: k)) :: Type where {}
    |                                           ^
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14887#comment:5>
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