[GHC] #15852: Bad axiom produced for polykinded data family

[GHC]

#15852: Bad axiom produced for polykinded data family
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           Reporter:  goldfire       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.7
           Keywords:  TypeFamilies,  |  Operating System:  Unknown/Multiple
  TypeInType                         |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 When I say

 {{{#!hs
 data family DF a (b :: k)
 data instance DF (Proxy c) :: Proxy j -> Type
 }}}

 with `-ddump-tc`, I get

 {{{
   axiom Scratch.D:R:DFProxyProxy0 ::
     forall k1 k2 (c :: k1) (j :: k2) (a :: Proxy j).
       DF (Proxy c) a = Scratch.R:DFProxyProxy k1 k2 c j
 }}}

 This is bogus, because `a` should be on the RHS, too. It's not clear to me
 whether this is a pretty-printing bug or a real one.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15852>
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