[GHC] #13998: Default Signature messes up arity of type constructor

[GHC]

#13998: Default Signature messes up arity of type constructor
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        Reporter:  andrewthad        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Woo, Simon to the rescue!

 Out of curiosity, are you trying to fix this by changing the visibility of
 `k`, or changing the codegen to spit out `$dmeqForallPoly @Atom @Value`
 instead? The former fix sounds more principled, but I think harder, since
 it seems that GHC doesn't track visibility for class tyvars like I would
 have imagined:

 {{{
 λ> :type +v eqForallPoly
 eqForallPoly
   :: forall k (f :: k -> *).
      EqForallPoly f =>
      forall (a :: k) (b :: k). f a -> f b -> Bool
 }}}

 I would have expected instead:

 {{{
 λ> :type +v eqForallPoly
 eqForallPoly
   :: forall {k} (f :: k -> *).
      EqForallPoly f =>
      forall (a :: k) (b :: k). f a -> f b -> Bool
 }}}

 But this should probably go in a separate ticket.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13998#comment:6>
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