[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):

 I'm 99% sure the culprit is
 [http://git.haskell.org/ghc.git/blob/d7b17517e26007f537feab490509c0e13e0e239a:/compiler/typecheck/TcTyDecls.hs#l771
 here]:

 {{{#!hs
 mkDefaultMethodType :: Class -> Id -> DefMethSpec Type -> Type
 -- Returns the top-level type of the default method
 mkDefaultMethodType _ sel_id VanillaDM        = idType sel_id
 mkDefaultMethodType cls _   (GenericDM dm_ty) = mkSpecSigmaTy cls_tvs
 [pred] dm_ty
    where
      cls_tvs = classTyVars cls
      pred    = mkClassPred cls (mkTyVarTys cls_tvs)
 }}}

 The second case (`GenericDM`) is what constructs the type of
 `$dmeqForallPoly`. Notice that it's using `mkSpecSigmaTy` to prepend the
 `forall k (f :: k -> *). EqForallPoly f => ...` bit to the rest of the
 type. However, because it's using `mkSpecSigmaTy`, every class tyvar
 binder has a visibility of `Specified`, meaning they'll all be available
 for visible type application, which isn't what we want.

 However, I'm not sure how to inform GHC that `k` should be invisible, but
 `f` should be visible, in this code. The problem is that the `Class` data
 type
 [http://git.haskell.org/ghc.git/blob/d7b17517e26007f537feab490509c0e13e0e239a:/compiler/types/Class.hs#l52
 doesn't store any visibility information for its type variables]...

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13998#comment:4>
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