[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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Changes (by RyanGlScott):

 * cc: RyanGlScott (added)


Comment:

 It works on GHC 8.0.1 and earlier, but not on 8.0.2 or later. This can be
 explained by the fact that the way GHC typechecks default implementations
 received a drastic overhaul in 8.0.2. Unfortunately, that's been the root
 of many bugs...

 That being said, I have a hunch what is going on here. From inspecting the
 `-ddump-tc-trace` output, one can observe that GHC gives the following
 type signature for `$dmeqForallPoly`:

 {{{
 tcInferId
   Main.$dmeqForallPoly :: forall k (f :: k -> *).
                           EqForallPoly f =>
                           forall (a :: k) (b :: k). TestEquality f => f a
 -> f b -> Bool
 }}}

 Notice that the `k` comes //before// `f`. However, in the generated
 `EqForallPoly Value` instance (which can be seen with `-ddump-deriv`):

 {{{
 Main.EqForallPoly [Main.Atom, Main.Value]
   Main.eqForallPoly = Main.$dmeqForallPoly @(Main.Value)
 }}}

 It's using `TypeApplications` to instantiate `k` to `Value`. This is
 wrong, however, since we really want to be instantiating //`f`// to
 `Value`. That is, we'd rather generate this code:

 {{{
 instance EqForallPoly Value where
   eqForallPoly = $dmeqForallPoly @Atom @Value
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13998#comment:2>
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