[GHC] #14331: Overzealous free-floating kind check causes deriving clause to be rejected

[GHC]

#14331: Overzealous free-floating kind check causes deriving clause to be rejected
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.2.2
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  deriving/should_compile/T14331
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Huh? That is not what I would have expected at all. I would have expected:

 {{{#!hs
 class C {k1 k2} (a :: k1) (b :: k2)
 data D {k3} (r :: k3) = D

 instance forall k1 (a :: k1). C a (D a) where ...
 }}}

 In other words, you should unify `r` with `a`. After all, the `a` in `data
 D a = D deriving (C (a :: k))` scopes over the data type, not the
 `deriving` clause!

 Now if you had chosen to use a different scoping with `data D a = D
 deriving (forall (a :: k). C a)`, //then// I could see the instance being
 derived that you suggested. But I don't think users would expect that
 behavior to be the default (that is, in lieu of explicit `forall`s on a
 `deriving` clause, one should assume that the user-written type variables
 are bound by the data type if they appear in the `<tvbs>` in `data D
 <tvbs>`).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14331#comment:14>
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