[GHC] #9858: Typeable instances should be kind-aware

[GHC]

#9858: Typeable instances should be kind-aware
-------------------------------------+-------------------------------------
        Reporter:  dreixel           |                   Owner:
            Type:  bug               |                  Status:  merge
        Priority:  highest           |               Milestone:  7.10.2
       Component:  Compiler          |                 Version:  7.9
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |  typecheck/should_fail/T9858a,
                                     |  should_run/T9858b
                                     |                Blocking:
                                     |  Differential Revisions:  Phab:D652
-------------------------------------+-------------------------------------

Comment (by goldfire):

 After some discussion, we propose the following:
  * Restore `Typeable` for constraints.
  * Forbid `Typeable` for types with `=>`, as those types are higher-rank-
 ish.
  * Ensure that the `TypeRep`s for `() :: *` and `() :: Constraint` are
 distinct, and similarly for higher arities.

 This seems to solve the problems stated here.

 For what it's worth, I agree the statements in comment:92, but we can do
 the trick for all arities, not just 0.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9858#comment:93>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler