[GHC] #12224: Replace placeHolderPunRhs with a PlaceHolder value

[GHC]

#12224: Replace placeHolderPunRhs with a PlaceHolder value
-------------------------------------+-------------------------------------
        Reporter:  alanz             |                Owner:  alanz
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
  (Parser)                           |
      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:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 alanz, can you show me the diff? From the error message, I'm wondering if
 you're trying to do something like `instance Functor (HsRecField' id)`,
 which won't work. You need to instantiate `id` with a concrete type in
 each `Functor` instance in order to `PostRn id` to reduce enough.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12224#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler