[GHC] #13214: Orphan instances in Backpack signatures don't work

[GHC]

#13214: Orphan instances in Backpack signatures don't work
-------------------------------------+-------------------------------------
           Reporter:  ezyang         |             Owner:  ezyang
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:  backpack       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{
 unit p where
     signature A where
         instance Show (a -> b)
     module B where
         import A
         f = show (\x -> x)
 }}}

 There are probably other problems too but here's the start.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13214>
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