[GHC] #13262: Allow type synonym family application in instance head if it has no free variables

[GHC]

#13262: Allow type synonym family application in instance head if it has no free
variables
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        Reporter:  ezyang            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  backpack
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:  12680
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ezyang):

 This seems to be more complicated than just lifting the restriction,
 because whenever we have a wanted Show (F T) (where F is the type family),
 we always flatten this into Show a, a ~ F T; and of course, there is never
 an instance of Show for a in the top-level.  I'm not entirely sure how the
 solver solves this when Show (F T) is a local constraint; some guidance
 here would be appreciated.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13262#comment:9>
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