[GHC] #13262: Allow type synonym family application in instance head if it reduces away

[GHC]

#13262: Allow type synonym family application in instance head if it reduces away
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        Reporter:  ezyang            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      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:                    |
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Comment (by rwbarton):

 Perhaps we should guard the "first, we reduce type families as much as
 possible" step with the `TypeSynonymInstances` extension (which is implied
 by `FlexibleInstances`, apparently).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13262#comment:1>
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