[GHC] #8441: Allow family instances in an hs-boot file

[GHC]

#8441: Allow family instances in an hs-boot file
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       Reporter:  goldfire          |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  low               |         Milestone:
      Component:  Compiler          |           Version:  7.7
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Compiling an .hs-boot file with a data instance leads to this error:

 {{{
     Illegal family instance in hs-boot file
 }}}

 Yet, data instances make good sense in an hs-boot file, in case another
 module wants to access a constructor declared in the instance.

 Furthermore, it may sometimes be desirable to allow type family instances
 in a hs-boot file, as well, in case some including modules need the
 instance for type simplification.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8441>
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