[GHC] #12551: Make type indices take local constraints into account in type instance declaration

[GHC]

#12551: Make type indices take local constraints into account in type instance
declaration
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        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  TypeFamilies
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 goldfire):

 What you wish to do was possible previously (7.8? 7.6?) but it caused
 quite a bit of complication in processing associated type instances. Simon
 proposed restricting the syntax of associated type instances to require
 that the instances be defined over precisely the types in the head of the
 class instance. No one objected. So he made the change. Your "workaround"
 is precisely the way to do what you want.

 It is a bit frustrating in this particular case, where what you want is
 "obviously" correct, but I don't quite think it's worth restoring the old,
 complicated implementation of associated types here.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12551#comment:3>
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