[GHC] #7543: Constraint synonym instances

[GHC]

#7543: Constraint synonym instances
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        Reporter:  monoidal          |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.6.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by goldfire):

 Building on comment:1, would it be easy to build this feature if the names
 of the methods have no scoping conflicts? That is, just do a normal (non-
 instance-style) lookup of method names, even in instances. If there is no
 ambiguity, proceed. If there is ambiguity, look at the instance head. If
 it's a class name, use that to disambiguate. Otherwise (if it's a synonym
 or bogus), report an error. That seems simple enough and without undue
 upheaval.

 Personally, I don't think disambiguating among multiply-in-scope names
 should hold this feature up.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7543#comment:7>
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