[GHC] #12175: Instance resolution regression

[GHC]

#12175: Instance resolution regression
-------------------------------------+-------------------------------------
        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:  indexed-
  valid program                      |  types/should_compile/T12175
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by simonpj):

 * testcase:   => indexed-types/should_compile/T12175
 * status:  new => merge
 * milestone:   => 8.0.2


Comment:

 Thanks for the report. It's a pretty delicate case, but it turned out to
 be easy to fix.

 Probably worth merging to 8.0.2. Not super-important if it doesn't go.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12175#comment:2>
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