[GHC] #15800: Overlapping instances error with single instance

[GHC]

#15800: Overlapping instances error with single instance
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        Reporter:  roland            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I've tried to improve the docs. The key is that GHC looks at ''both'' top
 level instances ''and'' in-scope constraints, not just the former.  In the
 error messages they are listed as "Matching instances" and "Matching
 givens" respectively.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15800#comment:2>
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