[GHC] #12832: GHC infers too simplified contexts

[GHC]

#12832: GHC infers too simplified contexts
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        Reporter:  danilo2           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by danilo2):

 @simon, I understand this note (to some degree) and its motivation. In its
 example it makes perfect sense not to throw an overlap error and compile
 just fine. But in my example it is just wrong nad I dont see any reason
 for it (maybe I dont understand something here). Anyway – when I create:

 {{{
 instance Run m Int where
     run = test
 }}}

 It could be possible that I wanted to choose the `Test IM a` as
 constraint. It could be possible and it would make sense here. Exactly the
 same sense as the second option available. Why is GHC just blindly
 choosing one option and throwing a very misguiding error to the user?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12832#comment:4>
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