[GHC] #12580: Eagerly simplify inherently-coherent instances

[GHC]

#12580: Eagerly simplify inherently-coherent instances
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        Reporter:  dfeuer            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
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):

 For the instance to be accepted, don't we also need (`constraintI` implies
 `constraintC`)? Perhaps that's what you mean by the constraints being
 identical.

 To rephrase your proposal, you wish solving for `C` to pretend that
 `IncoherentInstances` is in effect, reducing to
 `constraintC`/`constraintI` even when another given is in scope. This
 seems plausible at first glance.

 Do you have a full concrete example of where this would change behavior?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12580#comment:1>
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