[GHC] #12647: Can't capture improvement of functional dependencies

[GHC]

#12647: Can't capture improvement of functional dependencies
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        Reporter:  Iceland_jack      |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  None              |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  FunctionalDependencies
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):

 Instance improvement happens only when you have a "wanted" constraint --
 that is, when you're typechecking a function call of `f` where `f` has a
 class constraint with a functional dependency.

 Does that help?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12647#comment:3>
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