[GHC] #15351: QuantifiedConstraints ignore FunctionalDependencies

[GHC]

#15351: QuantifiedConstraints ignore FunctionalDependencies
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        Reporter:  aaronvargo        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
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):

 Yes, you are right.  Generally, when solving a constraint, GHC checks it
 against all in-scope instances in case there's fundep match.  The same
 should happen for in-scope quantified constraints.

 Would anyone like to fix this?

 It's intriguing -- I never expected people to explore the outer limits of
 quantified constraints so rapidly. I hope that means that they are proving
 useful (when not so close to the limits).  Is that so?  Experience reports
 or blog posts (like Ryan Scott's) would be fascinating.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15351#comment:6>
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