[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):

 > this doesn't explain what specifically GHC "checks it against" in this
 case

 It's worth reading the original paper -- though alas I cannot find PDF
 online.

 Given
 {{{
 class C a b | a -> b
 instance C Int Bool
 }}}
 if GHC finds a wanted constraint `[W] C Int t`, it compares it against all
 the top-level instances to see if the fist argument of `C` matches the one
 in the instance.  It succeeds in this case, and emits a new Derived
 constraint `[D] t ~ Bool`.

 Quantified constraints behave very like top-level instances, so they too
 should be examined in this search.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15351#comment:8>
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