[GHC] #15351: QuantifiedConstraints ignore FunctionalDependencies

[GHC]

#15351: QuantifiedConstraints ignore FunctionalDependencies
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------

Comment (by aaronvargo):

 Replying to [comment:6 simonpj]:
 > Would anyone like to fix this?

 How difficult would it be? I might like to try, but I've never contributed
 to GHC before. I could see it being pretty simple since most of the logic
 should already exist for top-level instances, but I wouldn't know. And I
 might need some hand-holding...

 > It's intriguing -- I never expected people to explore the outer limits
 of quantified constraints so rapidly.

 I would guess this is partly because we could already (somewhat painfully)
 manually represent such constraints with `Dict`s, so we already had some
 idea of what they might be useful for, and want them to be able to do
 everything we can do with `Dict`s (though that turns out to be
 unreasonable).

 I came across this particular issue while trying to work around #15347,
 but I'll write more about that on that ticket (at some point. Maybe after
 I can get my stupid example to work).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15351#comment:9>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler