[GHC] #15347: QuantifiedConstraints: Implication constraints with type families don't work

[GHC]

#15347: QuantifiedConstraints: Implication constraints with type families don't
work
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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 goldfire):

 OK. I see how this is different than #14860. But I still claim that GHC
 should not do this rewriting, because having a variable in a quantified
 constraint head casts a longer shadow than having a type family. And it's
 all about that shadow.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15347#comment:6>
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