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

[GHC]

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

 I don't see how this is different from #14860. It looks the same to me.

 But I think `(x ~ F b, forall a. a => x)` and `forall a. a => F b` really
 are different. It's all about shadowing. The first casts a much bigger
 shadow, one whose shape can be specified. The second's shadow is too oddly
 shaped to be predictable.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15347#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler