[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:                    |
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Comment (by aaronvargo):

 So I guess what I want is for the given `a => F b` (in `foo`) to be
 replaced with the givens:

 {{{
 a => x
 x ~ F b
 }}}

 (as in `qux`) and for the given

 {{{#!hs
 forall a. Ob (Dom f) a => Ob (Cod f) (f a)
 }}}

 to be replaced with:

 {{{#!hs
 forall a. Ob (Dom f) a => Ob x (f a)
 x ~ Cod f
 }}}

 or something like that.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15347#comment:2>
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