[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):

 But wait...this compiles:

 {{{#!hs
 type family F a :: Constraint

 uhh :: (a => Eq b, F b) => Dict (Eq Int)
 uhh = Dict
 }}}

 Now add:

 {{{#!hs
 type instance F b = b ~ Int
 }}}

 {{{
 • Could not deduce: a arising from a use of ‘Dict’
 }}}

 Maybe this sort of problem is unavoidable without backtracking.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15347#comment:12>
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