[GHC] #14968: QuantifiedConstraints: Can't be RHS of type family instances

[GHC]

#14968: QuantifiedConstraints: Can't be RHS of type family instances
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        Reporter:  josef             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9269             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 The challenge is knowing whether a type `(F t)` might reduce to a
 polytype.  Currently that cannot happen.  Allowing it to happen gives all
 the same problems as impredicativity in general. So I think that allowing
 a forall on the RHS of a `type instance` is a big step.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14968#comment:8>
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