[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 goldfire):

 The challenge impredicativity causes is in knowing where there is an
 invisible parameter. (I'm pretty sure that's the challenge, at least.) So
 if `x :: ty` and we don't know whether or not `ty` is a `forall`-type, we
 have a problem. This simply doesn't happen with constraints, so we avoid
 the challenge.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14968#comment:6>
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