[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):

 Actually, I think the OP's use case should be OK. There is a big
 difference between the ''type'' `(forall x. x -> x, Int)` and the
 ''constraint'' `(forall x. A x => B x, Show x)`. The former is an
 impredicative use of the `(,)` constructor, while the second is not
 considered to be so. I think this is a simple case of updating the
 validity checker to allow the new thing. :)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14968#comment:4>
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