[GHC] #10011: The Data instance for Ratio violates internal invariants.

[GHC]

#10011: The Data instance for Ratio violates internal invariants.
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        Reporter:  ekmett            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:  7.10.1
       Component:  Core Libraries    |                 Version:  7.10.1-rc1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:  Phab:D625
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Comment (by hvr):

 The following was suggested as regression test by Edward:

 {{{
 >>> :set -XScopedTypeVariables -XTypeOperators -XGADTs
 >>> :m + Control.Lens Data.Data.Lens Data.Ratio
 >>> let bad = gmapT (\(x :: b) -> case eqT :: Maybe (b :~: Integer) of
 Nothing -> x; Just Refl -> x * 2) (1 % 2) :: Rational in bad == numerator
 bad % denominator bad
 False
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10011#comment:12>
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