[GHC] #14488: Can't define a lens for a field with a constraint

[GHC]

#14488: Can't define a lens for a field with a constraint
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        Reporter:  int-index         |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4213
       Wiki Page:                    |
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Comment (by simonpj):

 Thanks!  Can someone pls commit?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14488#comment:3>
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