[GHC] #10488: Inconsistent reduction of type family

[GHC]

#10488: Inconsistent reduction of type family
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        Reporter:  goldfire          |                   Owner:
            Type:  bug               |                  Status:  merge
        Priority:  highest           |               Milestone:  7.10.2
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:  Phab:D955         |                Blocking:
                                     |  Differential Revisions:
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Comment (by goldfire):

 No test case added, as I was unable to tickle the bug without
 `singletons`. Besides, the fix is so straightforward, I'm not worried
 about a regression here.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10488#comment:4>
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