[GHC] #11599: Why is UndecidableInstances required for an obviously terminating type family?

[GHC]

#11599: Why is UndecidableInstances required for an obviously terminating type
family?
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        Reporter:  heisenbug         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 The test is conservative.  It could certainly be improved.  The trick is
 how.  A good project for someone.  Lots of prior art!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11599#comment:1>
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